Improve impl implements different signature from trait error message

[gaurikholkar-zz]

We need to improve the error message for the second case.

trait Foo {
    fn foo<'a>(x: &i32, y: &'a i32) -> &'a i32;
}

// First is when impl implements same signature as trait:
impl Foo for () {
    fn foo<'a>(x: &i32, y: &'a i32) -> &'a i32 {
        if x > y { x } else { y }
    }
}

// Second is when impl implements different signature from trait.
// Here we *could* suggest adding lifetime to `x`.
trait Bar {
    fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32;
}

impl Bar for () {
    fn foo<'a>(x: &i32, y: &'a i32) -> &'a i32 {
        // this body `y` would be OK
        if x > y { x } else { y }
    }
}

cc @nikomatsakis

[gaurikholkar-zz]

@nikomatsakis, maybe this needs mentoring?
Also for E0621, we don't handle Impl Items, should I open up an issue for that?

[steveklabnik]

Triage: here's the error today:

error[E0623]: lifetime mismatch
  --> src/lib.rs:10:20
   |
8  |     fn foo<'a>(x: &i32, y: &'a i32) -> &'a i32 {
   |                   ----                 -------
   |                   |
   |                   this parameter and the return type are declared with different lifetimes...
9  |         // this body `y` would be OK
10 |         if x > y { x } else { y }
   |                    ^ ...but data from `x` is returned here