Preliminaries (not part of Rust By Example)

RBE's Code Style

The code examples in RBE have a quirky, non-idiomatic style. In particular, I've never seen anyone format their where-clause-using functions the way that RBE does:

fn print_ref<'a, T>(t: &'a T) where
    T: Debug + 'a {
    println!("`print_ref`: t is {:?}", t);
}

Something more idiomatic would be:

fn print_ref<'a, T>(t: &'a T)
where
    T: Debug + 'a,
{
    println!("`print_ref`: t is {:?}", t);
}

If in doubt, run rustfmt. I don't agree with everything rustfmt does (or refuses to do), but it will at least be idiomatic, unlike many examples in RBE.

OK let's get started

The rest of these notes follow RBE's structure (at the time of writing).

Hello World

Almost no-one invokes rustc directly. A future section introduces cargo; you should use cargo instead of rustc directly.

If you do use rustc directly, you should specify the latest stable edition in order to get access to more modern language features and to help ensure your code is more idiomatic.

As I am writing this, that would be: rustc --edition=2021.

But you can also see which editions are available for yourself:

rustc --edition=
# error: argument for `--edition` must be one of: 2015|2018|2021|2024. (instead was ``)

rustc --edition=2024
# error: the crate requires edition 2024, but the latest edition supported by this Rust version is 2021

rustc --edition=2021
# error: no input filename given

echo yay

Comments

There are also block doc comments, starting with /** or /*!.

/** ...outer block comment like /// */
/*! ...inner block comment like //! */

(But they are less commonly used.)

Formatted print

This is at the bottom of the long example and thus easy to miss:

    // For Rust 1.58 and above, you can directly capture the argument from a
    // surrounding variable. Just like the above, this will output
    // "    1", 4 white spaces and a "1".
    let number: f64 = 1.0;
    let width: usize = 5;
    println!("{number:>width$}");

(Rust 1.58 was released on January 13th, 2022.)

This style is now more common than either positional or named parameters. Without format specifiers, it is even simpler than their example.

    println!("{number}");

Primitives

Arrays and slices

It's not uncommon for even official documentation to be sloppy with terminology around slices, and Rust By Example is no exception.

Let us be a bit more formal and say that:

[T] is a slice
&[T] is a shared slice
And there are other variations like a boxed slice (Box<[T]>)

Then a slice is the type of the contiguous collection of Ts of some length not known at compile time. Because the length is not known, the size of the type is also not known -- it is a dynamic property, depending on how many elements are in the slice. This also means that the following bound:

[T]: Sized

can never hold. This is also called being "unsized" or being a "dynamically sized type (DST)".

Incidentally, when you declare a generic parameter T, that parameter has an implicit T: Sized bound. You can remove it with T: ?Sized.

A shared slice (&[T]) is the type which consists of two usize-sized values, a pointer to the data and a count of the elements.

This two-value reference is also called a "wide reference" or "wide pointer" or "fat pointer", as it's twice the size of "normal" ("thin") reference to a Sized value.

The order between the length and the pointer is not specified, ignore what RBE says about "first" and "second" words. You almost surely won't need to destructure a shared slice until you're much further in your Rust journey, if ever, but if you do -- find out how to do so without unsafe; transmuting in particular is incorrect. (Constructing a wide reference often requires unsafe, but transmute is still not the answer.)

&[T]: Sized because its size is a compile-time property -- the two values we just described. Box<[T]> and the other variations are also generally also Sized and consist of a wide pointer.

It is common to call all of [T], &[T], &mut [T], etc "slices", so don't count on anything you read being careful about distinguishing the unsized type and the (Sized) references or pointers to the slice.

There's a diagram of a [T] and a &[T] (and a Vec<T> and &Vec<T>) further down these notes.

Custom Types

This is sloppy wording, as consts and statics are distinct things:

Constants can also be created via the const and static keywords.

More about that in the specific section.

Structures

Structs with named fields are sometimes called "nominal structs". I don't think it's common to call them "C structs". In fact, Rust does have features for creating data structures compatible with C-based FFI, but it requires specific annotations; referring to your typical named-field struct as a "C struct" is misleading.

Enums

`use`

If you import the variant names, take care not to make any typos or you'll introduce a binding instead.

Example thread about the hazard.

You'll generally get a warning or three about it.

I don't think the guide mentions this incidentally: Rust warnings are generally pretty good. Don't just ignore them like you might be used to from some other languages.

Full warnings and errors in the console (e.g. cargo check) are generally better than the abbreviated warnings and errors you might get in your IDE.

constants

statics are particular values that persist for the entire run time of your program. They are not duplicated.

consts are evaluated where you use them, as if you had pasted the definition in place. If the const happens to create a static value, for instance, the compiler is free to create multiple static values. See this issue for an example.

static mut is so wildly hard to use correctly that the experts get it wrong. It will probably be deprecated as better alternatives become available in the language and standard library, for example:

Don't use static mut.

Using a static when you want or need to have only one globally shared value, not because you want a "mutable constant".

Variable Bindings

Mutability

Without a mut binding, you can't

Create a &mut _ to the variable (including implicitly, e.g. when calling methods)
Overwrite the variable (variable = new_value)

But note that this is, roughly speaking, a compiler-enforced lint and not some additional intrisic property of the value in question; the type of the variable is the same with or without mut, for example. And you can still do things like this:

let supposedly_immutable = String::new();
let mut tmp = supposedly_immutable;
tmp.push_str("Gottem");
let supposedly_immutable = tmp;

(It also doesn't prevent destructors from running.)

In contrast, &mut T and &T are distinct types with distinct capabilities and other behavior. Despite the spelling, you should think of &mut T as an exclusive reference and &T as a shared reference. Why? Well, Rust has "interior mutability", a.k.a. "shared mutability", which allows mutation through a &T in some cases. By thinking of references as "exclusive vs. shared" from the start, you'll have less to unlearn and relearn once you encounter shared mutability types. (There's an example later in these notes.)

Additionally, ensuring exclusivity is actually how &mut T works/is defined.

Declare first

The wording here is pretty poor...

However, this form is seldom used, as it may lead to the use of uninitialized variables. [...] The compiler forbids use of uninitialized variables, as this would lead to undefined behavior.

Using unitialized variables can't be the reason it's seldom used, since that's not something that even compiles. I would say it's more that having potentially unitialized bindings is a rare need.

So how about an example where it's actually useful?

use std::sync::{Arc, Mutex};
fn some_condition() -> bool { true }

fn example(mtx: Arc<Mutex<Vec<String>>>, other_data: &mut [String]) {
    let mut lock;
    let data = if some_condition() {
        lock = mtx.lock().unwrap();
        &mut **lock
    } else {
        other_data
    };
    
    // ... do work on the data ...
}

Here, we may or may not need to lock the mutex to operate on some data. We don't want to lock the mutex if we don't have to. But if we do lock the mutex, the mutex guard (lock) needs to stick around until the end of the function. So this won't compile because the lock goes out of scope:

    let data = if some_condition() {
        let mut lock = mtx.lock().unwrap();
        &mut **lock
    } else {
        other_data
    };

Instead we declare outside the if and initialize only where we have to. The compiler will compile things in such a way that the lock gets dropped at the end of the function if and only if it was initialized.

Freezing

This is a weird example. For one, it can only work with types that implement Copy as it is currently written. For another, you can just as easily "unfreeze" a variable by moving it into a mut binding. For a third, this doesn't really prevent mutation in all cases, namely, when you have indirection via something like a reference.

fn example(v: &mut String) {
    {
        // Haha, no `mut`!
        let v = v;
        
        // Oh yeah, we don't need `mut v: &mut` in order to mutate *through* `v`
        v.push_str("...");
    }
}

More or less this just goes back to mut bindings being like a lint that prevents you from accidentally overwriting a variable or taking a &mut to it. Moving something to a non-mut binding turns on the lint, moving something to a mut binding turns off the lint.

You can use variable shadowing without moving the original variable to make it inaccessible, but this won't kill any outstanding borrows on its own...

fn example() {
    let mut s = String::new();
    let r = &mut s;
    
    // Haha, no `mut`!
    let s = ();

    // ...drat
    r.push_str("...");
}

...so you need something more convoluted if that's your goal.

fn example() {
    let mut s = String::new();
    let r = &mut s;
    
    // Haha, no `mut`!
    // *And*, taking the `&mut s` ensures there are no outstanding
    // borrows at this point.  Moving would be another way.
    let s = { let _tmp = &mut s; () };

    // This is now a borrow check error
    r.push_str("...");
}

Actually using these sorts of code patterns is rare.

The "unfreezing" pattern is discussed in the section on ownership for some reason. The two sections could be combined and rewritten in a way that just highlights that you can rebind things to add or remove the mut qualifier. And to be more clear about what omitting mut actually prevents (&mut and direct overwites).

Types

You can also type-erase values to dyn Trait, which is covered in a future section.

Aliasing

There is also a form of opaque aliasing related to impl Trait that I'll write about in that section.

Conversion

Other conversion traits include

AsRef and AsMut
Borrow and BorrowMut

One thing this section doesn't currently point out is that the blanket implementations on these traits limit or prevent you from writing blanket implementations for your own types, due to the possibility of overlapping implementations.

So this doesn't work, for example.

struct Mine(String);
impl<T: From<String>> From<T> for Mine { /* ... */ }

Flow control

`if`/`else`

More weird formatting here; you'd just have the if on the same line as the let big_n = typically.

`for` loops

`for` and iterators

for works with the IntoIterator trait, not the Iterator trait. It's true that every I: Iterator implements IntoIterator by returning itself, but IntoIterator::into_iter is actual the functionality that for uses. More on that here.

Not every I: IntoIterator implements Iterator.

RBE then says:

into_iter, iter and iter_mut all handle the conversion of a collection into an iterator in different ways, by providing different views on the data within.

iter and iter_mut are methods on individual collection types, not trait methods. (Some collections may also define an into_iter method on the collection type that does the same thing as their IntoIterator implementation.) Their iter and iter_mut names are only by convention. Some types have other methods too, like values_mut.

By convention,

collection.iter() will do the same thing as impl IntoIterator for &Collection
collection.iter_mut() will do the same thing as impl IntoIterator for &mut Collection

Which means you can write the examples like so.

fn main() {
    let names = vec!["Bob", "Frank", "Ferris"];

    // for name in names.iter() {
    for name in &names {
        // ...
    }
}

fn main() {
    let mut names = vec!["Bob", "Frank", "Ferris"];

    // for name in names.iter_mut() {
    for name in &mut names {
        // ...
    }
}

These are conventions, not requirements.

`match`

Destructuring

I recommend reading this article. It explains a lot about how patterns work in the absense of binding modes (more about that in a moment). The main things it doesn't cover are @, ref, and ref mut.

I'll assume after this you've read it.

pointers/`ref`

This page explains ref/ref mut adequately well.

So -- binding modes (a.k.a. "match ergonomics"). What's that? It's something that was introduced to the language so that you don't have to destructure references and use ref and ref mut so much.

The basic idea is that if you match on a reference without destructuring the reference, the "default binding mode" changes from by-value to ref or ref mut. If you later do explicitly destructure a reference, the default binding mode changes back to by value.

Binding modes can be pretty confusing when the patterns are complicated, but they're pretty intuitive at a basic level once you get used to them. Let's see an example:

fn example(opt: &Option<String>) {
    // Without binding modes (1):
    match *opt {
        None => {}
        Some(ref s) => {
            println!("{s}")
        }
    }

    // Without binding modes (2):
    match opt {
        &None => {}
        &Some(ref s) => {
            println!("{s}")
        }
    }

    // With binding modes:
    match opt {
        None => {}
        Some(s) => {
            println!("{s}")
        }
    }
}

These all do the same thing. In the last match, using the pattern Some(...) against the value which has type &Option<_> results in a ref binding mode. So when you introduce the s binding, it binds like ref s does in the first two matches.

Things get less obvious when the pattern moves in and out of different ref modes, or when there are otherwise a lot of different binding modes depending on where you are in the pattern. If you run into such a situation and want to "disable" binding modes so that the duality of patterns is restored, making the pattern more explicit, you can set the pattern_type_mismatch Clippy lint to deny.

Example. (You can run Clippy under Tools, top-right.)

Functions

Closures

As input parameters

This doesn't really cover the heirarchical nature of the Fn traits.

Everything that implements Fn implements FnMut
Everything that implements FnMut implements FnOnce

Or when you want to use which.

FnOnce is the easiest bound to meet; use it if you only need to call the closure once
FnMut is the next easiest, and usually not too restrictive for the callee; prefer this if you need to call the closure more than once
Fn is the most restrictive and is only rarely needed by the callee

On a variable-by-variable basis, the compiler will capture variables in the least restrictive manner possible.

When defined in a place annotated with a Fn-trait bound -- like when passing a closure into a method with such a bound -- the bound will influence how the compiler decides to capture variables and which Fn traits get implemented. The compiler's closure inference isn't perfect, and such annotations are one of the main ways of controlling the inference.

Type annonymity

The code style here is still weird.

meanwhile implementing the functionality via one of the traits: Fn, FnMut, or FnOnce

More than one, if required by the heirarchy mentioned above.

However, an unbounded type parameter would still be ambiguous and not be allowed.

It's not ambiguous, it's the fact that you can't use capabilities of generic types which aren't specified in the bounds.

Input Functions

As an additional note, the Fn, FnMut, and FnOnce traits dictate how a closure captures variables from the enclosing scope.

What they mean is that the bound will influence how the compiler decides to capture variables and which Fn traits are implemented, as mentioned above.

Other related notes...

Like closures, functions ("function items") each have their own unnameable type
But there are also function pointer types like fn() or fn(&str) -> String, etc.
Function items can be coerced to function pointers
Non-capturing closures can also be coerced to function pointers

As output parameters

so we have to use impl Trait to return them

No, you don't. You can return Box<dyn Fn()> or the like too.

Beyond this, the move keyword must be used, which signals that all captures occur by value.

No, it's not always needed. Not all closures capture, for one...

fn create_fn() -> impl Fn() {
    || println!("This is a: {}", "Fn")
}

fn create_fnmut() -> impl FnMut() {
    || println!("This is a: {}", "FnMut")
}

fn create_fnonce() -> impl FnOnce() {
    || println!("This is a: {}", "FnOnce")
}

...others don't need additional annotation to capture by value, for another...

fn create_fnonce() -> impl FnOnce() -> String {
    let s = "FnOnce".to_string();
    || s
}

...and closures that don't capture by value can also be returned:

fn create_fn(s: &str) -> impl Fn() + '_ {
    || println!("This is a: {}", &*s)
}

N.b. this last example relies on the edition 2021+ closure capture semantics, which illustrates why you should always use the latest edition / use cargo.

Higher Order Functions

These are functions that take one or more functions and/or produce a more useful function.

Uh, nothing in the example produces a function. They do take closures. There are other combinators that don't take closures, too.

"Higher order function" isn't terminology I see being used in the community.

Probably this section just needs some wording adjustment.

Diverging functions

Another way to describe ! is "uninhabited type".

Enums with no variants are also uninhabited, but don't have !'s magical coercive properties.

The property is useful for things like

let thing: Thing = match result {
    Ok(thing) => thing,
    Err(e) => {
        do_other_stuff(e);
        return
    }
}

Because both branches have to have the same type (Thing); the Err branch achieves this by coercing ! to Thing.

This page includes an example that uses a #![feature(...)]. You can only use features on nightly (which this guide hasn't introduced).

Diverging functions never return, but they might unwind due to non-aborting panic. I.e. they don't have to loop forever or stop thread execution. (Unwinds can be caught.)

Modules

Struct visibility

The fields of tuple structs are also private by default, but you can choose visibility.

pub struct Tup(i32, pub String);

The variants of enums all have the same visibility as the enum itself.

The section should mention non_exhaustive.

You can't destructure or use struct update syntax with types that have fields you can't name or foreign types that are non_exhaustive.

Hmm, do they cover SUS anywhere? I don't think they do. The general idea is that here:

struct User {
    active: bool,
    username: String,
    email: String,
    sign_in_count: u64,
}

fn example(user1: User) {
    // This part is the struct update syntax
    let user2 = User {
        email: String::from("another@example.com"),
        active: true,
        ..user1
    };
    
    let email = user1.email;
}

all the fields that are not explicitly specified (username, sign_in_count) are copied or moved from the expression after .. -- in this case, from user1.

Note that it does not act like this:

    let user2 = {
        let mut tmp = user1;
        tmp.email = String::from("another@example.com");
        tmp.active = true;
        tmp
    };

Instead it acts like this:

    let user2 = User {
        email: String::from("another@example.com"),
        active: true,
        username: user1.username,
        sign_in_count: user1.sign_in_count,
    };

And that is why you can use the unmoved fields of user1.

    let email = user1.email;

Because the thing after .. can be any expression that has the same type as the struct, a common idiom is:

    let user2 = User {
        email: String::from("another@example.com"),
        active: true,
        ..Default::default(),
    };

But note that this requires that User: Default, and constructs an entire User to pull fields out of. (Then the rest of the temporary value drops). That is, Default::default is not magical here, and it does not act field-by-field.

File hierarchy

my/mod.rs is an alternative to my.rs:

$ tree .
.
├── my
│   ├── inaccessible.rs
│   ├── mod.rs           // <---
│   └── nested.rs
└── split.rs

Some prefer this as it keeps all of the my module's code under the my directory.

Other projects use mod.rs for legacy reasons.

I'm not going to go into details, but many things about modules changed in edition 2018, so this topic is another reason to always specify an edition (or just use cargo).

Cargo

Cargo should be introduced in chapter 1 (or 0), with a forward link to advanced features.

Dependencies

You can add dependencies with cargo add.

Generics

They introduce turbofish in the next section, but turbofish isn't specific to functions. You need turbofish whenever you need to specify a generic paramter in an expression context (as opposed to a type context).

struct SingleGen<T> { field: T }

fn main() {
    //         vvvvvvvvvvvvvvvv annotations are a type position
    let _char: SingleGen<usize> = SingleGen { field: 0 };
    let _char = SingleGen::<usize> { field: 0 };
    //          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this is an expression
}}

Implementation

Similar to functions, implementations require care to remain generic.

Their examples "require care" because they named a struct S.

This isn't really a hazard unless you name your types the same way you name your generic type parameters.

Traits

Drop::drop doesn't take things by value. This section needs a better example. Add maybe.

Bounds

where should probably be introduced here, not later.

Testcase: empty bounds

Copy is a bad example. While it doesn't have methods, it definitely has different capabilities. Magical ones, even.

Where clauses

It's worth highlighting that there is no difference between a bound specified in a where clause and a bound specified inline:

fn printer<T: Display>(t: T) {
    println!("{}", t);
}

fn printer<T>(t: T)
where
    T: Display,
{
    println!("{}", t);
}

And that inline bounds are still called "where clauses".

This is a bad example:

// Because we would otherwise have to express this as `T: Debug` or 
// use another method of indirect approach, this requires a `where` clause:
impl<T> PrintInOption for T where
    Option<T>: Debug {

As "another indirect approach" is just to:

impl<T: Debug> PrintInOption for T { // ...

So this does not require an (explicit) where clause.

A better example would be one that literally can't be written inline, e.g. a bound on something that's not a parameter of the function.

trait Iterator {
    fn count(self) -> usize
    where
        Self: Sized,

New Type Idiom

Rust has "orphan rules" which basically mean that you can only implement a foreign trait for a foreign type in very specific circumstances.

But you can always implement a trait (foreign or local) for local types (provided any required bounds are met).

Because of that, another common use of the new type pattern is to wrap up a foreign type so that you can implement traits (or inherent methods!) on it.

Associated items

You can have associated consts too.

Associated types

You can have generic associated types (GATs) too.

trait Useful {}
trait Example {
    type Gat<'a>: Useful where Self: 'a;
    fn example(&self) -> Self::Gat<'_>;
}

impl Useful for &str {}
impl Example for String {
    type Gat<'a> = &'a str;
    fn example(&self) -> Self::Gat<'_> {
        &**self
    }
}

Phantom type parameters

Testcase: unit clarification

The goal of this section is to demonstrate why PhantomData is useful.

However, this could be rewritten to use zero-sized types instead of uninhabited enums:

struct Inch;
struct Mm;

(And IMO it would be an improvement.)

But there are other scenarios where you need PhantomData because

The type you need as a parameter isn't trivial
You need a lifetime or such
You need to influence auto-traits or variance
Probably more

Perhaps this section should just refer elsewhere like the official docs on PhantomData does.

Scope

I recommend taking care to distinguish between Rust lifetimes ('_), lexical scopes (like { /* ... */ } blocks), and the liveness scope of values -- when values go out of scope.

Lifetimes -- as in, those '_ things -- haven't been directly tied to lexical scopes since NLL (non-lexical lifetimes), 5+ years ago. Probably they don't want to restructure the book in a way that breaks a lot of links, but the naming of this chapter is unfortunate.

The main way lifetimes and scopes interact is that a value is considered to get used when it goes out of scope, and that use is incompatible with the value being borrowed.

{
    let x = ();
    let borrow = &x; // Say this has lifetime `'b`
}
// `x` goes out of scope at the `}`
// `'b` cannot be alive here as that would mean `x` is still borrowed

RAII

you'll never have to manually free memory or worry about memory leaks again!

Nit: You might have to worry about reference counter loops with Rc and Arc. And naturally you'll need some sort of manual garbage collection if you have a cache in a long-running process or the like.

Eh, don't sweat it until you run into it though. (E.g. if you make some cyclic structures.)

Destructor

Types that don't implement Drop can also have non-trivial destructors. Drop is more about custom destructors than having a destructor at all.

A T: Drop bound is more-or-less useless; a type that doesn't meet the bound may still have a destructor.

Ownership and moves

resources can only have one owner

Shared ownership can be modeled in Rust. They say this themselves in the sections on Rc and Arc later.

I'm not sure why they didn't bring up Copy in this section. Here, just go read the docs about "move semantics" versus "copy semantics".

Types that implement Copy don't have destructors (or have trivial no-op destructors, if you prefer).

Mutability

This is a weird thing to put under ownership in my opinion. I'm not sure why this wasn't part of the "freeze" section.

Anyway, we've already covered this above: You can move a variable between mut and non-mut bindings.

Partial moves

You can't partially move something that has a Drop implementation.

struct Foo {
    a: String,
    b: String,
}

impl Drop for Foo {
    fn drop(&mut self) {}
}

fn example(foo: Foo) {
    // compilation errors
    let a = foo.a;
    let Foo { a, ref b } = foo;
}

You can't destructure types from foreign traits that have private fields or the non_exhaustive attribute.

Borrowing

N.b. there's an implicit auto-deref going on in some of the code.

fn borrow_i32(borrowed_i32: &i32) {
    println!("This int is: {}", borrowed_i32);
}
// ...
    let boxed_i32 = Box::new(5_i32);
    borrow_i32(&boxed_i32); // Same as `&*boxed_i32`

Note: Shared refernces (&T) implement Copy.

Mutability

Again I suggest a more precise terminology:

&mut are exclusive references
&T are shared references
- "immutable" is inaccurate in the general case due to interior (shared) mutablity

This book doesn't seem to cover interior mutability at all. Interior mutability underlies mechanisms like Rc, Arc, Mutex, atomics, and other synchronization primitives.

Interior mutability removes the immutability guarantee for shared references.

Here's a simple example of mutating through a shared reference:

use core::cell::Cell;
fn example(i: &Cell<i32>) {
    i.set(42);
}

fn main() {
    // no `mut`!
    let i = Cell::new(0);
    example(&i);
    println!("{}", i.get());
}

Like the docs say, there is no way to relax the exclusiveness guarantee for &mut _.

Aliasing

&mut T does not implement Copy. But you can reborrow through it.

Instead of rewriting everything, I'll just link here.

Lifetimes

It's clear from the introduction that there have been some updates to dinstinguish between scopes and lifetimes. That's good! However, RBE is using the term "lifetime" both for the liveness scopes of values and for those '_ things.

That's not uncommon, but I find it leads to misunderstandings when trying to understand Rust ('_) lifetimes. I will take care to use the term "lifetime" only for those '_ things.

In the example, i doesn't have a lifetime. The diagram indicates where it goes out of scope.

When the compiler does borrow check analysis, it doesn't assign i a lifetime (one of those '_ things). It just knows that going out of scope is a use of i, and checks that against any potential borrows of i.

Explicit annotation

What's foo here? A function I guess?

Anyway, I think they're conflating concepts again. I think they should introduce lifetime bounds, aka outlives relationships, here instead of later. In particular, the bound

Foo<'a, 'b, T>: 'x

holds if and only if all of these hold:

'a: 'x,
'b: 'x,
T: 'x,

In which case we say "Foo<'a, 'b, 'T> outlives 'x". Or perhaps "is valid for 'x".

Note that outlives relationships are "greater or equal", not "strictly greater".

The wording on their examples here are also weird. I don't think I've ever seen anyone reason about function lifetimes by worrying about the functions "outliving" anything.

What they were attempting to get at is this: Any lifetimes supplied to a function that has a generic lifetime are chosen by the caller and must last at least just longer than the function body. In particular, this means that you can never borrow a local variable for as long as a generic lifetime, because local variables must be moved or go out of scope by the end of the function body.

More nits:

A short lifetime cannot be coerced into a longer one.

It's not something you generally think about, but contravariant lifetimes -- which can be coerced into a longer one (but not a shorter one) -- do exist.

Because the lifetime is never constrained, it defaults to 'static.

It doesn't matter what the unconstrained lifetime is, but it doesn't have a default. But you can pretend it's 'static if you want I suppose.

Functions

Ignoring elision, function signatures with lifetimes have a few constraints:

any reference must have an annotated lifetime.

Elision covers a majority of cases, so you should go read the rules. People generally only annotate lifetimes if they need to.

Here's the functions headers from their example when excercising elision.

// Didn't need any annotation
fn print_one(x: &i32) {

// Didn't need any annotation
fn add_one(x: &mut i32) {

// Didn't need any annotation
fn print_multi(x: &i32, y: &i32) {

// Finally we need an annotation... but for one lifetime, not two
fn pass_x<'a>(x: &'a i32, _: &i32) -> &'a i32 { x }

any reference being returned must have the same lifetime as an input or be static.

This part simply isn't true.

fn silly<'a>() -> &'a str {
    ""
}

Now, it's true this only compiles because the "" can be a &'static str, but that doesn't matter to the function signature. And there are other less silly use cases.

pub fn leak<'a>(b: Self) -> &'a mut T
where
    A: 'a,

But the vast majority of the time, yes, an output lifetime will correspond to an input lifetime, or be 'static.

Methods

Again you would elide the lifetimes here in practice.

They mentioned it elsewhere, but should probably reiterate here...

self is short for self: Self
&self is short for self: &Self
&mut self is short for self: &mut Self
etc

Traits

The lifetime parameters of traits are invariant, meaning they cannot be coerced to shorter or longer lifetimes.

Bounds

By "all references in T" they mean that bounds are recursive on their generic parameters, like I noted before.

Foo<'a, 'b, T>: 'x

holds if and only if all of these hold:

'a: 'x,
'b: 'x,
T: 'x,

(Note that not all lifetime-carrying types actually contain references.)

Again you would elide the lifetimes in print_ref. The T: 'a bound is implied by the presence of &T in the inputs. For reasons I don't want to go into here, the function is actually more general if you elide the lifetimes.

You can't elide lifetimes in the struct definition, but you can elide bounds implied by the fields. So these two are equivalent:

struct Ref<'a, T: 'a>(&'a T);
struct Ref<'a, T>(&'a T);

Because, similar to in print_ref, the presence of &'a T implies the T: 'a bound.

These actually do have a subtle difference when T = dyn Trait:

struct Ref<'a, T: 'a + ?Sized>(&'a T);
struct Ref<'a, T: ?Sized>(&'a T);

...but it's one that almost no-one knows or needs to care about in practice, as far as I know. (It also requires the ?Sized implicit-bound-removal to be relevant.)

Coercion

Lifetimes in covariant positions can be coerced to shorter lifetimes. That's the most common, and includes the (outer) lifetime of references (shared and exclusive).

However, Rust also has invariance and contravariance. You normally don't have to worry about the latter. But you will definitely run into invariance at some point. Invariant lifetimes cannot be coerced to a different lifetime.

Here's some more on the topic.

Static

Lifetimes can't use keyword names, but to date, 'static is the only special named lifetime you actually have to deal with. Well, and the wildcard '_ lifetime I guess, if you want to call that a name.

Reference lifetime

Again, I wouldn't conflate constants with statics.

The &'static str case applies to anything else that can undergo constant promotion, like a &i32 say.

fn example<T: 'static>(_: T) {}
fn main() {
    example(&0);
}

Trait bound

The implied definition of "owned" is circular here. Sometimes people use "owned" to mean "satisfies a 'static bound". But sometimes they use it to mean "responsible for cleaning up resources in a RAII sense".

I'd say Vec<&str> owns the references within, and the memory it allocated to store them. It doesn't own the str data that the references point to. Yet it only satisfies a 'static bound if it's a Vec<&'static str>.

Even if it's Vec<&'static str>, it doesn't own the str data that the references point to. (Arguably nothing does, since it's borrowed forever.)

Elision

Yeah... this should have came before all those examples. Or at least a big heads-up that having unnecessary named lifetimes is very unidiomatic.

Traits

A trait is a collection of methods defined for an unknown type: Self.

"unknown type" is odd phrasing. The implementing type is sort of like a generic of the trait. Trait do not have an implicit Sized bound, by the way.

Traits can have bounds that prevent implementing the trait for any type.

Derive

It's worth noting that if your struct has any generic type parameters, the built-in derive will put a bound on every type parameter that corresponds to the trait. For example,

#[derive(Default)]
struct MyOption<T>(Option<T>);

will do something like

impl<T: Default> Default for MyOption<T> {
    fn default() -> Self {
        Self(Option::<T>::default())
    }
}

even though the T: Default is not required for Option<T> to implement Default.

If you don't need and don't want this restriction, you have to implement the trait yourself.

If you want to match consts of your type in patterns (like match), you currently have to derive PartialEq and Eq. This may change in the future. (See here.)

Returning Traits with `dyn`

This section reads like it was written when you could just write Animal instead of dyn Animal for the trait object (edition 2015). Back then, it was contextual whether you meant the dyn Animal type or the Animal trait.

It needs a rewrite to consistently use dyn Trait when it means the trait object.

This means all your functions have to return a concrete type

Eh, I don't agree with this. I mean, all functions have to return a concrete type, because Rust is statically typed. Also, dyn Trait + '_ is a concrete type.

The problem is that there's no way to return unsized types. Like [T], dyn Trait is a dynamically sized type (DST) that does not implement Sized. All functions have to return a Sized type.

You're never "returning a trait", even when you box it up. You're type erasing your original value and returning a Box<dyn Animal> or whatever. That's its own type. Traits aren't types.

See here for an introduction to what dyn Trait is and isn't.

Operator Overloading

I'll just note that any time you read that an operator is "equivalent to" calling a method, it's not strictly true; there are subtle differences around type inference and the like.

Drop

At least today, String doesn't implement Drop.

// Fails today
fn witness<T: Drop>() {}
fn main() {
    witness::<String>();
}

As I mentioned before, types can have destructors even if they don't implement Drop. If a type has a non-trivial destructor, it's an implementation detail as to whether that's due to Drop or not. Technically removing a Drop implementation is a breaking change, but practically, it's not something anyone should be relying on.

Iterators

or automatically defined (as in arrays and ranges).

Not true but not important either really...

As mentioned before, it's IntoIterator that supplies the into_iter method.

`impl Trait`

This section should go over the capturing rules of return-position impl Trait, how it differs inside and outside of traits, how that may change across editions, the relation with async fn, and the future plans with type aliases.

OK fine, I'll do it here.

Return-position impl Trait (RPIT) is a form of thin, opaque aliasing. You can only return one concrete type from an RPIT function, so this doesn't work:

fn example(b: bool) -> impl std::fmt::Display {
    match b {
        false => 0,
        true => String::new(),
    }
}

There's no dyn Trait-like type erasure; the compiler still knows the underlying concrete type on some level. It just limits what consumers of the function can know about it. Among other things, this gives the function writer more flexibility -- they can change the concrete type used without breaking downstream.

Next, async fn are, roughly speaking, sugar for the following pattern:

// async fn foo<Input>(t: Input) -> OutputTy { /* ... */ }
fn foo<Input>(t: Input) -> impl Future<Output = OutputTy> {
    async move {
        _always_moves_everything = (&t,);
        /* ... */
    }
}

There are some subtle differences, which we'll return to soon. But the part to remember is that when we're talking about async fn, we're really talking about RPIT -- an opaque return type.

With a normal function (without an opaque return type), and a healthy understanding of lifetime elision, you can generally tell when the return type "captures" a generic input of the function; that is, when the output type is also parameterized by the generic input lifetime or type:

// "Captures" `T` but does not capture any lifetimes
fn example_1<T: Clone>(t: &T) -> Option<T> { None }

// "Captures" the lifetime but does not capture `T`
// You can tell due to the presence of `&`.  With less elision, the signature is:
// - `fn example_2<T: AsRef<str>>(t: &'_ T) -> &'_ str`, or
// - `fn example_2<'a, T: AsRef<str>>(t: &'a T) -> &'a str`
fn example_2<T: AsRef<str>>(t: &T) -> &str { "" }

// "Captures" both the lifetime and `T`
fn example_3<T: Debug>(t: &T) -> &T { t }

You can see how the types and/or lifetimes "flow into" the output type.

However, with RPIT, we don't currently have a great way to annotate generic lifetimes and types on the return type. So what do they capture? As it turns out, it's contextual.

async fn everywhere and RPIT in traits (RPITIT (I know, ugh)) capture all generic inputs
RPIT outside of traits before edition 2024 captures generic types, but not generic lifetimes
In edition 2024, the plan is for all RPITs to capture lifetimes

Is there a way to deal with undercapturing RPITs in edition 2021 -- to make them capture the lifetimes? There are ways, but they are clunky or imperfect.

The imperfect way is to slap + '_ on the end of the RPIT:

// Fails without the annotation                                         vvvv
fn example<T: ?Sized + AsRef<str>>(t: &T) -> impl Iterator<Item = char> + '_ {
    t.as_ref().chars()
}

It's imperfect as this actually imposes a bound on the opaque type -- and that bound applies to every generic type parameter.

The clunky way is to find some other way to mention the lifetime:

// This trait doesn't mean anything, and we implement it for everything.
// It's just a way to mention a lifetime.
trait Captures<'a> {}
impl<T: ?Sized> Captures<'_> for T {}

fn example<T: ?Sized + AsRef<str>>(t: &T) -> impl Iterator<Item = char> + Captures<'_> {
    t.as_ref().chars()
}

There are also some subtle differences between capturing by mention only, and capturing the concrete type containing the lifetime, which I won't go into here.

Alternatively you can wait for edition 2024, where undercapturing will supposedly go away.

Ok then, but, is there a way to deal with overcapturing? There is not one yet on stable, but it's planned. The core idea is called type alias impl Trait (TAIT), and it allows giving opaque types names by creating an alias -- and that alias can have parameters.

A TAIT captures whatever parameters you give it, but no more.

// Not available on stable yet
#![feature(type_alias_impl_trait)]
type MyIter<T: Clone> = impl Iterator<Item = (String, T)>;

// Captures `T`, but not the lifetime from the `&str`
fn example<T: Clone>(s: &str, t: T) -> MyIter<T> {
    // This is a silly (non-lazy) way to make an interator; for illustration only
    let vec: Vec<_> = s.split_whitespace().map(String::from).collect();
    vec.into_iter().map(move |s| (s, t.clone()))
}

Now, like non-opaque returning functions, the captures are "visible" again.

All of the RPIT and RPITIT and async fn versions can be described in terms of TAITs. The in-trait version of TAIT is a generic associated type (GAT) that is given the "type" impl Trait.

Clone

When dealing with resources, the default behavior is to transfer them during assignments or function calls.

...unless the type implements Copy.

"Without resources" is a false distinction (or a cyclic defintion). In order to implement Copy, all your fields must recursively and structurally also implement Copy. I wouldn't say an Option<i32> is "without resources", but it implements Copy. If you define a unit struct and don't implement Copy, you could say it is "without resources", but it still doesn't implement Copy.

Implementing Copy requires implementing Clone.

`macro_rules!`

I skipped this part :-)

Error handling

When there is a chance that things do go wrong and the caller has to deal with the problem, use Result. You can unwrap and expect them as well (please don't do that unless it's a test or quick prototype).

No good rule of thumb is as simple as "please don't do that".

https://blog.burntsushi.net/unwrap/

`panic`

Depending on the compilation, programs may abort on panic, instead of unwinding.

...oh, I see they correct themselves in the immediately following section. I don't know why they said it unwinds without qualifying the statement, then.

`Option` & `unwrap`

I only skimmed this part :-)

They might not have mentioned ok_or_else to turn an Option into a Result.

They might not have mentioned let else:

fn does_not_return_option(an_option: Option<Whatever>) {
    let Some(variable) = an_option else { return };
}

`Result`

I only skimmed this part :-)

There could be some re-ordering here. They "introduce ?", but they already did that for Option.

They might not have mentioned .ok() to turn a Result into an Option.

Multiple error types

Good error handling depends on a lot of things (perhaps the most fundamental is, who is your audience? Are you an end-user binary or a library?). If you care about good error handling, you're probably better off finding some dedicated blog posts or other resources on the topic.

Heads-up though, it's a large topic.

https://sabrinajewson.org/blog/errors

Other uses of `?`

? was previously explained as either unwrap or return Err(err). This is only mostly true. It actually means unwrap or return Err(From::from(err)).

It's actually more complicated than that.

But it acts mostly as if it did that, for Result.

Std library types

Box, stack and heap

It doesn't look like they cover how * is magical for Box. You can move non-Copy types out of a Box with * too. For instance you can remove the derive(Copy) from their example and it still compiles, even though it contains this code:

let unboxed_point: Point = *boxed_point;

Vectors

Like slices, their size is not known at compile time, but they can grow or shrink at any time.

The length of the data they point to is not known at compile time. Vec<_> itself is Sized.

+---+---+---+---+---+---+---+---+
| Pointer       | Length        | &[T] (or &str, &Path, &OsStr, ...)
+---+---+---+---+---+---+---+---+
  |
  V
+---+---+---+---+---+---+---+---+
| D | A | T | A | . | . | . | ......    [T] (or str, Path, OsStr, ...)
+---+---+---+---+---+---+---+---+   
  ^
  |
+---+---+---+---+---+---+---+---+---+---+---+---+
| Pointer       | Length        | Capacity      | Vec<T> (or PathBuf, String, OsString, ...)
+---+---+---+---+---+---+---+---+---+---+---+---+
  ^
  |
+---+---+---+---+
| Pointer       | &Vec<T> (or &PathBuf, &String, &OsString, ...)
+---+---+---+---+

N.b. the order of pointer/length/capacity is unspecified and subject to change.

Incidentally, I don't think this book mentions that you should prefer &[T] over &Vec<T> (et cetera) in your APIs. There's not much you can do with a &Vec<_> that you can't do with a &[_] (check capacity, I guess). And as the diagram above demonstrates, &Vec<_> introduces an extra layer of indirection.

(&mut Vec<T>, et cetera, is a different scenario.)

Option

Option is for values that might not be present generally. It has no special or intrinsic relation to failures or panic!. None doesn't inately mean something unfortunate is going on.

Result

`?`

? is not Result specific.

`panic!`

Panics might abort instead of unwinding the stack.

There are ways to catch unwinding panics, which I don't think this guide covers. (My notes here don't either.)

`HashMap`

Like vectors, HashMaps are growable, but HashMaps can also shrink themselves when they have excess space.

Uh, Vecs can do that too. They both have shrink* methods. As far as I know, neither of the std data structures sheds excess capacity unless you request it does so.

or use HashMap::new() to get a HashMap with a default initial capacity (recommended).

new is documented to start with a capacity of 0.

Probably this section is conveying implementation details of the pre-hashbrown std HashMap.

Alternate/custom key types

The mention of int and uint probably means this page hasn't been updated since before Rust 1.0.

`HashSet`

If you insert a value that is already present in the HashSet, (i.e. the new value is equal to the existing and they both have the same hash), then the new value will replace the old.

Exactly wrong, the new value is dropped and the old one retained.

`Rc`

This is one of the std shared ownership types mentioned above.

Reference count of an Rc increases by 1 whenever an Rc is cloned, and decreases by 1 whenever one cloned Rc is dropped out of the scope.

That's what happens to the strong count, yes...

When an Rc's reference count becomes zero (which means there are no remaining owners), both the Rc and the value are all dropped.

...but this is not correct (or at best very misleading). When the strong count reaches 0, the shared value is dropped. When the weak count reaches 0, then the shared backing memory is dropped.

`Arc`

Same drop behavior as Rc.

Std misc

Threads

Going to go out on a limb and guess they're not going to mention scoped threads, which can capture locals / need not be 'static.

Path

The "flavors" of Path is an implementation detail. Probably their note at the bottom to check out the methods are what evolved into the os module extension traits, like these.

A Path is immutable. The owned version of Path is PathBuf.

No, a Path is not immutable. You can have a &mut Path, no problem. You could use that to get a &mut OsStr and then lowercase all the ASCII characters, for example.

A Path is unsized, like str and [T]. It's not necessarily borrowed. You can have a (borrowed) &Path or &mut Path. Or you can have a Box<Path>.

The relation between Path and PathBuf is similar to that of str and String: a PathBuf can be mutated in-place, and can be dereferenced to a Path.

This is more accurate.

Everything I just said about mutability and ownership applies to str as well as Path.

Note that a Path is not internally represented as an UTF-8 string, but instead is stored as an OsString

PathBuf corresponds to OsString. Path corresponds to OsStr.

a Path can be freely converted to an OsString or &OsStr

Conversion from &Path to OsString (or PathBuf) isn't free, it allocates.

Maybe they meant infallibly.

File I/O

There's also a create_new.

`read_lines`

We also update read_lines to return an iterator instead of allocating new String objects in memory for each line.

BufRead allocates a new String per line too.

Filesystem operations

(I'm skimming at this point but) these are awful implementations of commands like cat. Files don't have to be UTF8 and reading the entire thing in first is inefficient, to pick one example.

Program arguments

Use args_os so you don't panic on non-UTF8 args.

Testing

I skipped this part :-)

Unsafe Operations

I only skimmed this part.

If you're going to venture into unsafe yourself, find a dedicated guide. You should know it exists and what it means, but no introductory text is going to give you enough information to use it soundly.

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