Show impl for RefCell, Ref & RefMut

[forticulous]

Show impl for RefCell and friends

[alexcrichton]

This is an interesting direction to take because it means that Show for RefCell is fallible. I don't think that there are many other fallible implementations.

[forticulous]

I was trying to stick to the idea that the Show impl would borrow a reference to the inner value and call fmt on that. Can you elaborate on the fallible part?

[huonw]

let x = RefCell::new(1);let y = x.borrow_mut();// this will fail, due to the `y` mutable borrow stopping// any other borrow/borrow_mut.println!("{}", x);

[forticulous]

This would also fail for the same reasons:

letmut x = 75u;let y = &mut x;println!("{}", x);

[alexcrichton]

Your example code, @forticulous, does not fail (you can try to run it). @huonw explained why this type is fallible.

[forticulous]

I did just run it on rust-lang.org

error: cannot borrow `x` as immutable because it is also borrowed as mutable
note: in expansion of format_args!
<std macros>:2:23: 2:77 note: expansion site
<std macros>:1:1: 3:2 note: in expansion of println!
error: aborting due to previous error

[alexcrichton]

Note that it is a compilation error, not a runtime error. RefCell yields runtime errors.

[huonw]

The RefCell version fails at runtime. The type is design as a way to get around the hard limits of &mut pointers and aliasability, but this ends up requiring runtime checks (and failure) instead of compile time ones.

[forticulous]

That's the whole idea of RefCell isn't it? Checking borrow rules at runtime instead of at compilation time.

[forticulous]

Here's maybe a better example (though this would also be my fault)

letmut x = Rc::new(75u);let y = x.make_unique();println!("{}", x);
error: cannot borrow `x` asimmutable because it is also borrowed asmutable

[huonw]

Yep, that's the idea of RefCell, the point is this Show impl differs to all other ones, because it can reasonably fail at runtime, no other implementation of Show will fail like this. (The examples you have given are just invalid code which fail to compile; also, the errors are not related to Show, in particular, replacing the println! with just let z = &x; will display the same errors.)

I would be very happy to merge Ref and RefMut and then consider the RefCell implementation in a separate PR.

[forticulous]

Yeah, it seems like when you choose to use RefCell you are intentionally giving up some of the compile time safety and relying on runtime checks so you're opening yourself up to possible runtime failures.

I can cut out the RefCell stuff for now and make an update

[forticulous]

Updated with just Ref and RefMut