Improve code completion

Top level identifiers are no longer parsed as rules, which improves
suggestions, as the following rules are now parsed correctly.

src/frontend/generated.rs

@@ -419,7 +419,7 @@ impl<'a> Parser<'a> {
             Token::Token => {
                 self.r#token_list(diags);
             }
-            Token::Id => {
+            Token::Id if self.predicate_decl_1() => {
                 self.r#rule_decl(diags);
             }
             Token::Start => {
@@ -431,6 +431,9 @@ impl<'a> Parser<'a> {
             Token::Skip => {
                 self.r#skip_decl(diags);
             }
+            Token::Id => {
+                self.advance_with_error(diags, err![self.span(),]);
+            }
             _ => {
                 self.error(
                     diags,
@@ -878,4 +881,5 @@ trait PredicatesAndActions {
     /// Called when a new syntax tree node is created
     #[allow(clippy::ptr_arg)]
     fn build(&mut self, _rule: Rule, _node: NodeRef, _diags: &mut Vec<Diagnostic>) {}
+    fn predicate_decl_1(&self) -> bool;
 }

src/frontend/lelwel.llw

@@ -15,7 +15,7 @@ skip Comment DocComment Whitespace;
 file: decl*;
 decl^:
   token_list
-| rule_decl
+| ?1 rule_decl
 | start_decl
 | right_decl
 | skip_decl

src/frontend/parser.rs

@@ -188,4 +188,9 @@ pub fn tokenize(
 
 include!("./generated.rs");
 
-impl PredicatesAndActions for Parser<'_> {}
+impl PredicatesAndActions for Parser<'_> {
+    fn predicate_decl_1(&self) -> bool {
+        let peek = self.peek(1);
+        peek == Token::Colon || (peek == Token::Hat && self.peek(2) == Token::Colon)
+    }
+}

tests/frontend.rs

@@ -285,6 +285,17 @@ fn syntax_error() {
     assert_eq!(lines.next(), None);
 }
 
+#[test]
+#[rustfmt::skip]
+fn top_level_ident() {
+    let diags = gen_diags("tests/frontend/top_level_ident.llw");
+    let mut lines = diags.lines();
+
+    assert_eq!(lines.next().unwrap(), "tests/frontend/top_level_ident.llw:3:1: error: invalid syntax");
+    assert_eq!(lines.next().unwrap(), "tests/frontend/top_level_ident.llw:5:1: warning[W005]: empty rule");
+    assert_eq!(lines.next(), None);
+}
+
 #[test]
 #[rustfmt::skip]
 fn undefined() {