error: `derive` may only be applied to structs, enums and unions
  --> $DIR/issue-43106-gating-of-derive.rs:4:1
   |
LL | #[derive(Debug)]
   | ^^^^^^^^^^^^^^^^

error: `derive` may only be applied to structs, enums and unions
  --> $DIR/issue-43106-gating-of-derive.rs:7:17
   |
LL |     mod inner { #![derive(Debug)] }
   |                 ^^^^^^^^^^^^^^^^^ help: try an outer attribute: `#[derive(Debug)]`

error: `derive` may only be applied to structs, enums and unions
  --> $DIR/issue-43106-gating-of-derive.rs:10:5
   |
LL |     #[derive(Debug)]
   |     ^^^^^^^^^^^^^^^^

error: `derive` may only be applied to structs, enums and unions
  --> $DIR/issue-43106-gating-of-derive.rs:23:5
   |
LL |     #[derive(Debug)]
   |     ^^^^^^^^^^^^^^^^

error: `derive` may only be applied to structs, enums and unions
  --> $DIR/issue-43106-gating-of-derive.rs:27:5
   |
LL |     #[derive(Debug)]
   |     ^^^^^^^^^^^^^^^^

error: aborting due to 5 previous errors