for loop expression can now have generated iterator's called (#21627)

A for expression now can have a generated iterator, allowing for more composable iterables

(cherry picked from commit 686c75cef0)

compiler/semstmts.nim

@@ -975,7 +975,8 @@ proc semFor(c: PContext, n: PNode; flags: TExprFlags): PNode =
   result = n
   n[^2] = semExprNoDeref(c, n[^2], {efWantIterator})
   var call = n[^2]
-  if call.kind == nkStmtListExpr and isTrivalStmtExpr(call):
+
+  if call.kind == nkStmtListExpr and (isTrivalStmtExpr(call) or (call.lastSon.kind in nkCallKinds and call.lastSon[0].sym.kind == skIterator)):
     call = call.lastSon
     n[^2] = call
   let isCallExpr = call.kind in nkCallKinds

tests/iter/tgeniteratorinblock.nim

@@ -0,0 +1,54 @@
+discard """
+  output: '''30
+60
+90
+150
+180
+210
+240
+60
+180
+240
+[60, 180, 240]
+[60, 180]'''
+"""
+import std/enumerate
+
+template map[T; Y](i: iterable[T], fn: proc(x: T): Y): untyped =
+  iterator internal(): Y  {.gensym.} =
+    for it in i:
+      yield fn(it)
+  internal()
+
+template filter[T](i: iterable[T], fn: proc(x: T): bool): untyped =
+  iterator internal(): T {.gensym.} =
+    for it in i:
+      if fn(it):
+        yield it
+  internal()
+
+template group[T](i: iterable[T], amount: static int): untyped =
+  iterator internal(): array[amount, T] {.gensym.} =
+    var val: array[amount, T]
+    for ind, it in enumerate i:
+      val[ind mod amount] = it
+      if ind mod amount == amount - 1:
+        yield val
+  internal()
+
+var a = [10, 20, 30, 50, 60, 70, 80]
+
+proc mapFn(x: int): int = x * 3
+proc filterFn(x: int): bool = x mod 20 == 0
+
+for x in a.items.map(mapFn):
+  echo x
+
+for y in a.items.map(mapFn).filter(filterFn):
+  echo y
+
+for y in a.items.map(mapFn).filter(filterFn).group(3):
+  echo y
+
+for y in a.items.map(mapFn).filter(filterFn).group(2):
+  echo y