Validate pragmas attached to for variables (#8749)

Fixes #8741

compiler/pragmas.nim

@@ -71,6 +71,7 @@ const
   letPragmas* = varPragmas
   procTypePragmas* = {FirstCallConv..LastCallConv, wVarargs, wNosideeffect,
                       wThread, wRaises, wLocks, wTags, wGcSafe}
+  forVarPragmas* = {wInject, wGensym}
   allRoutinePragmas* = methodPragmas + iteratorPragmas + lambdaPragmas
 
 proc getPragmaVal*(procAst: PNode; name: TSpecialWord): PNode =

compiler/semstmts.nim

@@ -567,6 +567,8 @@ proc symForVar(c: PContext, n: PNode): PSym =
   let m = if n.kind == nkPragmaExpr: n.sons[0] else: n
   result = newSymG(skForVar, m, c)
   styleCheckDef(c.config, result)
+  if n.kind == nkPragmaExpr:
+    pragma(c, result, n.sons[1], forVarPragmas)
 
 proc semForVars(c: PContext, n: PNode): PNode =
   result = n

tests/pragmas/t8741.nim

@@ -0,0 +1,10 @@
+discard """
+  line: 9
+  errormsg: "attempting to call undeclared routine: 'foobar'"
+"""
+
+for a {.gensym, inject.} in @[1,2,3]:
+  discard
+
+for a {.foobar.} in @[1,2,3]:
+  discard