TwigQI: The Twig Quality Inspector extension

This repo offers an extension to the Twig templating engine which adds compile-time and runtime code quality inspections for templates.

Unlike other projects like curlylint and djLint, which focus on HTML, this tool exclusively analyzes the Twig code.

While the Twig compiler will throw SyntaxErrors, there are few runtime checks.

For example, a RuntimeError is thrown when trying to use an undeclared variables (if strict_variables is enabled). However, this only happens if the code is actually executed. In the logic is non-trivial, neither developer nor tests (if any even exist) might detect obvious mistakes such as a misspelled variable name.

This project provides ways to increase code quality and prevent defects by adding compile-time inspections.

The two intended use cases are:

• Add the extension to the Twig\Environment during development
• Invoke a CLI command in CI and/or pre-commit hook which compiles all templates with the extension

Justification

Just in case you need convincing, please consider the following example:

{% macro userCard(user, showBadge = false) %}
  {# @var user \User #}
  {# @var showBadge bool #}
  
  {{ user.name }}
  {% if showBadge %}
    {% if usr.admin %} {# Oops #}
      (admin)
    {% else if user.role %}
      ({{ user.getRoleLabel(usr.role) }}) {# Uh oh! #}
    {% endif %}
  {% endif %}
{% endmacro %}

Here, usr.admin is obviously a typo. Fortunately, this bug is easily detected with strict_types enabled, but only if the macro is called with showBadge=true, which might be uncommon enough to go unnoticed during development. In this example, the (admin) badge will simply never be printed in production, where strict_types is likely disabled. A bug for sure, but perhaps not a critical one.

However, user.getRoleLabel(usr.role) will cause an uncaught TypeError if that method's parameter is not nullable, since Twig will call that method with null. Instead of just having a buggy badge, the whole page breaks.

Usage

Add the extension to your Twig\Environment.

Any issues will be reported using PHP's trigger_error with E_USER_* levels. Set up your app and/or CI build to report these as you see fit.

Design

The current design uses NodeVisitor classes for every inspection. That allows for easy testing and configurability.

The reason the inspections use trigger_error instead of Exceptions is that the latter would halt compilation, preventing the extension from reporting multiple issues in one go.

The level of error (error, warning, notice) depends entirely on the authors' opinions on code quality. E_USER_ERROR is used for, well, errors, that the author(s) deem actual errors in code. For more opinionated issues (e.g., relying on macro arguments always being optional), E_USER_WARNING is used.

Challenges and limitations

While implementing the first inspections, I ran into some limitations in the Twig extension design.

One seemingly simple is that there was no way to distinguish whether a macro argument has an explicit null default or an implicit one. ExpressionParser->parseArguments() (line 628) created identical ASTs for both. I've created a PR to add a distinction in the AST, which has been merged.

Another is that there is no way to specify types. While the @var comments in the example are supported by the Symfony Support plugin for PHPStorm, Twig's parser does not add comments to the AST, meaning there's no way for extensions to process them. (A PR to add support was closed.)

The good news is that Twig recently added the types tag.

Inspections

Here's a list of inspections considered relevant.

Those marked with ⌛ are (considered) possible to implement once the PRs mentioned above have been merged.

Note that most of these could also be analyzed by PHPStan if it could properly understand (compiled) templates and how they are rendered. This is the aim of Ruud Kamphuis's similar project, TwigQI.

Variable types

• ⌛ Invalid type declared (e.g., {% types {i: 'nit'} %})

• ⌛ Runtime type doesn't match declaration

• ⌛ Invalid object property or method (e.g., {{ user.nmae }})

• ⌛ Undocumented context variable (i.e., missing {% types %})

• ⌛ Use of short-hand form (e.g., {{ user.admin }} instead of isAdmin) [Notice]

  Rationale: makes it hard to find usage of properties/methods

• Non-stringable variable in string interpolation

Constants

• ✅ Invalid constant (e.g., constant('BAD'))

• ✅ Expressions as first argument (e.g., constant('UH' ~ 'OH'))

  This is opinionated, as it can work perfectly fine

• ✅ Second argument (object) is not a name (e.g., constant('CONST', {}))

  This is opinionated, too: constant('CONST', foo ?: bar) can work fine

Macros

• ⌛ Arguments not declared using types
• ✅ Use of undefined variables (arguments, {% set %}, etc)
• ✅ Calls with too many arguments (except if varargs is used)
• ✅ Calls with too few arguments (arguments with no default are considered required)
• ✅ Required argument (no explicit default) declared after optional
• ⌛ Type mismatch in macro call