[jarpadat]

That discussion is a bit strange. In that example Foo is implicitly #[repr(Rust)] meaning it has an undefined layout. In particular, a,b is unordered, and even if you don't care which order you get, there is the question of padding (a reasonable compiler will tightly pack a,b but that's not required).

For this reason, we never reach the question of alignment because we know neither i32, u8, nor any other type has the same layout as Foo (undefined layout).

It is certainly true that unsafe Rust is veeeeery unsafe, as perhaps evidenced by me being the first person to point out the repr issue. On the other hand this scheme has a lot of advantages for writing safe Rust.