Clarify semantics of the various pointer to pointer casts

[udoprog]

Relates #1448

I've tried to the best of my ability to reproduce the verbiage of the reference, some parts feel a bit wordy to me so any feedback would be appreciated.

To fully describe the semantics of unsized to sized pointer casts, I felt it was necessary to document that the reinterpreting sized to sized casts produce identical values. I couldn't immediately find this documented.

This is because unsized to sized pointer casts also permits a reintepreting cast in the same sweep (such as *const [u8] as *const u32).

[ehuss]

@RalfJung Can you review this?

[ehuss]

The word "metadata" isn't defined anywhere. Can you add a description here that defines what it means? (Or wherever seems appropriate.)

[udoprog]

I don't know if I'm just promoting kicking the can down the road, but I opted not to define metadata since:

• It's not necessary to define the semantics of this cast, the only distinction is that unsized pointers have metadata and this cast removes it.
• Another part of the reference "behaviors considered undefined" uses metadata without defining it.

That being said, if I were to define it, where do you think I should do it and to what extent?

The representation of pointers and references to DSTs is made up of two components; A base pointer and metadata. The metadata is the necessary extra data that has to be attached to pointers and references of DSTs which completes their representation. This is stored adjacent to the base pointer causing them to be wider than sized pointers earning them the name "wide" or "fat" pointers. The base pointer can be extracted through an unsized to sized pointer cast.

I'm a bit concerned that I'll end up having to define everything metadata adjacent 😓, so at least I'd want to avoid that as much as possible.

[RalfJung]

I think this might be better structured in a different way... I'd make a single big section for raw-to-raw casts, which also handles once and for all the mut/const part:

#### Pointer-to-pointer cast

`*const T`/`*mut T` can be cast to `*const U`/`*mut U` under the following conditions:

- If both `T` and `U` are sized, [...]
- If both `T` and `U` are unsized, [...]
- If `T` is unsized and `U` is sized, [...]

[udoprog]

Thanks for taking the time!

Note that if you want to change something, feel free to edit it directly (assuming that's enabled) if that is your preference. I only have an interest in the cast being documented - I don't mind how it's done or who does it.

[RalfJung]

LGTM, but since I now largely rewrote this it should be reviewed by someone else.

[udoprog]

For what it's worth, you have my +1.

[udoprog]

This seems a bit stalled. Since I'm pretty much OK with this change, can we say that I wrote it and move forward? 😄

I think this would pretty much emulate the same scenario as if this was fully an external change proposed by me which was reviewed by Ralf. Or maybe @ehuss can give this a quick look over?

[RalfJung]

@udoprog please squash the PR into a single commit.

But yeah, for review would be good if @ehuss took a look. Maybe someone from @rust-lang/opsem can help as well.

[udoprog]

Done

[ehuss]

Thanks!