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Non-'static Lifetimes in Const Generics #74052

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yodaldevoid opened this issue Jul 5, 2020 · 3 comments
Open

Non-'static Lifetimes in Const Generics #74052

yodaldevoid opened this issue Jul 5, 2020 · 3 comments
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A-const-generics Area: const generics (parameters and arguments) A-lifetimes Area: Lifetimes / regions C-enhancement Category: An issue proposing an enhancement or a PR with one. T-lang Relevant to the language team, which will review and decide on the PR/issue.

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@yodaldevoid
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We currently don't correctly handle non-'static lifetimes in const generics. Until #74051 is merged we ICE when a non-'static lifetime is hit, as seen in #60814.

Many uses for const generics does not hit this limitation, but the following case has been brought up:

fn test<'a, const VALUE: std::mem::Discriminant<Enum<'a>>(v: Enum<'a>) -> bool {
     std::mem::discriminant(&v) == VALUE
}

as quoted from @lcnr on Zulip

mem::Discriminant is invariant so VALUE has to be mem::Discriminant of Enum<'a> and can't use Enum<'static>.

@varkor @eddyb @nikomatsakis
@jonas-schievink jonas-schievink added A-const-generics Area: const generics (parameters and arguments) A-lifetimes Area: Lifetimes / regions F-const_generics `#![feature(const_generics)]` T-lang Relevant to the language team, which will review and decide on the PR/issue. labels Jul 5, 2020
@JohnTitor JohnTitor added the C-enhancement Category: An issue proposing an enhancement or a PR with one. label Jul 5, 2020
@nikomatsakis
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This is "just" an implementation limitation, correct?

@lcnr
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lcnr commented Jul 16, 2020

Mostly, it should be theoretically fine, but ends up with similar problems to what #74113 ended up solving.

It also has some open design questions IMO, which I don't want to care about for now 😆

@lcnr lcnr mentioned this issue Jul 17, 2020
Merged
github-actions bot pushed a commit to rust-lang/glacier that referenced this issue Jul 23, 2020
@rustbot
ices/56445-2.rs: fixed with errors
ce27e32 
=== stdout ===
=== stderr ===
warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /home/runner/work/glacier/glacier/ices/56445-2.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default
  = note: see issue #44580 <rust-lang/rust#44580> for more information

error[E0771]: use of non-static lifetime `'a` in const generic
 --> /home/runner/work/glacier/glacier/ices/56445-2.rs:6:26
  |
6 | struct Bug<'a, const S: &'a str>(PhantomData<&'a ()>);
  |                          ^^
  |
  = note: for more information, see issue #74052 <rust-lang/rust#74052>

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0771`.
==============
github-actions bot pushed a commit to rust-lang/glacier that referenced this issue Jul 23, 2020
@rustbot
ices/60814.rs: fixed with errors
6cda7fe 
=== stdout ===
=== stderr ===
warning: the feature `const_generics` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /home/runner/work/glacier/glacier/ices/60814.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default
  = note: see issue #44580 <rust-lang/rust#44580> for more information

error[E0771]: use of non-static lifetime `'a` in const generic
 --> /home/runner/work/glacier/glacier/ices/60814.rs:3:45
  |
3 | pub fn function_with_str<'a, const STRING: &'a str>() {
  |                                             ^^
  |
  = note: for more information, see issue #74052 <rust-lang/rust#74052>

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0771`.
==============
@BoxyUwU BoxyUwU added F-adt_const_params `#![feature(adt_const_params)]` and removed F-const_generics `#![feature(const_generics)]` labels Jun 28, 2022
@lcnr lcnr removed the F-adt_const_params `#![feature(adt_const_params)]` label Jun 28, 2022
@lcnr
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lcnr commented Jun 28, 2022

const generics triage

This is part of generic const parameter types. The design work for that is tracked in rust-lang/project-const-generics#28.

@chenyukang chenyukang mentioned this issue Mar 16, 2023
Closed
@github-actions github-actions bot mentioned this issue Apr 9, 2024
Merged
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A-const-generics Area: const generics (parameters and arguments) A-lifetimes Area: Lifetimes / regions C-enhancement Category: An issue proposing an enhancement or a PR with one. T-lang Relevant to the language team, which will review and decide on the PR/issue.
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@nikomatsakis @yodaldevoid @jonas-schievink @BoxyUwU @JohnTitor @lcnr