Error message suggests using `core` when it ought to suggest using `std`

[bstrie]

Program:

fn main() {
    Vec::from_iter();
}

Output:

fri.rs:2:5: 2:19 error: type `collections::vec::Vec<_>` does not implement any method in scope named `from_iter`
fri.rs:2     Vec::from_iter();
             ^~~~~~~~~~~~~~
fri.rs:2:19: 2:19 help: methods from traits can only be called if the trait is in scope; the following trait is implemented but not in scope, perhaps add a `use` for it:
fri.rs:2:19: 2:19 help: candidate #1: use `core::iter::FromIterator`
error: aborting due to previous error

Note that it suggests core::iter::FromIterator when it could be suggesting std::iter::FromIterator. Should a user follow this advice, they would need to also link libcore via extern crate core, which (IIRC) means that they will be opting out of 1.0-stable thanks to core's intentional lack of stability.

I don't know if it's worth nominating, but this is definitely a backcompat hazard for users.

Tempted to close this as a dupe of #23224.

[bstrie]

I think it's worth keeping this open as a more specific example of #23224, because it specifically has implications for stabilizing libcore. I can imagine a short-term hack to resolve this specific issue that would leave #23224 and #13065 for later.

[arielb1]

@bstrie

Such a hack will probably not be implemented.

[bstrie]

@arielb1 I as well don't think it's likely, merely a possibility. :P

I also think this is a much bigger pain point for beginners than most people realize. The only reason why I'm suggesting an ugly hack at all is because this must be addressed before libcore hits stable, and its FCP will be coming to a close any day now.

[arielb1]

@bstrie

Why does this need to be addressed?

[bstrie]

@arielb1 I was under the impression that using any API from core directly required a feature flag and hence was only possible on nightly, but I've just experimented and determined that's not the case. So I suppose stabilizing core at this point won't make us any worse off regarding this issue.