Rollup merge of #140175 - Kivooeo:new-fix-one, r=compiler-errors

`rc""` more clear error message

here is small fix that provides better error message when user is trying to use `rc""` the same way it was made for `rb""`

example of it's work

```rust
  |
2 |     rc"\n";
  |     ^^ unknown prefix
  |
  = note: prefixed identifiers and literals are reserved since Rust 2021
help: use `cr` for a raw C-string
  |
2 -     rc"\n";
2 +     cr"\n";
  |
```

**related issue**
fixes #140170

cc `@cyrgani` (issue author)

2 files changed, 31 insertions, 0 deletions

diff --git a/tests/ui/suggestions/raw-c-string-prefix.rs b/tests/ui/suggestions/raw-c-string-prefix.rs
new file mode 100644
index 00000000000..6af72df4024
--- /dev/null
+++ b/tests/ui/suggestions/raw-c-string-prefix.rs
@@ -0,0 +1,10 @@
+// `rc` and `cr` are easy to confuse; check that we issue a suggestion to help.
+
+//@ edition:2021
+
+fn main() {
+    rc"abc";
+    //~^ ERROR: prefix `rc` is unknown
+    //~| HELP: use `cr` for a raw C-string
+    //~| ERROR: expected one of
+}
diff --git a/tests/ui/suggestions/raw-c-string-prefix.stderr b/tests/ui/suggestions/raw-c-string-prefix.stderr
new file mode 100644
index 00000000000..5341e3d04e8
--- /dev/null
+++ b/tests/ui/suggestions/raw-c-string-prefix.stderr
@@ -0,0 +1,21 @@
+error: prefix `rc` is unknown
+  --> $DIR/raw-c-string-prefix.rs:6:5
+   |
+LL |     rc"abc";
+   |     ^^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: use `cr` for a raw C-string
+   |
+LL -     rc"abc";
+LL +     cr"abc";
+   |
+
+error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `"abc"`
+  --> $DIR/raw-c-string-prefix.rs:6:7
+   |
+LL |     rc"abc";
+   |       ^^^^^ expected one of 8 possible tokens
+
+error: aborting due to 2 previous errors
+