Add AST dumper for human-readable tree output

Produces indented text showing node kinds, named fields, and leaf
content. Unnamed tokens are hidden unless inside a named field.
Used by tests for readable assertions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

shared/yeast/src/dump.rs

@@ -0,0 +1,159 @@
+use std::fmt::Write;
+
+use crate::{Ast, Node, NodeContent, CHILD_FIELD};
+
+/// Options for controlling AST dump output.
+pub struct DumpOptions {
+    /// Whether to include source locations in the output.
+    pub show_locations: bool,
+    /// Whether to include source text for leaf nodes.
+    pub show_content: bool,
+}
+
+impl Default for DumpOptions {
+    fn default() -> Self {
+        Self {
+            show_locations: false,
+            show_content: true,
+        }
+    }
+}
+
+/// Dump a yeast AST as a human-readable indented text format.
+///
+/// Output format:
+/// ```text
+/// program
+///   assignment
+///     left:
+///       left_assignment_list
+///         identifier "x"
+///         identifier "y"
+///     right:
+///       call
+///         method:
+///           identifier "foo"
+/// ```
+pub fn dump_ast(ast: &Ast, root: usize, source: &str) -> String {
+    dump_ast_with_options(ast, root, source, &DumpOptions::default())
+}
+
+pub fn dump_ast_with_options(ast: &Ast, root: usize, source: &str, options: &DumpOptions) -> String {
+    let mut out = String::new();
+    dump_node(ast, root, source, options, 0, &mut out);
+    out
+}
+
+fn dump_node(ast: &Ast, id: usize, source: &str, options: &DumpOptions, indent: usize, out: &mut String) {
+    let node = match ast.get_node(id) {
+        Some(n) => n,
+        None => return,
+    };
+
+    let prefix = "  ".repeat(indent);
+
+    // Node kind
+    write!(out, "{}{}", prefix, node.kind_name()).unwrap();
+
+    // Location
+    if options.show_locations {
+        let start = node.start_position();
+        let end = node.end_position();
+        write!(out, " [{},{}]-[{},{}]",
+            start.row + 1, start.column + 1,
+            end.row + 1, end.column + 1
+        ).unwrap();
+    }
+
+    // Content for leaf nodes
+    if options.show_content && node.is_named() && is_leaf(node) {
+        let content = node_content(node, source);
+        if !content.is_empty() {
+            write!(out, " {:?}", content).unwrap();
+        }
+    }
+
+    writeln!(out).unwrap();
+
+    // Named fields first
+    for (&field_id, children) in &node.fields {
+        if field_id == CHILD_FIELD {
+            continue; // Handle unnamed children last
+        }
+        let field_name = ast.field_name_for_id(field_id).unwrap_or("?");
+        if children.len() == 1 {
+            write!(out, "{}  {}:", prefix, field_name).unwrap();
+            // Inline single child
+            let child = ast.get_node(children[0]);
+            if child.map_or(false, |c| is_leaf(c)) {
+                write!(out, " ").unwrap();
+                dump_node_inline(ast, children[0], source, options, out);
+            } else {
+                writeln!(out).unwrap();
+                dump_node(ast, children[0], source, options, indent + 2, out);
+            }
+        } else {
+            writeln!(out, "{}  {}:", prefix, field_name).unwrap();
+            for &child_id in children {
+                dump_node(ast, child_id, source, options, indent + 2, out);
+            }
+        }
+    }
+
+    // Unnamed children — skip unnamed tokens (keywords, punctuation)
+    if let Some(children) = node.fields.get(&CHILD_FIELD) {
+        for &child_id in children {
+            if let Some(child) = ast.get_node(child_id) {
+                if child.is_named() {
+                    dump_node(ast, child_id, source, options, indent + 1, out);
+                }
+            }
+        }
+    }
+}
+
+/// Dump a leaf node inline (no newline prefix, caller provides context).
+fn dump_node_inline(ast: &Ast, id: usize, source: &str, options: &DumpOptions, out: &mut String) {
+    let node = match ast.get_node(id) {
+        Some(n) => n,
+        None => return,
+    };
+
+    write!(out, "{}", node.kind_name()).unwrap();
+
+    if options.show_locations {
+        let start = node.start_position();
+        let end = node.end_position();
+        write!(out, " [{},{}]-[{},{}]",
+            start.row + 1, start.column + 1,
+            end.row + 1, end.column + 1
+        ).unwrap();
+    }
+
+    if options.show_content && node.is_named() {
+        let content = node_content(node, source);
+        if !content.is_empty() {
+            write!(out, " {:?}", content).unwrap();
+        }
+    }
+
+    writeln!(out).unwrap();
+}
+
+fn is_leaf(node: &Node) -> bool {
+    node.fields.is_empty()
+}
+
+fn node_content(node: &Node, source: &str) -> String {
+    match &node.content {
+        NodeContent::DynamicString(s) if !s.is_empty() => s.clone(),
+        _ => {
+            let range = node.byte_range();
+            if range.start < source.len() && range.end <= source.len() {
+                source[range.start..range.end].to_string()
+            } else {
+                String::new()
+            }
+        }
+    }
+}