possible bug with borrowing and moving things into tasks

[erickt]

I was trying to help out @ben0x539 with some code of his: https://gist.github.com/3846368. He's messing around with moving a channel into a task, which is then passed to a closure. This fails with

foo.rs:5:56: 5:58 error: copying a noncopyable value
foo.rs:5         let fail_: fn~(~str) -> ! = fn~(x: ~str) -> ! { wr.send(x); fail; };
                                                                 ^~
foo.rs:5:56: 5:58 note: non-copyable value cannot be copied into a ~fn closure
foo.rs:5         let fail_: fn~(~str) -> ! = fn~(x: ~str) -> ! { wr.send(x); fail; };

However, if we get rid of the task, it works fine. I would have thought that moving the channel into the task would be roughly equivalent to the non-task version, but I'm probably missing some subtly in the borrower.

In the meantime, you can use pipes::SharedChan to write this kind of function.

[nikomatsakis]

This is a dup of #2549. Basically the problem here is that you cannot move out of upvars right now, because Rust function types always assume they may be called more than once. To work around this, you must either use a channel to send the value to the task (rather than capturing it via an upvar) or use the so-called "option dance". Hopefully this will be fixed in 0.5.