Vendor things

third-party/vendor/static_assertions/src/assert_obj_safe.rs

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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;)+
+    };
+}