#[sealed]

This crate provides a convenient and simple way to implement the sealed trait pattern, as described in the Rust API Guidelines [1].

[dependencies]
sealed = "0.1"

Example

In the following code structs A and B implement the sealed trait T, the C struct, which is not sealed, will error during compilation.

You can see a demo in demo/.

use sealed::sealed;

#[sealed]
trait T {}

#[sealed]
pub struct A;

impl T for A {}

#[sealed]
pub struct B;

impl T for B {}

pub struct C;

impl T for C {} // compile error

fn main() {
    return
}

Details

The macro generates a private module when attached to a trait (this raises the limitation that the #[sealed] macro can only be added to a single trait per module), when attached to a struct the generated code simply implements the sealed trait for the respective structure.

Expansion

// #[sealed]
// trait T {}
trait T: private::Sealed {}
mod private {
    trait Sealed {}
}

// #[sealed]
// pub struct A;
pub struct A;
impl private::Sealed for A {}