[Explicit State Access] Error if attempting to read or write without feature flag

PiperOrigin-RevId: 868884341

Author: NL02

xls/dslx/frontend/parser.cc

@@ -2884,6 +2884,16 @@ absl::StatusOr<Expr*> Parser::BuildMacroOrInvocation(
                                                      parametrics);
       }
 
+      if (name == "read" || name == "write") {
+        if (!module_->attributes().contains(
+                ModuleAttribute::kExplicitStateAccess)) {
+          return ParseErrorStatus(
+              span,
+              absl::StrFormat(
+                  "%s() requires #![feature(explicit_state_access)]", name));
+        }
+      }
+
       if (name == "zero!") {
         if (parametrics.size() != 1) {
           return ParseErrorStatus(

xls/dslx/frontend/parser_test.cc

@@ -3498,6 +3498,26 @@ TEST_F(ParserTest, ParseExplicitStateAccessAttribute) {
               testing::ElementsAre(ModuleAttribute::kExplicitStateAccess));
 }
 
+TEST_F(ParserTest, ParseExplicitStateAccessNoFeatureFlag) {
+  constexpr std::string_view kProgram = R"(
+proc foo {
+    config() { () }
+    init { u32: 10 }
+    next(state: u32) {
+        read(state);
+        write(state, state + 1);
+        state
+    }
+}
+)";
+  Scanner s{file_table_, Fileno(0), std::string(kProgram)};
+  Parser parser{"test", &s};
+  EXPECT_THAT(parser.ParseModule(),
+              StatusIs(absl::StatusCode::kInvalidArgument,
+                       HasSubstr("read() requires "
+                                 "#![feature(explicit_state_access)]")));
+}
+
 TEST_F(ParserTest, ExplicitStateAccessProcNextReturnsEmptyTuple) {
   constexpr std::string_view kProgram = R"(#![feature(explicit_state_access)]
 proc simple {