Add data race to concurrency docs

Author: allonsy

src/doc/book/concurrency.md

@@ -162,7 +162,7 @@ The same [ownership system](ownership.html) that helps prevent using pointers
 incorrectly also helps rule out data races, one of the worst kinds of
 concurrency bugs.
 
-As an example, here is a Rust program that could have a data race in many
+As an example, here is a Rust program that would have a data race in many
 languages. It will not compile:
 
 ```ignore
@@ -174,7 +174,7 @@ fn main() {
 
     for i in 0..3 {
         thread::spawn(move || {
-            data[i] += 1;
+            data[0] += i;
         });
     }
 
@@ -186,7 +186,7 @@ This gives us an error:
 
 ```text
 8:17 error: capture of moved value: `data`
-        data[i] += 1;
+        data[0] += i;
         ^~~~
 ```
 
@@ -195,11 +195,6 @@ thread, and the thread takes ownership of the reference, we'd have three owners!
 `data` gets moved out of `main` in the first call to `spawn()`, so subsequent
 calls in the loop cannot use this variable.
 
-Note that this specific example will not cause a data race since different array
-indices are being accessed. But this can't be determined at compile time, and in
-a similar situation where `i` is a constant or is random, you would have a data
-race.
-
 So, we need some type that lets us have more than one owning reference to a
 value. Usually, we'd use `Rc<T>` for this, which is a reference counted type
 that provides shared ownership. It has some runtime bookkeeping that keeps track
@@ -223,7 +218,7 @@ fn main() {
 
         // use it in a thread
         thread::spawn(move || {
-            data_ref[i] += 1;
+            data_ref[0] += i;
         });
     }
 
@@ -266,7 +261,7 @@ fn main() {
     for i in 0..3 {
         let data = data.clone();
         thread::spawn(move || {
-            data[i] += 1;
+            data[0] += i;
         });
     }
 
@@ -281,7 +276,7 @@ And... still gives us an error.
 
 ```text
 <anon>:11:24 error: cannot borrow immutable borrowed content as mutable
-<anon>:11                    data[i] += 1;
+<anon>:11                    data[0] += i;
                              ^~~~
 ```
 
@@ -317,7 +312,7 @@ fn main() {
         let data = data.clone();
         thread::spawn(move || {
             let mut data = data.lock().unwrap();
-            data[i] += 1;
+            data[0] += i;
         });
     }
 
@@ -360,7 +355,7 @@ Let's examine the body of the thread more closely:
 #         let data = data.clone();
 thread::spawn(move || {
     let mut data = data.lock().unwrap();
-    data[i] += 1;
+    data[0] += i;
 });
 #     }
 #     thread::sleep(Duration::from_millis(50));