Heterogeneous comparisons for Option

[petrochenkov]

It would be useful to be able to compare Option<T> with Option<U> if T and U are comparable themselves. I.e.

impl<A, B> PartialEq<Option<B>> for Option<A> where A: PartialEq<B> { ... }

assert!(Some(String::from_str("str")) == Some("str"))

Heterogeneous comparisons for Option are clear and unambiguous and there are precedents - some other basic library types, like Vec or [T] support heterogeneous comparisons too.

cc @aturon
cc rust-lang/rust#22838

[petrochenkov]

For some reason after this change Some("str") == None can't infer the type of None despite the fact that PartialEq has default type parameter.
At the same time Some(0u8) == None still can infer the type of None.

[Gankra]

Default type parameters do not currently drive inference, iirc.

[aturon]

There is an issue open for generalizing comparison across all types in std. This is mainly a matter of what's been implemented, rather than a strong philosophical stance.

cc rust-lang/rust#20927 as well.

[bluss]

Potentially leads to widespread type inference regressions, when x == y can not hint the type for y anymore.

[petrochenkov]

Closing in favor of more general rust-lang/rust#20063.