Add basic plumbing for parser options

In the future, we will need the parser to support conflicting syntaxes
(E.g. TypeScript vs Flow, or ES Modules vs scripts). The user will need
to configure quick-lint-js to get the best diagnostics.

Add a way to internally tweak the parser's treatment of 'await' at the
module scope.

In the future, this will be configured using quick-lint-js.config. For
now, this is only used in tests.

Author: strager

src/parse.cpp

@@ -684,7 +684,10 @@ expression* parser::parse_async_expression_only(token async_token,
 
 expression* parser::parse_await_expression(token await_token, precedence prec) {
   bool is_identifier = [&]() -> bool {
-    if (this->in_async_function_) {
+    if (this->in_async_function_ ||
+        (this->in_top_level_ &&
+         this->options_.top_level_await_mode ==
+             parser_top_level_await_mode::await_operator)) {
       return false;
     } else {
       // await is a unary operator (in modules) or an identifier (in scripts).

src/quick-lint-js/parse.h

@@ -92,6 +92,17 @@
   } while (false)
 
 namespace quick_lint_js {
+enum class parser_top_level_await_mode {
+  auto_detect = 0,
+  await_operator,
+};
+
+// TODO(#465): Accept parser options from quick-lint-js.config or CLI options.
+struct parser_options {
+  parser_top_level_await_mode top_level_await_mode =
+      parser_top_level_await_mode::auto_detect;
+};
+
 // A parser reads JavaScript source code and calls the member functions of a
 // parse_visitor (visit_variable_declaration, visit_enter_function_scope, etc.).
 class parser {
@@ -105,7 +116,13 @@ class parser {
 
  public:
   explicit parser(padded_string_view input, error_reporter *error_reporter)
-      : lexer_(input, error_reporter), error_reporter_(error_reporter) {}
+      : parser(input, error_reporter, parser_options()) {}
+
+  explicit parser(padded_string_view input, error_reporter *error_reporter,
+                  parser_options options)
+      : lexer_(input, error_reporter),
+        error_reporter_(error_reporter),
+        options_(options) {}
 
   quick_lint_js::lexer &lexer() noexcept { return this->lexer_; }
 
@@ -3732,6 +3749,7 @@ class parser {
 
   quick_lint_js::lexer lexer_;
   error_reporter *error_reporter_;
+  parser_options options_;
   quick_lint_js::expression_arena expressions_;
 
   // Memory used for temporary memory allocations (e.g. vectors on the stack).

test/test-parse-var.cpp

@@ -1531,6 +1531,37 @@ TEST(test_parse, use_await_at_top_level_as_variable) {
   }
 }
 
+TEST(test_parse, forced_top_level_await_operator) {
+  {
+    padded_string code(u8"await p;"_sv);
+    spy_visitor v;
+    parser p(
+        &code, &v,
+        parser_options{
+            .top_level_await_mode = parser_top_level_await_mode::await_operator,
+        });
+    p.parse_and_visit_module(v);
+    EXPECT_THAT(v.visits, ElementsAre("visit_variable_use",  // p
+                                      "visit_end_of_module"));
+    EXPECT_THAT(v.errors, IsEmpty());
+  }
+
+  {
+    padded_string code(u8"await;"_sv);
+    spy_visitor v;
+    parser p(
+        &code, &v,
+        parser_options{
+            .top_level_await_mode = parser_top_level_await_mode::await_operator,
+        });
+    p.parse_and_visit_module(v);
+    EXPECT_THAT(v.visits, ElementsAre("visit_end_of_module"));
+    EXPECT_THAT(v.errors, ElementsAre(ERROR_TYPE_FIELD(
+                              error_missing_operand_for_operator, where,
+                              offsets_matcher(&code, 0, u8"await"))));
+  }
+}
+
 TEST(
     test_parse,
     declare_await_in_async_function_is_allowed_for_named_function_expressions) {