error[E0428]: the name `b` is defined multiple times
  --> $DIR/issue-58490.rs:18:5
   |
LL |         macro_rules! b { () => () }
   |         -------------- previous definition of the macro `b` here
...
LL |     macro_rules! b { () => () }
   |     ^^^^^^^^^^^^^^ `b` redefined here
   |
   = note: `b` must be defined only once in the macro namespace of this module

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0428`.