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    //! Implementation of [the WTF-8 encoding](https://simonsapin.github.io/wtf-8/).
    //!
    //! This library uses Rust’s type system to maintain
    //! [well-formedness](https://simonsapin.github.io/wtf-8/#well-formed),
    //! like the `String` and `&str` types do for UTF-8.
    //!
    //! Since [WTF-8 must not be used
    //! for interchange](https://simonsapin.github.io/wtf-8/#intended-audience),
    //! this library deliberately does not provide access to the underlying bytes
    //! of WTF-8 strings,
    //! nor can it decode WTF-8 from arbitrary bytes.
    //! WTF-8 strings can be obtained from UTF-8, UTF-16, or code points.

    // this module is imported from @SimonSapin's repo and has tons of dead code on
    // unix (it's mostly used on windows), so don't worry about dead code here.
    #![allow(dead_code)]

    #[cfg(test)]
    mod tests;

    use core::str::next_code_point;

    use crate::borrow::Cow;
    use crate::char;
    use crate::fmt;
    use crate::hash::{Hash, Hasher};
    use crate::iter::FromIterator;
    use crate::mem;
    use crate::ops;
    use crate::rc::Rc;
    use crate::slice;
    use crate::str;
    use crate::sync::Arc;
    use crate::sys_common::AsInner;

    const UTF8_REPLACEMENT_CHARACTER: &str = "\u{FFFD}";

    /// A Unicode code point: from U+0000 to U+10FFFF.
    ///
    /// Compares with the `char` type,
    /// which represents a Unicode scalar value:
    /// a code point that is not a surrogate (U+D800 to U+DFFF).
    #[derive(Eq, PartialEq, Ord, PartialOrd, Clone, Copy)]
    pub struct CodePoint {
        value: u32,
    }

    /// Format the code point as `U+` followed by four to six hexadecimal digits.
    /// Example: `U+1F4A9`
    impl fmt::Debug for CodePoint {
        #[inline]
        fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(formatter, "U+{:04X}", self.value)
        }
    }

    impl CodePoint {
        /// Unsafely creates a new `CodePoint` without checking the value.
        ///
        /// Only use when `value` is known to be less than or equal to 0x10FFFF.
        #[inline]
        pub unsafe fn from_u32_unchecked(value: u32) -> CodePoint {
            CodePoint { value }
        }

        /// Creates a new `CodePoint` if the value is a valid code point.
        ///
        /// Returns `None` if `value` is above 0x10FFFF.
        #[inline]
        pub fn from_u32(value: u32) -> Option<CodePoint> {
            match value {
                0..=0x10FFFF => Some(CodePoint { value }),
                _ => None,
            }
        }

        /// Creates a new `CodePoint` from a `char`.
        ///
        /// Since all Unicode scalar values are code points, this always succeeds.
        #[inline]
        pub fn from_char(value: char) -> CodePoint {
            CodePoint { value: value as u32 }
        }

        /// Returns the numeric value of the code point.
        #[inline]
        pub fn to_u32(&self) -> u32 {
            self.value
        }

        /// Optionally returns a Unicode scalar value for the code point.
        ///
        /// Returns `None` if the code point is a surrogate (from U+D800 to U+DFFF).
        #[inline]
        pub fn to_char(&self) -> Option<char> {
            match self.value {
                0xD800..=0xDFFF => None,
                _ => Some(unsafe { char::from_u32_unchecked(self.value) }),
            }
        }

        /// Returns a Unicode scalar value for the code point.
        ///
        /// Returns `'\u{FFFD}'` (the replacement character “�”)
        /// if the code point is a surrogate (from U+D800 to U+DFFF).
        #[inline]
        pub fn to_char_lossy(&self) -> char {
            self.to_char().unwrap_or('\u{FFFD}')
        }
    }

    /// An owned, growable string of well-formed WTF-8 data.
    ///
    /// Similar to `String`, but can additionally contain surrogate code points
    /// if they’re not in a surrogate pair.
    #[derive(Eq, PartialEq, Ord, PartialOrd, Clone)]
    pub struct Wtf8Buf {
        bytes: Vec<u8>,
    }

    impl ops::Deref for Wtf8Buf {
        type Target = Wtf8;

        fn deref(&self) -> &Wtf8 {
            self.as_slice()
        }
    }

    impl ops::DerefMut for Wtf8Buf {
        fn deref_mut(&mut self) -> &mut Wtf8 {
            self.as_mut_slice()
        }
    }

    /// Format the string with double quotes,
    /// and surrogates as `\u` followed by four hexadecimal digits.
    /// Example: `"a\u{D800}"` for a string with code points [U+0061, U+D800]
    impl fmt::Debug for Wtf8Buf {
        #[inline]
        fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Debug::fmt(&**self, formatter)
        }
    }

    impl Wtf8Buf {
        /// Creates a new, empty WTF-8 string.
        #[inline]
        pub fn new() -> Wtf8Buf {
            Wtf8Buf { bytes: Vec::new() }
        }

        /// Creates a new, empty WTF-8 string with pre-allocated capacity for `capacity` bytes.
        #[inline]
        pub fn with_capacity(capacity: usize) -> Wtf8Buf {
            Wtf8Buf { bytes: Vec::with_capacity(capacity) }
        }

        /// Creates a WTF-8 string from a UTF-8 `String`.
        ///
        /// This takes ownership of the `String` and does not copy.
        ///
        /// Since WTF-8 is a superset of UTF-8, this always succeeds.
        #[inline]
        pub fn from_string(string: String) -> Wtf8Buf {
            Wtf8Buf { bytes: string.into_bytes() }
        }

        /// Creates a WTF-8 string from a UTF-8 `&str` slice.
        ///
        /// This copies the content of the slice.
        ///
        /// Since WTF-8 is a superset of UTF-8, this always succeeds.
        #[inline]
        pub fn from_str(str: &str) -> Wtf8Buf {
            Wtf8Buf { bytes: <[_]>::to_vec(str.as_bytes()) }
        }

        pub fn clear(&mut self) {
            self.bytes.clear()
        }

        /// Creates a WTF-8 string from a potentially ill-formed UTF-16 slice of 16-bit code units.
        ///
        /// This is lossless: calling `.encode_wide()` on the resulting string
        /// will always return the original code units.
        pub fn from_wide(v: &[u16]) -> Wtf8Buf {
            let mut string = Wtf8Buf::with_capacity(v.len());
            for item in char::decode_utf16(v.iter().cloned()) {
                match item {
                    Ok(ch) => string.push_char(ch),
                    Err(surrogate) => {
                        let surrogate = surrogate.unpaired_surrogate();
                        // Surrogates are known to be in the code point range.
                        let code_point = unsafe { CodePoint::from_u32_unchecked(surrogate as u32) };
                        // Skip the WTF-8 concatenation check,
                        // surrogate pairs are already decoded by decode_utf16
                        string.push_code_point_unchecked(code_point)
                    }
                }
            }
            string
        }

        /// Copied from String::push
        /// This does **not** include the WTF-8 concatenation check.
        fn push_code_point_unchecked(&mut self, code_point: CodePoint) {
            let mut bytes = [0; 4];
            let bytes = char::encode_utf8_raw(code_point.value, &mut bytes);
            self.bytes.extend_from_slice(bytes)
        }

        #[inline]
        pub fn as_slice(&self) -> &Wtf8 {
            unsafe { Wtf8::from_bytes_unchecked(&self.bytes) }
        }

        #[inline]
        pub fn as_mut_slice(&mut self) -> &mut Wtf8 {
            unsafe { Wtf8::from_mut_bytes_unchecked(&mut self.bytes) }
        }

        /// Reserves capacity for at least `additional` more bytes to be inserted
        /// in the given `Wtf8Buf`.
        /// The collection may reserve more space to avoid frequent reallocations.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        #[inline]
        pub fn reserve(&mut self, additional: usize) {
            self.bytes.reserve(additional)
        }

        #[inline]
        pub fn reserve_exact(&mut self, additional: usize) {
            self.bytes.reserve_exact(additional)
        }

        #[inline]
        pub fn shrink_to_fit(&mut self) {
            self.bytes.shrink_to_fit()
        }

        #[inline]
        pub fn shrink_to(&mut self, min_capacity: usize) {
            self.bytes.shrink_to(min_capacity)
        }

        /// Returns the number of bytes that this string buffer can hold without reallocating.
        #[inline]
        pub fn capacity(&self) -> usize {
            self.bytes.capacity()
        }

        /// Append a UTF-8 slice at the end of the string.
        #[inline]
        pub fn push_str(&mut self, other: &str) {
            self.bytes.extend_from_slice(other.as_bytes())
        }

        /// Append a WTF-8 slice at the end of the string.
        ///
        /// This replaces newly paired surrogates at the boundary
        /// with a supplementary code point,
        /// like concatenating ill-formed UTF-16 strings effectively would.
        #[inline]
        pub fn push_wtf8(&mut self, other: &Wtf8) {
            match ((&*self).final_lead_surrogate(), other.initial_trail_surrogate()) {
                // Replace newly paired surrogates by a supplementary code point.
                (Some(lead), Some(trail)) => {
                    let len_without_lead_surrogate = self.len() - 3;
                    self.bytes.truncate(len_without_lead_surrogate);
                    let other_without_trail_surrogate = &other.bytes[3..];
                    // 4 bytes for the supplementary code point
                    self.bytes.reserve(4 + other_without_trail_surrogate.len());
                    self.push_char(decode_surrogate_pair(lead, trail));
                    self.bytes.extend_from_slice(other_without_trail_surrogate);
                }
                _ => self.bytes.extend_from_slice(&other.bytes),
            }
        }

        /// Append a Unicode scalar value at the end of the string.
        #[inline]
        pub fn push_char(&mut self, c: char) {
            self.push_code_point_unchecked(CodePoint::from_char(c))
        }

        /// Append a code point at the end of the string.
        ///
        /// This replaces newly paired surrogates at the boundary
        /// with a supplementary code point,
        /// like concatenating ill-formed UTF-16 strings effectively would.
        #[inline]
        pub fn push(&mut self, code_point: CodePoint) {
            if let trail @ 0xDC00..=0xDFFF = code_point.to_u32() {
                if let Some(lead) = (&*self).final_lead_surrogate() {
                    let len_without_lead_surrogate = self.len() - 3;
                    self.bytes.truncate(len_without_lead_surrogate);
                    self.push_char(decode_surrogate_pair(lead, trail as u16));
                    return;
                }
            }

            // No newly paired surrogates at the boundary.
            self.push_code_point_unchecked(code_point)
        }

        /// Shortens a string to the specified length.
        ///
        /// # Panics
        ///
        /// Panics if `new_len` > current length,
        /// or if `new_len` is not a code point boundary.
        #[inline]
        pub fn truncate(&mut self, new_len: usize) {
            assert!(is_code_point_boundary(self, new_len));
            self.bytes.truncate(new_len)
        }

        /// Consumes the WTF-8 string and tries to convert it to UTF-8.
        ///
        /// This does not copy the data.
        ///
        /// If the contents are not well-formed UTF-8
        /// (that is, if the string contains surrogates),
        /// the original WTF-8 string is returned instead.
        pub fn into_string(self) -> Result<String, Wtf8Buf> {
            match self.next_surrogate(0) {
                None => Ok(unsafe { String::from_utf8_unchecked(self.bytes) }),
                Some(_) => Err(self),
            }
        }

        /// Consumes the WTF-8 string and converts it lossily to UTF-8.
        ///
        /// This does not copy the data (but may overwrite parts of it in place).
        ///
        /// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”)
        pub fn into_string_lossy(mut self) -> String {
            let mut pos = 0;
            loop {
                match self.next_surrogate(pos) {
                    Some((surrogate_pos, _)) => {
                        pos = surrogate_pos + 3;
                        self.bytes[surrogate_pos..pos]
                            .copy_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
                    }
                    None => return unsafe { String::from_utf8_unchecked(self.bytes) },
                }
            }
        }

        /// Converts this `Wtf8Buf` into a boxed `Wtf8`.
        #[inline]
        pub fn into_box(self) -> Box<Wtf8> {
            unsafe { mem::transmute(self.bytes.into_boxed_slice()) }
        }

        /// Converts a `Box<Wtf8>` into a `Wtf8Buf`.
        pub fn from_box(boxed: Box<Wtf8>) -> Wtf8Buf {
            let bytes: Box<[u8]> = unsafe { mem::transmute(boxed) };
            Wtf8Buf { bytes: bytes.into_vec() }
        }
    }

    /// Creates a new WTF-8 string from an iterator of code points.
    ///
    /// This replaces surrogate code point pairs with supplementary code points,
    /// like concatenating ill-formed UTF-16 strings effectively would.
    impl FromIterator<CodePoint> for Wtf8Buf {
        fn from_iter<T: IntoIterator<Item = CodePoint>>(iter: T) -> Wtf8Buf {
            let mut string = Wtf8Buf::new();
            string.extend(iter);
            string
        }
    }

    /// Append code points from an iterator to the string.
    ///
    /// This replaces surrogate code point pairs with supplementary code points,
    /// like concatenating ill-formed UTF-16 strings effectively would.
    impl Extend<CodePoint> for Wtf8Buf {
        fn extend<T: IntoIterator<Item = CodePoint>>(&mut self, iter: T) {
            let iterator = iter.into_iter();
            let (low, _high) = iterator.size_hint();
            // Lower bound of one byte per code point (ASCII only)
            self.bytes.reserve(low);
            iterator.for_each(move |code_point| self.push(code_point));
        }

        #[inline]
        fn extend_one(&mut self, code_point: CodePoint) {
            self.push(code_point);
        }

        #[inline]
        fn extend_reserve(&mut self, additional: usize) {
            // Lower bound of one byte per code point (ASCII only)
            self.bytes.reserve(additional);
        }
    }

    /// A borrowed slice of well-formed WTF-8 data.
    ///
    /// Similar to `&str`, but can additionally contain surrogate code points
    /// if they’re not in a surrogate pair.
    #[derive(Eq, Ord, PartialEq, PartialOrd)]
    pub struct Wtf8 {
        bytes: [u8],
    }

    impl AsInner<[u8]> for Wtf8 {
        fn as_inner(&self) -> &[u8] {
            &self.bytes
        }
    }

    /// Format the slice with double quotes,
    /// and surrogates as `\u` followed by four hexadecimal digits.
    /// Example: `"a\u{D800}"` for a slice with code points [U+0061, U+D800]
    impl fmt::Debug for Wtf8 {
        fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
            fn write_str_escaped(f: &mut fmt::Formatter<'_>, s: &str) -> fmt::Result {
                use crate::fmt::Write;
                for c in s.chars().flat_map(|c| c.escape_debug()) {
                    f.write_char(c)?
                }
                Ok(())
            }

            formatter.write_str("\"")?;
            let mut pos = 0;
            while let Some((surrogate_pos, surrogate)) = self.next_surrogate(pos) {
                write_str_escaped(formatter, unsafe {
                    str::from_utf8_unchecked(&self.bytes[pos..surrogate_pos])
                })?;
                write!(formatter, "\\u{{{:x}}}", surrogate)?;
                pos = surrogate_pos + 3;
            }
            write_str_escaped(formatter, unsafe { str::from_utf8_unchecked(&self.bytes[pos..]) })?;
            formatter.write_str("\"")
        }
    }

    impl fmt::Display for Wtf8 {
        fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
            let wtf8_bytes = &self.bytes;
            let mut pos = 0;
            loop {
                match self.next_surrogate(pos) {
                    Some((surrogate_pos, _)) => {
                        formatter.write_str(unsafe {
                            str::from_utf8_unchecked(&wtf8_bytes[pos..surrogate_pos])
                        })?;
                        formatter.write_str(UTF8_REPLACEMENT_CHARACTER)?;
                        pos = surrogate_pos + 3;
                    }
                    None => {
                        let s = unsafe { str::from_utf8_unchecked(&wtf8_bytes[pos..]) };
                        if pos == 0 { return s.fmt(formatter) } else { return formatter.write_str(s) }
                    }
                }
            }
        }
    }

    impl Wtf8 {
        /// Creates a WTF-8 slice from a UTF-8 `&str` slice.
        ///
        /// Since WTF-8 is a superset of UTF-8, this always succeeds.
        #[inline]
        pub fn from_str(value: &str) -> &Wtf8 {
            unsafe { Wtf8::from_bytes_unchecked(value.as_bytes()) }
        }

        /// Creates a WTF-8 slice from a WTF-8 byte slice.
        ///
        /// Since the byte slice is not checked for valid WTF-8, this functions is
        /// marked unsafe.
        #[inline]
        unsafe fn from_bytes_unchecked(value: &[u8]) -> &Wtf8 {
            mem::transmute(value)
        }

        /// Creates a mutable WTF-8 slice from a mutable WTF-8 byte slice.
        ///
        /// Since the byte slice is not checked for valid WTF-8, this functions is
        /// marked unsafe.
        #[inline]
        unsafe fn from_mut_bytes_unchecked(value: &mut [u8]) -> &mut Wtf8 {
            mem::transmute(value)
        }

        /// Returns the length, in WTF-8 bytes.
        #[inline]
        pub fn len(&self) -> usize {
            self.bytes.len()
        }

        #[inline]
        pub fn is_empty(&self) -> bool {
            self.bytes.is_empty()
        }

        /// Returns the code point at `position` if it is in the ASCII range,
        /// or `b'\xFF' otherwise.
        ///
        /// # Panics
        ///
        /// Panics if `position` is beyond the end of the string.
        #[inline]
        pub fn ascii_byte_at(&self, position: usize) -> u8 {
            match self.bytes[position] {
                ascii_byte @ 0x00..=0x7F => ascii_byte,
                _ => 0xFF,
            }
        }

        /// Returns an iterator for the string’s code points.
        #[inline]
        pub fn code_points(&self) -> Wtf8CodePoints<'_> {
            Wtf8CodePoints { bytes: self.bytes.iter() }
        }

        /// Tries to convert the string to UTF-8 and return a `&str` slice.
        ///
        /// Returns `None` if the string contains surrogates.
        ///
        /// This does not copy the data.
        #[inline]
        pub fn as_str(&self) -> Option<&str> {
            // Well-formed WTF-8 is also well-formed UTF-8
            // if and only if it contains no surrogate.
            match self.next_surrogate(0) {
                None => Some(unsafe { str::from_utf8_unchecked(&self.bytes) }),
                Some(_) => None,
            }
        }

        /// Lossily converts the string to UTF-8.
        /// Returns a UTF-8 `&str` slice if the contents are well-formed in UTF-8.
        ///
        /// Surrogates are replaced with `"\u{FFFD}"` (the replacement character “�”).
        ///
        /// This only copies the data if necessary (if it contains any surrogate).
        pub fn to_string_lossy(&self) -> Cow<'_, str> {
            let surrogate_pos = match self.next_surrogate(0) {
                None => return Cow::Borrowed(unsafe { str::from_utf8_unchecked(&self.bytes) }),
                Some((pos, _)) => pos,
            };
            let wtf8_bytes = &self.bytes;
            let mut utf8_bytes = Vec::with_capacity(self.len());
            utf8_bytes.extend_from_slice(&wtf8_bytes[..surrogate_pos]);
            utf8_bytes.extend_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
            let mut pos = surrogate_pos + 3;
            loop {
                match self.next_surrogate(pos) {
                    Some((surrogate_pos, _)) => {
                        utf8_bytes.extend_from_slice(&wtf8_bytes[pos..surrogate_pos]);
                        utf8_bytes.extend_from_slice(UTF8_REPLACEMENT_CHARACTER.as_bytes());
                        pos = surrogate_pos + 3;
                    }
                    None => {
                        utf8_bytes.extend_from_slice(&wtf8_bytes[pos..]);
                        return Cow::Owned(unsafe { String::from_utf8_unchecked(utf8_bytes) });
                    }
                }
            }
        }

        /// Converts the WTF-8 string to potentially ill-formed UTF-16
        /// and return an iterator of 16-bit code units.
        ///
        /// This is lossless:
        /// calling `Wtf8Buf::from_ill_formed_utf16` on the resulting code units
        /// would always return the original WTF-8 string.
        #[inline]
        pub fn encode_wide(&self) -> EncodeWide<'_> {
            EncodeWide { code_points: self.code_points(), extra: 0 }
        }

        #[inline]
        fn next_surrogate(&self, mut pos: usize) -> Option<(usize, u16)> {
            let mut iter = self.bytes[pos..].iter();
            loop {
                let b = *iter.next()?;
                if b < 0x80 {
                    pos += 1;
                } else if b < 0xE0 {
                    iter.next();
                    pos += 2;
                } else if b == 0xED {
                    match (iter.next(), iter.next()) {
                        (Some(&b2), Some(&b3)) if b2 >= 0xA0 => {
                            return Some((pos, decode_surrogate(b2, b3)));
                        }
                        _ => pos += 3,
                    }
                } else if b < 0xF0 {
                    iter.next();
                    iter.next();
                    pos += 3;
                } else {
                    iter.next();
                    iter.next();
                    iter.next();
                    pos += 4;
                }
            }
        }

        #[inline]
        fn final_lead_surrogate(&self) -> Option<u16> {
            match self.bytes {
                [.., 0xED, b2 @ 0xA0..=0xAF, b3] => Some(decode_surrogate(b2, b3)),
                _ => None,
            }
        }

        #[inline]
        fn initial_trail_surrogate(&self) -> Option<u16> {
            match self.bytes {
                [0xED, b2 @ 0xB0..=0xBF, b3, ..] => Some(decode_surrogate(b2, b3)),
                _ => None,
            }
        }

        pub fn clone_into(&self, buf: &mut Wtf8Buf) {
            self.bytes.clone_into(&mut buf.bytes)
        }

        /// Boxes this `Wtf8`.
        #[inline]
        pub fn into_box(&self) -> Box<Wtf8> {
            let boxed: Box<[u8]> = self.bytes.into();
            unsafe { mem::transmute(boxed) }
        }

        /// Creates a boxed, empty `Wtf8`.
        pub fn empty_box() -> Box<Wtf8> {
            let boxed: Box<[u8]> = Default::default();
            unsafe { mem::transmute(boxed) }
        }

        #[inline]
        pub fn into_arc(&self) -> Arc<Wtf8> {
            let arc: Arc<[u8]> = Arc::from(&self.bytes);
            unsafe { Arc::from_raw(Arc::into_raw(arc) as *const Wtf8) }
        }

        #[inline]
        pub fn into_rc(&self) -> Rc<Wtf8> {
            let rc: Rc<[u8]> = Rc::from(&self.bytes);
            unsafe { Rc::from_raw(Rc::into_raw(rc) as *const Wtf8) }
        }

        #[inline]
        pub fn make_ascii_lowercase(&mut self) {
            self.bytes.make_ascii_lowercase()
        }

        #[inline]
        pub fn make_ascii_uppercase(&mut self) {
            self.bytes.make_ascii_uppercase()
        }

        #[inline]
        pub fn to_ascii_lowercase(&self) -> Wtf8Buf {
            Wtf8Buf { bytes: self.bytes.to_ascii_lowercase() }
        }

        #[inline]
        pub fn to_ascii_uppercase(&self) -> Wtf8Buf {
            Wtf8Buf { bytes: self.bytes.to_ascii_uppercase() }
        }

        #[inline]
        pub fn is_ascii(&self) -> bool {
            self.bytes.is_ascii()
        }

        #[inline]
        pub fn eq_ignore_ascii_case(&self, other: &Self) -> bool {
            self.bytes.eq_ignore_ascii_case(&other.bytes)
        }
    }

    /// Returns a slice of the given string for the byte range [`begin`..`end`).
    ///
    /// # Panics
    ///
    /// Panics when `begin` and `end` do not point to code point boundaries,
    /// or point beyond the end of the string.
    impl ops::Index<ops::Range<usize>> for Wtf8 {
        type Output = Wtf8;

        #[inline]
        fn index(&self, range: ops::Range<usize>) -> &Wtf8 {
            // is_code_point_boundary checks that the index is in [0, .len()]
            if range.start <= range.end
                && is_code_point_boundary(self, range.start)
                && is_code_point_boundary(self, range.end)
            {
                unsafe { slice_unchecked(self, range.start, range.end) }
            } else {
                slice_error_fail(self, range.start, range.end)
            }
        }
    }

    /// Returns a slice of the given string from byte `begin` to its end.
    ///
    /// # Panics
    ///
    /// Panics when `begin` is not at a code point boundary,
    /// or is beyond the end of the string.
    impl ops::Index<ops::RangeFrom<usize>> for Wtf8 {
        type Output = Wtf8;

        #[inline]
        fn index(&self, range: ops::RangeFrom<usize>) -> &Wtf8 {
            // is_code_point_boundary checks that the index is in [0, .len()]
            if is_code_point_boundary(self, range.start) {
                unsafe { slice_unchecked(self, range.start, self.len()) }
            } else {
                slice_error_fail(self, range.start, self.len())
            }
        }
    }

    /// Returns a slice of the given string from its beginning to byte `end`.
    ///
    /// # Panics
    ///
    /// Panics when `end` is not at a code point boundary,
    /// or is beyond the end of the string.
    impl ops::Index<ops::RangeTo<usize>> for Wtf8 {
        type Output = Wtf8;

        #[inline]
        fn index(&self, range: ops::RangeTo<usize>) -> &Wtf8 {
            // is_code_point_boundary checks that the index is in [0, .len()]
            if is_code_point_boundary(self, range.end) {
                unsafe { slice_unchecked(self, 0, range.end) }
            } else {
                slice_error_fail(self, 0, range.end)
            }
        }
    }

    impl ops::Index<ops::RangeFull> for Wtf8 {
        type Output = Wtf8;

        #[inline]
        fn index(&self, _range: ops::RangeFull) -> &Wtf8 {
            self
        }
    }

    #[inline]
    fn decode_surrogate(second_byte: u8, third_byte: u8) -> u16 {
        // The first byte is assumed to be 0xED
        0xD800 | (second_byte as u16 & 0x3F) << 6 | third_byte as u16 & 0x3F
    }

    #[inline]
    fn decode_surrogate_pair(lead: u16, trail: u16) -> char {
        let code_point = 0x10000 + ((((lead - 0xD800) as u32) << 10) | (trail - 0xDC00) as u32);
        unsafe { char::from_u32_unchecked(code_point) }
    }

    /// Copied from core::str::StrPrelude::is_char_boundary
    #[inline]
    pub fn is_code_point_boundary(slice: &Wtf8, index: usize) -> bool {
        if index == slice.len() {
            return true;
        }
        match slice.bytes.get(index) {
            None => false,
            Some(&b) => b < 128 || b >= 192,
        }
    }

    /// Copied from core::str::raw::slice_unchecked
    #[inline]
    pub unsafe fn slice_unchecked(s: &Wtf8, begin: usize, end: usize) -> &Wtf8 {
        // memory layout of an &[u8] and &Wtf8 are the same
        Wtf8::from_bytes_unchecked(slice::from_raw_parts(s.bytes.as_ptr().add(begin), end - begin))
    }

    /// Copied from core::str::raw::slice_error_fail
    #[inline(never)]
    pub fn slice_error_fail(s: &Wtf8, begin: usize, end: usize) -> ! {
        assert!(begin <= end);
        panic!("index {} and/or {} in `{:?}` do not lie on character boundary", begin, end, s);
    }

    /// Iterator for the code points of a WTF-8 string.
    ///
    /// Created with the method `.code_points()`.
    #[derive(Clone)]
    pub struct Wtf8CodePoints<'a> {
        bytes: slice::Iter<'a, u8>,
    }

    impl<'a> Iterator for Wtf8CodePoints<'a> {
        type Item = CodePoint;

        #[inline]
        fn next(&mut self) -> Option<CodePoint> {
            next_code_point(&mut self.bytes).map(|c| CodePoint { value: c })
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let len = self.bytes.len();
            (len.saturating_add(3) / 4, Some(len))
        }
    }

    /// Generates a wide character sequence for potentially ill-formed UTF-16.
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct EncodeWide<'a> {
        code_points: Wtf8CodePoints<'a>,
        extra: u16,
    }

    // Copied from libunicode/u_str.rs
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a> Iterator for EncodeWide<'a> {
        type Item = u16;

        #[inline]
        fn next(&mut self) -> Option<u16> {
            if self.extra != 0 {
                let tmp = self.extra;
                self.extra = 0;
                return Some(tmp);
            }

            let mut buf = [0; 2];
            self.code_points.next().map(|code_point| {
                let n = char::encode_utf16_raw(code_point.value, &mut buf).len();
                if n == 2 {
                    self.extra = buf[1];
                }
                buf[0]
            })
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (low, high) = self.code_points.size_hint();
            // every code point gets either one u16 or two u16,
            // so this iterator is between 1 or 2 times as
            // long as the underlying iterator.
            (low, high.and_then(|n| n.checked_mul(2)))
        }
    }

    impl Hash for CodePoint {
        #[inline]
        fn hash<H: Hasher>(&self, state: &mut H) {
            self.value.hash(state)
        }
    }

    impl Hash for Wtf8Buf {
        #[inline]
        fn hash<H: Hasher>(&self, state: &mut H) {
            state.write(&self.bytes);
            0xfeu8.hash(state)
        }
    }

    impl Hash for Wtf8 {
        #[inline]
        fn hash<H: Hasher>(&self, state: &mut H) {
            state.write(&self.bytes);
            0xfeu8.hash(state)
        }
    }