error: lifetime parameter `'a` only used once
  --> $DIR/fn-types.rs:9:10
   |
LL |   a: for<'a> fn(&'a u32),
   |          ^^      -- ...is used only here
   |          |
   |          this lifetime...
   |
note: the lint level is defined here
  --> $DIR/fn-types.rs:1:9
   |
LL | #![deny(single_use_lifetimes)]
   |         ^^^^^^^^^^^^^^^^^^^^
help: elide the single-use lifetime
   |
LL -   a: for<'a> fn(&'a u32),
LL +   a: fn(&u32),
   |

error[E0581]: return type references lifetime `'a`, which is not constrained by the fn input types
  --> $DIR/fn-types.rs:12:22
   |
LL |   d: for<'a> fn() -> &'a u32, // OK, used only in return type.
   |                      ^^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0581`.