Rollup merge of #70908 - estebank:suggest-add, r=nikomatsakis

Rollup merge of #70908 - estebank:suggest-add, r=nikomatsakis

Provide suggestions for type parameters missing bounds for associated types

When implementing the binary operator traits it is easy to forget to restrict the `Output` associated type. `rustc` now accounts for different cases to lead users in the right direction to add the necessary restrictions. The structured suggestions in the following output are new:

```
error: equality constraints are not yet supported in `where` clauses
  --> $DIR/missing-bounds.rs:37:33
   |
LL | impl<B: Add> Add for E<B> where <B as Add>::Output = B {
   |                                 ^^^^^^^^^^^^^^^^^^^^^^ not supported
   |
   = note: see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information
help: if `Output` is an associated type you're trying to set, use the associated type binding syntax
   |
LL | impl<B: Add> Add for E<B> where B: Add<Output = B> {
   |                                 ^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
  --> $DIR/missing-bounds.rs:11:11
   |
7  | impl<B> Add for A<B> where B: Add {
   |      - this type parameter
...
11 |         A(self.0 + rhs.0)
   |           ^^^^^^^^^^^^^^ expected type parameter `B`, found associated type
   |
   = note: expected type parameter `B`
             found associated type `<B as std::ops::Add>::Output`
help: consider further restricting this bound
   |
7  | impl<B> Add for A<B> where B: Add + std::ops::Add<Output = B> {
   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0369]: cannot add `B` to `B`
  --> $DIR/missing-bounds.rs:31:21
   |
31 |         Self(self.0 + rhs.0)
   |              ------ ^ ----- B
   |              |
   |              B
   |
help: consider restricting type parameter `B`
   |
27 | impl<B: std::ops::Add<Output = B>> Add for D<B> {
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

That output is given for the following cases:

```rust
struct A<B>(B);
impl<B> Add for A<B> where B: Add {
    type Output = Self;

    fn add(self, rhs: Self) -> Self {
        A(self.0 + rhs.0) //~ ERROR mismatched types
    }
}

struct D<B>(B);
impl<B> Add for D<B> {
    type Output = Self;

    fn add(self, rhs: Self) -> Self {
        Self(self.0 + rhs.0) //~ ERROR cannot add `B` to `B`
    }
}

struct E<B>(B);
impl<B: Add> Add for E<B> where <B as Add>::Output = B {
    type Output = Self;

    fn add(self, rhs: Self) -> Self {
        Self(self.0 + rhs.0)
    }
}
```