fix #14157 (#15877)

* fix #14157

* Update compiler/jsgen.nim

* add changelog

* Update compiler/jsgen.nim

* Update tests/js/tmodify_cstring.nim

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

changelog.md

@@ -35,6 +35,8 @@
 
 ## Language changes
 
+- The `cstring` doesn't support `[]=` operator in JS backend.
+
 
 
 ## Compiler changes

compiler/jsgen.nim

@@ -1049,6 +1049,10 @@ proc genAsgnAux(p: PProc, x, y: PNode, noCopyNeeded: bool) =
   var a, b: TCompRes
   var xtyp = mapType(p, x.typ)
 
+  # disable `[]=` for cstring
+  if x.kind == nkBracketExpr and x.len >= 2 and x[0].typ.skipTypes(abstractInst).kind == tyCString:
+    localError(p.config, x.info, "cstring doesn't support `[]=` operator")
+
   gen(p, x, a)
   genLineDir(p, y)
   gen(p, y, b)

tests/js/tmodify_cstring.nim

@@ -0,0 +1,6 @@
+discard """
+  errormsg: "cstring doesn't support `[]=` operator"
+"""
+
+var x = cstring"abcd"
+x[0] = 'x'