Compiler error wrongly suggests to borrow

[akiross]

We noticed that if a HashMap<&str, T> is instanced and a get is performed using String as a key, the rust compiler suggests to use a reference &String instead, but that solution won't compile.

For example, given this code

use std::collections::HashMap;

fn main() {
    let mut m: HashMap<&str, u32> = HashMap::new();
    let s: String = "hello".to_owned();
    m.get(s);
}

The compiler will print this error

6 |     m.get(s);
  |           ^
  |           |
  |           expected reference, found struct `std::string::String`
  |           help: consider borrowing here: `&s`
  |

and if that change is applied the error becomes

  |
6 |     m.get(&s);
  |       ^^^ the trait `std::borrow::Borrow<std::string::String>` is not implemented for `&str`

This is happening with rustc 1.42.0 on the playground.

Edit: sorry I submitted the issue by mistake while incomplete :P

[pnkfelix]

According to godbolt, we started making this misleading suggestion back on Rust version 1.18.

(The code will compile if you use &*s.)

Interesting, Rust version 1.17 uses a different suggestion strategy that does, by change, hit a suggestion that would work:

  = note: expected type `&_`
             found type `std::string::String`
  = help: here are some functions which might fulfill your needs:
          - .as_bytes()
          - .as_str()
          - .trim()
          - .trim_left()
          - .trim_right()

[estebank]

Triage: no change.

[estebank]

We still incorrectly suggest borrowing:

error[E0308]: mismatched types
   --> src/main.rs:6:11
    |
  6 |     m.get(s);
    |       --- ^ expected `&_`, found `String`
    |       |
    |       arguments to this method are incorrect
    |
    = note: expected reference `&_`
                  found struct `String`
note: method defined here
   --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/collections/hash/map.rs:909:12
    |
909 |     pub fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V>
    |            ^^^
help: consider borrowing here
    |
  6 |     m.get(&s);
    |           +