Rollup merge of #23712 - nikomatsakis:reflect-trait, r=FlaPer87

 This PR introduces a `Reflect` marker trait which is a supertrait of `Any`. The idea is that `Reflect` is defined for all concrete types, but is not defined for type parameters unless there is a `T:Reflect` bound. This is intended to preserve the parametricity property. This allows the `Any` interface to be stabilized without committing us to unbounded reflection that is not easily detectable by the caller.

The implementation of `Reflect` relies on an experimental variant of OIBIT. This variant behaves differently for objects, since it requires that all types exposed as part of the object's *interface* are `Reflect`, but isn't concerned about other types that may be closed over. In other words, you don't have to write `Foo+Reflect` in order for `Foo: Reflect` to hold (where `Foo` is a trait).

Given that `Any` is slated to stabilization and hence that we are committed to some form of reflection, the goal of this PR is to leave our options open with respect to parametricity. I see the options for full stabilization as follows (I think an RFC would be an appropriate way to confirm whichever of these three routes we take):

1. We make `Reflect` a lang-item.
2. We stabilize some version of the OIBIT variation I implemented as a general mechanism that may be appropriate for other use cases.
3. We give up on preserving parametricity here and just have `impl<T> Reflect for T` instead. In that case, `Reflect` is a harmless but not especially useful trait going forward.

cc @aturon
cc @alexcrichton
cc @glaebhoerl (this is more-or-less your proposal, as I understood it)
cc @reem (this is more-or-less what we discussed on IRC at some point)
cc @FlaPer87 (vaguely pertains to OIBIT)

1 files changed, 3 insertions, 3 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 8b18fbf554a..f9bd0ab2f1e 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -244,13 +244,13 @@ pub trait BoxAny {
     /// Returns the boxed value if it is of type `T`, or
     /// `Err(Self)` if it isn't.
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn downcast<T: 'static>(self) -> Result<Box<T>, Box<Any>>;
+    fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>>;
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl BoxAny for Box<Any> {
     #[inline]
-    fn downcast<T: 'static>(self) -> Result<Box<T>, Box<Any>> {
+    fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
         if self.is::<T>() {
             unsafe {
                 // Get the raw representation of the trait object
@@ -270,7 +270,7 @@ impl BoxAny for Box<Any> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl BoxAny for Box<Any+Send> {
     #[inline]
-    fn downcast<T: 'static>(self) -> Result<Box<T>, Box<Any>> {
+    fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
         <Box<Any>>::downcast(self)
     }
 }