Sub-optional error message with slice rest syntax in patterns

[leonardo-m]

fn foo(c: &[u32], n: u32) -> u32 {
    match *c {
        [h, ..] if h > n => 0,
        [h, ..] if h == n => 1,
        //[h, ref ts @..] => foo(c, n - h) + foo(ts, n), // OK
        [h, ref ts..] => foo(c, n - h) + foo(ts, n), // Wrong
        [] => 0,
    }
}
fn main() {}

Gives:

error: expected one of `,`, `@`, `]`, or `|`, found `..`
 --> ...\test.rs:6:19
  |
6 |         [h, ref ts..] => foo(c, n - h) + foo(ts, n),
  |                   -^
  |                   |
  |                   expected one of `,`, `@`, `]`, or `|`
  |                   help: missing `,`

error[E0308]: mismatched types
 --> ...\test.rs:6:46
  |
6 |         [h, ref ts..] => foo(c, n - h) + foo(ts, n),
  |                                              ^^ expected slice `[u32]`, found `u32`
  |
  = note: expected reference `&[u32]`
             found reference `&u32`

error: aborting due to 2 previous errors

I think a better error message should suggest the usage of ref ts @...

[chrissimpkins]

@rustbot claim

[chrissimpkins]

#72534

This gets us to:

error: expected one of `,`, `@`, `]`, or `|`, found `..`
 --> src/main.rs:5:19
  |
5 |         [h, ref ts..] => foo(c, n - h) + foo(ts, n),
  |                   ^^
  |                   |
  |                   expected one of `,`, `@`, `]`, or `|`
  |                   help: maybe missing `@` in binding pattern?

error: aborting due to previous error

[chrissimpkins]

Format and help message revised to:

error: expected one of `,`, `@`, `]`, or `|`, found `..`
 --> src/main.rs:5:20
  |
5 |         [h, ref ts..] => foo(c, n - h) + foo(ts, n),
  |                    ^^ expected one of `,`, `@`, `]`, or `|`
  |
help: if you meant to bind the contents of the rest of the array pattern into `ts`, use `@`
  |
5 |         [h, ref ts @ ..] => foo(c, n - h) + foo(ts, n),
  |                    ^

error: aborting due to previous error