feat(treesitter): upstream get_root_for_node() as a node method

Util from the nvim-treesitter project.

src/nvim/lua/treesitter.c

@@ -89,6 +89,8 @@ static struct luaL_Reg node_meta[] = {
   { "next_named_sibling", node_next_named_sibling },
   { "prev_named_sibling", node_prev_named_sibling },
   { "named_children", node_named_children },
+  { "root", node_root },
+
   { NULL, NULL }
 };
 
@@ -1088,6 +1090,27 @@ static int node_named_children(lua_State *L)
   return 1;
 }
 
+static int node_root(lua_State *L)
+{
+  TSNode parent;
+  TSNode result;
+
+  TSNode node;
+  if (!node_check(L, 1, &node)) {
+    return 0;
+  }
+  parent = node;
+  result = node;
+
+  while (!ts_node_is_null(parent)){
+    result = parent;
+    parent = ts_node_parent(result);
+  }
+
+  push_node(L, result, 1);
+  return 1;
+}
+
 /// assumes the match table being on top of the stack
 static void set_match(lua_State *L, TSQueryMatch *match, int nodeidx)
 {

test/functional/treesitter/node_spec.lua

@@ -77,4 +77,19 @@ describe('treesitter node API', function()
     eq(3, len)
     eq('<node compound_statement>', lua_eval('tostring(children[3])'))
   end)
+
+  it('can retrieve the tree root given a node', function()
+    insert([[
+      int main() {
+        int x = 3;
+      }]])
+
+    exec_lua([[
+      tree = vim.treesitter.get_parser(0, "c"):parse()[1]
+      root = tree:root()
+      node = root:child(0):child(2)
+    ]])
+
+    eq(lua_eval('tostring(root)'), lua_eval('tostring(node:root())'))
+  end)
 end)