Problem with type inference resolution

[Lapz]

A problem was found with the type inference resoultion when the env_logger crate was imported. The issue was reported on reddit and errors on the latest stable and nightly.

// Uncomment this and it breaks...
extern crate env_logger;

fn main() {
    let string = String::new();
    let str = <&str>::from(&string);

    println!("{:?}", str);
}

[cuviper]

Normally, <&str>::from(&string) will auto-deref that &String to &str, so we're using the identity impl From<T> for T here. If env_logger or its dependencies add a new impl From<X> for &str, then the type resolution is ambiguous, and auto-deref isn't used.

I had a somewhat similar issue in #49062 using Sum, but not involving auto-deref.

[steveklabnik]

Triage: here's the error message today:

error[E0277]: the trait bound `&str: std::convert::From<&std::string::String>` is not satisfied
 --> src/main.rs:6:15
  |
6 |     let str = <&str>::from(&string);
  |               ^^^^^^^^^^^^ the trait `std::convert::From<&std::string::String>` is not implemented for `&str`
  |
  = help: the following implementations were found:
            <&'t str as std::convert::From<regex::re_unicode::Match<'t>>>
  = note: required by `std::convert::From::from`