[DRAFT] Add ub_checks for downcast_unchecked

[Techcable]

Right now debug_assert! is used, which will not trigger in user code.

This is likely unacceptable for performance reasons, since the optimizer cannot understand virtual Any::type_id() calls.

This could potentially be fixed by applying something like #[ffi_const] to the Any::type_id function (issue #58328), which would have wider reaching performance benefits. Unfortunately, #[ffi_const] is not possible right now because it is limited to FFI calls (as its name suggests).

Ignoring the performance issue, I wasn't quite sure how to implement the actual assertion. It cannot use the assert_unsafe_precondition! macro because that requires the assertion to work in a const context. The closest thing I could find in the stdlib seems to be debug_assert_fd_is_open, which uses rtabort!

rust/library/std/src/sys/fs/unix.rs

Lines 848 to 853 in 040a98a

	// this is similar to assert_unsafe_precondition!() but it doesn't require const
	if core::ub_checks::check_library_ub() {
	if unsafe { libc::fcntl(fd, libc::F_GETFD) } == -1 && errno() == libc::EBADF {
	rtabort!("IO Safety violation: owned file descriptor already closed");
	}
	}

However, use of rtabort! requires std. The current choice of assert! has the possibility of triggering unwinding, which is inconsistent with the behavior of the other UB checks. Another possibility would be to outline the check into a helper function annotated with #[rustc_nounwind]

I have verified the old assertion doesn't trigger in user code, but I have not tested this PR because it is a very early draft and I don't have much rust compiler experience.

Cross Reference #90850 and #123499

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