Confusing error while calling contains_key on HashMap<&str, _>

[ishan]

I have this little program.

use std::collections::HashMap;
fn main() {
    let mut map: HashMap<&str, u32> = HashMap::new();
    let new = "TOMATO".to_string();
    if !map.contains_key(&new) {}
}

When I try to compile this program, I get an error that I believe is quite confusing.

error[E0277]: the trait bound `&str: std::borrow::Borrow<std::string::String>` is not satisfied
 --> src/main.rs:5:13
  |
5 |     if !map.contains_key(&new) {}
  |             ^^^^^^^^^^^^ the trait `std::borrow::Borrow<std::string::String>` is not implemented for `&str`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

I have already resolved this and there were two ways to resolve this.

1. Use HashMap<String, _> signature instead of HashMap<&str, _>
2. Replace, if !map.contains_key(&new) {} with if !map.contains_key(new.as_str()) {}.

I am wondering if anyone else also found the compiler error to be confusing and maybe if we can do anything to improve it? Playground link

[2e71828]

@ishan

the trait bound &str: std::borrow::Borrow<std::string::String> is not satisfied

After looking up what Borrow actually means from the docs, I was able to translate this to:

&str cannot be borrowed as a String

Which makes perfect sense; the relationship goes the other way around. To lookup a key in a HashMap, you to provide a reference that matches its owned key type.

For me, the most confisung part of the error message is the ambiguity in the trait name Borrow: It isn’t immediately clear what A: Borrow<B> means— Is A the reference type of B, or is B the reference type of A?