RFC: scope and borrowing

[via Digitalmars-d]

On Saturday, 4 October 2014 at 19:22:45 UTC, Marc Schütz wrote:
> On Saturday, 4 October 2014 at 18:13:03 UTC, Ivan Timokhin 
> wrote:
>> Also, would it really make much sense to track the owner 
>> further than the assignment of a function's return value? That 
>> seems to complicate things a lot by adding a hidden attribute 
>> to a variable that is not only invisible at the declaration 
>> (even though the full type is spelled out), but, in fact, 
>> cannot be specified explicitly (because there's no syntax for 
>> that, now that scope with owners is limited to function 
>> signatures).
>>
>> How about this:
>> ---
>>    scope(string) haystack, needle;
>>    // next assignment is okay, because `s` is guaranteed not 
>> to outlive
>>    // `haystack`.
>>    scope(string) s = findSubstring(haystack, needle);
>>    // type of `s` is now scope(string), no additional 
>> information
>>    // attached
>>
>>    // so the next assignment is disallowed:
>>    //needle = s; // error!
>> ---
>>
>> This could be unnecessarily limiting, but would it really 
>> cause much trouble?
>
> I think you're right, I thought about this after I replied to 
> you. It would be the logical next step. On the other hand, I 
> wouldn't want to lose const borrowing, because it turned out to 
> be a requirement for safe moving. But I think it can still be 
> tracked internally (owner tracking is necessary for 
> implementation anyway).

Owner tracking is then completely limited to one expression. I 
think this will simplify the implementation a lot. Besides, it 
has precedences: uniqueness is also only tracked inside an 
expression, AFAIK.