Add debug_assert! in a few places to protect invariants

Author: dralley

src/reader/buffered_reader.rs

@@ -238,6 +238,9 @@ impl<'b, R: BufRead> XmlSource<'b, &'b mut Vec<u8>> for R {
         buf: &'b mut Vec<u8>,
         position: &mut usize,
     ) -> Result<Option<&'b [u8]>> {
+        // search byte must be within the ascii range
+        debug_assert!(byte.is_ascii());
+
         let mut read = 0;
         let mut done = false;
         let start = buf.len();
@@ -397,6 +400,9 @@ impl<'b, R: BufRead> XmlSource<'b, &'b mut Vec<u8>> for R {
     /// Consume and discard one character if it matches the given byte. Return
     /// true if it matched.
     fn skip_one(&mut self, byte: u8, position: &mut usize) -> Result<bool> {
+        // search byte must be within the ascii range
+        debug_assert!(byte.is_ascii());
+
         match self.peek_one()? {
             Some(b) if b == byte => {
                 *position += 1;

src/reader/slice_reader.rs

@@ -147,6 +147,8 @@ impl<'a> XmlSource<'a, ()> for &'a [u8] {
         _buf: (),
         position: &mut usize,
     ) -> Result<Option<&'a [u8]>> {
+        // search byte must be within the ascii range
+        debug_assert!(byte.is_ascii());
         if self.is_empty() {
             return Ok(None);
         }
@@ -217,6 +219,8 @@ impl<'a> XmlSource<'a, ()> for &'a [u8] {
     }
 
     fn skip_one(&mut self, byte: u8, position: &mut usize) -> Result<bool> {
+        // search byte must be within the ascii range
+        debug_assert!(byte.is_ascii());
         if self.first() == Some(&byte) {
             *self = &self[1..];
             *position += 1;