Add a DynSized trait.

The DynSized trait is implemented by all types which have a size and alignment
known at runtime. This includes every type other than extern types, introduced
in RFC 1861 and implemented in rust-lang#44295, which are completely opaque.

The main motivation for this trait is to prevent the use of !DynSized types as
struct tails. Consider for example the following types :
```rust
extern {
  type foo;
}

struct A<T: ?Sized> {
    a_x: u8
    a_y: T
}

struct B<T: ?Sized> {
    b_x: u8
    b_y: T
}
```

Before this change, the type `A<B<foo>>` is considered well-formed. However,
the alignment of `B<foo>` and thus the offset of the `a_y` field depends on the
alignment of `foo`, which is unknown.

By introducing this new trait, struct tails are now required to implement
`DynSized`, such that their alignment is known. The trait is an implicit bound,
making `A<B<foo>>` ill-formed.

Just like the `Sized` trait, the default bound can be opted-out by using
`?DynSized`.

src/libcore/marker.rs

@@ -40,6 +40,16 @@ use hash::Hasher;
 /// [ub]: ../../reference/behavior-considered-undefined.html
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(stage0, lang = "send")]
+#[cfg(not(stage0))]
+#[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"]
+pub unsafe trait Send: ?DynSized {
+    // empty.
+}
+
+/// docs
+#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(stage0, lang = "send")]
+#[cfg(stage0)]
 #[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"]
 pub unsafe trait Send {
     // empty.
@@ -51,9 +61,9 @@ pub unsafe trait Send {
 unsafe impl Send for .. { }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> !Send for *const T { }
+impl<T: ?DynSized> !Send for *const T { }
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> !Send for *mut T { }
+impl<T: ?DynSized> !Send for *mut T { }
 
 /// Types with a constant size known at compile time.
 ///
@@ -96,6 +106,29 @@ pub trait Sized {
     // Empty.
 }
 
+/// Types with a size known at run time.
+///
+/// This trait is implemented both by `Sized` types, and by dynamically sized
+/// types such as slices and [trait objects]. [Extern types], whose size is not
+/// known, even at runtime, do not implement this trait.
+///
+/// All traits and type parameters have an implicit bound of `DynSized`. The
+/// special syntax `?DynSized` can be used to remove this bound if it's not
+/// appropriate.
+///
+/// [trait object]: ../../book/first-edition/trait-objects.html
+#[cfg(not(stage0))]
+#[unstable(feature = "dynsized", issue = "0")]
+#[lang = "dynsized"]
+#[rustc_on_unimplemented = "`{Self}` does not have a size known at run-time"]
+#[fundamental]
+pub trait DynSized: ?DynSized {
+    // Empty.
+}
+
+#[cfg(stage0)]
+use self::Sized as DynSized; // This is just so we don't have to stage too much stuff
+
 /// Types that can be "unsized" to a dynamically-sized type.
 ///
 /// For example, the sized array type `[i8; 2]` implements `Unsize<[i8]>` and
@@ -345,6 +378,16 @@ pub trait Copy : Clone {
 /// [transmute]: ../../std/mem/fn.transmute.html
 #[stable(feature = "rust1", since = "1.0.0")]
 #[lang = "sync"]
+#[cfg(not(stage0))]
+#[rustc_on_unimplemented = "`{Self}` cannot be shared between threads safely"]
+pub unsafe trait Sync: ?DynSized {
+    // Empty
+}
+
+/// docs
+#[stable(feature = "rust1", since = "1.0.0")]
+#[lang = "sync"]
+#[cfg(stage0)]
 #[rustc_on_unimplemented = "`{Self}` cannot be shared between threads safely"]
 pub unsafe trait Sync {
     // Empty
@@ -356,56 +399,56 @@ pub unsafe trait Sync {
 unsafe impl Sync for .. { }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> !Sync for *const T { }
+impl<T: ?DynSized> !Sync for *const T { }
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> !Sync for *mut T { }
+impl<T: ?DynSized> !Sync for *mut T { }
 
 macro_rules! impls{
     ($t: ident) => (
         #[stable(feature = "rust1", since = "1.0.0")]
-        impl<T:?Sized> Hash for $t<T> {
+        impl<T:?DynSized> Hash for $t<T> {
             #[inline]
             fn hash<H: Hasher>(&self, _: &mut H) {
             }
         }
 
         #[stable(feature = "rust1", since = "1.0.0")]
-        impl<T:?Sized> cmp::PartialEq for $t<T> {
+        impl<T:?DynSized> cmp::PartialEq for $t<T> {
             fn eq(&self, _other: &$t<T>) -> bool {
                 true
             }
         }
 
         #[stable(feature = "rust1", since = "1.0.0")]
-        impl<T:?Sized> cmp::Eq for $t<T> {
+        impl<T:?DynSized> cmp::Eq for $t<T> {
         }
 
         #[stable(feature = "rust1", since = "1.0.0")]
-        impl<T:?Sized> cmp::PartialOrd for $t<T> {
+        impl<T:?DynSized> cmp::PartialOrd for $t<T> {
             fn partial_cmp(&self, _other: &$t<T>) -> Option<cmp::Ordering> {
                 Option::Some(cmp::Ordering::Equal)
             }
         }
 
         #[stable(feature = "rust1", since = "1.0.0")]
-        impl<T:?Sized> cmp::Ord for $t<T> {
+        impl<T:?DynSized> cmp::Ord for $t<T> {
             fn cmp(&self, _other: &$t<T>) -> cmp::Ordering {
                 cmp::Ordering::Equal
             }
         }
 
         #[stable(feature = "rust1", since = "1.0.0")]
-        impl<T:?Sized> Copy for $t<T> { }
+        impl<T:?DynSized> Copy for $t<T> { }
 
         #[stable(feature = "rust1", since = "1.0.0")]
-        impl<T:?Sized> Clone for $t<T> {
+        impl<T:?DynSized> Clone for $t<T> {
             fn clone(&self) -> $t<T> {
                 $t
             }
         }
 
         #[stable(feature = "rust1", since = "1.0.0")]
-        impl<T:?Sized> Default for $t<T> {
+        impl<T:?DynSized> Default for $t<T> {
             fn default() -> $t<T> {
                 $t
             }
@@ -548,31 +591,35 @@ macro_rules! impls{
 /// [drop check]: ../../nomicon/dropck.html
 #[lang = "phantom_data"]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub struct PhantomData<T:?Sized>;
+pub struct PhantomData<T:?DynSized>;
 
 impls! { PhantomData }
 
-mod impls {
-    #[stable(feature = "rust1", since = "1.0.0")]
-    unsafe impl<'a, T: Sync + ?Sized> Send for &'a T {}
-    #[stable(feature = "rust1", since = "1.0.0")]
-    unsafe impl<'a, T: Send + ?Sized> Send for &'a mut T {}
-}
+#[stable(feature = "rust1", since = "1.0.0")]
+unsafe impl<'a, T: Sync + ?DynSized> Send for &'a T {}
+#[stable(feature = "rust1", since = "1.0.0")]
+unsafe impl<'a, T: Send + ?DynSized> Send for &'a mut T {}
 
 /// Compiler-internal trait used to determine whether a type contains
 /// any `UnsafeCell` internally, but not through an indirection.
 /// This affects, for example, whether a `static` of that type is
 /// placed in read-only static memory or writable static memory.
 #[lang = "freeze"]
+#[cfg(not(stage0))]
+unsafe trait Freeze: ?DynSized {}
+
+/// docs
+#[lang = "freeze"]
+#[cfg(stage0)]
 unsafe trait Freeze {}
 
 #[allow(unknown_lints)]
 #[allow(auto_impl)]
 unsafe impl Freeze for .. {}
 
-impl<T: ?Sized> !Freeze for UnsafeCell<T> {}
-unsafe impl<T: ?Sized> Freeze for PhantomData<T> {}
-unsafe impl<T: ?Sized> Freeze for *const T {}
-unsafe impl<T: ?Sized> Freeze for *mut T {}
-unsafe impl<'a, T: ?Sized> Freeze for &'a T {}
-unsafe impl<'a, T: ?Sized> Freeze for &'a mut T {}
+impl<T: ?DynSized> !Freeze for UnsafeCell<T> {}
+unsafe impl<T: ?DynSized> Freeze for PhantomData<T> {}
+unsafe impl<T: ?DynSized> Freeze for *const T {}
+unsafe impl<T: ?DynSized> Freeze for *mut T {}
+unsafe impl<'a, T: ?DynSized> Freeze for &'a T {}
+unsafe impl<'a, T: ?DynSized> Freeze for &'a mut T {}

src/libcore/ptr.rs

@@ -56,7 +56,7 @@ pub use intrinsics::write_bytes;
 #[stable(feature = "drop_in_place", since = "1.8.0")]
 #[lang = "drop_in_place"]
 #[allow(unconditional_recursion)]
-pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
+pub unsafe fn drop_in_place<T: ?DynSized>(to_drop: *mut T) {
     // Code here does not matter - this is replaced by the
     // real drop glue by the compiler.
     drop_in_place(to_drop);

src/librustc/dep_graph/dep_node.rs

@@ -509,6 +509,7 @@ define_dep_nodes!( <'tcx>
     [] IsForeignItem(DefId),
     [] TypeParamPredicates { item_id: DefId, param_id: DefId },
     [] SizedConstraint(DefId),
+    [] DynSizedConstraint(DefId),
     [] DtorckConstraint(DefId),
     [] AdtDestructor(DefId),
     [] AssociatedItemDefIds(DefId),
@@ -527,6 +528,7 @@ define_dep_nodes!( <'tcx>
 
     [] IsCopy { param_env: ParamEnvAnd<'tcx, Ty<'tcx>> },
     [] IsSized { param_env: ParamEnvAnd<'tcx, Ty<'tcx>> },
+    [] IsDynSized { param_env: ParamEnvAnd<'tcx, Ty<'tcx>> },
     [] IsFreeze { param_env: ParamEnvAnd<'tcx, Ty<'tcx>> },
     [] NeedsDrop { param_env: ParamEnvAnd<'tcx, Ty<'tcx>> },
     [] Layout { param_env: ParamEnvAnd<'tcx, Ty<'tcx>> },

src/librustc/middle/lang_items.rs

@@ -230,6 +230,7 @@ language_item_table! {
     F64ImplItem,                     "f64",                     f64_impl;
 
     SizedTraitLangItem,              "sized",                   sized_trait;
+    DynSizedTraitLangItem,           "dynsized",                dynsized_trait;
     UnsizeTraitLangItem,             "unsize",                  unsize_trait;
     CopyTraitLangItem,               "copy",                    copy_trait;
     CloneTraitLangItem,              "clone",                   clone_trait;

src/librustc/traits/error_reporting.rs

@@ -1243,6 +1243,9 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             ObligationCauseCode::StructInitializerSized => {
                 err.note("structs must have a statically known size to be initialized");
             }
+            ObligationCauseCode::FieldDynSized => {
+                err.note("the last field of a struct or tuple must have a dynamically sized type");
+            }
             ObligationCauseCode::FieldSized(ref item) => {
                 match *item {
                     AdtKind::Struct => {

src/librustc/traits/mod.rs

@@ -137,6 +137,9 @@ pub enum ObligationCauseCode<'tcx> {
     /// Types of fields (other than the last) in a struct must be sized.
     FieldSized(AdtKind),
 
+    /// Last field of a struct must be DynSized.
+    FieldDynSized,
+
     /// Constant expressions must be sized.
     ConstSized,
 

src/librustc/traits/select.rs

@@ -1348,6 +1348,12 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             let sized_conditions = self.sized_conditions(obligation);
             self.assemble_builtin_bound_candidates(sized_conditions,
                                                    &mut candidates)?;
+        } else if lang_items.dynsized_trait() == Some(def_id) {
+            // DynSized is never implementable by end-users, it is
+            // always automatically computed.
+            let dynsized_conditions = self.dynsized_conditions(obligation);
+            self.assemble_builtin_bound_candidates(dynsized_conditions,
+                                                   &mut candidates)?;
          } else if lang_items.unsize_trait() == Some(def_id) {
              self.assemble_candidates_for_unsizing(obligation, &mut candidates);
          } else {
@@ -2054,6 +2060,53 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
         }
     }
 
+    fn dynsized_conditions(&mut self, obligation: &TraitObligation<'tcx>)
+                     -> BuiltinImplConditions<'tcx>
+    {
+        use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
+
+        // NOTE: binder moved to (*)
+        let self_ty = self.infcx.shallow_resolve(
+            obligation.predicate.skip_binder().self_ty());
+
+        match self_ty.sty {
+            ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
+            ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
+            ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyRawPtr(..) |
+            ty::TyChar | ty::TyRef(..) | ty::TyGenerator(..) |
+            ty::TyArray(..) | ty::TyClosure(..) | ty::TyNever |
+            ty::TyStr | ty::TySlice(_) | ty::TyDynamic(..) |
+            ty::TyError  => {
+                // safe for everything
+                Where(ty::Binder(Vec::new()))
+            }
+
+            ty::TyTuple(tys, _) => {
+                Where(ty::Binder(tys.last().into_iter().cloned().collect()))
+            }
+
+            ty::TyAdt(def, substs) => {
+                let dynsized_crit = def.dynsized_constraint(self.tcx());
+                // (*) binder moved here
+                Where(ty::Binder(
+                    dynsized_crit.iter().map(|ty| ty.subst(self.tcx(), substs)).collect()
+                ))
+            }
+
+            ty::TyForeign(..) => Never,
+
+            ty::TyProjection(_) | ty::TyParam(_) | ty::TyAnon(..) => None,
+            ty::TyInfer(ty::TyVar(_)) => Ambiguous,
+
+            ty::TyInfer(ty::FreshTy(_))
+            | ty::TyInfer(ty::FreshIntTy(_))
+            | ty::TyInfer(ty::FreshFloatTy(_)) => {
+                bug!("asked to assemble builtin bounds of unexpected type: {:?}",
+                     self_ty);
+            }
+        }
+    }
+
     fn copy_clone_conditions(&mut self, obligation: &TraitObligation<'tcx>)
                      -> BuiltinImplConditions<'tcx>
     {
@@ -2382,6 +2435,9 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 _ if Some(trait_def) == lang_items.sized_trait() => {
                     self.sized_conditions(obligation)
                 }
+                _ if Some(trait_def) == lang_items.dynsized_trait() => {
+                    self.dynsized_conditions(obligation)
+                }
                 _ if Some(trait_def) == lang_items.copy_trait() => {
                     self.copy_clone_conditions(obligation)
                 }

src/librustc/traits/structural_impls.rs

@@ -204,6 +204,7 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
             super::SizedReturnType => Some(super::SizedReturnType),
             super::RepeatVec => Some(super::RepeatVec),
             super::FieldSized(item) => Some(super::FieldSized(item)),
+            super::FieldDynSized => Some(super::FieldDynSized),
             super::ConstSized => Some(super::ConstSized),
             super::SharedStatic => Some(super::SharedStatic),
             super::BuiltinDerivedObligation(ref cause) => {
@@ -522,6 +523,7 @@ impl<'tcx> TypeFoldable<'tcx> for traits::ObligationCauseCode<'tcx> {
             super::ReturnNoExpression |
             super::RepeatVec |
             super::FieldSized(_) |
+            super::FieldDynSized |
             super::ConstSized |
             super::SharedStatic |
             super::BlockTailExpression(_) |
@@ -570,6 +572,7 @@ impl<'tcx> TypeFoldable<'tcx> for traits::ObligationCauseCode<'tcx> {
             super::ReturnNoExpression |
             super::RepeatVec |
             super::FieldSized(_) |
+            super::FieldDynSized |
             super::ConstSized |
             super::SharedStatic |
             super::BlockTailExpression(_) |

src/librustc/ty/maps/config.rs

@@ -61,6 +61,12 @@ impl<'tcx> QueryDescription<'tcx> for queries::is_sized_raw<'tcx> {
     }
 }
 
+impl<'tcx> QueryDescription<'tcx> for queries::is_dynsized_raw<'tcx> {
+    fn describe(_tcx: TyCtxt, env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> String {
+        format!("computing whether `{}` is `DynSized`", env.value)
+    }
+}
+
 impl<'tcx> QueryDescription<'tcx> for queries::is_freeze_raw<'tcx> {
     fn describe(_tcx: TyCtxt, env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> String {
         format!("computing whether `{}` is freeze", env.value)

src/librustc/ty/maps/mod.rs

@@ -104,6 +104,7 @@ define_maps! { <'tcx>
     [] fn adt_def: AdtDefOfItem(DefId) -> &'tcx ty::AdtDef,
     [] fn adt_destructor: AdtDestructor(DefId) -> Option<ty::Destructor>,
     [] fn adt_sized_constraint: SizedConstraint(DefId) -> &'tcx [Ty<'tcx>],
+    [] fn adt_dynsized_constraint: DynSizedConstraint(DefId) -> &'tcx [Ty<'tcx>],
     [] fn adt_dtorck_constraint: DtorckConstraint(DefId) -> ty::DtorckConstraint<'tcx>,
 
     /// True if this is a const fn
@@ -261,6 +262,7 @@ define_maps! { <'tcx>
     // `ty.is_copy()`, etc, since that will prune the environment where possible.
     [] fn is_copy_raw: is_copy_dep_node(ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool,
     [] fn is_sized_raw: is_sized_dep_node(ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool,
+    [] fn is_dynsized_raw: is_dynsized_dep_node(ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool,
     [] fn is_freeze_raw: is_freeze_dep_node(ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool,
     [] fn needs_drop_raw: needs_drop_dep_node(ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool,
     [] fn layout_raw: layout_dep_node(ty::ParamEnvAnd<'tcx, Ty<'tcx>>)
@@ -438,6 +440,10 @@ fn is_freeze_dep_node<'tcx>(param_env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> DepCo
     DepConstructor::IsFreeze { param_env }
 }
 
+fn is_dynsized_dep_node<'tcx>(param_env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> DepConstructor<'tcx> {
+    DepConstructor::IsDynSized { param_env }
+}
+
 fn needs_drop_dep_node<'tcx>(param_env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> DepConstructor<'tcx> {
     DepConstructor::NeedsDrop { param_env }
 }