Add a hook for post-ANF transformation

For use by custom implementations of the async macro.

Author: retronym

src/main/scala/scala/async/internal/AsyncTransform.scala

@@ -20,7 +20,9 @@ trait AsyncTransform {
     // Transform to A-normal form:
     //  - no await calls in qualifiers or arguments,
     //  - if/match only used in statement position.
-    val anfTree: Block = anfTransform(body)
+    val anfTree0: Block = anfTransform(body)
+
+    val anfTree = futureSystemOps.postAnfTransform(anfTree0)
 
     val resumeFunTreeDummyBody = DefDef(Modifiers(), name.resume, Nil, List(Nil), Ident(definitions.UnitClass), Literal(Constant(())))
 

src/main/scala/scala/async/internal/FutureSystem.scala

@@ -51,6 +51,9 @@ trait FutureSystem {
 
     def spawn(tree: Tree, execContext: Tree): Tree =
       future(Expr[Unit](tree))(Expr[ExecContext](execContext)).tree
+
+    /** A hook for custom macros to transform the tree post-ANF transform */
+    def postAnfTransform(tree: Block): Block = tree
   }
 
   def mkOps(c: SymbolTable): Ops { val universe: c.type }