Make Pure a class in scala that gets erased

The Pure marker trait is now a class in the scala package that
gets erased to Object.

Author: odersky

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

@@ -976,6 +976,8 @@ class Definitions {
     @tu lazy val Caps_unsafeBoxFunArg: Symbol = CapsUnsafeModule.requiredMethod("unsafeBoxFunArg")
     @tu lazy val Caps_SealedAnnot: ClassSymbol = requiredClass("scala.caps.Sealed")
 
+  @tu lazy val PureClass: Symbol = requiredClass("scala.Pure")
+
   // Annotation base classes
   @tu lazy val AnnotationClass: ClassSymbol = requiredClass("scala.annotation.Annotation")
   @tu lazy val StaticAnnotationClass: ClassSymbol = requiredClass("scala.annotation.StaticAnnotation")
@@ -2041,15 +2043,17 @@ class Definitions {
   def isValueSubClass(sym1: Symbol, sym2: Symbol): Boolean =
     valueTypeEnc(sym2.asClass.name) % valueTypeEnc(sym1.asClass.name) == 0
 
-  @tu lazy val specialErasure: SimpleIdentityMap[Symbol, ClassSymbol] =
-    SimpleIdentityMap.empty[Symbol]
-      .updated(AnyClass, ObjectClass)
-      .updated(MatchableClass, ObjectClass)
-      .updated(AnyValClass, ObjectClass)
-      .updated(SingletonClass, ObjectClass)
-      .updated(TupleClass, ProductClass)
-      .updated(NonEmptyTupleClass, ProductClass)
-      .updated(PairClass, ObjectClass)
+  @tu lazy val specialErasure: collection.Map[Symbol, ClassSymbol] =
+    val m = mutable.Map[Symbol, ClassSymbol]()
+    m(AnyClass) = ObjectClass
+    m(MatchableClass) = ObjectClass
+    m(PureClass) = ObjectClass
+    m(AnyValClass) = ObjectClass
+    m(SingletonClass) = ObjectClass
+    m(TupleClass) = ProductClass
+    m(NonEmptyTupleClass) = ProductClass
+    m(PairClass) = ObjectClass
+    m
 
   // ----- Initialization ---------------------------------------------------
 

library/src/scala/Pure.scala

@@ -0,0 +1,9 @@
+package scala
+import annotation.experimental
+
+/** A marker trait that declares that all inheriting classes are "pure" in the
+ *  sense that their values retain no capabilities including capabilities needed
+ *  to perform effects. This has formal meaning only under capture checking.
+ */
+@experimental trait Pure:
+  this: Pure =>

library/src/scala/caps.scala

@@ -40,9 +40,3 @@ import annotation.experimental
    */
   @deprecated("The Sealed annotation should not be directly used in source code.\nUse the `sealed` modifier on type parameters instead.")
   class Sealed extends annotation.Annotation
-
-  /** Mixing in this trait forces a trait or class to be pure, i.e.
-   *  have no capabilities retained in its self type.
-   */
-  trait Pure:
-    this: Pure =>

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

@@ -1,4 +1,4 @@
-@annotation.experimental class C(x: () => Unit) extends caps.Pure // error
+@annotation.experimental class C(x: () => Unit) extends Pure // error
 
-@annotation.experimental class D(@annotation.constructorOnly x: () => Unit) extends caps.Pure // ok
+@annotation.experimental class D(@annotation.constructorOnly x: () => Unit) extends Pure // ok
 

tests/pos-with-compiler-cc/backend/jvm/BCodeHelpers.scala

@@ -209,7 +209,7 @@ trait BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
     }
   } // end of trait BCPickles
 
-  trait BCInnerClassGen extends caps.Pure {
+  trait BCInnerClassGen extends Pure {
 
     def debugLevel = 3 // 0 -> no debug info; 1-> filename; 2-> lines; 3-> varnames
 

tests/pos-with-compiler-cc/backend/jvm/BCodeIdiomatic.scala

@@ -17,7 +17,7 @@ import dotty.tools.dotc.report
  *  @version 1.0
  *
  */
-trait BCodeIdiomatic extends caps.Pure {
+trait BCodeIdiomatic extends Pure {
   val int: DottyBackendInterface
   final lazy val bTypes = new BTypesFromSymbols[int.type](int)
 

tests/pos-with-compiler-cc/backend/jvm/BTypes.scala

@@ -14,7 +14,7 @@ import scala.tools.asm
  * This representation is immutable and independent of the compiler data structures, hence it can
  * be queried by concurrent threads.
  */
-abstract class BTypes extends caps.Pure {
+abstract class BTypes extends Pure {
 
   val int: DottyBackendInterface
   import int.given
@@ -47,7 +47,7 @@ abstract class BTypes extends caps.Pure {
    * A BType is either a primitve type, a ClassBType, an ArrayBType of one of these, or a MethodType
    * referring to BTypes.
    */
-  /*sealed*/ trait BType extends caps.Pure { // Not sealed for now due to SI-8546
+  /*sealed*/ trait BType extends Pure { // Not sealed for now due to SI-8546
     final override def toString: String = this match {
       case UNIT   => "V"
       case BOOL   => "Z"

tests/pos-with-compiler-cc/backend/sjs/ScopedVar.scala

@@ -1,6 +1,6 @@
 package dotty.tools.backend.sjs
 
-class ScopedVar[A](init: A) extends caps.Pure {
+class ScopedVar[A](init: A) extends Pure {
   import ScopedVar.Assignment
 
   private[ScopedVar] var value = init

tests/pos-with-compiler-cc/dotc/ast/Positioned.scala

@@ -15,7 +15,7 @@ import annotation.internal.sharable
 
 /** A base class for things that have positions (currently: modifiers and trees)
  */
-abstract class Positioned(implicit @constructorOnly src: SourceFile) extends SrcPos, Product, Cloneable, caps.Pure {
+abstract class Positioned(implicit @constructorOnly src: SourceFile) extends SrcPos, Product, Cloneable, Pure {
   import Positioned.{ids, nextId, debugId}
 
   private var mySpan: Span = _

tests/pos-with-compiler-cc/dotc/cc/CaptureSet.scala

@@ -39,7 +39,7 @@ import annotation.retains
  *  if the mapped function is either a bijection or if it is idempotent
  *  on capture references (c.f. doc comment on `map` below).
  */
-sealed abstract class CaptureSet extends Showable, caps.Pure:
+sealed abstract class CaptureSet extends Showable, Pure:
   import CaptureSet.*
 
   /** The elements of this capture set. For capture variables,