Add capture-checking annotations to scala.util.boundary

[natsukagami]

Adds capture-tracking annotations to scala.util.boundary.

The following changes are made:

• Label[T] is now a capability, directly inheriting caps.Capability.
• Break exception:
  • now does not expose the .label field. This is a source-breaking change. - however, label is still private[boundary], which should make it TASTy- and binary-compatible.
  • While we don't expect any code to inspect the values inside a Break exception, if some code is manually implementing boundary.apply's logic, we instead provide a isSameLabelAs method, that compares the given Label[T] with the inner Label without leaking it.
• Note that catching Break exceptions may still possibly cause them to escape the boundary.

To demonstrate that capture-checking works, a test is included.

[hamzaremmal]

Shouldn't cc changes in the library go to https://github.com/scala/scala3/tree/main/scala2-library-cc/src/scala and not the actual library ?

[natsukagami]

Shouldn't cc changes in the library go to https://github.com/scala/scala3/tree/main/scala2-library-cc/src/scala and not the actual library ?

scala.util.boundary is not part of scala2-library

[natsukagami]

@sjrd what's your opinion on the breaking changes?

[sjrd]

Breaking change or not, I don't think we have consensus about taking the risk of introducing CC annotations in the standard standard library.

So far, it has always been pretty clear that CC goes to an alternate library that people can depend on if they want to. Here this is a much bigger "attack surface".

At the very least this requires a Core discussion.

[hamzaremmal]

scala.util.boundary is not part of scala2-library

Right, I'm overlooked that. Nevertheless, I agree with @sjrd, we don't have a consensus about adding cc-related code in the stdlib.