Value returned by `to_owned` doesn't always *own* the data

[BusyJay]

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.