Proof of concept for proc macros

[silvanshade]

Following the discussion from #411

[silvanshade]

We could probably make specifying even retain optional in the typical case since we could just determine this by looking at the return type of the signature.

[madsmtm]

The reason that I did it the @__retain_semantics extern_methods! originally is because of performance (otherwise it uses const stuff to figure out the retain method from the selector), but doing such work is possibly performant enough when running as a proc macro, so we might not need it at all.

[silvanshade]

Similar to my other comment, we could allow eliding the retain info here as well (by examining the type of this).

[silvanshade]

Thinking about this some more, it seems like we might be able to simplify things more than I anticipated.

What if we had something like this:

#[class(extern, super = NSActionCell, inherits = NSCell, NSObject)]
struct NSPathCell;

// `unsafe impl ClassType for NSPathCell` added automatically

#[class]
unsafe impl NSPathCell {
    // #[method] can often be skipped entirely
    pub unsafe fn pathStyle(&self) -> NSPathStyle;
    pub unsafe fn setPathStyle(&self, path_style: NSPathStyle);
    #[cfg(all(feature = "Foundation_NSArray", feature = "Foundation_NSString"))]
    pub unsafe fn allowedTypes(&self) -> Option<Id<NSArray<NSString>, Shared>>;
}

#[protocol] // `: NSObjectProtocol` can be added automatically
pub unsafe trait NSPathCellDelegate {
    #[cfg(all(feature = "AppKit_NSOpenPanel", feature = "AppKit_NSPathCell"))]
    #[method(optional)] // selector computed from name
    unsafe fn pathCell_willDisplayOpenPanel(
        &self,
        path_cell: &NSPathCell,
        open_panel: &NSOpenPanel,
    );
}

// `unsafe impl ProtocolType for dyn NSPathCellDelegate {}` added automatically

Anyway, I might try experimenting with a few different approaches to see what seems feasible.

[madsmtm]

// #[method] can often be skipped entirely

Don't spend too much time on this; we'll want different names for methods anyhow: #284

#[class(extern, super = NSActionCell, inherits = NSCell, NSObject)]
struct NSPathCell;

I'm not sure about this one, it would allow writing something like #[class(super = NSString)] struct NSCell; without unsafe anywhere. We'd have to make sure that case was sound (if not still nonsensical).

[madsmtm]

Also, I suspect I'd like to keep msg_send! and msg_send_id! as non proc-macros; that way, if users just want a better objc, they can opt out of the more advanced functionality, and just get the improvements that msg_send_id! provides.

[madsmtm]

Superseded by #425