Switch drop-flag to `u8` to allow special tags to instrument state.

Refactored code so that the drop-flag values for initialized
(`DTOR_NEEDED`) versus dropped (`DTOR_DONE`) are given explicit names.

Add `mem::dropped()` (which with `DTOR_DONE == 0` is semantically the
same as `mem::zeroed`, but the point is that it abstracts away from
the particular choice of value for `DTOR_DONE`).

Filling-drop needs to use something other than `ptr::read_and_zero`,
so I added such a function: `ptr::read_and_drop`.  But, libraries
should not use it if they can otherwise avoid it.

Fixes to tests to accommodate filling-drop.

1 files changed, 15 insertions, 0 deletions

diff --git a/src/libcore/ptr.rs b/src/libcore/ptr.rs
index 9b3ee3ef5e0..7a9c9274f3b 100644
--- a/src/libcore/ptr.rs
+++ b/src/libcore/ptr.rs
@@ -230,6 +230,21 @@ pub unsafe fn read_and_zero<T>(dest: *mut T) -> T {
     tmp
 }
 
+/// Variant of read_and_zero that writes the specific drop-flag byte
+/// (which may be more apropriate than zero).
+#[inline(always)]
+#[unstable(feature = "core",
+           reason = "may play a larger role in std::ptr future extensions")]
+pub unsafe fn read_and_drop<T>(dest: *mut T) -> T {
+    // Copy the data out from `dest`:
+    let tmp = read(&*dest);
+
+    // Now mark `dest` as dropped:
+    write_bytes(dest, mem::POST_DROP_U8, 1);
+
+    tmp
+}
+
 /// Overwrites a memory location with the given value without reading or
 /// dropping the old value.
 ///