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The main reason being that IoCapable doesn't >> actually provide any methods that provide a guarantee of being able to >> read/write from the IO space, but Io does - and IoKnownSize is making >> the guarantee the IO space is at least of size IoKnownSize::MIN_SIZE. > > Mmm I think you are correct. For instance, nothing prevents someone from > implementing `Io` with bogus `addr()` and `maxsize()` methods, which > would trigger undefined behavior with purely safe code. They can, but the `Io` trait just passes the wrong address to the `IoCapabl= e` trait, and nothing horrible can happen without doing things unsafely inside `IoCapable` impl, which is controlled by the user who implements `Io`. It l= ooks to me that unsafe code is still needed to do bogus things. Can you give a more detailed example on how safe code can cause issue with = a safe `Io` trait? Best, Gary