AOT-compile generators

[davidmorgan]

Currently build_runner has to JIT-compile generators because there are some uses of mirrors: source_gen for identifying types, and build_runner seems to have some internal mirrors use.

This is slower than if they were AOT-compiled, something like 30%.

So it would be good if we could switch to AOT-compiled.

[scheglov]

Hm... Why does it use mirrors? When working with analyzer's Element, you can ask it for the URI and name, and this is all you need to identify the element. There might be some complications with exports, e.g. you want to say: the element named Widget from package:flutter/widget.dart, not from package:flutter/src/widgets/framework.dart. But this is solvable, make it Matcher(exportingLibrary: Uri.parse('package:flutter/widget.dart'), name: 'Widget'). At the time of matching, you have Element, so have access to libraries of the analysis context.

Actually, now that I think about it, I'm even more confused. To use mirrors, such mirrors based generation would have to depend on package:flutter, which makes no sense. The generator is not a Flutter application.

[davidmorgan]

source_gen uses mirrors here

https://github.com/dart-lang/source_gen/blob/master/source_gen/lib/src/type_checker.dart

to check whether a type seen via analysis is the same as one the generator can see in its runtime.

Many generators have a small, pure Dart package that contains annotations/interfaces, so the generator can indeed depend on the annotations/interfaces and see them at runtime. I agree it wouldn't work for package:flutter/widget.

The type_checker library provides two other ways that don't use mirrors, but I think many generators use the mirrors one because it's more convenient; and anyway the mirrors dep is a problem even if not used.

build_runner uses mirrors here

https://github.com/dart-lang/build/blob/master/build_runner_core/lib/src/changes/build_script_updates.dart

to check whether its own sources changed.

[rrousselGit]

I've never used the mirror API of TypeChecker personally.
I maintain my own fork of TypeChecker that's mirror-free and use it in custom_lint.

Instead what I use is a custom API that uses the package name + class name:

final checker = TyperChecker.name('Riverpod', package: 'riverpod_annotation');

That's generally enough. Packages pretty much never expose two classes with the same name,
And that's less brittle than using the full path to a class like riverpod_annotation/lib/src/riverpod.dart as those may change over time