macro_rules! downcast_get_type_id {
() => {
/// A helper method to get the type ID of the type
/// this trait is implemented on.
/// This method is unsafe to *implement*, since `downcast_ref` relies
/// on the returned `TypeId` to perform a cast.
///
/// Unfortunately, Rust has no notion of a trait method that is
/// unsafe to implement (marking it as `unsafe` makes it unsafe
/// to *call*). As a workaround, we require this method
/// to return a private type along with the `TypeId`. This
/// private type (`PrivateHelper`) has a private constructor,
/// making it impossible for safe code to construct outside of
/// this module. This ensures that safe code cannot violate
/// type-safety by implementing this method.
///
/// We also take `PrivateHelper` as a parameter, to ensure that
/// safe code cannot obtain a `PrivateHelper` instance by
/// delegating to an existing implementation of `__private_get_type_id__`
#[doc(hidden)]
#[allow(dead_code)]
fn __private_get_type_id__(&self, _: PrivateHelper) -> (std::any::TypeId, PrivateHelper)
where
Self: 'static,
{
(std::any::TypeId::of::<Self>(), PrivateHelper(()))
}
};
}
// Generate implementation for dyn $name
macro_rules! downcast_dyn {
($name:ident) => {
/// A struct with a private constructor, for use with
/// `__private_get_type_id__`. Its single field is private,
/// ensuring that it can only be constructed from this module
#[doc(hidden)]
#[allow(dead_code)]
pub struct PrivateHelper(());
impl dyn $name + 'static {
/// Downcasts generic body to a specific type.
#[allow(dead_code)]
pub fn downcast_ref<T: $name + 'static>(&self) -> Option<&T> {
if self.__private_get_type_id__(PrivateHelper(())).0
== std::any::TypeId::of::<T>()
{
// SAFETY: external crates cannot override the default
// implementation of `__private_get_type_id__`, since
// it requires returning a private type. We can therefore
// rely on the returned `TypeId`, which ensures that this
// case is correct.
unsafe { Some(&*(self as *const dyn $name as *const T)) }
} else {
None
}
}
/// Downcasts a generic body to a mutable specific type.
#[allow(dead_code)]
pub fn downcast_mut<T: $name + 'static>(&mut self) -> Option<&mut T> {
if self.__private_get_type_id__(PrivateHelper(())).0
== std::any::TypeId::of::<T>()
{
// SAFETY: external crates cannot override the default
// implementation of `__private_get_type_id__`, since
// it requires returning a private type. We can therefore
// rely on the returned `TypeId`, which ensures that this
// case is correct.
unsafe { Some(&mut *(self as *const dyn $name as *const T as *mut T)) }
} else {
None
}
}
}
};
}
pub(crate) use {downcast_dyn, downcast_get_type_id};
#[cfg(test)]
mod tests {
#![allow(clippy::upper_case_acronyms)]
trait MB {
downcast_get_type_id!();
}
downcast_dyn!(MB);
impl MB for String {}
impl MB for () {}
#[actix_rt::test]
async fn test_any_casting() {
let mut body = String::from("hello cast");
let resp_body: &mut dyn MB = &mut body;
let body = resp_body.downcast_ref::<String>().unwrap();
assert_eq!(body, "hello cast");
let body = resp_body.downcast_mut::<String>().unwrap();
body.push('!');
let body = resp_body.downcast_ref::<String>().unwrap();
assert_eq!(body, "hello cast!");
let not_body = resp_body.downcast_ref::<()>();
assert!(not_body.is_none());
}
}