Fix a bug in negating a character class.

If the character class is empty and it is negated, then it should
yield all of Unicode. Previously, it was returning an empty class.

Author: BurntSushi

regex-syntax/src/lib.rs

@@ -374,7 +374,12 @@ impl CharClass {
     pub fn negate(mut self) -> CharClass {
         fn range(s: char, e: char) -> ClassRange { ClassRange::new(s, e) }
 
-        if self.is_empty() { return self; }
+        if self.is_empty() {
+            // Inverting an empty range yields all of Unicode.
+            return CharClass {
+                ranges: vec![ClassRange { start: '\x00', end: '\u{10ffff}' }],
+            };
+        }
         self = self.canonicalize();
         let mut inv = self.to_empty();
         if self[0].start > '\x00' {

regex-syntax/src/properties.rs

@@ -31,7 +31,9 @@ fn class(ranges: &[(char, char)]) -> CharClass {
 #[test]
 fn negate() {
     fn prop(ranges: Vec<(char, char)>) -> bool {
-        class(&ranges).canonicalize() == class(&ranges).negate().negate()
+        let expected = class(&ranges).canonicalize();
+        let got = class(&ranges).negate().negate();
+        expected == got
     }
     qc(prop as fn(Vec<(char, char)>) -> bool);
 }