Type inference regression in Rust 1.28.0

[ruuda]

The following file compiles on Rust 1.27.0:

use std::borrow::Cow;

struct Widget<'a> {
    handle: &'a u32,
}

impl<'a> Widget<'a> {
    pub fn new<S>(host: S) -> Result<Widget<'a>, String> where S: Into<Cow<'a, str>> {
        unimplemented!();
    }
}

pub fn main() {
    let host: String = "localhost".to_string();
    Widget::new(host.as_ref()).unwrap();
}

On 1.28.0, it fails with the following message:

error[E0283]: type annotations required: cannot resolve `std::string::String: std::convert::AsRef<_>`
  --> example.rs:15:22
   |
15 |     Widget::new(host.as_ref()).unwrap();
   |                      ^^^^^^

error: aborting due to previous error

I found #48668 which looks related, but it happened around 1.25.0.

The regression is still present in Rust 1.29.2.

[jonas-schievink]

Caused by #50170 - also see this list of other regressions caused by it: #51844

[emilyalbini]

Closing as a duplicate of #51844.