Handle custom diagnostic for &str + String

src/test/ui/span/issue-39018.rs

@@ -17,6 +17,9 @@ pub fn main() {
     // that won't output for the above string concatenation
     let y = World::Hello + World::Goodbye;
     //~^ ERROR cannot be applied to type
+
+    let x = "Hello " + "World!".to_owned();
+    //~^ ERROR cannot be applied to type
 }
 
 enum World {

src/test/ui/span/issue-39018.stderr

@@ -16,5 +16,19 @@ error[E0369]: binary operation `+` cannot be applied to type `World`
    |
    = note: an implementation of `std::ops::Add` might be missing for `World`
 
-error: aborting due to 2 previous errors
+error[E0369]: binary operation `+` cannot be applied to type `&str`
+  --> $DIR/issue-39018.rs:21:13
+   |
+21 |     let x = "Hello " + "World!".to_owned();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `+` can't be used to concatenate a `&str` with a `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
+   |
+21 |     let x = "Hello ".to_owned() + "World!".to_owned();
+   |             ^^^^^^^^^^^^^^^^^^^
+help: you also need to borrow the `String` on the right to get a `&str`
+   |
+21 |     let x = "Hello " + &"World!".to_owned();
+   |                        ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors