Function forget_unsized

Like forget, but also accepts unsized values.

While Rust does not permit unsized locals since its removal in #111942 it is still possible to call functions with unsized values from a function argument or place expression.

#![feature(unsized_fn_params, forget_unsized)]
#![allow(internal_features)]

use std::mem::forget_unsized;

pub fn in_place() {
    forget_unsized(*Box::<str>::from("str"));
}

pub fn param(x: str) {
    forget_unsized(x);
}

This works because the compiler will alter these functions to pass the parameter by reference instead. This trick is necessary to support Box<dyn FnOnce()>: FnOnce(). See #68304 and #71170 for more information.