> I just tried this, but rustc version 1.15.1 can't find the .iter() method for the Range. I'm assuming it's a small change (which I'd really like to see if you're willing), but that's quite a stack of legos you've snapped together there :-)

Yeah, ranges are iterators already; you don't need to create iterators out of them.

`let boxslice = (0..10).map(|_| func()).collect::<Vec<_>>().into_boxed_slice();` is something that will actually compile. The turbofish `::<Vec<_>>` is necessary because `collect()` can collect into arbitrary containers (like HashSets) and we need to tell it which one to collect into. A two-liner `let myvec: Vec<_> = ....collect(); let boxslice = myvec.into_boxed_slice();` would also work and wouldn't need the turbofish.

In case of functions returning Clone types, you can just do `vec![func(); n].into_boxed_slice();`. My example was the fully generic one that would be suitable for implementing a function in the stdlib, not exactly what you might use -- I didn't expect you to be able to piece it together :). For your purposes just using the vec syntax is fine, and would work for most types.

Using ranges as iterators is basically the go-to pattern for "iterate n times", for future reference.

> Does that create and then copy (possibly large) temporaries? Walking through the code, I see it calls RawVec::shrink_to_fit() - which looks like it's possibly a no-op if the capacity is the right size. Then it calls Unique::read() - which looks like a memcpy. I honestly don't know if this does make copies, but if it does, that cost can be significant sometimes.

In this case .collect() is operating on an ExactSizeIterator (runtime known length) so it uses Vec::with_capacity and shrink_to_fit would be a noop. In general .collect().into_boxed_slice() may do a shrink (which involves copying) if it operates on iterators of a-priori unknown length. This is not one of those cases. At most you may have a copy involved of each element when it is returned from func() and placed into the vector. I suspect it can get optimized out.

vec![func(), n] will call func once and then create n copies by calling .clone(). Usually that cost is about the same as calling func() n times.

> Let's instantiate T with a String, a File, or a HashTable - I don't see how adding MyBigInt could possibly make sense on either the left or the right. Maybe they make sense with the right bounds added.

Yeah, that's why I had a bound there. I personally feel these impls make sense, both for traits and for operators. Perhaps more for non-operator traits.

I think your usecase is a good one, and it's possible that the covered rule could be made to work with the current rules. I don't know. It would be nice to see a post exploring these possibilities. It might be worth looking at how #[fundamental] works (https://github.com/rust-lang/rfcs/blob/1f5d3a9512ba08390a222...) -- it's a coherence escape hatch put on Box<T> and some other stdlib types which makes a tradeoff: Box<T> and some other types can be used in certain places in impls without breaking coherence, but the stdlib is not allowed to make certain trait impls on Box without it being a breaking change (unless the trait is introduced in the same release as the impl). The operator traits may have a solution in a similar spirit -- restrict how the stdlib may use them, but open up more possibilities outside. It's possible that this may not even be necessary; the current coherence rules are still conservative and could be extended. I don't think I'm the right person to really help you figure this out, however, I recommend posting about this on the internals forum.

(I'm not sure if this discussion is over, but if it isn't I think it makes more sense to continue over email. username@gmail.com. Fine to continue here if you don't want to use email for whatever reason)