Merge pull request rust-lang#2239 from apatniv/use_references

traits as parameters: Use references instead of moving the values

listings/ch10-generic-types-traits-and-lifetimes/no-listing-04-traits-as-parameters/src/lib.rs

@@ -29,7 +29,7 @@ impl Summary for Tweet {
 }
 
 // ANCHOR: here
-pub fn notify(item: impl Summary) {
+pub fn notify(item: &impl Summary) {
     println!("Breaking news! {}", item.summarize());
 }
 // ANCHOR_END: here

src/ch10-02-traits.md

@@ -222,7 +222,7 @@ syntax sugar for a longer form, which is called a *trait bound*; it looks like
 this:
 
 ```rust,ignore
-pub fn notify<T: Summary>(item: T) {
+pub fn notify<T: Summary>(item: &T) {
     println!("Breaking news! {}", item.summarize());
 }
 ```
@@ -237,7 +237,7 @@ example, we can have two parameters that implement `Summary`. Using the `impl
 Trait` syntax looks like this:
 
 ```rust,ignore
-pub fn notify(item1: impl Summary, item2: impl Summary) {
+pub fn notify(item1: &impl Summary, item2: &impl Summary) {
 ```
 
 If we wanted this function to allow `item1` and `item2` to have different
@@ -246,7 +246,7 @@ types, using `impl Trait` would be appropriate (as long as both types implement
 only possible to express using a trait bound, like this:
 
 ```rust,ignore
-pub fn notify<T: Summary>(item1: T, item2: T) {
+pub fn notify<T: Summary>(item1: &T, item2: &T) {
 ```
 
 The generic type `T` specified as the type of the `item1` and `item2`
@@ -261,13 +261,13 @@ the `notify` definition that `item` must implement both `Display` and
 `Summary`. We can do so using the `+` syntax:
 
 ```rust,ignore
-pub fn notify(item: impl Summary + Display) {
+pub fn notify(item: &(impl Summary + Display)) {
 ```
 
 The `+` syntax is also valid with trait bounds on generic types:
 
 ```rust,ignore
-pub fn notify<T: Summary + Display>(item: T) {
+pub fn notify<T: Summary + Display>(item: &T) {
 ```
 
 With the two trait bounds specified, the body of `notify` can call `summarize`
@@ -283,13 +283,13 @@ syntax for specifying trait bounds inside a `where` clause after the function
 signature. So instead of writing this:
 
 ```rust,ignore
-fn some_function<T: Display + Clone, U: Clone + Debug>(t: T, u: U) -> i32 {
+fn some_function<T: Display + Clone, U: Clone + Debug>(t: &T, u: &U) -> i32 {
 ```
 
 we can use a `where` clause, like this:
 
 ```rust,ignore
-fn some_function<T, U>(t: T, u: U) -> i32
+fn some_function<T, U>(t: &T, u: &U) -> i32
     where T: Display + Clone,
           U: Clone + Debug
 {