Use MaybeUnformed in Optional.

Add builtins to form and detect null `MaybeUnformed(T*)` values, and use
those to represent `Optional(T*)`.

Author: zygoloid

core/prelude/types/optional.carbon

@@ -6,30 +6,83 @@ package Core library "prelude/types/optional";
 
 import library "prelude/copy";
 import library "prelude/destroy";
+import library "prelude/operators/as";
 import library "prelude/types/bool";
+import library "prelude/types/maybe_unformed";
+
+// TODO: Decide how to expose this in the public API.
+private interface OptionalStorage {
+  let Type:! type;
+  fn None() -> Type;
+  fn Some[self: Self]() -> Type;
+  fn Has(value: Type) -> bool;
+  fn Get(value: Type) -> Self;
+}
 
-// For now, an `Optional(T)` is stored as a pair of a `bool` and a `T`, with
-// the `T` left uninitialized if the `bool` is `false`. This isn't a viable
-// approach in the longer term, but is the best we can do for now.
-//
-// TODO: Revisit this once we have choice types implemented in the toolchain.
-//
 // 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:! OptionalStorage) {
   fn None() -> Self {
-    returned var me: Self;
-    me.has_value = false;
-    return var;
+    return {.value = T.None()};
   }
-
   fn Some(value: T) -> Self {
-    return {.has_value = true, .value = value};
+    return {.value = value.Some()};
+  }
+  fn HasValue[self: Self]() -> bool {
+    return T.Has(self.value);
+  }
+  fn Get[self: Self]() -> T {
+    return T.Get(self.value);
   }
 
-  fn HasValue[self: Self]() -> bool { return self.has_value; }
-  fn Get[self: Self]() -> T { return self.value; }
+  private var value: T.Type;
+}
 
-  private var has_value: bool;
-  private var value: T;
+// By default, an `Optional(T)` is stored as a pair of a `bool` and a
+// `MaybeUnformed(T)`, with the `MaybeUnformed(T)` left uninitialized if the
+// `bool` is `false`.
+//
+// TODO: Revisit this once we have choice types implemented in the toolchain.
+private class DefaultOptionalStorage(T:! Copy) {
+  var value: MaybeUnformed(T);
+  var has_value: bool;
+}
+
+impl forall [T:! Copy] T as OptionalStorage
+    where .Type = DefaultOptionalStorage(T) {
+  fn None() -> DefaultOptionalStorage(T) {
+    returned var me: DefaultOptionalStorage(T);
+    me.has_value = false;
+    return var;
+  }
+  fn Some[self: Self]() -> DefaultOptionalStorage(T) {
+    returned var me: DefaultOptionalStorage(T);
+    // TODO: Should be:
+    //   me.value = self as MaybeUnformed(T);
+    me.value unsafe as T = self;
+    me.has_value = true;
+    return var;
+  }
+  fn Has(value: DefaultOptionalStorage(T)) -> bool {
+    return value.has_value;
+  }
+  fn Get(value: DefaultOptionalStorage(T)) -> T {
+    return value.value unsafe as T;
+  }
+}
+
+// For pointers, we use a null pointer value as the "None" value. This allows
+// `Optional(T*)` to be ABI-compatible with a C++ nullable pointer.
+final impl forall [T:! type] T* as OptionalStorage
+    where .Type = MaybeUnformed(T*) {
+  fn None() -> MaybeUnformed(T*) = "pointer.make_null";
+  fn Some[self: Self]() -> MaybeUnformed(T*) {
+    returned var result: MaybeUnformed(T*);
+    result unsafe as T* = self;
+    return var;
+  }
+  fn Has(value: MaybeUnformed(T*)) -> bool = "pointer.is_null";
+  fn Get(value: MaybeUnformed(T*)) -> T* {
+    return value unsafe as T*;
+  }
 }

toolchain/check/eval.cpp

@@ -1693,7 +1693,9 @@ static auto MakeConstantForBuiltinCall(EvalContext& eval_context,
     case SemIR::BuiltinFunctionKind::IntOrAssign:
     case SemIR::BuiltinFunctionKind::IntXorAssign:
     case SemIR::BuiltinFunctionKind::IntLeftShiftAssign:
-    case SemIR::BuiltinFunctionKind::IntRightShiftAssign: {
+    case SemIR::BuiltinFunctionKind::IntRightShiftAssign:
+    case SemIR::BuiltinFunctionKind::PointerMakeNull:
+    case SemIR::BuiltinFunctionKind::PointerIsNull: {
       // These are runtime-only builtins.
       // TODO: Consider tracking this on the `BuiltinFunctionKind`.
       return SemIR::ConstantId::NotConstant;

toolchain/lower/handle_call.cpp

@@ -469,6 +469,26 @@ static auto HandleBuiltinCall(FunctionContext& context, SemIR::InstId inst_id,
       CARBON_FATAL("Missing constant value for call to comptime-only function");
     }
 
+    case SemIR::BuiltinFunctionKind::PointerMakeNull: {
+      // MaybeUnformed(T*) has an in-place initializing representation, so an
+      // out parameter will be passed.
+      context.builder().CreateStore(
+          llvm::ConstantPointerNull::get(
+              llvm::PointerType::get(context.llvm_context(),
+                                     /*AddressSpace=*/0)),
+          context.GetValue(arg_ids[1]));
+      context.SetLocal(inst_id,
+                       llvm::PoisonValue::get(context.GetTypeOfInst(inst_id)));
+      return;
+    }
+
+    case SemIR::BuiltinFunctionKind::PointerIsNull: {
+      auto* ptr = context.builder().CreateLoad(
+          context.GetTypeOfInst(arg_ids[0]), context.GetValue(arg_ids[0]));
+      context.SetLocal(inst_id, context.builder().CreateIsNull(ptr));
+      return;
+    }
+
     case SemIR::BuiltinFunctionKind::TypeAggregateDestroy:
       // TODO: Destroy aggregate members.
       return;

toolchain/sem_ir/builtin_function_kind.cpp

@@ -80,6 +80,22 @@ struct PointerTo {
   }
 };
 
+// Constraint that a type is MaybeUnformed<T>.
+template <typename T>
+struct MaybeUnformed {
+  static auto Check(const File& sem_ir, ValidateState& state, TypeId type_id)
+      -> bool {
+    auto maybe_unformed =
+        sem_ir.types().TryGetAs<SemIR::MaybeUnformedType>(type_id);
+    if (!maybe_unformed) {
+      return false;
+    }
+    return Check<T>(
+        sem_ir, state,
+        sem_ir.types().GetTypeIdForTypeInstId(maybe_unformed->inner_id));
+  }
+};
+
 // Constraint that a type is `()`, used as the return type of builtin functions
 // with no return value.
 struct NoReturn {
@@ -605,6 +621,18 @@ constexpr BuiltinInfo BoolEq = {"bool.eq",
 constexpr BuiltinInfo BoolNeq = {"bool.neq",
                                  ValidateSignature<auto(Bool, Bool)->Bool>};
 
+// "pointer.make_null": returns the representation of a null pointer value. This
+// is an unformed state for the pointer type.
+constexpr BuiltinInfo PointerMakeNull = {
+    "pointer.make_null",
+    ValidateSignature<auto()->MaybeUnformed<PointerTo<AnyType>>>};
+
+// "pointer.is_null": determines whether the given pointer representation is a
+// null pointer value.
+constexpr BuiltinInfo PointerIsNull = {
+    "pointer.is_null",
+    ValidateSignature<auto(MaybeUnformed<PointerTo<AnyType>>)->Bool>};
+
 // "type.and": facet type combination.
 constexpr BuiltinInfo TypeAnd = {"type.and",
                                  ValidateSignature<auto(Type, Type)->Type>};

toolchain/sem_ir/builtin_function_kind.def

@@ -123,6 +123,10 @@ CARBON_SEM_IR_BUILTIN_FUNCTION_KIND(FloatGreaterEq)
 CARBON_SEM_IR_BUILTIN_FUNCTION_KIND(BoolEq)
 CARBON_SEM_IR_BUILTIN_FUNCTION_KIND(BoolNeq)
 
+// Pointers.
+CARBON_SEM_IR_BUILTIN_FUNCTION_KIND(PointerMakeNull)
+CARBON_SEM_IR_BUILTIN_FUNCTION_KIND(PointerIsNull)
+
 // Facet type combination.
 CARBON_SEM_IR_BUILTIN_FUNCTION_KIND(TypeAnd)
 

toolchain/sem_ir/type_iterator.h

@@ -40,7 +40,9 @@ class TypeIterator {
   // The iterator will visit things in the reverse order that they are added.
   auto Add(InstId inst_id) -> void {
     auto type_id = sem_ir_->insts().Get(inst_id).type_id();
-    CARBON_CHECK(sem_ir_->types().IsFacetType(type_id));
+    CARBON_CHECK(sem_ir_->types().IsFacetType(type_id),
+                 "Type {0} of type inst is not a facet type",
+                 sem_ir_->types().GetAsInst(type_id).kind());
     PushInstId(inst_id);
   }