fixes #25122 #15353 #15353; Invalid C code generation with iterator with when nimvm

compiler/transf.nim

@@ -321,14 +321,27 @@ proc introduceNewLocalVars(c: PTransf, n: PNode): PNode =
     result = n
   of nkVarSection, nkLetSection:
     result = transformVarSection(c, n)
+    for i in 0..<result.len:
+      result[i] = introduceNewLocalVars(c, result[i])
+  of nkIdentDefs, nkVarTuple:
+    result = newTransNode(n)
+    for i in 0..<n.len-1:
+      result[i] = n[i]
+    result[^1] = introduceNewLocalVars(c, n[^1])
   of nkClosure:
     # it can happen that for-loop-inlining produced a fresh
     # set of variables, including some computed environment
     # (bug #2604). We need to patch this environment here too:
     let a = n[1]
     if a.kind == nkSym:
       n[1] = transformSymAux(c, a)
-    return n
+    if n[0].kind == nkSym:
+      result = n
+    else:
+      # introduces local vars for the lambda closures
+      result = newTransNode(n)
+      result[0] = introduceNewLocalVars(c, n[0])
+      result[1] = n[1]
   of nkProcDef, nkFuncDef, nkMethodDef, nkConverterDef: # todo optimize nosideeffects?
     result = newTransNode(n)
     let x = newSymNode(copySym(n[namePos].sym, c.idgen))
@@ -337,6 +350,12 @@ proc introduceNewLocalVars(c: PTransf, n: PNode): PNode =
     for i in 1..<n.len:
       result[i] = introduceNewLocalVars(c, n[i])
     result[namePos].sym.ast = result
+  of nkLambda:
+    result = newTransNode(n)
+    result[namePos] = n[namePos]
+    for i in 1..<n.len:
+      result[i] = introduceNewLocalVars(c, n[i])
+    result[namePos].sym.ast = result
   else:
     result = newTransNode(n)
     for i in 0..<n.len:

tests/iter/titer_issues.nim

@@ -432,3 +432,31 @@ block:
   let x = cast[typeof(aaa)](aaa)   # not even var
   for _ in x[]:
     discard
+
+iterator v(): int =
+  when nimvm:
+    yield 0
+  else:
+    yield 0
+for _ in v():
+  for c in v():
+    (; let _: proc() = proc() = discard c)
+
+proc foo =
+  for _ in v():
+    for c in v():
+      (; let _: proc() = proc() = discard c)
+
+foo()
+
+
+when not defined(js):
+  import std/[os, sugar]
+
+  for file in walkDirRec("./"):
+    let multiplier = 5
+    discard(toSeq(1..4).map(x => x * multiplier))
+
+  for unused_var in walkDir("."):
+    let outside = "x"
+    let closure = proc(): string = outside