fix #17159 items(cstring) works in VM (#17160)

* fix #17159 items(cstring) works in VM

* improve test coverage tests/stdlib/tcstring.nim; add helpers: whenRuntimeJs, whenVMorJs

* document items(cstring)

* address comments

tests/stdlib/tcstring.nim

@@ -1,19 +1,92 @@
 discard """
   targets: "c cpp js"
+  matrix: "; --gc:arc"
 """
 
+from std/sugar import collect
+from stdtest/testutils import whenRuntimeJs, whenVMorJs
 
-block: # bug #13859
-  let str = "abc".cstring
-  doAssert len(str).int8 == 3
-  doAssert len(str).int16 == 3
-  doAssert len(str).int32 == 3
-  var str2 = "cde".cstring
-  doAssert len(str2).int8 == 3
-  doAssert len(str2).int16 == 3
-  doAssert len(str2).int32 == 3
+template testMitems() =
+  block:
+    var a = "abc"
+    var b = a.cstring
+    let s = collect:
+      for bi in mitems(b):
+        if bi == 'b': bi = 'B'
+        bi
+    whenRuntimeJs:
+      discard # xxx mitems should give CT error instead of @['\x00', '\x00', '\x00']
+    do:
+      doAssert s == @['a', 'B', 'c']
 
-  const str3 = "abc".cstring
-  doAssert len(str3).int32 == 3
-  doAssert len("abc".cstring).int16 == 3
-  doAssert len("abc".cstring).float32 == 3.0
+  block:
+    var a = "abc\0def"
+    var b = a.cstring
+    let s = collect:
+      for bi in mitems(b):
+        if bi == 'b': bi = 'B'
+        bi
+    whenRuntimeJs:
+      discard # ditto
+    do:
+      doAssert s == @['a', 'B', 'c']
+
+proc mainProc() =
+  testMitems()
+
+template main() =
+  block: # bug #13859
+    let str = "abc".cstring
+    doAssert len(str).int8 == 3
+    doAssert len(str).int16 == 3
+    doAssert len(str).int32 == 3
+    var str2 = "cde".cstring
+    doAssert len(str2).int8 == 3
+    doAssert len(str2).int16 == 3
+    doAssert len(str2).int32 == 3
+
+    const str3 = "abc".cstring
+    doAssert len(str3).int32 == 3
+    doAssert len("abc".cstring).int16 == 3
+    doAssert len("abc".cstring).float32 == 3.0
+
+  block: # bug #17159
+    block:
+      var a = "abc"
+      var b = a.cstring
+      doAssert $(b, ) == """("abc",)"""
+      let s = collect:
+        for bi in b: bi
+      doAssert s == @['a', 'b', 'c']
+
+    block:
+      var a = "abc\0def"
+      var b = a.cstring
+      let s = collect:
+        for bi in b: bi
+      whenRuntimeJs:
+        doAssert $(b, ) == """("abc\x00def",)"""
+        doAssert s == @['a', 'b', 'c', '\x00', 'd', 'e', 'f']
+      do:
+        doAssert $(b, ) == """("abc",)"""
+        doAssert s == @['a', 'b', 'c']
+
+  block:
+    when defined(gcArc): # xxx SIGBUS
+      discard
+    else:
+      mainProc()
+    when false: # xxx bug vm: Error: unhandled exception: 'node' is not accessible using discriminant 'kind' of type 'TFullReg' [FieldDefect]
+      testMitems()
+
+  block: # bug #13321: [codegen] --gc:arc does not properly emit cstring, results in SIGSEGV
+    let a = "hello".cstring
+    doAssert $a == "hello"
+    doAssert $a[0] == "h"
+    doAssert $a[4] == "o"
+    whenVMorJs: discard # xxx this should work in vm, refs https://github.com/timotheecour/Nim/issues/619
+    do:
+      doAssert a[a.len] == '\0'
+
+static: main()
+main()