"Unexpected end of macro invocation" error in nested macro has misleading span, doesn't specify which macro

The following code (based on a test case by @apoelstra):

macro_rules! check_enum {
    ($e:ident, $($var:ident),*) => ({
        $(assert_eq!(0 == $e::$var);)*
    })
}

fn main() {
    check_enum!(Foo, Bar, Baz);
}

generates this error message:

error: unexpected end of macro invocation
 --> test.rs:9:25
9 |>     check_enum!(Foo, Bar, Baz);
  |>                         ^

The problem is that assert_eq! is expecting a comma. This is very hard to decipher though, because the span points to the outer macro invocation. (This may be related to #26615.) If a better span can't be found, it would be useful to at least include the name of the macro in the error message, e.g. "unepected end of assert_eq! invocation."

1. We now generate this error, which to me is still largely incomprehensible. This is better, though; @mbrubeck Could you provide the "ideal" error message so we can either implement that or as close as possible?

   error: unexpected end of macro invocation
    --> ./test.rs:3:31
     |
   3 |         $(assert_eq!(0 == $e::$var);)*
     |                               ^^^^
   ...
   8 |     check_enum!(Foo, Bar, Baz);
     |     --------------------------- in this macro invocation
   
   
   

   Mark Rousskov at 2017-05-04 12:03:36

2. Not mbrubeck, but imho the ideal message would expected "," not found with the point where the comma should be highlighted. When I stumbled across this with another macro, the "unexpected end" part made me think I had mismatching brackets.

   konsti at 2018-03-28 22:49:35

3. Current output:

   error: expected identifier, found `,`
    --> src/main.rs:3:31
     |
   3 |         $(assert_eq!(0 == $e::$var);)*
     |                               ^
     |                               |
     |                               expected identifier
     |                               help: remove this comma
   ...
   8 |     check_enum!(Foo, Bar, Baz);
     |     --------------------------- in this macro invocation
   

   Esteban Kuber at 2019-01-18 22:28:15

4. error: unexpected end of macro invocation
    --> src/main.rs:3:35
     |
   3 |         $(assert_eq!(0 == $e::$var);)*
     |                                   ^ missing tokens in macro arguments
     |
   note: while trying to match `,`
    --> /rustc/8ede3aae28fe6e4d52b38157d7bfe0d3bceef225/library/core/src/macros/mod.rs:37:16
   
   

   Esteban Kuber at 2023-08-03 19:11:41