optimize TypeFoldable for 2 element tuples

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -889,19 +889,6 @@ fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow
     }
 }
 
-impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<Ty<'tcx>> {
-    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
-        self,
-        folder: &mut F,
-    ) -> Result<Self, F::Error> {
-        ty::util::fold_list(self, folder, |tcx, v| tcx.intern_type_list(v))
-    }
-
-    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
-        self.iter().try_for_each(|t| t.visit_with(visitor))
-    }
-}
-
 impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<ProjectionKind> {
     fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
         self,

compiler/rustc_middle/src/ty/subst.rs

@@ -458,6 +458,45 @@ fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow
     }
 }
 
+impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::List<Ty<'tcx>> {
+    fn try_super_fold_with<F: FallibleTypeFolder<'tcx>>(
+        self,
+        folder: &mut F,
+    ) -> Result<Self, F::Error> {
+        // This code is fairly hot, though not as hot as `SubstsRef`.
+        //
+        // When compiling stage 2, I get the following results:
+        //
+        // len |   total   |   %
+        // --- | --------- | -----
+        //  2  |  15083590 |  48.1
+        //  3  |   7540067 |  24.0
+        //  1  |   5300377 |  16.9
+        //  4  |   1351897 |   4.3
+        //  0  |   1256849 |   4.0
+        //
+        // I've tried it with some private repositories and got
+        // close to the same result, with 4 and 0 swapping places
+        // sometimes.
+        match self.len() {
+            2 => {
+                let param0 = self[0].try_fold_with(folder)?;
+                let param1 = self[1].try_fold_with(folder)?;
+                if param0 == self[0] && param1 == self[1] {
+                    Ok(self)
+                } else {
+                    Ok(folder.tcx().intern_type_list(&[param0, param1]))
+                }
+            }
+            _ => ty::util::fold_list(self, folder, |tcx, v| tcx.intern_type_list(v)),
+        }
+    }
+
+    fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
+        self.iter().try_for_each(|t| t.visit_with(visitor))
+    }
+}
+
 ///////////////////////////////////////////////////////////////////////////
 // Public trait `Subst`
 //