new trait solver: only consider goal changed if response is not identity

compiler/rustc_middle/src/infer/canonical.rs

@@ -68,6 +68,22 @@ pub struct CanonicalVarValues<'tcx> {
     pub var_values: IndexVec<BoundVar, GenericArg<'tcx>>,
 }
 
+impl CanonicalVarValues<'_> {
+    pub fn is_identity(&self) -> bool {
+        self.var_values.iter_enumerated().all(|(bv, arg)| match arg.unpack() {
+            ty::GenericArgKind::Lifetime(r) => {
+                matches!(*r, ty::ReLateBound(ty::INNERMOST, br) if br.var == bv)
+            }
+            ty::GenericArgKind::Type(ty) => {
+                matches!(*ty.kind(), ty::Bound(ty::INNERMOST, bt) if bt.var == bv)
+            }
+            ty::GenericArgKind::Const(ct) => {
+                matches!(ct.kind(), ty::ConstKind::Bound(ty::INNERMOST, bc) if bc == bv)
+            }
+        })
+    }
+}
+
 /// When we canonicalize a value to form a query, we wind up replacing
 /// various parts of it with canonical variables. This struct stores
 /// those replaced bits to remember for when we process the query

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -178,7 +178,7 @@ impl<'tcx> EvalCtxt<'tcx> {
         let canonical_goal = infcx.canonicalize_query(goal, &mut orig_values);
         let canonical_response = self.evaluate_canonical_goal(canonical_goal)?;
         Ok((
-            true, // FIXME: check whether `var_values` are an identity substitution.
+            !canonical_response.value.var_values.is_identity(),
             instantiate_canonical_query_response(infcx, &orig_values, canonical_response),
         ))
     }