comments and code-cleanup in response to reviews.

Author: pnkfelix

src/librustc_trans/trans/_match.rs

@@ -952,9 +952,24 @@ fn insert_lllocals<'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
                         Lvalue::new("_match::insert_lllocals"),
                     TrByMoveIntoCopy(..) => {
                         // match_input moves from the input into a
-                        // separate stack slot; it must zero (at least
-                        // until we track drop flags for a fragmented
-                        // parent match input expression).
+                        // separate stack slot.
+                        //
+                        // E.g. consider moving the value `D(A)` out
+                        // of the tuple `(D(A), D(B))` and into the
+                        // local variable `x` via the pattern `(x,_)`,
+                        // leaving the remainder of the tuple `(_,
+                        // D(B))` still to be dropped in the future.
+                        //
+                        // Thus, here we must must zero the place that
+                        // we are moving *from*, because we do not yet
+                        // track drop flags for a fragmented parent
+                        // match input expression.
+                        //
+                        // Longer term we will be able to map the move
+                        // into `(x, _)` up to the parent path that
+                        // owns the whole tuple, and mark the
+                        // corresponding stack-local drop-flag
+                        // tracking the first component of the tuple.
                         let hint_kind = HintKind::ZeroAndMaintain;
                         Lvalue::new_with_hint("_match::insert_lllocals (match_input)",
                                               bcx, binding_info.id, hint_kind)

src/librustc_trans/trans/datum.rs

@@ -189,9 +189,27 @@ pub struct Rvalue {
     pub mode: RvalueMode
 }
 
+/// Classifies what action we should take when a value is moved away
+/// with respect to its drop-flag.
+///
+/// Long term there will be no need for this classification: all flags
+/// (which will be stored on the stack frame) will have the same
+/// interpretation and maintenance code associated with them.
 #[derive(Copy, Clone, Debug)]
 pub enum HintKind {
+    /// When the value is moved, set the drop-flag to "dropped"
+    /// (i.e. "zero the flag", even when the specific representation
+    /// is not literally 0) and when it is reinitialized, set the
+    /// drop-flag back to "initialized".
     ZeroAndMaintain,
+
+    /// When the value is moved, do not set the drop-flag to "dropped"
+    /// However, continue to read the drop-flag in deciding whether to
+    /// drop. (In essence, the path/fragment in question will never
+    /// need to be dropped at the points where it is moved away by
+    /// this code, but we are defending against the scenario where
+    /// some *other* code could move away (or drop) the value and thus
+    /// zero-the-flag, which is why we will still read from it.
     DontZeroJustUse,
 }
 
@@ -218,7 +236,8 @@ impl Lvalue { // Constructors for various Lvalues.
                     DropFlagInfo::ZeroAndMaintain(id),
                 HintKind::DontZeroJustUse if hint_available =>
                     DropFlagInfo::DontZeroJustUse(id),
-                _ => DropFlagInfo::None,
+                _ =>
+                    DropFlagInfo::None,
             };
             (Some(id), info)
         };