Expected Fn / fn, found [closure@...]

[jofas]

You need to nudge type inference a little here:

use std::collections::HashMap;

enum A {
    B,
    C,
}

fn donotwork() {
    let g: HashMap<u8, fn(A) -> A> = HashMap::from([(0u8, (|x: A| x) as _)]);
    let h: HashMap<u8, Box<dyn (Fn(A) -> A) + Sync>> = HashMap::from([(0, Box::new(|x: A| x) as _)]);
}

Playground.

See i.e. this topic for a similar issue I encountered when using collections with elements that need to be coerced:

Type inference fails for collection of trait objects

Recently I found a case where rust's type inference fails for collections of trait objects. Here an example: use std::fmt::Debug; #[derive(Debug)] struct D1; #[derive(Debug)] struct D2; fn debug_vec() -> Vec<Box<dyn Debug>> { // compiler: hmm, I don't know what type this is. Let's see how this one // unfolds. let mut v = Vec::new(); // compiler: got it, v must be Vec<Box<D1>>! v.push(Box::new(D1)); // compiler: gosh darn it, this is no Box<D1> add…

or this topic:

Implictly convert `&Arc<closure>` to `&Arc<dyn Fn>`

I'm a little surprised that the compiler cannot automatically convert a between &Arc<closure> and &Arc<dyn Fn>: fn test(f: &Arc<dyn Fn()>) {} fn main() { test(&Arc::new(|| {})) } The error is: error[E0308]: mismatched types --> src/main.rs:8:10 | 8 | test(&Arc::new(|| {})) | ---- ^^^^^^^^^^^^^^^^ expected trait object `dyn Fn`, found closure | | | arguments to this function are incorrect | = note: expected reference `&Arc<(dyn Fn() + 'static)>` …

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BTW, I created a macro that makes it easy for you to create a hashmap with values that need to be coerced to a different type before you can store them: map_macro::hash_map_e . It uses the same trick with explicit coercion to "something" (the _ placeholder) under the hood.