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> These two macros now let you safely create unaligned pointers. So now the compiler will generate calls to memcpy() for accesses to unaligned elements in pack

• kzrdude 6y ago
  Good question. I think the answer is no, and that `let value = *ptr;` is UB if the pointer is misaligned. The right way to go is `let value = ptr.read_unaligned();` But the docs will say this: > Accessing unaligned fields directly with e.g. packed.unaligned is safe however. https://doc.rust-lang.org/nightly/std/ptr/fn.read_unaligned.html https://doc.rust-lang.org/nightly/std/ptr/fn.read_unaligned....
• coolreader18 6y ago
  Creating unaligned references is UB, so it's illegal to take a reference to a packed struct field. However, even if you just wanted to create a raw pointer to that field, you would've had to do `&packed.f2 as *const u16`, which is technically UB since you do create a temporary reference before casting it to a pointer. I'm sure one way to solve that is to just allow that as a special case; say that `&<expr> as *const _` semantically doesn't actually create a reference. But, the way the lang team went is to have a separate macro that transforms `ptr::addr_of!(<expr>)` to something like C's `&<expr>` operator, so that it creates a raw pointer directly instead of a reference.
  • kzrdude 6y ago
    Making the existing syntax - `&<expr> as *const _` like you said - just work would be far and away the best solution. Otherwise we have a 5-year transition period before all code out there that touch this get the memo.