Macro static_assertions::assert_obj_safe

macro_rules! assert_obj_safe {
    ($($xs:path),+ $(,)?) => { ... };
}

Asserts that the traits support dynamic dispatch (object-safety).

This is useful for when changes are made to a trait that accidentally prevent it from being used as an object. Such a case would be adding a generic method and forgetting to add where Self: Sized after it. If left unnoticed, that mistake will affect crate users and break both forward and backward compatibility.

Examples

When exposing a public API, it’s important that traits that could previously use dynamic dispatch can still do so in future compatible crate versions.

trait MySafeTrait {
    fn foo(&self) -> u32;
}

assert_obj_safe!(std::fmt::Write, MySafeTrait);

Works with traits that are not in the calling module:

mod inner {
    pub trait BasicTrait {
        fn bar(&self);
    }
    assert_obj_safe!(BasicTrait);
}

assert_obj_safe!(inner::BasicTrait);

The following example fails to compile because raw pointers cannot be sent between threads safely:

assert_impl!(*const u8, Send);

The following example fails to compile because generics without where Self: Sized are not allowed in object-safe trait methods:

trait MyUnsafeTrait {
    fn baz<T>(&self) -> T;
}

assert_obj_safe!(MyUnsafeTrait);

When we fix that, the previous code will compile:

trait MyUnsafeTrait {
    fn baz<T>(&self) -> T where Self: Sized;
}

assert_obj_safe!(MyUnsafeTrait);