Using default and regular assignments doesn't guarantee overwriting everything

[cesarb]

As mentioned in #3, just assigning a Default::default() might not overwrite everything in case the value contains an enum (most probably an Option).

Simply using mem::zeroed is not a good idea if the type contains something for which an all-zero-bits value is invalid.

Possible solution: convert the &mut T to *mut T, and do a memset to zero immediately followed by a ptr::write of the Default::default(), which is valid regardless of dst's contents, since ptr::write ignores the target's contents (it's the official way to initialize a mem::uninitialized()), followed then by the hide_mem. The compiler should be able to elide the memset in the common case where just a Default is enough to overwrite everything.

cc #5, @burdges, @bluss

[burdges]

I'd imagine the reverted commit ccb563e#diff-ac56d72e9f4cbe36b5de6fb90bd33456R33 solves the problem because the ptr::write copies from Default::default() without dropping the value already there, so the only drop should be the *self = mem::zeroed::<Self>(); I'd imagine LLVM would optimizes it to write only once, but not sure.

[cesarb]

Fixed by the ptr::write_bytes in commit 517686e.