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    //! Defines utf8 error type.

    use crate::fmt;

    /// Errors which can occur when attempting to interpret a sequence of [`u8`]
    /// as a string.
    ///
    /// As such, the `from_utf8` family of functions and methods for both [`String`]s
    /// and [`&str`]s make use of this error, for example.
    ///
    /// [`String`]: ../../std/string/struct.String.html#method.from_utf8
    /// [`&str`]: super::from_utf8
    ///
    /// # Examples
    ///
    /// This error type’s methods can be used to create functionality
    /// similar to `String::from_utf8_lossy` without allocating heap memory:
    ///
    /// ```
    /// fn from_utf8_lossy<F>(mut input: &[u8], mut push: F) where F: FnMut(&str) {
    ///     loop {
    ///         match std::str::from_utf8(input) {
    ///             Ok(valid) => {
    ///                 push(valid);
    ///                 break
    ///             }
    ///             Err(error) => {
    ///                 let (valid, after_valid) = input.split_at(error.valid_up_to());
    ///                 unsafe {
    ///                     push(std::str::from_utf8_unchecked(valid))
    ///                 }
    ///                 push("\u{FFFD}");
    ///
    ///                 if let Some(invalid_sequence_length) = error.error_len() {
    ///                     input = &after_valid[invalid_sequence_length..]
    ///                 } else {
    ///                     break
    ///                 }
    ///             }
    ///         }
    ///     }
    /// }
    /// ```
    #[derive(Copy, Eq, PartialEq, Clone, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Utf8Error {
        pub(super) valid_up_to: usize,
        pub(super) error_len: Option<u8>,
    }

    impl Utf8Error {
        /// Returns the index in the given string up to which valid UTF-8 was
        /// verified.
        ///
        /// It is the maximum index such that `from_utf8(&input[..index])`
        /// would return `Ok(_)`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::str;
        ///
        /// // some invalid bytes, in a vector
        /// let sparkle_heart = vec![0, 159, 146, 150];
        ///
        /// // std::str::from_utf8 returns a Utf8Error
        /// let error = str::from_utf8(&sparkle_heart).unwrap_err();
        ///
        /// // the second byte is invalid here
        /// assert_eq!(1, error.valid_up_to());
        /// ```
        #[stable(feature = "utf8_error", since = "1.5.0")]
        #[inline]
        pub fn valid_up_to(&self) -> usize {
            self.valid_up_to
        }

        /// Provides more information about the failure:
        ///
        /// * `None`: the end of the input was reached unexpectedly.
        ///   `self.valid_up_to()` is 1 to 3 bytes from the end of the input.
        ///   If a byte stream (such as a file or a network socket) is being decoded incrementally,
        ///   this could be a valid `char` whose UTF-8 byte sequence is spanning multiple chunks.
        ///
        /// * `Some(len)`: an unexpected byte was encountered.
        ///   The length provided is that of the invalid byte sequence
        ///   that starts at the index given by `valid_up_to()`.
        ///   Decoding should resume after that sequence
        ///   (after inserting a [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD]) in case of
        ///   lossy decoding.
        ///
        /// [U+FFFD]: ../../std/char/constant.REPLACEMENT_CHARACTER.html
        #[stable(feature = "utf8_error_error_len", since = "1.20.0")]
        #[inline]
        pub fn error_len(&self) -> Option<usize> {
            self.error_len.map(|len| len as usize)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Display for Utf8Error {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            if let Some(error_len) = self.error_len {
                write!(
                    f,
                    "invalid utf-8 sequence of {} bytes from index {}",
                    error_len, self.valid_up_to
                )
            } else {
                write!(f, "incomplete utf-8 byte sequence from index {}", self.valid_up_to)
            }
        }
    }

    /// An error returned when parsing a `bool` using [`from_str`] fails
    ///
    /// [`from_str`]: super::FromStr::from_str
    #[derive(Debug, Clone, PartialEq, Eq)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct ParseBoolError {
        pub(super) _priv: (),
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Display for ParseBoolError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            "provided string was not `true` or `false`".fmt(f)
        }
    }