carbon-lang/toolchain/check/handle_index.cpp

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include <optional>

#include "toolchain/base/kind_switch.h"
#include "toolchain/check/context.h"
#include "toolchain/check/convert.h"
#include "toolchain/check/handle.h"
#include "toolchain/check/operator.h"
#include "toolchain/diagnostics/diagnostic.h"
#include "toolchain/sem_ir/builtin_inst_kind.h"
#include "toolchain/sem_ir/inst.h"
#include "toolchain/sem_ir/typed_insts.h"

namespace Carbon::Check {

auto HandleParseNode(Context& /*context*/, Parse::IndexExprStartId /*node_id*/)
    -> bool {
  // Leave the expression on the stack for IndexExpr.
  return true;
}

// Returns the argument values of the `IndexWith` interface. Arguments
// correspond to the `SubscriptType` and the `ElementType`. If no arguments are
// used to define `IndexWith`, this returns an empty array reference. If the
// class does not implement the said interface, this returns a `std::nullopt`.
// TODO: Switch to using an associated type instead of a parameter for the
// `ElementType`.
static auto GetIndexWithArgs(Context& context, Parse::NodeId node_id,
                             SemIR::TypeId self_id)
    -> std::optional<llvm::ArrayRef<SemIR::InstId>> {
  auto index_with_inst_id = context.LookupNameInCore(node_id, "IndexWith");
  // If the `IndexWith` interface doesn't have generic arguments then return an
  // empty reference.
  if (context.insts().Is<SemIR::InterfaceType>(index_with_inst_id)) {
    return llvm::ArrayRef<SemIR::InstId>();
  }

  auto index_with_inst =
      context.insts().TryGetAsIfValid<SemIR::StructValue>(index_with_inst_id);
  if (!index_with_inst) {
    return std::nullopt;
  }

  auto index_with_interface =
      context.types().TryGetAs<SemIR::GenericInterfaceType>(
          index_with_inst->type_id);
  if (!index_with_interface) {
    return std::nullopt;
  }

  for (const auto& impl : context.impls().array_ref()) {
    auto impl_self_type_id = context.GetTypeIdForTypeInst(impl.self_id);
    auto impl_constraint_type_id =
        context.GetTypeIdForTypeInst(impl.constraint_id);

    if (impl_self_type_id != self_id) {
      continue;
    }
    auto interface_type =
        context.types().TryGetAs<SemIR::InterfaceType>(impl_constraint_type_id);
    if (!interface_type) {
      continue;
    }

    if (index_with_interface->interface_id != interface_type->interface_id) {
      continue;
    }

    return context.inst_blocks().GetOrEmpty(
        context.specifics().Get(interface_type->specific_id).args_id);
  }

  return std::nullopt;
}

// Performs an index with base expression `operand_inst_id` and
// `operand_type_id` for types that are not an array. This checks if
// the base expression implements the `IndexWith` interface; if so, uses the
// `At` associative method, otherwise prints a diagnostic.
static auto PerformIndexWith(Context& context, Parse::NodeId node_id,
                             SemIR::InstId operand_inst_id,
                             SemIR::TypeId operand_type_id,
                             SemIR::InstId index_inst_id) -> SemIR::InstId {
  auto args = GetIndexWithArgs(context, node_id, operand_type_id);

  // If the type does not implement the `IndexWith` interface, then return
  // an error.
  if (!args) {
    CARBON_DIAGNOSTIC(TypeNotIndexable, Error,
                      "type {0} does not support indexing", SemIR::TypeId);
    context.emitter().Emit(node_id, TypeNotIndexable, operand_type_id);
    return SemIR::InstId::BuiltinError;
  }

  Operator op{
      .interface_name = "IndexWith",
      .interface_args_ref = *args,
      .op_name = "At",
  };

  // IndexWith is defined without generic arguments.
  if (args->empty()) {
    return BuildBinaryOperator(context, node_id, op, operand_inst_id,
                               index_inst_id);
  }

  // The first argument of the `IndexWith` interface corresponds to the
  // `SubscriptType`, so first cast `index_inst_id` to that type.
  auto subscript_type_id = context.GetTypeIdForTypeInst((*args)[0]);
  auto cast_index_id =
      ConvertToValueOfType(context, node_id, index_inst_id, subscript_type_id);

  return BuildBinaryOperator(context, node_id, op, operand_inst_id,
                             cast_index_id);
}

auto HandleParseNode(Context& context, Parse::IndexExprId node_id) -> bool {
  auto index_inst_id = context.node_stack().PopExpr();
  auto operand_inst_id = context.node_stack().PopExpr();
  operand_inst_id = ConvertToValueOrRefExpr(context, operand_inst_id);
  auto operand_inst = context.insts().Get(operand_inst_id);
  auto operand_type_id = operand_inst.type_id();

  CARBON_KIND_SWITCH(context.types().GetAsInst(operand_type_id)) {
    case CARBON_KIND(SemIR::ArrayType array_type): {
      auto index_loc_id = context.insts().GetLocId(index_inst_id);
      auto cast_index_id = ConvertToValueOfType(
          context, index_loc_id, index_inst_id,
          context.GetBuiltinType(SemIR::BuiltinInstKind::IntType));
      auto array_cat =
          SemIR::GetExprCategory(context.sem_ir(), operand_inst_id);
      if (array_cat == SemIR::ExprCategory::Value) {
        // If the operand is an array value, convert it to an ephemeral
        // reference to an array so we can perform a primitive indexing into it.
        operand_inst_id = context.AddInst<SemIR::ValueAsRef>(
            node_id, {.type_id = operand_type_id, .value_id = operand_inst_id});
      }
      // Constant evaluation will perform a bounds check on this array indexing
      // if the index is constant.
      auto elem_id = context.AddInst<SemIR::ArrayIndex>(
          node_id, {.type_id = array_type.element_type_id,
                    .array_id = operand_inst_id,
                    .index_id = cast_index_id});
      if (array_cat != SemIR::ExprCategory::DurableRef) {
        // Indexing a durable reference gives a durable reference expression.
        // Indexing anything else gives a value expression.
        // TODO: This should be replaced by a choice between using `IndexWith`
        // and `IndirectIndexWith`.
        elem_id = ConvertToValueExpr(context, elem_id);
      }
      context.node_stack().Push(node_id, elem_id);
      return true;
    }

    default: {
      auto elem_id = SemIR::InstId::BuiltinError;
      if (operand_type_id != SemIR::TypeId::Error) {
        elem_id = PerformIndexWith(context, node_id, operand_inst_id,
                                   operand_type_id, index_inst_id);
      }
      context.node_stack().Push(node_id, elem_id);
      return true;
    }
  }
}

}  // namespace Carbon::Check