ToStr::to_str() should be marked as "pure"

[andymatuschak]

Shouldn't it? Am I missing something?

pub trait ToStr { fn to_str() -> ~str; }

[nikomatsakis]

it should.

[erickt]

This unfortunately broke some of my code. I used the fact that I could implement ToStr by using the impure io::with_str_writer with a to_writer(wr: io::Writer) to make my ~strs.

I think we could also make io::with_str_writer pure, but we'd have to use unsafe blocks, at least until #3722 is implemented. Is that too much cheating?

[catamorphism]

@erickt That's what I did with some of the existing to_str code. It's okay to cheat like this for now, IMO, because (hopefully) the system will eventually be more expressive.