Implicit return working for async proc (#20933)

* Implicit return working for asyncdispatch proc

Closes #11558

* Test case

* Test that return value is actually used

* Update tests/async/t11558.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

lib/pure/asyncmacro.nim

@@ -214,9 +214,18 @@ proc asyncSingleProc(prc: NimNode): NimNode =
   # don't do anything with forward bodies (empty)
   if procBody.kind != nnkEmpty:
     # fix #13899, defer should not escape its original scope
-    procBody = newStmtList(newTree(nnkBlockStmt, newEmptyNode(), procBody))
+    let blockStmt = newStmtList(newTree(nnkBlockStmt, newEmptyNode(), procBody))
-    procBody.add(createFutureVarCompletions(futureVarIdents, nil))
+    procBody = newStmtList()
     let resultIdent = ident"result"
+    procBody.add quote do:
+      template setResult(x: `subRetType`) {.used.} =
+        # If the proc has implicit return then this will get called
+        `resultIdent` = x
+      template setResult(x: untyped) {.used.} =
+        # If the proc doesn't have implicit return then this will get called
+        x
+    procBody.add newCall(ident"setResult", blockStmt)
+    procBody.add(createFutureVarCompletions(futureVarIdents, nil))
     procBody.insert(0): quote do:
       {.push warning[resultshadowed]: off.}
       when `subRetType` isnot void:

tests/async/t11558.nim

@@ -0,0 +1,13 @@
+discard """
+output: "Hello\n9"
+"""
+import std/asyncdispatch
+
+proc foo(): Future[string] {.async.} =
+  "Hello"
+
+proc bar(): Future[int] {.async.} =
+  result = 9
+
+echo waitFor foo()
+echo waitFor bar()