[Runtime] Add function_cast, switch from std::bit_cast.

[mikeash]

Function types aren't always trivially copyable, e.g. with address-discriminated signed pointers on ARM64e. Introduce a function_cast helper and use that instead.

[bnbarham]

@swift-ci please test

[compnerd]

This adds explicit UB rather than implicit UB. The std::bit_cast is absolutely required even if it doesn’t work on arm64e. We should instead be stripping pointer signing and resigning after the copy for ptrauth enabled environments. That would allow this to work on all Von Neumann architectures.

The implicit UB that we form in the std::bit_cast approach is for Harvard architectures or more complex memory architectures such as GPU, which may have multiple address spaces. In both those scenarios, the pointer itself is inconvertible and we would form something which is invalid.

[mikeash]

Manually stripping and re-signing would be pretty annoying and error-prone. What if we implement function_cast in terms of bit_cast when there's no ptrauth, and implement it with reinterpret_cast only when __has_feature(ptrauth_calls)? Or to make the condition directly address the problem, only when the types are not is_trivially_copyable.

[rjmccall]

Stripping and resigning is not allowed; pointer authentication is, in fact, actually supposed to do something.

[mikeash]

Right, we'd really want to auth and re-sign. But doing that manually is still not an attractive prospect.

[mikeash]

Seems that it's even simpler. The ptrauth annotation drops off the source once we're inside function_cast, so we can implement it as just an unconditional wrapper around std::bit_cast. I've updated the PR with this approach.

[mikeash]

@compnerd Are you good with this new approach?

[compnerd]

Suggested change

	// Ptrauth attributes decay away here, so we can now bit_cast in peace.
	// ptrauth attributes decay away here, so we can now bit_cast in peace.

[compnerd]

Can we inline the std::bit_cast implementation for compatibility with older C++ standards?

[mikeash]

@swift-ci please test

[mikeash]

@swift-ci please smoke test macos platform

[mikeash]

@swift-ci please test