once_cell::Lazy fails to infer type parameter

[jonas-schievink]

This code fails infer the type of lazy (rust-analyzer gives it the type Lazy<{unknown}, || -> ()>, but the program compiles fine with rustc):

use std::ops::Deref;

struct Lazy<T, F> {
    _t: T,
    _f: F,
}

impl<T, F> Lazy<T, F> {
    fn new(_f: F) -> Self {
        loop {}
    }
}

impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        loop {}
    }
}

fn main() {
    let lazy = Lazy::new(|| ());

    let () = *lazy;
}

I believe this is caused by the autoderef logic not applying the F: FnOnce() -> T bound back to the inference table, as mentioned in this comment:

https://github.com/rust-analyzer/rust-analyzer/blob/3acbf94d29fb57f83666cf6538435b12d4a431b6/crates/hir_ty/src/autoderef.rs#L192-L196

[flodiebold]

Oh, interesting. Yeah, one way to approach this would be to change autoderef and method resolution to happen inside the inference context; the other might be to just record the non-builtin derefs (which we want to do anyway) and add obligations for them (which we probably have to do anyway, given that we perform the deref even when the answer is still ambiguous)... 🤔