[Bug]: `treesit-query-expand` writes `nil` as `()`

[oysmuc]

Bug summary

Given the same input, GNU Emacs and neomacs produce different output:

	treesit-query-expand '((nil) @f)
GNU Emacs 31.1	"(nil) @f"
neomacs 0.0.19	"(()) @f"

() and nil are not the same thing in a tree-sitter query.
The tree-sitter query syntax reference says a node pattern is

a pair of parentheses containing two things: the node's type

so (()) contains no type name and matches nothing the syntax defines.

Steps to reproduce

Side-by-side comparison

emacs   --batch -Q -l treesit-nil-demo.el
neomacs --batch -Q -l treesit-nil-demo.el

The acceptance check needs a grammar for any of go, ruby or lua.

;;; treesit-nil-demo.el --- -*- lexical-binding: t; -*-

(require 'treesit)

(defvar tnd-queries
  '(((nil) @f)
    ([(false) (nil) (true)] @f)
    ([(false) (true)] @f))
  "Query sexps to pass to `treesit-query-expand'.")

(defvar tnd-dirs
  (let ((dirs (list (expand-file-name "tree-sitter/" user-emacs-directory)
                    (expand-file-name "~/.local/share/emacs/tree-sitter/")
                    (expand-file-name "~/.emacs.d/tree-sitter/"))))
    (when (boundp 'treesit-extra-load-path)
      (setq treesit-extra-load-path
            (append (seq-filter #'file-directory-p dirs) treesit-extra-load-path)))
    dirs)
  "Where this script looks for grammars, so it needs no extra flags.")

(defvar tnd-lang (seq-find #'treesit-language-available-p '(go ruby lua)))

(dolist (q tnd-queries)
  (let ((out (treesit-query-expand q)))
    (princ (format "in        %S\n" q))
    (princ (format "out       %S\n" out))
    (princ (format "accepted  %s\n\n"
                   (cond ((null tnd-lang) "(no grammar found)")
                         ((ignore-errors (treesit-query-compile tnd-lang out t) t) "yes")
                         (t "no"))))))

(princ (if tnd-lang
           (format "(grammar: %s)\n" tnd-lang)
         (format "(no grammar found; looked in %s)\n" (string-join tnd-dirs " "))))

Output, same grammar files:

** GNU Emacs 31.1
in        ((nil) @f)
out       "(nil) @f"
accepted  yes

in        ([(false) (nil) (true)] @f)
out       "[(false) (nil) (true)] @f"
accepted  yes

in        ([(false) (true)] @f)
out       "[(false) (true)] @f"
accepted  yes

(grammar: go)

** neomacs 0.0.19
in        ((nil) @f)
out       "(()) @f"
accepted  no

in        ([(false) (nil) (true)] @f)
out       "[(false) (()) (true)] @f"
accepted  no

in        ([(false) (true)] @f)
out       "[(false) (true)] @f"
accepted  yes

(grammar: go)

The inputs are identical on both builds. The outputs are not, and each
build's own output is then handed back to the same grammar: GNU's is
accepted, neomacs's is rejected. The third query, which contains no nil,
expands the same way on both and is accepted by both.

go-ts-mode, ruby-ts-mode and lua-ts-mode all match nil literals in
their font-lock rules, so on neomacs opening a .go, .rb or
.lua file gives:

Warning (treesit-font-lock-rules-mismatch): Emacs cannot compile every
font-lock rules because a mismatch between the grammar and the rules. [...]
This error can be fixed by either downgrading the grammar (tree-sitter-go)
on your system, or upgrading the major mode package.  The following are the
temporarily disabled features:

- `constant' for go.

The same grammar files work on GNU Emacs.

Expected behavior

No response

Neomacs --version output

GNU Emacs 31.1 (emacs-plus)
macOS 27.0 (Darwin 27.0.0), arm64

Neomacs 0.0.19
Git commit: b715d50bd26cceb537e905adf1cc3bfbf46fd340
Source date: 2026-09-20T12:37:58.000000000-04:00
Build: release for aarch64-apple-darwin with rustc 1.96.1
Standalone Rust binary for Neomacs (no C dependency)

Platform

macOS

Install source

Prebuilt release asset (downloaded from Releases)

Extra context

No response