Type inference regression affecting app_dirs on MacOS

[PlasmaPower]

As seen in this Travis build, app_dirs currently builds on stable but not beta because of a type inference regression.

[nikomatsakis]

The failure is a call to into():

error[E0619]: the type of this value must be known in this context
  --> /Users/travis/.cargo/registry/src/github.spider-man.dpdns.org-1ecc6299db9ec823/app_dirs-1.1.1/src/imp/platform/macos.rs:10:40
   |
10 |         Ok(Component::RootDir.as_ref().into())
   |  

as_ref is not really meant to be used this kind of open-ended way, this is a common source of breakage -- that could be the problem.

cc @rust-lang/libs -- any new AsRef impls added recently?

[nikomatsakis]

Here is a minimized test case:

use std::path::Component;
use std::ffi::OsStr;

fn main() {
    let x: Box<OsStr> = Component::CurDir.as_ref().into();
}

[nikomatsakis]

Tagging as T-compiler for now since I don't immediately see what new impl could have caused this.

[sfackler]

Component implements AsRef<Path> in addition to the existing AsRef<OsStr> on beta.

[alexcrichton]

Switching relevant team to T-libs as this is almost for sure a T-libs issue rather than a T-compiler issue

[Mark-Simulacrum]

Closing in favor of #48716.