Sealing Trait §

Sealing traits with a supertrait(best practice) §

mod private {
    pub trait Sealed {}
}
 
pub trait SealedTrait : private::Sealed {
    fn method(&self);
}

pub struct TypeThatImplsSealed;
 
impl private::Sealed for TypeThatImplsSealed {}
 
impl SealedTrait for TypeThatImplsSealed {
    fn method(&self) {}
}

 Sealing the trait in this way doesn’t prevent downstream code from calling its methods. The following code in a downstream crate works just fine:

fn use_sealed(value: impl upstream::SealedTrait) {
    value.method()
}

Sealing traits via method signatures §

mod private {
    pub struct Token;
}
 
pub trait SealedTrait {
    fn method(&self, _: private::Token);
}

pub struct TypeThatImplsSealed;
 
impl SealedTrait for TypeThatImplsSealed {
    fn method(&self, _: private::Token) {
        // impl here
    }
}

Meanwhile, downstream code can both see and name the trait and its method, but cannot implement the trait nor call the method.

References §

• A definitive guide to sealed traits in Rust