fixes regression #17121; adding doc comment in importc proc makes it silently noop at CT (#20766)

* fixes regression #17121; adding doc comment in importc proc makes it silently noop at CT

* Update compiler/vmgen.nim

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

compiler/vmgen.nim

@@ -1596,11 +1596,20 @@ proc genTypeLit(c: PCtx; t: PType; dest: var TDest) =
   n.typ = t
   genLit(c, n, dest)
 
+proc isEmptyBody(n: PNode): bool =
+  case n.kind
+  of nkStmtList:
+    for i in 0..<n.len:
+      if not isEmptyBody(n[i]): return false
+    result = true
+  else:
+    result = n.kind in {nkCommentStmt, nkEmpty}
+
 proc importcCond*(c: PCtx; s: PSym): bool {.inline.} =
   ## return true to importc `s`, false to execute its body instead (refs #8405)
   if sfImportc in s.flags:
     if s.kind in routineKinds:
-      return getBody(c.graph, s).kind == nkEmpty
+      return isEmptyBody(getBody(c.graph, s))
 
 proc importcSym(c: PCtx; info: TLineInfo; s: PSym) =
   when hasFFI:

tests/vm/t17121.nim

@@ -0,0 +1,9 @@
+discard """
+  errormsg: "cannot 'importc' variable at compile time; c_printf"
+"""
+
+proc c_printf*(frmt: cstring): cint {.importc: "printf", header: "<stdio.h>", varargs, discardable.} =
+  ## foo bar
+  runnableExamples: discard
+static:
+  let a = c_printf("abc\n")