Don't fold ExternalConstraintsData when it's empty

Author: compiler-errors

compiler/rustc_middle/src/traits/solve.rs

@@ -49,6 +49,10 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ExternalConstraints<'tcx> {
         self,
         folder: &mut F,
     ) -> Result<Self, F::Error> {
+        if self.is_empty() {
+            return Ok(self);
+        }
+
         Ok(FallibleTypeFolder::cx(folder).mk_external_constraints(ExternalConstraintsData {
             region_constraints: self.region_constraints.clone().try_fold_with(folder)?,
             opaque_types: self
@@ -64,6 +68,10 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ExternalConstraints<'tcx> {
     }
 
     fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
+        if self.is_empty() {
+            return self;
+        }
+
         TypeFolder::cx(folder).mk_external_constraints(ExternalConstraintsData {
             region_constraints: self.region_constraints.clone().fold_with(folder),
             opaque_types: self.opaque_types.iter().map(|opaque| opaque.fold_with(folder)).collect(),

compiler/rustc_type_ir/src/solve/mod.rs

@@ -236,6 +236,14 @@ pub struct ExternalConstraintsData<I: Interner> {
     pub normalization_nested_goals: NestedNormalizationGoals<I>,
 }
 
+impl<I: Interner> ExternalConstraintsData<I> {
+    pub fn is_empty(&self) -> bool {
+        self.region_constraints.is_empty()
+            && self.opaque_types.is_empty()
+            && self.normalization_nested_goals.is_empty()
+    }
+}
+
 #[derive_where(Clone, Hash, PartialEq, Eq, Debug, Default; I: Interner)]
 #[derive(TypeVisitable_Generic, TypeFoldable_Generic)]
 #[cfg_attr(feature = "nightly", derive(HashStable_NoContext))]