remove trait-upcasting restriction on downstream users of provider

Author: yaahc

library/core/src/any.rs

@@ -110,9 +110,7 @@
 //! ## Examples
 //!
 //! ```
-//! # #![allow(incomplete_features)]
 //! # #![feature(provide_any)]
-//! # #![feature(trait_upcasting)]
 //! use std::any::{Provider, Demand, request_ref};
 //!
 //! // Definition of MyTrait
@@ -124,7 +122,7 @@
 //! impl dyn MyTrait + '_ {
 //!     /// Common case: get a reference to a field of the error.
 //!     pub fn get_context_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
-//!         request_ref::<T>(self)
+//!         request_ref(|demand| self.provide(demand))
 //!     }
 //! }
 //!
@@ -785,23 +783,32 @@ pub trait Provider {
 
 /// Request a value from the `Provider`.
 #[unstable(feature = "provide_any", issue = "none")]
-pub fn request_value<'a, T: 'static>(provider: &'a dyn Provider) -> Option<T> {
-    request_by_type_tag::<'a, tags::Value<T>>(provider)
+pub fn request_value<'a, T, F>(f: F) -> Option<T>
+where
+    T: 'static,
+    F: FnOnce(&mut Demand<'a>),
+{
+    request_by_type_tag::<'a, tags::Value<T>, _>(f)
 }
 
 /// Request a reference from the `Provider`.
 #[unstable(feature = "provide_any", issue = "none")]
-pub fn request_ref<'a, T: ?Sized + 'static>(provider: &'a dyn Provider) -> Option<&'a T> {
-    request_by_type_tag::<'a, tags::Ref<tags::MaybeSizedValue<T>>>(provider)
+pub fn request_ref<'a, T, F>(f: F) -> Option<&'a T>
+where
+    T: ?Sized + 'static,
+    F: FnOnce(&mut Demand<'a>),
+{
+    request_by_type_tag::<'a, tags::Ref<tags::MaybeSizedValue<T>>, _>(f)
 }
 
 /// Request a specific value by tag from the `Provider`.
-fn request_by_type_tag<'a, I>(provider: &'a dyn Provider) -> Option<I::Reified>
+fn request_by_type_tag<'a, I, F>(f: F) -> Option<I::Reified>
 where
     I: tags::Type<'a>,
+    F: FnOnce(&mut Demand<'a>),
 {
     let mut tagged = TaggedOption::<'a, I>(None);
-    provider.provide(tagged.as_demand());
+    f(tagged.as_demand());
     tagged.0
 }
 

src/test/ui/provider/provider_as_supertrait.rs

@@ -0,0 +1,50 @@
+// run-pass
+#![feature(provide_any)]
+use std::any::{Provider, Demand, request_ref, request_value};
+
+// Definition of MyTrait
+trait MyTrait: Provider {
+    // ...
+}
+
+// Methods on `MyTrait` trait objects.
+impl dyn MyTrait + '_ {
+    /// Common case: get a reference to a field of the error.
+    pub fn get_context_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
+        request_ref(|demand| self.provide(demand))
+    }
+
+    /// Common case: get a reference to a field of the error.
+    pub fn get_context<T: 'static>(&self) -> Option<T> {
+        request_value(|demand| self.provide(demand))
+    }
+}
+
+struct IsFatal;
+
+struct SomeFatalError;
+
+impl MyTrait for SomeFatalError {}
+
+impl Provider for SomeFatalError {
+    fn provide<'a>(&'a self, req: &mut Demand<'a>) {
+        req.provide_ref(&IsFatal).provide_value(|| IsFatal);
+    }
+}
+
+struct SomeNonFatalError;
+
+impl MyTrait for SomeNonFatalError {}
+
+impl Provider for SomeNonFatalError {
+    fn provide<'a>(&'a self, _req: &mut Demand<'a>) { }
+}
+
+fn is_fatal(obj: &dyn MyTrait) -> bool {
+    obj.get_context_ref::<IsFatal>().is_some() && obj.get_context::<IsFatal>().is_some()
+}
+
+fn main() {
+    assert!(is_fatal(&SomeFatalError));
+    assert!(!is_fatal(&SomeNonFatalError));
+}