---

yaml
---
r: 210738
b: refs/heads/try
c: 12c93aa
h: refs/heads/master
v: v3

Author: steveklabnik

[refs]

@@ -2,7 +2,7 @@
 refs/heads/master: 3e561f05c00cd180ec02db4ccab2840a4aba93d2
 refs/heads/snap-stage1: e33de59e47c5076a89eadeb38f4934f58a3618a6
 refs/heads/snap-stage3: ba0e1cd8147d452c356aacb29fb87568ca26f111
-refs/heads/try: 4b5b9668e0f13cb4cda51b06c098d4b34f86c1c5
+refs/heads/try: 12c93aa278ebbe8527b42d17065d3d5605c26bac
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/heads/dist-snap: ba4081a5a8573875fed17545846f6f6902c8ba8d
 refs/tags/release-0.2: c870d2dffb391e14efb05aa27898f1f6333a9596

branches/try/AUTHORS.txt

@@ -736,6 +736,7 @@ Rob Hoelz <[email protected]>
 Robert Buonpastore <[email protected]>
 Robert Clipsham <[email protected]>
 Robert Gawdzik <[email protected]>
+Robert Foss <[email protected]>
 Robert Irelan <[email protected]>
 Robert Knight <[email protected]>
 Robert Millar <[email protected]>

branches/try/src/doc/reference.md

@@ -1346,6 +1346,8 @@ vtable when the trait is used as a [trait object](#trait-objects).
 Traits are implemented for specific types through separate
 [implementations](#implementations).
 
+Consider the following trait:
+
 ```
 # type Surface = i32;
 # type BoundingBox = i32;
@@ -1360,6 +1362,20 @@ This defines a trait with two methods. All values that have
 `draw` and `bounding_box` methods called, using `value.bounding_box()`
 [syntax](#method-call-expressions).
 
+Traits can include default implementations of methods, as in:
+
+```
+trait Foo {
+    fn bar(&self);
+
+    fn baz(&self) { println!("We called baz."); }
+}
+```
+
+Here the `baz` method has a default implementation, so types that implement
+`Foo` need only implement `bar`. It is also possible for implementing types
+to override a method that has a default implementation.
+
 Type parameters can be specified for a trait to make it generic. These appear
 after the trait name, using the same syntax used in [generic
 functions](#generic-functions).
@@ -1372,6 +1388,30 @@ trait Seq<T> {
 }
 ```
 
+It is also possible to define associated types for a trait. Consider the
+following example of a `Container` trait. Notice how the type is available
+for use in the method signatures:
+
+```
+trait Container {
+    type E;
+    fn empty() -> Self;
+    fn insert(&mut self, Self::E);
+}
+```
+
+In order for a type to implement this trait, it must not only provide
+implementations for every method, but it must specify the type `E`. Here's
+an implementation of `Container` for the standard library type `Vec`:
+
+```
+impl<T> Container for Vec<T> {
+    type E = T;
+    fn empty() -> Vec<T> { Vec::new() }
+    fn insert(&mut self, x: T) { self.push(x); }
+}
+```
+
 Generic functions may use traits as _bounds_ on their type parameters. This
 will have two effects: only types that have the trait may instantiate the
 parameter, and within the generic function, the methods of the trait can be
@@ -3470,13 +3510,21 @@ more of the closure traits:
 
 ### Trait objects
 
-Every trait item (see [traits](#traits)) defines a type with the same name as
-the trait. This type is called the _trait object_ of the trait. Trait objects
-permit "late binding" of methods, dispatched using _virtual method tables_
-("vtables"). Whereas most calls to trait methods are "early bound" (statically
-resolved) to specific implementations at compile time, a call to a method on an
-trait objects is only resolved to a vtable entry at compile time. The actual
-implementation for each vtable entry can vary on an object-by-object basis.
+In Rust, a type like `&SomeTrait` or `Box<SomeTrait>` is called a _trait object_.
+Each instance of a trait object includes:
+
+ - a pointer to an instance of a type `T` that implements `SomeTrait`
+ - a _virtual method table_, often just called a _vtable_, which contains, for
+   each method of `SomeTrait` that `T` implements, a pointer to `T`'s
+   implementation (i.e. a function pointer).
+
+The purpose of trait objects is to permit "late binding" of methods. A call to
+a method on a trait object is only resolved to a vtable entry at compile time.
+The actual implementation for each vtable entry can vary on an object-by-object
+basis.
+
+Note that for a trait object to be instantiated, the trait must be
+_object-safe_. Object safety rules are defined in [RFC 255][rfc255].
 
 Given a pointer-typed expression `E` of type `&T` or `Box<T>`, where `T`
 implements trait `R`, casting `E` to the corresponding pointer type `&R` or

branches/try/src/liballoc/boxed.rs

@@ -62,6 +62,11 @@ use core::ops::{Deref, DerefMut};
 use core::ptr::{Unique};
 use core::raw::{TraitObject};
 
+#[cfg(not(stage0))]
+use core::marker::Unsize;
+#[cfg(not(stage0))]
+use core::ops::CoerceUnsized;
+
 /// A value that represents the heap. This is the default place that the `box`
 /// keyword allocates into when no place is supplied.
 ///
@@ -390,3 +395,6 @@ impl<'a,A,R> FnOnce<A> for Box<FnBox<A,Output=R>+Send+'a> {
         self.call_box(args)
     }
 }
+
+#[cfg(not(stage0))]
+impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}