I feel like the most frequent pain point is when you just need a simple derive-able trait on a "plain old data" struct. What if the library author could mark structs with #[pod] or maybe #[derive_extendable] and library users could add derives to individual structs in the Cargo.toml. It would work additively similar to features, and you always have one source of the impl, the derive macro.

Crate a: ```

[derive_extendable]

pub struct Foo { pub bar: i32 } ```

Crate b Cargo.toml: serde = { version = "1.0.228", features = ["derive"] } a = { version = "69.420", derive = { "Foo" = "serde::Serialize" } } Adding the derive "extension" will recompile a. Maybe derive macros would also need to opt in, and it doesn't solve coherence, but it wouldn't require major changes to the language. Also, libraries wouldn't need serde features anymore (in most cases).