offset_of returns a temporary

[tgross35]

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

Meta

rustc 1.79 nightly 2024-04-27

[tgross35]

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

[GKFX]

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]

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]

@rustbot claim