Unsized rvalues now making this lint inaccurate

The following code

#![feature(unsized_locals)]
fn a(f: Box<dyn FnOnce()>) {
    f()
}

fn main() {
    let f = || println!("Hello!");
    a(Box::new(f))
}

Playground

is now allowed in nightly. However, Clippy gives inaccurate lint:

    Checking playground v0.0.1 (/playground)
warning: this argument is passed by value, but not consumed in the function body
 --> src/main.rs:2:9
  |
2 | fn a(f: Box<dyn FnOnce()>) {
  |         ^^^^^^^^^^^^^^^^^ help: consider taking a reference instead: `&Box<dyn FnOnce()>`
  |
  = note: #[warn(clippy::needless_pass_by_value)] on by default
  = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#needless_pass_by_value

    Finished dev [unoptimized + debuginfo] target(s) in 0.66s

Because when run the code above, it do consumes the variable f when using unsized rvalues, and it cannot run without it.

Start with Clippy's needless_pass_by_value lint and reproduce the provided unsized_locals example on nightly. The work is done when the lint no longer reports an inaccurate suggestion for the consumed boxed FnOnce value and the behavior is covered by an appropriate regression test.