Implementing Borrow trait for a type with a lifetime

Question

I'm trying to use. strongly typed wrapper for a "keys" in my program, so that I don't mistake arbitrary strings for a Key. I have:

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
struct Key(String);

I have a HashMap<Key, _>, and I want to lookup values with a reference to a key type (i.e. not having to own the string). It seems like what I need to do is:

1. create a "ref" type for my Key:

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
struct KeyRef<'a>(&'a String);

(in reality I'd want KeyRef<'a>(&'a str), but using String makes for a clearer example)

2. implement Borrow<KeyRef<'_>> for Key

I've tried my best, here's a playground link

My most explicit attempt (annotating all lifetimes) is:

impl<'a> Borrow<KeyRef<'a>> for Key {
    fn borrow<'b>(&'b self) -> &'b KeyRef<'a> where 'b: 'a {
        let string_ref : &'a String = &self.0;
        let key_ref : &'a KeyRef<'a> = &KeyRef(string_ref);
        key_ref
    }
}

Which gives me the error: "lifetime parameters or bounds on method borrow do not match the trait declaration".

Intuitively it feels like this should be possible:

• KeyRef holds a reference of lifetime 'a, so any value of KeyRef cannot outlive 'a.
• In fn borrow<'b>(&'b self), 'b can't be greater than 'a due to the above

But the compiler doesn't seem to like my explicit attempt to demonstrate that (with where 'b: 'a), and leaving it off I get "cannot infer an appropriate lifetime for borrow expression due to conflicting requirements"

Answer 1

As far as I understand your situation, you are needlessly overcomplicating things. A straightforward implementation:

use std::collections::HashMap;
use std::borrow::Borrow;

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
struct Key(String);

impl Borrow<str> for Key {
    fn borrow(&self) -> &str {
        &self.0
    }
}

impl Borrow<String> for Key {
    fn borrow(&self) -> &String {
        &self.0
    }
}

fn main() {
    let mut map = HashMap::new();
    map.insert(Key("one".to_owned()), 1);

    // Because Key is Borrow<String>
    println!("{:?}", map.get("one".to_owned()));
    
    // Because Key is Borrow<str>
    println!("{:?}", map.get("one"));
}