boxed_local doesn't work with the FnBox pattern.

[Stebalien]

In my template library horrorshow, I have the following definition to allow RenderOnce::render_once to be called through a boxed RenderOnce.

pub trait RenderBox {
    /// Do not call. Called by RenderOnce impl on Box<RenderBox>
    #[doc(hidden)]
    fn render_box(self: Box<Self>, tmpl: &mut TemplateBuffer);

    /// Do not call. Called by RenderOnce impl on Box<RenderBox>
    #[doc(hidden)]
    fn size_hint_box(&self) -> usize;
}


impl<T> RenderBox for T where T: RenderOnce {
    fn render_box(self: Box<T>, tmpl: &mut TemplateBuffer) {
        (*self).render_once(tmpl);
    }

    fn size_hint_box(&self) -> usize {
        RenderOnce::size_hint(self)
    }
}

// Box<RenderBox>

impl<'b> RenderOnce for Box<RenderBox + 'b> {
    #[inline]
    fn render_once(self, tmpl: &mut TemplateBuffer) {
        RenderBox::render_box(self, tmpl);
    }

    #[inline]
    fn size_hint(&self) -> usize {
        RenderBox::size_hint_box(self)
    }
}

This is the same pattern as stdlib's FnBox/FnOnce. Unfortunately, clippy claims that this box is unnecessary:

src/render.rs:68:19: 68:23 warning: local variable doesn't need to be boxed here, #[warn(boxed_local)] on by default
src/render.rs:68     fn render_box(self: Box<T>, tmpl: &mut TemplateBuffer) {

[oli-obk]

well... it is "unnecessary" at the implementation site unless you also add a ?Sized bound.

[Stebalien]

RenderBox must only be implemented on sized T's because it moves a value of type T out of the box (Box<T>). Because RenderBox knows the size of the real type, RenderOnce can be implemented on Box<RenderBox> to recover the size and move the real T out of the box. Unfortunately, I can't give a better explanation because I don't really know how this works under the covers.

See FnBox: https://doc.rust-lang.org/src/alloc/up/src/liballoc/boxed.rs.html#529-533

[phansch]

Fixed via #3321 (feel free to re-open if it's still a problem)