diff --git a/src/librustc_data_structures/bitvec.rs b/src/librustc_data_structures/bitvec.rs
index cb648038c34..79d3f0cf688 100644
--- a/src/librustc_data_structures/bitvec.rs
+++ b/src/librustc_data_structures/bitvec.rs
@@ -11,7 +11,7 @@
 use std::iter::FromIterator;
 
 /// A very simple BitVector type.
-#[derive(Clone)]
+#[derive(Clone, Debug, PartialEq)]
 pub struct BitVector {
     data: Vec<u64>,
 }
diff --git a/src/librustc_mir/build/matches/mod.rs b/src/librustc_mir/build/matches/mod.rs
index c1a0e1f9a69..88d7e41bc61 100644
--- a/src/librustc_mir/build/matches/mod.rs
+++ b/src/librustc_mir/build/matches/mod.rs
@@ -15,6 +15,7 @@
 
 use build::{BlockAnd, BlockAndExtension, Builder};
 use rustc_data_structures::fnv::FnvHashMap;
+use rustc_data_structures::bitvec::BitVector;
 use rustc::middle::const_val::ConstVal;
 use rustc::ty::{AdtDef, Ty};
 use rustc::mir::repr::*;
@@ -266,6 +267,7 @@ enum TestKind<'tcx> {
     // test the branches of enum
     Switch {
         adt_def: AdtDef<'tcx>,
+        variants: BitVector,
     },
 
     // test the branches of enum
@@ -391,9 +393,11 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
 
     fn join_otherwise_blocks(&mut self,
                              span: Span,
-                             otherwise: Vec<BasicBlock>)
+                             mut otherwise: Vec<BasicBlock>)
                              -> BasicBlock
     {
+        otherwise.sort();
+        otherwise.dedup(); // variant switches can introduce duplicate target blocks
         let scope_id = self.innermost_scope_id();
         if otherwise.len() == 1 {
             otherwise[0]
@@ -502,6 +506,15 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                     }
                 }
             }
+            TestKind::Switch { adt_def: _, ref mut variants} => {
+                for candidate in candidates.iter() {
+                    if !self.add_variants_to_switch(&match_pair.lvalue,
+                                                    candidate,
+                                                    variants) {
+                        break;
+                    }
+                }
+            }
             _ => { }
         }
 
@@ -525,6 +538,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                                                           &mut target_candidates))
                       .count();
         assert!(tested_candidates > 0); // at least the last candidate ought to be tested
+        debug!("tested_candidates: {}", tested_candidates);
+        debug!("untested_candidates: {}", candidates.len() - tested_candidates);
 
         // For each outcome of test, process the candidates that still
         // apply. Collect a list of blocks where control flow will
diff --git a/src/librustc_mir/build/matches/test.rs b/src/librustc_mir/build/matches/test.rs
index 79656ea21f7..09c146537e7 100644
--- a/src/librustc_mir/build/matches/test.rs
+++ b/src/librustc_mir/build/matches/test.rs
@@ -19,6 +19,7 @@ use build::Builder;
 use build::matches::{Candidate, MatchPair, Test, TestKind};
 use hair::*;
 use rustc_data_structures::fnv::FnvHashMap;
+use rustc_data_structures::bitvec::BitVector;
 use rustc::middle::const_val::ConstVal;
 use rustc::ty::{self, Ty};
 use rustc::mir::repr::*;
@@ -33,7 +34,10 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             PatternKind::Variant { ref adt_def, variant_index: _, subpatterns: _ } => {
                 Test {
                     span: match_pair.pattern.span,
-                    kind: TestKind::Switch { adt_def: adt_def.clone() },
+                    kind: TestKind::Switch {
+                        adt_def: adt_def.clone(),
+                        variants: BitVector::new(self.hir.num_variants(adt_def)),
+                    },
                 }
             }
 
@@ -125,9 +129,10 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                        });
                 true
             }
-
+            PatternKind::Variant { .. } => {
+                panic!("you should have called add_variants_to_switch instead!");
+            }
             PatternKind::Range { .. } |
-            PatternKind::Variant { .. } |
             PatternKind::Slice { .. } |
             PatternKind::Array { .. } |
             PatternKind::Wild |
@@ -140,6 +145,31 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         }
     }
 
+    pub fn add_variants_to_switch<'pat>(&mut self,
+                                        test_lvalue: &Lvalue<'tcx>,
+                                        candidate: &Candidate<'pat, 'tcx>,
+                                        variants: &mut BitVector)
+                                        -> bool
+    {
+        let match_pair = match candidate.match_pairs.iter().find(|mp| mp.lvalue == *test_lvalue) {
+            Some(match_pair) => match_pair,
+            _ => { return false; }
+        };
+
+        match *match_pair.pattern.kind {
+            PatternKind::Variant { adt_def: _ , variant_index,  .. } => {
+                // We have a pattern testing for variant `variant_index`
+                // set the corresponding index to true
+                variants.insert(variant_index);
+                true
+            }
+            _ => {
+                // don't know how to add these patterns to a switch
+                false
+            }
+        }
+    }
+
     /// Generates the code to perform a test.
     pub fn perform_test(&mut self,
                         block: BasicBlock,
@@ -148,11 +178,21 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                         -> Vec<BasicBlock> {
         let scope_id = self.innermost_scope_id();
         match test.kind {
-            TestKind::Switch { adt_def } => {
+            TestKind::Switch { adt_def, ref variants } => {
                 let num_enum_variants = self.hir.num_variants(adt_def);
-                let target_blocks: Vec<_> =
-                    (0..num_enum_variants).map(|_| self.cfg.start_new_block())
-                                          .collect();
+                let mut otherwise_block = None;
+                let target_blocks: Vec<_> = (0..num_enum_variants).map(|i| {
+                    if variants.contains(i) {
+                        self.cfg.start_new_block()
+                    } else {
+                        if otherwise_block.is_none() {
+                            otherwise_block = Some(self.cfg.start_new_block());
+                        }
+                        otherwise_block.unwrap()
+                    }
+                }).collect();
+                debug!("num_enum_variants: {}, num tested variants: {}, variants: {:?}",
+                       num_enum_variants, variants.iter().count(), variants);
                 self.cfg.terminate(block, scope_id, test.span, TerminatorKind::Switch {
                     discr: lvalue.clone(),
                     adt_def: adt_def,
@@ -415,7 +455,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         match test.kind {
             // If we are performing a variant switch, then this
             // informs variant patterns, but nothing else.
-            TestKind::Switch { adt_def: tested_adt_def } => {
+            TestKind::Switch { adt_def: tested_adt_def , .. } => {
                 match *match_pair.pattern.kind {
                     PatternKind::Variant { adt_def, variant_index, ref subpatterns } => {
                         assert_eq!(adt_def, tested_adt_def);