How to avoid temporary allocations when using a complex key for a HashMap?

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It sounds like you want this.

Cow will accept a &str or String. 

use std::borrow::Cow;

#[derive(Debug, Eq, Hash, PartialEq)]
struct Complex<'a> {
    n: i32,
    s: Cow<'a, str>,
}

impl<'a> Complex<'a> {
    fn new<S: Into<Cow<'a, str>>>(n: i32, s: S) -> Self {
        Complex { n: n, s: s.into() }
    }
}

fn main() {
    let mut m = std::collections::HashMap::<Complex<'_>, i32>::new();
    m.insert(Complex::new(42, "foo"), 123);

    assert_eq!(123, *m.get(&Complex::new(42, "foo")).unwrap());
}

A comment about lifetime parameters:

If you don’t like the lifetime parameter and you only need to work with &'static str or String then you can use Cow<'static, str> and remove the other lifetime parameters from the impl block and struct definition.

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