Box<Iterator> (and &mut Iterator) does not implement Iterator

[BurntSushi]

This code fails to compile:

fn main() {
    let mut shrinker = Box::new(vec![1].into_iter()) as Box<Iterator<Item=i32>>;
    println!("{:?}", shrinker.next());
    for v in shrinker { println!("{:?}", v); }
}

error message:

[andrew@Liger play] rustc scratch2.rs 
scratch2.rs:32:14: 32:22 error: `for` loop expression has type `Box<core::iter::Iterator>` which does not implement the `Iterator` trait; maybe try .iter()
scratch2.rs:32     for v in shrinker { println!("{:?}", v); }
                            ^~~~~~~~
error: aborting due to previous error

A similar error occurs if &mut Iterator is used instead of Box<Iterator>. However, if you comment out the for loop, the call to next works just fine. I thought maybe it had something to do with auto-deref, so I tried dereferencing the iterator:

for v in *shrinker { println!("{:?}", v); }

but rustc dumps core:

[andrew@Liger play] rustc scratch2.rs 
rustc: /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/llvm/lib/IR/Instructions.cpp:1086: void llvm::StoreInst::AssertOK(): Assertion `getOperand(0)->getType() == cast<PointerType>(getOperand(1)->getType())->getElementType() && "Ptr must be a pointer to Val type!"' failed.
Aborted (core dumped)

I'm not sure, but this may be related to #20605? rustc dumps core when I try &mut Iterator too.

rustc version:

[andrew@Liger play] rustc --version
rustc 1.0.0-nightly (44a287e6e 2015-01-08 17:03:40 -0800)

[Gankra]

CC @nikomatsakis

[chris-morgan]

I think the problem is worse than this; trait objects in general don’t seem to be being recognised as implementing their traits:

trait Trait { }

fn a(x: &Trait) { try(x); }
fn b(x: &mut Trait) { try(x); }
fn c(x: Box<Trait>) { try(x); }
fn try<T: Trait>(t: T) { }

fn main() { }

All three fail:

i.rs:3:19: 3:22 error: the trait `Trait` is not implemented for the type `&Trait`
i.rs:3 fn a(x: &Trait) { try(x); }
                         ^~~
i.rs:4:23: 4:26 error: the trait `Trait` is not implemented for the type `&mut Trait`
i.rs:4 fn b(x: &mut Trait) { try(x); }
                             ^~~
i.rs:5:23: 5:26 error: the trait `Trait` is not implemented for the type `Box<Trait>`
i.rs:5 fn c(x: Box<Trait>) { try(x); }
                             ^~~
error: aborting due to 3 previous errors

[eddyb]

I'm not sure this is a bug at all. You're pretty much asking for an automatic impl<T> Trait for &mut T where T: Trait - I did come up with a Deref & DerefMut-based model that would kinda work for this and IIRC @nikomatsakis pointed out that object-safe traits would eventually qualify for this. We do have object safety now, so it might be doable, but I wouldn't bet on it making into 1.0.
In the meanwhile, does .by_ref() work? If it doesn't, is it just the case of missing ?Sized in a couple places?

[japaric]

This a particular case of #20617. See comments over there. @nikomatsakis is leaning towards an opt-in derive syntax extension that handles the boiler plate code, instead of automatic impls by the compiler.

[bluss]

ByRef for trait objects seems to hit the same llvm assertion:

fn main() {
    struct ByRef<'r, I: ?Sized>(&'r mut I) where I: 'r;
    impl<'r, I: ?Sized> Iterator for ByRef<'r, I> where
        I: 'r + Iterator
    {
        type Item = <I as Iterator>::Item;
        fn next(&mut self) -> Option<<I as Iterator>::Item>
        {
            self.0.next()
        }
    }

    let mut it = Box::new(0..10) as Box<Iterator<Item=i32>>;
    assert_eq!(it.next(), Some(0));

    let mut jt: &mut Iterator<Item=i32> = &mut *it;
    assert_eq!(jt.next(), Some(1));

    let mut r = ByRef(jt);
    assert_eq!(r.next(), Some(2));    // No ICE without this line.
}

output:

rustc: /build/rust-git/src/rust/src/llvm/lib/IR/Instructions.cpp:1086: void llvm::StoreInst::AssertOK(): Assertion `getOperand(0)->getType() == cast<PointerType>(getOperand(1)->getType())->getElementType() && "Ptr must be a pointer to Val type!"' failed.

[BurntSushi]

I'm not sure this is a bug at all. You're pretty much asking for an automatic impl<T> Trait for &mut T where T: Trait

Interesting. So if I have a value with type Box<T> where T is a trait, then we can't say that Box<T> implements T in the general case? (Assuming object safety.) So my guess is that Box<Iterator> can still call next because of auto deref? Shouldn't that mean it can work with for? Maybe that is what #20605 is about... You have to explicitly deref a Box<Iterator> to use it in for?

(Sorry for all the questions. I'm just confused!)

[eddyb]

@BurntSushi doing autoderef on the iterator in for seems like something reasonable to have.
#20605 looks like the compiler is mistakenly treating an unsized lvalue as an rvalue, in trans.

[nikomatsakis]

On Mon, Jan 12, 2015 at 01:33:08PM -0800, Andrew Gallant wrote:

Interesting. So if I have a value with type Box<T> where T is a
trait, then we can't say that Box<T> implements T in the general
case?

Actually, I believe it works with Box, but not with the other types.

[hicqu]

Just thank you for this issue. It resolves all my problems perfectly!