Fail to parse a sequence consisting of a single anchor

crates/metaslang/cst/src/query/parser.rs

@@ -67,9 +67,15 @@ pub(super) fn parse_matcher_alternatives<T: KindTypes>(
 pub(super) fn parse_matcher_sequence<T: KindTypes>(
     i: &str,
 ) -> IResult<&str, ASTNode<T>, VerboseError<&str>> {
-    many1(parse_sequence_item::<T>)
-        .map(|children| ASTNode::Sequence(Rc::new(SequenceASTNode { children })))
-        .parse(i)
+    verify(
+        many1(parse_sequence_item::<T>),
+        |children: &[ASTNode<T>]| {
+            // It doesn't make sense for a sequence to be a single anchor
+            children.len() > 1 || !matches!(children[0], ASTNode::Anchor)
+        },
+    )
+    .map(|children| ASTNode::Sequence(Rc::new(SequenceASTNode { children })))
+    .parse(i)
 }
 
 pub(super) fn parse_sequence_item<T: KindTypes>(
@@ -79,6 +85,7 @@ pub(super) fn parse_sequence_item<T: KindTypes>(
 }
 
 pub(super) fn parse_anchor<T: KindTypes>(i: &str) -> IResult<&str, ASTNode<T>, VerboseError<&str>> {
+    // An anchor is a single '.' character, and cannot be followed by another anchor
     pair(token('.'), cut(peek(none_of(". \t\r\n"))))
         .map(|_| ASTNode::Anchor)
         .parse(i)

crates/testlang/outputs/cargo/tests/src/query/parser_tests.rs

@@ -54,7 +54,7 @@ fn test_zero_or_more_canonicalisation() {
 fn test_parsing_error() {
     let result = Query::parse(r#"@root [_"#);
     match result {
-        Ok(_) => panic!("Expected error"),
+        Ok(_) => panic!("Expected parse failure"),
         Err(e) => assert_eq!(e.message, "Parse error:\nexpected ']' at: \nAlt at: [_\n"),
     }
 }
@@ -63,7 +63,7 @@ fn test_parsing_error() {
 fn test_fails_parsing_ellipsis() {
     let result = Query::parse(r#"[_ ...]"#);
     match result {
-        Ok(_) => panic!("Expected error"),
+        Ok(_) => panic!("Expected parse failure"),
         Err(e) => assert_eq!(e.message, "Parse error:\nNoneOf at: ..]\n"),
     }
 }
@@ -72,7 +72,16 @@ fn test_fails_parsing_ellipsis() {
 fn test_fails_consecutive_anchors() {
     let result = Query::parse(r#"[_ . .]"#);
     match result {
-        Ok(_) => panic!("Expected error"),
+        Ok(_) => panic!("Expected parse failure"),
         Err(e) => assert_eq!(e.message, "Parse error:\nNoneOf at: .]\n"),
     }
 }
+
+#[test]
+fn test_fails_single_anchor() {
+    let result = Query::parse(r#"[_ .]"#);
+    match result {
+        Ok(_) => panic!("Expected parse failure"),
+        Err(e) => assert_eq!(e.message, "Parse error:\nexpected ']' at: .]\nAlt at: [_ .]\n"),
+    }
+}