Deserialize: Detect string end quotes correctly

src/de/mod.rs

@@ -224,10 +224,37 @@ impl<'a> Deserializer<'a> {
         loop {
             match self.peek() {
                 Some(b'"') => {
-                    let end = self.index;
-                    self.eat_char();
-                    return from_utf8(&self.slice[start..end])
-                        .map_err(|_| Error::InvalidUnicodeCodePoint);
+                    // Counts the number of backslashes in front of the current index.
+                    //
+                    // "some string with \\\" included."
+                    //                  ^^^^^
+                    //                  |||||
+                    //       loop run:  4321|
+                    //                      |
+                    //                   `index`
+                    //
+                    // Since we only get in this code branch if we found a " starting the string and `index` is greater
+                    // than the start position, we know the loop will end no later than this point.
+                    let leading_backslashes = |index: usize| -> usize {
+                        let mut count = 0;
+                        loop {
+                            if self.slice[index - count - 1] == b'\\' {
+                                count += 1;
+                            } else {
+                                return count;
+                            }
+                        }
+                    };
+
+                    let is_escaped = leading_backslashes(self.index) % 2 == 1;
+                    if is_escaped {
+                        self.eat_char(); // just continue
+                    } else {
+                        let end = self.index;
+                        self.eat_char();
+                        return from_utf8(&self.slice[start..end])
+                            .map_err(|_| Error::InvalidUnicodeCodePoint);
+                    }
                 }
                 Some(_) => self.eat_char(),
                 None => return Err(Error::EofWhileParsingString),
@@ -774,10 +801,30 @@ mod tests {
 
     #[test]
     fn str() {
+        // simple
         assert_eq!(from_str(r#" "hello" "#), Ok("hello"));
         assert_eq!(from_str(r#" "" "#), Ok(""));
         assert_eq!(from_str(r#" " " "#), Ok(" "));
         assert_eq!(from_str(r#" "👏" "#), Ok("👏"));
+
+        // no unescaping is done (as documented as a known issue in lib.rs)
+        assert_eq!(from_str(r#" "hel\tlo" "#), Ok("hel\\tlo"));
+        assert_eq!(from_str(r#" "hello \\" "#), Ok("hello \\\\"));
+
+        // escaped " in the string content
+        assert_eq!(from_str(r#" "foo\"bar" "#), Ok(r#"foo\"bar"#));
+        assert_eq!(from_str(r#" "foo\\\"bar" "#), Ok(r#"foo\\\"bar"#));
+        assert_eq!(from_str(r#" "foo\"\"bar" "#), Ok(r#"foo\"\"bar"#));
+        assert_eq!(from_str(r#" "\"bar" "#), Ok(r#"\"bar"#));
+        assert_eq!(from_str(r#" "foo\"" "#), Ok(r#"foo\""#));
+        assert_eq!(from_str(r#" "\"" "#), Ok(r#"\""#));
+
+        // non-excaped " preceded by backslashes
+        assert_eq!(from_str(r#" "foo bar\\" "#), Ok(r#"foo bar\\"#));
+        assert_eq!(from_str(r#" "foo bar\\\\" "#), Ok(r#"foo bar\\\\"#));
+        assert_eq!(from_str(r#" "foo bar\\\\\\" "#), Ok(r#"foo bar\\\\\\"#));
+        assert_eq!(from_str(r#" "foo bar\\\\\\\\" "#), Ok(r#"foo bar\\\\\\\\"#));
+        assert_eq!(from_str(r#" "\\" "#), Ok(r#"\\"#));
     }
 
     #[test]
@@ -786,6 +833,10 @@ mod tests {
         assert_eq!(from_str(r#" "" "#), Ok(String::from("")));
         assert_eq!(from_str(r#" " " "#), Ok(String::from(" ")));
         assert_eq!(from_str(r#" "👏" "#), Ok(String::from("👏")));
+
+        // escaped " in the string content
+        // (note: no unescaping is performed, as documented as a known issue in lib.rs)
+        assert_eq!(from_str(r#" "foo\"bar" "#), Ok(String::from(r#"foo\"bar"#)));
     }
 
     #[test]