add Iterable traits

[thestinger]

Something like the following:

trait Iterable<'self, A, T: Iterator<&'self A>> {
    fn iter(&'self self) -> T;
}

trait ReverseIterable<'self, A, T: Iterator<&'self A>> {
    fn rev_iter(&'self self) -> T;
}

trait MutableIterable<'self, A, T: Iterator<&'self mut A>> {
    fn mut_iter(&'self mut self) -> T;
}

trait MutableReverseIterable<'self, A, T: Iterator<&'self mut A>> {
    fn mut_rev_iter(&'self mut self) -> T;
}

In the future we can define default methods like enumerate when method resolve is fixed. #5898

[thestinger]

This is not yet possible due to issues (including ICEs) related to lifetimes. At least some are already reported, but I'll look into this more.

[bluss]

This version of Iterable does not apply to str since it iterates by value, nor hashmap since it iterates by (&'self Key, &'self Value).

[thestinger]

@blake2-ppc: yup, but it gets really problematic if you omit the &'self A and use A instead - rustc just isn't capable of compiling it yet

[catamorphism]

Triage bump.

[aturon]

See also erickt's RFC proposing a somewhat similar design with additional convenience methods.

[japaric]

I think this should be closed (or moved to rust-lang/rfcs) since it requires an accepted RFC to be implemented, and there are two RFCs that address this issue: rust-lang/rfcs#17 (Iterable) and rust-lang/rfcs#235 (Collection reform).

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#397