parse error recovery could be better with missing commas after struct fields

[comex]

Test case: (playground link)

struct S {
    foo: u32 // <- missing comma
    bar: u32
}

fn main() {
    let s = S { foo: 5, bar: 6 };
}

Actual result (nightly):

error: expected `,`, or `}`, found `bar`
 --> src/main.rs:3:5
  |
3 |     bar: u32
  |     ^^^
  |
  = help: struct fields should be separated by commas

error[E0560]: struct `S` has no field named `foo`
 --> src/main.rs:7:17
  |
7 |     let s = S { foo: 5, bar: 6 };
  |                 ^^^ `S` does not have this field

error[E0560]: struct `S` has no field named `bar`
 --> src/main.rs:7:25
  |
7 |     let s = S { foo: 5, bar: 6 };
  |                         ^^^ `S` does not have this field

Expected result:

Ideally the parser would guess that there was meant to be a comma at the end of the line. Then:

• The "expected comma" error could highlight the line missing a comma (and suggest a fix-it), rather than highlighting the first token in the next line.
• After producing an error, it could recover by pretending the comma was there, adding the foo and bar fields to the struct to avoid the latter two errors.

[simartin]

I propose to work on that one.

[comex]

@simartin ❤️