Enum Ptr

This crate provides a custom derive macro EnumPtr to generate bridges between an enum T and Compact<T> with minimum cost. Compact<T> is the compact representation of T, and it is only one pointer wide. In other words, this crate is a library for defining tagged pointers in ergonomic way, even supporting different pointer types (&, Box, Arc, etc) as different enum variants.

For example, the following code

use enum_ptr::EnumPtr;

#[derive(EnumPtr)]
#[repr(C, usize)]
enum Foo<'a> {
    A(&'a i32),
    B(Box<i32>),
}

will generate

impl<'a> From<Foo<'a>> for Compact<Foo<'a>> {
    // ...
}

impl<'a> From<Compact<Foo<'a>>> for Foo<'a> {
    // ...
}

Since &i32 and Box<i32> are aligned by 4 bytes, the lowest 2 bits of them are always zeros. Compact<Foo<'a>> utilizes these bits to store the tag (discriminant).

Features

• No need to write unsafe pointer operations
• Supports various types and can be extended
• Supports no_std
• Minimum type conversion cost
• Passes cargo +nightly miri test with strict provenance enabled.

Testing

$ cargo test
$ cargo +nightly miri test

Credits

• Thanks to @oxalica for reviewing this crate and providing a lot of helpful suggestions.

License

This project is licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.