ZeroizeOnDrop auto-deref

[daxpedda]

This changes nothing from the users perspective, but will enable to implement ZeroizeOnDrop for fields that implement it replacing the need to skip them instead.

This will also validate that these fields types implement ZeroizeOnDrop.

See #652 for further discussion.
Builds on top of #699.

Fixes #652.

[tarcieri]

I’d suggest making these proper sealed traits by placing them in a private module, rather than merely hiding them from the docs. Otherwise they’re still a part of the public API.

[daxpedda]

They need to be part of the public API, otherwise the derive proc-macro can't access them. Some alternatives:

• Rename them further to __AssertZeroizeOnDrop.
• Remove them from zeroize and let the proc-macro insert them. This has the disadvantage of requiring every impl block to add them, which might increase compile time.
• Mark traits as unsafe.

Or a combination of these.

[tarcieri]

otherwise the derive proc-macro can't access them

Aah!

Rename them further to __AssertZeroizeOnDrop.

Another option would be to put them in a module like pub mod __internal, which would also let you do #[doc(hidden)] in a single place.

Mark traits as unsafe.

Since the trait doesn't deal strictly with memory safety I'd prefer not to do this.

[daxpedda]

Done the __internal thing!

[tarcieri]

Thanks!