I encountered an ugly problem. Actually, I had already run into
it in my first proposal, but Steven Schveighoffer just posted
about it here, which made me aware again:
http://forum.dlang.org/thread/mcqcor$aa$1...@digitalmars.com#post-mcqk4s:246qb:241:40digitalmars.com
class T {
void doSomething() scope;
}
struct S {
RC!T t;
}
void main() {
auto s = S(RC!T()); // `s.t`'s refcount is 1
foo(s, s.t); // borrowing, no refcount changes
}
void foo(ref S s, scope T t) {
s.t = RC!T(); // drops the old `s.t`
t.doSomething(); // oops, `t` is gone
}
This (and similar things) are the reason I introduced "const
borrowing", a way for an object to make itself temporarily const,
as long as borrowed references to it exist. Unfortunately, this
was broken in the presence of aliasing: When another alias (in
the above example, imagine another pointer to `s`) of the owning
struct existed before the borrowing took place, it was not
affected by the change to const.
Now that I know a bit more about linear type systems (but am not
an expert by any means), I understand why it happens. I suspect
that the only way to really prevent problems of this kind is a
full blown linear type system, i.e. one that guarantees that to
each object there is at most one mutable reference.
The question is: What do we do about it? Maybe there is actually
a way to fix this problem without a borrow checker? Any type
system gurus here?
Or we could simply live with it and make it a convention not to
pass RC objects (or related types) into situations where it can
be a problem. I don't like that option, though.
Or we implement a borrow checker... It doesn't have to be as
fancy as Rust's, i.e. we don't need to have data flow analysis.
Just a lexical scope based solution would work.
Any other ideas and opinions?
On a positive note, I did some experiments with the inference
algorithm, and I'm reasonably sure it works (absent un-@safe
operations like `delete` and `free()`, of course). Here are the
examples:
http://wiki.dlang.org/User_talk:Schuetzm/scope2
I'm going to try and formalize it during the next days.
