Tighten up the auto-deref algorithm

[brson]

@nikomatsakis has mentioned that it has some unpredictable interactions with type checking. Hard to spec and understand as-is. Nominating.

[eddyb]

Is this a dupe of #12825?

[nikomatsakis]

No. I think what @brson was thinking of was an idea I had to make auto-deref behavior more predictable. The basic plan was to say that we always autoderef until we find a type T where one of the following conditions is true:

1. The type T is defined in the current module or some ancestor of the current module.
2. The type T is not derefable (does not implement Deref, not a built-in derefable type).

Such a plan would probably best go through the RFC process, and it might need some fine-tuning as well. I had planned to do some local experiments first. There are a fair number of pre-requisites, among them #5527 (which I think encompasses #12825).

[pnkfelix]

Assigning P-backcompat-lang, 1.0.

[nikomatsakis]

Specifically: can we make it possible to add methods to smart pointers without breaking backwards compatibility for users of those smart pointers?

Also, is there a way to make smart pointers able to explicitly identify methods that they DO want to override? For example, Rc<T> may want clone() to be callable so users can write my_rc.clone().

One issue with my proposal in general is that if I implement a trait on a pointer type, you still won't be able to use that trait with method notation.

In other words,

trait Foo { fn foo(self); }
impl Foo for &T { fn foo(self); }

and

trait Foo { fn foo(&self); }
impl Foo for T { fn foo(&self); }

cannot be used in an equivalent way (in fact, they are not quite equivalent in the trait method resolution algorithm I already proposed; that's something I ... kind of forgot about and was meaning to rectify. But it's not too hard to rectify today).

[pcwalton]

@nikomatsakis Is there an RFC on this? If so, could you link to it? If not, could you make one?

[nikomatsakis]

I have not made an RFC. I've been hoping to finish #5527 / UFCS and do some experiments. Perhaps that is too conservative. I'll try to write something up, sure.

[pcwalton]

After talking with Niko I'd like to nominate this for removal from 1.0. We don't have clear ideas on what to do here.

[pnkfelix]

(postponing to triage where at least one of {patrick, niko} is present.)