| 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 | 
 |    | | 
 |    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no` | 
 |  | 
 | error: aborting due to 3 previous errors | 
 |  | 
 | For more information about this error, try `rustc --explain E0412`. |