Renamed variable str -> tail for clarity

compiler/rustc_lexer/src/unescape.rs

@@ -319,17 +319,17 @@ where
     where
         F: FnMut(Range<usize>, Result<char, EscapeError>),
     {
-        let str = chars.as_str();
-        let first_non_space = str
+        let tail = chars.as_str();
+        let first_non_space = tail
             .bytes()
             .position(|b| b != b' ' && b != b'\t' && b != b'\n' && b != b'\r')
-            .unwrap_or(str.len());
-        if str[1..first_non_space].contains('\n') {
+            .unwrap_or(tail.len());
+        if tail[1..first_non_space].contains('\n') {
             // The +1 accounts for the escaping slash.
             let end = start + first_non_space + 1;
             callback(start..end, Err(EscapeError::MultipleSkippedLinesWarning));
         }
-        let tail = &str[first_non_space..];
+        let tail = &tail[first_non_space..];
         if let Some(c) = tail.chars().nth(0) {
             // For error reporting, we would like the span to contain the character that was not
             // skipped.  The +1 is necessary to account for the leading \ that started the escape.