Need more helpful diagnositics for label values.

[crlf0710]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=781876a356578cbf3da13b05f8e40ba5

macro_rules! foo {
    ($v:expr, $l:lifetime) => {
        {
            dbg!($v);
            break $l
        }
    }
}
fn main () {
    'x: loop {
        // foo!(42, 'x); // correct usage
        foo!('x);
    }
}

The current output is:

error: expected `while`, `for`, `loop` or `{` after a label
  --> src/main.rs:12:14
   |
12 |         foo!('x);
   |              ^^ expected `while`, `for`, `loop` or `{` after a label

The error and diagnostics is ... kind of not pointing to the correct fix.

[estebank]

Current output:

error: expected `while`, `for`, `loop` or `{` after a label
  --> src/main.rs:12:14
   |
12 |         foo!('x);
   |              ^^ expected `while`, `for`, `loop` or `{` after a label

error: expected expression, found end of macro arguments
  --> src/main.rs:12:16
   |
 2 |     ($v:expr, $l:lifetime) => {
   |      ------- while parsing argument for this `expr` macro fragment
...
12 |         foo!('x);
   |                ^ expected expression

warning: unused label
  --> src/main.rs:10:5
   |
10 |     'x: loop {
   |     ^^
   |
   = note: `#[warn(unused_labels)]` (part of `#[warn(unused)]`) on by default

By pointing at line 2 it provides some guidance. I'm not sure how we can improve this in a general way, due to how free macros can be. We likely can try looking at all the matchers and try alternative modifications to the macro invocation and suggest if we find one or two good ones, like foo!(/* expr */, 'x); in this case.