string.rs.html -- source
Description: Source to the Rust file `src/liballoc/string.rs`.
Keywords: rust, rustlang, rust-lang

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    //! A UTF-8 encoded, growable string.
    //!
    //! This module contains the [`String`] type, a trait for converting
    //! [`ToString`]s, and several error types that may result from working with
    //! [`String`]s.
    //!
    //! [`ToString`]: trait.ToString.html
    //!
    //! # Examples
    //!
    //! There are multiple ways to create a new [`String`] from a string literal:
    //!
    //! ```
    //! let s = "Hello".to_string();
    //!
    //! let s = String::from("world");
    //! let s: String = "also this".into();
    //! ```
    //!
    //! You can create a new [`String`] from an existing one by concatenating with
    //! `+`:
    //!
    //! [`String`]: struct.String.html
    //!
    //! ```
    //! let s = "Hello".to_string();
    //!
    //! let message = s + " world!";
    //! ```
    //!
    //! If you have a vector of valid UTF-8 bytes, you can make a [`String`] out of
    //! it. You can do the reverse too.
    //!
    //! ```
    //! let sparkle_heart = vec![240, 159, 146, 150];
    //!
    //! // We know these bytes are valid, so we'll use `unwrap()`.
    //! let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
    //!
    //! assert_eq!("💖", sparkle_heart);
    //!
    //! let bytes = sparkle_heart.into_bytes();
    //!
    //! assert_eq!(bytes, [240, 159, 146, 150]);
    //! ```

    #![stable(feature = "rust1", since = "1.0.0")]

    use core::char::{decode_utf16, REPLACEMENT_CHARACTER};
    use core::fmt;
    use core::hash;
    use core::iter::{FromIterator, FusedIterator};
    use core::ops::{self, Add, AddAssign, Index, IndexMut, RangeBounds};
    use core::ops::Bound::{Excluded, Included, Unbounded};
    use core::ptr;
    use core::str::{pattern::Pattern, lossy};

    use crate::borrow::{Cow, ToOwned};
    use crate::collections::CollectionAllocErr;
    use crate::boxed::Box;
    use crate::str::{self, from_boxed_utf8_unchecked, FromStr, Utf8Error, Chars};
    use crate::vec::Vec;

    /// A UTF-8 encoded, growable string.
    ///
    /// The `String` type is the most common string type that has ownership over the
    /// contents of the string. It has a close relationship with its borrowed
    /// counterpart, the primitive [`str`].
    ///
    /// [`str`]: ../../std/primitive.str.html
    ///
    /// # Examples
    ///
    /// You can create a `String` from a literal string with [`String::from`]:
    ///
    /// ```
    /// let hello = String::from("Hello, world!");
    /// ```
    ///
    /// You can append a [`char`] to a `String` with the [`push`] method, and
    /// append a [`&str`] with the [`push_str`] method:
    ///
    /// ```
    /// let mut hello = String::from("Hello, ");
    ///
    /// hello.push('w');
    /// hello.push_str("orld!");
    /// ```
    ///
    /// [`String::from`]: #method.from
    /// [`char`]: ../../std/primitive.char.html
    /// [`push`]: #method.push
    /// [`push_str`]: #method.push_str
    ///
    /// If you have a vector of UTF-8 bytes, you can create a `String` from it with
    /// the [`from_utf8`] method:
    ///
    /// ```
    /// // some bytes, in a vector
    /// let sparkle_heart = vec![240, 159, 146, 150];
    ///
    /// // We know these bytes are valid, so we'll use `unwrap()`.
    /// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
    ///
    /// assert_eq!("💖", sparkle_heart);
    /// ```
    ///
    /// [`from_utf8`]: #method.from_utf8
    ///
    /// # UTF-8
    ///
    /// `String`s are always valid UTF-8. This has a few implications, the first of
    /// which is that if you need a non-UTF-8 string, consider [`OsString`]. It is
    /// similar, but without the UTF-8 constraint. The second implication is that
    /// you cannot index into a `String`:
    ///
    /// ```compile_fail,E0277
    /// let s = "hello";
    ///
    /// println!("The first letter of s is {}", s[0]); // ERROR!!!
    /// ```
    ///
    /// [`OsString`]: ../../std/ffi/struct.OsString.html
    ///
    /// Indexing is intended to be a constant-time operation, but UTF-8 encoding
    /// does not allow us to do this. Furthermore, it's not clear what sort of
    /// thing the index should return: a byte, a codepoint, or a grapheme cluster.
    /// The [`bytes`] and [`chars`] methods return iterators over the first
    /// two, respectively.
    ///
    /// [`bytes`]: #method.bytes
    /// [`chars`]: #method.chars
    ///
    /// # Deref
    ///
    /// `String`s implement [`Deref`]`<Target=str>`, and so inherit all of [`str`]'s
    /// methods. In addition, this means that you can pass a `String` to a
    /// function which takes a [`&str`] by using an ampersand (`&`):
    ///
    /// ```
    /// fn takes_str(s: &str) { }
    ///
    /// let s = String::from("Hello");
    ///
    /// takes_str(&s);
    /// ```
    ///
    /// This will create a [`&str`] from the `String` and pass it in. This
    /// conversion is very inexpensive, and so generally, functions will accept
    /// [`&str`]s as arguments unless they need a `String` for some specific
    /// reason.
    ///
    /// In certain cases Rust doesn't have enough information to make this
    /// conversion, known as [`Deref`] coercion. In the following example a string
    /// slice [`&'a str`][`&str`] implements the trait `TraitExample`, and the function
    /// `example_func` takes anything that implements the trait. In this case Rust
    /// would need to make two implicit conversions, which Rust doesn't have the
    /// means to do. For that reason, the following example will not compile.
    ///
    /// ```compile_fail,E0277
    /// trait TraitExample {}
    ///
    /// impl<'a> TraitExample for &'a str {}
    ///
    /// fn example_func<A: TraitExample>(example_arg: A) {}
    ///
    /// fn main() {
    ///     let example_string = String::from("example_string");
    ///     example_func(&example_string);
    /// }
    /// ```
    ///
    /// There are two options that would work instead. The first would be to
    /// change the line `example_func(&example_string);` to
    /// `example_func(example_string.as_str());`, using the method [`as_str()`]
    /// to explicitly extract the string slice containing the string. The second
    /// way changes `example_func(&example_string);` to
    /// `example_func(&*example_string);`. In this case we are dereferencing a
    /// `String` to a [`str`][`&str`], then referencing the [`str`][`&str`] back to
    /// [`&str`]. The second way is more idiomatic, however both work to do the
    /// conversion explicitly rather than relying on the implicit conversion.
    ///
    /// # Representation
    ///
    /// A `String` is made up of three components: a pointer to some bytes, a
    /// length, and a capacity. The pointer points to an internal buffer `String`
    /// uses to store its data. The length is the number of bytes currently stored
    /// in the buffer, and the capacity is the size of the buffer in bytes. As such,
    /// the length will always be less than or equal to the capacity.
    ///
    /// This buffer is always stored on the heap.
    ///
    /// You can look at these with the [`as_ptr`], [`len`], and [`capacity`]
    /// methods:
    ///
    /// ```
    /// use std::mem;
    ///
    /// let story = String::from("Once upon a time...");
    ///
    /// let ptr = story.as_ptr();
    /// let len = story.len();
    /// let capacity = story.capacity();
    ///
    /// // story has nineteen bytes
    /// assert_eq!(19, len);
    ///
    /// // Now that we have our parts, we throw the story away.
    /// mem::forget(story);
    ///
    /// // We can re-build a String out of ptr, len, and capacity. This is all
    /// // unsafe because we are responsible for making sure the components are
    /// // valid:
    /// let s = unsafe { String::from_raw_parts(ptr as *mut _, len, capacity) } ;
    ///
    /// assert_eq!(String::from("Once upon a time..."), s);
    /// ```
    ///
    /// [`as_ptr`]: #method.as_ptr
    /// [`len`]: #method.len
    /// [`capacity`]: #method.capacity
    ///
    /// If a `String` has enough capacity, adding elements to it will not
    /// re-allocate. For example, consider this program:
    ///
    /// ```
    /// let mut s = String::new();
    ///
    /// println!("{}", s.capacity());
    ///
    /// for _ in 0..5 {
    ///     s.push_str("hello");
    ///     println!("{}", s.capacity());
    /// }
    /// ```
    ///
    /// This will output the following:
    ///
    /// ```text
    /// 0
    /// 5
    /// 10
    /// 20
    /// 20
    /// 40
    /// ```
    ///
    /// At first, we have no memory allocated at all, but as we append to the
    /// string, it increases its capacity appropriately. If we instead use the
    /// [`with_capacity`] method to allocate the correct capacity initially:
    ///
    /// ```
    /// let mut s = String::with_capacity(25);
    ///
    /// println!("{}", s.capacity());
    ///
    /// for _ in 0..5 {
    ///     s.push_str("hello");
    ///     println!("{}", s.capacity());
    /// }
    /// ```
    ///
    /// [`with_capacity`]: #method.with_capacity
    ///
    /// We end up with a different output:
    ///
    /// ```text
    /// 25
    /// 25
    /// 25
    /// 25
    /// 25
    /// 25
    /// ```
    ///
    /// Here, there's no need to allocate more memory inside the loop.
    ///
    /// [`&str`]: ../../std/primitive.str.html
    /// [`Deref`]: ../../std/ops/trait.Deref.html
    /// [`as_str()`]: struct.String.html#method.as_str
    #[derive(PartialOrd, Eq, Ord)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct String {
        vec: Vec<u8>,
    }

    /// A possible error value when converting a `String` from a UTF-8 byte vector.
    ///
    /// This type is the error type for the [`from_utf8`] method on [`String`]. It
    /// is designed in such a way to carefully avoid reallocations: the
    /// [`into_bytes`] method will give back the byte vector that was used in the
    /// conversion attempt.
    ///
    /// [`from_utf8`]: struct.String.html#method.from_utf8
    /// [`String`]: struct.String.html
    /// [`into_bytes`]: struct.FromUtf8Error.html#method.into_bytes
    ///
    /// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
    /// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
    /// an analogue to `FromUtf8Error`, and you can get one from a `FromUtf8Error`
    /// through the [`utf8_error`] method.
    ///
    /// [`Utf8Error`]: ../../std/str/struct.Utf8Error.html
    /// [`std::str`]: ../../std/str/index.html
    /// [`u8`]: ../../std/primitive.u8.html
    /// [`&str`]: ../../std/primitive.str.html
    /// [`utf8_error`]: #method.utf8_error
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// // some invalid bytes, in a vector
    /// let bytes = vec![0, 159];
    ///
    /// let value = String::from_utf8(bytes);
    ///
    /// assert!(value.is_err());
    /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Debug)]
    pub struct FromUtf8Error {
        bytes: Vec<u8>,
        error: Utf8Error,
    }

    /// A possible error value when converting a `String` from a UTF-16 byte slice.
    ///
    /// This type is the error type for the [`from_utf16`] method on [`String`].
    ///
    /// [`from_utf16`]: struct.String.html#method.from_utf16
    /// [`String`]: struct.String.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// // 𝄞mu<invalid>ic
    /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
    ///           0xD800, 0x0069, 0x0063];
    ///
    /// assert!(String::from_utf16(v).is_err());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Debug)]
    pub struct FromUtf16Error(());

    impl String {
        /// Creates a new empty `String`.
        ///
        /// Given that the `String` is empty, this will not allocate any initial
        /// buffer. While that means that this initial operation is very
        /// inexpensive, it may cause excessive allocation later when you add
        /// data. If you have an idea of how much data the `String` will hold,
        /// consider the [`with_capacity`] method to prevent excessive
        /// re-allocation.
        ///
        /// [`with_capacity`]: #method.with_capacity
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s = String::new();
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_const_unstable(feature = "const_string_new")]
        pub const fn new() -> String {
            String { vec: Vec::new() }
        }

        /// Creates a new empty `String` with a particular capacity.
        ///
        /// `String`s have an internal buffer to hold their data. The capacity is
        /// the length of that buffer, and can be queried with the [`capacity`]
        /// method. This method creates an empty `String`, but one with an initial
        /// buffer that can hold `capacity` bytes. This is useful when you may be
        /// appending a bunch of data to the `String`, reducing the number of
        /// reallocations it needs to do.
        ///
        /// [`capacity`]: #method.capacity
        ///
        /// If the given capacity is `0`, no allocation will occur, and this method
        /// is identical to the [`new`] method.
        ///
        /// [`new`]: #method.new
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::with_capacity(10);
        ///
        /// // The String contains no chars, even though it has capacity for more
        /// assert_eq!(s.len(), 0);
        ///
        /// // These are all done without reallocating...
        /// let cap = s.capacity();
        /// for _ in 0..10 {
        ///     s.push('a');
        /// }
        ///
        /// assert_eq!(s.capacity(), cap);
        ///
        /// // ...but this may make the vector reallocate
        /// s.push('a');
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn with_capacity(capacity: usize) -> String {
            String { vec: Vec::with_capacity(capacity) }
        }

        // HACK(japaric): with cfg(test) the inherent `[T]::to_vec` method, which is
        // required for this method definition, is not available. Since we don't
        // require this method for testing purposes, I'll just stub it
        // NB see the slice::hack module in slice.rs for more information
        #[inline]
        #[cfg(test)]
        pub fn from_str(_: &str) -> String {
            panic!("not available with cfg(test)");
        }

        /// Converts a vector of bytes to a `String`.
        ///
        /// A string slice ([`&str`]) is made of bytes ([`u8`]), and a vector of bytes
        /// ([`Vec<u8>`]) is made of bytes, so this function converts between the
        /// two. Not all byte slices are valid `String`s, however: `String`
        /// requires that it is valid UTF-8. `from_utf8()` checks to ensure that
        /// the bytes are valid UTF-8, and then does the conversion.
        ///
        /// If you are sure that the byte slice is valid UTF-8, and you don't want
        /// to incur the overhead of the validity check, there is an unsafe version
        /// of this function, [`from_utf8_unchecked`], which has the same behavior
        /// but skips the check.
        ///
        /// This method will take care to not copy the vector, for efficiency's
        /// sake.
        ///
        /// If you need a [`&str`] instead of a `String`, consider
        /// [`str::from_utf8`].
        ///
        /// The inverse of this method is [`as_bytes`].
        ///
        /// # Errors
        ///
        /// Returns [`Err`] if the slice is not UTF-8 with a description as to why the
        /// provided bytes are not UTF-8. The vector you moved in is also included.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // some bytes, in a vector
        /// let sparkle_heart = vec![240, 159, 146, 150];
        ///
        /// // We know these bytes are valid, so we'll use `unwrap()`.
        /// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
        ///
        /// assert_eq!("💖", sparkle_heart);
        /// ```
        ///
        /// Incorrect bytes:
        ///
        /// ```
        /// // some invalid bytes, in a vector
        /// let sparkle_heart = vec![0, 159, 146, 150];
        ///
        /// assert!(String::from_utf8(sparkle_heart).is_err());
        /// ```
        ///
        /// See the docs for [`FromUtf8Error`] for more details on what you can do
        /// with this error.
        ///
        /// [`from_utf8_unchecked`]: struct.String.html#method.from_utf8_unchecked
        /// [`&str`]: ../../std/primitive.str.html
        /// [`u8`]: ../../std/primitive.u8.html
        /// [`Vec<u8>`]: ../../std/vec/struct.Vec.html
        /// [`str::from_utf8`]: ../../std/str/fn.from_utf8.html
        /// [`as_bytes`]: struct.String.html#method.as_bytes
        /// [`FromUtf8Error`]: struct.FromUtf8Error.html
        /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error> {
            match str::from_utf8(&vec) {
                Ok(..) => Ok(String { vec }),
                Err(e) => {
                    Err(FromUtf8Error {
                        bytes: vec,
                        error: e,
                    })
                }
            }
        }

        /// Converts a slice of bytes to a string, including invalid characters.
        ///
        /// Strings are made of bytes ([`u8`]), and a slice of bytes
        /// ([`&[u8]`][byteslice]) is made of bytes, so this function converts
        /// between the two. Not all byte slices are valid strings, however: strings
        /// are required to be valid UTF-8. During this conversion,
        /// `from_utf8_lossy()` will replace any invalid UTF-8 sequences with
        /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD], which looks like this: �
        ///
        /// [`u8`]: ../../std/primitive.u8.html
        /// [byteslice]: ../../std/primitive.slice.html
        /// [U+FFFD]: ../char/constant.REPLACEMENT_CHARACTER.html
        ///
        /// If you are sure that the byte slice is valid UTF-8, and you don't want
        /// to incur the overhead of the conversion, there is an unsafe version
        /// of this function, [`from_utf8_unchecked`], which has the same behavior
        /// but skips the checks.
        ///
        /// [`from_utf8_unchecked`]: struct.String.html#method.from_utf8_unchecked
        ///
        /// This function returns a [`Cow<'a, str>`]. If our byte slice is invalid
        /// UTF-8, then we need to insert the replacement characters, which will
        /// change the size of the string, and hence, require a `String`. But if
        /// it's already valid UTF-8, we don't need a new allocation. This return
        /// type allows us to handle both cases.
        ///
        /// [`Cow<'a, str>`]: ../../std/borrow/enum.Cow.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // some bytes, in a vector
        /// let sparkle_heart = vec![240, 159, 146, 150];
        ///
        /// let sparkle_heart = String::from_utf8_lossy(&sparkle_heart);
        ///
        /// assert_eq!("💖", sparkle_heart);
        /// ```
        ///
        /// Incorrect bytes:
        ///
        /// ```
        /// // some invalid bytes
        /// let input = b"Hello \xF0\x90\x80World";
        /// let output = String::from_utf8_lossy(input);
        ///
        /// assert_eq!("Hello �World", output);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn from_utf8_lossy<'a>(v: &'a [u8]) -> Cow<'a, str> {
            let mut iter = lossy::Utf8Lossy::from_bytes(v).chunks();

            let (first_valid, first_broken) = if let Some(chunk) = iter.next() {
                let lossy::Utf8LossyChunk { valid, broken } = chunk;
                if valid.len() == v.len() {
                    debug_assert!(broken.is_empty());
                    return Cow::Borrowed(valid);
                }
                (valid, broken)
            } else {
                return Cow::Borrowed("");
            };

            const REPLACEMENT: &str = "\u{FFFD}";

            let mut res = String::with_capacity(v.len());
            res.push_str(first_valid);
            if !first_broken.is_empty() {
                res.push_str(REPLACEMENT);
            }

            for lossy::Utf8LossyChunk { valid, broken } in iter {
                res.push_str(valid);
                if !broken.is_empty() {
                    res.push_str(REPLACEMENT);
                }
            }

            Cow::Owned(res)
        }

        /// Decode a UTF-16 encoded vector `v` into a `String`, returning [`Err`]
        /// if `v` contains any invalid data.
        ///
        /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // 𝄞music
        /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
        ///           0x0073, 0x0069, 0x0063];
        /// assert_eq!(String::from("𝄞music"),
        ///            String::from_utf16(v).unwrap());
        ///
        /// // 𝄞mu<invalid>ic
        /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
        ///           0xD800, 0x0069, 0x0063];
        /// assert!(String::from_utf16(v).is_err());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error> {
            // This isn't done via collect::<Result<_, _>>() for performance reasons.
            // FIXME: the function can be simplified again when #48994 is closed.
            let mut ret = String::with_capacity(v.len());
            for c in decode_utf16(v.iter().cloned()) {
                if let Ok(c) = c {
                    ret.push(c);
                } else {
                    return Err(FromUtf16Error(()));
                }
            }
            Ok(ret)
        }

        /// Decode a UTF-16 encoded slice `v` into a `String`, replacing
        /// invalid data with [the replacement character (`U+FFFD`)][U+FFFD].
        ///
        /// Unlike [`from_utf8_lossy`] which returns a [`Cow<'a, str>`],
        /// `from_utf16_lossy` returns a `String` since the UTF-16 to UTF-8
        /// conversion requires a memory allocation.
        ///
        /// [`from_utf8_lossy`]: #method.from_utf8_lossy
        /// [`Cow<'a, str>`]: ../borrow/enum.Cow.html
        /// [U+FFFD]: ../char/constant.REPLACEMENT_CHARACTER.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // 𝄞mus<invalid>ic<invalid>
        /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
        ///           0x0073, 0xDD1E, 0x0069, 0x0063,
        ///           0xD834];
        ///
        /// assert_eq!(String::from("𝄞mus\u{FFFD}ic\u{FFFD}"),
        ///            String::from_utf16_lossy(v));
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn from_utf16_lossy(v: &[u16]) -> String {
            decode_utf16(v.iter().cloned()).map(|r| r.unwrap_or(REPLACEMENT_CHARACTER)).collect()
        }

        /// Creates a new `String` from a length, capacity, and pointer.
        ///
        /// # Safety
        ///
        /// This is highly unsafe, due to the number of invariants that aren't
        /// checked:
        ///
        /// * The memory at `ptr` needs to have been previously allocated by the
        ///   same allocator the standard library uses.
        /// * `length` needs to be less than or equal to `capacity`.
        /// * `capacity` needs to be the correct value.
        ///
        /// Violating these may cause problems like corrupting the allocator's
        /// internal data structures.
        ///
        /// The ownership of `ptr` is effectively transferred to the
        /// `String` which may then deallocate, reallocate or change the
        /// contents of memory pointed to by the pointer at will. Ensure
        /// that nothing else uses the pointer after calling this
        /// function.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// use std::mem;
        ///
        /// unsafe {
        ///     let s = String::from("hello");
        ///     let ptr = s.as_ptr();
        ///     let len = s.len();
        ///     let capacity = s.capacity();
        ///
        ///     mem::forget(s);
        ///
        ///     let s = String::from_raw_parts(ptr as *mut _, len, capacity);
        ///
        ///     assert_eq!(String::from("hello"), s);
        /// }
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub unsafe fn from_raw_parts(buf: *mut u8, length: usize, capacity: usize) -> String {
            String { vec: Vec::from_raw_parts(buf, length, capacity) }
        }

        /// Converts a vector of bytes to a `String` without checking that the
        /// string contains valid UTF-8.
        ///
        /// See the safe version, [`from_utf8`], for more details.
        ///
        /// [`from_utf8`]: struct.String.html#method.from_utf8
        ///
        /// # Safety
        ///
        /// This function is unsafe because it does not check that the bytes passed
        /// to it are valid UTF-8. If this constraint is violated, it may cause
        /// memory unsafety issues with future users of the `String`, as the rest of
        /// the standard library assumes that `String`s are valid UTF-8.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // some bytes, in a vector
        /// let sparkle_heart = vec![240, 159, 146, 150];
        ///
        /// let sparkle_heart = unsafe {
        ///     String::from_utf8_unchecked(sparkle_heart)
        /// };
        ///
        /// assert_eq!("💖", sparkle_heart);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String {
            String { vec: bytes }
        }

        /// Converts a `String` into a byte vector.
        ///
        /// This consumes the `String`, so we do not need to copy its contents.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s = String::from("hello");
        /// let bytes = s.into_bytes();
        ///
        /// assert_eq!(&[104, 101, 108, 108, 111][..], &bytes[..]);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_bytes(self) -> Vec<u8> {
            self.vec
        }

        /// Extracts a string slice containing the entire `String`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s = String::from("foo");
        ///
        /// assert_eq!("foo", s.as_str());
        /// ```
        #[inline]
        #[stable(feature = "string_as_str", since = "1.7.0")]
        pub fn as_str(&self) -> &str {
            self
        }

        /// Converts a `String` into a mutable string slice.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("foobar");
        /// let s_mut_str = s.as_mut_str();
        ///
        /// s_mut_str.make_ascii_uppercase();
        ///
        /// assert_eq!("FOOBAR", s_mut_str);
        /// ```
        #[inline]
        #[stable(feature = "string_as_str", since = "1.7.0")]
        pub fn as_mut_str(&mut self) -> &mut str {
            self
        }

        /// Appends a given string slice onto the end of this `String`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("foo");
        ///
        /// s.push_str("bar");
        ///
        /// assert_eq!("foobar", s);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn push_str(&mut self, string: &str) {
            self.vec.extend_from_slice(string.as_bytes())
        }

        /// Returns this `String`'s capacity, in bytes.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s = String::with_capacity(10);
        ///
        /// assert!(s.capacity() >= 10);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn capacity(&self) -> usize {
            self.vec.capacity()
        }

        /// Ensures that this `String`'s capacity is at least `additional` bytes
        /// larger than its length.
        ///
        /// The capacity may be increased by more than `additional` bytes if it
        /// chooses, to prevent frequent reallocations.
        ///
        /// If you do not want this "at least" behavior, see the [`reserve_exact`]
        /// method.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows [`usize`].
        ///
        /// [`reserve_exact`]: struct.String.html#method.reserve_exact
        /// [`usize`]: ../../std/primitive.usize.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::new();
        ///
        /// s.reserve(10);
        ///
        /// assert!(s.capacity() >= 10);
        /// ```
        ///
        /// This may not actually increase the capacity:
        ///
        /// ```
        /// let mut s = String::with_capacity(10);
        /// s.push('a');
        /// s.push('b');
        ///
        /// // s now has a length of 2 and a capacity of 10
        /// assert_eq!(2, s.len());
        /// assert_eq!(10, s.capacity());
        ///
        /// // Since we already have an extra 8 capacity, calling this...
        /// s.reserve(8);
        ///
        /// // ... doesn't actually increase.
        /// assert_eq!(10, s.capacity());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn reserve(&mut self, additional: usize) {
            self.vec.reserve(additional)
        }

        /// Ensures that this `String`'s capacity is `additional` bytes
        /// larger than its length.
        ///
        /// Consider using the [`reserve`] method unless you absolutely know
        /// better than the allocator.
        ///
        /// [`reserve`]: #method.reserve
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::new();
        ///
        /// s.reserve_exact(10);
        ///
        /// assert!(s.capacity() >= 10);
        /// ```
        ///
        /// This may not actually increase the capacity:
        ///
        /// ```
        /// let mut s = String::with_capacity(10);
        /// s.push('a');
        /// s.push('b');
        ///
        /// // s now has a length of 2 and a capacity of 10
        /// assert_eq!(2, s.len());
        /// assert_eq!(10, s.capacity());
        ///
        /// // Since we already have an extra 8 capacity, calling this...
        /// s.reserve_exact(8);
        ///
        /// // ... doesn't actually increase.
        /// assert_eq!(10, s.capacity());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn reserve_exact(&mut self, additional: usize) {
            self.vec.reserve_exact(additional)
        }

        /// Tries to reserve capacity for at least `additional` more elements to be inserted
        /// in the given `String`. The collection may reserve more space to avoid
        /// frequent reallocations. After calling `reserve`, capacity will be
        /// greater than or equal to `self.len() + additional`. Does nothing if
        /// capacity is already sufficient.
        ///
        /// # Errors
        ///
        /// If the capacity overflows, or the allocator reports a failure, then an error
        /// is returned.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(try_reserve)]
        /// use std::collections::CollectionAllocErr;
        ///
        /// fn process_data(data: &str) -> Result<String, CollectionAllocErr> {
        ///     let mut output = String::new();
        ///
        ///     // Pre-reserve the memory, exiting if we can't
        ///     output.try_reserve(data.len())?;
        ///
        ///     // Now we know this can't OOM in the middle of our complex work
        ///     output.push_str(data);
        ///
        ///     Ok(output)
        /// }
        /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
        /// ```
        #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
        pub fn try_reserve(&mut self, additional: usize) -> Result<(), CollectionAllocErr> {
            self.vec.try_reserve(additional)
        }

        /// Tries to reserves the minimum capacity for exactly `additional` more elements to
        /// be inserted in the given `String`. After calling `reserve_exact`,
        /// capacity will be greater than or equal to `self.len() + additional`.
        /// Does nothing if the capacity is already sufficient.
        ///
        /// Note that the allocator may give the collection more space than it
        /// requests. Therefore, capacity can not be relied upon to be precisely
        /// minimal. Prefer `reserve` if future insertions are expected.
        ///
        /// # Errors
        ///
        /// If the capacity overflows, or the allocator reports a failure, then an error
        /// is returned.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(try_reserve)]
        /// use std::collections::CollectionAllocErr;
        ///
        /// fn process_data(data: &str) -> Result<String, CollectionAllocErr> {
        ///     let mut output = String::new();
        ///
        ///     // Pre-reserve the memory, exiting if we can't
        ///     output.try_reserve(data.len())?;
        ///
        ///     // Now we know this can't OOM in the middle of our complex work
        ///     output.push_str(data);
        ///
        ///     Ok(output)
        /// }
        /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
        /// ```
        #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
        pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), CollectionAllocErr>  {
            self.vec.try_reserve_exact(additional)
        }

        /// Shrinks the capacity of this `String` to match its length.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("foo");
        ///
        /// s.reserve(100);
        /// assert!(s.capacity() >= 100);
        ///
        /// s.shrink_to_fit();
        /// assert_eq!(3, s.capacity());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn shrink_to_fit(&mut self) {
            self.vec.shrink_to_fit()
        }

        /// Shrinks the capacity of this `String` with a lower bound.
        ///
        /// The capacity will remain at least as large as both the length
        /// and the supplied value.
        ///
        /// Panics if the current capacity is smaller than the supplied
        /// minimum capacity.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(shrink_to)]
        /// let mut s = String::from("foo");
        ///
        /// s.reserve(100);
        /// assert!(s.capacity() >= 100);
        ///
        /// s.shrink_to(10);
        /// assert!(s.capacity() >= 10);
        /// s.shrink_to(0);
        /// assert!(s.capacity() >= 3);
        /// ```
        #[inline]
        #[unstable(feature = "shrink_to", reason = "new API", issue="56431")]
        pub fn shrink_to(&mut self, min_capacity: usize) {
            self.vec.shrink_to(min_capacity)
        }

        /// Appends the given [`char`] to the end of this `String`.
        ///
        /// [`char`]: ../../std/primitive.char.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("abc");
        ///
        /// s.push('1');
        /// s.push('2');
        /// s.push('3');
        ///
        /// assert_eq!("abc123", s);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn push(&mut self, ch: char) {
            match ch.len_utf8() {
                1 => self.vec.push(ch as u8),
                _ => self.vec.extend_from_slice(ch.encode_utf8(&mut [0; 4]).as_bytes()),
            }
        }

        /// Returns a byte slice of this `String`'s contents.
        ///
        /// The inverse of this method is [`from_utf8`].
        ///
        /// [`from_utf8`]: #method.from_utf8
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s = String::from("hello");
        ///
        /// assert_eq!(&[104, 101, 108, 108, 111], s.as_bytes());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn as_bytes(&self) -> &[u8] {
            &self.vec
        }

        /// Shortens this `String` to the specified length.
        ///
        /// If `new_len` is greater than the string's current length, this has no
        /// effect.
        ///
        /// Note that this method has no effect on the allocated capacity
        /// of the string
        ///
        /// # Panics
        ///
        /// Panics if `new_len` does not lie on a [`char`] boundary.
        ///
        /// [`char`]: ../../std/primitive.char.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("hello");
        ///
        /// s.truncate(2);
        ///
        /// assert_eq!("he", s);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn truncate(&mut self, new_len: usize) {
            if new_len <= self.len() {
                assert!(self.is_char_boundary(new_len));
                self.vec.truncate(new_len)
            }
        }

        /// Removes the last character from the string buffer and returns it.
        ///
        /// Returns [`None`] if this `String` is empty.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("foo");
        ///
        /// assert_eq!(s.pop(), Some('o'));
        /// assert_eq!(s.pop(), Some('o'));
        /// assert_eq!(s.pop(), Some('f'));
        ///
        /// assert_eq!(s.pop(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn pop(&mut self) -> Option<char> {
            let ch = self.chars().rev().next()?;
            let newlen = self.len() - ch.len_utf8();
            unsafe {
                self.vec.set_len(newlen);
            }
            Some(ch)
        }

        /// Removes a [`char`] from this `String` at a byte position and returns it.
        ///
        /// This is an `O(n)` operation, as it requires copying every element in the
        /// buffer.
        ///
        /// # Panics
        ///
        /// Panics if `idx` is larger than or equal to the `String`'s length,
        /// or if it does not lie on a [`char`] boundary.
        ///
        /// [`char`]: ../../std/primitive.char.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("foo");
        ///
        /// assert_eq!(s.remove(0), 'f');
        /// assert_eq!(s.remove(1), 'o');
        /// assert_eq!(s.remove(0), 'o');
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn remove(&mut self, idx: usize) -> char {
            let ch = match self[idx..].chars().next() {
                Some(ch) => ch,
                None => panic!("cannot remove a char from the end of a string"),
            };

            let next = idx + ch.len_utf8();
            let len = self.len();
            unsafe {
                ptr::copy(self.vec.as_ptr().add(next),
                          self.vec.as_mut_ptr().add(idx),
                          len - next);
                self.vec.set_len(len - (next - idx));
            }
            ch
        }

        /// Retains only the characters specified by the predicate.
        ///
        /// In other words, remove all characters `c` such that `f(c)` returns `false`.
        /// This method operates in place and preserves the order of the retained
        /// characters.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut s = String::from("f_o_ob_ar");
        ///
        /// s.retain(|c| c != '_');
        ///
        /// assert_eq!(s, "foobar");
        /// ```
        #[inline]
        #[stable(feature = "string_retain", since = "1.26.0")]
        pub fn retain<F>(&mut self, mut f: F)
            where F: FnMut(char) -> bool
        {
            let len = self.len();
            let mut del_bytes = 0;
            let mut idx = 0;

            while idx < len {
                let ch = unsafe {
                    self.get_unchecked(idx..len).chars().next().unwrap()
                };
                let ch_len = ch.len_utf8();

                if !f(ch) {
                    del_bytes += ch_len;
                } else if del_bytes > 0 {
                    unsafe {
                        ptr::copy(self.vec.as_ptr().add(idx),
                                  self.vec.as_mut_ptr().add(idx - del_bytes),
                                  ch_len);
                    }
                }

                // Point idx to the next char
                idx += ch_len;
            }

            if del_bytes > 0 {
                unsafe { self.vec.set_len(len - del_bytes); }
            }
        }

        /// Inserts a character into this `String` at a byte position.
        ///
        /// This is an `O(n)` operation as it requires copying every element in the
        /// buffer.
        ///
        /// # Panics
        ///
        /// Panics if `idx` is larger than the `String`'s length, or if it does not
        /// lie on a [`char`] boundary.
        ///
        /// [`char`]: ../../std/primitive.char.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::with_capacity(3);
        ///
        /// s.insert(0, 'f');
        /// s.insert(1, 'o');
        /// s.insert(2, 'o');
        ///
        /// assert_eq!("foo", s);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn insert(&mut self, idx: usize, ch: char) {
            assert!(self.is_char_boundary(idx));
            let mut bits = [0; 4];
            let bits = ch.encode_utf8(&mut bits).as_bytes();

            unsafe {
                self.insert_bytes(idx, bits);
            }
        }

        unsafe fn insert_bytes(&mut self, idx: usize, bytes: &[u8]) {
            let len = self.len();
            let amt = bytes.len();
            self.vec.reserve(amt);

            ptr::copy(self.vec.as_ptr().add(idx),
                      self.vec.as_mut_ptr().add(idx + amt),
                      len - idx);
            ptr::copy(bytes.as_ptr(),
                      self.vec.as_mut_ptr().add(idx),
                      amt);
            self.vec.set_len(len + amt);
        }

        /// Inserts a string slice into this `String` at a byte position.
        ///
        /// This is an `O(n)` operation as it requires copying every element in the
        /// buffer.
        ///
        /// # Panics
        ///
        /// Panics if `idx` is larger than the `String`'s length, or if it does not
        /// lie on a [`char`] boundary.
        ///
        /// [`char`]: ../../std/primitive.char.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("bar");
        ///
        /// s.insert_str(0, "foo");
        ///
        /// assert_eq!("foobar", s);
        /// ```
        #[inline]
        #[stable(feature = "insert_str", since = "1.16.0")]
        pub fn insert_str(&mut self, idx: usize, string: &str) {
            assert!(self.is_char_boundary(idx));

            unsafe {
                self.insert_bytes(idx, string.as_bytes());
            }
        }

        /// Returns a mutable reference to the contents of this `String`.
        ///
        /// # Safety
        ///
        /// This function is unsafe because it does not check that the bytes passed
        /// to it are valid UTF-8. If this constraint is violated, it may cause
        /// memory unsafety issues with future users of the `String`, as the rest of
        /// the standard library assumes that `String`s are valid UTF-8.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("hello");
        ///
        /// unsafe {
        ///     let vec = s.as_mut_vec();
        ///     assert_eq!(&[104, 101, 108, 108, 111][..], &vec[..]);
        ///
        ///     vec.reverse();
        /// }
        /// assert_eq!(s, "olleh");
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8> {
            &mut self.vec
        }

        /// Returns the length of this `String`, in bytes.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let a = String::from("foo");
        ///
        /// assert_eq!(a.len(), 3);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn len(&self) -> usize {
            self.vec.len()
        }

        /// Returns `true` if this `String` has a length of zero, and `false` otherwise.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut v = String::new();
        /// assert!(v.is_empty());
        ///
        /// v.push('a');
        /// assert!(!v.is_empty());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_empty(&self) -> bool {
            self.len() == 0
        }

        /// Splits the string into two at the given index.
        ///
        /// Returns a newly allocated `String`. `self` contains bytes `[0, at)`, and
        /// the returned `String` contains bytes `[at, len)`. `at` must be on the
        /// boundary of a UTF-8 code point.
        ///
        /// Note that the capacity of `self` does not change.
        ///
        /// # Panics
        ///
        /// Panics if `at` is not on a `UTF-8` code point boundary, or if it is beyond the last
        /// code point of the string.
        ///
        /// # Examples
        ///
        /// ```
        /// # fn main() {
        /// let mut hello = String::from("Hello, World!");
        /// let world = hello.split_off(7);
        /// assert_eq!(hello, "Hello, ");
        /// assert_eq!(world, "World!");
        /// # }
        /// ```
        #[inline]
        #[stable(feature = "string_split_off", since = "1.16.0")]
        pub fn split_off(&mut self, at: usize) -> String {
            assert!(self.is_char_boundary(at));
            let other = self.vec.split_off(at);
            unsafe { String::from_utf8_unchecked(other) }
        }

        /// Truncates this `String`, removing all contents.
        ///
        /// While this means the `String` will have a length of zero, it does not
        /// touch its capacity.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("foo");
        ///
        /// s.clear();
        ///
        /// assert!(s.is_empty());
        /// assert_eq!(0, s.len());
        /// assert_eq!(3, s.capacity());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn clear(&mut self) {
            self.vec.clear()
        }

        /// Creates a draining iterator that removes the specified range in the `String`
        /// and yields the removed `chars`.
        ///
        /// Note: The element range is removed even if the iterator is not
        /// consumed until the end.
        ///
        /// # Panics
        ///
        /// Panics if the starting point or end point do not lie on a [`char`]
        /// boundary, or if they're out of bounds.
        ///
        /// [`char`]: ../../std/primitive.char.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("α is alpha, β is beta");
        /// let beta_offset = s.find('β').unwrap_or(s.len());
        ///
        /// // Remove the range up until the β from the string
        /// let t: String = s.drain(..beta_offset).collect();
        /// assert_eq!(t, "α is alpha, ");
        /// assert_eq!(s, "β is beta");
        ///
        /// // A full range clears the string
        /// s.drain(..);
        /// assert_eq!(s, "");
        /// ```
        #[stable(feature = "drain", since = "1.6.0")]
        pub fn drain<R>(&mut self, range: R) -> Drain<'_>
            where R: RangeBounds<usize>
        {
            // Memory safety
            //
            // The String version of Drain does not have the memory safety issues
            // of the vector version. The data is just plain bytes.
            // Because the range removal happens in Drop, if the Drain iterator is leaked,
            // the removal will not happen.
            let len = self.len();
            let start = match range.start_bound() {
                Included(&n) => n,
                Excluded(&n) => n + 1,
                Unbounded => 0,
            };
            let end = match range.end_bound() {
                Included(&n) => n + 1,
                Excluded(&n) => n,
                Unbounded => len,
            };

            // Take out two simultaneous borrows. The &mut String won't be accessed
            // until iteration is over, in Drop.
            let self_ptr = self as *mut _;
            // slicing does the appropriate bounds checks
            let chars_iter = self[start..end].chars();

            Drain {
                start,
                end,
                iter: chars_iter,
                string: self_ptr,
            }
        }

        /// Removes the specified range in the string,
        /// and replaces it with the given string.
        /// The given string doesn't need to be the same length as the range.
        ///
        /// # Panics
        ///
        /// Panics if the starting point or end point do not lie on a [`char`]
        /// boundary, or if they're out of bounds.
        ///
        /// [`char`]: ../../std/primitive.char.html
        /// [`Vec::splice`]: ../../std/vec/struct.Vec.html#method.splice
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = String::from("α is alpha, β is beta");
        /// let beta_offset = s.find('β').unwrap_or(s.len());
        ///
        /// // Replace the range up until the β from the string
        /// s.replace_range(..beta_offset, "Α is capital alpha; ");
        /// assert_eq!(s, "Α is capital alpha; β is beta");
        /// ```
        #[stable(feature = "splice", since = "1.27.0")]
        pub fn replace_range<R>(&mut self, range: R, replace_with: &str)
            where R: RangeBounds<usize>
        {
            // Memory safety
            //
            // Replace_range does not have the memory safety issues of a vector Splice.
            // of the vector version. The data is just plain bytes.

            match range.start_bound() {
                 Included(&n) => assert!(self.is_char_boundary(n)),
                 Excluded(&n) => assert!(self.is_char_boundary(n + 1)),
                 Unbounded => {},
            };
            match range.end_bound() {
                 Included(&n) => assert!(self.is_char_boundary(n + 1)),
                 Excluded(&n) => assert!(self.is_char_boundary(n)),
                 Unbounded => {},
            };

            unsafe {
                self.as_mut_vec()
            }.splice(range, replace_with.bytes());
        }

        /// Converts this `String` into a [`Box`]`<`[`str`]`>`.
        ///
        /// This will drop any excess capacity.
        ///
        /// [`Box`]: ../../std/boxed/struct.Box.html
        /// [`str`]: ../../std/primitive.str.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s = String::from("hello");
        ///
        /// let b = s.into_boxed_str();
        /// ```
        #[stable(feature = "box_str", since = "1.4.0")]
        #[inline]
        pub fn into_boxed_str(self) -> Box<str> {
            let slice = self.vec.into_boxed_slice();
            unsafe { from_boxed_utf8_unchecked(slice) }
        }
    }

    impl FromUtf8Error {
        /// Returns a slice of [`u8`]s bytes that were attempted to convert to a `String`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // some invalid bytes, in a vector
        /// let bytes = vec![0, 159];
        ///
        /// let value = String::from_utf8(bytes);
        ///
        /// assert_eq!(&[0, 159], value.unwrap_err().as_bytes());
        /// ```
        #[stable(feature = "from_utf8_error_as_bytes", since = "1.26.0")]
        pub fn as_bytes(&self) -> &[u8] {
            &self.bytes[..]
        }

        /// Returns the bytes that were attempted to convert to a `String`.
        ///
        /// This method is carefully constructed to avoid allocation. It will
        /// consume the error, moving out the bytes, so that a copy of the bytes
        /// does not need to be made.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // some invalid bytes, in a vector
        /// let bytes = vec![0, 159];
        ///
        /// let value = String::from_utf8(bytes);
        ///
        /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_bytes(self) -> Vec<u8> {
            self.bytes
        }

        /// Fetch a `Utf8Error` to get more details about the conversion failure.
        ///
        /// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
        /// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
        /// an analogue to `FromUtf8Error`. See its documentation for more details
        /// on using it.
        ///
        /// [`Utf8Error`]: ../../std/str/struct.Utf8Error.html
        /// [`std::str`]: ../../std/str/index.html
        /// [`u8`]: ../../std/primitive.u8.html
        /// [`&str`]: ../../std/primitive.str.html
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // some invalid bytes, in a vector
        /// let bytes = vec![0, 159];
        ///
        /// let error = String::from_utf8(bytes).unwrap_err().utf8_error();
        ///
        /// // the first byte is invalid here
        /// assert_eq!(1, error.valid_up_to());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn utf8_error(&self) -> Utf8Error {
            self.error
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Display for FromUtf8Error {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(&self.error, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Display for FromUtf16Error {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt("invalid utf-16: lone surrogate found", f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Clone for String {
        fn clone(&self) -> Self {
            String { vec: self.vec.clone() }
        }

        fn clone_from(&mut self, source: &Self) {
            self.vec.clone_from(&source.vec);
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl FromIterator<char> for String {
        fn from_iter<I: IntoIterator<Item = char>>(iter: I) -> String {
            let mut buf = String::new();
            buf.extend(iter);
            buf
        }
    }

    #[stable(feature = "string_from_iter_by_ref", since = "1.17.0")]
    impl<'a> FromIterator<&'a char> for String {
        fn from_iter<I: IntoIterator<Item = &'a char>>(iter: I) -> String {
            let mut buf = String::new();
            buf.extend(iter);
            buf
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a> FromIterator<&'a str> for String {
        fn from_iter<I: IntoIterator<Item = &'a str>>(iter: I) -> String {
            let mut buf = String::new();
            buf.extend(iter);
            buf
        }
    }

    #[stable(feature = "extend_string", since = "1.4.0")]
    impl FromIterator<String> for String {
        fn from_iter<I: IntoIterator<Item = String>>(iter: I) -> String {
            let mut iterator = iter.into_iter();

            // Because we're iterating over `String`s, we can avoid at least
            // one allocation by getting the first string from the iterator
            // and appending to it all the subsequent strings.
            match iterator.next() {
                None => String::new(),
                Some(mut buf) => {
                    buf.extend(iterator);
                    buf
                }
            }
        }
    }

    #[stable(feature = "herd_cows", since = "1.19.0")]
    impl<'a> FromIterator<Cow<'a, str>> for String {
        fn from_iter<I: IntoIterator<Item = Cow<'a, str>>>(iter: I) -> String {
            let mut iterator = iter.into_iter();

            // Because we're iterating over CoWs, we can (potentially) avoid at least
            // one allocation by getting the first item and appending to it all the
            // subsequent items.
            match iterator.next() {
                None => String::new(),
                Some(cow) => {
                    let mut buf = cow.into_owned();
                    buf.extend(iterator);
                    buf
                }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Extend<char> for String {
        fn extend<I: IntoIterator<Item = char>>(&mut self, iter: I) {
            let iterator = iter.into_iter();
            let (lower_bound, _) = iterator.size_hint();
            self.reserve(lower_bound);
            iterator.for_each(move |c| self.push(c));
        }
    }

    #[stable(feature = "extend_ref", since = "1.2.0")]
    impl<'a> Extend<&'a char> for String {
        fn extend<I: IntoIterator<Item = &'a char>>(&mut self, iter: I) {
            self.extend(iter.into_iter().cloned());
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a> Extend<&'a str> for String {
        fn extend<I: IntoIterator<Item = &'a str>>(&mut self, iter: I) {
            iter.into_iter().for_each(move |s| self.push_str(s));
        }
    }

    #[stable(feature = "extend_string", since = "1.4.0")]
    impl Extend<String> for String {
        fn extend<I: IntoIterator<Item = String>>(&mut self, iter: I) {
            iter.into_iter().for_each(move |s| self.push_str(&s));
        }
    }

    #[stable(feature = "herd_cows", since = "1.19.0")]
    impl<'a> Extend<Cow<'a, str>> for String {
        fn extend<I: IntoIterator<Item = Cow<'a, str>>>(&mut self, iter: I) {
            iter.into_iter().for_each(move |s| self.push_str(&s));
        }
    }

    /// A convenience impl that delegates to the impl for `&str`
    #[unstable(feature = "pattern",
               reason = "API not fully fleshed out and ready to be stabilized",
               issue = "27721")]
    impl<'a, 'b> Pattern<'a> for &'b String {
        type Searcher = <&'b str as Pattern<'a>>::Searcher;

        fn into_searcher(self, haystack: &'a str) -> <&'b str as Pattern<'a>>::Searcher {
            self[..].into_searcher(haystack)
        }

        #[inline]
        fn is_contained_in(self, haystack: &'a str) -> bool {
            self[..].is_contained_in(haystack)
        }

        #[inline]
        fn is_prefix_of(self, haystack: &'a str) -> bool {
            self[..].is_prefix_of(haystack)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl PartialEq for String {
        #[inline]
        fn eq(&self, other: &String) -> bool {
            PartialEq::eq(&self[..], &other[..])
        }
        #[inline]
        fn ne(&self, other: &String) -> bool {
            PartialEq::ne(&self[..], &other[..])
        }
    }

    macro_rules! impl_eq {
        ($lhs:ty, $rhs: ty) => {
            #[stable(feature = "rust1", since = "1.0.0")]
            impl<'a, 'b> PartialEq<$rhs> for $lhs {
                #[inline]
                fn eq(&self, other: &$rhs) -> bool { PartialEq::eq(&self[..], &other[..]) }
                #[inline]
                fn ne(&self, other: &$rhs) -> bool { PartialEq::ne(&self[..], &other[..]) }
            }

            #[stable(feature = "rust1", since = "1.0.0")]
            impl<'a, 'b> PartialEq<$lhs> for $rhs {
                #[inline]
                fn eq(&self, other: &$lhs) -> bool { PartialEq::eq(&self[..], &other[..]) }
                #[inline]
                fn ne(&self, other: &$lhs) -> bool { PartialEq::ne(&self[..], &other[..]) }
            }

        }
    }

    impl_eq! { String, str }
    impl_eq! { String, &'a str }
    impl_eq! { Cow<'a, str>, str }
    impl_eq! { Cow<'a, str>, &'b str }
    impl_eq! { Cow<'a, str>, String }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Default for String {
        /// Creates an empty `String`.
        #[inline]
        fn default() -> String {
            String::new()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Display for String {
        #[inline]
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(&**self, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Debug for String {
        #[inline]
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Debug::fmt(&**self, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl hash::Hash for String {
        #[inline]
        fn hash<H: hash::Hasher>(&self, hasher: &mut H) {
            (**self).hash(hasher)
        }
    }

    /// Implements the `+` operator for concatenating two strings.
    ///
    /// This consumes the `String` on the left-hand side and re-uses its buffer (growing it if
    /// necessary). This is done to avoid allocating a new `String` and copying the entire contents on
    /// every operation, which would lead to `O(n^2)` running time when building an `n`-byte string by
    /// repeated concatenation.
    ///
    /// The string on the right-hand side is only borrowed; its contents are copied into the returned
    /// `String`.
    ///
    /// # Examples
    ///
    /// Concatenating two `String`s takes the first by value and borrows the second:
    ///
    /// ```
    /// let a = String::from("hello");
    /// let b = String::from(" world");
    /// let c = a + &b;
    /// // `a` is moved and can no longer be used here.
    /// ```
    ///
    /// If you want to keep using the first `String`, you can clone it and append to the clone instead:
    ///
    /// ```
    /// let a = String::from("hello");
    /// let b = String::from(" world");
    /// let c = a.clone() + &b;
    /// // `a` is still valid here.
    /// ```
    ///
    /// Concatenating `&str` slices can be done by converting the first to a `String`:
    ///
    /// ```
    /// let a = "hello";
    /// let b = " world";
    /// let c = a.to_string() + b;
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    impl Add<&str> for String {
        type Output = String;

        #[inline]
        fn add(mut self, other: &str) -> String {
            self.push_str(other);
            self
        }
    }

    /// Implements the `+=` operator for appending to a `String`.
    ///
    /// This has the same behavior as the [`push_str`][String::push_str] method.
    #[stable(feature = "stringaddassign", since = "1.12.0")]
    impl AddAssign<&str> for String {
        #[inline]
        fn add_assign(&mut self, other: &str) {
            self.push_str(other);
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl ops::Index<ops::Range<usize>> for String {
        type Output = str;

        #[inline]
        fn index(&self, index: ops::Range<usize>) -> &str {
            &self[..][index]
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl ops::Index<ops::RangeTo<usize>> for String {
        type Output = str;

        #[inline]
        fn index(&self, index: ops::RangeTo<usize>) -> &str {
            &self[..][index]
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl ops::Index<ops::RangeFrom<usize>> for String {
        type Output = str;

        #[inline]
        fn index(&self, index: ops::RangeFrom<usize>) -> &str {
            &self[..][index]
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl ops::Index<ops::RangeFull> for String {
        type Output = str;

        #[inline]
        fn index(&self, _index: ops::RangeFull) -> &str {
            unsafe { str::from_utf8_unchecked(&self.vec) }
        }
    }
    #[stable(feature = "inclusive_range", since = "1.26.0")]
    impl ops::Index<ops::RangeInclusive<usize>> for String {
        type Output = str;

        #[inline]
        fn index(&self, index: ops::RangeInclusive<usize>) -> &str {
            Index::index(&**self, index)
        }
    }
    #[stable(feature = "inclusive_range", since = "1.26.0")]
    impl ops::Index<ops::RangeToInclusive<usize>> for String {
        type Output = str;

        #[inline]
        fn index(&self, index: ops::RangeToInclusive<usize>) -> &str {
            Index::index(&**self, index)
        }
    }

    #[stable(feature = "derefmut_for_string", since = "1.3.0")]
    impl ops::IndexMut<ops::Range<usize>> for String {
        #[inline]
        fn index_mut(&mut self, index: ops::Range<usize>) -> &mut str {
            &mut self[..][index]
        }
    }
    #[stable(feature = "derefmut_for_string", since = "1.3.0")]
    impl ops::IndexMut<ops::RangeTo<usize>> for String {
        #[inline]
        fn index_mut(&mut self, index: ops::RangeTo<usize>) -> &mut str {
            &mut self[..][index]
        }
    }
    #[stable(feature = "derefmut_for_string", since = "1.3.0")]
    impl ops::IndexMut<ops::RangeFrom<usize>> for String {
        #[inline]
        fn index_mut(&mut self, index: ops::RangeFrom<usize>) -> &mut str {
            &mut self[..][index]
        }
    }
    #[stable(feature = "derefmut_for_string", since = "1.3.0")]
    impl ops::IndexMut<ops::RangeFull> for String {
        #[inline]
        fn index_mut(&mut self, _index: ops::RangeFull) -> &mut str {
            unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
        }
    }
    #[stable(feature = "inclusive_range", since = "1.26.0")]
    impl ops::IndexMut<ops::RangeInclusive<usize>> for String {
        #[inline]
        fn index_mut(&mut self, index: ops::RangeInclusive<usize>) -> &mut str {
            IndexMut::index_mut(&mut **self, index)
        }
    }
    #[stable(feature = "inclusive_range", since = "1.26.0")]
    impl ops::IndexMut<ops::RangeToInclusive<usize>> for String {
        #[inline]
        fn index_mut(&mut self, index: ops::RangeToInclusive<usize>) -> &mut str {
            IndexMut::index_mut(&mut **self, index)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl ops::Deref for String {
        type Target = str;

        #[inline]
        fn deref(&self) -> &str {
            unsafe { str::from_utf8_unchecked(&self.vec) }
        }
    }

    #[stable(feature = "derefmut_for_string", since = "1.3.0")]
    impl ops::DerefMut for String {
        #[inline]
        fn deref_mut(&mut self) -> &mut str {
            unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
        }
    }

    /// An error when parsing a `String`.
    ///
    /// This `enum` is slightly awkward: it will never actually exist. This error is
    /// part of the type signature of the implementation of [`FromStr`] on
    /// [`String`]. The return type of [`from_str`], requires that an error be
    /// defined, but, given that a [`String`] can always be made into a new
    /// [`String`] without error, this type will never actually be returned. As
    /// such, it is only here to satisfy said signature, and is useless otherwise.
    ///
    /// [`FromStr`]: ../../std/str/trait.FromStr.html
    /// [`String`]: struct.String.html
    /// [`from_str`]: ../../std/str/trait.FromStr.html#tymethod.from_str
    #[stable(feature = "str_parse_error", since = "1.5.0")]
    pub type ParseError = core::convert::Infallible;

    #[stable(feature = "rust1", since = "1.0.0")]
    impl FromStr for String {
        type Err = core::convert::Infallible;
        #[inline]
        fn from_str(s: &str) -> Result<String, ParseError> {
            Ok(String::from(s))
        }
    }

    /// A trait for converting a value to a `String`.
    ///
    /// This trait is automatically implemented for any type which implements the
    /// [`Display`] trait. As such, `ToString` shouldn't be implemented directly:
    /// [`Display`] should be implemented instead, and you get the `ToString`
    /// implementation for free.
    ///
    /// [`Display`]: ../../std/fmt/trait.Display.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait ToString {
        /// Converts the given value to a `String`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let i = 5;
        /// let five = String::from("5");
        ///
        /// assert_eq!(five, i.to_string());
        /// ```
        #[rustc_conversion_suggestion]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn to_string(&self) -> String;
    }

    /// # Panics
    ///
    /// In this implementation, the `to_string` method panics
    /// if the `Display` implementation returns an error.
    /// This indicates an incorrect `Display` implementation
    /// since `fmt::Write for String` never returns an error itself.
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: fmt::Display + ?Sized> ToString for T {
        #[inline]
        default fn to_string(&self) -> String {
            use fmt::Write;
            let mut buf = String::new();
            buf.write_fmt(format_args!("{}", self))
               .expect("a Display implementation returned an error unexpectedly");
            buf.shrink_to_fit();
            buf
        }
    }

    #[stable(feature = "str_to_string_specialization", since = "1.9.0")]
    impl ToString for str {
        #[inline]
        fn to_string(&self) -> String {
            String::from(self)
        }
    }

    #[stable(feature = "cow_str_to_string_specialization", since = "1.17.0")]
    impl ToString for Cow<'_, str> {
        #[inline]
        fn to_string(&self) -> String {
            self[..].to_owned()
        }
    }

    #[stable(feature = "string_to_string_specialization", since = "1.17.0")]
    impl ToString for String {
        #[inline]
        fn to_string(&self) -> String {
            self.to_owned()
        }
    }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl AsRef<[u8]> for String {
        #[inline]
        fn as_ref(&self) -> &[u8] {
            self.as_bytes()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a> From<&'a str> for String {
        #[inline]
        fn from(s: &'a str) -> String {
            s.to_owned()
        }
    }

    // note: test pulls in libstd, which causes errors here
    #[cfg(not(test))]
    #[stable(feature = "string_from_box", since = "1.18.0")]
    impl From<Box<str>> for String {
        /// Converts the given boxed `str` slice to a `String`.
        /// It is notable that the `str` slice is owned.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s1: String = String::from("hello world");
        /// let s2: Box<str> = s1.into_boxed_str();
        /// let s3: String = String::from(s2);
        ///
        /// assert_eq!("hello world", s3)
        /// ```
        fn from(s: Box<str>) -> String {
            s.into_string()
        }
    }

    #[stable(feature = "box_from_str", since = "1.20.0")]
    impl From<String> for Box<str> {
        /// Converts the given `String` to a boxed `str` slice that is owned.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s1: String = String::from("hello world");
        /// let s2: Box<str> = Box::from(s1);
        /// let s3: String = String::from(s2);
        ///
        /// assert_eq!("hello world", s3)
        /// ```
        fn from(s: String) -> Box<str> {
            s.into_boxed_str()
        }
    }

    #[stable(feature = "string_from_cow_str", since = "1.14.0")]
    impl<'a> From<Cow<'a, str>> for String {
        fn from(s: Cow<'a, str>) -> String {
            s.into_owned()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a> From<&'a str> for Cow<'a, str> {
        #[inline]
        fn from(s: &'a str) -> Cow<'a, str> {
            Cow::Borrowed(s)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a> From<String> for Cow<'a, str> {
        #[inline]
        fn from(s: String) -> Cow<'a, str> {
            Cow::Owned(s)
        }
    }

    #[stable(feature = "cow_from_string_ref", since = "1.28.0")]
    impl<'a> From<&'a String> for Cow<'a, str> {
        #[inline]
        fn from(s: &'a String) -> Cow<'a, str> {
            Cow::Borrowed(s.as_str())
        }
    }

    #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
    impl<'a> FromIterator<char> for Cow<'a, str> {
        fn from_iter<I: IntoIterator<Item = char>>(it: I) -> Cow<'a, str> {
            Cow::Owned(FromIterator::from_iter(it))
        }
    }

    #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
    impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str> {
        fn from_iter<I: IntoIterator<Item = &'b str>>(it: I) -> Cow<'a, str> {
            Cow::Owned(FromIterator::from_iter(it))
        }
    }

    #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
    impl<'a> FromIterator<String> for Cow<'a, str> {
        fn from_iter<I: IntoIterator<Item = String>>(it: I) -> Cow<'a, str> {
            Cow::Owned(FromIterator::from_iter(it))
        }
    }

    #[stable(feature = "from_string_for_vec_u8", since = "1.14.0")]
    impl From<String> for Vec<u8> {
        /// Converts the given `String` to a vector `Vec` that holds values of type `u8`.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s1 = String::from("hello world");
        /// let v1 = Vec::from(s1);
        ///
        /// for b in v1 {
        ///     println!("{}", b);
        /// }
        /// ```
        fn from(string: String) -> Vec<u8> {
            string.into_bytes()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Write for String {
        #[inline]
        fn write_str(&mut self, s: &str) -> fmt::Result {
            self.push_str(s);
            Ok(())
        }

        #[inline]
        fn write_char(&mut self, c: char) -> fmt::Result {
            self.push(c);
            Ok(())
        }
    }

    /// A draining iterator for `String`.
    ///
    /// This struct is created by the [`drain`] method on [`String`]. See its
    /// documentation for more.
    ///
    /// [`drain`]: struct.String.html#method.drain
    /// [`String`]: struct.String.html
    #[stable(feature = "drain", since = "1.6.0")]
    pub struct Drain<'a> {
        /// Will be used as &'a mut String in the destructor
        string: *mut String,
        /// Start of part to remove
        start: usize,
        /// End of part to remove
        end: usize,
        /// Current remaining range to remove
        iter: Chars<'a>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl fmt::Debug for Drain<'_> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("Drain { .. }")
        }
    }

    #[stable(feature = "drain", since = "1.6.0")]
    unsafe impl Sync for Drain<'_> {}
    #[stable(feature = "drain", since = "1.6.0")]
    unsafe impl Send for Drain<'_> {}

    #[stable(feature = "drain", since = "1.6.0")]
    impl Drop for Drain<'_> {
        fn drop(&mut self) {
            unsafe {
                // Use Vec::drain. "Reaffirm" the bounds checks to avoid
                // panic code being inserted again.
                let self_vec = (*self.string).as_mut_vec();
                if self.start <= self.end && self.end <= self_vec.len() {
                    self_vec.drain(self.start..self.end);
                }
            }
        }
    }

    #[stable(feature = "drain", since = "1.6.0")]
    impl Iterator for Drain<'_> {
        type Item = char;

        #[inline]
        fn next(&mut self) -> Option<char> {
            self.iter.next()
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    #[stable(feature = "drain", since = "1.6.0")]
    impl DoubleEndedIterator for Drain<'_> {
        #[inline]
        fn next_back(&mut self) -> Option<char> {
            self.iter.next_back()
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl FusedIterator for Drain<'_> {}

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