Say I want to create a cache using a HashMap. My keys are structs that contain a font name and size, something like:

#[derive(Hash, Eq, PartialEq, Debug)]
struct FontKey { 
    font_name: String, 
    font_size: NotNan<f32> 
}

I don't want to have to allocate that String to look something up. For a lookup, I'd prefer something like:

struct FontKey2 { font_name: &'a str, font_size: NotNan<f32> }

But I can’t put those in my HashMap field. I thought maybe I could Borrow, but I guess not. Don’t see how to fill in the todo!() here. (playground link).

Is there a way to do this? Another path: maybe I can intern the font names and use something like a struct FontNameId(u32) in the cache keys.

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

#[derive(Eq, PartialEq, Hash, Debug)]
struct Key {
    font_name: String,
    font_size: u32,
}
#[derive(Eq, PartialEq, Hash, Debug)]
struct Key2<'a> {
    font_name: &'a str,
    font_size: u32,
}
impl<'a> Borrow<Key2<'a>> for Key {
    fn borrow(&self) -> &Key2<'a> {
        todo!()
    }
}
fn main() {
    let mut cache: HashMap<Key, i32> = HashMap::new();
    let font_name: String = "Roboto".to_owned();
    cache.insert(Key { font_name: font_name.clone(), font_size: 14 }, 123);
    let k2 = Key2 { font_name: font_name.as_str(), font_size: 14 };
    let val = cache.get(&k2);
    println!("val = {:?}", val);
}

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