TwigQI: Static code analysis for Twig templates

Twig Quality Inspections is an extension to the Twig templating engine which adds static analysis (i.e., compile-time) and runtime assertions to increase templates' quality.

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 other compile-time or 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.

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 enabled.

This won't solve every single edge case or possibility, plus it's opinionated. You've been warned! ;p

Justification

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

{% macro userCard(user, showBadge = false) %}
  {% types {
    user: '\\User',
    showBadge: 'boolean',
  } %}
  
  {{ 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.

Typing system and syntax

Many inspections rely on proper typing. However, the documentation for the types tag explicitly avoids specifying the syntax or contents of types.

So how should developers declare types? While PHP developers are often familiar with PHPStan, Twig template designers may instead be used to TypeScript.

The Twig documentation sums up its stance succinctly:

Twig tries to abstract PHP types as much as possible and works with a few basic types[.]

Therefore, TwigQI uses the basic types described by Twig, while defining syntax for iterables. The goal is to have a simple type system that's easy to learn and use, and which should cover the vast majority of use cases.

Your preferences and/or requirements may very well differ.

Here's the list of types supported by TwigQI:

• Scalar: string, number, boolean, null, object (although a class is preferred)

• Classes, interfaces and traits (FQN, with a starting backslash. Note that backslashes must be escaped in Twig strings!)

• Three types of iterables, with increasing specificity

  • iterable declares nothing more or less than that the variable is iterable
  • iterable<ValueType> declares the values' type
  • iterable<number, ValueType> and iterable<string, ValueType> does the same for keys

  You can create recursive types: iterable<string, iterable<number, iterable<string>>>

• Lastly, mixed allows you to declare that a variable is defined without specifying a concrete type.

Any type can be prefixed with ? to make it nullable.

Note that there's no dedicated syntax for iterables with particular, known keys. Nor can you declare that values have different types. You could use one of the iterable variants (e.g., iterable<string, mixed>), but I would humbly recommend using a readonly class to act as a view model.

Inspections

Here's the list of inspections already considered relevant and feasible.

Those marked with ⌛ are planned / considered, while ✅ means the inspection is implemented.

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 a similar project: TwigStan.

Typed variables

• ✅ Declared types is invalid (e.g., {% types {i: 'nit'} %})

• ✅ Runtime: non-optional variable is not defined

• ✅ Runtime: non-nullable variable is null

• ✅ Runtime: variable does not match type

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

• ⌛ Undocumented context variable (i.e., missing in {% 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

While Twig considers all macro arguments optional (and provides null as a default), TwigQI considers arguments with no explicit default value as required.

• ⌛ Arguments not declared using types
• ✅ Undefined variable used (arguments, {% set %}, etc)
• ✅ Call with too many arguments (except if varargs is used)
• ✅ Call with too few arguments
• ✅ Required argument declared after optional
• ⌛ Type mismatch in macro call

Acknowledgments

Big thanks to Ruud Kamphuis for TwigStan, and for helping on this very project.