error[E0124]: field `foo` is already declared
  --> $DIR/struct-fields-decl-dupe.rs:13:5
   |
12 |     foo: isize,
   |     ---------- `foo` first declared here
13 |     foo: isize,
   |     ^^^^^^^^^^ field already declared

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0124"