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    // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Character manipulation.
    //!
    //! For more details, see ::std_unicode::char (a.k.a. std::char)

    #![allow(non_snake_case)]
    #![stable(feature = "core_char", since = "1.2.0")]

    use char_private::is_printable;
    use convert::TryFrom;
    use fmt::{self, Write};
    use slice;
    use str::{from_utf8_unchecked_mut, FromStr};
    use iter::FusedIterator;
    use mem::transmute;

    // UTF-8 ranges and tags for encoding characters
    const TAG_CONT: u8    = 0b1000_0000;
    const TAG_TWO_B: u8   = 0b1100_0000;
    const TAG_THREE_B: u8 = 0b1110_0000;
    const TAG_FOUR_B: u8  = 0b1111_0000;
    const MAX_ONE_B: u32   =     0x80;
    const MAX_TWO_B: u32   =    0x800;
    const MAX_THREE_B: u32 =  0x10000;

    /*
        Lu  Uppercase_Letter        an uppercase letter
        Ll  Lowercase_Letter        a lowercase letter
        Lt  Titlecase_Letter        a digraphic character, with first part uppercase
        Lm  Modifier_Letter         a modifier letter
        Lo  Other_Letter            other letters, including syllables and ideographs
        Mn  Nonspacing_Mark         a nonspacing combining mark (zero advance width)
        Mc  Spacing_Mark            a spacing combining mark (positive advance width)
        Me  Enclosing_Mark          an enclosing combining mark
        Nd  Decimal_Number          a decimal digit
        Nl  Letter_Number           a letterlike numeric character
        No  Other_Number            a numeric character of other type
        Pc  Connector_Punctuation   a connecting punctuation mark, like a tie
        Pd  Dash_Punctuation        a dash or hyphen punctuation mark
        Ps  Open_Punctuation        an opening punctuation mark (of a pair)
        Pe  Close_Punctuation       a closing punctuation mark (of a pair)
        Pi  Initial_Punctuation     an initial quotation mark
        Pf  Final_Punctuation       a final quotation mark
        Po  Other_Punctuation       a punctuation mark of other type
        Sm  Math_Symbol             a symbol of primarily mathematical use
        Sc  Currency_Symbol         a currency sign
        Sk  Modifier_Symbol         a non-letterlike modifier symbol
        So  Other_Symbol            a symbol of other type
        Zs  Space_Separator         a space character (of various non-zero widths)
        Zl  Line_Separator          U+2028 LINE SEPARATOR only
        Zp  Paragraph_Separator     U+2029 PARAGRAPH SEPARATOR only
        Cc  Control                 a C0 or C1 control code
        Cf  Format                  a format control character
        Cs  Surrogate               a surrogate code point
        Co  Private_Use             a private-use character
        Cn  Unassigned              a reserved unassigned code point or a noncharacter
    */

    /// The highest valid code point a `char` can have.
    ///
    /// A [`char`] is a [Unicode Scalar Value], which means that it is a [Code
    /// Point], but only ones within a certain range. `MAX` is the highest valid
    /// code point that's a valid [Unicode Scalar Value].
    ///
    /// [`char`]: ../../std/primitive.char.html
    /// [Unicode Scalar Value]: http://www.unicode.org/glossary/#unicode_scalar_value
    /// [Code Point]: http://www.unicode.org/glossary/#code_point
    #[stable(feature = "rust1", since = "1.0.0")]
    pub const MAX: char = '\u{10ffff}';

    /// Converts a `u32` to a `char`.
    ///
    /// Note that all [`char`]s are valid [`u32`]s, and can be casted to one with
    /// [`as`]:
    ///
    /// ```
    /// let c = '💯';
    /// let i = c as u32;
    ///
    /// assert_eq!(128175, i);
    /// ```
    ///
    /// However, the reverse is not true: not all valid [`u32`]s are valid
    /// [`char`]s. `from_u32()` will return `None` if the input is not a valid value
    /// for a [`char`].
    ///
    /// [`char`]: ../../std/primitive.char.html
    /// [`u32`]: ../../std/primitive.u32.html
    /// [`as`]: ../../book/first-edition/casting-between-types.html#as
    ///
    /// For an unsafe version of this function which ignores these checks, see
    /// [`from_u32_unchecked`].
    ///
    /// [`from_u32_unchecked`]: fn.from_u32_unchecked.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::char;
    ///
    /// let c = char::from_u32(0x2764);
    ///
    /// assert_eq!(Some('❤'), c);
    /// ```
    ///
    /// Returning `None` when the input is not a valid [`char`]:
    ///
    /// ```
    /// use std::char;
    ///
    /// let c = char::from_u32(0x110000);
    ///
    /// assert_eq!(None, c);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn from_u32(i: u32) -> Option<char> {
        char::try_from(i).ok()
    }

    /// Converts a `u32` to a `char`, ignoring validity.
    ///
    /// Note that all [`char`]s are valid [`u32`]s, and can be casted to one with
    /// [`as`]:
    ///
    /// ```
    /// let c = '💯';
    /// let i = c as u32;
    ///
    /// assert_eq!(128175, i);
    /// ```
    ///
    /// However, the reverse is not true: not all valid [`u32`]s are valid
    /// [`char`]s. `from_u32_unchecked()` will ignore this, and blindly cast to
    /// [`char`], possibly creating an invalid one.
    ///
    /// [`char`]: ../../std/primitive.char.html
    /// [`u32`]: ../../std/primitive.u32.html
    /// [`as`]: ../../book/first-edition/casting-between-types.html#as
    ///
    /// # Safety
    ///
    /// This function is unsafe, as it may construct invalid `char` values.
    ///
    /// For a safe version of this function, see the [`from_u32`] function.
    ///
    /// [`from_u32`]: fn.from_u32.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::char;
    ///
    /// let c = unsafe { char::from_u32_unchecked(0x2764) };
    ///
    /// assert_eq!('❤', c);
    /// ```
    #[inline]
    #[stable(feature = "char_from_unchecked", since = "1.5.0")]
    pub unsafe fn from_u32_unchecked(i: u32) -> char {
        transmute(i)
    }

    #[stable(feature = "char_convert", since = "1.13.0")]
    impl From<char> for u32 {
        #[inline]
        fn from(c: char) -> Self {
            c as u32
        }
    }

    /// Maps a byte in 0x00...0xFF to a `char` whose code point has the same value, in U+0000 to U+00FF.
    ///
    /// Unicode is designed such that this effectively decodes bytes
    /// with the character encoding that IANA calls ISO-8859-1.
    /// This encoding is compatible with ASCII.
    ///
    /// Note that this is different from ISO/IEC 8859-1 a.k.a. ISO 8859-1 (with one less hyphen),
    /// which leaves some "blanks", byte values that are not assigned to any character.
    /// ISO-8859-1 (the IANA one) assigns them to the C0 and C1 control codes.
    ///
    /// Note that this is *also* different from Windows-1252 a.k.a. code page 1252,
    /// which is a superset ISO/IEC 8859-1 that assigns some (not all!) blanks
    /// to punctuation and various Latin characters.
    ///
    /// To confuse things further, [on the Web](https://encoding.spec.whatwg.org/)
    /// `ascii`, `iso-8859-1`, and `windows-1252` are all aliases
    /// for a superset of Windows-1252 that fills the remaining blanks with corresponding
    /// C0 and C1 control codes.
    #[stable(feature = "char_convert", since = "1.13.0")]
    impl From<u8> for char {
        #[inline]
        fn from(i: u8) -> Self {
            i as char
        }
    }

    /// An error which can be returned when parsing a char.
    #[stable(feature = "char_from_str", since = "1.20.0")]
    #[derive(Clone, Debug)]
    pub struct ParseCharError {
        kind: CharErrorKind,
    }

    impl ParseCharError {
        #[unstable(feature = "char_error_internals",
                   reason = "this method should not be available publicly",
                   issue = "0")]
        #[doc(hidden)]
        pub fn __description(&self) -> &str {
            match self.kind {
                CharErrorKind::EmptyString => {
                    "cannot parse char from empty string"
                },
                CharErrorKind::TooManyChars => "too many characters in string"
            }
        }
    }

    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
    enum CharErrorKind {
        EmptyString,
        TooManyChars,
    }

    #[stable(feature = "char_from_str", since = "1.20.0")]
    impl fmt::Display for ParseCharError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            self.__description().fmt(f)
        }
    }

    #[stable(feature = "char_from_str", since = "1.20.0")]
    impl FromStr for char {
        type Err = ParseCharError;

        #[inline]
        fn from_str(s: &str) -> Result<Self, Self::Err> {
            let mut chars = s.chars();
            match (chars.next(), chars.next()) {
                (None, _) => {
                    Err(ParseCharError { kind: CharErrorKind::EmptyString })
                },
                (Some(c), None) => Ok(c),
                _ => {
                    Err(ParseCharError { kind: CharErrorKind::TooManyChars })
                }
            }
        }
    }

    #[unstable(feature = "try_from", issue = "33417")]
    impl TryFrom<u32> for char {
        type Error = CharTryFromError;

        #[inline]
        fn try_from(i: u32) -> Result<Self, Self::Error> {
            if (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF) {
                Err(CharTryFromError(()))
            } else {
                Ok(unsafe { from_u32_unchecked(i) })
            }
        }
    }

    /// The error type returned when a conversion from u32 to char fails.
    #[unstable(feature = "try_from", issue = "33417")]
    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
    pub struct CharTryFromError(());

    #[unstable(feature = "try_from", issue = "33417")]
    impl fmt::Display for CharTryFromError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            "converted integer out of range for `char`".fmt(f)
        }
    }

    /// Converts a digit in the given radix to a `char`.
    ///
    /// A 'radix' here is sometimes also called a 'base'. A radix of two
    /// indicates a binary number, a radix of ten, decimal, and a radix of
    /// sixteen, hexadecimal, to give some common values. Arbitrary
    /// radices are supported.
    ///
    /// `from_digit()` will return `None` if the input is not a digit in
    /// the given radix.
    ///
    /// # Panics
    ///
    /// Panics if given a radix larger than 36.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::char;
    ///
    /// let c = char::from_digit(4, 10);
    ///
    /// assert_eq!(Some('4'), c);
    ///
    /// // Decimal 11 is a single digit in base 16
    /// let c = char::from_digit(11, 16);
    ///
    /// assert_eq!(Some('b'), c);
    /// ```
    ///
    /// Returning `None` when the input is not a digit:
    ///
    /// ```
    /// use std::char;
    ///
    /// let c = char::from_digit(20, 10);
    ///
    /// assert_eq!(None, c);
    /// ```
    ///
    /// Passing a large radix, causing a panic:
    ///
    /// ```
    /// use std::thread;
    /// use std::char;
    ///
    /// let result = thread::spawn(|| {
    ///     // this panics
    ///     let c = char::from_digit(1, 37);
    /// }).join();
    ///
    /// assert!(result.is_err());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn from_digit(num: u32, radix: u32) -> Option<char> {
        if radix > 36 {
            panic!("from_digit: radix is too high (maximum 36)");
        }
        if num < radix {
            let num = num as u8;
            if num < 10 {
                Some((b'0' + num) as char)
            } else {
                Some((b'a' + num - 10) as char)
            }
        } else {
            None
        }
    }

    // NB: the stabilization and documentation for this trait is in
    // unicode/char.rs, not here
    #[allow(missing_docs)] // docs in libunicode/u_char.rs
    #[doc(hidden)]
    #[unstable(feature = "core_char_ext",
               reason = "the stable interface is `impl char` in later crate",
               issue = "32110")]
    pub trait CharExt {
        #[stable(feature = "core", since = "1.6.0")]
        fn is_digit(self, radix: u32) -> bool;
        #[stable(feature = "core", since = "1.6.0")]
        fn to_digit(self, radix: u32) -> Option<u32>;
        #[stable(feature = "core", since = "1.6.0")]
        fn escape_unicode(self) -> EscapeUnicode;
        #[stable(feature = "core", since = "1.6.0")]
        fn escape_default(self) -> EscapeDefault;
        #[stable(feature = "char_escape_debug", since = "1.20.0")]
        fn escape_debug(self) -> EscapeDebug;
        #[stable(feature = "core", since = "1.6.0")]
        fn len_utf8(self) -> usize;
        #[stable(feature = "core", since = "1.6.0")]
        fn len_utf16(self) -> usize;
        #[stable(feature = "unicode_encode_char", since = "1.15.0")]
        fn encode_utf8(self, dst: &mut [u8]) -> &mut str;
        #[stable(feature = "unicode_encode_char", since = "1.15.0")]
        fn encode_utf16(self, dst: &mut [u16]) -> &mut [u16];
    }

    #[stable(feature = "core", since = "1.6.0")]
    impl CharExt for char {
        #[inline]
        fn is_digit(self, radix: u32) -> bool {
            self.to_digit(radix).is_some()
        }

        #[inline]
        fn to_digit(self, radix: u32) -> Option<u32> {
            if radix > 36 {
                panic!("to_digit: radix is too high (maximum 36)");
            }
            let val = match self {
              '0' ... '9' => self as u32 - '0' as u32,
              'a' ... 'z' => self as u32 - 'a' as u32 + 10,
              'A' ... 'Z' => self as u32 - 'A' as u32 + 10,
              _ => return None,
            };
            if val < radix { Some(val) }
            else { None }
        }

        #[inline]
        fn escape_unicode(self) -> EscapeUnicode {
            let c = self as u32;

            // or-ing 1 ensures that for c==0 the code computes that one
            // digit should be printed and (which is the same) avoids the
            // (31 - 32) underflow
            let msb = 31 - (c | 1).leading_zeros();

            // the index of the most significant hex digit
            let ms_hex_digit = msb / 4;
            EscapeUnicode {
                c: self,
                state: EscapeUnicodeState::Backslash,
                hex_digit_idx: ms_hex_digit as usize,
            }
        }

        #[inline]
        fn escape_default(self) -> EscapeDefault {
            let init_state = match self {
                '\t' => EscapeDefaultState::Backslash('t'),
                '\r' => EscapeDefaultState::Backslash('r'),
                '\n' => EscapeDefaultState::Backslash('n'),
                '\\' | '\'' | '"' => EscapeDefaultState::Backslash(self),
                '\x20' ... '\x7e' => EscapeDefaultState::Char(self),
                _ => EscapeDefaultState::Unicode(self.escape_unicode())
            };
            EscapeDefault { state: init_state }
        }

        #[inline]
        fn escape_debug(self) -> EscapeDebug {
            let init_state = match self {
                '\t' => EscapeDefaultState::Backslash('t'),
                '\r' => EscapeDefaultState::Backslash('r'),
                '\n' => EscapeDefaultState::Backslash('n'),
                '\\' | '\'' | '"' => EscapeDefaultState::Backslash(self),
                c if is_printable(c) => EscapeDefaultState::Char(c),
                c => EscapeDefaultState::Unicode(c.escape_unicode()),
            };
            EscapeDebug(EscapeDefault { state: init_state })
        }

        #[inline]
        fn len_utf8(self) -> usize {
            let code = self as u32;
            if code < MAX_ONE_B {
                1
            } else if code < MAX_TWO_B {
                2
            } else if code < MAX_THREE_B {
                3
            } else {
                4
            }
        }

        #[inline]
        fn len_utf16(self) -> usize {
            let ch = self as u32;
            if (ch & 0xFFFF) == ch { 1 } else { 2 }
        }

        #[inline]
        fn encode_utf8(self, dst: &mut [u8]) -> &mut str {
            let code = self as u32;
            unsafe {
                let len =
                if code < MAX_ONE_B && !dst.is_empty() {
                    *dst.get_unchecked_mut(0) = code as u8;
                    1
                } else if code < MAX_TWO_B && dst.len() >= 2 {
                    *dst.get_unchecked_mut(0) = (code >> 6 & 0x1F) as u8 | TAG_TWO_B;
                    *dst.get_unchecked_mut(1) = (code & 0x3F) as u8 | TAG_CONT;
                    2
                } else if code < MAX_THREE_B && dst.len() >= 3  {
                    *dst.get_unchecked_mut(0) = (code >> 12 & 0x0F) as u8 | TAG_THREE_B;
                    *dst.get_unchecked_mut(1) = (code >>  6 & 0x3F) as u8 | TAG_CONT;
                    *dst.get_unchecked_mut(2) = (code & 0x3F) as u8 | TAG_CONT;
                    3
                } else if dst.len() >= 4 {
                    *dst.get_unchecked_mut(0) = (code >> 18 & 0x07) as u8 | TAG_FOUR_B;
                    *dst.get_unchecked_mut(1) = (code >> 12 & 0x3F) as u8 | TAG_CONT;
                    *dst.get_unchecked_mut(2) = (code >>  6 & 0x3F) as u8 | TAG_CONT;
                    *dst.get_unchecked_mut(3) = (code & 0x3F) as u8 | TAG_CONT;
                    4
                } else {
                    panic!("encode_utf8: need {} bytes to encode U+{:X}, but the buffer has {}",
                        from_u32_unchecked(code).len_utf8(),
                        code,
                        dst.len())
                };
                from_utf8_unchecked_mut(dst.get_unchecked_mut(..len))
            }
        }

        #[inline]
        fn encode_utf16(self, dst: &mut [u16]) -> &mut [u16] {
            let mut code = self as u32;
            unsafe {
                if (code & 0xFFFF) == code && !dst.is_empty() {
                    // The BMP falls through (assuming non-surrogate, as it should)
                    *dst.get_unchecked_mut(0) = code as u16;
                    slice::from_raw_parts_mut(dst.as_mut_ptr(), 1)
                } else if dst.len() >= 2 {
                    // Supplementary planes break into surrogates.
                    code -= 0x1_0000;
                    *dst.get_unchecked_mut(0) = 0xD800 | ((code >> 10) as u16);
                    *dst.get_unchecked_mut(1) = 0xDC00 | ((code as u16) & 0x3FF);
                    slice::from_raw_parts_mut(dst.as_mut_ptr(), 2)
                } else {
                    panic!("encode_utf16: need {} units to encode U+{:X}, but the buffer has {}",
                        from_u32_unchecked(code).len_utf16(),
                        code,
                        dst.len())
                }
            }
        }
    }

    /// Returns an iterator that yields the hexadecimal Unicode escape of a
    /// character, as `char`s.
    ///
    /// This `struct` is created by the [`escape_unicode`] method on [`char`]. See
    /// its documentation for more.
    ///
    /// [`escape_unicode`]: ../../std/primitive.char.html#method.escape_unicode
    /// [`char`]: ../../std/primitive.char.html
    #[derive(Clone, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct EscapeUnicode {
        c: char,
        state: EscapeUnicodeState,

        // The index of the next hex digit to be printed (0 if none),
        // i.e. the number of remaining hex digits to be printed;
        // increasing from the least significant digit: 0x543210
        hex_digit_idx: usize,
    }

    // The enum values are ordered so that their representation is the
    // same as the remaining length (besides the hexadecimal digits). This
    // likely makes `len()` a single load from memory) and inline-worth.
    #[derive(Clone, Debug)]
    enum EscapeUnicodeState {
        Done,
        RightBrace,
        Value,
        LeftBrace,
        Type,
        Backslash,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Iterator for EscapeUnicode {
        type Item = char;

        fn next(&mut self) -> Option<char> {
            match self.state {
                EscapeUnicodeState::Backslash => {
                    self.state = EscapeUnicodeState::Type;
                    Some('\\')
                }
                EscapeUnicodeState::Type => {
                    self.state = EscapeUnicodeState::LeftBrace;
                    Some('u')
                }
                EscapeUnicodeState::LeftBrace => {
                    self.state = EscapeUnicodeState::Value;
                    Some('{')
                }
                EscapeUnicodeState::Value => {
                    let hex_digit = ((self.c as u32) >> (self.hex_digit_idx * 4)) & 0xf;
                    let c = from_digit(hex_digit, 16).unwrap();
                    if self.hex_digit_idx == 0 {
                        self.state = EscapeUnicodeState::RightBrace;
                    } else {
                        self.hex_digit_idx -= 1;
                    }
                    Some(c)
                }
                EscapeUnicodeState::RightBrace => {
                    self.state = EscapeUnicodeState::Done;
                    Some('}')
                }
                EscapeUnicodeState::Done => None,
            }
        }

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

        #[inline]
        fn count(self) -> usize {
            self.len()
        }

        fn last(self) -> Option<char> {
            match self.state {
                EscapeUnicodeState::Done => None,

                EscapeUnicodeState::RightBrace |
                EscapeUnicodeState::Value |
                EscapeUnicodeState::LeftBrace |
                EscapeUnicodeState::Type |
                EscapeUnicodeState::Backslash => Some('}'),
            }
        }
    }

    #[stable(feature = "exact_size_escape", since = "1.11.0")]
    impl ExactSizeIterator for EscapeUnicode {
        #[inline]
        fn len(&self) -> usize {
            // The match is a single memory access with no branching
            self.hex_digit_idx + match self.state {
                EscapeUnicodeState::Done => 0,
                EscapeUnicodeState::RightBrace => 1,
                EscapeUnicodeState::Value => 2,
                EscapeUnicodeState::LeftBrace => 3,
                EscapeUnicodeState::Type => 4,
                EscapeUnicodeState::Backslash => 5,
            }
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl FusedIterator for EscapeUnicode {}

    #[stable(feature = "char_struct_display", since = "1.16.0")]
    impl fmt::Display for EscapeUnicode {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            for c in self.clone() {
                f.write_char(c)?;
            }
            Ok(())
        }
    }

    /// An iterator that yields the literal escape code of a `char`.
    ///
    /// This `struct` is created by the [`escape_default`] method on [`char`]. See
    /// its documentation for more.
    ///
    /// [`escape_default`]: ../../std/primitive.char.html#method.escape_default
    /// [`char`]: ../../std/primitive.char.html
    #[derive(Clone, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct EscapeDefault {
        state: EscapeDefaultState
    }

    #[derive(Clone, Debug)]
    enum EscapeDefaultState {
        Done,
        Char(char),
        Backslash(char),
        Unicode(EscapeUnicode),
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Iterator for EscapeDefault {
        type Item = char;

        fn next(&mut self) -> Option<char> {
            match self.state {
                EscapeDefaultState::Backslash(c) => {
                    self.state = EscapeDefaultState::Char(c);
                    Some('\\')
                }
                EscapeDefaultState::Char(c) => {
                    self.state = EscapeDefaultState::Done;
                    Some(c)
                }
                EscapeDefaultState::Done => None,
                EscapeDefaultState::Unicode(ref mut iter) => iter.next(),
            }
        }

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

        #[inline]
        fn count(self) -> usize {
            self.len()
        }

        fn nth(&mut self, n: usize) -> Option<char> {
            match self.state {
                EscapeDefaultState::Backslash(c) if n == 0 => {
                    self.state = EscapeDefaultState::Char(c);
                    Some('\\')
                },
                EscapeDefaultState::Backslash(c) if n == 1 => {
                    self.state = EscapeDefaultState::Done;
                    Some(c)
                },
                EscapeDefaultState::Backslash(_) => {
                    self.state = EscapeDefaultState::Done;
                    None
                },
                EscapeDefaultState::Char(c) => {
                    self.state = EscapeDefaultState::Done;

                    if n == 0 {
                        Some(c)
                    } else {
                        None
                    }
                },
                EscapeDefaultState::Done => return None,
                EscapeDefaultState::Unicode(ref mut i) => return i.nth(n),
            }
        }

        fn last(self) -> Option<char> {
            match self.state {
                EscapeDefaultState::Unicode(iter) => iter.last(),
                EscapeDefaultState::Done => None,
                EscapeDefaultState::Backslash(c) | EscapeDefaultState::Char(c) => Some(c),
            }
        }
    }

    #[stable(feature = "exact_size_escape", since = "1.11.0")]
    impl ExactSizeIterator for EscapeDefault {
        fn len(&self) -> usize {
            match self.state {
                EscapeDefaultState::Done => 0,
                EscapeDefaultState::Char(_) => 1,
                EscapeDefaultState::Backslash(_) => 2,
                EscapeDefaultState::Unicode(ref iter) => iter.len(),
            }
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl FusedIterator for EscapeDefault {}

    #[stable(feature = "char_struct_display", since = "1.16.0")]
    impl fmt::Display for EscapeDefault {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            for c in self.clone() {
                f.write_char(c)?;
            }
            Ok(())
        }
    }

    /// An iterator that yields the literal escape code of a `char`.
    ///
    /// This `struct` is created by the [`escape_debug`] method on [`char`]. See its
    /// documentation for more.
    ///
    /// [`escape_debug`]: ../../std/primitive.char.html#method.escape_debug
    /// [`char`]: ../../std/primitive.char.html
    #[stable(feature = "char_escape_debug", since = "1.20.0")]
    #[derive(Clone, Debug)]
    pub struct EscapeDebug(EscapeDefault);

    #[stable(feature = "char_escape_debug", since = "1.20.0")]
    impl Iterator for EscapeDebug {
        type Item = char;
        fn next(&mut self) -> Option<char> { self.0.next() }
        fn size_hint(&self) -> (usize, Option<usize>) { self.0.size_hint() }
    }

    #[stable(feature = "char_escape_debug", since = "1.20.0")]
    impl ExactSizeIterator for EscapeDebug { }

    #[unstable(feature = "fused", issue = "35602")]
    impl FusedIterator for EscapeDebug {}

    #[stable(feature = "char_escape_debug", since = "1.20.0")]
    impl fmt::Display for EscapeDebug {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Display::fmt(&self.0, f)
        }
    }

    /// An iterator over an iterator of bytes of the characters the bytes represent
    /// as UTF-8
    #[unstable(feature = "decode_utf8", issue = "33906")]
    #[derive(Clone, Debug)]
    pub struct DecodeUtf8<I: Iterator<Item = u8>>(::iter::Peekable<I>);

    /// Decodes an `Iterator` of bytes as UTF-8.
    #[unstable(feature = "decode_utf8", issue = "33906")]
    #[inline]
    pub fn decode_utf8<I: IntoIterator<Item = u8>>(i: I) -> DecodeUtf8<I::IntoIter> {
        DecodeUtf8(i.into_iter().peekable())
    }

    /// `<DecodeUtf8 as Iterator>::next` returns this for an invalid input sequence.
    #[unstable(feature = "decode_utf8", issue = "33906")]
    #[derive(PartialEq, Eq, Debug)]
    pub struct InvalidSequence(());

    #[unstable(feature = "decode_utf8", issue = "33906")]
    impl<I: Iterator<Item = u8>> Iterator for DecodeUtf8<I> {
        type Item = Result<char, InvalidSequence>;
        #[inline]

        fn next(&mut self) -> Option<Result<char, InvalidSequence>> {
            self.0.next().map(|first_byte| {
                // Emit InvalidSequence according to
                // Unicode §5.22 Best Practice for U+FFFD Substitution
                // http://www.unicode.org/versions/Unicode9.0.0/ch05.pdf#G40630

                // Roughly: consume at least one byte,
                // then validate one byte at a time and stop before the first unexpected byte
                // (which might be the valid start of the next byte sequence).

                let mut code_point;
                macro_rules! first_byte {
                    ($mask: expr) => {
                        code_point = u32::from(first_byte & $mask)
                    }
                }
                macro_rules! continuation_byte {
                    () => { continuation_byte!(0x80...0xBF) };
                    ($range: pat) => {
                        match self.0.peek() {
                            Some(&byte @ $range) => {
                                code_point = (code_point << 6) | u32::from(byte & 0b0011_1111);
                                self.0.next();
                            }
                            _ => return Err(InvalidSequence(()))
                        }
                    }
                }

                match first_byte {
                    0x00...0x7F => {
                        first_byte!(0b1111_1111);
                    }
                    0xC2...0xDF => {
                        first_byte!(0b0001_1111);
                        continuation_byte!();
                    }
                    0xE0 => {
                        first_byte!(0b0000_1111);
                        continuation_byte!(0xA0...0xBF);  // 0x80...0x9F here are overlong
                        continuation_byte!();
                    }
                    0xE1...0xEC | 0xEE...0xEF => {
                        first_byte!(0b0000_1111);
                        continuation_byte!();
                        continuation_byte!();
                    }
                    0xED => {
                        first_byte!(0b0000_1111);
                        continuation_byte!(0x80...0x9F);  // 0xA0..0xBF here are surrogates
                        continuation_byte!();
                    }
                    0xF0 => {
                        first_byte!(0b0000_0111);
                        continuation_byte!(0x90...0xBF);  // 0x80..0x8F here are overlong
                        continuation_byte!();
                        continuation_byte!();
                    }
                    0xF1...0xF3 => {
                        first_byte!(0b0000_0111);
                        continuation_byte!();
                        continuation_byte!();
                        continuation_byte!();
                    }
                    0xF4 => {
                        first_byte!(0b0000_0111);
                        continuation_byte!(0x80...0x8F);  // 0x90..0xBF here are beyond char::MAX
                        continuation_byte!();
                        continuation_byte!();
                    }
                    _ => return Err(InvalidSequence(()))  // Illegal first byte, overlong, or beyond MAX
                }
                unsafe {
                    Ok(from_u32_unchecked(code_point))
                }
            })
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<I: FusedIterator<Item = u8>> FusedIterator for DecodeUtf8<I> {}

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