error[E0369]: cannot add `&str` to `&str`
  --> $DIR/issue-47377.rs:4:14
   |
LL |      let _a = b + ", World!";
   |               - ^ ---------- &str
   |               | |
   |               | `+` cannot be used to concatenate two `&str` strings
   |               &str
   |
   = note: string concatenation requires an owned `String` on the left
help: create an owned `String` from a string reference
   |
LL |      let _a = b.to_owned() + ", World!";
   |                +++++++++++

error: aborting due to 1 previous error

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