Add initial parsing for 'extern library' (#4173)

toolchain/parse/context.cpp

@@ -437,6 +437,43 @@ auto Context::RecoverFromDeclError(StateStackEntry state, NodeKind node_kind,
           /*has_error=*/true);
 }
 
+auto Context::ParseLibraryName(bool accept_default)
+    -> std::optional<StringLiteralValueId> {
+  if (auto library_name_token = ConsumeIf(Lex::TokenKind::StringLiteral)) {
+    AddLeafNode(NodeKind::LibraryName, *library_name_token);
+    return tokens().GetStringLiteralValue(*library_name_token);
+  }
+
+  if (accept_default) {
+    if (auto default_token = ConsumeIf(Lex::TokenKind::Default)) {
+      AddLeafNode(NodeKind::DefaultLibrary, *default_token);
+      return StringLiteralValueId::Invalid;
+    }
+  }
+
+  CARBON_DIAGNOSTIC(
+      ExpectedLibraryNameOrDefault, Error,
+      "Expected `default` or a string literal to specify the library name.");
+  CARBON_DIAGNOSTIC(ExpectedLibraryName, Error,
+                    "Expected a string literal to specify the library name.");
+  emitter().Emit(*position(), accept_default ? ExpectedLibraryNameOrDefault
+                                             : ExpectedLibraryName);
+  return std::nullopt;
+}
+
+auto Context::ParseLibrarySpecifier(bool accept_default)
+    -> std::optional<StringLiteralValueId> {
+  auto library_token = ConsumeChecked(Lex::TokenKind::Library);
+  auto library_subtree_start = tree().size();
+  auto library_id = ParseLibraryName(accept_default);
+  if (!library_id) {
+    AddLeafNode(NodeKind::LibraryName, *position_, /*has_error=*/true);
+  }
+  AddNode(NodeKind::LibrarySpecifier, library_token, library_subtree_start,
+          /*has_error=*/false);
+  return library_id;
+}
+
 auto Context::DiagnoseExpectedDeclSemi(Lex::TokenKind expected_kind) -> void {
   CARBON_DIAGNOSTIC(ExpectedDeclSemi, Error,
                     "`{0}` declarations must end with a `;`.", Lex::TokenKind);