error[E0277]: expected a `Fn<()>` closure, found `unsafe fn() -> i32`
  --> $DIR/fn-trait.rs:20:16
   |
LL |     require_fn(f as unsafe fn() -> i32);
   |     ---------- ^^^^^^^^^^^^^^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
   |     |
   |     required by a bound introduced by this call
   |
   = help: the trait `Fn<()>` is not implemented for `unsafe fn() -> i32`
   = note: wrap the `unsafe fn() -> i32` in a closure with no arguments: `|| { /* code */ }`
note: required by a bound in `require_fn`
  --> $DIR/fn-trait.rs:3:23
   |
LL | fn require_fn(_: impl Fn() -> i32) {}
   |                       ^^^^^^^^^^^ required by this bound in `require_fn`

error[E0271]: type mismatch resolving `<unsafe fn() -> i32 as FnOnce<()>>::Output == i32`
  --> $DIR/fn-trait.rs:20:16
   |
LL |     require_fn(f as unsafe fn() -> i32);
   |     ---------- ^^^^^^^^^^^^^^^^^^^^^^^ types differ
   |     |
   |     required by a bound introduced by this call
   |
note: required by a bound in `require_fn`
  --> $DIR/fn-trait.rs:3:31
   |
LL | fn require_fn(_: impl Fn() -> i32) {}
   |                               ^^^ required by this bound in `require_fn`

error[E0277]: expected a `Fn<()>` closure, found `extern "C" fn() -> i32`
  --> $DIR/fn-trait.rs:24:16
   |
LL |     require_fn(g as extern "C" fn() -> i32);
   |     ---------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an `Fn<()>` closure, found `extern "C" fn() -> i32`
   |     |
   |     required by a bound introduced by this call
   |
   = help: the trait `Fn<()>` is not implemented for `extern "C" fn() -> i32`
   = note: wrap the `extern "C" fn() -> i32` in a closure with no arguments: `|| { /* code */ }`
note: required by a bound in `require_fn`
  --> $DIR/fn-trait.rs:3:23
   |
LL | fn require_fn(_: impl Fn() -> i32) {}
   |                       ^^^^^^^^^^^ required by this bound in `require_fn`

error[E0271]: type mismatch resolving `<extern "C" fn() -> i32 as FnOnce<()>>::Output == i32`
  --> $DIR/fn-trait.rs:24:16
   |
LL |     require_fn(g as extern "C" fn() -> i32);
   |     ---------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ
   |     |
   |     required by a bound introduced by this call
   |
note: required by a bound in `require_fn`
  --> $DIR/fn-trait.rs:3:31
   |
LL | fn require_fn(_: impl Fn() -> i32) {}
   |                               ^^^ required by this bound in `require_fn`

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0271, E0277.
For more information about an error, try `rustc --explain E0271`.