Mention move of individual struct fields in struct update syntax

[jpmelos]

I would like to propose improvements to the text under the section "Creating Instances from Other Instances with Struct Update Syntax" in Chapter 5. I would welcome any improvements to the new text that I'm proposing.

When using the struct update syntax, the original struct is not invalidated: only those fields that were individually moved into the new struct.

In the original text, watch out for the emphases that I added:

Note that the struct update syntax uses = like an assignment; this is because it moves the data, just as we saw in the “Variables and Data Interacting with Move” section. In this example, we can no longer use user1 as a whole 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. Both active and sign_in_count are types that implement the Copy trait, so the behavior we discussed in the “Stack-Only Data: Copy” section would apply.

It reads:

• we can no longer use user1 as a whole: When you say "as a whole", it reads to me like the entirety of the struct has been invalidated and cannot be used anymore, which is not true. I am not a native English speaker, so please forgive me if this is just because of my poor English communication skills.
• then user1 would still be valid: which implies that user1 is not valid. But it remains valid (in the sense that you can still access any fields that weren't moved), as least in the latest version of the compiler. Maybe this is a remnant from older versions?

For proof, here's a Rust program (based on the examples in the book) that compiles, and runs, and outputs as expected:

struct User {
    active: bool,
    username: String,
    email: String,
    sign_in_count: u64,
}

fn main() {
    let user1 = User {
        active: true,
        username: String::from("someusername123"),
        email: String::from("someone@example.com"),
        sign_in_count: 1,
    };

    let user2 = User {
        email: String::from("another@example.com"),
        ..user1
    };

    println!("{} active: {}", user1.email, user1.active);  // Note that I am using `user1.email` here.
    println!("{} active: {}", user2.email, user2.active);
}

Then, the following code will not compile, as expected:

struct User {
    active: bool,
    username: String,
    email: String,
    sign_in_count: u64,
}

fn main() {
    let user1 = User {
        active: true,
        username: String::from("someusername123"),
        email: String::from("someone@example.com"),
        sign_in_count: 1,
    };

    let user2 = User {
        email: String::from("another@example.com"),
        ..user1
    };

    println!("{} active: {}", user1.email, user1.active);
    println!("{} active: {}", user2.email, user2.active);

    println!("{}", user1.username); // This tries to use `user1.username`,
                                    // but it was moved into `user2`.
}

This is the compiler message that I'm getting with rustc 1.81.0 (eeb90cda1 2024-09-04).

   Compiling learn_rust v0.1.0 (/Users/jpmelos/devel/learn_rust)
error[E0382]: borrow of moved value: `user1.username`
  --> src/main.rs:24:20
   |
16 |       let user2 = User {
   |  _________________-
17 | |         email: String::from("another@example.com"),
18 | |         ..user1
19 | |     };
   | |_____- value moved here
...
24 |       println!("{}", user1.username);
   |                      ^^^^^^^^^^^^^^ value borrowed here after move
   |
   = note: move occurs because `user1.username` has type `String`, which does not implement the `Copy` trait
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0382`.
error: could not compile `learn_rust` (bin "learn_rust") due to 1 previous error

[chriskrycho]

This is an interesting one! I absolutely see what you’re going for, but the new text you suggested is actually getting at something substantially different from the original. Here’s code showing what the original text is trying to illustrate (playground):

#[derive(Debug)]
struct User {
    active: bool,
    username: String,
    email: String,
    sign_in_count: u64,
}

fn main() {
    let user1 = User {
        email: String::from("someone@example.com"),
        username: String::from("someusername123"),
        active: true,
        sign_in_count: 1,
    };

    let user2 = User {
        email: String::from("another@example.com"),
        ..user1
    };
    
    println!("{user1:?}"); // We cannot use the struct *as a whole*.
}

We can still use individual parts of it, as your example shows, but we cannot use the struct as a whole, because parts of it have been moved out. So I think we should keep the phrase as is, but perhaps add a note after the bits about Copy indicating that we can also still use user1.email in this example, since its value was not moved out.

[jpmelos]

Oh, I see it now! I was misreading "as a whole", I understand now that it means "as a package", or "everything as a unit". Okay, I get that.

I agree about adding that note. I'll give it a try writing it if you'll allow me.

[jpmelos]

we can also still use user1.email in this example, since its value was not moved out.

Well, there isn't much to write, actually, so I used your words. There isn't much to elaborate on, it seems, and the paragraph is already pretty dense.

[chriskrycho]

👍🏼 Thanks!