error: expected one of `)` or `,`, found `(`
  --> $DIR/test2.rs:5:26
   |
LL |         struct S3(pub $t ());
   |                         -^ expected one of `)` or `,`
   |                         |
   |                         help: missing `,`
...
LL |     define_struct! { (foo) }
   |     ------------------------ in this macro invocation
   |
   = note: this error originates in the macro `define_struct` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0412]: cannot find type `foo` in this scope
  --> $DIR/test2.rs:11:23
   |
LL |     define_struct! { (foo) }
   |                       ^^^ not found in this scope

error[E0412]: cannot find type `foo` in this scope
  --> $DIR/test2.rs:11:23
   |
LL |     define_struct! { (foo) }
   |                       ^^^ not found in this scope

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0412`.