improve suggestion when writing "fn Trait for Ty" instead of "impl Trait for Ty"

[matthiaskrgr]

Given the following code:

struct Foo;

fn From<i32> for Foo {
    fn from(a: i32) -> Self {
        Foo
    }
}

fn main() {}

The current output is:

error: expected `(`, found keyword `for`
 --> src/main.rs:3:14
  |
3 | fn From<i32> for Foo {
  |              ^^^ expected `(`

I wonder if we can detect that this is supposed to be trait impl, maybe look for <Trait> for <Ty> or something?

Ideally the output should look like:

 --> src/main.rs:3:14
  |
3 | fn From<i32> for Foo {
  | ^^ expected `impl`, found "fn"

[chenyukang]

@rustbot claim