Error for incorrect identifier patterns could be improved

[tesuji]

The compiler could suggest that e identifier must precede the pattern in the following code:

playpen link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=ca2dc7094a56b0478edefe0faefefe0e

fn main() {
    let x = 2;

    match x {
        1 ..= 5 @ e => {}
        _ => {}
    }
}

Current error message:

error: expected one of `=>`, `if`, or `|`, found `@`
 --> src/main.rs:5:17
  |
5 |         1 ..= 5 @ e => {}
  |                 ^ expected one of `=>`, `if`, or `|` here

error: aborting due to previous error

It could be improved as:

error: expected one of `=>`, `if`, or `|`, found `@`
 --> src/main.rs:5:17
  |
5 |         1 ..= 5 @ e => {}
  |                 ^ expected one of `=>`, `if`, or `|` here
                    | help: try `e @ 1 ..= 5`


error: aborting due to previous error

[Centril]

So what happens after 1 ..= 5 is that we've parsed a PatKind::Range and then @ follows which is not a valid token after a parsed $pat. To fix this we can, after having parsed let pat = ...; in parse_pat_with_range_pat, check if @ follows and then emit appropriate suggestions. We will need to use some heuristics to figure out what the user wants based on the structure of pat: PatKind. Some of the suggestions would be MachineApplicable and some not.

cc @estebank