Allow implicit coercions for cross-borrows.

[nrc]

Wherever a type implements Deref<T>, allow an implicit coercion to &T. Removes the need for most occurrences of &*.

[lilyball]

I've been a big fan of this idea in the past. I'm not as much of a fan today, primarily because it makes it slightly harder to figure out whether a given expression is a move/copy or a cross-borrow, but I think I still fall in the 👍 camp.

I'm also in favor of implementing Deref on Vec/String in light of this RFC, to get rid of all those .as_string() calls. I'm in favor of doing that even without this RFC, because saying &*strvar is still better than strvar.as_slice().

For what it's worth, we already have some inconsistency in moves vs borrows with macros. I can say

let s: String = foo();
println!("{}", s); // note: bare `s`
consume_string(s);

But I can't say

let s: String = foo();
io::println(s); // bare `s` again, this time it's moved
consume_string(s);

This RFC coupled with a Deref impl on String would let the latter snippet work just as the former snippet does.

[lilyball]

This also seems related to rust-lang/rust#13910, which needs an RFC (extending support for box in patterns to work with arbitrary smart pointers).

[lilyball]

FWIW, UFCS provides an argument in favor of more drastic auto-borrowing, because if any type T could be borrowed to &T when given as a function argument, then any expression foo.bar() (where foo: T) can be rewritten under UFCS as T::bar(foo) without having to know whether the receiver type is by-value or by-reference.

That said, I think it's still appropriate to stick with just cross-borrowing. Full auto-borrowing just seems potentially too problematic.

[huonw]

I don't understand how this can work?

It seems let x = &&**e; is equivalent to the following today:

let tmp = e.deref().deref();
let x = &tmp;

while the latter seems like it should just be e.deref() (as &*e is now)? Or is there some deep coercions going on here, if so, that doesn't seem like it's backwards compatible, particularly, since &*e could mean two different things.

[nrc]

It might be wrong actually. I was imagining *e would happen first, then we would implicitly coerce to &**e and finally the explicit & would give &&**e. If we allow coercions at &e but not at *e, then I think there is no ambiguity. This distinction does seem a bit ad-hoc though (it follows where we have an expected type in the type checker at the moment). I have another RFC which goes in to depth on sites for coercions, for this RFC I'm just kind of assuming that is nailed down elsewhere.

[Ericson2314]

Sounds good, my only reservation is that being able to write &* instead of &&** in the double box case might look misleading. Of course, on the other hand, that follows from the rule, and preventing it would be a crude hack.

Maybe deep cross-borrowing wouldn't be so bad?

[aturon]

I've posted an alternative design that achieves similar goals but is somewhat more conservative: it only allows coercions on already-borrowed data, and thereby does not introduce new borrows implicitly. The RFC gives detail about the downsided of implicit borrowing.

[nrc]

Some data and a little discussion: https://gist.github.com/nick29581/809614adb2bbb38232b7

[nrc]

Closed in favour of #241 or something like that (that idea is superior because it better respects the borrowing/ownership principles of Rust and substitutability).