PartialEq between reference and non-reference type?

[LukasKalbertodt]

I'm not sure, if that's intended, but it's "impossible" to compare a reference type with a non-reference type via PartialEq. Short example code:

let a = 3;
let b = 4;
&a == &b; // works
a == &b;   // doesn't work

This should work in my opinion. There already are these impls in std (I removed some noise -- original):

impl<A, B> PartialEq<&    B> for &    A where A: PartialEq<B>
impl<A, B> PartialEq<&mut B> for &mut A where A: PartialEq<B>
impl<A, B> PartialEq<&mut B> for &    A where A: PartialEq<B>
impl<A, B> PartialEq<&    B> for &mut A where A: PartialEq<B>

The third line in my example works because of this.

Why not something like this?

impl<A, B> PartialEq<&B> for  A where A: PartialEq<B>
impl<A, B> PartialEq< B> for &A where A: PartialEq<B>

Or do I misunderstand something?

[petrochenkov]

Trait coherence prevents it :(

See rust-lang/rust#23521 for one (unsuccessful) attempt to fix it with help of OIBITs.
I'll try again when specialization is implemented.

[LukasKalbertodt]

@petrochenkov It seems like specialization is mostly implemented (tracking issue, PR). I just came across this issue again and wanted to ask if there's anything new? :)

[varkor]

I'm not sure specialisation helps in this situation. The problem as I see it is that if you something where: A: PartialEq<B> implies A: PartialEq<&B> and &A: PartialEq<B>, then there arise two ways to get to &A: PartialEq<&B> (depending on the order you apply the impls). These implementations are conflicting, even if you have a more specialised &A: PartialEq<&B> impl, because specialisation doesn't address this sort of case. What you want is some way for specialisation to "cover" the conflict-cases for multiple traits.

EDIT: Alternatively, if you could specify that a trait only applies to non-reference types, this should also solve the problem, along with the existing rules for referencing both types.

[nico-abram]

Would something like rust-lang/rust#42721 help there? (Referring to EDIT: Alternatively, if you could specify that a trait only applies to non-reference types)