Tracking issue for exclusive range patterns #37854

oli-obk · 2016-11-18T11:33:43Z

Summary

This feature allows exclusive integer ranges (such as 0..100) in patterns.

fn is_digit(x: u32) -> bool {
    match x {
        0..10 => true,
        10.. => false,
    }
}

Concerns

~~Conflict with the syntax of slice patterns Tracking issue for RFC #495 (features slice_patterns and advanced_slice_patterns) #23121~~
- Tracking issue for #![feature(slice_patterns)] #62254 (comment)
General concern about the syntax of inclusive/exclusive patterns

History and status

Proposed and implemented in PR exclusive range patterns #35712
- approved provisionally by lang team without an RFC

The text was updated successfully, but these errors were encountered:

ghost · 2017-07-30T23:57:14Z

EDIT: the following is no longer true 12 jan 2019 nightly debug 2018:

Missing warning: unreachable pattern in the case when 1...10 then 1..10 are present in that order. That warning is present in case of 1..12 then 1..11 for example.

Example (playground link):

#![feature(exclusive_range_pattern)]
fn main() {
    let i = 9; //this hits whichever branch is first in the match without warning
    match i {
        1...10 => println!("hit inclusive"),
        1..10 => println!("hit exclusive"),//FIXME: there's no warning here
        // ^ can shuffle the above two, still no warnings.
        1...10 => println!("hit inclusive2"),//this does get warning: unreachable pattern
        1..10 => println!("hit exclusive2"),//this also gets warning: unreachable pattern

        _ => (),
    };

    //expecting a warning like in this:
    match i {
        1..12 => println!("hit 12"),
        1..11 => println!("hit 11"),//warning: unreachable pattern
        _ => (),
    }
}

derekdreery · 2018-06-05T12:50:17Z

I've just been saved for the second time by the compiler when I did 1 .. 10, but meant 1 ..= 10. It might be worth considering that the compiler won't catch this error if exclusive ranges are allowed.

This will print AC if on external power supply. The coloring can be simplified as soon as range matches are stabilized: rust-lang/rust#37854

donbright · 2019-01-12T22:20:23Z

seems like exclusive range match would help to improve consistency ... why can i do range in a loop but not in a pattern?

 const MAXN:usize = 14;
 ... 
   for x in 0..MAXN  { blahblahblah(); }
 ...
   let s = match n {
      0..MAXN=>borkbork(),
      MAXN=>lastone(),
      _=>urgh(),
   };

also note that the following is not possible currently because the -1 will break the compile with "expected one of ::, =>, if, or | here"

   let s = match n {
      0..=MAXN-1=>borkbork(),
      MAXN=>lastone(),
      _=>urgh(),
   };

thank you

Getting the following compilation error when directive is not specified: error[E0658]: exclusive range pattern syntax is experimental --> src/vga_buffer.rs:74:17 | 74 | 0x20..0x7e | b'\n' => self.write_byte(byte), | ^^^^^^^^^^ | = note: for more information, see rust-lang/rust#37854 = help: add #![feature(exclusive_range_pattern)] to the crate attributes to enable

kentfredric · 2019-06-26T02:50:39Z

@donbright I know its a poor consolation, but a good workaround which ensures backwards compatibility might be:

        match i {
            0..=MAXN if i < MAXN => println!("Exclusive: {}", i),
            MAXN => println!("Limit: {}", i),
            _ => println!("Out of range: {}", i)
        }

playground

Figured this suggestion might help some poor soul who came here and also battled with not being allowed to use 0..=MAXN-1

bcantrill · 2020-01-16T18:52:59Z

It doesn't sound like much is going to happen on this issue, but (FWIW) I would argue against its inclusion in stable Rust: exclusive ranges on pattern matching makes it very easy to introduce bugs, as exclusive ranges are very rarely what the programmer wants in a pattern match (emphasis on match!). In these cases, I would argue that having the programmer be explicit is helpful; there is no reason to offer syntactic sugar in this case. (I landed here because of finding several bugs in a project that had turned this feature on; see tock/tock#1544 for details.)

@matthewjasper

…=matthewjasper Stabilize `#![feature(slice_patterns)]` in 1.42.0 # Stabilization report The following is the stabilization report for `#![feature(slice_patterns)]`. This report is the collaborative effort of @matthewjasper and @Centril. Tracking issue: rust-lang#62254 [Version target](https://forge.rust-lang.org/#current-release-versions): 1.42 (2020-01-30 => beta, 2020-03-12 => stable). ## Backstory: slice patterns It is already possible to use slice patterns on stable Rust to match on arrays and slices. For example, to match on a slice, you may write: ```rust fn foo(slice: &[&str]) { match slice { [] => { dbg!() } [a] => { dbg!(a); } [a, b] => { dbg!(a, b); } _ => {} // ^ Fallback -- necessary because the length is unknown! } } ``` To match on an array, you may instead write: ```rust fn bar([a, b, c]: [u8; 3]) {} // --------- Length is known, so pattern is irrefutable. ``` However, on stable Rust, it is not yet possible to match on a subslice or subarray. ## A quick user guide: Subslice patterns The ability to match on a subslice or subarray is gated under `#![feature(slice_patterns)]` and is what is proposed for stabilization here. ### The syntax of subslice patterns Subslice / subarray patterns come in two flavors syntactically. Common to both flavors is they use the token `..`, referred as a *"rest pattern"* in a pattern context. This rest pattern functions as a variable-length pattern, matching whatever amount of elements that haven't been matched already before and after. When `..` is used syntactically as an element of a slice-pattern, either directly (1), or as part of a binding pattern (2), it becomes a subslice pattern. On stable Rust, a rest pattern `..` can also be used in a tuple or tuple-struct pattern with `let (x, ..) = (1, 2, 3);` and `let TS(x, ..) = TS(1, 2, 3);` respectively. ### (1) Matching on a subslice without binding it ```rust fn base(string: &str) -> u8 { match string.as_bytes() { [b'0', b'x', ..] => 16, [b'0', b'o', ..] => 8, [b'0', b'b', ..] => 2, _ => 10, } } fn main() { assert_eq!(base("0xFF"), 16); assert_eq!(base("0x"), 16); } ``` In the function `base`, the pattern `[b'0', b'x', ..]` will match on any byte-string slice with the *prefix* `0x`. Note that `..` may match on nothing, so `0x` is a valid match. ### (2) Binding a subslice: ```rust fn main() { #[derive(PartialEq, Debug)] struct X(u8); let xs: Vec<X> = vec![X(0), X(1), X(2)]; if let [start @ .., end] = &*xs { // --- bind on last element, assuming there is one. // ---------- bind the initial elements, if there are any. assert_eq!(start, &[X(0), X(1)] as &[X]); assert_eq!(end, &X(2)); let _: &[X] = start; let _: &X = end; } } ``` In this case, `[start @ .., end]` will match any non-empty slice, binding the last element to `end` and any elements before that to `start`. Note in particular that, as above, `start` may match on the empty slice. ### Only one `..` per slice pattern In today's stable Rust, a tuple (struct) pattern `(a, b, c)` can only have one subtuple pattern (e.g., `(a, .., c)`). That is, if there is a rest pattern, it may only occur once. Any `..` that follow, as in e.g., `(a, .., b, ..)` will cause an error, as there is no way for the compiler to know what `b` applies to. This rule also applies to slice patterns. That is, you may also not write `[a, .., b, ..]`. ## Motivation [PR rust-lang#67569]: https://github.com/rust-lang/rust/pull/67569/files Slice patterns provide a natural and efficient way to pattern match on slices and arrays. This is particularly useful as slices and arrays are quite a common occurence in modern software targeting modern hardware. However, as aforementioned, it's not yet possible to perform incomplete matches, which is seen in `fn base`, an example taken from the `rustc` codebase itself. This is where subslice patterns come in and extend slice patterns with the natural syntax `xs @ ..` and `..`, where the latter is already used for tuples and tuple structs. As an example of how subslice patterns can be used to clean up code, we have [PR rust-lang#67569]. In this PR, slice patterns enabled us to improve readability and reduce unsafety, at no loss to performance. ## Technical specification ### Grammar The following specification is a *sub-set* of the grammar necessary to explain what interests us here. Note that stabilizing subslice patterns does not alter the stable grammar. The stabilization contains purely semantic changes. ```rust Binding = reference:"ref"? mutable:"mut"? name:IDENT; Pat = | ... // elided | Rest: ".." | Binding:{ binding:Binding { "@" subpat:Pat }? } | Slice:{ "[" elems:Pat* %% "," "]" } | Paren:{ "(" pat:Pat ")" } | Tuple:{ path:Path? "(" elems:Pat* &% "," ")" } ; ``` Notes: 1. `(..)` is interpreted as a `Tuple`, not a `Paren`. This means that `[a, (..)]` is interpreted as `Slice[Binding(a), Tuple[Rest]]` and not `Slice[Binding(a), Paren(Rest)]`. ### Name resolution [resolve_pattern_inner]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.LateResolutionVisitor.html#method.resolve_pattern_inner [product context]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/enum.PatBoundCtx.html#variant.Product A slice pattern is [resolved][resolve_pattern_inner] as a [product context] and `..` is given no special treatment. ### Abstract syntax of slice patterns The abstract syntax (HIR level) is defined like so: ```rust enum PatKind { ... // Other unimportant stuff. Wild, Binding { binding: Binding, subpat: Option<Pat>, }, Slice { before: List<Pat>, slice: Option<Pat>, after: List<Pat>, }, } ``` [`hir::PatKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/hir/enum.PatKind.html The executable definition is found in [`hir::PatKind`]. ### Lowering to abstract syntax Lowering a slice pattern to its abstract syntax proceeds by: 1. Lowering each element pattern of the slice pattern, where: 1. `..` is lowered to `_`, recording that it was a subslice pattern, 2. `binding @ ..` is lowered to `binding @ _`, recording that it was a subslice pattern, 3. and all other patterns are lowered as normal, recording that it was not a subslice pattern. 2. Taking all lowered elements until the first subslice pattern. 3. Take all following elements. If there are any, 1. The head is the sub-`slice` pattern. 2. The tail (`after`) must not contain a subslice pattern, or an error occurs. [`LoweringContext::lower_pat_slice`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/hir/lowering/struct.LoweringContext.html#method.lower_pat_slice The full executable definition can be found in [`LoweringContext::lower_pat_slice`]. ### Type checking slice patterns #### Default binding modes [non-reference pattern]: https://doc.rust-lang.org/nightly/reference/patterns.html#binding-modes [`is_non_ref_pat`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/check/struct.FnCtxt.html#method.is_non_ref_pat [peel_off_references]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/check/struct.FnCtxt.html#method.peel_off_references A slice pattern is a [non-reference pattern] as defined in [`is_non_ref_pat`]. This means that when type checking a slice pattern, as many immediate reference types are [peeled off][peel_off_references] from the `expected` type as possible and the default binding mode is adjusted to by-reference before checking the slice pattern. See rust-lang#63118 for an algorithmic description. [RFC 2359]: https://github.com/rust-lang/rfcs/blob/master/text/2359-subslice-pattern-syntax.md [rfc-2359-gle]: https://github.com/rust-lang/rfcs/blob/master/text/2359-subslice-pattern-syntax.md#guide-level-explanation See [RFC 2359]'s [guide-level explanation][rfc-2359-gle] and the tests listed below for examples of what effect this has. #### Checking the pattern Type checking a slice pattern proceeds as follows: 1. Resolve any type variables by a single level. If the result still is a type variable, error. 2. Determine the expected type for any subslice pattern (`slice_ty`) and for elements (`inner_ty`) depending on the expected type. 1. If the expected type is an array (`[E; N]`): 1. Evaluate the length of the array. If the length couldn't be evaluated, error. This may occur when we have e.g., `const N: usize`. Now `N` is known. 2. If there is no sub-`slice` pattern, check `len(before) == N`, and otherwise error. 3. Otherwise, set `S = N - len(before) - len(after)`, and check `N >= 0` and otherwise error. Set `slice_ty = [E; S]`. Set `inner_ty = E`. 2. If the expected type is a slice (`[E]`), set `inner_ty = E` and `slice_ty = [E]`. 3. Otherwise, error. 3. Check each element in `before` and `after` against `inner_ty`. 4. If it exists, check `slice` against `slice_ty`. [`check_pat_slice`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/check/struct.FnCtxt.html#method.check_pat_slice For an executable definition, see [`check_pat_slice`]. ### Typed abstract syntax of slice and array patterns The typed abstract syntax (HAIR level) is defined like so: ```rust enum PatKind { ... // Other unimportant stuff. Wild, Binding { ... // Elided. } Slice { prefix: List<Pat>, slice: Option<Pat>, suffix: List<Pat>, }, Array { prefix: List<Pat>, slice: Option<Pat>, suffix: List<Pat>, }, } ``` [`hair::pattern::PatKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/hair/pattern/enum.PatKind.html The executable definition is found in [`hair::pattern::PatKind`]. ### Lowering to typed abstract syntax Lowering a slice pattern to its typed abstract syntax proceeds by: 1. Lowering each pattern in `before` into `prefix`. 2. Lowering the `slice`, if it exists, into `slice`. 1. A `Wild` pattern in abstract syntax is lowered to `Wild`. 2. A `Binding` pattern in abstract syntax is lowered to `Binding { .. }`. 3. Lowering each pattern in `after` into `after`. 4. If the type is `[E; N]`, construct `PatKind::Array { prefix, slice, after }`, otherwise `PatKind::Slice { prefix, slice, after }`. [`PatCtxt::slice_or_array_pattern`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/hair/pattern/struct.PatCtxt.html#method.slice_or_array_pattern The executable definition is found in [`PatCtxt::slice_or_array_pattern`]. ### Exhaustiveness checking Let `E` be the element type of a slice or array. - For array types, `[E; N]` with a known length `N`, the full set of constructors required for an exahustive match is the sequence `ctors(E)^N` where `ctors` denotes the constructors required for an exhaustive match of `E`. - Otherwise, for slice types `[E]`, or for an array type with an unknown length `[E; ?L]`, the full set of constructors is the infinite sequence `⋃_i=0^∞ ctors(E)^i`. This entails that an exhaustive match without a cover-all pattern (e.g. `_` or `binding`) or a subslice pattern (e.g., `[..]` or `[_, _, ..]`) is impossible. - `PatKind::{Slice, Array}(prefix, None, suffix @ [])` cover a sequence of of `len(prefix)` covered by `patterns`. Note that `suffix.len() > 0` with `slice == None` is unrepresentable. - `PatKind::{Slice, Array}(prefix, Some(s), suffix)` cover a `sequence` with `prefix` as the start and `suffix` as the end and where `len(prefix) + len(suffix) <= len(sequence)`. The `..` in the middle is interpreted as an unbounded number of `_`s in terms of exhaustiveness checking. ### MIR representation The relevant MIR representation for the lowering into MIR, which is discussed in the next section, includes: ```rust enum Rvalue { // ... /// The length of a `[X]` or `[X; N]` value. Len(Place), } struct Place { base: PlaceBase, projection: List<PlaceElem>, } enum ProjectionElem { // ... ConstantIndex { offset: Nat, min_length: Nat, from_end: bool, }, Subslice { from: Nat, to: Nat, from_end: bool, }, } ``` ### Lowering to MIR * For a slice pattern matching a slice, where the pattern has `N` elements specified, there is a check that the `Rvalue::Len` of the slice is at least `N` to decide if the pattern can match. * There are two kinds of `ProjectionElem` used for slice patterns: 1. `ProjectionElem::ConstantIndex` is an array or slice element with a known index. As a shorthand it's written `base[offset of min_length]` if `from_end` is false and `base[-offset of min_length]` if `from_end` is true. `base[-offset of min_length]` is the `len(base) - offset`th element of `base`. 2. `ProjectionElem::Subslice` is a subslice of an array or slice with known bounds. As a shorthand it's written `base[from..to]` if `from_end` is false and `base[from:-to]` if `from_end` is true. `base[from:-to]` is the subslice `base[from..len(base) - to]`. * Note that `ProjectionElem::Index` is used for indexing expressions, but not for slice patterns. It's written `base[idx]`. * When binding an array pattern, any individual element binding is lowered to an assignment or borrow of `base[offset of len]` where `offset` is the element's index in the array and `len` is the array's length. * When binding a slice pattern, let `N` be the number of elements that have patterns. Elements before the subslice pattern (`prefix`) are lowered to `base[offset of N]` where `offset` is the element's index from the start. Elements after the subslice pattern (`suffix`) are lowered to `base[-offset of N]` where `offset` is the element's index from the end, plus 1. * Subslices of arrays are lowered to `base[from..to]` where `from` is the number of elements before the subslice pattern and `to = len(array) - len(suffix)` is the length of the array minus the number of elements after the subslice pattern. * Subslices of slices are lowered to `base[from:-to]` where `from` is the number of elements before the subslice pattern (`len(prefix)`) and `to` is the number of elements after the subslice pattern (`len(suffix)`). ### Safety and const checking * Subslice patterns do not introduce any new unsafe operations. * As subslice patterns for arrays are irrefutable, they are allowed in const contexts. As are `[..]` and `[ref y @ ..]` patterns for slices. However, `ref mut` bindings are only allowed with `feature(const_mut_refs)` for now. * As other subslice patterns for slices require a `match`, `if let`, or `while let`, they are only allowed with `feature(const_if_match, const_fn)` for now. * Subslice patterns may occur in promoted constants. ### Borrow and move checking * A subslice pattern can be moved from if it has an array type `[E; N]` and the parent array can be moved from. * Moving from an array subslice pattern moves from all of the elements of the array within the subslice. * If the subslice contains at least one element, this means that dynamic indexing (`arr[idx]`) is no longer allowed on the array. * The array can be reinitialized and can still be matched with another slice pattern that uses a disjoint set of elements. * A subslice pattern can be mutably borrowed if the parent array/slice can be mutably borrowed. * When determining whether an access conflicts with a borrow and at least one is a slice pattern: * `x[from..to]` always conflicts with `x` and `x[idx]` (where `idx` is a variable). * `x[from..to]` conflicts with `x[idx of len]` if `from <= idx` and `idx < to` (that is, `idx ∈ from..to`). * `x[from..to]` conflicts with `x[from2..to2]` if `from < to2` and `from2 < to` (that is, `(from..to) ∩ (from2..to2) ≠ ∅`). * `x[from:-to]` always conflicts with `x`, `x[idx]`, and `x[from2:-to2]`. * `x[from:-to]` conflicts with `x[idx of len]` if `from <= idx`. * `x[from:-to]` conflicts with `x[-idx of len]` if `to < idx`. * A constant index from the end conflicts with other elements as follows: * `x[-idx of len]` always conflicts with `x` and `x[idx]`. * `x[-idx of len]` conflicts with `x[-idx2 of len2]` if `idx == idx2`. * `x[-idx of len]` conflicts with `x[idx2 of len2]` if `idx + idx2 >= max(len, len2)`. ## Tests The tests can be primarily seen in the PR itself. Here are some of them: ### Parsing (3) * Testing that `..` patterns are syntactically allowed in all pattern contexts (2) * [pattern/rest-pat-syntactic.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/rest-pat-syntactic.rs) * [ignore-all-the-things.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/ignore-allthe-things.rs) * Slice patterns allow a trailing comma, including after `..` (1) * [trailing-comma.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/trailing-comma.rs) ### Lowering (2) * `@ ..` isn't allowed outside of slice patterns and only allowed once in each pattern (1) * [pattern/rest-pat-semantic-disallowed.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/rest-pat-semantic-disallowed.rs) * Mulitple `..` patterns are not allowed (1) * [parser/match-vec-invalid.rs](https://github.com/rust-lang/rust/blob/53712f8637dbe326df569a90814aae1cc5429710/src/test/ui/parser/match-vec-invalid.rs) ### Type checking (5) * Default binding modes apply to slice patterns (2) * [rfc-2005-default-binding-mode/slice.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/rfc-2005-default-binding-mode/slice.rs) * [rfcs/rfc-2005-default-binding-mode/slice.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/rfcs/rfc-2005-default-binding-mode/slice.rs) * Array patterns cannot have more elements in the pattern than in the array (2) * [match/match-vec-mismatch.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/match/match-vec-mismatch.rs) * [error-codes/E0528.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/error-codes/E0528.rs) * Array subslice patterns have array types (1) * [array-slice-vec/subslice-patterns-pass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-pass.rs) ### Exhaustiveness and usefulness checking (20) * Large subslice matches don't stack-overflow the exhaustiveness checker (1) * [pattern/issue-53820-slice-pattern-large-array.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/issue-53820-slice-pattern-large-array.rs) * Array patterns with subslices are irrefutable (1) * [issues/issue-7784.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-7784.rs) * `[xs @ ..]` slice patterns are irrefutable (1) * [binding/irrefutable-slice-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/irrefutable-slice-patterns.rs) * Subslice patterns can match zero-length slices (2) * [issues/issue-15080.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-15080.rs) * [issues/issue-15104.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-15104.rs) * General tests (13) * [issues/issue-12369.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-12369.rs) * [issues/issue-37598.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-37598.rs) * [pattern/usefulness/match-vec-unreachable.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/match-vec-unreachable.rs) * [pattern/usefulness/non-exhaustive-match.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/non-exhaustive-match.rs) * [pattern/usefulness/non-exhaustive-match-nested.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/non-exhaustive-match-nested.rs) * [pattern/usefulness/non-exhaustive-pattern-witness.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/non-exhaustive-pattern-witness.rs) * [pattern/usefulness/65413-constants-and-slices-exhaustiveness.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/65413-constants-and-slices-exhaustiveness.rs) * [pattern/usefulness/match-byte-array-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/match-byte-array-patterns.rs) * [pattern/usefulness/match-slice-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/match-slice-patterns.rs) * [pattern/usefulness/slice-patterns-exhaustiveness.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/slice-patterns-exhaustiveness.rs) * [pattern/usefulness/slice-patterns-irrefutable.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/slice-patterns-irrefutable.rs) * [pattern/usefulness/slice-patterns-reachability.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/slice-patterns-reachability.rs) * [uninhabited/uninhabited-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/uninhabited/uninhabited-patterns.rs) * Interactions with or-patterns (2) * [or-patterns/exhaustiveness-pass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/or-patterns/exhaustiveness-pass.rs) * [or-patterns/exhaustiveness-unreachable-pattern.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/or-patterns/exhaustiveness-unreachable-pattern.rs) ### Borrow checking (28) * Slice patterns can only move from owned, fixed-length arrays (4) * [borrowck/borrowck-move-out-of-vec-tail.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-move-out-of-vec-tail.rs) * [moves/move-out-of-slice-2.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/moves/move-out-of-slice-2.rs) * [moves/move-out-of-array-ref.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/moves/move-out-of-array-ref.rs) * [issues/issue-12567.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-12567.rs) * Moves from arrays are tracked by element (2) * [borrowck/borrowck-move-out-from-array-no-overlap.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-move-out-from-array-no-overlap.rs) * [borrowck/borrowck-move-out-from-array-use-no-overlap.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-move-out-from-array-use-no-overlap.rs) * Slice patterns cannot be used on moved-from slices/arrays (2) * [borrowck/borrowck-move-out-from-array.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-move-out-from-array.rs) * [borrowck/borrowck-move-out-from-array-use.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-move-out-from-array-use.rs) * Slice patterns cannot be used with conflicting borrows (3) * [borrowck/borrowck-describe-lvalue.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-describe-lvalue.rs) * [borrowck/borrowck-slice-pattern-element-loan-array.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-array.rs) * [borrowck/borrowck-slice-pattern-element-loan-slice.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-slice.rs) * Borrows from slice patterns are tracked and only conflict when there is possible overlap (6) * [borrowck/borrowck-slice-pattern-element-loan-array-no-overlap.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-array-no-overlap.rs) * [borrowck/borrowck-slice-pattern-element-loan-slice-no-overlap.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-slice-no-overlap.rs) * [borrowck/borrowck-slice-pattern-element-loan-rpass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-rpass.rs) * [borrowck/borrowck-vec-pattern-element-loan.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-vec-pattern-element-loan.rs) * [borrowck/borrowck-vec-pattern-loan-from-mut.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-vec-pattern-loan-from-mut.rs) * [borrowck/borrowck-vec-pattern-tail-element-loan.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-vec-pattern-tail-element-loan.rs) * Slice patterns affect indexing expressions (1) * [borrowck/borrowck-vec-pattern-move-tail.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-vec-pattern-move-tail.rs) * Borrow and move interactions with `box` patterns (1) * [borrowck/borrowck-vec-pattern-move-tail.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-vec-pattern-move-tail.rs) * Slice patterns correctly affect inference of closure captures (2) * [borrowck/borrowck-closures-slice-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-closures-slice-patterns.rs) * [borrowck/borrowck-closures-slice-patterns-ok.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-closures-slice-patterns-ok.rs) * Interactions with `#![feature(bindings_after_at)]` (7) * [pattern/bindings-after-at/borrowck-move-and-move.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-move-and-move.rs) * [pattern/bindings-after-at/borrowck-pat-at-and-box-pass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-at-and-box-pass.rs) * [pattern/bindings-after-at/borrowck-pat-at-and-box.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-at-and-box.rs) * [pattern/bindings-after-at/borrowck-pat-by-copy-bindings-in-at.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-by-copy-bindings-in-at.rs) * [pattern/bindings-after-at/borrowck-pat-ref-both-sides.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-ref-both-sides.rs) * [pattern/bindings-after-at/borrowck-pat-ref-mut-and-ref.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-ref-mut-and-ref.rs) * [pattern/bindings-after-at/borrowck-pat-ref-mut-twice.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-ref-mut-twice.rs) * Misc (1) * [issues/issue-26619.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-26619.rs) ### MIR lowering (1) * [uniform_array_move_out.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/mir-opt/uniform_array_move_out.rs) ### Evaluation (19) * Slice patterns don't cause leaks or double drops (2) * [drop/dynamic-drop.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/drop/dynamic-drop.rs) * [drop/dynamic-drop-async.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/drop/dynamic-drop-async.rs) * General run-pass tests (10) * [array-slice-vec/subslice-patterns-pass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-pass.rs) * [array-slice-vec/vec-matching-fixed.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/vec-matching-fixed.rs) * [array-slice-vec/vec-matching-fold.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/vec-matching-fold.rs) * [array-slice-vec/vec-matching-legal-tail-element-borrow.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/vec-matching-legal-tail-element-borrow.rs) * [array-slice-vec/vec-matching.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/vec-matching.rs) * [array-slice-vec/vec-tail-matching.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/vec-tail-matching.rs) * [binding/irrefutable-slice-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/irrefutable-slice-patterns.rs) * [binding/match-byte-array-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/match-byte-array-patterns.rs) * [binding/match-vec-alternatives.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/match-vec-alternatives.rs) * [borrowck/borrowck-slice-pattern-element-loan-rpass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-rpass.rs) * Matching a large by-value array (1) * [issues/issue-17877.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-17877.rs) * Uninhabited elements (1) * [binding/empty-types-in-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/empty-types-in-patterns.rs) * Zero-sized elements (3) * [binding/zero_sized_subslice_match.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/zero_sized_subslice_match.rs) * [array-slice-vec/subslice-patterns-const-eval.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-const-eval.rs) * [array-slice-vec/subslice-patterns-const-eval-match.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-const-eval-match.rs) * Evaluation in const contexts (2) * [array-slice-vec/subslice-patterns-const-eval.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-const-eval.rs) * [array-slice-vec/subslice-patterns-const-eval-match.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-const-eval-match.rs) ## Misc (1) * Exercising a case where const-prop cased an ICE (1) * [consts/const_prop_slice_pat_ice.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/consts/const_prop_slice_pat_ice.rs) ## History - 2012-12-08, commit rust-lang@1968cb3 Author: Jakub Wieczorek Reviewers: @graydon This is where slice patterns were first implemented. It is particularly instructive to read the `vec-tail-matching.rs` test. - 2013-08-20, issue rust-lang#8636 Author: @huonw Fixed by @mikhail-m1 in rust-lang#51894 The issue describes a problem wherein the borrow-checker would not consider disjointness when checking mutable references in slice patterns. - 2014-09-03, RFC rust-lang/rfcs#164 Author: @brson Reviewers: The Core Team The RFC decided to feature gate slice patterns due to concerns over lack of oversight and the exhaustiveness checking logic not having seen much love. Since then, the exhaustivenss checking algorithm, in particular for slice patterns, has been substantially refactored and tests have been added. - 2014-09-03, RFC rust-lang/rfcs#202 Author: @krdln Reviewers: The Core Team > Change syntax of subslices matching from `..xs` to `xs..` to be more consistent with the rest of the language and allow future backwards compatible improvements. In 2019, rust-lang/rfcs#2359 changed the syntax again in favor of `..` and `xs @ ..`. - 2014-09-08, PR rust-lang#17052 Author: @pcwalton Reviewers: @alexcrichton and @sfackler This implemented the feature gating as specified in rust-lang/rfcs#164. - 2015-03-06, RFC rust-lang/rfcs#495 Author: @P1start Reviewers: The Core Team The RFC changed array and slice patterns like so: - Made them only match on arrays (`[T; N]`) and slice types (`[T]`), not references to slice types (`& mut? [T]`). - Made subslice matching yield a value of type `[T; N]` or `[T]`, not `& mut? [T]`. - Allowed multiple mutable references to be made to different parts of the same array or slice in array patterns. These changes were made to fit with the introduction of DSTs like `[T]` as well as with e.g. `box [a, b, c]` (`Box<[T]>`) in the future. All points remain true today, in particular with the advent of default binding modes. - 2015-03-22, PR rust-lang#23361 Author: @petrochenkov Reviewers: Unknown The PR adjusted codegen ("trans") such that `let ref a = *"abcdef"` would no longer ICE, paving the way for rust-lang/rfcs#495. - 2015-05-28, PR rust-lang#23794 Author: @brson Reviewers: @nrc The PR feature gated slice patterns in more contexts. - 2016-06-09, PR rust-lang#32202 Author: @arielb1 Reviewers: @eddyb and @nikomatsakis This implemented RFC rust-lang/rfcs#495 via a MIR based implementation fixing some bugs. - 2016-09-16, PR rust-lang#36353 Author: @arielb1 Reviewers: @nagisa, @pnkfelix, and @nikomatsakis The PR made move-checker improvements prohibiting moves out of slices. - 2018-02-17, PR rust-lang#47926 Author: @mikhail-m1 Reviewers: @nikomatsakis This added the `UniformArrayMoveOut` which converted move-out-from-array by `Subslice` and `ConstIndex {.., from_end: true }` to `ConstIndex` move out(s) from the beginning of the array. This fixed some problems with the MIR borrow-checker and drop-elaboration of arrays. Unfortunately, the transformation ultimately proved insufficient for soundness and was removed and replaced in rust-lang#66650. - 2018-02-19, PR rust-lang#48355 Author: @mikhail-m1 Reviewers: @nikomatsakis After rust-lang#47926, this restored some MIR optimizations after drop-elaboration and borrow-checking. - 2018-03-20, PR rust-lang#48516 Author: @petrochenkov Reviewers: @nikomatsakis This stabilized fixed length slice patterns `[a, b, c]` without variable length subslices and moved subslice patterns into `#![feature(slice_patterns)`. See rust-lang#48836 wherein the language team accepted the proposal to stabilize. - 2018-07-06, PR rust-lang#51894 Author: @mikhail-m1 Reviewers: @nikomatsakis rust-lang#8636 was fixed such that the borrow-checker would consider disjointness with respect to mutable references in slice patterns. - 2019-06-30, RFC rust-lang/rfcs#2359 Author: @petrochenkov Reviewers: The Language Team The RFC switched the syntax of subslice patterns to `{$binding @}? ..` as opposed to `.. $pat?` (which was what the RFC originally proposed). This RFC reignited the work towards finishing the implementation and the testing of slice patterns which eventually lead to this stabilization proposal. - 2019-06-30, RFC rust-lang/rfcs#2707 Author: @petrochenkov Reviewers: The Language Team This RFC built upon rust-lang/rfcs#2359 turning `..` into a full-fledged pattern (`Pat |= Rest:".." ;`), as opposed to a special part of slice and tuple patterns, moving previously syntactic restrictions into semantic ones. - 2019-07-03, PR rust-lang#62255 Author: @Centril Reviewers: @varkor This closed the old tracking issue (rust-lang#23121) in favor of the new one (rust-lang#62254) due to the new RFCs having been accepted. - 2019-07-28, PR rust-lang#62550 Author: @Centril Reviewers: @petrochenkov and @eddyb Implemented RFCs rust-lang/rfcs#2707 and rust-lang/rfcs#2359 by introducing the `..` syntactic rest pattern form as well as changing the lowering to subslice and subtuple patterns and the necessary semantic restrictions as per the RFCs. Moreover, the parser was cleaned up to use a more generic framework for parsing sequences of things. This framework was employed in parsing slice patterns. Finally, the PR introduced parser recovery for half-open ranges (e.g., `..X`, `..=X`, and `X..`), demonstrating in practice that the RFCs proposed syntax will enable half-open ranges if we want to add those (which is done in rust-lang#67258). - 2019-07-30, PR rust-lang#63111 Author: @Centril Reviewers: @estebank Added a test which comprehensively exercised the parsing of `..` rest patterns. That is, the PR exercised the specification in rust-lang/rfcs#2707. Moreover, a test was added for the semantic restrictions noted in the RFC. - 2019-07-31, PR rust-lang#63129 Author: @Centril Reviewers: @oli-obk Hardened the test-suite for subslice and subarray patterns with a run-pass tests. This test exercises both type checking and dynamic semantics. - 2019-09-15, PR rust-lang/rust-analyzer#1848 Author: @ecstatic-morse Reviewers: @matklad This implemented the syntactic change (rest patterns, `..`) in rust-analyzer. - 2019-11-05, PR rust-lang#65874 Author: @Nadrieril Reviewers: @varkor, @arielb1, and @Centril Usefulness / exhaustiveness checking saw a major refactoring clarifying the analysis by emphasizing that each row of the matrix can be seen as a sort of stack from which we pop constructors. - 2019-11-12, PR rust-lang#66129 Author: @Nadrieril Reviewers: @varkor, @Centril, and @estebank Usefulness / exhaustiveness checking of slice patterns were refactored in favor of clearer code. Before the PR, variable-length slice patterns were eagerly expanded into a union of fixed-length slices. They now have their own special constructor, which allows expanding them more lazily. As a side-effect, this improved diagnostics. Moreover, the test suite for exhaustiveness checking of slice patterns was hardened. - 2019-11-20, PR rust-lang#66497 Author: @Nadrieril Reviewers: @varkor and @Centril Building on the previous PR, this one fixed a bug rust-lang#53820 wherein sufficiently large subarray patterns (`match [0u8; 16*1024] { [..] => {}}`) would result in crashing the compiler with a stack-overflow. The PR did this by treating array patterns in a more first-class way (using a variable-length mechanism also used for slices) rather than like large tuples. This also had the effect of improving diagnostics for non-exhaustive matches. - 2019-11-28, PR rust-lang#66603 Author: @Nadrieril Reviewers: @varkor Fixed a bug rust-lang#65413 wherein constants, slice patterns, and exhaustiveness checking interacted in a suboptimal way conspiring to suggest that a reachable arm was in fact unreachable. - 2019-12-12, PR rust-lang#66650 Author: @matthewjasper Reviewers: @pnkfelix and @Centril Removed the `UniformArrayMoveOut` MIR transformation pass in favor of baking the necessary logic into the borrow-checker, drop elaboration and MIR building itself. This fixed a number of bugs, including a soundness hole rust-lang#66502. Moreover, the PR added a slew of tests for borrow- and move-checking of slice patterns as well as a test for the dynamic semantics of dropping subslice patterns. - 2019-12-16, PR rust-lang#67318 Author: @Centril Reviewers: @matthewjasper Improved documentation for AST->HIR lowering + type checking of slice as well as minor code simplification. - 2019-12-21, PR rust-lang#67467 Author: @matthewjasper Reviewers: @oli-obk, @RalfJung, and @Centril Fixed bugs in the const evaluation of slice patterns and added tests for const evaluation as well as borrow- and move-checking. - 2019-12-22, PR rust-lang#67439 Author: @Centril Reviewers: @matthewjasper Cleaned up HAIR lowering of slice patterns, removing special cased dead code for the unrepresentable `[a, b] @ ..`. The PR also refactored type checking for slice patterns. - 2019-12-23, PR rust-lang#67546 Author: @oli-obk Reviewers: @varkor and @RalfJung Fixed an ICE in the MIR interpretation of slice patterns. - 2019-12-24, PR rust-lang#66296 Author: @Centril Reviewers: @pnkfelix and @matthewjasper This implemented `#![feature(bindings_after_at)]` which allows writing e.g. `a @ Some([_, b @ ..])`. This is not directly linked to slice patterns other than with patterns in general. However, the combination of the feature and `slice_patterns` received some testing in the PR. - 2020-01-09, PR rust-lang#67990 Author: @Centril Reviewers: @matthewjasper This hardened move-checker tests for `match` expressions in relation to rust-lang#53114. - This PR stabilizes `slice_patterns`. ## Related / possible future work There is on-going work to improve pattern matching in other ways (the relevance of some of these are indirect, and only by composition): - OR-patterns, `pat_0 | .. | pat_n` is almost implemented. Tracking issue: rust-lang#54883 - Bindings after `@`, e.g., `x @ Some(y)` is implemented. Tracking issue: rust-lang#65490 - Half-open range patterns, e.g., `X..`, `..X`, and `..=X` as well as exclusive range patterns, e.g., `X..Y`. Tracking issue: rust-lang#67264 and rust-lang#37854 The relevance here is that this work demonstrates, in practice, that there are no syntactic conflicts introduced by the stabilization of subslice patterns. As for more direct improvements to slice patterns, some avenues could be: - Box patterns, e.g., `box [a, b, .., c]` to match on `Box<[T]>`. Tracking issue: rust-lang#29641 This issue currently has no path to stabilization. Note that it is currently possible to match on `Box<[T]>` or `Vec<T>` by first dereferencing them to slices. - `DerefPure`, which would allow e.g., using slice patterns to match on `Vec<T>` (e.g., moving out of it). Another idea which was raised by [RFC 2707](https://github.com/rust-lang/rfcs/blob/master/text/2707-dotdot-patterns.md#future-possibilities) and [RFC 2359](https://github.com/rust-lang/rfcs/blob/master/text/2359-subslice-pattern-syntax.md#pat-vs-pat) was to allow binding a subtuple pattern. That is, we could allow `(a, xs @ .., b)`. However, while we could allow by-value bindings to `..` as in `xs @ ..` at zero cost, the same cannot be said of by-reference bindings, e.g. `(a, ref xs @ .., b)`. The issue here becomes that for a reference to be legal, we have to represent `xs` contiguously in memory. In effect, we are forced into a [`HList`](https://docs.rs/frunk/0.3.1/frunk/hlist/struct.HCons.html) based representation for tuples.

@matthewjasper

…=matthewjasper Stabilize `#![feature(slice_patterns)]` in 1.42.0 # Stabilization report The following is the stabilization report for `#![feature(slice_patterns)]`. This report is the collaborative effort of @matthewjasper and @Centril. Tracking issue: rust-lang#62254 [Version target](https://forge.rust-lang.org/#current-release-versions): 1.42 (2020-01-30 => beta, 2020-03-12 => stable). ## Backstory: slice patterns It is already possible to use slice patterns on stable Rust to match on arrays and slices. For example, to match on a slice, you may write: ```rust fn foo(slice: &[&str]) { match slice { [] => { dbg!() } [a] => { dbg!(a); } [a, b] => { dbg!(a, b); } _ => {} // ^ Fallback -- necessary because the length is unknown! } } ``` To match on an array, you may instead write: ```rust fn bar([a, b, c]: [u8; 3]) {} // --------- Length is known, so pattern is irrefutable. ``` However, on stable Rust, it is not yet possible to match on a subslice or subarray. ## A quick user guide: Subslice patterns The ability to match on a subslice or subarray is gated under `#![feature(slice_patterns)]` and is what is proposed for stabilization here. ### The syntax of subslice patterns Subslice / subarray patterns come in two flavors syntactically. Common to both flavors is they use the token `..`, referred as a *"rest pattern"* in a pattern context. This rest pattern functions as a variable-length pattern, matching whatever amount of elements that haven't been matched already before and after. When `..` is used syntactically as an element of a slice-pattern, either directly (1), or as part of a binding pattern (2), it becomes a subslice pattern. On stable Rust, a rest pattern `..` can also be used in a tuple or tuple-struct pattern with `let (x, ..) = (1, 2, 3);` and `let TS(x, ..) = TS(1, 2, 3);` respectively. ### (1) Matching on a subslice without binding it ```rust fn base(string: &str) -> u8 { match string.as_bytes() { [b'0', b'x', ..] => 16, [b'0', b'o', ..] => 8, [b'0', b'b', ..] => 2, _ => 10, } } fn main() { assert_eq!(base("0xFF"), 16); assert_eq!(base("0x"), 16); } ``` In the function `base`, the pattern `[b'0', b'x', ..]` will match on any byte-string slice with the *prefix* `0x`. Note that `..` may match on nothing, so `0x` is a valid match. ### (2) Binding a subslice: ```rust fn main() { #[derive(PartialEq, Debug)] struct X(u8); let xs: Vec<X> = vec![X(0), X(1), X(2)]; if let [start @ .., end] = &*xs { // --- bind on last element, assuming there is one. // ---------- bind the initial elements, if there are any. assert_eq!(start, &[X(0), X(1)] as &[X]); assert_eq!(end, &X(2)); let _: &[X] = start; let _: &X = end; } } ``` In this case, `[start @ .., end]` will match any non-empty slice, binding the last element to `end` and any elements before that to `start`. Note in particular that, as above, `start` may match on the empty slice. ### Only one `..` per slice pattern In today's stable Rust, a tuple (struct) pattern `(a, b, c)` can only have one subtuple pattern (e.g., `(a, .., c)`). That is, if there is a rest pattern, it may only occur once. Any `..` that follow, as in e.g., `(a, .., b, ..)` will cause an error, as there is no way for the compiler to know what `b` applies to. This rule also applies to slice patterns. That is, you may also not write `[a, .., b, ..]`. ## Motivation [PR rust-lang#67569]: https://github.com/rust-lang/rust/pull/67569/files Slice patterns provide a natural and efficient way to pattern match on slices and arrays. This is particularly useful as slices and arrays are quite a common occurence in modern software targeting modern hardware. However, as aforementioned, it's not yet possible to perform incomplete matches, which is seen in `fn base`, an example taken from the `rustc` codebase itself. This is where subslice patterns come in and extend slice patterns with the natural syntax `xs @ ..` and `..`, where the latter is already used for tuples and tuple structs. As an example of how subslice patterns can be used to clean up code, we have [PR rust-lang#67569]. In this PR, slice patterns enabled us to improve readability and reduce unsafety, at no loss to performance. ## Technical specification ### Grammar The following specification is a *sub-set* of the grammar necessary to explain what interests us here. Note that stabilizing subslice patterns does not alter the stable grammar. The stabilization contains purely semantic changes. ```rust Binding = reference:"ref"? mutable:"mut"? name:IDENT; Pat = | ... // elided | Rest: ".." | Binding:{ binding:Binding { "@" subpat:Pat }? } | Slice:{ "[" elems:Pat* %% "," "]" } | Paren:{ "(" pat:Pat ")" } | Tuple:{ path:Path? "(" elems:Pat* &% "," ")" } ; ``` Notes: 1. `(..)` is interpreted as a `Tuple`, not a `Paren`. This means that `[a, (..)]` is interpreted as `Slice[Binding(a), Tuple[Rest]]` and not `Slice[Binding(a), Paren(Rest)]`. ### Name resolution [resolve_pattern_inner]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/struct.LateResolutionVisitor.html#method.resolve_pattern_inner [product context]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_resolve/late/enum.PatBoundCtx.html#variant.Product A slice pattern is [resolved][resolve_pattern_inner] as a [product context] and `..` is given no special treatment. ### Abstract syntax of slice patterns The abstract syntax (HIR level) is defined like so: ```rust enum PatKind { ... // Other unimportant stuff. Wild, Binding { binding: Binding, subpat: Option<Pat>, }, Slice { before: List<Pat>, slice: Option<Pat>, after: List<Pat>, }, } ``` [`hir::PatKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/hir/enum.PatKind.html The executable definition is found in [`hir::PatKind`]. ### Lowering to abstract syntax Lowering a slice pattern to its abstract syntax proceeds by: 1. Lowering each element pattern of the slice pattern, where: 1. `..` is lowered to `_`, recording that it was a subslice pattern, 2. `binding @ ..` is lowered to `binding @ _`, recording that it was a subslice pattern, 3. and all other patterns are lowered as normal, recording that it was not a subslice pattern. 2. Taking all lowered elements until the first subslice pattern. 3. Take all following elements. If there are any, 1. The head is the sub-`slice` pattern. 2. The tail (`after`) must not contain a subslice pattern, or an error occurs. [`LoweringContext::lower_pat_slice`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/hir/lowering/struct.LoweringContext.html#method.lower_pat_slice The full executable definition can be found in [`LoweringContext::lower_pat_slice`]. ### Type checking slice patterns #### Default binding modes [non-reference pattern]: https://doc.rust-lang.org/nightly/reference/patterns.html#binding-modes [`is_non_ref_pat`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/check/struct.FnCtxt.html#method.is_non_ref_pat [peel_off_references]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/check/struct.FnCtxt.html#method.peel_off_references A slice pattern is a [non-reference pattern] as defined in [`is_non_ref_pat`]. This means that when type checking a slice pattern, as many immediate reference types are [peeled off][peel_off_references] from the `expected` type as possible and the default binding mode is adjusted to by-reference before checking the slice pattern. See rust-lang#63118 for an algorithmic description. [RFC 2359]: https://github.com/rust-lang/rfcs/blob/master/text/2359-subslice-pattern-syntax.md [rfc-2359-gle]: https://github.com/rust-lang/rfcs/blob/master/text/2359-subslice-pattern-syntax.md#guide-level-explanation See [RFC 2359]'s [guide-level explanation][rfc-2359-gle] and the tests listed below for examples of what effect this has. #### Checking the pattern Type checking a slice pattern proceeds as follows: 1. Resolve any type variables by a single level. If the result still is a type variable, error. 2. Determine the expected type for any subslice pattern (`slice_ty`) and for elements (`inner_ty`) depending on the expected type. 1. If the expected type is an array (`[E; N]`): 1. Evaluate the length of the array. If the length couldn't be evaluated, error. This may occur when we have e.g., `const N: usize`. Now `N` is known. 2. If there is no sub-`slice` pattern, check `len(before) == N`, and otherwise error. 3. Otherwise, set `S = N - len(before) - len(after)`, and check `N >= 0` and otherwise error. Set `slice_ty = [E; S]`. Set `inner_ty = E`. 2. If the expected type is a slice (`[E]`), set `inner_ty = E` and `slice_ty = [E]`. 3. Otherwise, error. 3. Check each element in `before` and `after` against `inner_ty`. 4. If it exists, check `slice` against `slice_ty`. [`check_pat_slice`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_typeck/check/struct.FnCtxt.html#method.check_pat_slice For an executable definition, see [`check_pat_slice`]. ### Typed abstract syntax of slice and array patterns The typed abstract syntax (HAIR level) is defined like so: ```rust enum PatKind { ... // Other unimportant stuff. Wild, Binding { ... // Elided. } Slice { prefix: List<Pat>, slice: Option<Pat>, suffix: List<Pat>, }, Array { prefix: List<Pat>, slice: Option<Pat>, suffix: List<Pat>, }, } ``` [`hair::pattern::PatKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/hair/pattern/enum.PatKind.html The executable definition is found in [`hair::pattern::PatKind`]. ### Lowering to typed abstract syntax Lowering a slice pattern to its typed abstract syntax proceeds by: 1. Lowering each pattern in `before` into `prefix`. 2. Lowering the `slice`, if it exists, into `slice`. 1. A `Wild` pattern in abstract syntax is lowered to `Wild`. 2. A `Binding` pattern in abstract syntax is lowered to `Binding { .. }`. 3. Lowering each pattern in `after` into `after`. 4. If the type is `[E; N]`, construct `PatKind::Array { prefix, slice, after }`, otherwise `PatKind::Slice { prefix, slice, after }`. [`PatCtxt::slice_or_array_pattern`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/hair/pattern/struct.PatCtxt.html#method.slice_or_array_pattern The executable definition is found in [`PatCtxt::slice_or_array_pattern`]. ### Exhaustiveness checking Let `E` be the element type of a slice or array. - For array types, `[E; N]` with a known length `N`, the full set of constructors required for an exahustive match is the sequence `ctors(E)^N` where `ctors` denotes the constructors required for an exhaustive match of `E`. - Otherwise, for slice types `[E]`, or for an array type with an unknown length `[E; ?L]`, the full set of constructors is the infinite sequence `⋃_i=0^∞ ctors(E)^i`. This entails that an exhaustive match without a cover-all pattern (e.g. `_` or `binding`) or a subslice pattern (e.g., `[..]` or `[_, _, ..]`) is impossible. - `PatKind::{Slice, Array}(prefix, None, suffix @ [])` cover a sequence of of `len(prefix)` covered by `patterns`. Note that `suffix.len() > 0` with `slice == None` is unrepresentable. - `PatKind::{Slice, Array}(prefix, Some(s), suffix)` cover a `sequence` with `prefix` as the start and `suffix` as the end and where `len(prefix) + len(suffix) <= len(sequence)`. The `..` in the middle is interpreted as an unbounded number of `_`s in terms of exhaustiveness checking. ### MIR representation The relevant MIR representation for the lowering into MIR, which is discussed in the next section, includes: ```rust enum Rvalue { // ... /// The length of a `[X]` or `[X; N]` value. Len(Place), } struct Place { base: PlaceBase, projection: List<PlaceElem>, } enum ProjectionElem { // ... ConstantIndex { offset: Nat, min_length: Nat, from_end: bool, }, Subslice { from: Nat, to: Nat, from_end: bool, }, } ``` ### Lowering to MIR * For a slice pattern matching a slice, where the pattern has `N` elements specified, there is a check that the `Rvalue::Len` of the slice is at least `N` to decide if the pattern can match. * There are two kinds of `ProjectionElem` used for slice patterns: 1. `ProjectionElem::ConstantIndex` is an array or slice element with a known index. As a shorthand it's written `base[offset of min_length]` if `from_end` is false and `base[-offset of min_length]` if `from_end` is true. `base[-offset of min_length]` is the `len(base) - offset`th element of `base`. 2. `ProjectionElem::Subslice` is a subslice of an array or slice with known bounds. As a shorthand it's written `base[from..to]` if `from_end` is false and `base[from:-to]` if `from_end` is true. `base[from:-to]` is the subslice `base[from..len(base) - to]`. * Note that `ProjectionElem::Index` is used for indexing expressions, but not for slice patterns. It's written `base[idx]`. * When binding an array pattern, any individual element binding is lowered to an assignment or borrow of `base[offset of len]` where `offset` is the element's index in the array and `len` is the array's length. * When binding a slice pattern, let `N` be the number of elements that have patterns. Elements before the subslice pattern (`prefix`) are lowered to `base[offset of N]` where `offset` is the element's index from the start. Elements after the subslice pattern (`suffix`) are lowered to `base[-offset of N]` where `offset` is the element's index from the end, plus 1. * Subslices of arrays are lowered to `base[from..to]` where `from` is the number of elements before the subslice pattern and `to = len(array) - len(suffix)` is the length of the array minus the number of elements after the subslice pattern. * Subslices of slices are lowered to `base[from:-to]` where `from` is the number of elements before the subslice pattern (`len(prefix)`) and `to` is the number of elements after the subslice pattern (`len(suffix)`). ### Safety and const checking * Subslice patterns do not introduce any new unsafe operations. * As subslice patterns for arrays are irrefutable, they are allowed in const contexts. As are `[..]` and `[ref y @ ..]` patterns for slices. However, `ref mut` bindings are only allowed with `feature(const_mut_refs)` for now. * As other subslice patterns for slices require a `match`, `if let`, or `while let`, they are only allowed with `feature(const_if_match, const_fn)` for now. * Subslice patterns may occur in promoted constants. ### Borrow and move checking * A subslice pattern can be moved from if it has an array type `[E; N]` and the parent array can be moved from. * Moving from an array subslice pattern moves from all of the elements of the array within the subslice. * If the subslice contains at least one element, this means that dynamic indexing (`arr[idx]`) is no longer allowed on the array. * The array can be reinitialized and can still be matched with another slice pattern that uses a disjoint set of elements. * A subslice pattern can be mutably borrowed if the parent array/slice can be mutably borrowed. * When determining whether an access conflicts with a borrow and at least one is a slice pattern: * `x[from..to]` always conflicts with `x` and `x[idx]` (where `idx` is a variable). * `x[from..to]` conflicts with `x[idx of len]` if `from <= idx` and `idx < to` (that is, `idx ∈ from..to`). * `x[from..to]` conflicts with `x[from2..to2]` if `from < to2` and `from2 < to` (that is, `(from..to) ∩ (from2..to2) ≠ ∅`). * `x[from:-to]` always conflicts with `x`, `x[idx]`, and `x[from2:-to2]`. * `x[from:-to]` conflicts with `x[idx of len]` if `from <= idx`. * `x[from:-to]` conflicts with `x[-idx of len]` if `to < idx`. * A constant index from the end conflicts with other elements as follows: * `x[-idx of len]` always conflicts with `x` and `x[idx]`. * `x[-idx of len]` conflicts with `x[-idx2 of len2]` if `idx == idx2`. * `x[-idx of len]` conflicts with `x[idx2 of len2]` if `idx + idx2 >= max(len, len2)`. ## Tests The tests can be primarily seen in the PR itself. Here are some of them: ### Parsing (3) * Testing that `..` patterns are syntactically allowed in all pattern contexts (2) * [pattern/rest-pat-syntactic.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/rest-pat-syntactic.rs) * [ignore-all-the-things.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/ignore-allthe-things.rs) * Slice patterns allow a trailing comma, including after `..` (1) * [trailing-comma.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/trailing-comma.rs) ### Lowering (2) * `@ ..` isn't allowed outside of slice patterns and only allowed once in each pattern (1) * [pattern/rest-pat-semantic-disallowed.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/rest-pat-semantic-disallowed.rs) * Mulitple `..` patterns are not allowed (1) * [parser/match-vec-invalid.rs](https://github.com/rust-lang/rust/blob/53712f8637dbe326df569a90814aae1cc5429710/src/test/ui/parser/match-vec-invalid.rs) ### Type checking (5) * Default binding modes apply to slice patterns (2) * [rfc-2005-default-binding-mode/slice.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/rfc-2005-default-binding-mode/slice.rs) * [rfcs/rfc-2005-default-binding-mode/slice.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/rfcs/rfc-2005-default-binding-mode/slice.rs) * Array patterns cannot have more elements in the pattern than in the array (2) * [match/match-vec-mismatch.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/match/match-vec-mismatch.rs) * [error-codes/E0528.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/error-codes/E0528.rs) * Array subslice patterns have array types (1) * [array-slice-vec/subslice-patterns-pass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-pass.rs) ### Exhaustiveness and usefulness checking (20) * Large subslice matches don't stack-overflow the exhaustiveness checker (1) * [pattern/issue-53820-slice-pattern-large-array.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/issue-53820-slice-pattern-large-array.rs) * Array patterns with subslices are irrefutable (1) * [issues/issue-7784.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-7784.rs) * `[xs @ ..]` slice patterns are irrefutable (1) * [binding/irrefutable-slice-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/irrefutable-slice-patterns.rs) * Subslice patterns can match zero-length slices (2) * [issues/issue-15080.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-15080.rs) * [issues/issue-15104.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-15104.rs) * General tests (13) * [issues/issue-12369.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-12369.rs) * [issues/issue-37598.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-37598.rs) * [pattern/usefulness/match-vec-unreachable.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/match-vec-unreachable.rs) * [pattern/usefulness/non-exhaustive-match.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/non-exhaustive-match.rs) * [pattern/usefulness/non-exhaustive-match-nested.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/non-exhaustive-match-nested.rs) * [pattern/usefulness/non-exhaustive-pattern-witness.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/non-exhaustive-pattern-witness.rs) * [pattern/usefulness/65413-constants-and-slices-exhaustiveness.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/65413-constants-and-slices-exhaustiveness.rs) * [pattern/usefulness/match-byte-array-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/match-byte-array-patterns.rs) * [pattern/usefulness/match-slice-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/match-slice-patterns.rs) * [pattern/usefulness/slice-patterns-exhaustiveness.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/slice-patterns-exhaustiveness.rs) * [pattern/usefulness/slice-patterns-irrefutable.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/slice-patterns-irrefutable.rs) * [pattern/usefulness/slice-patterns-reachability.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/usefulness/slice-patterns-reachability.rs) * [uninhabited/uninhabited-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/uninhabited/uninhabited-patterns.rs) * Interactions with or-patterns (2) * [or-patterns/exhaustiveness-pass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/or-patterns/exhaustiveness-pass.rs) * [or-patterns/exhaustiveness-unreachable-pattern.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/or-patterns/exhaustiveness-unreachable-pattern.rs) ### Borrow checking (28) * Slice patterns can only move from owned, fixed-length arrays (4) * [borrowck/borrowck-move-out-of-vec-tail.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-move-out-of-vec-tail.rs) * [moves/move-out-of-slice-2.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/moves/move-out-of-slice-2.rs) * [moves/move-out-of-array-ref.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/moves/move-out-of-array-ref.rs) * [issues/issue-12567.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-12567.rs) * Moves from arrays are tracked by element (2) * [borrowck/borrowck-move-out-from-array-no-overlap.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-move-out-from-array-no-overlap.rs) * [borrowck/borrowck-move-out-from-array-use-no-overlap.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-move-out-from-array-use-no-overlap.rs) * Slice patterns cannot be used on moved-from slices/arrays (2) * [borrowck/borrowck-move-out-from-array.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-move-out-from-array.rs) * [borrowck/borrowck-move-out-from-array-use.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-move-out-from-array-use.rs) * Slice patterns cannot be used with conflicting borrows (3) * [borrowck/borrowck-describe-lvalue.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-describe-lvalue.rs) * [borrowck/borrowck-slice-pattern-element-loan-array.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-array.rs) * [borrowck/borrowck-slice-pattern-element-loan-slice.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-slice.rs) * Borrows from slice patterns are tracked and only conflict when there is possible overlap (6) * [borrowck/borrowck-slice-pattern-element-loan-array-no-overlap.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-array-no-overlap.rs) * [borrowck/borrowck-slice-pattern-element-loan-slice-no-overlap.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-slice-no-overlap.rs) * [borrowck/borrowck-slice-pattern-element-loan-rpass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-rpass.rs) * [borrowck/borrowck-vec-pattern-element-loan.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-vec-pattern-element-loan.rs) * [borrowck/borrowck-vec-pattern-loan-from-mut.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-vec-pattern-loan-from-mut.rs) * [borrowck/borrowck-vec-pattern-tail-element-loan.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-vec-pattern-tail-element-loan.rs) * Slice patterns affect indexing expressions (1) * [borrowck/borrowck-vec-pattern-move-tail.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-vec-pattern-move-tail.rs) * Borrow and move interactions with `box` patterns (1) * [borrowck/borrowck-vec-pattern-move-tail.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-vec-pattern-move-tail.rs) * Slice patterns correctly affect inference of closure captures (2) * [borrowck/borrowck-closures-slice-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-closures-slice-patterns.rs) * [borrowck/borrowck-closures-slice-patterns-ok.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-closures-slice-patterns-ok.rs) * Interactions with `#![feature(bindings_after_at)]` (7) * [pattern/bindings-after-at/borrowck-move-and-move.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-move-and-move.rs) * [pattern/bindings-after-at/borrowck-pat-at-and-box-pass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-at-and-box-pass.rs) * [pattern/bindings-after-at/borrowck-pat-at-and-box.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-at-and-box.rs) * [pattern/bindings-after-at/borrowck-pat-by-copy-bindings-in-at.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-by-copy-bindings-in-at.rs) * [pattern/bindings-after-at/borrowck-pat-ref-both-sides.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-ref-both-sides.rs) * [pattern/bindings-after-at/borrowck-pat-ref-mut-and-ref.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-ref-mut-and-ref.rs) * [pattern/bindings-after-at/borrowck-pat-ref-mut-twice.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/pattern/bindings-after-at/borrowck-pat-ref-mut-twice.rs) * Misc (1) * [issues/issue-26619.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-26619.rs) ### MIR lowering (1) * [uniform_array_move_out.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/mir-opt/uniform_array_move_out.rs) ### Evaluation (19) * Slice patterns don't cause leaks or double drops (2) * [drop/dynamic-drop.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/drop/dynamic-drop.rs) * [drop/dynamic-drop-async.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/drop/dynamic-drop-async.rs) * General run-pass tests (10) * [array-slice-vec/subslice-patterns-pass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-pass.rs) * [array-slice-vec/vec-matching-fixed.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/vec-matching-fixed.rs) * [array-slice-vec/vec-matching-fold.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/vec-matching-fold.rs) * [array-slice-vec/vec-matching-legal-tail-element-borrow.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/vec-matching-legal-tail-element-borrow.rs) * [array-slice-vec/vec-matching.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/vec-matching.rs) * [array-slice-vec/vec-tail-matching.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/vec-tail-matching.rs) * [binding/irrefutable-slice-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/irrefutable-slice-patterns.rs) * [binding/match-byte-array-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/match-byte-array-patterns.rs) * [binding/match-vec-alternatives.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/match-vec-alternatives.rs) * [borrowck/borrowck-slice-pattern-element-loan-rpass.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/borrowck/borrowck-slice-pattern-element-loan-rpass.rs) * Matching a large by-value array (1) * [issues/issue-17877.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/issues/issue-17877.rs) * Uninhabited elements (1) * [binding/empty-types-in-patterns.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/empty-types-in-patterns.rs) * Zero-sized elements (3) * [binding/zero_sized_subslice_match.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/binding/zero_sized_subslice_match.rs) * [array-slice-vec/subslice-patterns-const-eval.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-const-eval.rs) * [array-slice-vec/subslice-patterns-const-eval-match.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-const-eval-match.rs) * Evaluation in const contexts (2) * [array-slice-vec/subslice-patterns-const-eval.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-const-eval.rs) * [array-slice-vec/subslice-patterns-const-eval-match.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/array-slice-vec/subslice-patterns-const-eval-match.rs) ## Misc (1) * Exercising a case where const-prop cased an ICE (1) * [consts/const_prop_slice_pat_ice.rs](https://github.com/rust-lang/rust/blob/acb6690e1d58fc5f262ada5b5030fe73e601f1e8/src/test/ui/consts/const_prop_slice_pat_ice.rs) ## History - 2012-12-08, commit rust-lang@1968cb3 Author: Jakub Wieczorek Reviewers: @graydon This is where slice patterns were first implemented. It is particularly instructive to read the `vec-tail-matching.rs` test. - 2013-08-20, issue rust-lang#8636 Author: @huonw Fixed by @mikhail-m1 in rust-lang#51894 The issue describes a problem wherein the borrow-checker would not consider disjointness when checking mutable references in slice patterns. - 2014-09-03, RFC rust-lang/rfcs#164 Author: @brson Reviewers: The Core Team The RFC decided to feature gate slice patterns due to concerns over lack of oversight and the exhaustiveness checking logic not having seen much love. Since then, the exhaustivenss checking algorithm, in particular for slice patterns, has been substantially refactored and tests have been added. - 2014-09-03, RFC rust-lang/rfcs#202 Author: @krdln Reviewers: The Core Team > Change syntax of subslices matching from `..xs` to `xs..` to be more consistent with the rest of the language and allow future backwards compatible improvements. In 2019, rust-lang/rfcs#2359 changed the syntax again in favor of `..` and `xs @ ..`. - 2014-09-08, PR rust-lang#17052 Author: @pcwalton Reviewers: @alexcrichton and @sfackler This implemented the feature gating as specified in rust-lang/rfcs#164. - 2015-03-06, RFC rust-lang/rfcs#495 Author: @P1start Reviewers: The Core Team The RFC changed array and slice patterns like so: - Made them only match on arrays (`[T; N]`) and slice types (`[T]`), not references to slice types (`& mut? [T]`). - Made subslice matching yield a value of type `[T; N]` or `[T]`, not `& mut? [T]`. - Allowed multiple mutable references to be made to different parts of the same array or slice in array patterns. These changes were made to fit with the introduction of DSTs like `[T]` as well as with e.g. `box [a, b, c]` (`Box<[T]>`) in the future. All points remain true today, in particular with the advent of default binding modes. - 2015-03-22, PR rust-lang#23361 Author: @petrochenkov Reviewers: Unknown The PR adjusted codegen ("trans") such that `let ref a = *"abcdef"` would no longer ICE, paving the way for rust-lang/rfcs#495. - 2015-05-28, PR rust-lang#23794 Author: @brson Reviewers: @nrc The PR feature gated slice patterns in more contexts. - 2016-06-09, PR rust-lang#32202 Author: @arielb1 Reviewers: @eddyb and @nikomatsakis This implemented RFC rust-lang/rfcs#495 via a MIR based implementation fixing some bugs. - 2016-09-16, PR rust-lang#36353 Author: @arielb1 Reviewers: @nagisa, @pnkfelix, and @nikomatsakis The PR made move-checker improvements prohibiting moves out of slices. - 2018-02-17, PR rust-lang#47926 Author: @mikhail-m1 Reviewers: @nikomatsakis This added the `UniformArrayMoveOut` which converted move-out-from-array by `Subslice` and `ConstIndex {.., from_end: true }` to `ConstIndex` move out(s) from the beginning of the array. This fixed some problems with the MIR borrow-checker and drop-elaboration of arrays. Unfortunately, the transformation ultimately proved insufficient for soundness and was removed and replaced in rust-lang#66650. - 2018-02-19, PR rust-lang#48355 Author: @mikhail-m1 Reviewers: @nikomatsakis After rust-lang#47926, this restored some MIR optimizations after drop-elaboration and borrow-checking. - 2018-03-20, PR rust-lang#48516 Author: @petrochenkov Reviewers: @nikomatsakis This stabilized fixed length slice patterns `[a, b, c]` without variable length subslices and moved subslice patterns into `#![feature(slice_patterns)`. See rust-lang#48836 wherein the language team accepted the proposal to stabilize. - 2018-07-06, PR rust-lang#51894 Author: @mikhail-m1 Reviewers: @nikomatsakis rust-lang#8636 was fixed such that the borrow-checker would consider disjointness with respect to mutable references in slice patterns. - 2019-06-30, RFC rust-lang/rfcs#2359 Author: @petrochenkov Reviewers: The Language Team The RFC switched the syntax of subslice patterns to `{$binding @}? ..` as opposed to `.. $pat?` (which was what the RFC originally proposed). This RFC reignited the work towards finishing the implementation and the testing of slice patterns which eventually lead to this stabilization proposal. - 2019-06-30, RFC rust-lang/rfcs#2707 Author: @petrochenkov Reviewers: The Language Team This RFC built upon rust-lang/rfcs#2359 turning `..` into a full-fledged pattern (`Pat |= Rest:".." ;`), as opposed to a special part of slice and tuple patterns, moving previously syntactic restrictions into semantic ones. - 2019-07-03, PR rust-lang#62255 Author: @Centril Reviewers: @varkor This closed the old tracking issue (rust-lang#23121) in favor of the new one (rust-lang#62254) due to the new RFCs having been accepted. - 2019-07-28, PR rust-lang#62550 Author: @Centril Reviewers: @petrochenkov and @eddyb Implemented RFCs rust-lang/rfcs#2707 and rust-lang/rfcs#2359 by introducing the `..` syntactic rest pattern form as well as changing the lowering to subslice and subtuple patterns and the necessary semantic restrictions as per the RFCs. Moreover, the parser was cleaned up to use a more generic framework for parsing sequences of things. This framework was employed in parsing slice patterns. Finally, the PR introduced parser recovery for half-open ranges (e.g., `..X`, `..=X`, and `X..`), demonstrating in practice that the RFCs proposed syntax will enable half-open ranges if we want to add those (which is done in rust-lang#67258). - 2019-07-30, PR rust-lang#63111 Author: @Centril Reviewers: @estebank Added a test which comprehensively exercised the parsing of `..` rest patterns. That is, the PR exercised the specification in rust-lang/rfcs#2707. Moreover, a test was added for the semantic restrictions noted in the RFC. - 2019-07-31, PR rust-lang#63129 Author: @Centril Reviewers: @oli-obk Hardened the test-suite for subslice and subarray patterns with a run-pass tests. This test exercises both type checking and dynamic semantics. - 2019-09-15, PR rust-lang/rust-analyzer#1848 Author: @ecstatic-morse Reviewers: @matklad This implemented the syntactic change (rest patterns, `..`) in rust-analyzer. - 2019-11-05, PR rust-lang#65874 Author: @Nadrieril Reviewers: @varkor, @arielb1, and @Centril Usefulness / exhaustiveness checking saw a major refactoring clarifying the analysis by emphasizing that each row of the matrix can be seen as a sort of stack from which we pop constructors. - 2019-11-12, PR rust-lang#66129 Author: @Nadrieril Reviewers: @varkor, @Centril, and @estebank Usefulness / exhaustiveness checking of slice patterns were refactored in favor of clearer code. Before the PR, variable-length slice patterns were eagerly expanded into a union of fixed-length slices. They now have their own special constructor, which allows expanding them more lazily. As a side-effect, this improved diagnostics. Moreover, the test suite for exhaustiveness checking of slice patterns was hardened. - 2019-11-20, PR rust-lang#66497 Author: @Nadrieril Reviewers: @varkor and @Centril Building on the previous PR, this one fixed a bug rust-lang#53820 wherein sufficiently large subarray patterns (`match [0u8; 16*1024] { [..] => {}}`) would result in crashing the compiler with a stack-overflow. The PR did this by treating array patterns in a more first-class way (using a variable-length mechanism also used for slices) rather than like large tuples. This also had the effect of improving diagnostics for non-exhaustive matches. - 2019-11-28, PR rust-lang#66603 Author: @Nadrieril Reviewers: @varkor Fixed a bug rust-lang#65413 wherein constants, slice patterns, and exhaustiveness checking interacted in a suboptimal way conspiring to suggest that a reachable arm was in fact unreachable. - 2019-12-12, PR rust-lang#66650 Author: @matthewjasper Reviewers: @pnkfelix and @Centril Removed the `UniformArrayMoveOut` MIR transformation pass in favor of baking the necessary logic into the borrow-checker, drop elaboration and MIR building itself. This fixed a number of bugs, including a soundness hole rust-lang#66502. Moreover, the PR added a slew of tests for borrow- and move-checking of slice patterns as well as a test for the dynamic semantics of dropping subslice patterns. - 2019-12-16, PR rust-lang#67318 Author: @Centril Reviewers: @matthewjasper Improved documentation for AST->HIR lowering + type checking of slice as well as minor code simplification. - 2019-12-21, PR rust-lang#67467 Author: @matthewjasper Reviewers: @oli-obk, @RalfJung, and @Centril Fixed bugs in the const evaluation of slice patterns and added tests for const evaluation as well as borrow- and move-checking. - 2019-12-22, PR rust-lang#67439 Author: @Centril Reviewers: @matthewjasper Cleaned up HAIR lowering of slice patterns, removing special cased dead code for the unrepresentable `[a, b] @ ..`. The PR also refactored type checking for slice patterns. - 2019-12-23, PR rust-lang#67546 Author: @oli-obk Reviewers: @varkor and @RalfJung Fixed an ICE in the MIR interpretation of slice patterns. - 2019-12-24, PR rust-lang#66296 Author: @Centril Reviewers: @pnkfelix and @matthewjasper This implemented `#![feature(bindings_after_at)]` which allows writing e.g. `a @ Some([_, b @ ..])`. This is not directly linked to slice patterns other than with patterns in general. However, the combination of the feature and `slice_patterns` received some testing in the PR. - 2020-01-09, PR rust-lang#67990 Author: @Centril Reviewers: @matthewjasper This hardened move-checker tests for `match` expressions in relation to rust-lang#53114. - This PR stabilizes `slice_patterns`. ## Related / possible future work There is on-going work to improve pattern matching in other ways (the relevance of some of these are indirect, and only by composition): - OR-patterns, `pat_0 | .. | pat_n` is almost implemented. Tracking issue: rust-lang#54883 - Bindings after `@`, e.g., `x @ Some(y)` is implemented. Tracking issue: rust-lang#65490 - Half-open range patterns, e.g., `X..`, `..X`, and `..=X` as well as exclusive range patterns, e.g., `X..Y`. Tracking issue: rust-lang#67264 and rust-lang#37854 The relevance here is that this work demonstrates, in practice, that there are no syntactic conflicts introduced by the stabilization of subslice patterns. As for more direct improvements to slice patterns, some avenues could be: - Box patterns, e.g., `box [a, b, .., c]` to match on `Box<[T]>`. Tracking issue: rust-lang#29641 This issue currently has no path to stabilization. Note that it is currently possible to match on `Box<[T]>` or `Vec<T>` by first dereferencing them to slices. - `DerefPure`, which would allow e.g., using slice patterns to match on `Vec<T>` (e.g., moving out of it). Another idea which was raised by [RFC 2707](https://github.com/rust-lang/rfcs/blob/master/text/2707-dotdot-patterns.md#future-possibilities) and [RFC 2359](https://github.com/rust-lang/rfcs/blob/master/text/2359-subslice-pattern-syntax.md#pat-vs-pat) was to allow binding a subtuple pattern. That is, we could allow `(a, xs @ .., b)`. However, while we could allow by-value bindings to `..` as in `xs @ ..` at zero cost, the same cannot be said of by-reference bindings, e.g. `(a, ref xs @ .., b)`. The issue here becomes that for a reference to be legal, we have to represent `xs` contiguously in memory. In effect, we are forced into a [`HList`](https://docs.rs/frunk/0.3.1/frunk/hlist/struct.HCons.html) based representation for tuples.

dumindu · 2020-05-19T00:25:18Z

To consistent with for and other cases,

either we have to support both inclusive and exclusive patterns in match.
or either we should deprecate exclusive patterns in both places.

// 0 to 10 (10 exclusive)
for a in 0..10 { 
  println!("Current value : {}", a);
}

// 0 to 10 (10 inclusive)
for a in 0..=10 { 
  println!("Current value : {}", a);
}

let tshirt_width = 20;
let tshirt_size = match tshirt_width {
    16 => "S",       // 16
    17 | 18 => "M",  // 17 or 18
    19..21 => "L",   // 19 to 21; 21 excusive (19,20)
    21..=24 => "xL", // 21 to 24; 24 inclusive (21,22,23,24)
    25 => "XXL",
    _ => "Not Available",
};

While we having inconsistencies like this even in smallest things, it's more harder for newcomers to learn and master Rust.

kentfredric · 2020-05-28T14:28:49Z

ranges are very rarely what the programmer wants in a pattern match (emphasis on match!)

Arguments like that IMO are better grounds for lints, not "you can't have this".

Otherwise I'd be arguing you shouldn't be allowed to have a thing.is_file(), because it sometimes isn't what you expect: Usually you just want a readable thing that isn't a directory, and things like pipes and symbolic links and device nodes are acceptable, even if weird... but .is_file() will likely exclude those options for you. ( Hence why I proposed a lint for that )

And lets not even get into the rabbit warren of "stating files is bad, because you could have the underlying file change between the stat time and the time you modify it, use proper error handling instead" argument, because while I agree in part, its a decision you should make after understanding a lint telling you "why this is probably bad" and you decide whether or not it matters for your code.

harrysarson · 2020-08-14T16:09:54Z

If I may add a datapoint. I tried to use an exclusive range today and the compiler error took me here. Turned out I wanted an inclusive range, so I for one am greatful that rust does not (stabley) allow exclusive ranges and I would argue against stablising as it would have caused me to shoot myself in the foot. :)

avl · 2020-08-20T20:57:21Z

I wanted exclusive ranges today, for a case where I had a variable that needed to be handled differently depending on which of the following ranges it fell into: 0..10, 10..100, 100..1000 and 1000..10000. In my opinion the code would have been cleaner if exclusive ranges were supported.

phaux · 2020-10-07T20:38:08Z

for a case where I had a variable that needed to be handled differently depending on which of the following ranges it fell into: 0..10, 10..100, 100..1000 and 1000..10000.

You could just do a match on x.log10().floor()

avl · 2020-10-07T20:52:13Z

for a case where I had a variable that needed to be handled differently depending on which of the following ranges it fell into: 0..10, 10..100, 100..1000 and 1000..10000.

You could just do a match on x.log10().floor()

Yeah, good point! But at least to me, it wouldn't be as readable. The match would be something like:

0 => /* handle  case 0..10 */,
1 => /* handle case 10..100 */,
2 => /* handle case 100..1000 */,

Which isn't bad, but without the comment it wouldn't be immediately obvious that the case "0" handled the range 0..10 .

workingjubilee · 2020-10-21T23:53:43Z

It is not clear that

0..=9
10..=99
100..=999
1000..=9999

is less desirable.

However I would like to voice that I prefer having the option to have an exclusive range pattern for the sole case of the "half-open range" (#67264) that goes from a certain value unto the natural end of that range

MultisampledNight · 2024-02-26T17:47:18Z

Hi! By chance, what is the status of this? The FCP seems to be complete and successful, do I see correctly that this is just missing a stabilization PR?

Nadrieril · 2024-02-26T18:09:22Z

Hi, this is currently blocked on this lint being approved and merged: #118879. After that we'll be able to make the stabilization PR

Nadrieril · 2024-03-08T14:30:38Z

The lint got approved! If anyone's motivated, we can now make the stabilization PR for exclusive range patterns (instructions here)

Lint singleton gaps after exclusive ranges In the discussion to stabilize exclusive range patterns (rust-lang#37854), it has often come up that they're likely to cause off-by-one mistakes. We already have the `overlapping_range_endpoints` lint, so I [proposed](rust-lang#37854 (comment)) a lint to catch the complementary mistake. This PR adds a new `non_contiguous_range_endpoints` lint that catches likely off-by-one errors with exclusive range patterns. Here's the idea (see the test file for more examples): ```rust match x { 0..10 => ..., // WARN: this range doesn't match `10_u8` because `..` is an exclusive range 11..20 => ..., // this could appear to continue range `0_u8..10_u8`, but `10_u8` isn't matched by either of them _ => ..., } // help: use an inclusive range instead: `0_u8..=10_u8` ``` More precisely: for any exclusive range `lo..hi`, if `hi+1` is matched by another range but `hi` isn't, we suggest writing an inclusive range `lo..=hi` instead. We also catch `lo..T::MAX`.

Lint singleton gaps after exclusive ranges In the discussion to stabilize exclusive range patterns (rust-lang/rust#37854), it has often come up that they're likely to cause off-by-one mistakes. We already have the `overlapping_range_endpoints` lint, so I [proposed](rust-lang/rust#37854 (comment)) a lint to catch the complementary mistake. This PR adds a new `non_contiguous_range_endpoints` lint that catches likely off-by-one errors with exclusive range patterns. Here's the idea (see the test file for more examples): ```rust match x { 0..10 => ..., // WARN: this range doesn't match `10_u8` because `..` is an exclusive range 11..20 => ..., // this could appear to continue range `0_u8..10_u8`, but `10_u8` isn't matched by either of them _ => ..., } // help: use an inclusive range instead: `0_u8..=10_u8` ``` More precisely: for any exclusive range `lo..hi`, if `hi+1` is matched by another range but `hi` isn't, we suggest writing an inclusive range `lo..=hi` instead. We also catch `lo..T::MAX`.

Nadrieril · 2024-03-11T17:47:15Z

This is not the place to report bugs :)

Nadrieril · 2024-03-11T17:49:03Z

(you're right that this is surprising though, feel free to open an issue to suggest a diagnostic that explains the confusion)

MultisampledNight · 2024-03-19T12:59:08Z

I'd want to tackle making a stabilization PR this weekend, if no one else has done it until then!

Lint singleton gaps after exclusive ranges In the discussion to stabilize exclusive range patterns (rust-lang/rust#37854), it has often come up that they're likely to cause off-by-one mistakes. We already have the `overlapping_range_endpoints` lint, so I [proposed](rust-lang/rust#37854 (comment)) a lint to catch the complementary mistake. This PR adds a new `non_contiguous_range_endpoints` lint that catches likely off-by-one errors with exclusive range patterns. Here's the idea (see the test file for more examples): ```rust match x { 0..10 => ..., // WARN: this range doesn't match `10_u8` because `..` is an exclusive range 11..20 => ..., // this could appear to continue range `0_u8..10_u8`, but `10_u8` isn't matched by either of them _ => ..., } // help: use an inclusive range instead: `0_u8..=10_u8` ``` More precisely: for any exclusive range `lo..hi`, if `hi+1` is matched by another range but `hi` isn't, we suggest writing an inclusive range `lo..=hi` instead. We also catch `lo..T::MAX`.

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Tracking issue for exclusive range patterns #37854

Tracking issue for exclusive range patterns #37854

oli-obk commented Nov 18, 2016 •

edited by Nadrieril

ghost commented Jul 30, 2017 •

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donbright commented Jan 12, 2019 •

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avl commented Aug 20, 2020

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workingjubilee commented Oct 21, 2020 •

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Nadrieril commented May 5, 2024

Tracking issue for exclusive range patterns #37854

Tracking issue for exclusive range patterns #37854

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oli-obk commented Nov 18, 2016 • edited by Nadrieril

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ghost commented Jul 30, 2017 • edited by ghost

derekdreery commented Jun 5, 2018

donbright commented Jan 12, 2019 • edited

kentfredric commented Jun 26, 2019

bcantrill commented Jan 16, 2020

dumindu commented May 19, 2020 • edited

kentfredric commented May 28, 2020 • edited

harrysarson commented Aug 14, 2020 • edited

avl commented Aug 20, 2020

phaux commented Oct 7, 2020

avl commented Oct 7, 2020

workingjubilee commented Oct 21, 2020 • edited

MultisampledNight commented Feb 26, 2024

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Nadrieril commented May 5, 2024

oli-obk commented Nov 18, 2016 •

edited by Nadrieril

ghost commented Jul 30, 2017 •

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donbright commented Jan 12, 2019 •

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dumindu commented May 19, 2020 •

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kentfredric commented May 28, 2020 •

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harrysarson commented Aug 14, 2020 •

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workingjubilee commented Oct 21, 2020 •

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