rollup merge of #23712: nikomatsakis/reflect-trait

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, 4 insertions, 4 deletions

diff --git a/src/test/auxiliary/typeid-intrinsic.rs b/src/test/auxiliary/typeid-intrinsic.rs
index 82d07a9df4e..bd47054f093 100644
--- a/src/test/auxiliary/typeid-intrinsic.rs
+++ b/src/test/auxiliary/typeid-intrinsic.rs
@@ -10,7 +10,7 @@
 
 #![feature(core)]
 
-use std::any::TypeId;
+use std::any::{Any, TypeId};
 
 pub struct A;
 pub struct B(Option<A>);
@@ -31,4 +31,4 @@ pub unsafe fn id_F() -> TypeId { TypeId::of::<F>() }
 pub unsafe fn id_G() -> TypeId { TypeId::of::<G>() }
 pub unsafe fn id_H() -> TypeId { TypeId::of::<H>() }
 
-pub unsafe fn foo<T: 'static>() -> TypeId { TypeId::of::<T>() }
+pub unsafe fn foo<T: Any>() -> TypeId { TypeId::of::<T>() }
diff --git a/src/test/auxiliary/typeid-intrinsic2.rs b/src/test/auxiliary/typeid-intrinsic2.rs
index 82d07a9df4e..5e81bf50ae4 100644
--- a/src/test/auxiliary/typeid-intrinsic2.rs
+++ b/src/test/auxiliary/typeid-intrinsic2.rs
@@ -10,7 +10,7 @@
 
 #![feature(core)]
 
-use std::any::TypeId;
+use std::any::{Any, TypeId};
 
 pub struct A;
 pub struct B(Option<A>);
@@ -31,4 +31,4 @@ pub unsafe fn id_F() -> TypeId { TypeId::of::<F>() }
 pub unsafe fn id_G() -> TypeId { TypeId::of::<G>() }
 pub unsafe fn id_H() -> TypeId { TypeId::of::<H>() }
 
-pub unsafe fn foo<T: 'static>() -> TypeId { TypeId::of::<T>() }
+pub unsafe fn foo<T:Any>() -> TypeId { TypeId::of::<T>() }