CC: Various fixes and simplifications

[odersky]

• Propagate expected type into blocks

  The level checking should do the proper avoidance so that local symbols
  would not leak into type variables of the expected type.

• Don't automatically add a FreshCap owner to its hidden set

• Don't use span capture sets for function types as the underlying set

Based on #23874

[Linyxus]

The changes looks great to me!

Though I believe the original issue remains unsound. A mini example:

import language.experimental.captureChecking
import caps.*
class Console extends SharedCapability:
  def println(msg: Any): Unit = ()
class C1:
  this: C1^ =>
  val console: Console^ = Console()
  val foo: () ->{this.console} Unit = () =>
    console.println(42)
def expect(x: C1^)(op: () ->{x.console} Unit): () ->{x} Unit = op
def test(whatever: () => Unit): Unit =
  val c1: C1^{} = C1()
  val pureFoo: () -> Unit = expect(c1)(c1.foo)
  // ^^^ the same console effect, but pure!

The problem is that although this.console is properly tracked inside the class, but the class that creates and uses the console capability (class C1) remains pure. Then, with a simple trick, we can treat an operation with I/O effect as pure (the pureFoo line).

But maybe this can be addressed in a follow-up PR? I am approving this one for now.

[odersky]

Yes, we are not there yet. There will be a follow-up PR to address the following:

• Currently markFree treats FreshCaps generated by fields as non-visible since they are contained in the class symbol. So they are dropped from the capset of the class. We need to treat them as visible, except for FreshCaps in parameters, which are already handled in augmentConstructor.
• We further need to convert field FreshCaps to ResultCaps in the return type of the constructor, so that they are freshly instantiated on each call.