[m-rev.] for review: Tighten up string.from_code_unit_list et al.

[Mark Brown]

Hi Peter,

On Mon, Nov 4, 2019 at 4:51 PM Peter Wang <novalazy at gmail.com> wrote:
>
> library/string.m:
>     Document that from_code_unit_list fails if the result string would
>     contain a null character, and enforce that in the Java and C#
>     implementations. It was already enforced in the C implementation.
>
>     Make from_code_unit_list fail if the code unit list contains an
>     invalid value (negative or >0xff or >0xffff).
>
>     Document that from_utf{8,16}_code_unit_list fail if the result
>     string would contain a null character.

s/fail/fails/

> @@ -2027,11 +2034,9 @@ encode_utf16(Char, CodeList0, CodeList) :-
>      size = 0;
>      list_ptr = CodeList;
>      while (! MR_list_is_empty(list_ptr)) {
> -        int c;
> -        c = MR_list_head(list_ptr);
> -        // It is an error to put a null character in a string
> -        // (see the comments at the top of this file).
> -        if (c == '\\0' || c > 0xff) {
> +        unsigned c = (unsigned) MR_list_head(list_ptr);
> +        // Check for null character or invalid code unit.
> +        if (c == 0 || c > 0xff) {

Well spotted!

Mark