Show generic bounds in E0046

[JustAPerson]

Consider:

use std::iter::FromIterator;
struct X;
impl FromIterator<()> for X {
}

which causes the following error on stable and nightly:

error[E0046]: not all trait items implemented, missing: `from_iter`
 --> src/main.rs:3:1
  |
3 | impl FromIterator<()> for X {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `from_iter` in implementation
  |
  = note: `from_iter` from trait: `fn(T) -> Self`

It would be super helpful to print the other relevant bounds for this method:

pub trait FromIterator<A> {
    fn from_iter<T>(iter: T) -> Self
    where
        T: IntoIterator<Item = A>;
}

Just trying to add a partial definition fn from_iter<I>(i: I) -> Self { Self } will be accepted without any bound on I. Of course you can't really do anything with it yet. Thus it would be pleasant to remind the user what bound they probably need.

[estebank]

Current output, no real change other than using a copy/paste-able signature:

error[E0046]: not all trait items implemented, missing: `from_iter`
 --> src/lib.rs:3:1
  |
3 | impl FromIterator<()> for X {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `from_iter` in implementation
  |
  = help: implement the missing item: `fn from_iter(_: T) -> Self { unimplemented!() }`

[estebank]

The output from #68689 is not perfect because it uses incorrect syntax, but gets us closer to the correct output:

error[E0046]: not all trait items implemented, missing: `from_iter`
  --> $DIR/missing-assoc-fn.rs:19:1
   |
LL | impl FromIterator<()> for X {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `from_iter` in implementation
   |
   = help: implement the missing item: `fn from_iter<T>(_: T) -> Self where T: std::iter::IntoIterator, std::iter::IntoIterator::Item = A { unimplemented!() }`

[estebank]

Current output:

error[E0046]: not all trait items implemented, missing: `from_iter`
 --> src/lib.rs:3:1
  |
3 | impl FromIterator<()> for X {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `from_iter` in implementation
  |
  = help: implement the missing item: `fn from_iter<T: IntoIterator<Item = ()>>(_: T) -> Self { todo!() }`