sealed.rs

// 1: Only my crate can implement *and* use it (plain visibilty)
pub(crate) trait AllocImpl {}

// 2: Only my module can implement the trait, others can use it
mod private {
  pub trait Sealed {}
}

pub trait AllocImpl: private::Sealed {}

// 3: Other crates can implement but I have some private stuff only my module must do
pub trait AllocImpl {}

trait AllocImplExt: AllocImpl {
  // the private stuff, implementation can only be in terms of things exposed in AllocImpl
}

impl<T: AllocImpl> AllocImplExt for T {
  // ...
}