fixes #20954; bounchecks for len(toOpenArray()) [backport] (#20956)

* bounchecks for len(toOpenArray())

* add a testcase

compiler/ccgexprs.nim

@@ -1879,9 +1879,13 @@ proc genArrayLen(p: BProc, e: PNode, d: var TLoc, op: TMagic) =
     # Bug #9279, len(toOpenArray()) has to work:
     if a.kind in nkCallKinds and a[0].kind == nkSym and a[0].sym.magic == mSlice:
       # magic: pass slice to openArray:
+      var m: TLoc
       var b, c: TLoc
+      initLocExpr(p, a[1], m)
       initLocExpr(p, a[2], b)
       initLocExpr(p, a[3], c)
+      if optBoundsCheck in p.options:
+        genBoundsCheck(p, m, b, c)
       if op == mHigh:
         putIntoDest(p, d, e, ropecg(p.module, "($2)-($1)", [rdLoc(b), rdLoc(c)]))
       else:

tests/arc/topenarray.nim

@@ -22,3 +22,19 @@ block: # bug 18627
     for i in params.toOpenArray(0, params.len - 1):
       echo i
   uciLoop2()
+
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
+block: # bug #20954
+  block:
+    doAssertRaises(IndexDefect):
+      var v: array[10, int]
+
+      echo len(toOpenArray(v, 20, 30))
+
+  block:
+    doAssertRaises(IndexDefect):
+      var v: seq[int]
+
+      echo len(toOpenArray(v, 20, 30))