Add Stateful and Separate to caps package

Also: drop redundant statement in captureSetImpliedByFields

Author: odersky

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -980,8 +980,6 @@ class CheckCaptures extends Recheck, SymTransformer:
       def impliedClassifiers(cls: Symbol): List[ClassSymbol] = cls match
         case cls: ClassSymbol =>
           var fieldClassifiers = knownFields(cls).flatMap(classifiersOfFreshInType)
-          if cls.typeRef.isMutableType then
-            fieldClassifiers = cls.classifier :: fieldClassifiers
           val parentClassifiers =
             cls.parentSyms.map(impliedClassifiers).filter(_.nonEmpty)
           if fieldClassifiers.isEmpty && parentClassifiers.isEmpty

library/src/scala/caps/package.scala

@@ -78,21 +78,32 @@ type Exclusive = ExclusiveCapability
  */
 trait Control extends SharedCapability, Classifier
 
-/** Marker trait for classes with methods that require an exclusive reference. */
+/** Marker trait for classes that can consult and change the global program state.
+ *  These  classes typically contain mutable variables and/or update methods.
+ */
 @experimental
-trait Mutable extends ExclusiveCapability
+trait Stateful extends ExclusiveCapability
 
-/** Marker trait for classes with reader methods, typically extended by Mutable classes */
+/** Marker trait for classes that produce fresh capabilities with their values. If a value of a type
+ *  extending Separate is created, a fresh `cap` is automatically added to the value's capture set.
+ */
 @experimental
-@deprecated
-trait Read extends Mutable
+trait Separate extends ExclusiveCapability
 
-/** Marker trait for classes that are not subject to scoping restrictions
- *  of captured capabilities.
+/** Marker trait for classes that are not subject to scoping restrictions of captured capabilities.
  */
 @experimental
 trait Unscoped extends ExclusiveCapability, Classifier
 
+@experimental
+trait Mutable extends Stateful, Separate //, Unscoped
+
+/** Marker trait for classes with reader methods, typically extended by Mutable classes */
+@experimental
+@deprecated
+trait Read extends Mutable
+
+
 /** Carrier trait for capture set type parameters */
 @experimental
 trait CapSet extends Any