Improve string concatenation suggestion

[camelid]

Before:

error[E0369]: cannot add `&str` to `&str`
 --> file.rs:2:22
  |
2 |     let _x = "hello" + " world";
  |              ------- ^ -------- &str
  |              |       |
  |              |       `+` cannot be used to concatenate two `&str` strings
  |              &str
  |
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
  |
2 |     let _x = "hello".to_owned() + " world";
  |              ~~~~~~~~~~~~~~~~~~

After:

error[E0369]: cannot add `&str` to `&str`
 --> file.rs:2:22
  |
2 |     let _x = "hello" + " 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
  |
2 |     let _x = "hello".to_owned() + " world";
  |                     +++++++++++

[rust-highfive]

r? @matthewjasper

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[camelid]

r? @davidtwco

[davidtwco]

I like these changes, I've left some nitpicks and comments regarding the specific messages but this looks good otherwise.

[camelid]

I went with a shorter and slightly different version of the note you suggested. What do you think?

[davidtwco]

@bors r+ rollup

[bors]

📌 Commit 7c4eca0 has been approved by davidtwco