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Add explanation for custom trait object

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This commit is contained in:
Josef Reinhard Brandl 2018-07-02 18:55:42 +02:00
parent 9eee0d2288
commit cb2c7570db
1 changed files with 18 additions and 1 deletions
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19
src/libcore/future/future_obj.rs
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@ -20,7 +20,15 @@ use task::{Context, Poll};
/// A custom trait object for polling futures, roughly akin to
/// `Box<dyn Future<Output = T> + 'a>`.
/// Contrary to `FutureObj`, `LocalFutureObj` does not have a `Send` bound.
///
/// This custom trait object was introduced for two reasons:
/// - Currently it is not possible to take `dyn Trait` by value and
/// `Box<dyn Trait>` is not available in no_std contexts.
/// - The `Future` trait is currently not object safe: The `Future::poll`
/// method makes uses the arbitrary self types feature and traits in which
/// this feature is used are currently not object safe due to current compiler
/// limitations. (See tracking issue for arbitray self types for more
/// information #44874)
pub struct LocalFutureObj<'a, T> {
ptr: *mut (),
poll_fn: unsafe fn(*mut (), &mut Context) -> Poll<T>,
@ -87,6 +95,15 @@ impl<'a, T> Drop for LocalFutureObj<'a, T> {
/// A custom trait object for polling futures, roughly akin to
/// `Box<dyn Future<Output = T> + Send + 'a>`.
///
/// This custom trait object was introduced for two reasons:
/// - Currently it is not possible to take `dyn Trait` by value and
/// `Box<dyn Trait>` is not available in no_std contexts.
/// - The `Future` trait is currently not object safe: The `Future::poll`
/// method makes uses the arbitrary self types feature and traits in which
/// this feature is used are currently not object safe due to current compiler
/// limitations. (See tracking issue for arbitray self types for more
/// information #44874)
pub struct FutureObj<'a, T>(LocalFutureObj<'a, T>);
unsafe impl<'a, T> Send for FutureObj<'a, T> {}