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.

2 files changed, 3 insertions, 3 deletions

diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 105a61d0857..d4451cc7b71 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -40,7 +40,7 @@ impl<T> MoveMap<T> for Vec<T> {
         for p in &mut self {
             unsafe {
                 // FIXME(#5016) this shouldn't need to zero to be safe.
-                ptr::write(p, f(ptr::read_and_zero(p)));
+                ptr::write(p, f(ptr::read_and_drop(p)));
             }
         }
         self
diff --git a/src/libsyntax/ptr.rs b/src/libsyntax/ptr.rs
index ca3a1848c3a..7e0bcd3e1dc 100644
--- a/src/libsyntax/ptr.rs
+++ b/src/libsyntax/ptr.rs
@@ -71,8 +71,8 @@ impl<T: 'static> P<T> {
     {
         unsafe {
             let p = &mut *self.ptr;
-            // FIXME(#5016) this shouldn't need to zero to be safe.
-            ptr::write(p, f(ptr::read_and_zero(p)));
+            // FIXME(#5016) this shouldn't need to drop-fill to be safe.
+            ptr::write(p, f(ptr::read_and_drop(p)));
         }
         self
     }