Enhancement: Introduce caps.Pure trait

This can replace the more obscure idiom of declaring a
non-capturing self type.

Author: odersky

library/src/scala/caps.scala

@@ -16,3 +16,9 @@ import annotation.experimental
    *  avoids the error that would be raised when unboxing `*`.
    */
   extension [T](x: T) def unsafeUnbox: T = x
+
+  /** 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 =>

project/MiMaFilters.scala

@@ -19,5 +19,6 @@ object MiMaFilters {
     ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$experimental$pureFunctions$"),
     ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$experimental$captureChecking$"),
     ProblemFilters.exclude[MissingClassProblem]("scala.caps"),
+    ProblemFilters.exclude[MissingClassProblem]("scala.caps$Pure"),
   )
 }