error[E0124]: field `a` is already declared
  --> $DIR/fields-definition.rs:14:13
   |
LL |             a: u8,
   |             ----- `a` first declared here
LL |             $a: u8,
   |             ^^^^^^ field already declared
...
LL | legacy!(a);
   | ---------- in this macro invocation
   |
   = note: this error originates in the macro `legacy` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0124`.