Make Mutable extend Unscoped

Fix: the APPLY rule had to be changed so that it does not kick in if the
result has Unscoped capabilities. These need to be preserved, we cannot
approximate by the (possibly pure) function type.

Author: odersky

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

@@ -566,6 +566,7 @@ object Capabilities:
     def derivesFromCapability(using Context): Boolean = derivesFromCapTrait(defn.Caps_Capability)
     def derivesFromStateful(using Context): Boolean = derivesFromCapTrait(defn.Caps_Stateful)
     def derivesFromShared(using Context): Boolean = derivesFromCapTrait(defn.Caps_SharedCapability)
+    def derivesFromUnscoped(using Context): Boolean = derivesFromCapTrait(defn.Caps_Unscoped)
 
     /** The capture set consisting of exactly this reference */
     def singletonCaptureSet(using Context): CaptureSet.Const =

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

@@ -397,7 +397,7 @@ extension (tp: Type)
   def derivesFromCapability(using Context): Boolean = derivesFromCapTrait(defn.Caps_Capability)
   def derivesFromStateful(using Context): Boolean = derivesFromCapTrait(defn.Caps_Stateful)
   def derivesFromShared(using Context): Boolean = derivesFromCapTrait(defn.Caps_SharedCapability)
-  def derivesFromExclusive(using Context): Boolean = derivesFromCapTrait(defn.Caps_ExclusiveCapability)
+  def derivesFromUnscoped(using Context): Boolean = derivesFromCapTrait(defn.Caps_Unscoped)
 
   /** Drop @retains annotations everywhere */
   def dropAllRetains(using Context): Type = // TODO we should drop retains from inferred types before unpickling

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

@@ -864,6 +864,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             && !resultType.isBoxedCapturing
             && !tree.fun.symbol.isConstructor
             && !resultType.captureSet.containsResultCapability
+            && !resultType.captureSet.elems.exists(_.derivesFromUnscoped)
             && qualCaptures.mightSubcapture(refs)
             && argCaptures.forall(_.mightSubcapture(refs)) =>
           val callCaptures = argCaptures.foldLeft(qualCaptures)(_ ++ _)

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -288,7 +288,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
         if elideCapabilityCaps
             && parent.derivesFromCapability
             && refs.containsTerminalCapability
-            && (!parent.derivesFromExclusive || refs.isReadOnly)
+            && (!parent.derivesFromStateful || refs.isReadOnly)
         then toText(parent)
         else toTextCapturing(parent, refs, boxText)
       case tp @ RetainingType(parent, refSet) =>

library/src/scala/caps/package.scala

@@ -96,7 +96,7 @@ trait Separate extends Stateful
 trait Unscoped extends Stateful, Classifier
 
 @experimental
-trait Mutable extends Stateful, Separate//, Unscoped
+trait Mutable extends Stateful, Separate, Unscoped
 
 /** Marker trait for classes with reader methods, typically extended by Mutable classes */
 @experimental

tests/neg-custom-args/captures/check-inferred.check

@@ -58,4 +58,4 @@
    |           Fields capturing a root capability need to be given an explicit type unless the capability is already
    |           subsumed by the computed capability of the enclosing class.
    |
-   |           where:    ^ refers to a fresh root capability in the type of value count
+   |           where:    ^ refers to a fresh root capability classified as Unscoped in the type of value count

tests/neg-custom-args/captures/i23726.check

@@ -13,8 +13,8 @@
    |  Footprint set of function result      : {a}
    |  The two sets overlap at               : {a}
    |
-   |where:    ^  refers to a fresh root capability in the type of value a
-   |          ^² refers to a fresh root capability created in method test1 when checking argument to parameter x of method apply
+   |where:    ^  refers to a fresh root capability classified as Unscoped in the type of value a
+   |          ^² refers to a fresh root capability classified as Unscoped created in method test1 when checking argument to parameter x of method apply
 -- Error: tests/neg-custom-args/captures/i23726.scala:16:5 -------------------------------------------------------------
 16 |  f3(b)  // error
    |     ^
@@ -30,8 +30,8 @@
    |  Footprint set of function result      : {op, b}
    |  The two sets overlap at               : {b}
    |
-   |where:    ^  refers to a fresh root capability in the type of value b
-   |          ^² refers to a fresh root capability created in method test1 when checking argument to parameter x of method apply
+   |where:    ^  refers to a fresh root capability classified as Unscoped in the type of value b
+   |          ^² refers to a fresh root capability classified as Unscoped created in method test1 when checking argument to parameter x of method apply
 -- Error: tests/neg-custom-args/captures/i23726.scala:24:5 -------------------------------------------------------------
 24 |  f7(a)  // error
    |     ^
@@ -47,5 +47,5 @@
    |  Footprint set of function prefix      : {f7*, a, b}
    |  The two sets overlap at               : {a}
    |
-   |where:    ^  refers to a fresh root capability in the type of value a
-   |          ^² refers to a fresh root capability created in method test1 when checking argument to parameter x of method apply
+   |where:    ^  refers to a fresh root capability classified as Unscoped in the type of value a
+   |          ^² refers to a fresh root capability classified as Unscoped created in method test1 when checking argument to parameter x of method apply

tests/neg-custom-args/captures/i24310.scala

@@ -5,7 +5,7 @@ class Ref extends Mutable:
   def get(): Int = value
   update def set(v: Int): Unit = value = v
 
-class Matrix(val f: () => Int) extends Mutable:
+class Matrix(val f: () => Int) extends Stateful:
   self: Matrix^ =>
     def run() = f() // error <- note the missing update
     update def add(): Unit = ()

tests/neg-custom-args/captures/i24335.check

@@ -14,4 +14,4 @@
    |       Fields capturing a root capability need to be given an explicit type unless the capability is already
    |       subsumed by the computed capability of the enclosing class.
    |
-   |       where:    ^ refers to a fresh root capability in the type of value r
+   |       where:    ^ refers to a fresh root capability classified as Unscoped in the type of value r

tests/neg-custom-args/captures/lazyvals-sep.check

@@ -32,7 +32,7 @@
    |Note that {cap} is an exclusive capture set of the stateful type Ref^,
    |it cannot subsume a read-only capture set of the stateful type Ref^{TestClass.this.r.rd}.
    |
-   |where:    ^ and cap refer to a fresh root capability created in lazy value lazyVal8 when checking argument to parameter ref of constructor Wrapper
+   |where:    ^ and cap refer to a fresh root capability classified as Unscoped created in lazy value lazyVal8 when checking argument to parameter ref of constructor Wrapper
    |
    | longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/lazyvals-sep.scala:77:12 ---------------------------------
@@ -44,7 +44,7 @@
    |Note that {cap} is an exclusive capture set of the stateful type Ref^,
    |it cannot subsume a read-only capture set of the stateful type Ref^{TestClass.this.r.rd}.
    |
-   |where:    ^ and cap refer to a fresh root capability created in lazy value lazyVal9 when checking argument to parameter ref of constructor Wrapper
+   |where:    ^ and cap refer to a fresh root capability classified as Unscoped created in lazy value lazyVal9 when checking argument to parameter ref of constructor Wrapper
    |
    | longer explanation available when compiling with `-explain`
 -- Error: tests/neg-custom-args/captures/lazyvals-sep.scala:82:8 -------------------------------------------------------