Allow � character references

We should either restrict all invalid characters both in literal form and
as character references, or none of them. Disallowing only the one character
is inconsistently. Because checking literal forms means that we should decode
and check all the input, this will influence performance. We are not ready to
get that performance lost for now. Users of the Reader API could do their own
checks themselves.

Changelog.md

@@ -80,6 +80,7 @@ resolve predefined entities.
 - [#734]: No longer resolve predefined entities (`lt`, `gt`, `apos`, `quot`, `amp`)
   in `unescape_with` family of methods. You should do that by yourself using the methods
   listed above.
+- [#750]: Remove `EscapeError::EntityWithNull` and allow `�` character reference.
 
 [#275]: https://github.com/tafia/quick-xml/issues/275
 [#362]: https://github.com/tafia/quick-xml/issues/362
@@ -98,6 +99,7 @@ resolve predefined entities.
 [#738]: https://github.com/tafia/quick-xml/pull/738
 [#743]: https://github.com/tafia/quick-xml/pull/743
 [#748]: https://github.com/tafia/quick-xml/pull/748
+[#750]: https://github.com/tafia/quick-xml/pull/750
 [`DeEvent`]: https://docs.rs/quick-xml/latest/quick_xml/de/enum.DeEvent.html
 [`PayloadEvent`]: https://docs.rs/quick-xml/latest/quick_xml/de/enum.PayloadEvent.html
 [`Text`]: https://docs.rs/quick-xml/latest/quick_xml/de/struct.Text.html

src/escape.rs

@@ -10,8 +10,6 @@ use pretty_assertions::assert_eq;
 /// Error for XML escape / unescape.
 #[derive(Clone, Debug)]
 pub enum EscapeError {
-    /// Entity with Null character
-    EntityWithNull(Range<usize>),
     /// Unrecognized escape symbol
     UnrecognizedSymbol(Range<usize>, String),
     /// Cannot find `;` after `&`
@@ -31,11 +29,6 @@ pub enum EscapeError {
 impl std::fmt::Display for EscapeError {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         match self {
-            EscapeError::EntityWithNull(e) => write!(
-                f,
-                "Error while escaping character at range {:?}: Null character entity not allowed",
-                e
-            ),
             EscapeError::UnrecognizedSymbol(rge, res) => write!(
                 f,
                 "Error while escaping character at range {:?}: Unrecognized escape symbol: {:?}",
@@ -251,7 +244,7 @@ where
                 // search for character correctness
                 let pat = &raw[start + 1..end];
                 if let Some(entity) = pat.strip_prefix('#') {
-                    let codepoint = parse_number(entity, start..end)?;
+                    let codepoint = parse_number(entity)?;
                     unescaped.push_str(codepoint.encode_utf8(&mut [0u8; 4]));
                 } else if let Some(value) = resolve_entity(pat) {
                     unescaped.push_str(value);
@@ -1792,15 +1785,12 @@ pub fn resolve_html5_entity(entity: &str) -> Option<&'static str> {
     Some(s)
 }
 
-fn parse_number(bytes: &str, range: Range<usize>) -> Result<char, EscapeError> {
+fn parse_number(bytes: &str) -> Result<char, EscapeError> {
     let code = if let Some(hex_digits) = bytes.strip_prefix('x') {
         parse_hexadecimal(hex_digits)
     } else {
         parse_decimal(bytes)
     }?;
-    if code == 0 {
-        return Err(EscapeError::EntityWithNull(range));
-    }
     match std::char::from_u32(code) {
         Some(c) => Ok(c),
         None => Err(EscapeError::InvalidCodepoint(code)),