rc"" more clear error message

compiler/rustc_parse/messages.ftl

@@ -893,6 +893,7 @@ parse_unknown_prefix = prefix `{$prefix}` is unknown
     .label = unknown prefix
     .note =  prefixed identifiers and literals are reserved since Rust 2021
     .suggestion_br = use `br` for a raw byte string
+    .suggestion_cr = use `cr` for a raw C-string
     .suggestion_str = if you meant to write a string literal, use double quotes
     .suggestion_whitespace = consider inserting whitespace here
 

compiler/rustc_parse/src/errors.rs

@@ -2140,6 +2140,13 @@ pub(crate) enum UnknownPrefixSugg {
         style = "verbose"
     )]
     UseBr(#[primary_span] Span),
+    #[suggestion(
+        parse_suggestion_cr,
+        code = "cr",
+        applicability = "maybe-incorrect",
+        style = "verbose"
+    )]
+    UseCr(#[primary_span] Span),
     #[suggestion(
         parse_suggestion_whitespace,
         code = " ",

compiler/rustc_parse/src/lexer/mod.rs

@@ -256,7 +256,6 @@ impl<'psess, 'src> Lexer<'psess, 'src> {
                     let lit_start = start + BytePos(prefix_len);
                     self.pos = lit_start;
                     self.cursor = Cursor::new(&str_before[prefix_len as usize..]);
-
                     self.report_unknown_prefix(start);
                     let prefix_span = self.mk_sp(start, lit_start);
                     return (Token::new(self.ident(start), prefix_span), preceded_by_whitespace);
@@ -789,13 +788,14 @@ impl<'psess, 'src> Lexer<'psess, 'src> {
     fn report_unknown_prefix(&self, start: BytePos) {
         let prefix_span = self.mk_sp(start, self.pos);
         let prefix = self.str_from_to(start, self.pos);
-
         let expn_data = prefix_span.ctxt().outer_expn_data();
 
         if expn_data.edition.at_least_rust_2021() {
             // In Rust 2021, this is a hard error.
             let sugg = if prefix == "rb" {
                 Some(errors::UnknownPrefixSugg::UseBr(prefix_span))
+            } else if prefix == "rc" {
+                Some(errors::UnknownPrefixSugg::UseCr(prefix_span))
             } else if expn_data.is_root() {
                 if self.cursor.first() == '\''
                     && let Some(start) = self.last_lifetime

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
+}

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
+