feat: Warn on using (...) instead of {...} for sequences

crates/plotnik-lib/src/diagnostics/message.rs

@@ -54,6 +54,7 @@ pub enum DiagnosticKind {
     FieldNameHasHyphens,
     FieldNameUppercase,
     TypeNameInvalidChars,
+    TreeSitterSequenceSyntax,
 
     // Valid syntax, invalid semantics
     DuplicateDefinition,
@@ -87,7 +88,7 @@ impl DiagnosticKind {
     /// Default severity for this kind. Can be overridden by policy.
     pub fn default_severity(&self) -> Severity {
         match self {
-            Self::UnusedBranchLabels => Severity::Warning,
+            Self::UnusedBranchLabels | Self::TreeSitterSequenceSyntax => Severity::Warning,
             _ => Severity::Error,
         }
     }
@@ -171,6 +172,7 @@ impl DiagnosticKind {
             Self::FieldNameHasHyphens => "field names cannot contain `-`",
             Self::FieldNameUppercase => "field names must be lowercase",
             Self::TypeNameInvalidChars => "type names cannot contain `.` or `-`",
+            Self::TreeSitterSequenceSyntax => "Tree-sitter sequence syntax",
 
             // Semantic errors
             Self::DuplicateDefinition => "name already defined",

crates/plotnik-lib/src/parser/grammar/structures.rs

@@ -15,6 +15,7 @@ impl Parser<'_, '_> {
     pub(crate) fn parse_tree(&mut self) {
         let checkpoint = self.checkpoint();
         self.push_delimiter(SyntaxKind::ParenOpen);
+        let open_paren_span = self.current_span(); // save span before bump
         self.bump(); // consume '('
 
         let mut is_ref = false;
@@ -45,6 +46,18 @@ impl Parser<'_, '_> {
             }
             _ => {
                 self.start_node_at(checkpoint, SyntaxKind::Tree);
+                // Warn about tree-sitter style sequence: ((a) (b)) instead of {(a) (b)}
+                // Only warn when it looks like an expression (not predicates or other invalid tokens)
+                if self.currently_is_one_of(EXPR_FIRST_TOKENS) {
+                    self.diagnostics
+                        .report(
+                            self.source_id,
+                            DiagnosticKind::TreeSitterSequenceSyntax,
+                            open_paren_span,
+                        )
+                        .hint("use `{...}` for sequences")
+                        .emit();
+                }
             }
         }
 

crates/plotnik-lib/src/parser/tests/recovery/validation_tests.rs

@@ -1309,3 +1309,42 @@ fn single_colon_primitive_type() {
       |        ^^^^^^
     ");
 }
+
+#[test]
+fn treesitter_sequence_syntax_warning() {
+    // Tree-sitter uses ((a) (b)) for sequences, Plotnik uses {(a) (b)}
+    let input = "Test = ((a) (b))";
+
+    let res = Query::expect_warning(input);
+
+    insta::assert_snapshot!(res, @r"
+    warning: Tree-sitter sequence syntax
+      |
+    1 | Test = ((a) (b))
+      |        ^
+      |
+    help: use `{...}` for sequences
+    ");
+}
+
+#[test]
+fn treesitter_sequence_single_child_warning() {
+    let input = "Test = ((a))";
+
+    let res = Query::expect_warning(input);
+
+    insta::assert_snapshot!(res, @r"
+    warning: Tree-sitter sequence syntax
+      |
+    1 | Test = ((a))
+      |        ^
+      |
+    help: use `{...}` for sequences
+    ");
+}
+
+#[test]
+fn named_node_with_children_no_warning() {
+    // Normal node with children - NOT a tree-sitter sequence
+    Query::expect_valid("Test = (identifier (child))");
+}

crates/plotnik-lib/src/query/query_tests.rs

@@ -97,6 +97,25 @@ impl QueryAnalyzed {
         }
         query.dump_diagnostics()
     }
+
+    #[track_caller]
+    pub fn expect_warning(src: &str) -> String {
+        let source_map = SourceMap::one_liner(src);
+        let query = QueryBuilder::new(source_map).parse().unwrap().analyze();
+
+        if !query.is_valid() {
+            panic!(
+                "Expected valid query with warning, got error:\n{}",
+                query.dump_diagnostics()
+            );
+        }
+
+        if !query.diagnostics().has_warnings() {
+            panic!("Expected warning, got none:\n{}", query.dump_cst());
+        }
+
+        query.dump_diagnostics()
+    }
 }
 
 #[test]