remove the ins set altogether

src/librustc_mir/borrow_check/nll/mod.rs

@@ -261,13 +261,6 @@ fn dump_mir_results<'a, 'gcx, 'tcx>(
                     }
                 }
 
-                // Before each basic block, dump out the values
-                // that are live on entry to the basic block.
-                PassWhere::BeforeBlock(bb) => {
-                    let s = live_variable_set(&liveness.regular.ins[bb], &liveness.drop.ins[bb]);
-                    writeln!(out, "    | Live variables on entry to {:?}: {}", bb, s)?;
-                }
-
                 PassWhere::BeforeLocation(location) => {
                     let s = live_variable_set(
                         &regular_liveness_per_location[&location],
@@ -283,7 +276,14 @@ fn dump_mir_results<'a, 'gcx, 'tcx>(
                     )?;
                 }
 
-                PassWhere::AfterLocation(_) | PassWhere::AfterCFG => {}
+                // After each basic block, dump out the values
+                // that are live on exit from the basic block.
+                PassWhere::AfterTerminator(bb) => {
+                    let s = live_variable_set(&liveness.regular.outs[bb], &liveness.drop.outs[bb]);
+                    writeln!(out, "    | Live variables on entry to {:?}: {}", bb, s)?;
+                }
+
+                PassWhere::BeforeBlock(_) | PassWhere::AfterLocation(_) | PassWhere::AfterCFG => {}
             }
             Ok(())
         },

src/librustc_mir/util/liveness.rs

@@ -56,9 +56,6 @@ pub struct LivenessResult {
     /// Liveness mode in use when these results were computed.
     pub mode: LivenessMode,
 
-    /// Live variables on entry to each basic block.
-    pub ins: IndexVec<BasicBlock, LocalSet>,
-
     /// Live variables on exit to each basic block. This is equal to
     /// the union of the `ins` for each successor.
     pub outs: IndexVec<BasicBlock, LocalSet>,
@@ -125,11 +122,10 @@ pub fn liveness_of_locals<'tcx>(mir: &Mir<'tcx>, mode: LivenessMode) -> Liveness
         .map(|b| block(mode, b, locals))
         .collect();
 
-    let mut ins: IndexVec<_, _> = mir.basic_blocks()
+    let mut outs: IndexVec<_, _> = mir.basic_blocks()
         .indices()
         .map(|_| LocalSet::new_empty(locals))
         .collect();
-    let mut outs = ins.clone();
 
     let mut bits = LocalSet::new_empty(locals);
 
@@ -140,28 +136,21 @@ pub fn liveness_of_locals<'tcx>(mir: &Mir<'tcx>, mode: LivenessMode) -> Liveness
     let predecessors = mir.predecessors();
 
     while let Some(bb) = dirty_queue.pop() {
-        // outs[b] = ∪ {ins of successors}
-        bits.clear();
-        for &successor in mir[bb].terminator().successors() {
-            bits.union(&ins[successor]);
-        }
-        outs[bb].overwrite(&bits);
-
         // bits = use ∪ (bits - def)
+        bits.overwrite(&outs[bb]);
         def_use[bb].apply(&mut bits);
 
-        // update bits on entry and, if they have changed, enqueue all
-        // of our predecessors, since their inputs have now changed
-        if ins[bb] != bits {
-            ins[bb].overwrite(&bits);
-
-            for &pred_bb in &predecessors[bb] {
+        // add `bits` to the out set for each predecessor; if those
+        // bits were not already present, then enqueue the predecessor
+        // as dirty.
+        for &pred_bb in &predecessors[bb] {
+            if outs[pred_bb].union(&bits) {
                 dirty_queue.insert(pred_bb);
             }
         }
     }
 
-    LivenessResult { mode, ins, outs }
+    LivenessResult { mode, outs }
 }
 
 impl LivenessResult {
@@ -202,8 +191,6 @@ impl LivenessResult {
             statement_defs_uses.apply(&mut bits);
             callback(statement_location, &bits);
         }
-
-        assert_eq!(bits, self.ins[block]);
     }
 
     fn defs_uses<'tcx, V>(&self, mir: &Mir<'tcx>, location: Location, thing: &V) -> DefsUses
@@ -445,7 +432,6 @@ pub fn write_mir_fn<'a, 'tcx>(
                 .collect();
             writeln!(w, "{} {{{}}}", prefix, live.join(", "))
         };
-        print(w, "   ", &result.ins)?;
         write_basic_block(tcx, block, mir, &mut |_, _| Ok(()), w)?;
         print(w, "   ", &result.outs)?;
         if block.index() + 1 != mir.basic_blocks().len() {

src/librustc_mir/util/pretty.rs

@@ -44,6 +44,9 @@ pub enum PassWhere {
 
     /// We just dumped the given statement or terminator.
     AfterLocation(Location),
+
+    /// We just dumped the terminator for a block but not the closing `}`.
+    AfterTerminator(BasicBlock),
 }
 
 /// If the session is properly configured, dumps a human-readable
@@ -351,6 +354,7 @@ where
     })?;
 
     extra_data(PassWhere::AfterLocation(current_location), w)?;
+    extra_data(PassWhere::AfterTerminator(block), w)?;
 
     writeln!(w, "{}}}", INDENT)
 }