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// FIXME: Link below is required to render in index /// Asserts that the traits support dynamic dispatch /// ([object-safety](https://doc.rust-lang.org/book/ch17-02-trait-objects.html#object-safety-is-required-for-trait-objects)). /// /// 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. /// /// ``` /// # #[macro_use] extern crate static_assertions; fn main() {} /// trait MySafeTrait { /// fn foo(&self) -> u32; /// } /// /// assert_obj_safe!(std::fmt::Write, MySafeTrait); /// ``` /// /// Works with traits that are not in the calling module: /// /// ``` /// # #[macro_use] extern crate static_assertions; fn main() {} /// 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: /// /// ```compile_fail /// # #[macro_use] extern crate static_assertions; fn main() {} /// assert_impl!(*const u8, Send); /// ``` /// /// The following example fails to compile because generics without /// `where Self: Sized` are not allowed in [object-safe][object] trait methods: /// /// ```compile_fail /// # #[macro_use] extern crate static_assertions; fn main() {} /// trait MyUnsafeTrait { /// fn baz<T>(&self) -> T; /// } /// /// assert_obj_safe!(MyUnsafeTrait); /// ``` /// /// When we fix that, the previous code will compile: /// /// ``` /// # #[macro_use] extern crate static_assertions; fn main() {} /// trait MyUnsafeTrait { /// fn baz<T>(&self) -> T where Self: Sized; /// } /// /// assert_obj_safe!(MyUnsafeTrait); /// ``` /// /// [object]: https://doc.rust-lang.org/book/ch17-02-trait-objects.html#object-safety-is-required-for-trait-objects #[macro_export] macro_rules! assert_obj_safe { ($($xs:path),+ $(,)?) => { $(const _: Option<&$xs> = None;)+ }; }