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, 6 insertions, 1 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 60f81dac1e9..46115ae468f 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -74,6 +74,7 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
 
     ("rustc_diagnostic_macros", "1.0.0", Active),
     ("unboxed_closures", "1.0.0", Active),
+    ("reflect", "1.0.0", Active),
     ("import_shadowing", "1.0.0", Removed),
     ("advanced_slice_patterns", "1.0.0", Active),
     ("tuple_indexing", "1.0.0", Accepted),
@@ -281,7 +282,11 @@ pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType)] = &[
     // FIXME: #19470 this shouldn't be needed forever
     ("old_orphan_check", Whitelisted),
     ("old_impl_check", Whitelisted),
-    ("rustc_paren_sugar", Whitelisted), // FIXME: #18101 temporary unboxed closure hack
+
+    ("rustc_paren_sugar", Gated("unboxed_closures",
+                                "unboxed_closures are still evolving")),
+    ("rustc_reflect_like", Gated("reflect",
+                                 "defining reflective traits is still evolving")),
 
     // Crate level attributes
     ("crate_name", CrateLevel),