error[E0121]: the placeholder `_` is not allowed within types on item signatures for functions
  --> $DIR/issue-81885.rs:1:22
   |
LL | const TEST4: fn() -> _ = 42;
   |                      ^ not allowed in type signatures

error[E0121]: the placeholder `_` is not allowed within types on item signatures for constant items
  --> $DIR/issue-81885.rs:1:22
   |
LL | const TEST4: fn() -> _ = 42;
   |                      ^ not allowed in type signatures

error[E0121]: the placeholder `_` is not allowed within types on item signatures for functions
  --> $DIR/issue-81885.rs:6:26
   |
LL |     const TEST5: fn() -> _ = 42;
   |                          ^ not allowed in type signatures

error[E0121]: the placeholder `_` is not allowed within types on item signatures for constant items
  --> $DIR/issue-81885.rs:6:26
   |
LL |     const TEST5: fn() -> _ = 42;
   |                          ^ not allowed in type signatures

error: aborting due to 4 previous errors

For more information about this error, try `rustc --explain E0121`.