Add Thread::Backtrace::Location#syntax_tree

Returns the AST node at the location: the node of the surrounding
method or block is retrieved and validated in the same way as
Proc#syntax_tree, and then the node at the program counter is searched
within it ([Feature #21795]).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ast.c

@@ -312,6 +312,18 @@ node_id_for_backtrace_location(rb_execution_context_t *ec, VALUE module, VALUE l
     return INT2NUM(node_id);
 }
 
+static VALUE
+iseq_of_backtrace_location(rb_execution_context_t *ec, VALUE module, VALUE location)
+{
+    if (!rb_frame_info_p(location)) {
+        rb_raise(rb_eTypeError, "Thread::Backtrace::Location object expected");
+    }
+
+    const rb_iseq_t *iseq = rb_get_iseq_from_frame_info(location);
+    if (!iseq) return Qnil;
+    return rb_iseqw_new(iseq);
+}
+
 static VALUE
 ast_s_of(rb_execution_context_t *ec, VALUE module, VALUE body, VALUE keep_script_lines, VALUE error_tolerant, VALUE keep_tokens)
 {

ast.rb

@@ -472,3 +472,39 @@ class UnboundMethod
     RubyVM::InstructionSequence.of(self)&.syntax_tree
   end
 end
+
+class Thread::Backtrace::Location
+  #  call-seq:
+  #     location.syntax_tree -> Prism::Node | RubyVM::AbstractSyntaxTree::Node | nil
+  #
+  #  Returns the AST node at this location, by re-parsing the source file. See
+  #  RubyVM::InstructionSequence#syntax_tree for when +nil+ is returned.
+  #
+  #  This method is experimental and might change without notice.
+  def syntax_tree
+    iseq = Primitive.iseq_of_backtrace_location(self)
+    return nil unless iseq
+
+    node_id = Primitive.node_id_for_backtrace_location(self)
+    return nil unless node_id
+
+    scope = iseq.syntax_tree
+    return nil unless scope
+
+    if scope.is_a?(RubyVM::AbstractSyntaxTree::Node)
+      return Primitive.ast_node_find(scope, node_id)
+    end
+
+    return scope if scope.node_id == node_id
+
+    queue = [scope]
+    while (node = queue.shift)
+      node.compact_child_nodes.each do |child|
+        return child if child.node_id == node_id
+        queue << child
+      end
+    end
+
+    nil
+  end
+end

test/ruby/test_proc_syntax_tree.rb

@@ -97,6 +97,19 @@ class TestProcSyntaxTree < Test::Unit::TestCase
     assert_equal iseq.source_hash, iseq.to_a[4][:source_hash]
   end
 
+  def test_backtrace_location_syntax_tree
+    with_loaded_file("def proc_ast_test_loc = caller_locations(0, 1).first\n") do
+      node = proc_ast_test_loc.syntax_tree
+      if PRISM
+        assert_equal :call_node, node.type
+        assert_equal "caller_locations(0, 1)", node.slice
+      else
+        assert_equal :FCALL, node.type
+      end
+    ensure
+      Object.remove_method(:proc_ast_test_loc)
+    end
+  end
 
   def test_parse_y_syntax_tree
     assert_separately(%w[--parser=parse.y], "#{<<~"begin;"}\n#{<<~'end;'}")