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    //! Character conversions.

    use crate::convert::TryFrom;
    use crate::fmt;
    use crate::mem::transmute;
    use crate::str::FromStr;

    use super::MAX;

    /// Converts a `u32` to a `char`.
    ///
    /// Note that all [`char`]s are valid [`u32`]s, and can be cast 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
    ///
    /// 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 cast 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
    ///
    /// # 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 {
        /// Converts a [`char`] into a [`u32`].
        ///
        /// # Examples
        ///
        /// ```
        /// use std::mem;
        ///
        /// let c = 'c';
        /// let u = u32::from(c);
        /// assert!(4 == mem::size_of_val(&u))
        /// ```
        #[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..=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 {
        /// Converts a [`u8`] into a [`char`].
        ///
        /// # Examples
        ///
        /// ```
        /// use std::mem;
        ///
        /// let u = 32 as u8;
        /// let c = char::from(u);
        /// assert!(4 == mem::size_of_val(&c))
        /// ```
        #[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, PartialEq, Eq)]
    pub struct ParseCharError {
        kind: CharErrorKind,
    }

    impl ParseCharError {
        #[unstable(
            feature = "char_error_internals",
            reason = "this method should not be available publicly",
            issue = "none"
        )]
        #[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 }),
            }
        }
    }

    #[stable(feature = "try_from", since = "1.34.0")]
    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 {
                // SAFETY: checked that it's a legal unicode value
                Ok(unsafe { from_u32_unchecked(i) })
            }
        }
    }

    /// The error type returned when a conversion from u32 to char fails.
    #[stable(feature = "try_from", since = "1.34.0")]
    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
    pub struct CharTryFromError(());

    #[stable(feature = "try_from", since = "1.34.0")]
    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
        }
    }