Use unescape_unicode for raw C string literals.

They can't contain `\x` escapes, which means they can't contain high
bytes, which means we can used `unescape_unicode` instead of
`unescape_mixed` to unescape them. This avoids unnecessary used of
`MixedUnit`.

compiler/rustc_lexer/src/unescape.rs

@@ -97,7 +97,13 @@ where
         }
         Str | ByteStr => unescape_non_raw_common(src, mode, callback),
         RawStr | RawByteStr => check_raw_common(src, mode, callback),
-        CStr | RawCStr => unreachable!(),
+        RawCStr => check_raw_common(src, mode, &mut |r, mut result| {
+            if let Ok('\0') = result {
+                result = Err(EscapeError::NulInCStr);
+            }
+            callback(r, result)
+        }),
+        CStr => unreachable!(),
     }
 }
 
@@ -141,24 +147,13 @@ where
     F: FnMut(Range<usize>, Result<MixedUnit, EscapeError>),
 {
     match mode {
-        CStr => {
-            unescape_non_raw_common(src, mode, &mut |r, mut result| {
-                if let Ok(MixedUnit::Char('\0')) = result {
-                    result = Err(EscapeError::NulInCStr);
-                }
-                callback(r, result)
-            });
-        }
-        RawCStr => {
-            check_raw_common(src, mode, &mut |r, mut result| {
-                if let Ok('\0') = result {
-                    result = Err(EscapeError::NulInCStr);
-                }
-                // High bytes aren't possible in raw strings.
-                callback(r, result.map(MixedUnit::Char))
-            });
-        }
-        Char | Byte | Str | RawStr | ByteStr | RawByteStr => unreachable!(),
+        CStr => unescape_non_raw_common(src, mode, &mut |r, mut result| {
+            if let Ok(MixedUnit::Char('\0')) = result {
+                result = Err(EscapeError::NulInCStr);
+            }
+            callback(r, result)
+        }),
+        Char | Byte | Str | RawStr | ByteStr | RawByteStr | RawCStr => unreachable!(),
     }
 }
 

compiler/rustc_parse/src/lexer/mod.rs

@@ -472,7 +472,7 @@ impl<'sess, 'src> StringReader<'sess, 'src> {
                 if let Some(n_hashes) = n_hashes {
                     let n = u32::from(n_hashes);
                     let kind = token::CStrRaw(n_hashes);
-                    self.cook_mixed(kind, Mode::RawCStr, start, end, 3 + n, 1 + n) // cr##" "##
+                    self.cook_unicode(kind, Mode::RawCStr, start, end, 3 + n, 1 + n) // cr##" "##
                 } else {
                     self.report_raw_str_error(start, 2);
                 }