Derived PartialEq cannot be used in a `const` function

[mbartlett21]

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

[jonas-schievink]

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]

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

[mbartlett21]

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]

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

[eminence]

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
}