Avoid needless allocations in liveness_of_locals.

src/librustc_data_structures/indexed_set.rs

@@ -233,7 +233,9 @@ impl<T: Idx> IdxSet<T> {
         &mut self.bits
     }
 
-    pub fn clone_from(&mut self, other: &IdxSet<T>) {
+    /// Efficiently overwrite `self` with `other`. Panics if `self` and `other`
+    /// don't have the same length.
+    pub fn overwrite(&mut self, other: &IdxSet<T>) {
         self.words_mut().clone_from_slice(other.words());
     }
 

src/librustc_mir/dataflow/at_location.rs

@@ -139,7 +139,7 @@ impl<BD> FlowsAtLocation for FlowAtLocation<BD>
     where BD: BitDenotation
 {
     fn reset_to_entry_of(&mut self, bb: BasicBlock) {
-        (*self.curr_state).clone_from(self.base_results.sets().on_entry_set_for(bb.index()));
+        self.curr_state.overwrite(self.base_results.sets().on_entry_set_for(bb.index()));
     }
 
     fn reconstruct_statement_effect(&mut self, loc: Location) {

src/librustc_mir/dataflow/mod.rs

@@ -242,7 +242,7 @@ impl<'b, 'a: 'b, 'tcx: 'a, BD> PropagationContext<'b, 'a, 'tcx, BD> where BD: Bi
             {
                 let sets = builder.flow_state.sets.for_block(bb_idx);
                 debug_assert!(in_out.words().len() == sets.on_entry.words().len());
-                in_out.clone_from(sets.on_entry);
+                in_out.overwrite(sets.on_entry);
                 in_out.union(sets.gen_set);
                 in_out.subtract(sets.kill_set);
             }

src/librustc_mir/util/liveness.rs

@@ -141,14 +141,14 @@ pub fn liveness_of_locals<'tcx>(mir: &Mir<'tcx>, mode: LivenessMode) -> Liveness
             for &successor in mir.basic_blocks()[b].terminator().successors() {
                 bits.union(&ins[successor]);
             }
-            outs[b].clone_from(&bits);
+            outs[b].overwrite(&bits);
 
             // bits = use ∪ (bits - def)
             def_use[b].apply(&mut bits);
 
             // update bits on entry and flag if they have changed
             if ins[b] != bits {
-                ins[b].clone_from(&bits);
+                ins[b].overwrite(&bits);
                 changed = true;
             }
         }