Constant parameters

There is a request to add an annotation for parameters whose semantics ensures that the argument corresponding to the annotated parameter is a constant expression.

It occurred to me that this might be an interesting language feature. The proposal would be to allow the modifier const on parameter declarations and for the semantics to match those requested for the annotation. For example,

void f(const List<int> x) {}

void g(int a) {
  f(const [1, 2, 3]); // OK because it's explicitly const.
  f([1, 2, 3]); // OK because it's implicitly const.
  f([a]); // ERROR because `a` isn't a valid const expression
}

It might additionally provide some possibilities for compiler optimizations if it's known that the value of the parameter can't be modified.

I suspect that we'd want to introduce the annotation first and see how/whether it's used in practice, but I thought I'd mention it in case you'd want to track the work on the annotation.

Start by reading the linked Dart SDK request and the examples in this issue. Determine whether the annotation should be pursued before a const parameter modifier, then document the accepted semantics and language-specification changes; no implementation file or test is identified here.