Remove subtyping for object types and replace with an *upcast* coercion.

This upcast coercion currently preserves the vtable for the object, but
eventually it can be used to create a derived vtable. The upcast
coercion is not introduced into method dispatch; see comment on #18737
for information about why. Fixes #18737.

src/liballoc/boxed.rs

@@ -241,7 +241,7 @@ 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>, Self>;
+    fn downcast<T: 'static>(self) -> Result<Box<T>, Box<Any>>;
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -264,6 +264,15 @@ impl BoxAny for Box<Any> {
     }
 }
 
+#[cfg(not(stage0))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl BoxAny for Box<Any+Send> {
+    #[inline]
+    fn downcast<T: 'static>(self) -> Result<Box<T>, Box<Any>> {
+        <Box<Any>>::downcast(self)
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

src/libcore/any.rs

@@ -71,6 +71,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use marker::Send;
 use mem::transmute;
 use option::Option::{self, Some, None};
 use raw::TraitObject;
@@ -154,6 +155,31 @@ impl Any {
     }
 }
 
+#[cfg(not(stage0))]
+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 {
+        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> {
+        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> {
+        Any::downcast_mut::<T>(self)
+    }
+}
+
+
 ///////////////////////////////////////////////////////////////////////////////
 // TypeID and its methods
 ///////////////////////////////////////////////////////////////////////////////

src/librustc/middle/astencode.rs

@@ -1102,6 +1102,11 @@ impl<'a, 'tcx> rbml_writer_helpers<'tcx> for Encoder<'a> {
                         this.emit_enum_variant_arg(1, |this| Ok(this.emit_ty(ecx, self_ty)))
                     })
                 }
+                ty::UnsizeUpcast(target_ty) => {
+                    this.emit_enum_variant("UnsizeUpcast", 3, 1, |this| {
+                        this.emit_enum_variant_arg(0, |this| Ok(this.emit_ty(ecx, target_ty)))
+                    })
+                }
             }
         });
     }
@@ -1707,7 +1712,7 @@ impl<'a, 'tcx> rbml_decoder_decoder_helpers<'tcx> for reader::Decoder<'a> {
     fn read_unsize_kind<'b, 'c>(&mut self, dcx: &DecodeContext<'b, 'c, 'tcx>)
                                 -> ty::UnsizeKind<'tcx> {
         self.read_enum("UnsizeKind", |this| {
-            let variants = &["UnsizeLength", "UnsizeStruct", "UnsizeVtable"];
+            let variants = &["UnsizeLength", "UnsizeStruct", "UnsizeVtable", "UnsizeUpcast"];
             this.read_enum_variant(variants, |this, i| {
                 Ok(match i {
                     0 => {
@@ -1741,6 +1746,11 @@ impl<'a, 'tcx> rbml_decoder_decoder_helpers<'tcx> for reader::Decoder<'a> {
                             this.read_enum_variant_arg(1, |this| Ok(this.read_ty(dcx))).unwrap();
                         ty::UnsizeVtable(ty_trait, self_ty)
                     }
+                    3 => {
+                        let target_ty =
+                            this.read_enum_variant_arg(0, |this| Ok(this.read_ty(dcx))).unwrap();
+                        ty::UnsizeUpcast(target_ty)
+                    }
                     _ => panic!("bad enum variant for ty::UnsizeKind")
                 })
             })

src/librustc/middle/infer/combine.rs

@@ -314,9 +314,18 @@ pub trait Combine<'tcx> : Sized {
     }
 
     fn builtin_bounds(&self,
-                      a: ty::BuiltinBounds,
-                      b: ty::BuiltinBounds)
-                      -> cres<'tcx, ty::BuiltinBounds>;
+                      a: BuiltinBounds,
+                      b: BuiltinBounds)
+                      -> cres<'tcx, BuiltinBounds>
+    {
+        // Two sets of builtin bounds are only relatable if they are
+        // precisely the same (but see the coercion code).
+        if a != b {
+            Err(ty::terr_builtin_bounds(expected_found(self, a, b)))
+        } else {
+            Ok(a)
+        }
+    }
 
     fn trait_refs(&self,
                   a: &ty::TraitRef<'tcx>,

src/librustc/middle/infer/equate.rs

@@ -8,7 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use middle::ty::{BuiltinBounds};
 use middle::ty::{self, Ty};
 use middle::ty::TyVar;
 use middle::infer::combine::*;
@@ -73,23 +72,6 @@ impl<'f, 'tcx> Combine<'tcx> for Equate<'f, 'tcx> {
         }
     }
 
-    fn builtin_bounds(&self,
-                      a: BuiltinBounds,
-                      b: BuiltinBounds)
-                      -> cres<'tcx, BuiltinBounds>
-    {
-        // More bounds is a subtype of fewer bounds.
-        //
-        // e.g., fn:Copy() <: fn(), because the former is a function
-        // that only closes over copyable things, but the latter is
-        // any function at all.
-        if a != b {
-            Err(ty::terr_builtin_bounds(expected_found(self, a, b)))
-        } else {
-            Ok(a)
-        }
-    }
-
     fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, Ty<'tcx>> {
         debug!("{}.tys({}, {})", self.tag(),
                a.repr(self.fields.infcx.tcx), b.repr(self.fields.infcx.tcx));

src/librustc/middle/infer/glb.rs

@@ -14,7 +14,6 @@ use super::higher_ranked::HigherRankedRelations;
 use super::{cres};
 use super::Subtype;
 
-use middle::ty::{BuiltinBounds};
 use middle::ty::{self, Ty};
 use syntax::ast::{MutImmutable, MutMutable, Unsafety};
 use util::ppaux::mt_to_string;
@@ -94,15 +93,6 @@ impl<'f, 'tcx> Combine<'tcx> for Glb<'f, 'tcx> {
         }
     }
 
-    fn builtin_bounds(&self,
-                      a: ty::BuiltinBounds,
-                      b: ty::BuiltinBounds)
-                      -> cres<'tcx, ty::BuiltinBounds> {
-        // More bounds is a subtype of fewer bounds, so
-        // the GLB (mutual subtype) is the union.
-        Ok(a.union(b))
-    }
-
     fn regions(&self, a: ty::Region, b: ty::Region) -> cres<'tcx, ty::Region> {
         debug!("{}.regions({}, {})",
                self.tag(),

src/librustc/middle/infer/lub.rs

@@ -14,7 +14,6 @@ use super::lattice::*;
 use super::{cres};
 use super::{Subtype};
 
-use middle::ty::{BuiltinBounds};
 use middle::ty::{self, Ty};
 use syntax::ast::{MutMutable, MutImmutable, Unsafety};
 use util::ppaux::mt_to_string;
@@ -89,15 +88,6 @@ impl<'f, 'tcx> Combine<'tcx> for Lub<'f, 'tcx> {
         }
     }
 
-    fn builtin_bounds(&self,
-                      a: ty::BuiltinBounds,
-                      b: ty::BuiltinBounds)
-                      -> cres<'tcx, ty::BuiltinBounds> {
-        // More bounds is a subtype of fewer bounds, so
-        // the LUB (mutual supertype) is the intersection.
-        Ok(a.intersection(b))
-    }
-
     fn regions(&self, a: ty::Region, b: ty::Region) -> cres<'tcx, ty::Region> {
         debug!("{}.regions({}, {})",
                self.tag(),

src/librustc/middle/infer/sub.rs

@@ -14,7 +14,6 @@ use super::higher_ranked::HigherRankedRelations;
 use super::{Subtype};
 use super::type_variable::{SubtypeOf, SupertypeOf};
 
-use middle::ty::{BuiltinBounds};
 use middle::ty::{self, Ty};
 use middle::ty::TyVar;
 use util::ppaux::{Repr};
@@ -97,20 +96,6 @@ impl<'f, 'tcx> Combine<'tcx> for Sub<'f, 'tcx> {
         })
     }
 
-    fn builtin_bounds(&self, a: BuiltinBounds, b: BuiltinBounds)
-                      -> cres<'tcx, BuiltinBounds> {
-        // More bounds is a subtype of fewer bounds.
-        //
-        // e.g., fn:Copy() <: fn(), because the former is a function
-        // that only closes over copyable things, but the latter is
-        // any function at all.
-        if a.is_superset(&b) {
-            Ok(a)
-        } else {
-            Err(ty::terr_builtin_bounds(expected_found(self, a, b)))
-        }
-    }
-
     fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, Ty<'tcx>> {
         debug!("{}.tys({}, {})", self.tag(),
                a.repr(self.tcx()), b.repr(self.tcx()));

src/librustc/middle/ty.rs

@@ -292,7 +292,8 @@ pub enum UnsizeKind<'tcx> {
     // An unsize coercion applied to the tail field of a struct.
     // The uint is the index of the type parameter which is unsized.
     UnsizeStruct(Box<UnsizeKind<'tcx>>, uint),
-    UnsizeVtable(TyTrait<'tcx>, /* the self type of the trait */ Ty<'tcx>)
+    UnsizeVtable(TyTrait<'tcx>, /* the self type of the trait */ Ty<'tcx>),
+    UnsizeUpcast(Ty<'tcx>),
 }
 
 #[derive(Clone, Debug)]
@@ -4627,6 +4628,9 @@ pub fn unsize_ty<'tcx>(cx: &ctxt<'tcx>,
         &UnsizeVtable(TyTrait { ref principal, ref bounds }, _) => {
             mk_trait(cx, principal.clone(), bounds.clone())
         }
+        &UnsizeUpcast(target_ty) => {
+            target_ty
+        }
     }
 }
 
@@ -6830,6 +6834,7 @@ impl<'tcx> Repr<'tcx> for UnsizeKind<'tcx> {
             UnsizeLength(n) => format!("UnsizeLength({})", n),
             UnsizeStruct(ref k, n) => format!("UnsizeStruct({},{})", k.repr(tcx), n),
             UnsizeVtable(ref a, ref b) => format!("UnsizeVtable({},{})", a.repr(tcx), b.repr(tcx)),
+            UnsizeUpcast(ref a) => format!("UnsizeUpcast({})", a.repr(tcx)),
         }
     }
 }

src/librustc/middle/ty_fold.rs

@@ -480,6 +480,7 @@ impl<'tcx> TypeFoldable<'tcx> for ty::UnsizeKind<'tcx> {
                     },
                     self_ty.fold_with(folder))
             }
+            ty::UnsizeUpcast(t) => ty::UnsizeUpcast(t.fold_with(folder)),
         }
     }
 }

src/librustc/util/ppaux.rs

@@ -1214,17 +1214,17 @@ impl<'tcx> Repr<'tcx> for ty::ExistentialBounds<'tcx> {
     fn repr(&self, tcx: &ctxt<'tcx>) -> String {
         let mut res = Vec::new();
 
-        let region_str = self.region_bound.user_string(tcx);
+        let region_str = self.region_bound.repr(tcx);
         if !region_str.is_empty() {
             res.push(region_str);
         }
 
         for bound in &self.builtin_bounds {
-            res.push(bound.user_string(tcx));
+            res.push(bound.repr(tcx));
         }
 
         for projection_bound in &self.projection_bounds {
-            res.push(projection_bound.user_string(tcx));
+            res.push(projection_bound.repr(tcx));
         }
 
         res.connect("+")

src/librustc_trans/trans/consts.rs

@@ -311,12 +311,16 @@ pub fn const_expr<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
                     llconst = addr_of(cx, llconst, "autoref", e.id);
                 }
                 Some(box ty::AutoUnsize(ref k)) => {
-                    let info = expr::unsized_info(cx, k, e.id, ty, param_substs,
-                        |t| ty::mk_imm_rptr(cx.tcx(), cx.tcx().mk_region(ty::ReStatic), t));
+                    let info =
+                        expr::unsized_info(
+                            cx, k, e.id, ty, param_substs,
+                            |t| ty::mk_imm_rptr(cx.tcx(), cx.tcx().mk_region(ty::ReStatic), t),
+                            || const_get_elt(cx, llconst, &[abi::FAT_PTR_EXTRA as u32]));
 
                     let unsized_ty = ty::unsize_ty(cx.tcx(), ty, k, e.span);
                     let ptr_ty = type_of::in_memory_type_of(cx, unsized_ty).ptr_to();
                     let base = ptrcast(llconst, ptr_ty);
+
                     let prev_const = cx.const_unsized().borrow_mut()
                                        .insert(base, llconst);
                     assert!(prev_const.is_none() || prev_const == Some(llconst));