minor code cleanups

compiler/rustc_lexer/src/unescape.rs

@@ -204,14 +204,13 @@ fn scan_escape(chars: &mut Chars<'_>, is_byte: bool) -> Result<char, EscapeError
                         })?;
                     }
                     Some(c) => {
-                        let digit =
+                        let digit: u32 =
                             c.to_digit(16).ok_or(EscapeError::InvalidCharInUnicodeEscape)?;
                         n_digits += 1;
                         if n_digits > 6 {
                             // Stop updating value since we're sure that it's incorrect already.
                             continue;
                         }
-                        let digit = digit as u32;
                         value = value * 16 + digit;
                     }
                 };