Attribute Macro seal

#[seal]

Seal a trait.

Sealed Traits

Sealed traits are documented in the Rust API Guidelines as a way to prevent downstream implementations while providing a public trait API. This implementation enforces that the trait is implemented for the exact set of types specified, no more and no less.

use traitful::seal;

// Make it so that only cats and cat-people can meow
#[seal(Cat, CatPerson)]
pub trait Meow {
    fn meow() {
        println!("meow");
    }
}

pub struct Cat;

impl Meow for Cat {}

pub struct CatPerson;

// Will fail to compile if this line is commented out
impl Meow for CatPerson {}

Generics

You can also seal traits with generics:

use traitful::seal;

pub struct Newtype<T>(T);

impl<T> Generic<T> for Newtype<T> {}

#[seal(Newtype<T>)]
pub trait Generic<T> {}

Generics with for syntax

You can also seal traits to types with generics without adding them to the trait by using the for syntax.

use traitful::seal;

pub struct Newtype<T>(T);

impl<T> MaybeGeneric for Newtype<T> {}

pub struct UnitStruct;

impl MaybeGeneric for UnitStruct {}

#[seal(for<T> Newtype<T>, UnitStruct)]
pub trait MaybeGeneric {}