Expand TypeCanDestroy to handle more types

Author: jonmeow

core/prelude/iterate.carbon

@@ -11,13 +11,13 @@ export import library "prelude/operators";
 
 interface Iterate {
   // TODO: Support iterating ranges of non-copyable values.
-  let ElementType:! Copy;
+  let ElementType:! Copy & Destroy;
   let CursorType:! type;
   fn NewCursor[self: Self]() -> CursorType;
   fn Next[self: Self](cursor: CursorType*) -> Optional(ElementType);
 }
 
-impl forall [T:! Copy, N:! IntLiteral()]
+impl forall [T:! Copy & Destroy, N:! IntLiteral()]
     array(T, N) as Iterate
     where .ElementType = T and .CursorType = i32 {
   fn NewCursor[self: Self]() -> i32 { return 0; }

core/prelude/types/maybe_unformed.carbon

@@ -8,6 +8,7 @@ import library "prelude/destroy";
 
 private fn MakeMaybeUnformed(t: type) -> type = "maybe_unformed.make_type";
 
-class MaybeUnformed(T:! type) {
+// TODO: There should probably support for non-destructible types.
+class MaybeUnformed(T:! Destroy) {
   adapt MakeMaybeUnformed(T);
 }

core/prelude/types/optional.carbon

@@ -6,6 +6,7 @@ package Core library "prelude/types/optional";
 
 import library "prelude/copy";
 import library "prelude/destroy";
+import library "prelude/operators/bitwise";
 import library "prelude/types/bool";
 
 // For now, an `Optional(T)` is stored as a pair of a `bool` and a `T`, with
@@ -16,7 +17,7 @@ import library "prelude/types/bool";
 //
 // TODO: We don't have an approved design for an `Optional` type yet, but it's
 // used by the design for `Iterate`. The API here is a placeholder.
-class Optional(T:! Copy) {
+class Optional(T:! Copy & Destroy) {
   fn None() -> Self {
     returned var me: Self;
     me.has_value = false;

toolchain/check/impl_lookup.cpp

@@ -18,6 +18,7 @@
 #include "toolchain/check/impl.h"
 #include "toolchain/check/import_ref.h"
 #include "toolchain/check/inst.h"
+#include "toolchain/check/name_lookup.h"
 #include "toolchain/check/subst.h"
 #include "toolchain/check/type.h"
 #include "toolchain/check/type_completion.h"
@@ -26,6 +27,7 @@
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/impl.h"
 #include "toolchain/sem_ir/inst.h"
+#include "toolchain/sem_ir/specific_interface.h"
 #include "toolchain/sem_ir/typed_insts.h"
 
 namespace Carbon::Check {
@@ -523,48 +525,187 @@ static auto GetOrAddLookupImplWitness(Context& context, SemIR::LocId loc_id,
   return context.constant_values().GetInstId(witness_const_id);
 }
 
-// Returns true if the `Self` should impl `Destroy`.
-static auto TypeCanDestroy(Context& context,
-                           SemIR::ConstantId query_self_const_id) -> bool {
-  auto inst = context.insts().Get(context.constant_values().GetInstId(
-      GetCanonicalFacetOrTypeValue(context, query_self_const_id)));
-
-  // For facet values, look if the FacetType provides the same.
-  if (auto facet_type =
-          context.types().TryGetAs<SemIR::FacetType>(inst.type_id())) {
-    const auto& info = context.facet_types().Get(facet_type->facet_type_id);
-    if (info.builtin_constraint_mask.HasAnyOf(
-            SemIR::BuiltinConstraintMask::TypeCanDestroy)) {
-      return true;
+// Determines whether `type_id` is a `FacetType` which impls `Destroy`.
+//
+// This handles cases where the facet type provides either `CanDestroy` or
+// `Destroy`; the former case will always have a `Destroy` implementation.
+static auto CanDestroyFacetType(Context& context, SemIR::LocId loc_id,
+                                SemIR::FacetType facet_type,
+                                SemIR::InterfaceId& destroy_interface_id)
+    -> bool {
+  const auto& info = context.facet_types().Get(facet_type.facet_type_id);
+  if (info.builtin_constraint_mask.HasAnyOf(
+          SemIR::BuiltinConstraintMask::TypeCanDestroy)) {
+    return true;
+  }
+
+  if (info.extend_constraints.empty()) {
+    return false;
+  }
+
+  // Fetch `Core.Destroy` if it isn't yet known.
+  if (!destroy_interface_id.has_value()) {
+    auto destroy_id = LookupNameInCore(context, loc_id, "Destroy");
+    auto destroy_facet_type =
+        context.insts().TryGetAs<SemIR::FacetType>(destroy_id);
+    if (!destroy_facet_type) {
+      return false;
     }
+    const auto& destroy_facet_type_info =
+        context.facet_types().Get(destroy_facet_type->facet_type_id);
+    auto destroy_specific_interface =
+        destroy_facet_type_info.TryAsSingleInterface();
+    if (!destroy_specific_interface) {
+      return false;
+    }
+
+    destroy_interface_id = destroy_specific_interface->interface_id;
   }
 
-  CARBON_KIND_SWITCH(inst) {
-    case CARBON_KIND(SemIR::ClassType class_type): {
-      auto class_info = context.classes().Get(class_type.class_id);
-      // Incomplete and abstract classes can't be destroyed.
-      // TODO: Return false if the object repr doesn't impl `Destroy`.
-      // TODO: Return false for C++ types that lack a destructor.
-      return class_info.is_complete() &&
-             class_info.inheritance_kind !=
-                 SemIR::Class::InheritanceKind::Abstract;
-    }
-    case SemIR::ArrayType::Kind:
-    case SemIR::ConstType::Kind:
-    case SemIR::MaybeUnformedType::Kind:
-    case SemIR::PartialType::Kind:
-    case SemIR::StructType::Kind:
-    case SemIR::TupleType::Kind:
-      // TODO: Return false for types that indirectly reference a type that
-      // doesn't impl `Destroy`.
-      return true;
-    case SemIR::BoolType::Kind:
-    case SemIR::PointerType::Kind:
-      // Trivially destructible.
+  for (const auto& constraint : info.extend_constraints) {
+    if (constraint.interface_id == destroy_interface_id) {
       return true;
-    default:
-      return false;
+    }
+  }
+
+  return false;
+}
+
+// Returns true if the `Self` should impl `Destroy`.
+static auto TypeCanDestroy(Context& context, SemIR::LocId loc_id,
+                           SemIR::ConstantId query_self_const_id) -> bool {
+  // A lazily cached `Destroy` interface.
+  auto destroy_interface_id = SemIR::InterfaceId::None;
+
+  auto query_inst_id = context.constant_values().GetInstId(
+      GetCanonicalFacetOrTypeValue(context, query_self_const_id));
+
+  // When querying a facet type, we can return early.
+  if (auto facet_type = context.types().TryGetAs<SemIR::FacetType>(
+          context.insts().Get(query_inst_id).type_id())) {
+    return CanDestroyFacetType(context, loc_id, *facet_type,
+                               destroy_interface_id);
   }
+
+  // A stack of IDs to check (ordering shouldn't be important).
+  struct WorkItem {
+    SemIR::InstId id;
+    // The only case `abstract` is allowed during a type walk is when looking at
+    // a `base` type of a class.
+    bool allow_abstract = false;
+  };
+  llvm::SmallVector<WorkItem> work;
+
+  // Start by checking the queried instruction.
+  work.push_back({.id = query_inst_id});
+
+  // Loop through type structures recursively, looking for any types
+  // incompatible with `Destroy`.
+  while (!work.empty()) {
+    auto work_item = work.pop_back_val();
+    auto inst = context.insts().Get(work_item.id);
+
+    CARBON_KIND_SWITCH(inst) {
+      case CARBON_KIND(SemIR::ArrayType array_type): {
+        work.push_back({.id = array_type.element_type_inst_id});
+        break;
+      }
+
+      case CARBON_KIND(SemIR::BaseDecl base_decl): {
+        work.push_back(
+            {.id = base_decl.base_type_inst_id, .allow_abstract = true});
+        break;
+      }
+
+      case CARBON_KIND(SemIR::ConstType const_type): {
+        work.push_back({.id = const_type.inner_id,
+                        .allow_abstract = work_item.allow_abstract});
+        break;
+      }
+
+      case CARBON_KIND(SemIR::ClassType class_type): {
+        auto class_info = context.classes().Get(class_type.class_id);
+        // Incomplete and abstract classes can't be destroyed.
+        if (!class_info.is_complete() ||
+            (!work_item.allow_abstract &&
+             class_info.inheritance_kind ==
+                 SemIR::Class::InheritanceKind::Abstract)) {
+          return false;
+        }
+
+        // For C++ types, use Clang to determine whether they can be destructed.
+        // TODO: Needs appropriate calls.
+        if (context.name_scopes().Get(class_info.scope_id).is_cpp_scope()) {
+          break;
+        }
+
+        auto obj_repr_id =
+            class_info.GetObjectRepr(context.sem_ir(), SemIR::SpecificId::None);
+        work.push_back({.id = context.types().GetInstId(obj_repr_id)});
+        break;
+      }
+
+      case CARBON_KIND(SemIR::FacetAccessType facet_access_type): {
+        // For facet types, see if they impl `Core.Destroy` directly.
+        if (!facet_access_type.facet_value_inst_id.has_value()) {
+          return false;
+        }
+        auto facet_value =
+            context.insts().Get(facet_access_type.facet_value_inst_id);
+        auto facet_type =
+            context.types().GetAs<SemIR::FacetType>(facet_value.type_id());
+        if (!CanDestroyFacetType(context, loc_id, facet_type,
+                                 destroy_interface_id)) {
+          return false;
+        }
+        break;
+      }
+
+      case CARBON_KIND(SemIR::StructType struct_type): {
+        for (auto field :
+             context.struct_type_fields().Get(struct_type.fields_id)) {
+          work.push_back({.id = field.type_inst_id});
+        }
+        break;
+      }
+
+      case CARBON_KIND(SemIR::TupleType tuple_type): {
+        for (auto element_id :
+             context.inst_blocks().Get(tuple_type.type_elements_id)) {
+          work.push_back({.id = element_id});
+        }
+        break;
+      }
+
+      case SemIR::FloatLiteralType::Kind:
+      case SemIR::FloatType::Kind:
+      case SemIR::IntLiteralType::Kind:
+      case SemIR::IntType::Kind:
+      case SemIR::BoolType::Kind:
+      case SemIR::PointerType::Kind:
+      case SemIR::TypeType::Kind:
+        // Trivially destructible.
+        break;
+
+      case SemIR::MaybeUnformedType::Kind:
+        // TODO: Need to dig into the type to ensure it does `Destroy`.
+        break;
+
+      case SemIR::BindSymbolicName::Kind:
+      case SemIR::ImplWitnessAccess::Kind:
+        // TODO: Figure out what to do with these.
+        return false;
+
+      case SemIR::PartialType::Kind:
+        // Never destructible.
+        return false;
+
+      default:
+        CARBON_FATAL("TypeCanDestroy found unexpected inst: {0}", inst);
+    }
+  }
+
+  return true;
 }
 
 auto LookupImplWitness(Context& context, SemIR::LocId loc_id,
@@ -597,7 +738,7 @@ auto LookupImplWitness(Context& context, SemIR::LocId loc_id,
   }
   if (builtin_constraint_mask.HasAnyOf(
           SemIR::BuiltinConstraintMask::TypeCanDestroy) &&
-      !TypeCanDestroy(context, query_self_const_id)) {
+      !TypeCanDestroy(context, loc_id, query_self_const_id)) {
     return SemIR::InstBlockId::None;
   }
   if (interfaces.empty()) {