Implement Reflect trait with a variant on the standard OIBIT

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

Author: nikomatsakis

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)
     }
 }

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 .. { }
+

src/librustc/middle/traits/select.rs

@@ -138,6 +138,7 @@ enum SelectionCandidate<'tcx> {
     ParamCandidate(ty::PolyTraitRef<'tcx>),
     ImplCandidate(ast::DefId),
     DefaultImplCandidate(ast::DefId),
+    DefaultImplObjectCandidate(ast::DefId),
 
     /// This is a trait matching with a projected type as `Self`, and
     /// we found an applicable bound in the trait definition.
@@ -1160,7 +1161,20 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         if ty::trait_has_default_impl(self.tcx(), def_id) {
             match self_ty.sty {
-                ty::ty_trait(..) |
+                ty::ty_trait(..) => {
+                    // For object types, we don't know what the closed
+                    // over types are. For most traits, this means we
+                    // conservatively say nothing; a candidate may be
+                    // added by `assemble_candidates_from_object_ty`.
+                    // However, for the kind of magic reflect trait,
+                    // we consider it to be implemented even for
+                    // object types, because it just lets you reflect
+                    // onto the object type, not into the object's
+                    // interior.
+                    if ty::has_attr(self.tcx(), def_id, "rustc_reflect_like") {
+                        candidates.vec.push(DefaultImplObjectCandidate(def_id));
+                    }
+                }
                 ty::ty_param(..) |
                 ty::ty_projection(..) => {
                     // In these cases, we don't know what the actual
@@ -1798,7 +1812,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             DefaultImplCandidate(trait_def_id) => {
-                let data = try!(self.confirm_default_impl_candidate(obligation, trait_def_id));
+                let data = self.confirm_default_impl_candidate(obligation, trait_def_id);
+                Ok(VtableDefaultImpl(data))
+            }
+
+            DefaultImplObjectCandidate(trait_def_id) => {
+                let data = self.confirm_default_impl_object_candidate(obligation, trait_def_id);
                 Ok(VtableDefaultImpl(data))
             }
 
@@ -1927,17 +1946,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
     fn confirm_default_impl_candidate(&mut self,
                                       obligation: &TraitObligation<'tcx>,
-                                      impl_def_id: ast::DefId)
-                              -> Result<VtableDefaultImplData<PredicateObligation<'tcx>>,
-                                        SelectionError<'tcx>>
+                                      trait_def_id: ast::DefId)
+                                      -> VtableDefaultImplData<PredicateObligation<'tcx>>
     {
         debug!("confirm_default_impl_candidate({}, {})",
                obligation.repr(self.tcx()),
-               impl_def_id.repr(self.tcx()));
+               trait_def_id.repr(self.tcx()));
 
         let self_ty = self.infcx.shallow_resolve(obligation.predicate.0.self_ty());
         match self.constituent_types_for_ty(self_ty) {
-            Some(types) => Ok(self.vtable_default_impl(obligation, impl_def_id, types)),
+            Some(types) => self.vtable_default_impl(obligation, trait_def_id, types),
             None => {
                 self.tcx().sess.bug(
                     &format!(
@@ -1947,6 +1965,39 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
+    fn confirm_default_impl_object_candidate(&mut self,
+                                             obligation: &TraitObligation<'tcx>,
+                                             trait_def_id: ast::DefId)
+                                             -> VtableDefaultImplData<PredicateObligation<'tcx>>
+    {
+        debug!("confirm_default_impl_object_candidate({}, {})",
+               obligation.repr(self.tcx()),
+               trait_def_id.repr(self.tcx()));
+
+        assert!(ty::has_attr(self.tcx(), trait_def_id, "rustc_reflect_like"));
+
+        let self_ty = self.infcx.shallow_resolve(obligation.predicate.0.self_ty());
+        match self_ty.sty {
+            ty::ty_trait(ref data) => {
+                // OK to skip the binder, since vtable_default_impl reintroduces it
+                let input_types = data.principal.skip_binder().substs.types.get_slice(TypeSpace);
+                let assoc_types = data.bounds.projection_bounds
+                                             .iter()
+                                             .map(|pb| pb.skip_binder().ty);
+                let all_types: Vec<_> = input_types.iter().cloned()
+                                                          .chain(assoc_types)
+                                                          .collect();
+                self.vtable_default_impl(obligation, trait_def_id, all_types)
+            }
+            _ => {
+                self.tcx().sess.bug(
+                    &format!(
+                        "asked to confirm default object implementation for non-object type: {}",
+                        self_ty.repr(self.tcx())));
+            }
+        }
+    }
+
     /// See `confirm_default_impl_candidate`
     fn vtable_default_impl(&mut self,
                            obligation: &TraitObligation<'tcx>,
@@ -2530,6 +2581,7 @@ impl<'tcx> Repr<'tcx> for SelectionCandidate<'tcx> {
             ParamCandidate(ref a) => format!("ParamCandidate({})", a.repr(tcx)),
             ImplCandidate(a) => format!("ImplCandidate({})", a.repr(tcx)),
             DefaultImplCandidate(t) => format!("DefaultImplCandidate({:?})", t),
+            DefaultImplObjectCandidate(t) => format!("DefaultImplObjectCandidate({:?})", t),
             ProjectionCandidate => format!("ProjectionCandidate"),
             FnPointerCandidate => format!("FnPointerCandidate"),
             ObjectCandidate => format!("ObjectCandidate"),

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),

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>() }

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>() }

src/test/compile-fail/reflect-assoc.rs

@@ -0,0 +1,35 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that types that appear in assoc bindings in an object
+// type are subject to the reflect check.
+
+use std::marker::Reflect;
+use std::io::Write;
+
+trait Get {
+    type Output;
+    fn get(self) -> Self::Output;
+}
+
+struct Struct<T>(T);
+
+fn is_reflect<T:Reflect>() { }
+
+fn a<T>() {
+    is_reflect::<Box<Get<Output=T>>>(); //~ ERROR not implemented
+}
+
+fn ok_a<T: Reflect>() {
+    is_reflect::<Box<Get<Output=T>>>(); // OK
+}
+
+fn main() {
+}