String::as_ref() Into<String> type inference regression

[oherrala]

The following code has been working with stable compiler (rustc 1.34.2), but doesn't compile with nightly (rustc 1.36.0-nightly (963184b 2019-05-18)).

fn takes_string<S: Into<String>>(input: S) {
    let s = input.into();
    println!("{}", s);
}

fn main() {
    let foo = String::from("Hello world");
    takes_string(foo.as_ref());
}

code in playground

Error the nightly compiler is given:

error[E0283]: type annotations required: cannot resolve `std::string::String: std::convert::AsRef<_>`
  --> src/main.rs:10:22
   |
10 |     takes_string(foo.as_ref());
   |                      ^^^^^^

error: aborting due to previous error

However, stable compiler is happy with the code and prints the expected hello world.

Stable compiler:

rustc 1.34.2 (6c2484dc3 2019-05-13)
binary: rustc
commit-hash: 6c2484dc3c532c052f159264e970278d8b77cdc9
commit-date: 2019-05-13
host: x86_64-apple-darwin
release: 1.34.2
LLVM version: 8.0

Nightly compiler:

rustc 1.36.0-nightly (963184bbb 2019-05-18)
binary: rustc
commit-hash: 963184bbb670c1ffa97fc28a98cd5e8473118859
commit-date: 2019-05-18
host: x86_64-apple-darwin
release: 1.36.0-nightly
LLVM version: 8.0

[jonas-schievink]

Likely caused by #59825, but considered acceptable breakage.

[smarnach]

This breaks some crates that compiled before, e.g. dipstick and lettre_email. Just curious, why is this considered acceptable in this case?

[jonas-schievink]

These cases can be made to work again by locally elaborating types or the used impl (eg. using <Type as Trait>::method(...) syntax), so the fallout is easy to fix.

[Mark-Simulacrum]

Specifically, since this is essentially inference breakage it is permitted per our policies. If there is some reason this causes significant difficulty for you, though, we'd like to hear about that and may reconsider.

[smarnach]

@Mark-Simulacrum Thanks for your response. This doesn't cause any problem at all for me – I was just curious about what kind of breakage is permitted and what isn't. I found the commitments in this blog post as well.