Tag a bunch of destructors that need mutable self with FIXME for #4330.

Close #4943.

src/libstd/pipes.rs

@@ -315,6 +315,7 @@ struct BufferResource<T> {
 impl<T> Drop for BufferResource<T> {
     fn finalize(&self) {
         unsafe {
+            // FIXME(#4330) Need self by value to get mutability.
             let this: &mut BufferResource<T> = transmute(self);
 
             let mut b = move_it!(this.buffer);

src/libstd/rt/rc.rs

@@ -78,7 +78,7 @@ impl<T> Drop for RC<T> {
         assert!(self.refcount() > 0);
 
         unsafe {
-            // XXX: Mutable finalizer
+            // FIXME(#4330) Need self by value to get mutability.
             let this: &mut RC<T> = cast::transmute_mut(self);
 
             match *this.get_mut_state() {

src/libstd/run.rs

@@ -429,7 +429,7 @@ impl Process {
 
 impl Drop for Process {
     fn finalize(&self) {
-        // FIXME #4943: transmute is bad.
+        // FIXME(#4330) Need self by value to get mutability.
         let mut_self: &mut Process = unsafe { cast::transmute(self) };
 
         mut_self.finish();

src/libstd/task/spawn.rs

@@ -323,6 +323,7 @@ impl Drop for TCB {
     // Runs on task exit.
     fn finalize(&self) {
         unsafe {
+            // FIXME(#4330) Need self by value to get mutability.
             let this: &mut TCB = transmute(self);
 
             // If we are failing, the whole taskgroup needs to die.

src/libstd/unstable/atomics.rs

@@ -275,6 +275,7 @@ impl<T> Drop for AtomicOption<T> {
         // This will ensure that the contained data is
         // destroyed, unless it's null.
         unsafe {
+            // FIXME(#4330) Need self by value to get mutability.
             let this : &mut AtomicOption<T> = cast::transmute(self);
             let _ = this.take(SeqCst);
         }