Const equality disagrees when using in match and if

[Pzixel]

Consider following code:

#[derive(Debug, PartialEq, Eq)]
struct Foo([u32; 4]);

const fn bar() {
    const ONE: Foo = Foo([1, 0, 0, 0]);
    match Foo([0, 0, 0, 0]) {
        ONE => unreachable!(),
        _ => {},
    }

    if ONE == Foo([0, 0, 0, 0]) {
        unreachable!()
    }
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=f2b2c2421583fbb47472f8e8746b394f
It fails with "the trait PartialEq<_> is implemented for Foo, but that implementation is not const".

But if we do add a const implementation:

#![feature(const_trait_impl)]

#[derive(Debug, Eq)]
struct Foo([u32; 4]);

impl const PartialEq for Foo {
    fn eq(&self, other: &Self) -> bool {
        self.0[0] == other.0[0]
        && self.0[1] == other.0[1]
        && self.0[2] == other.0[2]
        && self.0[3] == other.0[3]
    }
}


const fn bar() {
    const ONE: Foo = Foo([1, 0, 0, 0]);
    match Foo([0, 0, 0, 0]) {
        ONE => unreachable!(),
        _ => {},
    }

    if ONE == Foo([0, 0, 0, 0]) {
        unreachable!()
    }
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=a980e3ab7d72b9545886de5547cf4963
then it fails with "to use a constant of type Foo in a pattern, Foo must be annotated with `#[derive]"

So we found ourselves between rock and hard place where we have to choose between using pattern matching and equality.

[mejrs]

Alternatively you can write

#![feature(derive_const)]

#[derive_const(PartialEq)]
#[derive(Debug, Eq)]
struct Foo([u32; 4]);

const fn bar() {
    const ONE: Foo = Foo([1, 0, 0, 0]);
    match Foo([0, 0, 0, 0]) {
        ONE => unreachable!(),
        _ => {},
    }

    if ONE == Foo([0, 0, 0, 0]) {
        unreachable!()
    }
}

but this fails as well:

error[[E0277]](https://doc.rust-lang.org/nightly/error_codes/E0277.html): can't compare `[u32; 4]` with `_` in const contexts
 --> src/lib.rs:5:12
  |
3 | #[derive_const(PartialEq)]
  |                --------- in this derive macro expansion
4 | #[derive(Debug, Eq)]
5 | struct Foo([u32; 4]);
  |            ^^^^^^^^ no implementation for `[u32; 4] == _`
  |
  = help: the trait `~const PartialEq<_>` is not implemented for `[u32; 4]`
note: the trait `PartialEq<_>` is implemented for `[u32; 4]`, but that implementation is not `const`