Basic checking for match_first blocks (#7481)

Push a scope_stack entry for match_first blocks, and handle impls being
inside those scope entries. An impl should not use the `match_first`
block as its "enclosing scope" for the purpose of deciding if the impl
is a redecl of another impl. We should look through it to the class or
namespace the impl (and match_first) are located inside.

We don't yet store the relationship between the impl and its match_first
block, nor then can we use it in impl lookup for prioritization.

toolchain/check/handle_impl.cpp

@@ -258,6 +258,16 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id)
   // The impl decl has a scope stack entry for the DeclNameStack, so we look at
   // the parent scope of that.
   auto parent_scope_inst_id = context.scope_stack().PeekParentInstId();
+  while (parent_scope_inst_id.has_value()) {
+    if (auto match_first = context.insts().TryGetAs<SemIR::MatchFirstDecl>(
+            parent_scope_inst_id)) {
+      parent_scope_inst_id = match_first->enclosing_scope_inst_id;
+      // TODO: Save the match first as an identifier on the Impl for choosing a
+      // best candidate later.
+    } else {
+      break;
+    }
+  }
 
   auto impl_id = SemIR::ImplId::None;
   {

toolchain/check/handle_match_first.cpp

@@ -8,18 +8,24 @@
 
 namespace Carbon::Check {
 
-auto HandleParseNode(Context& context, Parse::MatchFirstIntroducerId node_id)
-    -> bool {
-  return context.TODO(node_id, "HandleMatchFirstIntroducer");
+auto HandleParseNode(Context& /*context*/,
+                     Parse::MatchFirstIntroducerId /*node_id*/) -> bool {
+  return true;
 }
 
 auto HandleParseNode(Context& context,
                      Parse::MatchFirstDefinitionStartId node_id) -> bool {
-  return context.TODO(node_id, "HandleMatchFirstDefinitionStart");
+  auto enclosing_scope_inst_id = context.scope_stack().PeekInstId();
+  auto decl_id = AddInst<SemIR::MatchFirstDecl>(
+      context, node_id, {.enclosing_scope_inst_id = enclosing_scope_inst_id});
+  context.scope_stack().PushForMatchFirstBlock(decl_id);
+  return true;
 }
 
-auto HandleParseNode(Context& context, Parse::MatchFirstId node_id) -> bool {
-  return context.TODO(node_id, "HandleMatchFirst");
+auto HandleParseNode(Context& context, Parse::MatchFirstId /*node_id*/)
+    -> bool {
+  context.scope_stack().Pop();
+  return true;
 }
 
 }  // namespace Carbon::Check

toolchain/check/scope_stack.cpp

@@ -128,6 +128,11 @@ auto ScopeStack::PushForFunctionBody(SemIR::InstId scope_inst_id) -> void {
   destroy_id_stack_.PushArray();
 }
 
+auto ScopeStack::PushForMatchFirstBlock(SemIR::InstId scope_inst_id) -> void {
+  Push(scope_inst_id, SemIR::NameScopeId::None, SemIR::SpecificId::None,
+       /*lexical_lookup_has_load_error=*/false);
+}
+
 auto ScopeStack::Pop(bool check_unused) -> void {
   auto scope = scope_stack_.pop_back_val();
 

toolchain/check/scope_stack.h

@@ -64,6 +64,9 @@ class ScopeStack {
   // Pushes a function scope.
   auto PushForFunctionBody(SemIR::InstId scope_inst_id) -> void;
 
+  // Pushes a match_first block scope.
+  auto PushForMatchFirstBlock(SemIR::InstId scope_inst_id) -> void;
+
   // Pops the top scope from scope_stack_. Removes names from lexical_lookup_.
   // If `check_unused` is set, checks and emits diagnostics for unused names.
   auto Pop(bool check_unused = false) -> void;

toolchain/check/testdata/match_first/basic.carbon

@@ -0,0 +1,114 @@
+// 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-FILE: toolchain/testing/testdata/min_prelude/none.carbon
+// ^ARGS: -v compile %s
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/match_first/basic.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/match_first/basic.carbon
+
+// --- empty.carbon
+library "[[@TEST_NAME]]";
+
+match_first {}
+
+// --- todo_fail_class_inside.carbon
+library "[[@TEST_NAME]]";
+
+match_first {
+  // TODO: Only impls are allowed inside match_first.
+  class C;
+}
+
+// --- todo_fail_interface_inside.carbon
+library "[[@TEST_NAME]]";
+
+match_first {
+  // TODO: Only impls are allowed inside match_first.
+  interface Z;
+}
+
+// --- todo_fail_constraint_inside.carbon
+library "[[@TEST_NAME]]";
+
+match_first {
+  // TODO: Only impls are allowed inside match_first.
+  constraint N;
+}
+
+// --- todo_fail_function_inside.carbon
+library "[[@TEST_NAME]]";
+
+match_first {
+  // TODO: Only impls are allowed inside match_first.
+  fn F();
+}
+
+// --- todo_fail_final_inside.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+match_first {
+  // TODO: Can't write `final` inside the match first.
+  final impl () as Z {}
+}
+
+// --- decl.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+match_first {
+  impl () as Z;
+}
+
+impl () as Z {}
+
+// --- definition.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+match_first {
+  impl () as Z {}
+}
+
+// --- redecl.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+impl () as Z;
+
+match_first {
+  impl () as Z {}
+}
+
+// --- inside_class.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+class C {
+  match_first {
+    impl Self as Z {}
+  }
+}
+
+// --- todo_fail_inside_match_first.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+match_first {
+  // TODO: Can't write match_first in another match_first, that puts the same
+  // impl in two match_first blocks.
+  match_first {
+    impl () as Z {}
+  }
+}

toolchain/sem_ir/inst_kind.def

@@ -103,6 +103,7 @@ CARBON_SEM_IR_INST_KIND(InterfaceDecl)
 CARBON_SEM_IR_INST_KIND(InterfaceWithSelfDecl)
 CARBON_SEM_IR_INST_KIND(LookupImplWitness)
 CARBON_SEM_IR_INST_KIND(MarkInPlaceInit)
+CARBON_SEM_IR_INST_KIND(MatchFirstDecl)
 CARBON_SEM_IR_INST_KIND(MaybeUnformedType)
 CARBON_SEM_IR_INST_KIND(NameBindingDecl)
 CARBON_SEM_IR_INST_KIND(NamedConstraintDecl)

toolchain/sem_ir/typed_insts.h

@@ -942,6 +942,16 @@ struct GenericNamedConstraintType {
   SpecificId enclosing_specific_id;
 };
 
+// A `match_first` declaration.
+struct MatchFirstDecl {
+  static constexpr auto Kind =
+      InstKind::MatchFirstDecl.Define<Parse::MatchFirstDefinitionStartId>(
+          {.ir_name = "match_first",
+           .constant_kind = InstConstantKind::AlwaysUnique,
+           .is_lowered = false});
+  SemIR::InstId enclosing_scope_inst_id;
+};
+
 // An `impl` declaration.
 struct ImplDecl {
   static constexpr auto Kind = InstKind::ImplDecl.Define<Parse::AnyImplDeclId>(