The `vec![..]` macro can be used in pattern position with bad diagnostics

[czipperz]

fn main() {
    match Some(vec![3]) {
        Some(vec![x]) => (),
        _ => (),
    }
}

Gives error message:

error: unexpected `(` after qualified path
 --> src/main.rs:3:14
  |
3 |         Some(vec![x]) => (),
  |              ^^^^^^^
  |              |
  |              unexpected `(` after qualified path
  |              in this macro invocation
  |
  = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

But this is clearly wrong as there is no ( in the span pointed to.

[estebank]

This is caused by macro expansion:

$ rustc +nightly file3.rs -Zexternal-macro-backtrace
error: unexpected `(` after qualified path
 --> <::alloc::macros::vec macros>:3:23
  |
1 | / ( $ elem : expr ; $ n : expr ) => (
2 | | $ crate :: vec :: from_elem ( $ elem , $ n ) ) ; ( $ ( $ x : expr ) , * ) => (
3 | | < [ _ ] > :: into_vec ( box [ $ ( $ x ) , * ] ) ) ; ( $ ( $ x : expr , ) * )
  | |                       ^ unexpected `(` after qualified path
4 | | => ( $ crate :: vec ! [ $ ( $ x ) , * ] )
  | |_________________________________________- in this expansion of `vec!`
  |
 ::: file3.rs:3:14
  |
3 |           Some(vec![x]) => (),
  |                -------
  |                |
  |                in this macro invocation
  |                in this macro invocation

error: aborting due to previous error

You cannot pattern match on vectors, but you can pattern match on slices (and even rely on match ergonomics to make it look like you're matching on an array).

[czipperz]

I think it would be nice if this was special cased since it's easily fixable.