Why is compiler reporting partial move in error and not move?

user_622883 0
Viewed 357

In the below program why does the compiler error mention partially moved and not just moved? How is partial move happening here from x to z? If &mut T doesn't implement Copy trait, it should be moved and not partial move?

fn main() {
    let mut y = 2;
    let x = Some(&mut y);
    match x {
        None => (),
        Some(z) => {
        *z = 3; 
        println!("{}", z)},
    };
    println!("{:?}",x);
}

Below is the error from the playground

Compiling playground v0.0.1 (/playground)
error[E0382]: borrow of partially moved value: `x`
  --> src/main.rs:10:21
   |
6  |         Some(z) => {
   |              - value partially moved here
...
10 |     println!("{:?}",x);
   |                     ^ value borrowed here after partial move
   |
   = note: partial move occurs because value has type `&mut i32`, which does not implement the `Copy` trait
help: borrow this field in the pattern to avoid moving `x.0`
   |
6  |         Some(ref z) => {
   |              ^^^

For more information about this error, try `rustc --explain E0382`.
error: could not compile `playground` due to previous error
rust
1 Answers

Consider the Option enum:

enum Option<T> {
    Some(T),
    None
}

Within that enum, Some(T) is actually a tuple struct, containing one field.

When you use pattern matching to extract the value of the inner T, that would be a partial move of one field of the single field tuple struct Some.

A partial move of a field from a struct that only has one field might not seem useful, but more generally there could be any number of fields in the struct.

See also:

user_14114570
Related