Regression in Beta and Nightly in type inference

[Razican]

Stable build: https://travis-ci.org/SUPERAndroidAnalyzer/super/jobs/158500096
Beta build: https://travis-ci.org/SUPERAndroidAnalyzer/super/jobs/158500095
Nightly build: https://travis-ci.org/SUPERAndroidAnalyzer/super/jobs/158500094

Beta and nightly fail when trying to get a file name as a String from a PathBuf. The code is the following:

let dir_iter = try!(fs::read_dir("some/random/path"));
for f in dir_iter {
    let path_file = match f.unwrap().path().file_name() {
            Some(n) => String::from(n.to_string_lossy().borrow()),
            None => String::new(),
    };
    println!("{}", path_file);
}

And it also happens when getting the value from an AsRef<str>, in this example code:

pub fn new<S: AsRef<str>>(somedata: S) -> String {
    String::from(somedata.as_ref())
}

In both cases the error is the same:
type annotations required: cannot resolve std::string::String: std::convert::From<&_>

[arielb1]

That's because of the impl<'a> From<&'a [char]> for String impl added in ac73335f.

[alexcrichton]

Thanks for the investigation @arielb1! This is essentially expected breakage as per our library stability conventions (e.g. fixable by a local type annotation).

I believe @sfackler is going to open an issue, however, to track a system to better deal with these, however.

[sfackler]

#36443

[alexcrichton]

Thanks @sfackler! Closing in favor of #36443

[brson]

This was also reported 5 times on crater late in the beta cycle: #36352

[brson]

I'm opening this to consider reverting.