Macro with_safety 

macro_rules! with_safety {
    ( safe, $ty:ty ) => { ... };
    ( unsafe, $ty:ty ) => { ... };
    ( $safety:path, $ty:ty ) => { ... };
    ( $safety:tt, $ty:ty ) => { ... };
    ( @inner $safety:ty, $ty:ty ) => { ... };
}

Construct a function-pointer type identical to the given one but using the specified safety.

Accepts either:

• a Safety marker type (Safe or Unsafe)
• safety keyword (safe or unsafe).
• a boolean literal.

Examples

type F = with_safety!(safety::Safe, unsafe extern "C" fn(i32) -> i32);
// `F` is `extern "C" fn(i32) -> i32`

type G = with_safety!(unsafe, fn());
// `G` is `unsafe fn()`