Type inference for the expected type of arguments doesn't work when coercing from a closure expression. For example, this code does not compile https://is.gd/FNoLWg (at least, not without an explicit x: Vec<u32> annotation):

#![feature(closure_to_fn_coercion)]

fn foo(f: fn(Vec<u32>) -> usize) { }

fn main() {
    foo(|x| x.len())
}