Package: rust-core-arch As far as I can tell the core_arch crate on crates.io was a testing ground for what became core::arch in the standard library and is now abandoned. The package relies on "nightly" compiler features and has FTBFS for over a year without anyone caring enough to fix it. IMO it's time for it to go.
If noone objects I will turn this bug into a removal request in a week or two.