Fix so the compiler can infer msg_send! return types (#1227)

* Fix so the compiler can infer msg_send! return type

Currently, due to a quirk in Rust's type inference interacting with the
structure of the msg_send! macro, a () return type will be inferred when
the compiler cannot otherwise determine the return type. This behavior
is expected to change, and in the future could resolve to a ! return
type, which results in undefined behavior.

Linting has previously been added for this in rust-lang/rust#39216, but
it did not catch these cases due to SSheldon/rust-objc#62. An upcoming
version of objc will be fixed to stop hiding these errors, at which
point they will become compile errors.

This change fixes these errors and allows winit to compile with the
fixed version of objc.

* Bump cocoa to 0.19.1

Cargo.toml

@@ -37,7 +37,7 @@ version = "0.2"
 objc = "0.2.3"
 
 [target.'cfg(target_os = "macos")'.dependencies]
-cocoa = "0.18.4"
+cocoa = "0.19.1"
 core-foundation = "0.6"
 core-graphics = "0.17.3"
 dispatch = "0.1.4"

src/platform_impl/macos/window.rs

@@ -786,7 +786,7 @@ impl UnownedWindow {
                 // of the menu bar, and this looks broken, so we must make sure
                 // that the menu bar is disabled. This is done in the window
                 // delegate in `window:willUseFullScreenPresentationOptions:`.
-                msg_send![*self.ns_window, setLevel: ffi::CGShieldingWindowLevel() + 1];
+                let () = msg_send![*self.ns_window, setLevel: ffi::CGShieldingWindowLevel() + 1];
             },
             (
                 &Some(Fullscreen::Exclusive(RootVideoMode { ref video_mode })),