Fail compilation on targets w/o soundness proof

[joshlf]

TODO: Figure out how to make sure we block the next minor version release on adding back all of the targets that people actually rely on.

Makes progress on #383

[jswrenn]

Re. this:

TODO: Figure out how to make sure we block the next minor version release on adding back all of the targets that people actually rely on.

We could potentially take the initial, weaker step of merely warning on unproven architectures:

Suggested change

	compile_error!(
	r#"Zerocopy has not been proven sound on this architecture.
	This doesn't mean that it is unsound - just that we haven't gotten around to
	proving it sound. If you need support for this architecture, let us know and
	we'll prioritize it: https://github.com/google/zerocopy/issues/383"#
	);
	const _: () = {
	#[deprecated = r#"Zerocopy has not been proven sound on this architecture.
	This doesn't mean that it is unsound - just that we haven't gotten around to
	proving it sound. If you need support for this architecture, let us know and
	we'll prioritize it: https://github.com/google/zerocopy/issues/383"#]
	const _WARNING: () = ();
	#[warn(deprecated)]
	_WARNING
	};

[jswrenn]

I think there's almost certainly merit in providing an escape hatch for weird architectures, too:

Suggested change

	compile_error!(
	r#"Zerocopy has not been proven sound on this architecture.
	This doesn't mean that it is unsound - just that we haven't gotten around to
	proving it sound. If you need support for this architecture, let us know and
	we'll prioritize it: https://github.com/google/zerocopy/issues/383"#
	);
	#[cfg(not(ZEROCOPY_ALLOW_EXPERIMENTAL_ARCH))]
	compile_error!(
	r#"Zerocopy has not been proven sound on this architecture.
	This doesn't mean that it is unsound - just that we haven't gotten around to
	proving it sound. If you need support for this architecture, let us know and
	we'll prioritize it: https://github.com/google/zerocopy/issues/383
	You can temporarily enable this architecture by setting:
	export RUSTFLAGS="--cfg ZEROCOPY_ALLOW_EXPERIMENTAL_ARCH"
	"#
	);
	#[cfg(ZEROCOPY_ALLOW_EXPERIMENTAL_ARCH)]
	const _: () = {
	#[deprecated = r#"If you need support for this architecture, let us know and
	we'll prioritize it: https://github.com/google/zerocopy/issues/383"#]
	const _WARNING: () = ();
	#[warn(deprecated)]
	_WARNING
	};

[jswrenn]

When an architecture is expressly unsupported, we should report asmuch:

Suggested change

	#[cfg(target_arch = "spirv")]
	compile_error!(
	r#"Zerocopy is not sound on this architecture. SPIRV's memory model
	prohibits freely reinterpreting memory. If you believe this limitation is
	incorrect, let us know: https://github.com/google/zerocopy/issues/383"#
	);

[joshlf]

Given the discussion in rust-lang/unsafe-code-guidelines#461, I'm going to close this on the assumption that we'll adopt a policy of not reasoning about target architecture. We can always re-open if that's not how it plays out.