Merge pull request #965 from guswynn/patch-1

Clarify object safety rules for methods striked from the vtable

Author: Havvy

src/items/traits.md

@@ -67,23 +67,23 @@ trait Seq<T> {
 Object safe traits can be the base trait of a [trait object]. A trait is
 *object safe* if it has the following qualities (defined in [RFC 255]):
 
-* It must not require `Self: Sized`
-* All associated functions must either have a `where Self: Sized` bound, or
-    * Not have any type parameters (although lifetime parameters are allowed),
-      and
-    * Be a [method] that does not use `Self` except in the type of the receiver.
+* All [supertraits] must also be object safe.
+* `Sized` must not be a [supertrait][supertraits]. In other words, it must not require `Self: Sized`.
 * It must not have any associated constants.
-* All supertraits must also be object safe.
-
-When there isn't a `Self: Sized` bound on a method, the type of a method
-receiver must be one of the following types:
-
-* `&Self`
-* `&mut Self`
-* [`Box<Self>`]
-* [`Rc<Self>`]
-* [`Arc<Self>`]
-* [`Pin<P>`] where `P` is one of the types above
+* All associated functions must either be dispatchable from a trait object or be explicitly non-dispatchable:
+    * Dispatchable functions require:
+        * Not have any type parameters (although lifetime parameters are allowed),
+        * Be a [method] that does not use `Self` except in the type of the receiver.
+        * Have a receiver with one of the following types:
+            * `&Self` (i.e. `&self`)
+            * `&mut Self` (i.e `&mut self`)
+            * [`Box<Self>`]
+            * [`Rc<Self>`]
+            * [`Arc<Self>`]
+            * [`Pin<P>`] where `P` is one of the types above
+        * Does not have a `where Self: Sized` bound (reciever type of `Self` (i.e. `self`) implies this).
+    * Explicitly non-dispatchable functions require:
+        * Have a `where Self: Sized` bound (reciever type of `Self` (i.e. `self`) implies this).
 
 ```rust
 # use std::rc::Rc;
@@ -325,6 +325,7 @@ fn main() {
 [RFC 255]: https://github.com/rust-lang/rfcs/blob/master/text/0255-object-safety.md
 [associated items]: associated-items.md
 [method]: associated-items.md#methods
+[supertraits]: #supertraits
 [implementations]: implementations.md
 [generics]: generics.md
 [where clauses]: generics.md#where-clauses