treesitter: add node:field() to get field children

runtime/doc/lua.txt

@@ -628,6 +628,9 @@ tsnode:iter_children()					*tsnode:iter_children()*
 	Returns the child node plus the eventual field name corresponding to
 	this child node.
 
+tsnode:field({name})					*tsnode:field()*
+	Returns a table of the nodes corresponding to the {name} field.
+
 tsnode:child_count()					*tsnode:child_count()*
 	Get the node's number of children.
 

src/nvim/lua/treesitter.c

@@ -62,6 +62,7 @@ static struct luaL_Reg node_meta[] = {
   { "end_", node_end },
   { "type", node_type },
   { "symbol", node_symbol },
+  { "field", node_field },
   { "named", node_named },
   { "missing", node_missing },
   { "has_error", node_has_error },
@@ -653,6 +654,34 @@ static int node_symbol(lua_State *L)
   return 1;
 }
 
+static int node_field(lua_State *L)
+{
+  TSNode node;
+  if (!node_check(L, 1, &node)) {
+    return 0;
+  }
+
+  size_t name_len;
+  const char *field_name = luaL_checklstring(L, 2, &name_len);
+
+  TSTreeCursor cursor = ts_tree_cursor_new(node);
+
+  lua_newtable(L);  // [table]
+  unsigned int curr_index = 0;
+
+  if (ts_tree_cursor_goto_first_child(&cursor)) {
+    do {
+      if (!STRCMP(field_name, ts_tree_cursor_current_field_name(&cursor))) {
+        push_node(L, ts_tree_cursor_current_node(&cursor), 1);  // [table, node]
+        lua_rawseti(L, -2, ++curr_index);
+      }
+    } while (ts_tree_cursor_goto_next_sibling(&cursor));
+  }
+
+  ts_tree_cursor_delete(&cursor);
+  return 1;
+}
+
 static int node_named(lua_State *L)
 {
   TSNode node;

test/functional/lua/treesitter_spec.lua

@@ -151,6 +151,34 @@ void ui_refresh(void)
     }, res)
   end)
 
+  it('allows to get a child by field', function()
+    if not check_parser() then return end
+
+    insert(test_text);
+
+    local res = exec_lua([[
+      parser = vim.treesitter.get_parser(0, "c")
+
+      func_node = parser:parse():root():child(0)
+
+      local res = {}
+      for _, node in ipairs(func_node:field("type")) do
+        table.insert(res, {node:type(), node:range()})
+      end
+      return res
+    ]])
+
+    eq({{ "primitive_type", 0, 0, 0, 4 }}, res)
+
+    local res_fail = exec_lua([[
+      parser = vim.treesitter.get_parser(0, "c")
+
+      return #func_node:field("foo") == 0
+    ]])
+
+    assert(res_fail)
+  end)
+
   local query = [[
     ((call_expression function: (identifier) @minfunc (argument_list (identifier) @min_id)) (eq? @minfunc "MIN"))
     "for" @keyword