Implement Reflect trait with a variant on the standard OIBIT

semantics that tests the *interface* of trait objects, rather
than what they close over.

src/libcore/any.rs

@@ -55,7 +55,7 @@
 //! }
 //!
 //! // This function wants to log its parameter out prior to doing work with it.
-//! fn do_work<T: Debug + 'static>(value: &T) {
+//! fn do_work<T: Any + Debug>(value: &T) {
 //!     log(value);
 //!     // ...do some other work
 //! }
@@ -76,7 +76,7 @@ use mem::transmute;
 use option::Option::{self, Some, None};
 use raw::TraitObject;
 use intrinsics;
-use marker::Sized;
+use marker::{Reflect, Sized};
 
 ///////////////////////////////////////////////////////////////////////////////
 // Any trait
@@ -88,14 +88,16 @@ use marker::Sized;
 ///
 /// [mod]: ../index.html
 #[stable(feature = "rust1", since = "1.0.0")]
-pub trait Any: 'static {
+pub trait Any: Reflect + 'static {
     /// Get the `TypeId` of `self`
     #[unstable(feature = "core",
                reason = "this method will likely be replaced by an associated static")]
     fn get_type_id(&self) -> TypeId;
 }
 
-impl<T: 'static> Any for T {
+impl<T> Any for T
+    where T: Reflect + 'static
+{
     fn get_type_id(&self) -> TypeId { TypeId::of::<T>() }
 }
 
@@ -107,7 +109,7 @@ impl Any {
     /// Returns true if the boxed type is the same as `T`
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
-    pub fn is<T: 'static>(&self) -> bool {
+    pub fn is<T: Any>(&self) -> bool {
         // Get TypeId of the type this function is instantiated with
         let t = TypeId::of::<T>();
 
@@ -122,7 +124,7 @@ impl Any {
     /// `None` if it isn't.
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
-    pub fn downcast_ref<T: 'static>(&self) -> Option<&T> {
+    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
         if self.is::<T>() {
             unsafe {
                 // Get the raw representation of the trait object
@@ -140,7 +142,7 @@ impl Any {
     /// `None` if it isn't.
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
-    pub fn downcast_mut<T: 'static>(&mut self) -> Option<&mut T> {
+    pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
         if self.is::<T>() {
             unsafe {
                 // Get the raw representation of the trait object
@@ -159,21 +161,21 @@ impl Any+Send {
     /// Forwards to the method defined on the type `Any`.
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
-    pub fn is<T: 'static>(&self) -> bool {
+    pub fn is<T: Any>(&self) -> bool {
         Any::is::<T>(self)
     }
 
     /// Forwards to the method defined on the type `Any`.
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
-    pub fn downcast_ref<T: 'static>(&self) -> Option<&T> {
+    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
         Any::downcast_ref::<T>(self)
     }
 
     /// Forwards to the method defined on the type `Any`.
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
-    pub fn downcast_mut<T: 'static>(&mut self) -> Option<&mut T> {
+    pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
         Any::downcast_mut::<T>(self)
     }
 }
@@ -202,7 +204,7 @@ impl TypeId {
     /// instantiated with
     #[unstable(feature = "core",
                reason = "may grow a `Reflect` bound soon via marker traits")]
-    pub fn of<T: ?Sized + 'static>() -> TypeId {
+    pub fn of<T: ?Sized + Any>() -> TypeId {
         TypeId {
             t: unsafe { intrinsics::type_id::<T>() },
         }

src/libcore/lib.rs

@@ -72,6 +72,7 @@
 #![feature(rustc_attrs)]
 #![feature(optin_builtin_traits)]
 #![feature(concat_idents)]
+#![feature(reflect)]
 
 #[macro_use]
 mod macros;

src/libcore/marker.rs

@@ -450,3 +450,45 @@ pub struct CovariantType<T>;
 #[deprecated(since = "1.0.0", reason = "Replace with `PhantomData<Cell<T>>`")]
 #[lang="invariant_type"]
 pub struct InvariantType<T>;
+
+/// A marker trait indicates a type that can be reflected over. This
+/// trait is implemented for all types. Its purpose is to ensure that
+/// when you write a generic function that will employ reflection,
+/// that must be reflected (no pun intended) in the generic bounds of
+/// that function. Here is an example:
+///
+/// ```
+/// use std::marker::Reflect;
+/// use std::any::Any;
+/// fn foo<T:Reflect+'static>(x: &T) {
+///     let any: &Any = x;
+///     if any.is::<u32>() { println!("u32"); }
+/// }
+/// ```
+///
+/// Without the declaration `T:Reflect`, `foo` would not type check
+/// (note: as a matter of style, it would be preferable to to write
+/// `T:Any`, because `T:Any` implies `T:Reflect` and `T:'static`, but
+/// we use `Reflect` here to show how it works). The `Reflect` bound
+/// thus serves to alert `foo`'s caller to the fact that `foo` may
+/// behave differently depending on whether `T=u32` or not. In
+/// particular, thanks to the `Reflect` bound, callers know that a
+/// function declared like `fn bar<T>(...)` will always act in
+/// precisely the same way no matter what type `T` is supplied,
+/// beacuse there are no bounds declared on `T`. (The ability for a
+/// caller to reason about what a function may do based solely on what
+/// generic bounds are declared is often called the ["parametricity
+/// property"][1].)
+///
+/// [1]: http://en.wikipedia.org/wiki/Parametricity
+#[rustc_reflect_like]
+#[unstable(feature = "core", reason = "requires RFC and more experience")]
+pub trait Reflect : MarkerTrait {
+}
+
+#[cfg(stage0)]
+impl<T> Reflect for T { }
+
+#[cfg(not(stage0))]
+impl Reflect for .. { }
+