Discuss how struct update syntax moves

Fixes #2789.

listings/ch05-using-structs-to-structure-related-data/listing-05-06/src/main.rs

@@ -16,7 +16,7 @@ fn main() {
     // ANCHOR: here
     let user2 = User {
         active: user1.active,
-        username: String::from("anotherusername567"),
+        username: user1.username,
         email: String::from("another@example.com"),
         sign_in_count: user1.sign_in_count,
     };

listings/ch05-using-structs-to-structure-related-data/listing-05-07/src/main.rs

@@ -15,7 +15,6 @@ fn main() {
 
     // ANCHOR: here
     let user2 = User {
-        username: String::from("anotherusername567"),
         email: String::from("another@example.com"),
         ..user1
     };

src/ch05-01-defining-structs.md

@@ -94,17 +94,18 @@ than `email: email`.
 ### Creating Instances From Other Instances With Struct Update Syntax
 
 It’s often useful to create a new instance of a struct that uses most of an old
-instance’s values but changes some. You’ll do this using *struct update syntax*.
+instance’s values but changes some. You can do this using *struct update
+syntax*.
 
 First, Listing 5-6 shows how we create a new `User` instance in `user2` without
-the update syntax. We set new values for `email` and `username` but otherwise
-use the same values from `user1` that we created in Listing 5-2.
+the update syntax. We set a new value for `email` but otherwise use the same
+values from `user1` that we created in Listing 5-2.
 
 ```rust
 {{#rustdoc_include ../listings/ch05-using-structs-to-structure-related-data/listing-05-06/src/main.rs:here}}
 ```
 
-<span class="caption">Listing 5-6: Creating a new `User` instance using some of
+<span class="caption">Listing 5-6: Creating a new `User` instance using one of
 the values from `user1`</span>
 
 Using struct update syntax, we can achieve the same effect with less code, as
@@ -115,18 +116,29 @@ explicitly set should have the same value as the fields in the given instance.
 {{#rustdoc_include ../listings/ch05-using-structs-to-structure-related-data/listing-05-07/src/main.rs:here}}
 ```
 
-<span class="caption">Listing 5-7: Using struct update syntax to set new
-`email` and `username` values for a `User` instance but use the rest of the
-values from the fields of the instance in the `user1` variable</span>
+<span class="caption">Listing 5-7: Using struct update syntax to set a new
+`email` value for a `User` instance but use the rest of the values from
+`user1`</span>
 
 The code in Listing 5-7 also creates an instance in `user2` that has a
-different value for `email` and `username` but has the same values for the
-`active` and `sign_in_count` fields from `user1`. The `..user1` must come last
+different value for `email` but has the same values for the `username`,
+`active`, and `sign_in_count` fields from `user1`. The `..user1` must come last
 to specify that any remaining fields should get their values from the
 corresponding fields in `user1`, but we can choose to specify values for as
 many fields as we want in any order, regardless of the order of the fields in
 the struct’s definition.
 
+Note that the struct update syntax is like assignment with `=` because it moves
+the data, just as we saw in the [“Ways Variables and Data Interact: Move”
+section][move]<!-- ignore -->. In this example, we can no longer use `user1`
+after creating `user2` because the `String` in the `username` field of `user1`
+was moved into `user2`. If we had given `user2` new `String` values for both
+`email` and `username`, and thus only used the `active` and `sign_in_count`
+values from `user1`, then `user1` would still be valid after creating `user2`.
+The types of `active` and `sign_in_count` are types that implement the `Copy`
+trait, so the behavior we discussed in the [“Stack-Only Data: Copy”
+section][copy]<!-- ignore --> would apply.
+
 ### Using Tuple Structs without Named Fields to Create Different Types
 
 You can also define structs that look similar to tuples, called *tuple
@@ -262,3 +274,5 @@ paste above
 add `> ` before every line -->
 
 [tuples]: ch03-02-data-types.html#the-tuple-type
+[move]: ch04-01-what-is-ownership.html#ways-variables-and-data-interact-move
+[copy]: ch04-01-what-is-ownership.html#stack-only-data-copy