Regression: Strange behavior with labels and error messages

[ThePuzzlemaker]

I apologize that I wasn't able to minimize this one.

Here's the code.
Additionally, here's the code that calls the parser:

    let (items, parse_errs) = parse::item()
        .separated_by(parse::maybe_nls())
        .collect::<Vec<_>>()
        .then_ignore(end())
        .parse_with_state(stream, &mut &*gcx)
        .into_output_errors();

With this input:

fn invalid_syntax_here() -> fn true -> 0

As of bbb0d70, this error is generated:

Error: Unexpected token `true`, expected: number, `true`, `false`, `-`, `!`, `(`
   ╭─[<input>:1:32]
   │
 1 │ fn invalid_syntax_here() -> fn true -> 0
   │                                ──┬─  
   │                                  ╰─── didn't expect this token
───╯

Before 6837537, this error is generated, which is sensible (as true cannot possibly match in this context):

Error: Unexpected token `true`, expected: `(`, expression
   ╭─[<input>:1:32]
   │
 1 │ fn invalid_syntax_here() -> fn true -> 0
   │                                ──┬─  
   │                                  ╰─── didn't expect this token
───╯

This line of code is to blame; reverting this one-line change returns to the original behavior.