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Exclusive capabilities 3rd attempt #22588

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@odersky odersky commented Feb 12, 2025

Cherry-picked exclusive capabilities PR and fixes on top of main.

We should use this as a basis for experiments to find the root issue for the CI timeouts. We should merge only if all merge tests are green (last time it was test-non-bootstrapped that failed).

When printing a type `C^` where `C` extends `Capability`, don't show the `^`.
This is overridden under -Yprint-debug.
 - Add Mutable trait and mut modifier.
 - Add dedicated tests `isMutableVar` and `isMutableVarOrAccessor`
   so that update methods can share the same flag `Mutable` with mutable vars.
 - Disallow update methods overriding normal methods
 - Disallow update methods which are not members of classes extending Mutable
 - Add design document from papers repo to docs/internals
 - Add readOnly capabilities
 - Implement raeadOnly access
 - Check that update methods are only called on references with exclusive capture sets.
 - Use cap.rd as default capture set of Capability subtypes
 - Make Mutable a Capability, this means Mutable class references get {cap.rd} as
   default capture set.
 - Use {cap} as captu
….toCap

If existentials are mapped to fresh, it matters where they are opened. Pure or not
arguments don't have anything to do with that.
These are represented as Fresh.Cap(hidden) where hidden is the set of
capabilities subsumed by a fresh. The underlying representation is as
an annotated type `T @annotation.internal.freshCapability`.

Require -source `3.7` for caps to be converted to Fresh.Cap

Also:

 - Refacture and document CaputureSet
 - Make SimpleIdentitySets showable
 - Refactor VarState
    - Drop Frozen enum
    - Make VarState subclasses inner classes of companion object
    - Rename them
    - Give implicit parameter VarState of subCapture method a default value
 - Fix printing of capturesets containing cap and some other capability
 - Revise handing of @uncheckedAnnotation
Check separation from source 3.7 on.

We currently only check applications, other areas of separation checking
are still to be implemented.
Check that a capability that gets hidden in the (result-)type of some
definition is not used afterwards in the same or a nested scope.
When checking whether two items overlap we should always check their deep capture sets.
Buried aliases should count as well.
This is necessary since capability sets are IdentitySets.
TODO:

 - check that only @consume parameters flow to @consume parameters
Downgrade to -source 3.6 to turn it off.
Also: Fixes to computations of overlapWith and -- on Refs that take
account of pathss, where shorter paths cover deeper ones.
This partially reverts commit 7b3d3f4.
It looks like this fixes the problems we had with CI timeouts as well.
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