cannot resolve `std::borrow::Cow<'_, str>: std::borrow::Borrow<_>`

[jebrosen]

This code fails to compile on nightly 2018-05-20 (playground):

use std::borrow::{Borrow, Cow};

fn takes_into_cow<'a, T: Into<Cow<'a, str>>>(x: T) {
    let s = x.into();
    println!("{}", s);
}

fn forward_cow<'a>(x: Cow<'a, str>) {
    takes_into_cow(x.borrow())
}

fn main() {
    forward_cow("test".into());
}

With this error:

error[E0283]: type annotations required: cannot resolve `std::borrow::Cow<'_, str>: std::borrow::Borrow<_>`
 --> src/main.rs:9:22
  |
9 |     takes_into_cow(x.borrow())
  |                      ^^^^^^

This compiles fine on stable and beta, and probably on nightly as well until some time in the last week or so.

This is a simplified version of the code involved in rwf2/Rocket#643.

[leoyvens]

probably caused by #50170, we didn't do a crater run for that one. In that PR we added:

impl<'a> From<&'a String> for Cow<'a, str>

But we already have:

impl<'a> From<&'a str> for Cow<'a, str>

This makes the following Borrow impl ambiguous in this case:

impl<'a, B> Borrow<B> for Cow<'a, B> where B: ToOwned

It seems all impls (actually not all, see comment below) in that PR can cause the same problem, is it possible to see how many crates were broken by it? Hopefully crater can do that.

[pietroalbini]

Scheduled a check-only crater run. Should start in a few days.

[jebrosen]

I think the only problematic impls added were impl From<&'a T::Owned> for Cow<'a, T>. This is the last one in each "chunk" of #50170 (comment).

The same error occurs with this CString case:

use std::borrow::{Borrow, Cow};
use std::ffi::{CStr, CString};

fn main() {
    let x = CString::new("x").unwrap();
    let y: Cow<CStr> = x.borrow().into();
}

No code should be doing this one already, but it appears that the existence of impl From<&'a CString> for Cow<'a, CStr> makes impl From<&'a CStr> for Cow<'a, CStr> less useful, so I would argue against those ones.

[pietroalbini]

Check-only crater run started. Should be finished in ~3 days.

[pietroalbini]

Hi @leodasvacas (crater requester)! Crater results are at: http://cargobomb-reports.s3.amazonaws.com/issue-50954/index.html. 'Blacklisted' crates (spurious failures etc) can be found here. If you see any spurious failures not on the list, please make a PR against that file.

(interested observers: Crater is a tool for testing the impact of changes on the crates.io ecosystem. You can find out more at the repo if you're curious)

[kennytm]

ogrep-0.2.1:

May 25 11:49:29.066 INFO kablam! error[E0283]: type annotations required: cannot resolve `std::borrow::Cow<'_, str>: std::convert::From<&_>`
May 25 11:49:29.066 INFO kablam!    --> src/main.rs:541:17
May 25 11:49:29.066 INFO kablam!     |
May 25 11:49:29.066 INFO kablam! 541 |                 Cow::from(options.pattern.as_ref())
May 25 11:49:29.066 INFO kablam!     |                 ^^^^^^^^^
May 25 11:49:29.066 INFO kablam!     |
May 25 11:49:29.066 INFO kablam!     = note: required by `std::convert::From::from`

screen-api-0.4.1:

May 25 17:33:44.418 INFO kablam! error[E0283]: type annotations required: cannot resolve `std::string::String: std::convert::AsRef<_>`
May 25 17:33:44.418 INFO kablam!    --> src/websocket/types/mod.rs:163:76
May 25 17:33:44.419 INFO kablam!     |
May 25 17:33:44.419 INFO kablam! 163 |             } => Channel::room_map_view(room_name, shard_name.as_ref().map(AsRef::as_ref)),
May 25 17:33:44.419 INFO kablam!     |                                                                            ^^^^^^^^^^^^^
May 25 17:33:44.419 INFO kablam!     |
May 25 17:33:44.419 INFO kablam!     = note: required by `std::convert::AsRef::as_ref`

travis-0.1.1:

May 25 22:28:02.573 INFO kablam! error[E0283]: type annotations required: cannot resolve `std::string::String: std::convert::AsRef<_>`
May 25 22:28:02.573 INFO kablam!   --> examples/demo.rs:60:50
May 25 22:28:02.573 INFO kablam!    |
May 25 22:28:02.573 INFO kablam! 60 |             let builds = travis.builds(repo.slug.as_ref());
May 25 22:28:02.573 INFO kablam!    |                                                  ^^^^^^

[leoyvens]

This also happens with AsRef, I think it's because of the following impl:

impl<'a, T: ?Sized + ToOwned> AsRef<T> for Cow<'a, T>

I'm not sure what the motivation for the problematic impls was, but if it was to improve functions that take T: Into<Cow<_>> then it might have backfired. Though the set of regressions is small enough that we could consider it acceptable, if we want the impls.

[jebrosen]

All three of those look like a case of String -> AsRef::as_ref() -> Cow::from() failing to resolve, so I don't see how impl<'a, T: ?Sized + ToOwned> AsRef<T> for Cow<'a, T> could come into it. I thought there was some blanket impl <T> AsRef<T> for T but that's not the case, so I can't tell what the ambiguity is here.

[alexander-irbis]

http://play.rust-lang.org/?gist=d540658d847fed9d6bcd5ca5c74828b8&version=nightly&mode=debug

// #![feature(type_ascription)]

use std::borrow::Cow;

fn get_cow(string: &String) -> Cow<str> {
    // Fails on nightly with "type annotations required: 
    // cannot resolve `std::borrow::Cow<'_, str>: std::convert::From<&_>`"
    // https://github.com/rust-lang/rust/issues/50954
    // This works fine in rust 1.27 and earlier
    Cow::from(string.as_ref())
    
    // In rust nightly-1.28 It is easy to fix as in examples below
    // Cow::from(string.as_ref(): &str)
    // Cow::from(string)
    
    // But I found only these are acceptable in both cases
    // Cow::from(string.as_str())
    // Cow::Borrowed(string.as_ref())
    // Cow::Borrowed(string)
}

fn main() {
    let owned = String::from("Hello, world!");
    let _cow = get_cow(&owned);
}

This is a kind of breaking change, that form of from cannot co-exist in the old and the new stable rust.
However in my case (rsmorphy) I can painlessly replace it with Cow::from(string.as_str()).