Copy doesn't work for a closure argument

[yongqli]

I would expect this code [1] to work:

(0..5).flat_map(|i| (0..5).map(|j| i + j)).collect::<Vec<_>>()

since i is Copy. But rustc complains with

error: `i` does not live long enough

I could use move, but in the real code it would take ownership of too much. Shouldn't variables be copied if necessary and what's a succinct workaround?

[1] https://play.rust-lang.org/?gist=3ef7699396e1b712ccd4&version=stable

[Gankra]

It sounds like what you're really asking for is

• more aggressive inference of captures to enable "move" inference
• hybrid by-value/by-ref capturing closures

Note that the second bullet can be hacked as follows:

let to_cap_by_ref = 0;
let to_cap_by_val = 0;

let explicit_ref = &to_cap_by_ref;
foo.map(move |x| {
    to_cap_by_val + *explicit_ref;
});

Instead of closing over some variables by-reference, we're instead capturing references to those variables by-value.

[yongqli]

Thanks for the workaround. I was also able to solve my particular problem using iproduct! instead of flat_map.

[yongqli]

It looks like there's some related discussions here: https://internals.rust-lang.org/t/on-the-topic-of-inferring-move-closures/1438

[steveklabnik]

/cc @rust-lang/lang , are these sorts of changes large enough that they'd need an RFC, or is this a bug of some kind?

[Gankra]

This is definitely a feature request that would need an RFC.