Make static TermRefs trackable capabilities

Also:

 - Add interpolation of types of module vals, so that we can decide whether they are tracked or not
 - Treat non-capture checked global refs as non-trackable

Author: odersky

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

@@ -432,11 +432,18 @@ object Capabilities:
       core.isInstanceOf[RootCapability]
 
     /** Is the reference tracked? This is true if it can be tracked and the capture
-     *  set of the underlying type is not always empty.
+     *  set of the underlying type is not always empty. Also excluded are references
+     *  that come from source files that were not capture checked and that have
+     *  `Fluid` capture sets.
      */
     final def isTracked(using Context): Boolean = this.core match
       case _: RootCapability => true
-      case tp: CoreCapability => tp.isTrackableRef && !captureSetOfInfo.isAlwaysEmpty
+      case tp: CoreCapability =>
+        tp.isTrackableRef
+        && {
+          val cs = captureSetOfInfo
+          !cs.isAlwaysEmpty && cs != CaptureSet.Fluid
+        }
 
     /** An exclusive capability is a capability that derives
      *  indirectly from a maximal capability without going through

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

@@ -117,7 +117,7 @@ extension (tp: Type)
       !tp.underlying.exists // might happen during construction of lambdas with annotations on parameters
       ||
         ((tp.prefix eq NoPrefix)
-        || tp.symbol.isField && !tp.symbol.isStatic && tp.prefix.isTrackableRef
+        || tp.symbol.isField && tp.prefix.isTrackableRef
         ) && !tp.symbol.isOneOf(UnstableValueFlags)
     case tp: TypeRef =>
       tp.symbol.isType && tp.derivesFrom(defn.Caps_CapSet)

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

@@ -389,6 +389,9 @@ class CheckCaptures extends Recheck, SymTransformer:
 
     /** If `tpt` is an inferred type, interpolate capture set variables appearing contra-
      *  variantly in it. Also anchor Fresh instances with anchorCaps.
+     *  Note: module vals don't have inferred types but still hold capture set variables.
+     *  These capture set variables are interpolated after the associated module class
+     *  has been rechecked.
      */
     private def interpolateIfInferred(tpt: Tree, sym: Symbol)(using Context): Unit =
       if tpt.isInstanceOf[InferredTypeTree] then
@@ -1517,6 +1520,8 @@ class CheckCaptures extends Recheck, SymTransformer:
       finally
         checkExplicitUses(cls)
         checkExplicitUses(cls.primaryConstructor)
+        if cls.is(ModuleClass) then
+          interpolate(cls.sourceModule.info, cls.sourceModule)
         completed += cls
         curEnv = saved
     end recheckClassDef

compiler/src/dotty/tools/dotc/core/SymUtils.scala

@@ -313,7 +313,7 @@ class SymUtils:
       }
 
     def isField(using Context): Boolean =
-      self.isTerm && !self.isOneOf(Method | PhantomSymbol | NonMember)
+      self.isTerm && !self.isOneOf(Method | PhantomSymbol | NonMember | Package)
 
     def isEnumCase(using Context): Boolean =
       self.isAllOf(EnumCase, butNot = JavaDefined)

library/src/scala/collection/mutable/LongMap.scala

@@ -594,7 +594,7 @@ object LongMap {
   private final val VacantBit  = 0x4000_0000
   private final val MissVacant = 0xC000_0000
 
-  private val exceptionDefault: Long => Nothing = (k: Long) => throw new NoSuchElementException(k.toString)
+  private val exceptionDefault: Long -> Nothing = (k: Long) => throw new NoSuchElementException(k.toString)
 
   /** A builder for instances of `LongMap`.
     *

tests/neg/mixed_cc.check

@@ -0,0 +1,10 @@
+-- Warning: tests/neg/mixed_cc/B.scala:4:4 -----------------------------------------------------------------------------
+4 |val x: Object^{A.B} = ??? // error
+  |    ^
+  |    value x has a non-empty capture set but will not be added as
+  |    a capability to computed capture sets since it is globally accessible
+  |    as a top-level definition. Global values cannot be capabilities.
+-- Error: tests/neg/mixed_cc/B.scala:4:7 -------------------------------------------------------------------------------
+4 |val x: Object^{A.B} = ??? // error
+  |       ^^^^^^^^^^^^
+  |       mixed_cc.A.B.type cannot be tracked since its capture set is empty

tests/neg/mixed_cc/A.scala

@@ -0,0 +1,4 @@
+package mixed_cc
+
+object A:
+  object B

tests/neg/mixed_cc/B.scala

@@ -0,0 +1,4 @@
+package mixed_cc
+import language.experimental.captureChecking
+
+val x: Object^{A.B} = ??? // error