test(borrow): add a test to ensure borrow works

[Ryman]

Ran into this when trying to remove some unsafety from hyper.

This works on stable but seems to be a regression on beta and nightly. Placing this here for easy reference/repro.

src/lib.rs:142:17: 142:68 error: cannot infer an appropriate lifetime due to conflicting requirements [E0495]
src/lib.rs:142             map.get::<UniCase<Cow<'key, str>>>(&UniCase(fn_borrow))
                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/lib.rs:140:9: 143:10 help: consider using an explicit lifetime parameter as shown: fn get_shorter<'key>(map: &'key HashMap<UniCase<Cow<'static, str>>, u32>,
                     s: &'key str) -> Option<&'key u32>
src/lib.rs:140         fn get_shorter<'map, 'key>(map: &'map HashMap<UniCase<Cow<'static, str>>, u32>, s: &'key str) -> Option<&'map u32>{
src/lib.rs:141             let fn_borrow: Cow<'key, str> = s.into();
src/lib.rs:142             map.get::<UniCase<Cow<'key, str>>>(&UniCase(fn_borrow))
src/lib.rs:143         }
error: aborting due to previous error

Note that it works if you adjust the lifetimes to the suggestion, but that would limit the return value's lifetime to the lifetime of the key which would be undesirable for many cases.

[Ryman]

This get works with and without the type-hint on stable (1.4), fails in both cases on beta and nightly.

[seanmonstar]

Is this testing something specific about UniCase, or just lifetimes and Cow?

[Ryman]

The main intention was to test that Borrow actually worked through unicase, ensuring it works with HashMaps, but you're right in that it's not a very specific test. The verdict on the rust issue seems to be that it's a soundness issue and it's unlikely to compile as-is in the future.

[Ryman]

Closing for now, don't think there's a good way to do this currently.