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offset_of returns a temporary #124478

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tgross35 opened this issue Apr 28, 2024 · 4 comments · Fixed by #124484 or #124527
Closed

offset_of returns a temporary #124478

tgross35 opened this issue Apr 28, 2024 · 4 comments · Fixed by #124484 or #124527
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@GKFX
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C-bug Category: This is a bug. F-offset_of `#![feature(offset_of)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@tgross35
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tgross35 commented Apr 28, 2024 •
edited by rustbot

I tried this code:

struct Foo {
    a: u32,
}

impl Foo {
    fn bar() -> &'static [usize] {
        &[std::mem::offset_of!(Self, a)]
    }
}

Playground link

I expected to see this happen: no error

Instead, this happened:

error[E0515]: cannot return reference to temporary value
 --> src/lib.rs:7:9
  |
7 |         &[std::mem::offset_of!(Self, a)]
  |         ^-------------------------------
  |         ||
  |         |temporary value created here
  |         returns a reference to data owned by the current function

For more information about this error, try `rustc --explain E0515`.
error: could not compile `playground` (lib) due to 1 previous error

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rustc 1.79 nightly 2024-04-27
@tgross35 tgross35 added the C-bug Category: This is a bug. label Apr 28, 2024
@rustbot rustbot added the needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. label Apr 28, 2024
@tgross35 tgross35 changed the title offset_of offset_of returns a temporary Apr 28, 2024
@tgross35
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I can't think of any reason why it would create the temporary here, considering you can assign the result to a const just fine.

cc @GKFX who worked on this most recently

@rustbot label +T-compiler +F-offset_of -needs-triage

@rustbot rustbot added F-offset_of `#![feature(offset_of)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. and removed needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. labels Apr 28, 2024
@GKFX
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GKFX commented Apr 28, 2024

It's the core::hint::must_use that does it: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=bd518763786b97f87434d5c986cc52cf. builtin # offset_of alone doesn't cause the problem.

I'm not sure what the original reason was for making this must_use, or whether it would be OK to just remove it? Wrapping the macro in const {} works, but that would interfere with array indexing in offset_of if that ever gets supported.

let x = offset_of!(S, a[y]); // not const.

@asquared31415
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making it must_use makes sense, but if it's not possible to keep that behavior and make it not a temporary, i think it's better UX to remove the must_use hint.

@GKFX
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GKFX commented Apr 28, 2024

@rustbot claim

@GKFX GKFX mentioned this issue Apr 28, 2024
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This was referenced Apr 28, 2024
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jieyouxu added a commit to jieyouxu/rust that referenced this issue Apr 29, 2024
@jieyouxu
Rollup merge of rust-lang#124484 - GKFX:offset_of_must_use, r=jieyouxu
0580588 
Fix rust-lang#124478 - offset_of! returns a temporary

This was due to the must_use() call. Adding HIR's `OffsetOf` to the must_use checking within the compiler avoids this issue while maintaining the lint output.

Fixes rust-lang#124478. `@tgross35`
@jieyouxu jieyouxu mentioned this issue Apr 29, 2024
Merged
bors added a commit to rust-lang-ci/rust that referenced this issue Apr 29, 2024
@bors
Auto merge of rust-lang#124527 - jieyouxu:rollup-eslzncy, r=jieyouxu
a8a1d3a 
Rollup of 7 pull requests

Successful merges:

 - rust-lang#124269 (Pretty-print parenthesis around binary in postfix match)
 - rust-lang#124415 (Use probes more aggressively in new solver)
 - rust-lang#124475 (Remove direct dependencies on lazy_static, once_cell and byteorder)
 - rust-lang#124484 (Fix rust-lang#124478 - offset_of! returns a temporary)
 - rust-lang#124504 (Mark unions non-const-propagatable in `KnownPanicsLint` without calling layout)
 - rust-lang#124508 (coverage: Avoid hard-coded values when visiting logical ops)
 - rust-lang#124522 ([Refactor] Rename `Lint` and `LintGroup`'s `is_loaded` to `is_externally_loaded` )

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors closed this as completed in ca79086 Apr 29, 2024
rust-timer added a commit to rust-lang-ci/rust that referenced this issue Apr 29, 2024
@rust-timer
Unrolled build for rust-lang#124484
07e8ef2 
Rollup merge of rust-lang#124484 - GKFX:offset_of_must_use, r=jieyouxu

Fix rust-lang#124478 - offset_of! returns a temporary

This was due to the must_use() call. Adding HIR's `OffsetOf` to the must_use checking within the compiler avoids this issue while maintaining the lint output.

Fixes rust-lang#124478. `@tgross35`
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C-bug Category: This is a bug. F-offset_of `#![feature(offset_of)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Fix #124478 - offset_of! returns a temporary GKFX/rust
Rollup of 7 pull requests jieyouxu/rust
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@GKFX @tgross35 @asquared31415 @rustbot