extobj

A tiny, zero-cost library that lets you extend a struct with new fields across crate boundaries without recompiling upstream crates.

Quick start

use extobj::{extobj, ExtObj};

// 1. Declare the extension object type.
extobj!(struct Player);

// 2. In any downstream crate, add fields.
extobj!(impl Player {
    pub health: i32,
    level: u32,
});

// 3. Use it.
let mut p = ExtObj::<Player>::new();
assert_eq!(p[*health], 0);     // Default
p[*health] = 100;
p[*level]  = 5;
assert_eq!(p[*health], 100);

How it works

• Every field becomes a static Var<Player, T> singleton.
• ExtObj is a grow-only vector whose slot i stores a T created by the corresponding Var.
• Construction / destruction are routed through an internal v-table generated by the macro.
• All public APIs are safe; unsafe internals are confined to the crate.

Cross-crate usage

Upstream crate:

// game_core/src/lib.rs
pub use extobj::{extobj, ExtObj};

Downstream crate:

// game_ui/src/lib.rs
use game_core::{extobj, ExtObj};

extobj!(impl game_core::Player {
    pub visible: bool,
});

No recompilation of game_core is required.

Optional crate rename

If you re-export extobj under a different name, tell the macro:

extobj!(struct Foo, crate_path = my_reexport);

License

MIT OR Apache-2.0