fix unused result warning on implict return (fixes #414) (#417)

* fix unused result warning on implict return

* describe `{.used.}` tradeoffs

* oops

chronos/asyncmacro2.nim

@@ -175,9 +175,25 @@ proc asyncSingleProc(prc: NimNode): NimNode {.compileTime.} =
           nnkElseExpr.newTree(
             newStmtList(
               quote do: {.push warning[resultshadowed]: off.},
-              # var result: `baseType`
+              # var result {.used.}: `baseType`
-              nnkVarSection.newTree(
+              # In the proc body, result may or may not end up being used
-                nnkIdentDefs.newTree(ident "result", baseType, newEmptyNode())),
+              # depending on how the body is written - with implicit returns /
+              # expressions in particular, it is likely but not guaranteed that
+              # it is not used. Ideally, we would avoid emitting it in this
+              # case to avoid the default initializaiton. {.used.} typically
+              # works better than {.push.} which has a tendency to leak out of
+              # scope.
+              # TODO figure out if there's a way to detect `result` usage in
+              #      the proc body _after_ template exapnsion, and therefore
+              #      avoid creating this variable - one option is to create an
+              #      addtional when branch witha fake `result` and check
+              #      `compiles(procBody)` - this is not without cost though
+              nnkVarSection.newTree(nnkIdentDefs.newTree(
+                nnkPragmaExpr.newTree(
+                  ident "result",
+                  nnkPragma.newTree(ident "used")),
+                baseType, newEmptyNode())
+                ),
               quote do: {.pop.},
             )
           )

tests/testmacro.nim

@@ -177,6 +177,10 @@ suite "Macro transformations test suite":
       of false: await implicit7(v)
       of true: 42
 
+    proc implicit9(): Future[int] {.async.} =
+      result = 42
+      result
+
     let fin = new int
     check:
       waitFor(implicit()) == 42
@@ -193,6 +197,8 @@ suite "Macro transformations test suite":
       waitFor(implicit8(true)) == 42
       waitFor(implicit8(false)) == 33
 
+      waitFor(implicit9()) == 42
+
 suite "Closure iterator's exception transformation issues":
   test "Nested defer/finally not called on return":
     # issue #288