Odd type inference failure: unable to infer enough type information about `_`

[bgamari]

This,

/// A binary tree with nodes labelled with `T`
pub struct Tree<T> {
    pub value: T,
    pub left: Option<Box<Tree<T>>>,
    pub right: Option<Box<Tree<T>>>,
}

impl<T> Tree<T> {
    pub fn map_step<F, V>(self, f: &mut FnMut(T) -> V) -> Tree<V> {
        Tree {
            left: self.left.map(|x| { let a: Tree<V> = x.map_step(f);
                                      Box::new(a) }),
            right: self.right.map(|x| Box::new(x.map_step(f))),
            value: f(self.value),
        }
    }
}

fn main() {}

fails with,

hi.rs:12:58: 12:69 error: unable to infer enough type information about `_`; type annotations required [E0282]
hi.rs:12             left: self.left.map(|x| { let a: Tree<V> = x.map_step(f);
                                                                  ^~~~~~~~~~~

All of my attempts at helping the compiler find the right types have failed, therefore I suspect it's a bug.

[Aatch]

There isn't enough information for the compiler to infer the type of F in this case, since it's not used anywhere in the function signature.

The message could be more clear though, closing as a duplicate of #19477

[bgamari]

@Aatch, I'm not sure I follow. The type of f is given explicitly in the signature of map_step.

[bgamari]

Ahh, I missed the F type variable. It was a remnant from refactoring. Thanks again!