syntax: fix empty char class bug in HIR printer

When a character class is empty, the HIR printer would emit '[]', which
is not a valid regex. (Since if a ']' immediately follows an opening
'[', then the ']' is interpreted literally and not a closing bracket.)

Instead, we write '[a&&b]'. We could also do things like '(?u:\P{any})'
or '(?-u:[\x00-\xFF])', but '[a&&b]' doesn't require any flags and also
seems really obvious: the intersection of two distinct characters is
obviously empty.

Author: BurntSushi

regex-syntax/src/hir/print.rs

@@ -125,6 +125,9 @@ impl<W: fmt::Write> Visitor for Writer<W> {
                 }
             }
             HirKind::Class(hir::Class::Unicode(ref cls)) => {
+                if cls.ranges().is_empty() {
+                    return self.wtr.write_str("[a&&b]");
+                }
                 self.wtr.write_str("[")?;
                 for range in cls.iter() {
                     if range.start() == range.end() {
@@ -143,6 +146,9 @@ impl<W: fmt::Write> Visitor for Writer<W> {
                 self.wtr.write_str("]")?;
             }
             HirKind::Class(hir::Class::Bytes(ref cls)) => {
+                if cls.ranges().is_empty() {
+                    return self.wtr.write_str("[a&&b]");
+                }
                 self.wtr.write_str("(?-u:[")?;
                 for range in cls.iter() {
                     if range.start() == range.end() {
@@ -359,6 +365,11 @@ mod tests {
         roundtrip_bytes(r"(?-u)[\[]", r"\[");
         roundtrip_bytes(r"(?-u)[Z-_]", r"(?-u:[Z-_])");
         roundtrip_bytes(r"(?-u)[Z-_--Z]", r"(?-u:[\[-_])");
+
+        // This tests that an empty character class is correctly roundtripped.
+        #[cfg(feature = "unicode-gencat")]
+        roundtrip(r"\P{any}", r"[a&&b]");
+        roundtrip_bytes(r"(?-u)[^\x00-\xFF]", r"[a&&b]");
     }
 
     #[test]