Suggest br if the unknown string prefix rb is found

[FabianWolff]

Currently, for the following code:

fn main() {
    rb"abc";
}

we issue the following suggestion:

help: consider inserting whitespace here
  |
2 |     rb "abc";
  |       --

With my changes (only in edition 2021, where unknown prefixes became an error), I get:

help: use `br` for a raw byte string
  |
2 |     br"abc";
  |     ^^

[rust-highfive]

r? @estebank

(rust-highfive has picked a reviewer for you, use r? to override)

[estebank]

@bors r+

[bors]

📌 Commit f2c9654 has been approved by estebank

[bors]

⌛ Testing commit f2c9654 with merge 4e28279...

[bors]

☀️ Test successful - checks-actions
Approved by: estebank
Pushing 4e28279 to master...