Only error raw lifetime followed by \' in edition 2021+

compiler/rustc_parse/src/lexer/mod.rs

@@ -300,6 +300,26 @@ impl<'psess, 'src> Lexer<'psess, 'src> {
                     let prefix_span = self.mk_sp(start, ident_start);
 
                     if prefix_span.at_least_rust_2021() {
+                        // If the raw lifetime is followed by \' then treat it a normal
+                        // lifetime followed by a \', which is to interpret it as a character
+                        // literal. In this case, it's always an invalid character literal
+                        // since the literal must necessarily have >3 characters (r#...) inside
+                        // of it, which is invalid.
+                        if self.cursor.as_str().starts_with('\'') {
+                            let lit_span = self.mk_sp(start, self.pos + BytePos(1));
+                            let contents = self.str_from_to(start + BytePos(1), self.pos);
+                            emit_unescape_error(
+                                self.dcx(),
+                                contents,
+                                lit_span,
+                                lit_span,
+                                Mode::Char,
+                                0..contents.len(),
+                                EscapeError::MoreThanOneChar,
+                            )
+                            .expect("expected error");
+                        }
+
                         let span = self.mk_sp(start, self.pos);
 
                         let lifetime_name_without_tick =
@@ -371,8 +391,7 @@ impl<'psess, 'src> Lexer<'psess, 'src> {
                 rustc_lexer::TokenKind::Caret => token::BinOp(token::Caret),
                 rustc_lexer::TokenKind::Percent => token::BinOp(token::Percent),
 
-                rustc_lexer::TokenKind::Unknown
+                rustc_lexer::TokenKind::Unknown | rustc_lexer::TokenKind::InvalidIdent => {
-                | rustc_lexer::TokenKind::InvalidIdent => {
                     // Don't emit diagnostics for sequences of the same invalid token
                     if swallow_next_invalid > 0 {
                         swallow_next_invalid -= 1;

tests/ui/lifetimes/raw/immediately-followed-by-lt.e2021.stderr

@@ -1,5 +1,5 @@
 error: character literal may only contain one codepoint
-  --> $DIR/immediately-followed-by-lt.rs:11:4
+  --> $DIR/immediately-followed-by-lt.rs:15:4
    |
 LL | w!('r#long'id);
    |    ^^^^^^^^

tests/ui/lifetimes/raw/immediately-followed-by-lt.rs

@@ -1,4 +1,8 @@
-//@ edition: 2021
+//@ revisions: e2015 e2021
+
+//@[e2021] edition: 2021
+//@[e2015] edition: 2015
+//@[e2015] check-pass
 
 // Make sure we reject the case where a raw lifetime is immediately followed by another
 // lifetime. This reserves a modest amount of space for changing lexing to, for example,
@@ -9,6 +13,6 @@ macro_rules! w {
 }
 
 w!('r#long'id);
-//~^ ERROR character literal may only contain one codepoint
+//[e2021]~^ ERROR character literal may only contain one codepoint
 
 fn main() {}