move occurs because `data` has type `Vec`, which does not implement the `Copy` trait

Question

I am trying to print some data for debugging purposes when I encountered an error. You can have a look at the full code here.

The error occurred when I tried to print data returned from the prepare_hash_data() function, which returns a Vec<u8>.

let data = self.prepare_hash_data()?;      // returns Vec<u8>
println!("{}", String::from_utf8(data)?);  // error here
let mut hasher = Sha256::new();
hasher.input(&data[..]);
println!("{}", hasher.result_str().as_str());
self.hash = hasher.result_str();

The prepare_hash_data() function is given below. Other details are omitted. Simply, it is just a function that returns Vec<u8>

fn prepare_hash_data(&self) -> Result<Vec<u8>, failure::Error> {
        let content = (
            self.hash_prev_block.clone(),
            self.transactions.clone(),
            self.timestamp,
            DIFFICULTY,
            self.nonce
        );
        let bytes = bincode::serialize(&content)?;
        Ok(bytes)
    }

The error given is

error[E0382]: borrow of moved value: `data`
  --> src/block.rs:63:23
   |
60 |         let data = self.prepare_hash_data()?;
   |             ---- move occurs because `data` has type `Vec<u8>`, which does not implement the `Copy` trait
61 |         println!("{}", String::from_utf8(data)?);
   |                                          ---- value moved here
62 |         let mut hasher = Sha256::new();
63 |         hasher.input(&data[..]);
   |                       ^^^^ value borrowed here after move

I tried the following ways

• Implementing Copy trait. But, Vec<u8> can't have the Copy trait as described here.

• Looking at E0382 given in the error message, there are two ways suggested.

  • Using a reference, we can let another function borrow the value without changing its ownership.

    • But how should I use reference in this example?
    • Should I change the function signature to something like this fn prepare_hash_data(&self) -> Result<&Vec<u8>, failure::Error>?

  • With Rc, a value cannot be owned by more than one variable.

    • Don't know how to implement.

• I tried cloning the data by println!("{}", String::from_utf8(data.clone())?);

  • But, it gives another error backtrace::backtrace::trace_unsynchronized
  • For the full error log, click here

What should be the correct approach to printing some data that can't be copied or cloned without moving it for later usage in subsequent lines?

I did look at the following solutions but can't relate the answer.

• move occurs because value has type Vec, which does not implement the Copy trait
• 'move occurs because value has type' Rust error

Answer 1

I am trying to print some data for debugging purposes

The code you provided:

println!("{}", String::from_utf8(data)?);

Is not the correct way to debug-print data.

For one, String::from_utf8 consumes the data, destroying it in the process. Further, your data most likely isn't valid UTF8 data, so String::from_utf8 will only throw an error.

Use debug printing instead, it works out of the box:

println!("{:?}, data);

 

---

I retract my previous answer because it partially missed your problem.

It still contained valuable information, so I have kept it below. If someone disagrees, feel free to delete it together with this sentence.

 

--Previous Answer--

Your problem is that String::from_utf8 consumes its argument, meaning, data cannot be accessed any more afterwards.

Here is your problem in a more compact example:

fn main() {
    let data: Vec<u8> = "Hello!".as_bytes().to_vec();

    // Consumes `data`
    println!("{}", String::from_utf8(data).unwrap());

    // `data` cannot be accessed any more, as it got moved
    // into `String::from_utf8`
    println!("Length of vector: {}", data.len());
}

error[E0382]: borrow of moved value: `data`
 --> src/main.rs:9:38
  |
2 |     let data: Vec<u8> = "Hello!".as_bytes().to_vec();
  |         ---- move occurs because `data` has type `Vec<u8>`, which does not implement the `Copy` trait
...
5 |     println!("{}", String::from_utf8(data).unwrap());
  |                                      ---- value moved here
...
9 |     println!("Length of vector: {}", data.len());
  |                                      ^^^^^^^^^^ value borrowed here after move
  |
help: consider cloning the value if the performance cost is acceptable
  |
5 |     println!("{}", String::from_utf8(data.clone()).unwrap());
  |                                          ++++++++

In your case this can be easily fixed, though. The reason it consumes the data is because String is an owning variable. It owns its data, meaning, if you create it from a Vec, it stores the Vec data internally.

There is another type: &str, a string slice. It's very similar to String, just that it doesn't own its data, but merely references it. That means you can create it without destroying data:

fn main() {
    let data: Vec<u8> = "Hello!".as_bytes().to_vec();

    // Borrows `data`
    println!("{}", std::str::from_utf8(&data).unwrap());

    // `data` is still accessible because it was only borrowed
    println!("Length of vector: {}", data.len());
}

Hello!
Length of vector: 6

---

That said, the given solution is only a workaround. The proper way to fix this is to implement Display.

As your data object clearly has more meaning than just a bunch of bytes, create a proper struct that represents its meaning.

There are two ways I would consider:

• A newtype struct, like struct MyData(Vec<u8>), for which you can implement Display.
• A String. As you convert it to a string anyway, why not just make it a string right away, as the return value of prepare_hash_data? Note that if your reason to have it a Vec is that it is binary data, then you shouldn't convert it to a string via from_utf8, as it's not UTF-8 data. If it is valid UTF-8 data, however, use a String right away, not a Vec<u8>. And String already implements Display and can be printed without further conversion.