#[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 = "0.3"

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.

Examples are available in examples/, you can also see a demo in demo/.

use sealed::sealed;

#[sealed]
trait T {}

pub struct A;

#[sealed]
impl T for A {}

pub struct B;

#[sealed]
impl T for B {}

pub struct C;

impl T for C {} // compile error

Arguments

This is the list of arguments that can be used in a #[sealed] attribute:

• #[sealed(erase)]: turns on trait bounds erasure. This is useful when using the #[sealed] macro inside a function. For an example, see bound-erasure-fn example.

• #[sealed(pub(crate))] or #[sealed(pub(in some::path))]: allows to tune visibility of the generated sealing module (the default one is private). This useful when the trait and its impls are defined in different modules. For an example, see nesting example. Notice, that just pub is disallowed as breaks the whole idea of sealing.

Details

The #[sealed] attribute can be attached to either a trait or an impl. It supports:

• Several traits per module
• Generic parameters
• Foreign types
• Blanket impls

Expansion

Input

use sealed::sealed;

pub struct A;
pub struct B(i32);

#[sealed]
pub trait T {}

#[sealed]
impl T for A {}

#[sealed]
impl T for B {}

Expanded

use sealed::sealed;

pub struct A;
pub struct B(i32);

mod __seal_t {
    pub trait Sealed {}
}
pub trait T: __seal_t::Sealed {}

impl __seal_t::Sealed for A {}
impl T for A {}

impl __seal_t::Sealed for B {}
impl T for B {}