Avoid exponential behaviour when relating types

src/librustc/infer/equate.rs

@@ -125,7 +125,13 @@ impl TypeRelation<'tcx> for Equate<'combine, 'infcx, 'tcx> {
     where
         T: Relate<'tcx>,
     {
-        self.fields.higher_ranked_sub(a, b, self.a_is_expected)?;
-        self.fields.higher_ranked_sub(b, a, self.a_is_expected)
+        if a.skip_binder().has_escaping_bound_vars() || b.skip_binder().has_escaping_bound_vars() {
+            self.fields.higher_ranked_sub(a, b, self.a_is_expected)?;
+            self.fields.higher_ranked_sub(b, a, self.a_is_expected)
+        } else {
+            // Fast path for the common case.
+            self.relate(a.skip_binder(), b.skip_binder())?;
+            return Ok(a.clone());
+        }
     }
 }

src/librustc/infer/nll_relate/mod.rs

@@ -529,6 +529,10 @@ where
             b = self.infcx.shallow_resolve(b);
         }
 
+        if a == b {
+            return Ok(a);
+        }
+
         match (&a.kind, &b.kind) {
             (_, &ty::Infer(ty::TyVar(vid))) => {
                 if D::forbid_inference_vars() {
@@ -638,6 +642,13 @@ where
 
         debug!("binders({:?}: {:?}, ambient_variance={:?})", a, b, self.ambient_variance);
 
+        if !a.skip_binder().has_escaping_bound_vars() && !b.skip_binder().has_escaping_bound_vars()
+        {
+            // Fast path for the common case.
+            self.relate(a.skip_binder(), b.skip_binder())?;
+            return Ok(a.clone());
+        }
+
         if self.ambient_covariance() {
             // Covariance, so we want `for<..> A <: for<..> B` --
             // therefore we compare any instantiation of A (i.e., A

src/test/ui/closures/deeply-nested_closures.rs

@@ -0,0 +1,23 @@
+// Check that this can be compiled in a reasonable time.
+
+// build-pass
+
+fn main() {
+    // 96 nested closures
+    let x = ();
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+    || || || || || || || ||
+    [&(), &x];
+}