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    //! Operations on ASCII strings and characters.
    //!
    //! Most string operations in Rust act on UTF-8 strings. However, at times it
    //! makes more sense to only consider the ASCII character set for a specific
    //! operation.
    //!
    //! The [`escape_default`] function provides an iterator over the bytes of an
    //! escaped version of the character given.

    #![stable(feature = "core_ascii", since = "1.26.0")]

    use crate::fmt;
    use crate::iter::FusedIterator;
    use crate::ops::Range;
    use crate::str::from_utf8_unchecked;

    /// An iterator over the escaped version of a byte.
    ///
    /// This `struct` is created by the [`escape_default`] function. See its
    /// documentation for more.
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct EscapeDefault {
        range: Range<usize>,
        data: [u8; 4],
    }

    /// Returns an iterator that produces an escaped version of a `u8`.
    ///
    /// The default is chosen with a bias toward producing literals that are
    /// legal in a variety of languages, including C++11 and similar C-family
    /// languages. The exact rules are:
    ///
    /// * Tab is escaped as `\t`.
    /// * Carriage return is escaped as `\r`.
    /// * Line feed is escaped as `\n`.
    /// * Single quote is escaped as `\'`.
    /// * Double quote is escaped as `\"`.
    /// * Backslash is escaped as `\\`.
    /// * Any character in the 'printable ASCII' range `0x20` .. `0x7e`
    ///   inclusive is not escaped.
    /// * Any other chars are given hex escapes of the form '\xNN'.
    /// * Unicode escapes are never generated by this function.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ascii;
    ///
    /// let escaped = ascii::escape_default(b'0').next().unwrap();
    /// assert_eq!(b'0', escaped);
    ///
    /// let mut escaped = ascii::escape_default(b'\t');
    ///
    /// assert_eq!(b'\\', escaped.next().unwrap());
    /// assert_eq!(b't', escaped.next().unwrap());
    ///
    /// let mut escaped = ascii::escape_default(b'\r');
    ///
    /// assert_eq!(b'\\', escaped.next().unwrap());
    /// assert_eq!(b'r', escaped.next().unwrap());
    ///
    /// let mut escaped = ascii::escape_default(b'\n');
    ///
    /// assert_eq!(b'\\', escaped.next().unwrap());
    /// assert_eq!(b'n', escaped.next().unwrap());
    ///
    /// let mut escaped = ascii::escape_default(b'\'');
    ///
    /// assert_eq!(b'\\', escaped.next().unwrap());
    /// assert_eq!(b'\'', escaped.next().unwrap());
    ///
    /// let mut escaped = ascii::escape_default(b'"');
    ///
    /// assert_eq!(b'\\', escaped.next().unwrap());
    /// assert_eq!(b'"', escaped.next().unwrap());
    ///
    /// let mut escaped = ascii::escape_default(b'\\');
    ///
    /// assert_eq!(b'\\', escaped.next().unwrap());
    /// assert_eq!(b'\\', escaped.next().unwrap());
    ///
    /// let mut escaped = ascii::escape_default(b'\x9d');
    ///
    /// assert_eq!(b'\\', escaped.next().unwrap());
    /// assert_eq!(b'x', escaped.next().unwrap());
    /// assert_eq!(b'9', escaped.next().unwrap());
    /// assert_eq!(b'd', escaped.next().unwrap());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn escape_default(c: u8) -> EscapeDefault {
        let (data, len) = match c {
            b'\t' => ([b'\\', b't', 0, 0], 2),
            b'\r' => ([b'\\', b'r', 0, 0], 2),
            b'\n' => ([b'\\', b'n', 0, 0], 2),
            b'\\' => ([b'\\', b'\\', 0, 0], 2),
            b'\'' => ([b'\\', b'\'', 0, 0], 2),
            b'"' => ([b'\\', b'"', 0, 0], 2),
            b'\x20'..=b'\x7e' => ([c, 0, 0, 0], 1),
            _ => ([b'\\', b'x', hexify(c >> 4), hexify(c & 0xf)], 4),
        };

        return EscapeDefault { range: 0..len, data };

        fn hexify(b: u8) -> u8 {
            match b {
                0..=9 => b'0' + b,
                _ => b'a' + b - 10,
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Iterator for EscapeDefault {
        type Item = u8;
        fn next(&mut self) -> Option<u8> {
            self.range.next().map(|i| self.data[i])
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.range.size_hint()
        }
        fn last(mut self) -> Option<u8> {
            self.next_back()
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl DoubleEndedIterator for EscapeDefault {
        fn next_back(&mut self) -> Option<u8> {
            self.range.next_back().map(|i| self.data[i])
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl ExactSizeIterator for EscapeDefault {}
    #[stable(feature = "fused", since = "1.26.0")]
    impl FusedIterator for EscapeDefault {}

    #[stable(feature = "ascii_escape_display", since = "1.39.0")]
    impl fmt::Display for EscapeDefault {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            // SAFETY: ok because `escape_default` created only valid utf-8 data
            f.write_str(unsafe { from_utf8_unchecked(&self.data[self.range.clone()]) })
        }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl fmt::Debug for EscapeDefault {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("EscapeDefault { .. }")
        }
    }