Implement @use scheme

[odersky]

• Allow annotations like @use after capture types
• Implement @use scheme:
  • We allow @use on parameter types.
  • If we narrow cap to a reach capability under a @use, the reach capability becomes use-declared.
  • We print such reach capabilities as x*@use for debugging.
  • A use-declared capability can leak into the method owner the parameter.
  • User-written reach capabilities of parameters whose type carries a @use annotation are also allowed to leak into the enclosing method..
  • On the other hand, if we pass an actual argument(-part) to (part-of) a formal parameter type that is annotated with @use, we charge the deep capture set of the argument to the currently enclosing environments.
• Check that overridden methods are @use correct. This means if an overridden method has @use in some parameter types, the overriding method must have these parameter types under @use as well.
• Drop previous @unbox schme.
• Infer @use annotations. If a parameter has a type containing occurrences of cap and the parameter is not referred to from a function result, add an implied @use to the parameter type. The idea is that we'd pass a parameter with universal capturing type only if the parameter is either returned or used in the method. So if it is not returned we can infer an @use.
• Generalize the scheme to also handle capture set variables in addition to reach capabilities.

[odersky]

Built on the paths PR