error[E0369]: cannot add `&str` to `&String`
  --> $DIR/str-concat-on-double-ref.rs:4:15
   |
LL |     let c = a + b;
   |             - ^ - &str
   |             | |
   |             | `+` cannot be used to concatenate two `&str` strings
   |             &String
   |
help: `to_owned()` can be used to create an owned `String` from a string reference. String concatenation appends the string on the right to the string on the left and may require reallocation. This requires ownership of the string on the left
   |
LL |     let c = a.to_owned() + b;
   |             ^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0369`.