The title. I have OSX, and the problem occurs if I have a dependancy of the cocoa crate with any version ≤ 0.17 (Everything 0.18+ works). The error I'm getting is that msg_send![] needs a type annotation.
So I'm wondering if there would be a way to fix that (I'm not using cocoa directly, it's a dependency of other crates, specifically v0.11.0 and v0.14.0), so I'd need to change the Cargo.toml files of other crates, which I'm not so comfortable to do, especially because there are so many crates to check.
Unfortunately, you'd need the failing crate to change its code or upgrade the dependency. [patch] section can help you overwrite code of a dependency if you just need a quick fix for yourself.
IIRC the msg_send! macro used to rely on a bug/edge-case in type inference in the compiler, and when the compiler stopped assuming that everything unknown is (), then msg_send stopped defaulting to returning (), and now it needs annotation for its unspecified return type.