[RFC] Allow packed types to transitively contain aligned types

[PixelDust22]

Rendered

This issue was introduced in the original implementation of #[repr(packed(N))] and have since underwent extensive community discussions:

• #[repr(align(N))] fields not allowed in #[repr(packed(M>=N))] structs
• repr(C) does not always match the current target's C toolchain (when that target is windows-msvc)
• repr(C) is unsound on MSVC targets
• E0587 error on packed and aligned structures from C
• E0587 error on packed and aligned structures from C (bindgen)
• Support for both packed and aligned (in repr(C)
• bindgen wanted features & bugfixes (Rust-for-Linux)
• packed type cannot transitively contain a #[repr(align)] type
• structure layout using aligned attribute is incorrect

The solution proposed in this RFC mostly reflects what was proposed here