Provide #[archive(as = "...")] for users to provide their own archive types

[djkoloski]

With the changes around ArchiveCopy (#119) the idea of a type being archiveable as itself needs to be changed too.

Adding #[archive(self)] to a type should:

• Add T: Archive<Archived = Self> bounds for each of the fields
• Error on any uses of #[archive_attr] or other attributes except #[archive(copy_safe)]
• Ensure that enums have an appropriate repr
• Not generate an archived version of the type

[djkoloski]

Also provide the syntax #[archive(self = "...")] for these kinds of cases:

#[archive(self = "Self<T::Archived>")]
struct Example<T>(T);

Which currently generate an ArchivedExample<T> when they could just archive to Example<Archived<T>>. This conflicts with the bound T: Archive<Archived = Self> but I think it will be okay once project_struct gets switched to ptr::addr_of_mut because it will still cause a compiler error.

[djkoloski]

This should probably be reworked into #[archive(as = "Self")] and #[archive(as = "Example<T::Archived>")].

[djkoloski]

Fixed by 818e9f0

[dvc94ch]

Fantastic, I was just looking for how to do this!

[dvc94ch]

I wonder if there is a way to implement AsRef<[u8]> for types that are #[archive(as = "Self")]?

[djkoloski]

You can implement AsRef the same as you normally would, this parameter just swaps out the generated archived struct for the one you specify. So if you implement AsRef<[u8]> for a struct, it will archive as itself with #[archive(as = "Self")].

[dvc94ch]

Yes, using the zerocopy crate to derive AsBytes for simple structs.