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    //! Unicode string slices.
    //!
    //! *[See also the `str` primitive type](../../std/primitive.str.html).*
    //!
    //! The `&str` type is one of the two main string types, the other being `String`.
    //! Unlike its `String` counterpart, its contents are borrowed.
    //!
    //! # Basic Usage
    //!
    //! A basic string declaration of `&str` type:
    //!
    //! ```
    //! let hello_world = "Hello, World!";
    //! ```
    //!
    //! Here we have declared a string literal, also known as a string slice.
    //! String literals have a static lifetime, which means the string `hello_world`
    //! is guaranteed to be valid for the duration of the entire program.
    //! We can explicitly specify `hello_world`'s lifetime as well:
    //!
    //! ```
    //! let hello_world: &'static str = "Hello, world!";
    //! ```

    #![stable(feature = "rust1", since = "1.0.0")]
    // Many of the usings in this module are only used in the test configuration.
    // It's cleaner to just turn off the unused_imports warning than to fix them.
    #![allow(unused_imports)]

    use core::borrow::{Borrow, BorrowMut};
    use core::iter::FusedIterator;
    use core::mem;
    use core::ptr;
    use core::str::pattern::{DoubleEndedSearcher, Pattern, ReverseSearcher, Searcher};
    use core::unicode::conversions;

    use crate::borrow::ToOwned;
    use crate::boxed::Box;
    use crate::slice::{Concat, Join, SliceIndex};
    use crate::string::String;
    use crate::vec::Vec;

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::str::pattern;
    #[stable(feature = "encode_utf16", since = "1.8.0")]
    pub use core::str::EncodeUtf16;
    #[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
    pub use core::str::SplitAsciiWhitespace;
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::str::SplitWhitespace;
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::str::{from_utf8, from_utf8_mut, Bytes, CharIndices, Chars};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::str::{from_utf8_unchecked, from_utf8_unchecked_mut, ParseBoolError};
    #[stable(feature = "str_escape", since = "1.34.0")]
    pub use core::str::{EscapeDebug, EscapeDefault, EscapeUnicode};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::str::{FromStr, Utf8Error};
    #[allow(deprecated)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::str::{Lines, LinesAny};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::str::{MatchIndices, RMatchIndices};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::str::{Matches, RMatches};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::str::{RSplit, Split};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::str::{RSplitN, SplitN};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use core::str::{RSplitTerminator, SplitTerminator};

    /// Note: `str` in `Concat<str>` is not meaningful here.
    /// This type parameter of the trait only exists to enable another impl.
    #[unstable(feature = "slice_concat_ext", issue = "27747")]
    impl<S: Borrow<str>> Concat<str> for [S] {
        type Output = String;

        fn concat(slice: &Self) -> String {
            Join::join(slice, "")
        }
    }

    #[unstable(feature = "slice_concat_ext", issue = "27747")]
    impl<S: Borrow<str>> Join<&str> for [S] {
        type Output = String;

        fn join(slice: &Self, sep: &str) -> String {
            unsafe { String::from_utf8_unchecked(join_generic_copy(slice, sep.as_bytes())) }
        }
    }

    macro_rules! spezialize_for_lengths {
        ($separator:expr, $target:expr, $iter:expr; $($num:expr),*) => {
            let mut target = $target;
            let iter = $iter;
            let sep_bytes = $separator;
            match $separator.len() {
                $(
                    // loops with hardcoded sizes run much faster
                    // specialize the cases with small separator lengths
                    $num => {
                        for s in iter {
                            copy_slice_and_advance!(target, sep_bytes);
                            copy_slice_and_advance!(target, s.borrow().as_ref());
                        }
                    },
                )*
                _ => {
                    // arbitrary non-zero size fallback
                    for s in iter {
                        copy_slice_and_advance!(target, sep_bytes);
                        copy_slice_and_advance!(target, s.borrow().as_ref());
                    }
                }
            }
        };
    }

    macro_rules! copy_slice_and_advance {
        ($target:expr, $bytes:expr) => {
            let len = $bytes.len();
            let (head, tail) = { $target }.split_at_mut(len);
            head.copy_from_slice($bytes);
            $target = tail;
        };
    }

    // Optimized join implementation that works for both Vec<T> (T: Copy) and String's inner vec
    // Currently (2018-05-13) there is a bug with type inference and specialization (see issue #36262)
    // For this reason SliceConcat<T> is not specialized for T: Copy and SliceConcat<str> is the
    // only user of this function. It is left in place for the time when that is fixed.
    //
    // the bounds for String-join are S: Borrow<str> and for Vec-join Borrow<[T]>
    // [T] and str both impl AsRef<[T]> for some T
    // => s.borrow().as_ref() and we always have slices
    fn join_generic_copy<B, T, S>(slice: &[S], sep: &[T]) -> Vec<T>
    where
        T: Copy,
        B: AsRef<[T]> + ?Sized,
        S: Borrow<B>,
    {
        let sep_len = sep.len();
        let mut iter = slice.iter();

        // the first slice is the only one without a separator preceding it
        let first = match iter.next() {
            Some(first) => first,
            None => return vec![],
        };

        // compute the exact total length of the joined Vec
        // if the `len` calculation overflows, we'll panic
        // we would have run out of memory anyway and the rest of the function requires
        // the entire Vec pre-allocated for safety
        let len = sep_len
            .checked_mul(iter.len())
            .and_then(|n| {
                slice.iter().map(|s| s.borrow().as_ref().len()).try_fold(n, usize::checked_add)
            })
            .expect("attempt to join into collection with len > usize::MAX");

        // crucial for safety
        let mut result = Vec::with_capacity(len);
        assert!(result.capacity() >= len);

        result.extend_from_slice(first.borrow().as_ref());

        unsafe {
            {
                let pos = result.len();
                let target = result.get_unchecked_mut(pos..len);

                // copy separator and slices over without bounds checks
                // generate loops with hardcoded offsets for small separators
                // massive improvements possible (~ x2)
                spezialize_for_lengths!(sep, target, iter; 0, 1, 2, 3, 4);
            }
            result.set_len(len);
        }
        result
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Borrow<str> for String {
        #[inline]
        fn borrow(&self) -> &str {
            &self[..]
        }
    }

    #[stable(feature = "string_borrow_mut", since = "1.36.0")]
    impl BorrowMut<str> for String {
        #[inline]
        fn borrow_mut(&mut self) -> &mut str {
            &mut self[..]
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl ToOwned for str {
        type Owned = String;
        #[inline]
        fn to_owned(&self) -> String {
            unsafe { String::from_utf8_unchecked(self.as_bytes().to_owned()) }
        }

        fn clone_into(&self, target: &mut String) {
            let mut b = mem::take(target).into_bytes();
            self.as_bytes().clone_into(&mut b);
            *target = unsafe { String::from_utf8_unchecked(b) }
        }
    }

    /// Methods for string slices.
    #[lang = "str_alloc"]
    #[cfg(not(test))]
    impl str {
        /// Converts a `Box<str>` into a `Box<[u8]>` without copying or allocating.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s = "this is a string";
        /// let boxed_str = s.to_owned().into_boxed_str();
        /// let boxed_bytes = boxed_str.into_boxed_bytes();
        /// assert_eq!(*boxed_bytes, *s.as_bytes());
        /// ```
        #[stable(feature = "str_box_extras", since = "1.20.0")]
        #[inline]
        pub fn into_boxed_bytes(self: Box<str>) -> Box<[u8]> {
            self.into()
        }

        /// Replaces all matches of a pattern with another string.
        ///
        /// `replace` creates a new [`String`], and copies the data from this string slice into it.
        /// While doing so, it attempts to find matches of a pattern. If it finds any, it
        /// replaces them with the replacement string slice.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s = "this is old";
        ///
        /// assert_eq!("this is new", s.replace("old", "new"));
        /// ```
        ///
        /// When the pattern doesn't match:
        ///
        /// ```
        /// let s = "this is old";
        /// assert_eq!(s, s.replace("cookie monster", "little lamb"));
        /// ```
        #[must_use = "this returns the replaced string as a new allocation, \
                      without modifying the original"]
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn replace<'a, P: Pattern<'a>>(&'a self, from: P, to: &str) -> String {
            let mut result = String::new();
            let mut last_end = 0;
            for (start, part) in self.match_indices(from) {
                result.push_str(unsafe { self.get_unchecked(last_end..start) });
                result.push_str(to);
                last_end = start + part.len();
            }
            result.push_str(unsafe { self.get_unchecked(last_end..self.len()) });
            result
        }

        /// Replaces first N matches of a pattern with another string.
        ///
        /// `replacen` creates a new [`String`], and copies the data from this string slice into it.
        /// While doing so, it attempts to find matches of a pattern. If it finds any, it
        /// replaces them with the replacement string slice at most `count` times.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s = "foo foo 123 foo";
        /// assert_eq!("new new 123 foo", s.replacen("foo", "new", 2));
        /// assert_eq!("faa fao 123 foo", s.replacen('o', "a", 3));
        /// assert_eq!("foo foo new23 foo", s.replacen(char::is_numeric, "new", 1));
        /// ```
        ///
        /// When the pattern doesn't match:
        ///
        /// ```
        /// let s = "this is old";
        /// assert_eq!(s, s.replacen("cookie monster", "little lamb", 10));
        /// ```
        #[must_use = "this returns the replaced string as a new allocation, \
                      without modifying the original"]
        #[stable(feature = "str_replacen", since = "1.16.0")]
        pub fn replacen<'a, P: Pattern<'a>>(&'a self, pat: P, to: &str, count: usize) -> String {
            // Hope to reduce the times of re-allocation
            let mut result = String::with_capacity(32);
            let mut last_end = 0;
            for (start, part) in self.match_indices(pat).take(count) {
                result.push_str(unsafe { self.get_unchecked(last_end..start) });
                result.push_str(to);
                last_end = start + part.len();
            }
            result.push_str(unsafe { self.get_unchecked(last_end..self.len()) });
            result
        }

        /// Returns the lowercase equivalent of this string slice, as a new [`String`].
        ///
        /// 'Lowercase' is defined according to the terms of the Unicode Derived Core Property
        /// `Lowercase`.
        ///
        /// Since some characters can expand into multiple characters when changing
        /// the case, this function returns a [`String`] instead of modifying the
        /// parameter in-place.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s = "HELLO";
        ///
        /// assert_eq!("hello", s.to_lowercase());
        /// ```
        ///
        /// A tricky example, with sigma:
        ///
        /// ```
        /// let sigma = "Σ";
        ///
        /// assert_eq!("σ", sigma.to_lowercase());
        ///
        /// // but at the end of a word, it's ς, not σ:
        /// let odysseus = "ὈΔΥΣΣΕΎΣ";
        ///
        /// assert_eq!("ὀδυσσεύς", odysseus.to_lowercase());
        /// ```
        ///
        /// Languages without case are not changed:
        ///
        /// ```
        /// let new_year = "农历新年";
        ///
        /// assert_eq!(new_year, new_year.to_lowercase());
        /// ```
        #[stable(feature = "unicode_case_mapping", since = "1.2.0")]
        pub fn to_lowercase(&self) -> String {
            let mut s = String::with_capacity(self.len());
            for (i, c) in self[..].char_indices() {
                if c == 'Σ' {
                    // Σ maps to σ, except at the end of a word where it maps to ς.
                    // This is the only conditional (contextual) but language-independent mapping
                    // in `SpecialCasing.txt`,
                    // so hard-code it rather than have a generic "condition" mechanism.
                    // See https://github.com/rust-lang/rust/issues/26035
                    map_uppercase_sigma(self, i, &mut s)
                } else {
                    match conversions::to_lower(c) {
                        [a, '\0', _] => s.push(a),
                        [a, b, '\0'] => {
                            s.push(a);
                            s.push(b);
                        }
                        [a, b, c] => {
                            s.push(a);
                            s.push(b);
                            s.push(c);
                        }
                    }
                }
            }
            return s;

            fn map_uppercase_sigma(from: &str, i: usize, to: &mut String) {
                // See http://www.unicode.org/versions/Unicode7.0.0/ch03.pdf#G33992
                // for the definition of `Final_Sigma`.
                debug_assert!('Σ'.len_utf8() == 2);
                let is_word_final = case_ignoreable_then_cased(from[..i].chars().rev())
                    && !case_ignoreable_then_cased(from[i + 2..].chars());
                to.push_str(if is_word_final { "ς" } else { "σ" });
            }

            fn case_ignoreable_then_cased<I: Iterator<Item = char>>(iter: I) -> bool {
                use core::unicode::{Case_Ignorable, Cased};
                match iter.skip_while(|&c| Case_Ignorable(c)).next() {
                    Some(c) => Cased(c),
                    None => false,
                }
            }
        }

        /// Returns the uppercase equivalent of this string slice, as a new [`String`].
        ///
        /// 'Uppercase' is defined according to the terms of the Unicode Derived Core Property
        /// `Uppercase`.
        ///
        /// Since some characters can expand into multiple characters when changing
        /// the case, this function returns a [`String`] instead of modifying the
        /// parameter in-place.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s = "hello";
        ///
        /// assert_eq!("HELLO", s.to_uppercase());
        /// ```
        ///
        /// Scripts without case are not changed:
        ///
        /// ```
        /// let new_year = "农历新年";
        ///
        /// assert_eq!(new_year, new_year.to_uppercase());
        /// ```
        ///
        /// One character can become multiple:
        /// ```
        /// let s = "tschüß";
        ///
        /// assert_eq!("TSCHÜSS", s.to_uppercase());
        /// ```
        #[stable(feature = "unicode_case_mapping", since = "1.2.0")]
        pub fn to_uppercase(&self) -> String {
            let mut s = String::with_capacity(self.len());
            for c in self[..].chars() {
                match conversions::to_upper(c) {
                    [a, '\0', _] => s.push(a),
                    [a, b, '\0'] => {
                        s.push(a);
                        s.push(b);
                    }
                    [a, b, c] => {
                        s.push(a);
                        s.push(b);
                        s.push(c);
                    }
                }
            }
            s
        }

        /// Converts a [`Box<str>`] into a [`String`] without copying or allocating.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let string = String::from("birthday gift");
        /// let boxed_str = string.clone().into_boxed_str();
        ///
        /// assert_eq!(boxed_str.into_string(), string);
        /// ```
        #[stable(feature = "box_str", since = "1.4.0")]
        #[inline]
        pub fn into_string(self: Box<str>) -> String {
            let slice = Box::<[u8]>::from(self);
            unsafe { String::from_utf8_unchecked(slice.into_vec()) }
        }

        /// Creates a new [`String`] by repeating a string `n` times.
        ///
        /// # Panics
        ///
        /// This function will panic if the capacity would overflow.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// assert_eq!("abc".repeat(4), String::from("abcabcabcabc"));
        /// ```
        ///
        /// A panic upon overflow:
        ///
        /// ```should_panic
        /// // this will panic at runtime
        /// "0123456789abcdef".repeat(usize::MAX);
        /// ```
        #[stable(feature = "repeat_str", since = "1.16.0")]
        pub fn repeat(&self, n: usize) -> String {
            unsafe { String::from_utf8_unchecked(self.as_bytes().repeat(n)) }
        }

        /// Returns a copy of this string where each character is mapped to its
        /// ASCII upper case equivalent.
        ///
        /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
        /// but non-ASCII letters are unchanged.
        ///
        /// To uppercase the value in-place, use [`make_ascii_uppercase`].
        ///
        /// To uppercase ASCII characters in addition to non-ASCII characters, use
        /// [`to_uppercase`].
        ///
        /// # Examples
        ///
        /// ```
        /// let s = "Grüße, Jürgen ❤";
        ///
        /// assert_eq!("GRüßE, JüRGEN ❤", s.to_ascii_uppercase());
        /// ```
        ///
        /// [`make_ascii_uppercase`]: str::make_ascii_uppercase
        /// [`to_uppercase`]: #method.to_uppercase
        #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
        #[inline]
        pub fn to_ascii_uppercase(&self) -> String {
            let mut bytes = self.as_bytes().to_vec();
            bytes.make_ascii_uppercase();
            // make_ascii_uppercase() preserves the UTF-8 invariant.
            unsafe { String::from_utf8_unchecked(bytes) }
        }

        /// Returns a copy of this string where each character is mapped to its
        /// ASCII lower case equivalent.
        ///
        /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
        /// but non-ASCII letters are unchanged.
        ///
        /// To lowercase the value in-place, use [`make_ascii_lowercase`].
        ///
        /// To lowercase ASCII characters in addition to non-ASCII characters, use
        /// [`to_lowercase`].
        ///
        /// # Examples
        ///
        /// ```
        /// let s = "Grüße, Jürgen ❤";
        ///
        /// assert_eq!("grüße, jürgen ❤", s.to_ascii_lowercase());
        /// ```
        ///
        /// [`make_ascii_lowercase`]: str::make_ascii_lowercase
        /// [`to_lowercase`]: #method.to_lowercase
        #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
        #[inline]
        pub fn to_ascii_lowercase(&self) -> String {
            let mut bytes = self.as_bytes().to_vec();
            bytes.make_ascii_lowercase();
            // make_ascii_lowercase() preserves the UTF-8 invariant.
            unsafe { String::from_utf8_unchecked(bytes) }
        }
    }

    /// Converts a boxed slice of bytes to a boxed string slice without checking
    /// that the string contains valid UTF-8.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let smile_utf8 = Box::new([226, 152, 186]);
    /// let smile = unsafe { std::str::from_boxed_utf8_unchecked(smile_utf8) };
    ///
    /// assert_eq!("☺", &*smile);
    /// ```
    #[stable(feature = "str_box_extras", since = "1.20.0")]
    #[inline]
    pub unsafe fn from_boxed_utf8_unchecked(v: Box<[u8]>) -> Box<str> {
        unsafe { Box::from_raw(Box::into_raw(v) as *mut str) }
    }