> +pub unsafe trait FnCall<Args, R = ()>: 'static + Sync + Sized {
> + /// Referring to this internal constant asserts that the `Self` type is
> + /// zero-sized. Can be replaced by an inline const expression in
> + /// Rust 1.79.0+.
> + const ASSERT_ZERO_SIZED: () = { assert!(mem::size_of::<Self>() == 0) };
> +
> + /// Call the function with the arguments in args.
> + fn call(a: Args) -> R;
> +}
> +
> +macro_rules! impl_call {
> + ($($args:ident,)* ) => (
> + // SAFETY: because each function is treated as a separate type,
> + // accessing `FnCall` is only possible in code that would be
> + // allowed to call the function.
> + unsafe impl<F, $($args,)* R> FnCall<($($args,)*), R> for F
> + where
> + F: 'static + Sync + Sized + Fn($($args, )*) -> R,
> + {
> + #[inline(always)]
> + fn call(a: ($($args,)*)) -> R {
> + let _: () = Self::ASSERT_ZERO_SIZED;
> +
> + // SAFETY: the safety of this method is the condition for
> implementing
> + // `FnCall`. As to the `NonNull` idiom to create a
> zero-sized type,
> + // see https://github.com/rust-lang/libs-team/issues/292.
> + let f: &'static F = unsafe {
> &*NonNull::<Self>::dangling().as_ptr() };
Awesome! The definition of FnCall and this trick are both very elegant!
I've learned a lot.
> + let ($($args,)*) = a;
> + f($($args,)*)
> + }
> + }
> + )
> +}
> +
The examples and test both show the power of this callback pattern,
Reviewed-by: Zhao Liu <zhao1....@intel.com>
