Relationship to the Transmute pre-rfc

[asajeffrey]

I read this and thought of you... https://internals.rust-lang.org/t/pre-rfc-safe-transmute/11347

[lovesegfault]

Thanks Alan! @Ekleog will definitely have some thoughts on that, as well as @nagisa

[lovesegfault]

Thoughts as I read:

• I find the idea of a trait that cannot be impl'd to be weird; maybe they mean you can only unsafe impl it?
• ToFromBytes is a horrible name, I think overall they mean that the type is isomorphic to a byte slice.
• Maybe having the compiler verify type isomorphisms is where this thing is going?
• All Transmutable types are isomorphic to one another (via bytes) (?)
• Maybe they should just be two traits, FromBytes and ToBytes

[Ekleog]

We're not really using transmute anywhere that I can remember, nor expose a safe way of doing so in our API. So I'm not really sure I can think of a place where we could use such a mechanism here?

The point of the interprocess traits as currently defined here is to guarantee things assuming the references are to the same types on both sides of the process boundary, and that processes are cooperative; meaning that no transmute should be happening, no padding should be exposed, etc.

[asajeffrey]

There must be somewhere a transmute is happening, even if it's hidden in read_volatile.

[asajeffrey]

And yes, all transmutable types of the same size are isomorphic.

[Ekleog]

Well, in inteprocess-traits, we do assume that processes are cooperative, and that the unsafe requirements are satisfied on both sides.

With the unsafe requirements being satisfied, it is impossible to retrieve references to the same memory region as two distinct types. In other words, checking the absence of transmute is pushed to the user as a requirement to verify.

Also, because that is the API of ProcSync, we allow multiple & references to exist to the same memory region from different processes at the same time -- assuming the type is ProcSync. So we don't really need to do read_volatile either. We just do regular memory reads, and they're guaranteed to be of the same type as what was written by the unsafe requirements.

Is that more clear?