analyzer_plugin_toolkit

Shared building blocks for annotation-driven Dart analyzer plugins.

These are the pieces such a plugin needs and none of them should have to write twice: recognising your annotation rather than a same-named one from somebody else's package, doing that fast enough to run on every node of every file, comparing elements the analyzer hands back in more than one shape, and following a value through the local aliases it hides behind.

Install

dependencies:
  analyzer_plugin_toolkit: ^1.0.0

import 'package:analyzer_plugin_toolkit/analyzer_plugin_toolkit.dart';

This package is for people writing an analyzer plugin. If you are looking to use lint rules in your own project, you want a plugin package, not this.

Requires Dart 3.13.2 or later. Analyzer plugins first appeared in 3.10, but the element APIs this package is written against settled after that.

What it gives you

API	Solves
AnnotationFinder	Does this declaration carry my annotation? Resolved by package, fast enough to ask everywhere.
ElementCache	Memoize a per-element answer without leaking memory in a long-running server.
normalizeElement	Compare elements the analyzer returns in more than one shape.
referencedElement	Find the declaration an expression denotes, through (), ! and this..
AliasResolver	Follow a value through the local names it hides behind.

The example/ directory sketches a complete rule using all five.

AnnotationFinder

The core of an annotation-driven rule: does this declaration carry my annotation?

final _annotations = AnnotationFinder('my_package');

bool isMarked(Element? element) => _annotations.has(element, 'Marker');

String? reasonFor(Element? element) =>
    _annotations.valueOf(element, 'Marker')?.getField('reason')?.toStringValue();

Two things make this more than a name comparison.

It resolves by package. Matching on the class name alone would let somebody else's Throws or Disposable drive rules that know nothing about it. Matching on the exact library URI would be too strict instead: a package may re-export its annotations from several libraries, and a rule should not care which one the user imported. So the finder compares the declaring package, which is the thing that actually identifies an annotation.

It is fast enough to run everywhere. Evaluating a constant is the most expensive thing a rule does, and real code is full of annotations belonging to somebody else (@override, @immutable, a generator's), so that evaluation is almost always wasted. A candidate is therefore rejected by name before it is evaluated. The remaining answer is memoized per element, because the same element is asked about once per mention though the answer depends only on the element.

ElementCache

The memoization underneath, usable directly:

final _isWidget = ElementCache<InterfaceElement, bool>('isWidget');

bool isWidget(InterfaceType type) =>
    _isWidget.of(type.element, () => _computeIsWidget(type));

Keyed with an Expando, so an entry lives exactly as long as the element it describes. That matters in the analysis server, which is long-running and rebuilds element models as files change: a plain Map would pin every element of every file ever analysed, and an LRU would need a size nobody can pick correctly.

Values are boxed so that null can be cached. "This element carries no annotation" is the common answer, and a cache that could not tell it apart from "not computed yet" would recompute on every mention, which is the case the cache exists for.

This is sound only because editing a file yields fresh element objects, so a memoized answer can never be read back for changed source. That guarantee is the analyzer's rather than this package's, so it is checked empirically rather than assumed:

dart run tool/verify_cache_invalidation.dart

Element helpers

normalizeElement undoes the two analyzer details that stop elements comparing by identity: reading a field resolves to its synthetic getter, and a member reached through a generic class resolves to a *Member wrapper. A rule comparing the raw elements would fail to recognise the field it is tracking.

referencedElement looks through parentheses, null-assertions and this. access to the declaration an expression denotes. Its anyTarget flag decides whose state counts: by default only the enclosing instance's own members resolve, so a rule about an object cleaning up after itself is not satisfied by somebody else's field of the same name.

AliasResolver

Follows a value through the local aliases it hides behind, within one function body:

final aliases = AliasResolver.forBody(body);
if (aliases.refersTo(expression, field)) { /* ... */ }

Resolution returns a set, because one expression can refer to several declarations: cond ? _a : _b is either one, and the loop variable of for (final c in [_a, _b]) takes both in turn. A local is an alias for its initializer only when it is never reassigned. The walk over the body is deferred until the first question is asked, so building a resolver for a member that turns out to have nothing of interest costs nothing.

Testing quick fixes

analyzer_testing ships a harness for rules but not for fixes. This package does not ship one either, deliberately: such a harness must import package:test, a library under lib/ may only import from dependencies, and making test a real dependency here would push it onto every plugin built on the toolkit, where it cannot coexist with the test_api that flutter_test pins.

The plugins built on this toolkit each keep one in their own test/src/, under about 200 lines. If you want the same thing, copy test/src/fix_harness.dart from any of them: it is MIT licensed.

Development

dart analyze --fatal-infos                    # must be clean
dart test                                     # the toolkit's own behaviour
dart run tool/verify_cache_invalidation.dart  # the cache cannot go stale

The package is developed inside the arxdeus/qol_lints pub workspace, alongside the plugins built on it, which is why its pubspec carries resolution: workspace. See CONTRIBUTING.md for working on it standalone, for the commit conventions, and for how a release is cut.

License

MIT. See LICENSE.