Use TreeCursor to pretty-print :tree-sitter-subtree (#4606)

The current `:tree-sitter-subtree` has a bug for field-names when the
field name belongs to an unnamed child node. Take this ruby example:

    def self.method_name
      true
    end

The subtree given by tree-sitter-cli is:

    (singleton_method [2, 0] - [4, 3]
      object: (self [2, 4] - [2, 8])
      name: (identifier [2, 9] - [2, 20])
      body: (body_statement [3, 2] - [3, 6]
        (true [3, 2] - [3, 6])))

But the `:tree-sitter-subtree` output was

    (singleton_method
      object: (self)
      body: (identifier)
      (body_statement (true)))

The `singleton_method` rule defines the `name` and `body` fields in an
unnamed helper rule `_method_rest` and the old implementation of
`pretty_print_tree_impl` would pass the `field_name` down from the
named `singleton_method` node.

To fix it we switch to the [TreeCursor] API which is recommended by
the tree-sitter docs for traversing the tree. `TreeCursor::field_name`
accurately determines the field name for the current cursor position
even when the node is unnamed.

[TreeCursor]: https://docs.rs/tree-sitter/0.20.9/tree_sitter/struct.TreeCursor.html

helix-core/src/syntax.rs

@@ -1283,7 +1283,7 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 use std::{iter, mem, ops, str, usize};
 use tree_sitter::{
     Language as Grammar, Node, Parser, Point, Query, QueryCaptures, QueryCursor, QueryError,
-    QueryMatch, Range, TextProvider, Tree,
+    QueryMatch, Range, TextProvider, Tree, TreeCursor,
 };
 
 const CANCELLATION_CHECK_INTERVAL: usize = 100;
@@ -2153,57 +2153,68 @@ impl<I: Iterator<Item = HighlightEvent>> Iterator for Merge<I> {
     }
 }
 
+fn node_is_visible(node: &Node) -> bool {
+    node.is_missing() || (node.is_named() && node.language().node_kind_is_visible(node.kind_id()))
+}
+
 pub fn pretty_print_tree<W: fmt::Write>(fmt: &mut W, node: Node) -> fmt::Result {
-    pretty_print_tree_impl(fmt, node, true, None, 0)
+    if node.child_count() == 0 {
+        if node_is_visible(&node) {
+            write!(fmt, "({})", node.kind())
+        } else {
+            write!(fmt, "\"{}\"", node.kind())
+        }
+    } else {
+        pretty_print_tree_impl(fmt, &mut node.walk(), 0)
+    }
 }
 
 fn pretty_print_tree_impl<W: fmt::Write>(
     fmt: &mut W,
-    node: Node,
-    is_root: bool,
-    field_name: Option<&str>,
+    cursor: &mut TreeCursor,
     depth: usize,
 ) -> fmt::Result {
-    fn is_visible(node: Node) -> bool {
-        node.is_missing()
-            || (node.is_named() && node.language().node_kind_is_visible(node.kind_id()))
-    }
+    let node = cursor.node();
+    let visible = node_is_visible(&node);
 
-    if is_visible(node) {
+    if visible {
         let indentation_columns = depth * 2;
         write!(fmt, "{:indentation_columns$}", "")?;
 
-        if let Some(field_name) = field_name {
+        if let Some(field_name) = cursor.field_name() {
             write!(fmt, "{}: ", field_name)?;
         }
 
         write!(fmt, "({}", node.kind())?;
-    } else if is_root {
-        write!(fmt, "(\"{}\")", node.kind())?;
     }
 
-    for child_idx in 0..node.child_count() {
-        if let Some(child) = node.child(child_idx) {
-            if is_visible(child) {
+    // Handle children.
+    if cursor.goto_first_child() {
+        loop {
+            if node_is_visible(&cursor.node()) {
                 fmt.write_char('\n')?;
             }
 
-            pretty_print_tree_impl(
-                fmt,
-                child,
-                false,
-                node.field_name_for_child(child_idx as u32),
-                depth + 1,
-            )?;
+            pretty_print_tree_impl(fmt, cursor, depth + 1)?;
+
+            if !cursor.goto_next_sibling() {
+                break;
+            }
         }
+
+        let moved = cursor.goto_parent();
+        // The parent of the first child must exist, and must be `node`.
+        debug_assert!(moved);
+        debug_assert!(cursor.node() == node);
     }
 
-    if is_visible(node) {
-        write!(fmt, ")")?;
+    if visible {
+        fmt.write_char(')')?;
     }
 
     Ok(())
 }
+
 #[cfg(test)]
 mod test {
     use super::*;
@@ -2376,11 +2387,17 @@ mod test {
     }
 
     #[track_caller]
-    fn assert_pretty_print(source: &str, expected: &str, start: usize, end: usize) {
+    fn assert_pretty_print(
+        language_name: &str,
+        source: &str,
+        expected: &str,
+        start: usize,
+        end: usize,
+    ) {
         let source = Rope::from_str(source);
 
         let loader = Loader::new(Configuration { language: vec![] });
-        let language = get_language("rust").unwrap();
+        let language = get_language(language_name).unwrap();
 
         let config = HighlightConfiguration::new(language, "", "", "").unwrap();
         let syntax = Syntax::new(&source, Arc::new(config), Arc::new(loader));
@@ -2400,13 +2417,14 @@ mod test {
     #[test]
     fn test_pretty_print() {
         let source = r#"/// Hello"#;
-        assert_pretty_print(source, "(line_comment)", 0, source.len());
+        assert_pretty_print("rust", source, "(line_comment)", 0, source.len());
 
         // A large tree should be indented with fields:
         let source = r#"fn main() {
             println!("Hello, World!");
         }"#;
         assert_pretty_print(
+            "rust",
             source,
             concat!(
                 "(function_item\n",
@@ -2425,11 +2443,34 @@ mod test {
 
         // Selecting a token should print just that token:
         let source = r#"fn main() {}"#;
-        assert_pretty_print(source, r#"("fn")"#, 0, 1);
+        assert_pretty_print("rust", source, r#""fn""#, 0, 1);
 
         // Error nodes are printed as errors:
         let source = r#"}{"#;
-        assert_pretty_print(source, "(ERROR)", 0, source.len());
+        assert_pretty_print("rust", source, "(ERROR)", 0, source.len());
+
+        // Fields broken under unnamed nodes are determined correctly.
+        // In the following source, `object` belongs to the `singleton_method`
+        // rule but `name` and `body` belong to an unnamed helper `_method_rest`.
+        // This can cause a bug with a pretty-printing implementation that
+        // uses `Node::field_name_for_child` to determine field names but is
+        // fixed when using `TreeCursor::field_name`.
+        let source = "def self.method_name
+          true
+        end";
+        assert_pretty_print(
+            "ruby",
+            source,
+            concat!(
+                "(singleton_method\n",
+                "  object: (self)\n",
+                "  name: (identifier)\n",
+                "  body: (body_statement\n",
+                "    (true)))"
+            ),
+            0,
+            source.len(),
+        );
     }
 
     #[test]