Fix use-after-free printing the name of an interface that might have been invalidated by lazy import. (#4509)

While here, also change the diagnostic emission to pass the interface
type rather than the interface name. This prepares us to include the
arguments in the diagnostic.

Author: zygoloid

toolchain/check/context.cpp

@@ -1299,6 +1299,12 @@ auto Context::GetGenericInterfaceType(SemIR::InterfaceId interface_id,
       *this, interface_id, enclosing_specific_id);
 }
 
+auto Context::GetInterfaceType(SemIR::InterfaceId interface_id,
+                               SemIR::SpecificId specific_id) -> SemIR::TypeId {
+  return GetTypeImpl<SemIR::FacetType>(
+      *this, FacetTypeFromInterface(interface_id, specific_id).facet_type_id);
+}
+
 auto Context::GetPointerType(SemIR::TypeId pointee_type_id) -> SemIR::TypeId {
   return GetTypeImpl<SemIR::PointerType>(*this, pointee_type_id);
 }

toolchain/check/context.h

@@ -397,6 +397,10 @@ class Context {
                                SemIR::SpecificId enclosing_specific_id)
       -> SemIR::TypeId;
 
+  // Gets the facet type corresponding to a particular interface.
+  auto GetInterfaceType(SemIR::InterfaceId interface_id,
+                        SemIR::SpecificId specific_id) -> SemIR::TypeId;
+
   // Returns a pointer type whose pointee type is `pointee_type_id`.
   auto GetPointerType(SemIR::TypeId pointee_type_id) -> SemIR::TypeId;
 

toolchain/check/member_access.cpp

@@ -193,28 +193,29 @@ static auto PerformImplLookup(
     return SemIR::InstId::BuiltinError;
   }
 
-  auto& interface = context.interfaces().Get(interface_type->interface_id);
   auto witness_id =
       LookupInterfaceWitness(context, loc_id, type_const_id,
                              assoc_type.interface_type_id.AsConstantId());
   if (!witness_id.is_valid()) {
+    auto interface_type_id = context.GetInterfaceType(
+        interface_type->interface_id, interface_type->specific_id);
     if (missing_impl_diagnoser) {
       // TODO: Pass in the expression whose type we are printing.
       CARBON_DIAGNOSTIC(MissingImplInMemberAccessNote, Note,
-                        "type {1} does not implement interface `{0}`",
-                        SemIR::NameId, SemIR::TypeId);
+                        "type {1} does not implement interface {0}",
+                        SemIR::TypeId, SemIR::TypeId);
       missing_impl_diagnoser()
-          .Note(loc_id, MissingImplInMemberAccessNote, interface.name_id,
+          .Note(loc_id, MissingImplInMemberAccessNote, interface_type_id,
                 context.GetTypeIdForTypeConstant(type_const_id))
           .Emit();
     } else {
       // TODO: Pass in the expression whose type we are printing.
       CARBON_DIAGNOSTIC(MissingImplInMemberAccess, Error,
-                        "cannot access member of interface `{0}` in type {1} "
+                        "cannot access member of interface {0} in type {1} "
                         "that does not implement that interface",
-                        SemIR::NameId, SemIR::TypeId);
+                        SemIR::TypeId, SemIR::TypeId);
       context.emitter().Emit(loc_id, MissingImplInMemberAccess,
-                             interface.name_id,
+                             interface_type_id,
                              context.GetTypeIdForTypeConstant(type_const_id));
     }
     return SemIR::InstId::BuiltinError;