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)

2 files changed, 3 insertions, 3 deletions

diff --git a/src/test/run-pass/object-one-type-two-traits.rs b/src/test/run-pass/object-one-type-two-traits.rs
index baf8c6e4c97..f4e056b3f21 100644
--- a/src/test/run-pass/object-one-type-two-traits.rs
+++ b/src/test/run-pass/object-one-type-two-traits.rs
@@ -30,7 +30,7 @@ impl Wrap for int {
     }
 }
 
-fn is<T:'static>(x: &Any) -> bool {
+fn is<T:Any>(x: &Any) -> bool {
     x.is::<T>()
 }
 
diff --git a/src/test/run-pass/type-id-higher-rank.rs b/src/test/run-pass/type-id-higher-rank.rs
index 5670c45b68a..a40989d4e37 100644
--- a/src/test/run-pass/type-id-higher-rank.rs
+++ b/src/test/run-pass/type-id-higher-rank.rs
@@ -15,7 +15,7 @@
 
 #![feature(unboxed_closures, core)]
 
-use std::any::TypeId;
+use std::any::{Any, TypeId};
 
 fn main() {
     // Bare fns
@@ -63,7 +63,7 @@ fn main() {
         assert!(a != b);
     }
 
-    fn id<T:'static>(_: T) -> TypeId {
+    fn id<T:Any>(_: T) -> TypeId {
         TypeId::of::<T>()
     }
 }