syntax: forbid \P{any}

Previously, the translator would forbid constructs like [^\w\W] that
compiled to empty character classes. These things are forbidden not
because the translator can't handle it, but because the compile in
'regex' proper can't handle it. Once we migrate to the compiler in
regex-automata, which supports empty classes, then we can lift this
restriction. But until then, we should ban all such instances. It turns
out that \P{any} was another way to utter this, so we ban it in this
commit.

This was found by OSS-Fuzz:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=26505

Fixes #722

regex-syntax/src/hir/translate.rs

@@ -322,7 +322,7 @@ impl<'t, 'p> Visitor for TranslatorI<'t, 'p> {
                         ast.negated,
                         &mut cls,
                     )?;
-                    if cls.iter().next().is_none() {
+                    if cls.ranges().is_empty() {
                         return Err(self.error(
                             ast.span,
                             ErrorKind::EmptyClassNotAllowed,
@@ -337,7 +337,7 @@ impl<'t, 'p> Visitor for TranslatorI<'t, 'p> {
                         ast.negated,
                         &mut cls,
                     )?;
-                    if cls.iter().next().is_none() {
+                    if cls.ranges().is_empty() {
                         return Err(self.error(
                             ast.span,
                             ErrorKind::EmptyClassNotAllowed,
@@ -844,6 +844,11 @@ impl<'t, 'p> TranslatorI<'t, 'p> {
                 ast_class.negated,
                 class,
             )?;
+            if class.ranges().is_empty() {
+                let err = self
+                    .error(ast_class.span, ErrorKind::EmptyClassNotAllowed);
+                return Err(err);
+            }
         }
         result
     }
@@ -2317,6 +2322,21 @@ mod tests {
         );
     }
 
+    #[test]
+    #[cfg(feature = "unicode-gencat")]
+    fn class_unicode_any_empty() {
+        assert_eq!(
+            t_err(r"\P{any}"),
+            TestError {
+                kind: hir::ErrorKind::EmptyClassNotAllowed,
+                span: Span::new(
+                    Position::new(0, 1, 1),
+                    Position::new(7, 1, 8)
+                ),
+            }
+        );
+    }
+
     #[test]
     #[cfg(not(feature = "unicode-age"))]
     fn class_unicode_age_disabled() {

tests/regression_fuzz.rs

@@ -10,3 +10,10 @@
 fn fuzz1() {
     regex!(r"1}{55}{0}*{1}{55}{55}{5}*{1}{55}+{56}|;**");
 }
+
+// See: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=26505
+// See: https://github.com/rust-lang/regex/issues/722
+#[test]
+fn empty_any_errors_no_panic() {
+    assert!(regex_new!(r"\P{any}").is_err());
+}