Auto merge of #68772 - matthewjasper:relate-opt, r=davidtwco

Avoid exponential behaviour when relating types

When equating bound types we check subtyping in both directions. Since closures are invariant in their substs, we end up comparing the two types an exponential number of times. If there are no bound variables this isn't needed.

Closes #68061

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];
+}