---

yaml
---
r: 233516
b: refs/heads/beta
c: a1f64c2
h: refs/heads/master
v: v3

Author: steveklabnik

[refs]

@@ -23,7 +23,7 @@ refs/tags/0.9: 36870b185fc5f5486636d4515f0e22677493f225
 refs/tags/0.10: ac33f2b15782272ae348dbd7b14b8257b2148b5a
 refs/tags/0.11.0: e1247cb1d0d681be034adb4b558b5a0c0d5720f9
 refs/tags/0.12.0: f0c419429ef30723ceaf6b42f9b5a2aeb5d2e2d1
-refs/heads/beta: 6f6ea4687e9d596d7136dc591dee312e51fa0ad7
+refs/heads/beta: a1f64c2204687263ca1230f08af1cc7317cc4a26
 refs/tags/1.0.0-alpha: e42bd6d93a1d3433c486200587f8f9e12590a4d7
 refs/heads/tmp: 370fe2786109360f7c35b8ba552b83b773dd71d6
 refs/tags/1.0.0-alpha.2: 4c705f6bc559886632d3871b04f58aab093bfa2f

branches/beta/src/doc/trpl/trait-objects.md

@@ -300,3 +300,41 @@ let y = TraitObject {
 // y.method();
 (y.vtable.method)(y.data);
 ```
+
+## Object Safety
+
+Not every trait can be used to make a trait object. For example, vectors implement
+`Clone`, but if we try to make a trait object:
+
+```ignore
+let v = vec![1, 2, 3];
+let o = &v as &Clone;
+```
+
+We get an error:
+
+```text
+error: cannot convert to a trait object because trait `core::clone::Clone` is not object-safe [E0038]
+let o = &v as &Clone;
+        ^~
+note: the trait cannot require that `Self : Sized`
+let o = &v as &Clone;
+        ^~
+```
+
+The error says that `Clone` is not ‘object-safe’. Only traits that are
+object-safe can be made into trait objects. A trait is object-safe if both of
+these are true:
+
+* the trait does not require that `Self: Sized`
+* all of its methods are object-safe
+
+So what makes a method object-safe? Each method must require that `Self: Sized`
+or all of the following:
+
+* must not have any type parameters
+* must not use `Self`
+
+Whew! As we can see, almost all of these rules talk about `Self`. A good intuition
+is “except in special circumstances, if your trait’s method uses `Self`, it is not
+object-safe.”

branches/beta/src/libstd/io/mod.rs

@@ -1060,9 +1060,6 @@ pub trait Seek {
     /// A seek beyond the end of a stream is allowed, but implementation
     /// defined.
     ///
-    /// The behavior when seeking past the end of the stream is implementation
-    /// defined.
-    ///
     /// If the seek operation completed successfully,
     /// this method returns the new position from the start of the stream.
     /// That position can be used later with `SeekFrom::Start`.