Update to edition 2021 and fix tests.

Author: ehuss

book.toml

@@ -12,3 +12,6 @@ enable = true
 
 [output.html]
 git-repository-url = "https://github.com/rust-lang/rust-by-example"
+
+[rust]
+edition = "2021"

src/error/result/enter_question_mark.md

@@ -43,7 +43,7 @@ The `?` operator is now recommended, but you may still find `try!` when looking
 at older code. The same `multiply` function from the previous example
 would look like this using `try!`:
 
-```rust,editable
+```rust,editable,edition2015
 // To compile and run this example without errors, while using Cargo, change the value 
 // of the `edition` field, in the `[package]` section of the `Cargo.toml` file, to "2015".
 

src/fn/closures/closure_examples/iter_any.md

@@ -35,7 +35,7 @@ fn main() {
     // `iter()` for arrays yields `&i32`.
     println!("2 in array1: {}", array1.iter()     .any(|&x| x == 2));
     // `into_iter()` for arrays unusually yields `&i32`.
-    println!("2 in array2: {}", array2.into_iter().any(|&x| x == 2));
+    println!("2 in array2: {}", array2.iter().any(|&x| x == 2));
 }
 ```
 

src/generics/assoc_items/types.md

@@ -13,7 +13,7 @@ trait Contains {
     type B;
 
     // Updated syntax to refer to these new types generically.
-    fn contains(&self, &Self::A, &Self::B) -> bool;
+    fn contains(&self, _: &Self::A, _: &Self::B) -> bool;
 }
 ```