Fixes rust-lang#2417. Get the index from user input instead of a const.

The compiler has gotten sufficiently smart regarding const evaluation to
catch many trivial instances of array-index-out-of-bounds at compile
time. However, the compiler can't read peoples' minds... yet.

listings/ch03-common-programming-concepts/no-listing-15-invalid-array-access/src/main.rs

@@ -1,8 +1,25 @@
+use std::io;
+
 fn main() {
     let a = [1, 2, 3, 4, 5];
-    let index = 10;
+
+    println!("Please enter an array index.");
+
+    let mut index = String::new();
+
+    io::stdin()
+        .read_line(&mut index)
+        .expect("Failed to read line");
+
+    let index: usize = index
+        .trim()
+        .parse()
+        .expect("Index entered was not a number");
 
     let element = a[index];
 
-    println!("The value of element is: {}", element);
+    println!(
+        "The value of the element at index {} is: {}",
+        index, element
+    );
 }

src/ch03-02-data-types.md

@@ -324,26 +324,39 @@ get the value `2` from index `[1]` in the array.
 ##### Invalid Array Element Access
 
 What happens if you try to access an element of an array that is past the end
-of the array? Say you change the example to the following code, which will
-compile but exit with an error when it runs:
+of the array? Say you change the example to the following, which uses code
+similar to the guessing game in Chapter 2 to get an array index from the user:
 
 <span class="filename">Filename: src/main.rs</span>
 
 ```rust,ignore,does_not_compile
 {{#rustdoc_include ../listings/ch03-common-programming-concepts/no-listing-15-invalid-array-access/src/main.rs}}
 ```
 
-Running this code using `cargo run` produces the following result:
+This code compiles successfully. If you run this code using `cargo run` and
+enter 0, 1, 2, 3, or 4, the program will print out the corresponding value at
+that index in the array. If you instead enter a number past the end of the
+array, such as 10, you'll see output like this:
+
+<!-- manual-regeneration
+cd listings/ch03-common-programming-concepts/no-listing-15-invalid-array-access
+cargo run
+10
+-->
 
 ```console
-{{#include ../listings/ch03-common-programming-concepts/no-listing-15-invalid-array-access/output.txt}}
+thread 'main' panicked at 'index out of bounds: the len is 5 but the index is 10', src/main.rs:19:19
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 ```
 
-The compilation didn’t produce any errors, but the program resulted in a
-*runtime* error and didn’t exit successfully. When you attempt to access an
+The program resulted in a *runtime* error at the point of using an invalid
+value in the indexing operation. The program exited at that point with an error
+message and didn't execute the final `println!`. When you attempt to access an
 element using indexing, Rust will check that the index you’ve specified is less
 than the array length. If the index is greater than or equal to the array
-length, Rust will panic.
+length, Rust will panic. This check has to happen at runtime, especially in
+this case, because the compiler can't possibly know what the value a user
+running the code will later enter.
 
 This is the first example of Rust’s safety principles in action. In many
 low-level languages, this kind of check is not done, and when you provide an