Recover keywords in trait bounds

[WaffleLapkin]

(this pr was inspired by this tweet)

Recover keywords in trait bound, motivational example:

fn f(_: impl fn()) {} // mistyped, meant `Fn`

Current nightly (3 needless and confusing errors!)

error: expected identifier, found keyword `fn`
 --> ./t.rs:1:15
  |
1 | fn _f(_: impl fn()) {}
  |               ^^ expected identifier, found keyword
  |
help: escape `fn` to use it as an identifier
  |
1 | fn _f(_: impl r#fn()) {}
  |               ++

error: expected one of `:` or `|`, found `)`
 --> ./t.rs:1:19
  |
1 | fn _f(_: impl fn()) {}
  |                   ^ expected one of `:` or `|`

error: expected one of `!`, `(`, `)`, `,`, `?`, `for`, `~`, lifetime, or path, found keyword `fn`
 --> ./t.rs:1:15
  |
1 | fn _f(_: impl fn()) {}
  |              -^^ expected one of 9 possible tokens
  |              |
  |              help: missing `,`

error: at least one trait must be specified
 --> ./t.rs:1:10
  |
1 | fn _f(_: impl fn()) {}
  |          ^^^^

This PR:

error: expected identifier, found keyword `fn`
 --> ./t.rs:1:15
  |
1 | fn _f(_: impl fn()) {}
  |               ^^ expected identifier, found keyword
  |
help: escape `fn` to use it as an identifier
  |
1 | fn _f(_: impl r#fn()) {}
  |               ++

error[E0405]: cannot find trait `r#fn` in this scope
  --> ./t.rs:1:15
   |
1  | fn _f(_: impl fn()) {}
   |               ^^ help: a trait with a similar name exists (notice the capitalization): `Fn`
   |
  ::: /home/waffle/projects/repos/rust/library/core/src/ops/function.rs:74:1
   |
74 | pub trait Fn<Args>: FnMut<Args> {
   | ------------------------------- similarly named trait `Fn` defined here

It would be nice to have suggestion in the first error like "have you meant Fn trait", instead of a separate error, but the recovery is deep inside ident parsing, which makes it a lot harder to do.

r? @compiler-errors

[compiler-errors]

I'm not sure if ~const Trait is called a "const" bound technically-- I would call it a "const if const" bound. But whatever, should be fine.

[compiler-errors]

@bors r+

[bors]

📌 Commit 5d5e451 has been approved by compiler-errors

It is now in the queue for this repository.