Unhelpful message for syntax error in pattern bindings

[benschulz]

When changing a pattern binding from something like

match some_value {
    SomeEnum::SomeVariant { ref some_field } => { /* do something with some_field */ },
    _ => {}
}

to something like

match some_value {
    SomeEnum::SomeVariant { some_field: ref alias } => { /* do something with alias */ },
    _ => {}
}

it's easy to end up with

match some_value {
    SomeEnum::SomeVariant { ref some_field: alias } => { /* do something with alias */ },
    _ => {}
}

which yields an unhelpful syntax error message (playground). I don't believe it's obvious why ref some_field and some_field: alias are valid, but ref some_field: alias is not. The same holds for ref mut bindings, of course.

It would be nice if rustc's parser accepted the invalid syntax and provided one of its helpful "did you mean to write some_field: ref alias?" error messages.

[estebank]

Current output:

error: expected `,`
  --> src/main.rs:13:47
   |
13 |         SomeEnum::SomeVariant { ref some_field: alias } => { /* do something with alias */ }
   |         ---------------------                 ^
   |         |
   |         while parsing the fields for this pattern