No expansion backtrace for macro where contents does not parse

[huonw]

macro_rules! foo {
    (item) => { fn not_a_pattern() {} };
    (pattern) => { ref _x }
}
fn main() {
    foo!(pattern);
    let foo!(item) = ();
}

<anon>:3:20: 3:23 error: expected identifier, found keyword `ref`
<anon>:3     (pattern) => { ref _x }
                            ^~~
<anon>:3:24: 3:26 error: macro expansion ignores token `_x` and any following
<anon>:3     (pattern) => { ref _x }
                                ^~
<anon>:2:17: 2:19 error: expected identifier, found keyword `fn`
<anon>:2     (item) => { fn not_a_pattern() {} };
                         ^~
<anon>:2:20: 2:33 error: macro expansion ignores token `not_a_pattern` and any following
<anon>:2     (item) => { fn not_a_pattern() {} };
                            ^~~~~~~~~~~~~
error: aborting due to 4 previous errors

The error messages should point to the respective foo! invocations, not just the contents of the macro, like

<anon>:3:20: 3:23 error: expected identifier, found keyword `ref`
<anon>:3     (pattern) => { ref _x }
                            ^~~
<anon>:1:1: 4:2 note: in expansion of foo!
<anon>:6:5: 6:19 note: expansion site

[steveklabnik]

Triage: only the first and third errors show now, but don't mention foo! at all.

[estebank]

Current output points at macro invocation of every syntax error inside of a macro definition:

error: expected expression, found keyword `ref`
 --> src/main.rs:3:20
  |
3 |     (pattern) => { ref _x }
  |                    ^^^
...
6 |     foo!(pattern);
  |     -------------- in this macro invocation

error: expected pattern, found keyword `fn`
 --> src/main.rs:2:17
  |
2 |     (item) => { fn not_a_pattern() {} };
  |                 ^^
...
7 |     let foo!(item) = ();
  |         ---------- in this macro invocation