Add MaybeDangling

[WaffleLapkin]

cc #118166

[theemathas]

You also need to implement StructuralPartialEq, since ManuallyDrop in a const can currently be used in patterns on stable.

[joboet]

I don't know how much this is still an experiment, but if you're looking for more coverage, you might want to replace the MaybeUninit-based manual MaybeDangling implementation in std:

rust/library/std/src/thread/lifecycle.rs

Lines 61 to 82 in 864339a

	// Pass `f` in `MaybeUninit` because actually that closure might *run longer than the lifetime of `F`*.
	// See <https://github.com/rust-lang/rust/issues/101983> for more details.
	// To prevent leaks we use a wrapper that drops its contents.
	#[repr(transparent)]
	struct MaybeDangling<T>(MaybeUninit<T>);
	impl<T> MaybeDangling<T> {
	fn new(x: T) -> Self {
	MaybeDangling(MaybeUninit::new(x))
	}
	fn into_inner(self) -> T {
	// Make sure we don't drop.
	let this = ManuallyDrop::new(self);
	// SAFETY: we are always initialized.
	unsafe { this.0.assume_init_read() }
	}
	}
	impl<T> Drop for MaybeDangling<T> {
	fn drop(&mut self) {
	// SAFETY: we are always initialized.
	unsafe { self.0.assume_init_drop() };
	}
	}