improve comments

src/librustc_data_structures/indexed_set.rs

@@ -239,14 +239,20 @@ pub fn overwrite(&mut self, other: &IdxSet<T>) {
         self.words_mut().clone_from_slice(other.words());
     }
 
+    /// Set `self = self | other` and return true if `self` changed
+    /// (i.e., if new bits were added).
     pub fn union(&mut self, other: &IdxSet<T>) -> bool {
         bitwise(self.words_mut(), other.words(), &Union)
     }
 
+    /// Set `self = self - other` and return true if `self` changed.
+    /// (i.e., if any bits were removed).
     pub fn subtract(&mut self, other: &IdxSet<T>) -> bool {
         bitwise(self.words_mut(), other.words(), &Subtract)
     }
 
+    /// Set `self = self & other` and return true if `self` changed.
+    /// (i.e., if any bits were removed).
     pub fn intersect(&mut self, other: &IdxSet<T>) -> bool {
         bitwise(self.words_mut(), other.words(), &Intersect)
     }

src/librustc_mir/util/liveness.rs

@@ -140,9 +140,12 @@ pub fn liveness_of_locals<'tcx>(mir: &Mir<'tcx>, mode: LivenessMode) -> Liveness
         bits.overwrite(&outs[bb]);
         def_use[bb].apply(&mut bits);
 
-        // add `bits` to the out set for each predecessor; if those
+        // `bits` now contains the live variables on entry. Therefore,
+        // add `bits` to the `out` set for each predecessor; if those
         // bits were not already present, then enqueue the predecessor
         // as dirty.
+        //
+        // (note that `union` returns true if the `self` set changed)
         for &pred_bb in &predecessors[bb] {
             if outs[pred_bb].union(&bits) {
                 dirty_queue.insert(pred_bb);