I'm relatively new to Rust. I want to write a function that will create a HashMap from a collection given a closure that generates a key from a value, e.g.
[derive(Debug)]
struct Foo {
id: u32,
name: String,
}
let foos = vec![
Foo { id: 1, name: "Bill".to_string() },
Foo { id: 2, name: "Mary".to_string() },
];
println!("{:?}", foos);
println!("{:?}", map_by(foos.iter(), |f| f.id)); // borrow the Foos
println!("{:?}", map_by(foos.iter(), |f| &f.name)); // borrow
println!("{:?}", map_by(foos.iter(), |f| f.name.as_str())); // borrow
println!("{:?}", map_by(foos, |f| f.id)); // consume the Foos
I wrote this, and it works for the above uses:
fn map_by<I,K,V>(iterable: I, f: impl Fn(&V) -> K) -> HashMap<K,V>
where I: IntoIterator<Item = V>,
K: Eq + Hash
{
iterable.into_iter().map(|v| (f(&v), v)).collect()
}
[Foo { id: 1, name: "Bill" }, Foo { id: 2, name: "Mary" }]
{1: Foo { id: 1, name: "Bill" }, 2: Foo { id: 2, name: "Mary" }}
{"Bill": Foo { id: 1, name: "Bill" }, "Mary": Foo { id: 2, name: "Mary" }}
{"Bill": Foo { id: 1, name: "Bill" }, "Mary": Foo { id: 2, name: "Mary" }}
{2: Foo { id: 2, name: "Mary" }, 1: Foo { id: 1, name: "Bill" }}
Great! But it doesn't work when I let the HashMap OWN the values when the keys refer to them:
println!("{:?}", map_by(foos, |f| f.name.as_str()));
error[E0495]: cannot infer an appropriate lifetime for borrow expression due to conflicting requirements
And unfortunately the error message melted my brain. I tried specifying that the key and value have the same lifetime, but it didn't help. Here it is in a playground. What is the compiler trying to tell me, and what am I doing wrong? Surely if this can work when we borrow the values, it can work when we own them.
