Non-ZST stop static is probably unsound

[GoldsteinE]

Currently linkme generates an extern static of Element type for the “stop section” symbol:

linkme/impl/src/declaration.rs

Lines 139 to 143 in 0e5e28b

	#[cfg_attr(any(target_os = "none", target_os = "linux", target_os = "android", target_os = "fuchsia", target_os = "psp"), link_name = #linux_section_stop)]
	#[cfg_attr(any(target_os = "macos", target_os = "ios", target_os = "tvos"), link_name = #macho_section_stop)]
	#[cfg_attr(target_os = "illumos", link_name = #illumos_section_stop)]
	#[cfg_attr(any(target_os = "freebsd", target_os = "openbsd"), link_name = #bsd_section_stop)]
	static LINKME_STOP: <#ty as #linkme_path::__private::Slice>::Element;

As pointed out in rust-lang/rust#46188 (comment), statics are considered always-dereferenceable, so it’s probably unsound. As the stop static is never actually read from, I think it could be made zero-sized to remove the unsoundness?

[RalfJung]

Yes, this is indeed unsound and should be made zero-sized.

LINKME_START is also unsound if the array ends up being empty.

[GoldsteinE]

Yes, this is indeed unsound and should be made zero-sized.

Are zero-sized extern statics portable? I’m unsure because C/++ doesn’t have ZSTs, so I don’t know if LLVM portably supports it.

[RalfJung]

Yes they are. LLVM supports them just fine.