Use a BitSet in LexicalResolver::iterate_until_fixed_point().

This wins 3% on `unicode_normalization`.

src/librustc/infer/lexical_region_resolve/mod.rs

@@ -19,8 +19,8 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::graph::implementation::{
     Direction, Graph, NodeIndex, INCOMING, OUTGOING,
 };
+use rustc_index::bit_set::BitSet;
 use rustc_index::vec::{Idx, IndexVec};
-use smallvec::SmallVec;
 use std::fmt;
 use syntax_pos::Span;
 
@@ -870,21 +870,34 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
     where
         F: FnMut(&Constraint<'tcx>) -> (bool, bool),
     {
-        let mut constraints: SmallVec<[_; 16]> = self.data.constraints.keys().collect();
+        // Using bitsets to track the remaining elements is faster than using a
+        // `Vec` by itself (which requires removing elements, which requires
+        // element shuffling, which is slow).
+        let constraints: Vec<_> = self.data.constraints.keys().collect();
+        let mut live_indices: BitSet<usize> = BitSet::new_filled(constraints.len());
+        let mut killed_indices: BitSet<usize> = BitSet::new_empty(constraints.len());
         let mut iteration = 0;
         let mut changed = true;
         while changed {
             changed = false;
             iteration += 1;
             debug!("---- Expansion iteration {}", iteration);
-            constraints.retain(|constraint| {
+            for index in live_indices.iter() {
+                let constraint = constraints[index];
                 let (edge_changed, retain) = body(constraint);
                 if edge_changed {
                     debug!("updated due to constraint {:?}", constraint);
                     changed = true;
                 }
-                retain
-            });
+                if !retain {
+                    let changed = killed_indices.insert(index);
+                    debug_assert!(changed);
+                }
+            }
+            live_indices.subtract(&killed_indices);
+
+            // We could clear `killed_indices` here, but we don't need to and
+            // it's cheaper not to.
         }
         debug!("---- Expansion complete after {} iteration(s)", iteration);
     }