Provide better type hints when a type doesn't support a binary operator
For example, when checking whether `vec![A] == vec![A]` holds, we first evaluate the LHS's ty, then probe for any `PartialEq` implementations for that. If none is found, we report an error by evaluating `Vec<A>: PartialEq<?0>` for fulfillment errors, but the RHS is not yet evaluated and remains an inference variable `?0`!
To fix this, we evaluate the RHS and equate it to that RHS infer var `?0`, so that we are able to provide more detailed fulfillment errors for why `Vec<A>: PartialEq<Vec<A>>` doesn't hold (namely, the nested obligation `A: PartialEq<A>` doesn't hold).
Fixes#95285Fixes#110867
aba9fb4696