feat/ Add @mustBeConst lint

[jeffkwoh]

Add a lint for dart parameters that allows the analyzer to check if parameters are const.

Sample Use cases

When parameters are compile time constants, it provides the guarantee that the parameters cannot be user supplied. This involves the overall security of our applications, and reduces the need for security and privacy reviews when code can be formally verified to be safe. This lead to the following use cases:

Designing API for SQL methods that are guaranteed against SQL injection

/// Any call to this method cannot suffer from SQL injection, since param1 and /// param2 must be const. If the strings are interpolated strings, the
/// parameters in the interpolated strings should be const as well.
void makeDbCall(@mustBeConst String param1, @mustBeConst String param2) {
  db.query("FROM DBLICIOUS SELECT $PARAM1, $PARAM2");
}

Logging libraries can ensure that there is no PII in log messages from prod

/// Any call to this method guarantees that there is no PII since const 
/// parameters cannot be user generated.
void logInfoNoPii(
  @mustBeConst String logMessage, 
  @mustBeConst DartObject object, 
  @mustBeConst Error error
) {
  logger.info(...);
}

• Dart SDK Version (dart --version)
  google's internal version

Internal link: b/190345386

[davidmorgan]

Another use case: it looks like I could use this to add const constructors to built_collection classes, which is an oft-requested feature.

const BuiltList.compileTime(@mustBeConst List<E> list) : _list = list;

Without the annotation this is incorrect because list might be mutable, in which case it needs to be copied.

[simolus3]

This would also be useful to denote functions that end up being sent across isolates. These have to be top-level or static, so effectively constant:

Future<T> compute(@mustBeConst FutureOr<T> Function() computation) {
  // computation will be sent over a port
}

Future<Isolate> spawn(@mustBeConst Function entrypoint) {
  // entrypoint will be passed to Isolate.spawn
}

[vsmenon]

Any updates here?

[simolus3]

I'm happy to work on this if no one else is yet.

[vsmenon]

@jeffkwoh is investigating with this internally ( b/190345386 ). If that goes well, we'll move it to the public linter.