Fixes nim-lang#5738

lib/pure/asyncmacro.nim

@@ -28,7 +28,7 @@ template createCb(retFutureSym, iteratorNameSym,
                   name, futureVarCompletions: untyped) =
   var nameIterVar = iteratorNameSym
   #{.push stackTrace: off.}
-  proc cb {.closure,gcsafe.} =
+  proc cb {.closure.} =
     try:
       if not nameIterVar.finished:
         var next = nameIterVar()
@@ -38,7 +38,8 @@ template createCb(retFutureSym, iteratorNameSym,
                     "`nil` Future?"
             raise newException(AssertionError, msg % name)
         else:
-          next.callback = cb
+          {.gcsafe.}:
+            next.callback = (proc() {.closure, gcsafe.})(cb)
     except:
       futureVarCompletions
 
@@ -379,7 +380,10 @@ proc asyncSingleProc(prc: NimNode): NimNode {.compileTime.} =
                                   procBody, nnkIteratorDef)
     closureIterator.pragma = newNimNode(nnkPragma, lineInfoFrom=prc.body)
     closureIterator.addPragma(newIdentNode("closure"))
-    closureIterator.addPragma(newIdentNode("gcsafe"))
+
+    # If proc has an explicit gcsafe pragma, we add it to iterator as well.
+    if prc.pragma.findChild(it.kind in {nnkSym, nnkIdent} and $it == "gcsafe") != nil:
+      closureIterator.addPragma(newIdentNode("gcsafe"))
     outerProcBody.add(closureIterator)
 
     # -> createCb(retFuture)

tests/async/tasync_gcsafe.nim

@@ -0,0 +1,36 @@
+discard """
+  cmd: "nim c --threads:on $file"
+  output: '''
+1
+2
+3
+'''
+"""
+
+assert compileOption("threads"), "this test will not do anything useful without --threads:on"
+
+import asyncdispatch
+
+var globalDummy: ref int
+proc gcUnsafeProc() =
+    if not globalDummy.isNil:
+        echo globalDummy[]
+    echo "1"
+
+proc gcSafeAsyncProcWithNoAnnotation() {.async.} =
+    echo "2"
+
+proc gcSafeAsyncProcWithAnnotation() {.gcsafe, async.} =
+    echo "3"
+
+proc gcUnsafeAsyncProc() {.async.} =
+    # We should be able to call gcUnsafe
+    gcUnsafeProc()
+
+    # We should be able to call async implicitly gcsafe
+    await gcSafeAsyncProcWithNoAnnotation()
+
+    # We should be able to call async explicitly gcsafe
+    await gcSafeAsyncProcWithAnnotation()
+
+waitFor gcUnsafeAsyncProc()

tests/async/tasync_gcunsafe.nim

@@ -0,0 +1,30 @@
+discard """
+  cmd: "nim c --threads:on $file"
+  file: "asyncmacro.nim"
+  errormsg: "'anotherGCSafeAsyncProcIter' is not GC-safe as it calls 'asyncGCUnsafeProc'"
+"""
+
+assert compileOption("threads"), "this test will not do anything useful without --threads:on"
+
+import asyncdispatch
+
+var globalDummy: ref int
+proc gcUnsafeProc() =
+    if not globalDummy.isNil:
+        echo globalDummy[]
+
+proc asyncExplicitlyGCSafeProc() {.gcsafe, async.} =
+    echo "hi"
+
+proc asyncImplicitlyGCSafeProc() {.async.} =
+    echo "hi"
+
+proc asyncGCUnsafeProc() {.async.} =
+    gcUnsafeProc()
+
+proc anotherGCSafeAsyncProc() {.async, gcsafe.} =
+    # We should be able to call other gcsafe procs
+    await asyncExplicitlyGCSafeProc()
+    await asyncImplicitlyGCSafeProc()
+    # But we can't call gcunsafe procs
+    await asyncGCUnsafeProc()