Here's some code incorrectly defining a macro, because it's missing a fragment specifier. Playground

macro_rules! m {
    ($x [$y: ident]) => {}
}

Here's the error message rustc gives me:

error: missing fragment specifier
 [--> src/lib.rs:2:9
](https://play.rust-lang.org/#)  |
2 |     ($x [$y: ident]) => {}
  |         ^^^^^^^^^^^
  |

It's potentially confusing that the error points to the source after the name with the missing fragment specifier. Especially in a complicated macro, it would be better if it pointed to the item which was actually missing the fragment specifier.