I'm working on a library that has a function that I would like to be able to accept a HashMap of any hashing algorithm. But when I try to call it with a particular type of HashMap, such as a FnvHashMap, the compiler complains.
pub fn func(map: HashMap<u32, u32>) {}
let map: FnvHashMap<u32, u32> = FnvHashMap::default();
func(map);
error[E0308]: mismatched types
--> lib.rs:42:10
|
42 | func(map);
| ^^^ expected struct `RandomState`, found struct `BuildHasherDefault`
|
= note: expected struct `HashMap<_, _, RandomState>`
found struct `HashMap<_, _, BuildHasherDefault<FnvHasher>>`
As FnvHashMap is only a type alias I thought this wouldn't be a problem.
I'm sure there's a way to do this, but I haven't found it yet. What's the correct way?
The definition of HashMap
in the docs is:
pub struct HashMap<K, V, S = RandomState> { /* fields omitted */ }
RandomState
is the default type, but it can be overridden with another type, including a generic. It may not be immediately clear, but most of the HashMap
methods are declared in an impl
block that constraints S: BuildHasher
, which is the trait you need to bound it by:
use std::hash::BuildHasher;
pub fn func<S: BuildHasher>(map: HashMap<u32, u32, S>) {}