Value returned by to_owned doesn't always own the data

For example:

let a = vec![1, 2];
let s = Some(&a);
let r = vec![];
if let Some(ref v) = s {
    r = v.to_owned();
}

This doesn't compile:

error[E0308]: mismatched types
 --> src/main.rs:6:13
  |
6 |         r = v.to_owned();
  |             ^^^^^^^^^^^^ expected struct `std::vec::Vec`, found reference
  |
  = note: expected type `std::vec::Vec<_>`
             found type `&std::vec::Vec<{integer}>`

Relates to #43236.

1. We now suggest using .to_vec() (without asking to remove to_owned()):

   error[E0308]: mismatched types
    --> src/main.rs:6:9
     |
   6 |     r = v.to_owned();
     |         ^^^^^^^^^^^^
     |         |
     |         expected struct `std::vec::Vec`, found reference
     |         help: try using a conversion method: `v.to_owned().to_vec()`
     |
     = note: expected type `std::vec::Vec<_>`
                found type `&std::vec::Vec<{integer}>`
   

   Without the to_owned call, we also suggest the right thing:

   error[E0308]: mismatched types
    --> src/main.rs:6:9
     |
   6 |     r = v;
     |         ^
     |         |
     |         expected struct `std::vec::Vec`, found reference
     |         help: try using a conversion method: `v.to_vec()`
     |
     = note: expected type `std::vec::Vec<_>`
                found type `&&std::vec::Vec<{integer}>`
   

   Esteban Kuber at 2019-05-29 17:19:24

2. I may not express my concern correctly. It's not about the error messages, but the way to_owned works. When I calling to_owned, I'm expecting to get an owned data instead of another reference. This applies to other types other than Vec.

   Jay at 2019-05-31 06:24:55

3. Hi, I have the exact same concern. In the example I provide, I struggled understanding how was to_owned() implemented when neither Copy or Clone is.

   //#[derive(Clone)] //Uncomment and it will suddenly works
   pub struct Example {
   	pub name : String
   }
   
   fn main() {
   	let ex = Example{name: "Name".to_string()};
   	let ex_ref = &ex;
   	let ex_owned : Example = ex_ref.to_owned();  // Error: expected struct `Example`, found &Example
   }
   

   As far as I understood, it is cloning the reference using the fact that &T implements Clone in

   #[stable(feature = "rust1", since = "1.0.0")]
   impl<T> ToOwned for T
   where
       T: Clone,
   {
       type Owned = T;
       fn to_owned(&self) -> T {
           self.clone()
       }
   
       fn clone_into(&self, target: &mut T) {
           target.clone_from(self);
       }
   }
   

   QDoussot at 2021-06-09 09:59:39

4. close this?

   evbo at 2024-04-07 20:32:23