Expand description
§#[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 = "0.5"§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/.
#[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§Details
The attribute generates a private uniquely named module when attached to a
trait definition, when attached to an impl block the generated code simply
implements the sealed trait for the respective type.
// #[sealed]
// trait T {}
trait T: __seal_t::Sealed {}
mod __seal_t {
pub trait Sealed {}
}
pub struct A;
// #[sealed]
// impl T for A {}
impl T for A {}
impl __seal_t::Sealed for A {}§Arguments
The expanded code may be customized with the following attribute arguments.
§erase
Turns on trait bounds erasure. This is useful when using the #[sealed]
attribute inside a function. By default, all the bounds are propagated to
the generated Sealed trait.
// #[sealed(erase)]
// trait Trait<T: ?Sized + Default> {}
trait Trait<T: ?Sized + Default>: __seal_trait::Sealed<T> {}
mod __seal_trait {
pub trait Sealed<T> {}
}§pub(crate) or 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.
mod lets {
pub mod attempt {
pub mod some {
pub mod nesting {
#[sealed(pub(in super::super::super::super))]
pub trait T {}
}
}
}
}
pub struct A;
#[sealed]
impl lets::attempt::some::nesting::T for A {}Notice, that just pub is disallowed as breaks the whole idea of sealing.
#[sealed(pub)] // compile error
trait T {}
pub struct A;
#[sealed]
impl T for A {}