pkgs/record_use/README.md

This package provides the data classes for the usage recording feature in the
Dart SDK.

During compilation, usages of declarations annotated with `@RecordUse()` in
reachable code are recorded, and information about these usages is made available
to post-compile steps (such as link hooks). Usages in unreachable code are not
recorded.

- If placed on a statically resolved function or member (such as a top-level
  function, static method, non-redirecting factory constructor, getter,
  setter, operator, extension method, or extension type method), all calls
  to or tear-offs of that member in reachable code will be recorded, along
  with arguments passed to the call as far as they can be evaluated as
  constant expressions at compile time. Generative constructors and
  redirecting factory constructors cannot be annotated directly.
- If placed on a class, any constant instance of the class (including
  instances created via `const` redirecting factory constructors), any
  non-const generative constructor invocation, and any generative
  constructor tear-off in reachable code will be recorded. Calls to
  non-const factory constructors are not recorded directly by annotating the
  class; rather, any generative constructor invocation within the factory
  body will be recorded.

Only usages in executable code are recorded. Usages appearing within metadata
(annotations) are ignored.

> [!NOTE]
> The `@RecordUse()` annotation is only allowed on declarations within a package's
> `lib/` directory.

## Example

<!-- file://./example/api/usage.dart#usage -->
```dart
void main() {
  PirateTranslator.speak('Hello');
  print(const PirateShip('Black Pearl', 50));
}

abstract class PirateTranslator {
  @RecordUse()
  static String speak(String english) => 'Ahoy $english';
}

@RecordUse()
final class PirateShip {
  final String name;
  final int cannons;

  const PirateShip(this.name, this.cannons);
}
```
This code will generate a data file that contains both the field values of
the `PirateShip` instances, as well as the arguments for the `speak`
method annotated with `@RecordUse()`.

This information can then be accessed in a link hook as follows:
<!-- file://./example/api/usage_link.dart#link -->
```dart
void main(List<String> arguments) {
  link(arguments, (input, output) async {
    final uses = input.recordedUses;
    if (uses == null) return;

    final calls = uses.calls[methodId] ?? [];
    for (final call in calls) {
      switch (call) {
        case CallWithArguments(
          positionalArguments: [StringConstant(value: final english), ...],
        ):
          // Shrink a translations file based on all the different translation
          // keys.
          print('Translating to pirate: $english');
        case _:
          print('Cannot determine which translations are used.');
      }
    }

    final ships = uses.instances[classId] ?? [];
    for (final ship in ships) {
      switch (ship) {
        case InstanceConstantReference(
          instanceConstant: InstanceConstant(
            fields: {'name': StringConstant(value: final name)},
          ),
        ):
          // Include the 3d model for this ship in the application but not
          // bundle the other ships.
          print('Pirate ship found: $name');
        case _:
          print('Cannot determine which ships are used.');
      }
    }
  });
}
```

## Contributing
Contributions are welcome! Please open an issue or submit a pull request.