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core::hint::must_use returns a temporary #124493
Description
Originally noticed in #124478 (comment), @GKFX identified it as a problem in must_use in #124478 (comment)
I tried this code:
#![feature(hint_must_use)]
fn foo() -> &'static [usize] {
&[core::hint::must_use(42)]
}I expected to see this happen: no error, the value is a constant
Instead, this happened:
error[E0515]: cannot return reference to temporary value
--> src/lib.rs:4:5
|
4 | &[core::hint::must_use(42)]
| ^--------------------------
| ||
| |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
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tgross35 commentedon Apr 28, 2024
@rustbot label +T-compiler +A-lifetimes +requires-nightly -needs-triage
GKFX commentedon Apr 28, 2024
Is this likely to be fixable with
must_usebeing a function? As I understand it,fooonly knows the signature ofmust_use, not its implementation, so cannot know thatmust_useis an identity function and cannot avoid a temporary. It would be ideal if the#[must_use]attribute applied to expressions.A magic
#[rustc_identity_function]attribute onmust_usecould be used to let the compiler know infoothat it can avoid the temporary, but seems like overkill.git grep 'hint.*must_use'doesn't show any other usage (besides offset_of) ofcore::hint::must_usewhich makes me wonder how useful it is. Tagging #94745.tgross35 commentedon Apr 28, 2024
Yup you are right, I guess we can't actually do anything with the function signature so I will close this. Agreed that it may not be overly useful in its current form.
tgross35 commentedon May 3, 2024
Per @RalfJung in #124484 (comment), this may indeed be fixable by using
#[rustc_promotable]dlight commentedon Apr 26, 2025
Maybe turning
core::hint::must_useinto a macro could fix this issue?