Unknown type from Lazy.lock()

[Timmmm]

I'm using once_cell's Lazy type, like this:

use once_cell::sync::Lazy;

struct State {
}

static STATE: Lazy<Arc<Mutex<State>>> = Lazy::new(|| {
  Arc::new(Mutex::new(Default::default()))
});

fn foo() {
  let a = STATE.lock();

If I hover STATE then it does show the type as static STATE: Lazy<Arc<Mutex<State>>>, however completion does not work (it doesn't offer .lock()), and if you hover a or lock it says {unknown}. I guess this is something to do with the Deref trait.

PS: This is with master from today (de24097).

[flodiebold]

The Deref implementation is

impl<T, F: FnOnce() -> T> Deref for Lazy<T, F>

and Lazy is

pub struct Lazy<T, F = fn() -> T>

so the problem here is that we don't provide the built-in implementation of FnOnce for function pointers yet.

[Timmmm]

Is this hard to implement? I might have a go if you can point me in the right direction.

[flodiebold]

It's not trivial, but it's very similar to the existing built-in implementation of the Fn traits for closures, so it should be doable 🙂

Edit: removed outdated instructions. The current situation is that these impls should be added on the Chalk side; see rust-lang/chalk#363.

[lnicola]

Fixed by #5124.