More accurate suggestion for missing binop trait bounds

[estebank]

#94034 (comment)

We currently emit

LL | impl<B: std::ops::Add> Add for D<B> {
   |       +++++++++++++++

but it should be

LL | impl<B: std::ops::Add<Output = B>> Add for D<B> {
   |       +++++++++++++++++++++++++++

[willcrichton]

I will take a stab at this soon. I am fairly certain I know how to do it -- there's an Expectation type that describes the expected return type, and if that expectation exists, we can use it to bound Output=<Expectation>.

I tried implementing that today. This is the core change: willcrichton@e652939#diff-ea6f9d55e90333a1f5fc5e202d46862a6a031410831f2df42317ac024f821162R519-R539

This change seems to introduce some noise in other tests involving binops that will require manual fixing. But maybe next weekend I will try to finish this.

@rustbot claim

[estebank]

Fixed:

rust/tests/ui/generic-associated-types/missing-bounds.stderr

Lines 62 to 65 in ddbf3b9

	help: consider restricting type parameter `B` with trait `Add`
	|
	LL | impl<B: std::ops::Add<Output = B>> Add for D<B> {
	| +++++++++++++++++++++++++++