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    use crate::char;
    use crate::str as core_str;
    use crate::fmt::{self, Write};
    use crate::mem;

    /// Lossy UTF-8 string.
    #[unstable(feature = "str_internals", issue = "0")]
    pub struct Utf8Lossy {
        bytes: [u8]
    }

    impl Utf8Lossy {
        pub fn from_str(s: &str) -> &Utf8Lossy {
            Utf8Lossy::from_bytes(s.as_bytes())
        }

        pub fn from_bytes(bytes: &[u8]) -> &Utf8Lossy {
            unsafe { mem::transmute(bytes) }
        }

        pub fn chunks(&self) -> Utf8LossyChunksIter<'_> {
            Utf8LossyChunksIter { source: &self.bytes }
        }
    }

    /// Iterator over lossy UTF-8 string
    #[unstable(feature = "str_internals", issue = "0")]
    #[allow(missing_debug_implementations)]
    pub struct Utf8LossyChunksIter<'a> {
        source: &'a [u8],
    }

    #[unstable(feature = "str_internals", issue = "0")]
    #[derive(PartialEq, Eq, Debug)]
    pub struct Utf8LossyChunk<'a> {
        /// Sequence of valid chars.
        /// Can be empty between broken UTF-8 chars.
        pub valid: &'a str,
        /// Single broken char, empty if none.
        /// Empty iff iterator item is last.
        pub broken: &'a [u8],
    }

    impl<'a> Iterator for Utf8LossyChunksIter<'a> {
        type Item = Utf8LossyChunk<'a>;

        fn next(&mut self) -> Option<Utf8LossyChunk<'a>> {
            if self.source.is_empty() {
                return None;
            }

            const TAG_CONT_U8: u8 = 128;
            fn safe_get(xs: &[u8], i: usize) -> u8 {
                *xs.get(i).unwrap_or(&0)
            }

            let mut i = 0;
            while i < self.source.len() {
                let i_ = i;

                let byte = unsafe { *self.source.get_unchecked(i) };
                i += 1;

                if byte < 128 {

                } else {
                    let w = core_str::utf8_char_width(byte);

                    macro_rules! error { () => ({
                        unsafe {
                            let r = Utf8LossyChunk {
                                valid: core_str::from_utf8_unchecked(&self.source[0..i_]),
                                broken: &self.source[i_..i],
                            };
                            self.source = &self.source[i..];
                            return Some(r);
                        }
                    })}

                    match w {
                        2 => {
                            if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
                                error!();
                            }
                            i += 1;
                        }
                        3 => {
                            match (byte, safe_get(self.source, i)) {
                                (0xE0, 0xA0 ..= 0xBF) => (),
                                (0xE1 ..= 0xEC, 0x80 ..= 0xBF) => (),
                                (0xED, 0x80 ..= 0x9F) => (),
                                (0xEE ..= 0xEF, 0x80 ..= 0xBF) => (),
                                _ => {
                                    error!();
                                }
                            }
                            i += 1;
                            if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
                                error!();
                            }
                            i += 1;
                        }
                        4 => {
                            match (byte, safe_get(self.source, i)) {
                                (0xF0, 0x90 ..= 0xBF) => (),
                                (0xF1 ..= 0xF3, 0x80 ..= 0xBF) => (),
                                (0xF4, 0x80 ..= 0x8F) => (),
                                _ => {
                                    error!();
                                }
                            }
                            i += 1;
                            if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
                                error!();
                            }
                            i += 1;
                            if safe_get(self.source, i) & 192 != TAG_CONT_U8 {
                                error!();
                            }
                            i += 1;
                        }
                        _ => {
                            error!();
                        }
                    }
                }
            }

            let r = Utf8LossyChunk {
                valid: unsafe { core_str::from_utf8_unchecked(self.source) },
                broken: &[],
            };
            self.source = &[];
            Some(r)
        }
    }

    impl fmt::Display for Utf8Lossy {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            // If we're the empty string then our iterator won't actually yield
            // anything, so perform the formatting manually
            if self.bytes.is_empty() {
                return "".fmt(f)
            }

            for Utf8LossyChunk { valid, broken } in self.chunks() {
                // If we successfully decoded the whole chunk as a valid string then
                // we can return a direct formatting of the string which will also
                // respect various formatting flags if possible.
                if valid.len() == self.bytes.len() {
                    assert!(broken.is_empty());
                    return valid.fmt(f)
                }

                f.write_str(valid)?;
                if !broken.is_empty() {
                    f.write_char(char::REPLACEMENT_CHARACTER)?;
                }
            }
            Ok(())
        }
    }

    impl fmt::Debug for Utf8Lossy {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_char('"')?;

            for Utf8LossyChunk { valid, broken } in self.chunks() {

                // Valid part.
                // Here we partially parse UTF-8 again which is suboptimal.
                {
                    let mut from = 0;
                    for (i, c) in valid.char_indices() {
                        let esc = c.escape_debug();
                        // If char needs escaping, flush backlog so far and write, else skip
                        if esc.len() != 1 {
                            f.write_str(&valid[from..i])?;
                            for c in esc {
                                f.write_char(c)?;
                            }
                            from = i + c.len_utf8();
                        }
                    }
                    f.write_str(&valid[from..])?;
                }

                // Broken parts of string as hex escape.
                for &b in broken {
                    write!(f, "\\x{:02x}", b)?;
                }
            }

            f.write_char('"')
        }
    }

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