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Derived PartialEq cannot be used in a const function #77695

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mbartlett21 opened this issue Oct 8, 2020 · 5 comments
Closed

Derived PartialEq cannot be used in a const function #77695

mbartlett21 opened this issue Oct 8, 2020 · 5 comments
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A-const-eval Area: Constant evaluation, covers all const contexts (static, const fn, ...) C-feature-request Category: A feature request, i.e: not implemented / a PR. F-const_trait_impl `#![feature(const_trait_impl)]` T-lang Relevant to the language team, which will review and decide on the PR/issue.

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@mbartlett21
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The derived PartialEq implementation cannot be used in a const context.

The code below gives error E0015

#[derive(PartialEq)]
struct A(u8);

const fn is_eq(left: &A, right: &A) -> bool {
    left == right
}
error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 --> derive_eq_partialeq.rs:5:2
  |
5 |     left == right
  |     ^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0015`.

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (91a79fb29 2020-10-07)
binary: rustc
commit-hash: 91a79fb29ac78d057d04dbe86be13d5dcc64309a
commit-date: 2020-10-07
host: x86_64-pc-windows-msvc
release: 1.49.0-nightly
LLVM version: 11.0
@mbartlett21 mbartlett21 added the C-bug Category: This is a bug. label Oct 8, 2020
@matthewjasper matthewjasper added A-const-fn C-enhancement Category: An issue proposing an enhancement or a PR with one. F-const_trait_impl `#![feature(const_trait_impl)]` and removed C-bug Category: This is a bug. labels Oct 8, 2020
@jonas-schievink
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I'm not sure this will ever work. It would require the macro to generate a const impl (#67792), but there is no way it can know that that is possible at expansion time.

@leonardo-m
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What's the solution? A #[derive(ConstPartialEq)]? :-)

@mbartlett21
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What I may have to do for the moment is make a const fn on the struct.

I had previously implemented const PartialEq manually, but when I tried to use it a const_generic parameter, it said that they had to be derived.

@oli-obk
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oli-obk commented Nov 17, 2020

yea, this will require opt-in, so schemes like #[derive(const PartialEq)] could be possible.

@jonas-schievink jonas-schievink added C-feature-request Category: A feature request, i.e: not implemented / a PR. T-lang Relevant to the language team, which will review and decide on the PR/issue. and removed C-enhancement Category: An issue proposing an enhancement or a PR with one. labels Nov 21, 2020
@eminence
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Now that #102049 has landed, it's possible to do const derive PartialEq in the latest nightly:

#![feature(const_cmp)]
#![feature(const_trait_impl)]
#![feature(derive_const)]

#[derive_const(PartialEq)]
struct A(u8);

const fn is_eq(left: &A, right: &A) -> bool {
    left == right
}

@oli-obk oli-obk closed this as completed Nov 22, 2022
@RalfJung RalfJung added the A-const-eval Area: Constant evaluation, covers all const contexts (static, const fn, ...) label Dec 1, 2024
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Labels
A-const-eval Area: Constant evaluation, covers all const contexts (static, const fn, ...) C-feature-request Category: A feature request, i.e: not implemented / a PR. F-const_trait_impl `#![feature(const_trait_impl)]` T-lang Relevant to the language team, which will review and decide on the PR/issue.
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