Suggest fix for misplaced generic params on `fn` item

[jruderman]

Given the following code (playground):

fn<T> id(x: T) -> T { x }

The current output is:

error: expected identifier, found `<`
 --> src/lib.rs:1:3
  |
1 | fn<T> id(x: T) -> T { x }
  |   ^ expected identifier

Ideally the output should look also include:

help: place the generic parameter list after the function name:
  |
1 | fn id<T>(x: T) -> T { x }
  |      ~~~

This is an easy mistake to make because impl items do require the generic parameters to go immediately after the keyword.

[compiler-errors]

this probably should be generalized for the other item kinds as well (struct, enum, trait, and union), if someone is planning on implementing this.

[spanishpear]

Would this make a good first issue to look at? If so, would be happy to take a stab :)

[spanishpear]

@rustbot claim

Let's give it a try anyways 🥳

[jruderman]

Here are some potentially tricky cases:

Complicated bounds

aka "the programmer probably should have moved some of this into a where clause"

fn<'a, B: 'a + std::ops::Add<Output = u32>> f(_x: B) { }

fn main() {
    f(1_u32);
}

Generic bounds also present in the correct place

fn<T: Send> g<T>(x: T) -> T { x }

Here, the compiler should avoid suggesting modified code that also won't parse. Simply moving <T: Send> to after the function name in this case would result in fn g<T: Send><T>, which isn't right.

Ideally, for this specific example, it would suggest replacing the second with the first, creating fn g<T: Send>. But it's probably not worth the complexity to try to determine "how compatible" two generics lists are and how best to merge them.

For this case, I'd be happy enough with an error message that doesn't include a suggestion, or simply suggesting the removal of the first generics list.