Auto merge of #113476 - fee1-dead-contrib:c-str-lit, r=petrochenkov

Reimplement C-str literals

This reverts #113334, cc `@fmease.`

While converting lexer tokens to ast Tokens in `rustc_parse`, we check the edition of the span of the token. If the edition < 2021, we split the token into two, one being the identifier and other being the str literal.

compiler/rustc_lexer/src/cursor.rs

@@ -24,6 +24,10 @@ impl<'a> Cursor<'a> {
         }
     }
 
+    pub fn as_str(&self) -> &'a str {
+        self.chars.as_str()
+    }
+
     /// Returns the last eaten symbol (or `'\0'` in release builds).
     /// (For debug assertions only.)
     pub(crate) fn prev(&self) -> char {

compiler/rustc_lexer/src/lib.rs

@@ -367,6 +367,13 @@ impl Cursor<'_> {
                 Some(|terminated| Byte { terminated }),
             ),
 
+            // c-string literal, raw c-string literal or identifier.
+            'c' => self.c_or_byte_string(
+                |terminated| CStr { terminated },
+                |n_hashes| RawCStr { n_hashes },
+                None,
+            ),
+
             // Identifier (this should be checked after other variant that can
             // start as identifier).
             c if is_id_start(c) => self.ident_or_unknown_prefix(),