Subst `.Self` in `impl as` rewrites (#7105)

This makes rewrites of a constraint that requires a generic `I(Self)`
work. The `Self` there is replaced in the identified facet type by the
impl-as self type. But the rewrite is for the interface `.I(.Self)`, so
they don't match. Once `.Self` is replaced with the impl-as self type,
then they match and the rewrite is applied.

toolchain/check/impl.cpp

@@ -325,11 +325,24 @@ auto AddImpl(Context& context, const SemIR::Impl& impl,
 
 // Returns whether the `LookupImplWitness` of `witness_id` matches `interface`.
 static auto WitnessQueryMatchesInterface(
-    Context& context, SemIR::InstId witness_id,
-    const SemIR::SpecificInterface& interface) -> bool {
-  auto lookup = context.insts().GetAs<SemIR::LookupImplWitness>(witness_id);
-  return interface ==
-         context.specific_interfaces().Get(lookup.query_specific_interface_id);
+    Context& context, SemIR::LocId loc_id, SemIR::InstId impl_self,
+    SemIR::InstId access_witness_id,
+    const SemIR::SpecificInterface& impl_interface) -> bool {
+  auto lookup =
+      context.insts().GetAs<SemIR::LookupImplWitness>(access_witness_id);
+  auto access_interface =
+      context.specific_interfaces().Get(lookup.query_specific_interface_id);
+
+  // The `impl_interface` comes from an IdentifiedFacetType so it has `.Self`
+  // replaced. The access comes from a rewrite constraint, which do not have
+  // `.Self` replaced, so we need to do that here.
+  //
+  // TODO: Do this more eagerly as soon as we know the full decl before we
+  // construct the witness table from it?
+  SubstPeriodSelfCallbacks callbacks(&context, loc_id,
+                                     context.constant_values().Get(impl_self));
+  access_interface = SubstPeriodSelf(context, callbacks, access_interface);
+  return access_interface == impl_interface;
 }
 
 auto AddImplWitnessForDeclaration(Context& context, SemIR::LocId loc_id,
@@ -351,8 +364,8 @@ auto AddImplWitnessForDeclaration(Context& context, SemIR::LocId loc_id,
       [&](const SemIR::FacetTypeInfo::RewriteConstraint& rewrite) {
         auto access = context.insts().GetAs<SemIR::ImplWitnessAccess>(
             GetImplWitnessAccessWithoutSubstitution(context, rewrite.lhs_id));
-        return WitnessQueryMatchesInterface(context, access.witness_id,
-                                            impl.interface);
+        return WitnessQueryMatchesInterface(context, loc_id, impl.self_id,
+                                            access.witness_id, impl.interface);
       });
 
   if (rewrites_into_interface_to_witness.empty()) {

toolchain/check/testdata/impl/impl_as_named_constraint.carbon

@@ -210,7 +210,7 @@ fn G() {
 //   F(C);
 }
 
-// --- fail_todo_impl_as_symbolic_generic_constraint_where_rewrite.carbon
+// --- todo_impl_as_symbolic_generic_constraint_where_rewrite.carbon
 library "[[@TEST_NAME]]";
 
 interface I(T:! type) {
@@ -222,13 +222,6 @@ constraint N {
 }
 
 class C;
-// CHECK:STDERR: fail_todo_impl_as_symbolic_generic_constraint_where_rewrite.carbon:[[@LINE+7]]:1: error: associated constant I1 not given a value in impl of interface I [ImplAssociatedConstantNeedsValue]
-// CHECK:STDERR: impl C as N where .I1 = C {}
-// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~
-// CHECK:STDERR: fail_todo_impl_as_symbolic_generic_constraint_where_rewrite.carbon:[[@LINE-11]]:7: note: associated constant declared here [AssociatedConstantHere]
-// CHECK:STDERR:   let I1:! type;
-// CHECK:STDERR:       ^~~~~~~~~
-// CHECK:STDERR:
 impl C as N where .I1 = C {}
 
 fn F(_:! I(.Self) where .I1 = .Self) {}