PathBuf incorrectly transmutes OsString to Vec<u8>

[RalfJung]

PathBuf contains this gem:

rust/library/std/src/path.rs

Lines 1172 to 1175 in f56afa0

	#[inline]
	fn as_mut_vec(&mut self) -> &mut Vec<u8> {
	unsafe { &mut *(self as *mut PathBuf as *mut Vec<u8>) }
	}

This effectively transmutes an OsString to Vec<u8>. But on Windows, OsString is Wtf8Buf:

rust/library/std/src/sys_common/wtf8.rs

Lines 131 to 146 in 51a7396

	/// An owned, growable string of well-formed WTF-8 data.
	///
	/// Similar to `String`, but can additionally contain surrogate code points
	/// if they’re not in a surrogate pair.
	#[derive(Eq, PartialEq, Ord, PartialOrd, Clone)]
	pub struct Wtf8Buf {
	bytes: Vec<u8>,
	/// Do we know that `bytes` holds a valid UTF-8 encoding? We can easily
	/// know this if we're constructed from a `String` or `&str`.
	///
	/// It is possible for `bytes` to have valid UTF-8 without this being
	/// set, such as when we're concatenating `&Wtf8`'s and surrogates become
	/// paired, as we don't bother to rescan the entire string.
	is_known_utf8: bool,
	}

This is not repr(transparent), so there is no guarantee that we can just transmute this. And I think with layout randomization this can actually fail -- that's probably what happened here.

Is there a reason why this uses transmutes rather than some sort of private accessor that exposes a *mut Vec<u8> through all these layers?