1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! A UTF-8 encoded, growable string.
13 //! This module contains the [`String`] type, a trait for converting
14 //! [`ToString`]s, and several error types that may result from working with
17 //! [`String`]: struct.String.html
18 //! [`ToString`]: trait.ToString.html
22 //! There are multiple ways to create a new `String` from a string literal:
25 //! let s = "Hello".to_string();
27 //! let s = String::from("world");
28 //! let s: String = "also this".into();
31 //! You can create a new `String` from an existing one by concatenating with
35 //! let s = "Hello".to_string();
37 //! let message = s + " world!";
40 //! If you have a vector of valid UTF-8 bytes, you can make a `String` out of
41 //! it. You can do the reverse too.
44 //! let sparkle_heart = vec![240, 159, 146, 150];
46 //! // We know these bytes are valid, so we'll use `unwrap()`.
47 //! let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
49 //! assert_eq!("💖", sparkle_heart);
51 //! let bytes = sparkle_heart.into_bytes();
53 //! assert_eq!(bytes, [240, 159, 146, 150]);
56 #![stable(feature = "rust1", since = "1.0.0")]
60 use core::iter::FromIterator;
62 use core::ops::{self, Add, AddAssign, Index, IndexMut};
64 use core::str::pattern::Pattern;
65 use rustc_unicode::char::{decode_utf16, REPLACEMENT_CHARACTER};
66 use rustc_unicode::str as unicode_str;
68 use borrow::{Cow, ToOwned};
69 use range::RangeArgument;
70 use str::{self, FromStr, Utf8Error, Chars};
74 /// A UTF-8 encoded, growable string.
76 /// The `String` type is the most common string type that has ownership over the
77 /// contents of the string. It has a close relationship with its borrowed
78 /// counterpart, the primitive [`str`].
80 /// [`str`]: ../../std/primitive.str.html
84 /// You can create a `String` from a literal string with `String::from`:
87 /// let hello = String::from("Hello, world!");
90 /// You can append a [`char`] to a `String` with the [`push()`] method, and
91 /// append a [`&str`] with the [`push_str()`] method:
94 /// let mut hello = String::from("Hello, ");
97 /// hello.push_str("orld!");
100 /// [`char`]: ../../std/primitive.char.html
101 /// [`push()`]: #method.push
102 /// [`push_str()`]: #method.push_str
104 /// If you have a vector of UTF-8 bytes, you can create a `String` from it with
105 /// the [`from_utf8()`] method:
108 /// // some bytes, in a vector
109 /// let sparkle_heart = vec![240, 159, 146, 150];
111 /// // We know these bytes are valid, so we'll use `unwrap()`.
112 /// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
114 /// assert_eq!("💖", sparkle_heart);
117 /// [`from_utf8()`]: #method.from_utf8
121 /// `String`s are always valid UTF-8. This has a few implications, the first of
122 /// which is that if you need a non-UTF-8 string, consider [`OsString`]. It is
123 /// similar, but without the UTF-8 constraint. The second implication is that
124 /// you cannot index into a `String`:
129 /// println!("The first letter of s is {}", s[0]); // ERROR!!!
132 /// [`OsString`]: ../../std/ffi/struct.OsString.html
134 /// Indexing is intended to be a constant-time operation, but UTF-8 encoding
135 /// does not allow us to do this. Furtheremore, it's not clear what sort of
136 /// thing the index should return: a byte, a codepoint, or a grapheme cluster.
137 /// The [`as_bytes()`] and [`chars()`] methods return iterators over the first
138 /// two, respectively.
140 /// [`as_bytes()`]: #method.as_bytes
141 /// [`chars()`]: #method.chars
145 /// `String`s implement [`Deref`]`<Target=str>`, and so inherit all of [`str`]'s
146 /// methods. In addition, this means that you can pass a `String` to any
147 /// function which takes a [`&str`] by using an ampersand (`&`):
150 /// fn takes_str(s: &str) { }
152 /// let s = String::from("Hello");
157 /// [`&str`]: ../../std/primitive.str.html
158 /// [`Deref`]: ../../std/ops/trait.Deref.html
160 /// This will create a [`&str`] from the `String` and pass it in. This
161 /// conversion is very inexpensive, and so generally, functions will accept
162 /// [`&str`]s as arguments unless they need a `String` for some specific reason.
167 /// A `String` is made up of three components: a pointer to some bytes, a
168 /// length, and a capacity. The pointer points to an internal buffer `String`
169 /// uses to store its data. The length is the number of bytes currently stored
170 /// in the buffer, and the capacity is the size of the buffer in bytes. As such,
171 /// the length will always be less than or equal to the capacity.
173 /// This buffer is always stored on the heap.
175 /// You can look at these with the [`as_ptr()`], [`len()`], and [`capacity()`]
181 /// let story = String::from("Once upon a time...");
183 /// let ptr = story.as_ptr();
184 /// let len = story.len();
185 /// let capacity = story.capacity();
187 /// // story has nineteen bytes
188 /// assert_eq!(19, len);
190 /// // Now that we have our parts, we throw the story away.
191 /// mem::forget(story);
193 /// // We can re-build a String out of ptr, len, and capacity. This is all
194 /// // unsafe because we are responsible for making sure the components are
196 /// let s = unsafe { String::from_raw_parts(ptr as *mut _, len, capacity) } ;
198 /// assert_eq!(String::from("Once upon a time..."), s);
201 /// [`as_ptr()`]: #method.as_ptr
202 /// [`len()`]: #method.len
203 /// [`capacity()`]: #method.capacity
205 /// If a `String` has enough capacity, adding elements to it will not
206 /// re-allocate. For example, consider this program:
209 /// let mut s = String::new();
211 /// println!("{}", s.capacity());
214 /// s.push_str("hello");
215 /// println!("{}", s.capacity());
219 /// This will output the following:
230 /// At first, we have no memory allocated at all, but as we append to the
231 /// string, it increases its capacity appropriately. If we instead use the
232 /// [`with_capacity()`] method to allocate the correct capacity initially:
235 /// let mut s = String::with_capacity(25);
237 /// println!("{}", s.capacity());
240 /// s.push_str("hello");
241 /// println!("{}", s.capacity());
245 /// [`with_capacity()`]: #method.with_capacity
247 /// We end up with a different output:
258 /// Here, there's no need to allocate more memory inside the loop.
259 #[derive(PartialOrd, Eq, Ord)]
260 #[stable(feature = "rust1", since = "1.0.0")]
265 /// A possible error value when converting a `String` from a UTF-8 byte vector.
267 /// This type is the error type for the [`from_utf8()`] method on [`String`]. It
268 /// is designed in such a way to carefully avoid reallocations: the
269 /// [`into_bytes()`] method will give back the byte vector that was used in the
270 /// conversion attempt.
272 /// [`from_utf8()`]: struct.String.html#method.from_utf8
273 /// [`String`]: struct.String.html
274 /// [`into_bytes()`]: struct.FromUtf8Error.html#method.into_bytes
276 /// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
277 /// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
278 /// an analogue to `FromUtf8Error`, and you can get one from a `FromUtf8Error`
279 /// through the [`utf8_error()`] method.
281 /// [`Utf8Error`]: ../../std/str/struct.Utf8Error.html
282 /// [`std::str`]: ../../std/str/index.html
283 /// [`u8`]: ../../std/primitive.u8.html
284 /// [`&str`]: ../../std/primitive.str.html
285 /// [`utf8_error()`]: #method.utf8_error
292 /// // some invalid bytes, in a vector
293 /// let bytes = vec![0, 159];
295 /// let value = String::from_utf8(bytes);
297 /// assert!(value.is_err());
298 /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
300 #[stable(feature = "rust1", since = "1.0.0")]
302 pub struct FromUtf8Error {
307 /// A possible error value when converting a `String` from a UTF-16 byte slice.
309 /// This type is the error type for the [`from_utf16()`] method on [`String`].
311 /// [`from_utf16()`]: struct.String.html#method.from_utf16
312 /// [`String`]: struct.String.html
319 /// // 𝄞mu<invalid>ic
320 /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
321 /// 0xD800, 0x0069, 0x0063];
323 /// assert!(String::from_utf16(v).is_err());
325 #[stable(feature = "rust1", since = "1.0.0")]
327 pub struct FromUtf16Error(());
330 /// Creates a new empty `String`.
332 /// Given that the `String` is empty, this will not allocate any initial
333 /// buffer. While that means that this initial operation is very
334 /// inexpensive, but may cause excessive allocation later, when you add
335 /// data. If you have an idea of how much data the `String` will hold,
336 /// consider the [`with_capacity()`] method to prevent excessive
339 /// [`with_capacity()`]: #method.with_capacity
346 /// let s = String::new();
349 #[stable(feature = "rust1", since = "1.0.0")]
350 pub fn new() -> String {
351 String { vec: Vec::new() }
354 /// Creates a new empty `String` with a particular capacity.
356 /// `String`s have an internal buffer to hold their data. The capacity is
357 /// the length of that buffer, and can be queried with the [`capacity()`]
358 /// method. This method creates an empty `String`, but one with an initial
359 /// buffer that can hold `capacity` bytes. This is useful when you may be
360 /// appending a bunch of data to the `String`, reducing the number of
361 /// reallocations it needs to do.
363 /// [`capacity()`]: #method.capacity
365 /// If the given capacity is `0`, no allocation will occur, and this method
366 /// is identical to the [`new()`] method.
368 /// [`new()`]: #method.new
375 /// let mut s = String::with_capacity(10);
377 /// // The String contains no chars, even though it has capacity for more
378 /// assert_eq!(s.len(), 0);
380 /// // These are all done without reallocating...
381 /// let cap = s.capacity();
386 /// assert_eq!(s.capacity(), cap);
388 /// // ...but this may make the vector reallocate
392 #[stable(feature = "rust1", since = "1.0.0")]
393 pub fn with_capacity(capacity: usize) -> String {
394 String { vec: Vec::with_capacity(capacity) }
397 // HACK(japaric): with cfg(test) the inherent `[T]::to_vec` method, which is
398 // required for this method definition, is not available. Since we don't
399 // require this method for testing purposes, I'll just stub it
400 // NB see the slice::hack module in slice.rs for more information
403 pub fn from_str(_: &str) -> String {
404 panic!("not available with cfg(test)");
407 /// Converts a vector of bytes to a `String`.
409 /// A string slice ([`&str`]) is made of bytes ([`u8`]), and a vector of bytes
410 /// ([`Vec<u8>`]) is made of bytes, so this function converts between the
411 /// two. Not all byte slices are valid `String`s, however: `String`
412 /// requires that it is valid UTF-8. `from_utf8()` checks to ensure that
413 /// the bytes are valid UTF-8, and then does the conversion.
415 /// [`&str`]: ../../std/primitive.str.html
416 /// [`u8`]: ../../std/primitive.u8.html
417 /// [`Vec<u8>`]: ../../std/vec/struct.Vec.html
419 /// If you are sure that the byte slice is valid UTF-8, and you don't want
420 /// to incur the overhead of the validity check, there is an unsafe version
421 /// of this function, [`from_utf8_unchecked()`], which has the same behavior
422 /// but skips the check.
424 /// [`from_utf8_unchecked()`]: struct.String.html#method.from_utf8_unchecked
426 /// This method will take care to not copy the vector, for efficiency's
429 /// If you need a `&str` instead of a `String`, consider
430 /// [`str::from_utf8()`].
432 /// [`str::from_utf8()`]: ../../std/str/fn.from_utf8.html
436 /// Returns `Err` if the slice is not UTF-8 with a description as to why the
437 /// provided bytes are not UTF-8. The vector you moved in is also included.
444 /// // some bytes, in a vector
445 /// let sparkle_heart = vec![240, 159, 146, 150];
447 /// // We know these bytes are valid, so we'll use `unwrap()`.
448 /// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
450 /// assert_eq!("💖", sparkle_heart);
456 /// // some invalid bytes, in a vector
457 /// let sparkle_heart = vec![0, 159, 146, 150];
459 /// assert!(String::from_utf8(sparkle_heart).is_err());
462 /// See the docs for [`FromUtf8Error`] for more details on what you can do
465 /// [`FromUtf8Error`]: struct.FromUtf8Error.html
467 #[stable(feature = "rust1", since = "1.0.0")]
468 pub fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error> {
469 match str::from_utf8(&vec) {
470 Ok(..) => Ok(String { vec: vec }),
480 /// Converts a slice of bytes to a string, including invalid characters.
482 /// Strings are made of bytes ([`u8`]), and a slice of bytes
483 /// ([`&[u8]`][byteslice]) is made of bytes, so this function converts
484 /// between the two. Not all byte slices are valid strings, however: strings
485 /// are required to be valid UTF-8. During this conversion,
486 /// `from_utf8_lossy()` will replace any invalid UTF-8 sequences with
487 /// `U+FFFD REPLACEMENT CHARACTER`, which looks like this: �
489 /// [`u8`]: ../../std/primitive.u8.html
490 /// [byteslice]: ../../std/primitive.slice.html
492 /// If you are sure that the byte slice is valid UTF-8, and you don't want
493 /// to incur the overhead of the conversion, there is an unsafe version
494 /// of this function, [`from_utf8_unchecked()`], which has the same behavior
495 /// but skips the checks.
497 /// [`from_utf8_unchecked()`]: struct.String.html#method.from_utf8_unchecked
499 /// This function returns a [`Cow<'a, str>`]. If our byte slice is invalid
500 /// UTF-8, then we need to insert the replacement characters, which will
501 /// change the size of the string, and hence, require a `String`. But if
502 /// it's already valid UTF-8, we don't need a new allocation. This return
503 /// type allows us to handle both cases.
505 /// [`Cow<'a, str>`]: ../../std/borrow/enum.Cow.html
512 /// // some bytes, in a vector
513 /// let sparkle_heart = vec![240, 159, 146, 150];
515 /// let sparkle_heart = String::from_utf8_lossy(&sparkle_heart);
517 /// assert_eq!("💖", sparkle_heart);
523 /// // some invalid bytes
524 /// let input = b"Hello \xF0\x90\x80World";
525 /// let output = String::from_utf8_lossy(input);
527 /// assert_eq!("Hello �World", output);
529 #[stable(feature = "rust1", since = "1.0.0")]
530 pub fn from_utf8_lossy<'a>(v: &'a [u8]) -> Cow<'a, str> {
532 match str::from_utf8(v) {
533 Ok(s) => return Cow::Borrowed(s),
534 Err(e) => i = e.valid_up_to(),
537 const TAG_CONT_U8: u8 = 128;
538 const REPLACEMENT: &'static [u8] = b"\xEF\xBF\xBD"; // U+FFFD in UTF-8
540 fn unsafe_get(xs: &[u8], i: usize) -> u8 {
541 unsafe { *xs.get_unchecked(i) }
543 fn safe_get(xs: &[u8], i: usize, total: usize) -> u8 {
551 let mut res = String::with_capacity(total);
554 unsafe { res.as_mut_vec().extend_from_slice(&v[..i]) };
557 // subseqidx is the index of the first byte of the subsequence we're
558 // looking at. It's used to copy a bunch of contiguous good codepoints
559 // at once instead of copying them one by one.
560 let mut subseqidx = i;
564 let byte = unsafe_get(v, i);
567 macro_rules! error { () => ({
570 res.as_mut_vec().extend_from_slice(&v[subseqidx..i_]);
573 res.as_mut_vec().extend_from_slice(REPLACEMENT);
578 // subseqidx handles this
580 let w = unicode_str::utf8_char_width(byte);
584 if safe_get(v, i, total) & 192 != TAG_CONT_U8 {
591 match (byte, safe_get(v, i, total)) {
592 (0xE0, 0xA0...0xBF) => (),
593 (0xE1...0xEC, 0x80...0xBF) => (),
594 (0xED, 0x80...0x9F) => (),
595 (0xEE...0xEF, 0x80...0xBF) => (),
602 if safe_get(v, i, total) & 192 != TAG_CONT_U8 {
609 match (byte, safe_get(v, i, total)) {
610 (0xF0, 0x90...0xBF) => (),
611 (0xF1...0xF3, 0x80...0xBF) => (),
612 (0xF4, 0x80...0x8F) => (),
619 if safe_get(v, i, total) & 192 != TAG_CONT_U8 {
624 if safe_get(v, i, total) & 192 != TAG_CONT_U8 {
637 if subseqidx < total {
638 unsafe { res.as_mut_vec().extend_from_slice(&v[subseqidx..total]) };
643 /// Decode a UTF-16 encoded vector `v` into a `String`, returning `Err`
644 /// if `v` contains any invalid data.
652 /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
653 /// 0x0073, 0x0069, 0x0063];
654 /// assert_eq!(String::from("𝄞music"),
655 /// String::from_utf16(v).unwrap());
657 /// // 𝄞mu<invalid>ic
658 /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
659 /// 0xD800, 0x0069, 0x0063];
660 /// assert!(String::from_utf16(v).is_err());
662 #[stable(feature = "rust1", since = "1.0.0")]
663 pub fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error> {
664 decode_utf16(v.iter().cloned()).collect::<Result<_, _>>().map_err(|_| FromUtf16Error(()))
667 /// Decode a UTF-16 encoded vector `v` into a string, replacing
668 /// invalid data with the replacement character (U+FFFD).
675 /// // 𝄞mus<invalid>ic<invalid>
676 /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
677 /// 0x0073, 0xDD1E, 0x0069, 0x0063,
680 /// assert_eq!(String::from("𝄞mus\u{FFFD}ic\u{FFFD}"),
681 /// String::from_utf16_lossy(v));
684 #[stable(feature = "rust1", since = "1.0.0")]
685 pub fn from_utf16_lossy(v: &[u16]) -> String {
686 decode_utf16(v.iter().cloned()).map(|r| r.unwrap_or(REPLACEMENT_CHARACTER)).collect()
689 /// Creates a new `String` from a length, capacity, and pointer.
693 /// This is highly unsafe, due to the number of invariants that aren't
696 /// * The memory at `ptr` needs to have been previously allocated by the
697 /// same allocator the standard library uses.
698 /// * `length` needs to be less than or equal to `capacity`.
699 /// * `capacity` needs to be the correct value.
701 /// Violating these may cause problems like corrupting the allocator's
702 /// internal datastructures.
712 /// let s = String::from("hello");
713 /// let ptr = s.as_ptr();
714 /// let len = s.len();
715 /// let capacity = s.capacity();
719 /// let s = String::from_raw_parts(ptr as *mut _, len, capacity);
721 /// assert_eq!(String::from("hello"), s);
725 #[stable(feature = "rust1", since = "1.0.0")]
726 pub unsafe fn from_raw_parts(buf: *mut u8, length: usize, capacity: usize) -> String {
727 String { vec: Vec::from_raw_parts(buf, length, capacity) }
730 /// Converts a vector of bytes to a `String` without checking that the
731 /// string contains valid UTF-8.
733 /// See the safe version, [`from_utf8()`], for more details.
735 /// [`from_utf8()`]: struct.String.html#method.from_utf8
739 /// This function is unsafe because it does not check that the bytes passed
740 /// to it are valid UTF-8. If this constraint is violated, it may cause
741 /// memory unsafety issues with future users of the `String`, as the rest of
742 /// the standard library assumes that `String`s are valid UTF-8.
749 /// // some bytes, in a vector
750 /// let sparkle_heart = vec![240, 159, 146, 150];
752 /// let sparkle_heart = unsafe {
753 /// String::from_utf8_unchecked(sparkle_heart)
756 /// assert_eq!("💖", sparkle_heart);
759 #[stable(feature = "rust1", since = "1.0.0")]
760 pub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String {
761 String { vec: bytes }
764 /// Converts a `String` into a byte vector.
766 /// This consumes the `String`, so we do not need to copy its contents.
773 /// let s = String::from("hello");
774 /// let bytes = s.into_bytes();
776 /// assert_eq!(&[104, 101, 108, 108, 111][..], &bytes[..]);
779 #[stable(feature = "rust1", since = "1.0.0")]
780 pub fn into_bytes(self) -> Vec<u8> {
784 /// Extracts a string slice containing the entire string.
786 #[stable(feature = "string_as_str", since = "1.7.0")]
787 pub fn as_str(&self) -> &str {
791 /// Extracts a string slice containing the entire string.
793 #[stable(feature = "string_as_str", since = "1.7.0")]
794 pub fn as_mut_str(&mut self) -> &mut str {
798 /// Appends a given string slice onto the end of this `String`.
805 /// let mut s = String::from("foo");
807 /// s.push_str("bar");
809 /// assert_eq!("foobar", s);
812 #[stable(feature = "rust1", since = "1.0.0")]
813 pub fn push_str(&mut self, string: &str) {
814 self.vec.extend_from_slice(string.as_bytes())
817 /// Returns this `String`'s capacity, in bytes.
824 /// let s = String::with_capacity(10);
826 /// assert!(s.capacity() >= 10);
829 #[stable(feature = "rust1", since = "1.0.0")]
830 pub fn capacity(&self) -> usize {
834 /// Ensures that this `String`'s capacity is at least `additional` bytes
835 /// larger than its length.
837 /// The capacity may be increased by more than `additional` bytes if it
838 /// chooses, to prevent frequent reallocations.
840 /// If you do not want this "at least" behavior, see the [`reserve_exact()`]
843 /// [`reserve_exact()`]: #method.reserve_exact
847 /// Panics if the new capacity overflows `usize`.
854 /// let mut s = String::new();
858 /// assert!(s.capacity() >= 10);
861 /// This may not actually increase the capacity:
864 /// let mut s = String::with_capacity(10);
868 /// // s now has a length of 2 and a capacity of 10
869 /// assert_eq!(2, s.len());
870 /// assert_eq!(10, s.capacity());
872 /// // Since we already have an extra 8 capacity, calling this...
875 /// // ... doesn't actually increase.
876 /// assert_eq!(10, s.capacity());
879 #[stable(feature = "rust1", since = "1.0.0")]
880 pub fn reserve(&mut self, additional: usize) {
881 self.vec.reserve(additional)
884 /// Ensures that this `String`'s capacity is `additional` bytes
885 /// larger than its length.
887 /// Consider using the [`reserve()`] method unless you absolutely know
888 /// better than the allocator.
890 /// [`reserve()`]: #method.reserve
894 /// Panics if the new capacity overflows `usize`.
901 /// let mut s = String::new();
903 /// s.reserve_exact(10);
905 /// assert!(s.capacity() >= 10);
908 /// This may not actually increase the capacity:
911 /// let mut s = String::with_capacity(10);
915 /// // s now has a length of 2 and a capacity of 10
916 /// assert_eq!(2, s.len());
917 /// assert_eq!(10, s.capacity());
919 /// // Since we already have an extra 8 capacity, calling this...
920 /// s.reserve_exact(8);
922 /// // ... doesn't actually increase.
923 /// assert_eq!(10, s.capacity());
926 #[stable(feature = "rust1", since = "1.0.0")]
927 pub fn reserve_exact(&mut self, additional: usize) {
928 self.vec.reserve_exact(additional)
931 /// Shrinks the capacity of this `String` to match its length.
938 /// let mut s = String::from("foo");
941 /// assert!(s.capacity() >= 100);
943 /// s.shrink_to_fit();
944 /// assert_eq!(3, s.capacity());
947 #[stable(feature = "rust1", since = "1.0.0")]
948 pub fn shrink_to_fit(&mut self) {
949 self.vec.shrink_to_fit()
952 /// Appends the given `char` to the end of this `String`.
959 /// let mut s = String::from("abc");
965 /// assert_eq!("abc123", s);
968 #[stable(feature = "rust1", since = "1.0.0")]
969 pub fn push(&mut self, ch: char) {
970 match ch.len_utf8() {
971 1 => self.vec.push(ch as u8),
972 _ => self.vec.extend_from_slice(ch.encode_utf8().as_slice()),
976 /// Returns a byte slice of this `String`'s contents.
983 /// let s = String::from("hello");
985 /// assert_eq!(&[104, 101, 108, 108, 111], s.as_bytes());
988 #[stable(feature = "rust1", since = "1.0.0")]
989 pub fn as_bytes(&self) -> &[u8] {
993 /// Shortens this `String` to the specified length.
995 /// If `new_len` is greater than the string's current length, this has no
1000 /// Panics if `new_len` does not lie on a [`char`] boundary.
1002 /// [`char`]: ../../std/primitive.char.html
1009 /// let mut s = String::from("hello");
1013 /// assert_eq!("he", s);
1016 #[stable(feature = "rust1", since = "1.0.0")]
1017 pub fn truncate(&mut self, new_len: usize) {
1018 if new_len <= self.len() {
1019 assert!(self.is_char_boundary(new_len));
1020 self.vec.truncate(new_len)
1024 /// Removes the last character from the string buffer and returns it.
1026 /// Returns `None` if this `String` is empty.
1033 /// let mut s = String::from("foo");
1035 /// assert_eq!(s.pop(), Some('o'));
1036 /// assert_eq!(s.pop(), Some('o'));
1037 /// assert_eq!(s.pop(), Some('f'));
1039 /// assert_eq!(s.pop(), None);
1042 #[stable(feature = "rust1", since = "1.0.0")]
1043 pub fn pop(&mut self) -> Option<char> {
1044 let ch = match self.chars().rev().next() {
1046 None => return None,
1048 let newlen = self.len() - ch.len_utf8();
1050 self.vec.set_len(newlen);
1055 /// Removes a `char` from this `String` at a byte position and returns it.
1057 /// This is an `O(n)` operation, as it requires copying every element in the
1062 /// Panics if `idx` is larger than or equal to the `String`'s length,
1063 /// or if it does not lie on a [`char`] boundary.
1065 /// [`char`]: ../../std/primitive.char.html
1072 /// let mut s = String::from("foo");
1074 /// assert_eq!(s.remove(0), 'f');
1075 /// assert_eq!(s.remove(1), 'o');
1076 /// assert_eq!(s.remove(0), 'o');
1079 #[stable(feature = "rust1", since = "1.0.0")]
1080 pub fn remove(&mut self, idx: usize) -> char {
1081 let ch = match self[idx..].chars().next() {
1083 None => panic!("cannot remove a char from the end of a string"),
1086 let next = idx + ch.len_utf8();
1087 let len = self.len();
1089 ptr::copy(self.vec.as_ptr().offset(next as isize),
1090 self.vec.as_mut_ptr().offset(idx as isize),
1092 self.vec.set_len(len - (next - idx));
1097 /// Inserts a character into this `String` at a byte position.
1099 /// This is an `O(n)` operation as it requires copying every element in the
1104 /// Panics if `idx` is larger than the `String`'s length, or if it does not
1105 /// lie on a [`char`] boundary.
1107 /// [`char`]: ../../std/primitive.char.html
1114 /// let mut s = String::with_capacity(3);
1116 /// s.insert(0, 'f');
1117 /// s.insert(1, 'o');
1118 /// s.insert(2, 'o');
1120 /// assert_eq!("foo", s);
1123 #[stable(feature = "rust1", since = "1.0.0")]
1124 pub fn insert(&mut self, idx: usize, ch: char) {
1125 let len = self.len();
1126 assert!(idx <= len);
1127 assert!(self.is_char_boundary(idx));
1128 let bits = ch.encode_utf8();
1129 let bits = bits.as_slice();
1130 let amt = bits.len();
1131 self.vec.reserve(amt);
1134 ptr::copy(self.vec.as_ptr().offset(idx as isize),
1135 self.vec.as_mut_ptr().offset((idx + amt) as isize),
1137 ptr::copy(bits.as_ptr(),
1138 self.vec.as_mut_ptr().offset(idx as isize),
1140 self.vec.set_len(len + amt);
1144 /// Returns a mutable reference to the contents of this `String`.
1148 /// This function is unsafe because it does not check that the bytes passed
1149 /// to it are valid UTF-8. If this constraint is violated, it may cause
1150 /// memory unsafety issues with future users of the `String`, as the rest of
1151 /// the standard library assumes that `String`s are valid UTF-8.
1158 /// let mut s = String::from("hello");
1161 /// let vec = s.as_mut_vec();
1162 /// assert_eq!(&[104, 101, 108, 108, 111][..], &vec[..]);
1166 /// assert_eq!(s, "olleh");
1169 #[stable(feature = "rust1", since = "1.0.0")]
1170 pub unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8> {
1174 /// Returns the length of this `String`, in bytes.
1181 /// let a = String::from("foo");
1183 /// assert_eq!(a.len(), 3);
1186 #[stable(feature = "rust1", since = "1.0.0")]
1187 pub fn len(&self) -> usize {
1191 /// Returns `true` if this `String` has a length of zero.
1193 /// Returns `false` otherwise.
1200 /// let mut v = String::new();
1201 /// assert!(v.is_empty());
1204 /// assert!(!v.is_empty());
1207 #[stable(feature = "rust1", since = "1.0.0")]
1208 pub fn is_empty(&self) -> bool {
1212 /// Truncates this `String`, removing all contents.
1214 /// While this means the `String` will have a length of zero, it does not
1215 /// touch its capacity.
1222 /// let mut s = String::from("foo");
1226 /// assert!(s.is_empty());
1227 /// assert_eq!(0, s.len());
1228 /// assert_eq!(3, s.capacity());
1231 #[stable(feature = "rust1", since = "1.0.0")]
1232 pub fn clear(&mut self) {
1236 /// Create a draining iterator that removes the specified range in the string
1237 /// and yields the removed chars.
1239 /// Note: The element range is removed even if the iterator is not
1240 /// consumed until the end.
1244 /// Panics if the starting point or end point do not lie on a [`char`]
1245 /// boundary, or if they're out of bounds.
1247 /// [`char`]: ../../std/primitive.char.html
1254 /// let mut s = String::from("α is alpha, β is beta");
1255 /// let beta_offset = s.find('β').unwrap_or(s.len());
1257 /// // Remove the range up until the β from the string
1258 /// let t: String = s.drain(..beta_offset).collect();
1259 /// assert_eq!(t, "α is alpha, ");
1260 /// assert_eq!(s, "β is beta");
1262 /// // A full range clears the string
1264 /// assert_eq!(s, "");
1266 #[stable(feature = "drain", since = "1.6.0")]
1267 pub fn drain<R>(&mut self, range: R) -> Drain
1268 where R: RangeArgument<usize>
1272 // The String version of Drain does not have the memory safety issues
1273 // of the vector version. The data is just plain bytes.
1274 // Because the range removal happens in Drop, if the Drain iterator is leaked,
1275 // the removal will not happen.
1276 let len = self.len();
1277 let start = *range.start().unwrap_or(&0);
1278 let end = *range.end().unwrap_or(&len);
1280 // Take out two simultaneous borrows. The &mut String won't be accessed
1281 // until iteration is over, in Drop.
1282 let self_ptr = self as *mut _;
1283 // slicing does the appropriate bounds checks
1284 let chars_iter = self[start..end].chars();
1294 /// Converts this `String` into a `Box<str>`.
1296 /// This will drop any excess capacity.
1303 /// let s = String::from("hello");
1305 /// let b = s.into_boxed_str();
1307 #[stable(feature = "box_str", since = "1.4.0")]
1308 pub fn into_boxed_str(self) -> Box<str> {
1309 let slice = self.vec.into_boxed_slice();
1310 unsafe { mem::transmute::<Box<[u8]>, Box<str>>(slice) }
1314 impl FromUtf8Error {
1315 /// Returns the bytes that were attempted to convert to a `String`.
1317 /// This method is carefully constructed to avoid allocation. It will
1318 /// consume the error, moving out the bytes, so that a copy of the bytes
1319 /// does not need to be made.
1326 /// // some invalid bytes, in a vector
1327 /// let bytes = vec![0, 159];
1329 /// let value = String::from_utf8(bytes);
1331 /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
1333 #[stable(feature = "rust1", since = "1.0.0")]
1334 pub fn into_bytes(self) -> Vec<u8> {
1338 /// Fetch a `Utf8Error` to get more details about the conversion failure.
1340 /// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
1341 /// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
1342 /// an analogue to `FromUtf8Error`. See its documentation for more details
1345 /// [`Utf8Error`]: ../../std/str/struct.Utf8Error.html
1346 /// [`std::str`]: ../../std/str/index.html
1347 /// [`u8`]: ../../std/primitive.u8.html
1348 /// [`&str`]: ../../std/primitive.str.html
1355 /// // some invalid bytes, in a vector
1356 /// let bytes = vec![0, 159];
1358 /// let error = String::from_utf8(bytes).unwrap_err().utf8_error();
1360 /// // the first byte is invalid here
1361 /// assert_eq!(1, error.valid_up_to());
1363 #[stable(feature = "rust1", since = "1.0.0")]
1364 pub fn utf8_error(&self) -> Utf8Error {
1369 #[stable(feature = "rust1", since = "1.0.0")]
1370 impl fmt::Display for FromUtf8Error {
1371 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1372 fmt::Display::fmt(&self.error, f)
1376 #[stable(feature = "rust1", since = "1.0.0")]
1377 impl fmt::Display for FromUtf16Error {
1378 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1379 fmt::Display::fmt("invalid utf-16: lone surrogate found", f)
1383 #[stable(feature = "rust1", since = "1.0.0")]
1384 impl Clone for String {
1385 fn clone(&self) -> Self {
1386 String { vec: self.vec.clone() }
1389 fn clone_from(&mut self, source: &Self) {
1390 self.vec.clone_from(&source.vec);
1394 #[stable(feature = "rust1", since = "1.0.0")]
1395 impl FromIterator<char> for String {
1396 fn from_iter<I: IntoIterator<Item = char>>(iter: I) -> String {
1397 let mut buf = String::new();
1403 #[stable(feature = "rust1", since = "1.0.0")]
1404 impl<'a> FromIterator<&'a str> for String {
1405 fn from_iter<I: IntoIterator<Item = &'a str>>(iter: I) -> String {
1406 let mut buf = String::new();
1412 #[stable(feature = "extend_string", since = "1.4.0")]
1413 impl FromIterator<String> for String {
1414 fn from_iter<I: IntoIterator<Item = String>>(iter: I) -> String {
1415 let mut buf = String::new();
1421 #[stable(feature = "rust1", since = "1.0.0")]
1422 impl Extend<char> for String {
1423 fn extend<I: IntoIterator<Item = char>>(&mut self, iter: I) {
1424 let iterator = iter.into_iter();
1425 let (lower_bound, _) = iterator.size_hint();
1426 self.reserve(lower_bound);
1427 for ch in iterator {
1433 #[stable(feature = "extend_ref", since = "1.2.0")]
1434 impl<'a> Extend<&'a char> for String {
1435 fn extend<I: IntoIterator<Item = &'a char>>(&mut self, iter: I) {
1436 self.extend(iter.into_iter().cloned());
1440 #[stable(feature = "rust1", since = "1.0.0")]
1441 impl<'a> Extend<&'a str> for String {
1442 fn extend<I: IntoIterator<Item = &'a str>>(&mut self, iter: I) {
1449 #[stable(feature = "extend_string", since = "1.4.0")]
1450 impl Extend<String> for String {
1451 fn extend<I: IntoIterator<Item = String>>(&mut self, iter: I) {
1458 /// A convenience impl that delegates to the impl for `&str`
1459 #[unstable(feature = "pattern",
1460 reason = "API not fully fleshed out and ready to be stabilized",
1462 impl<'a, 'b> Pattern<'a> for &'b String {
1463 type Searcher = <&'b str as Pattern<'a>>::Searcher;
1465 fn into_searcher(self, haystack: &'a str) -> <&'b str as Pattern<'a>>::Searcher {
1466 self[..].into_searcher(haystack)
1470 fn is_contained_in(self, haystack: &'a str) -> bool {
1471 self[..].is_contained_in(haystack)
1475 fn is_prefix_of(self, haystack: &'a str) -> bool {
1476 self[..].is_prefix_of(haystack)
1480 #[stable(feature = "rust1", since = "1.0.0")]
1481 impl PartialEq for String {
1483 fn eq(&self, other: &String) -> bool {
1484 PartialEq::eq(&self[..], &other[..])
1487 fn ne(&self, other: &String) -> bool {
1488 PartialEq::ne(&self[..], &other[..])
1492 macro_rules! impl_eq {
1493 ($lhs:ty, $rhs: ty) => {
1494 #[stable(feature = "rust1", since = "1.0.0")]
1495 impl<'a, 'b> PartialEq<$rhs> for $lhs {
1497 fn eq(&self, other: &$rhs) -> bool { PartialEq::eq(&self[..], &other[..]) }
1499 fn ne(&self, other: &$rhs) -> bool { PartialEq::ne(&self[..], &other[..]) }
1502 #[stable(feature = "rust1", since = "1.0.0")]
1503 impl<'a, 'b> PartialEq<$lhs> for $rhs {
1505 fn eq(&self, other: &$lhs) -> bool { PartialEq::eq(&self[..], &other[..]) }
1507 fn ne(&self, other: &$lhs) -> bool { PartialEq::ne(&self[..], &other[..]) }
1513 impl_eq! { String, str }
1514 impl_eq! { String, &'a str }
1515 impl_eq! { Cow<'a, str>, str }
1516 impl_eq! { Cow<'a, str>, &'b str }
1517 impl_eq! { Cow<'a, str>, String }
1519 #[stable(feature = "rust1", since = "1.0.0")]
1520 impl Default for String {
1522 fn default() -> String {
1527 #[stable(feature = "rust1", since = "1.0.0")]
1528 impl fmt::Display for String {
1530 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1531 fmt::Display::fmt(&**self, f)
1535 #[stable(feature = "rust1", since = "1.0.0")]
1536 impl fmt::Debug for String {
1538 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1539 fmt::Debug::fmt(&**self, f)
1543 #[stable(feature = "rust1", since = "1.0.0")]
1544 impl hash::Hash for String {
1546 fn hash<H: hash::Hasher>(&self, hasher: &mut H) {
1547 (**self).hash(hasher)
1551 #[stable(feature = "rust1", since = "1.0.0")]
1552 impl<'a> Add<&'a str> for String {
1553 type Output = String;
1556 fn add(mut self, other: &str) -> String {
1557 self.push_str(other);
1562 #[stable(feature = "stringaddassign", since = "1.12.0")]
1563 impl<'a> AddAssign<&'a str> for String {
1565 fn add_assign(&mut self, other: &str) {
1566 self.push_str(other);
1570 #[stable(feature = "rust1", since = "1.0.0")]
1571 impl ops::Index<ops::Range<usize>> for String {
1575 fn index(&self, index: ops::Range<usize>) -> &str {
1579 #[stable(feature = "rust1", since = "1.0.0")]
1580 impl ops::Index<ops::RangeTo<usize>> for String {
1584 fn index(&self, index: ops::RangeTo<usize>) -> &str {
1588 #[stable(feature = "rust1", since = "1.0.0")]
1589 impl ops::Index<ops::RangeFrom<usize>> for String {
1593 fn index(&self, index: ops::RangeFrom<usize>) -> &str {
1597 #[stable(feature = "rust1", since = "1.0.0")]
1598 impl ops::Index<ops::RangeFull> for String {
1602 fn index(&self, _index: ops::RangeFull) -> &str {
1603 unsafe { str::from_utf8_unchecked(&self.vec) }
1606 #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
1607 impl ops::Index<ops::RangeInclusive<usize>> for String {
1611 fn index(&self, index: ops::RangeInclusive<usize>) -> &str {
1612 Index::index(&**self, index)
1615 #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
1616 impl ops::Index<ops::RangeToInclusive<usize>> for String {
1620 fn index(&self, index: ops::RangeToInclusive<usize>) -> &str {
1621 Index::index(&**self, index)
1625 #[stable(feature = "derefmut_for_string", since = "1.2.0")]
1626 impl ops::IndexMut<ops::Range<usize>> for String {
1628 fn index_mut(&mut self, index: ops::Range<usize>) -> &mut str {
1629 &mut self[..][index]
1632 #[stable(feature = "derefmut_for_string", since = "1.2.0")]
1633 impl ops::IndexMut<ops::RangeTo<usize>> for String {
1635 fn index_mut(&mut self, index: ops::RangeTo<usize>) -> &mut str {
1636 &mut self[..][index]
1639 #[stable(feature = "derefmut_for_string", since = "1.2.0")]
1640 impl ops::IndexMut<ops::RangeFrom<usize>> for String {
1642 fn index_mut(&mut self, index: ops::RangeFrom<usize>) -> &mut str {
1643 &mut self[..][index]
1646 #[stable(feature = "derefmut_for_string", since = "1.2.0")]
1647 impl ops::IndexMut<ops::RangeFull> for String {
1649 fn index_mut(&mut self, _index: ops::RangeFull) -> &mut str {
1650 unsafe { mem::transmute(&mut *self.vec) }
1653 #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
1654 impl ops::IndexMut<ops::RangeInclusive<usize>> for String {
1656 fn index_mut(&mut self, index: ops::RangeInclusive<usize>) -> &mut str {
1657 IndexMut::index_mut(&mut **self, index)
1660 #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
1661 impl ops::IndexMut<ops::RangeToInclusive<usize>> for String {
1663 fn index_mut(&mut self, index: ops::RangeToInclusive<usize>) -> &mut str {
1664 IndexMut::index_mut(&mut **self, index)
1668 #[stable(feature = "rust1", since = "1.0.0")]
1669 impl ops::Deref for String {
1673 fn deref(&self) -> &str {
1674 unsafe { str::from_utf8_unchecked(&self.vec) }
1678 #[stable(feature = "derefmut_for_string", since = "1.2.0")]
1679 impl ops::DerefMut for String {
1681 fn deref_mut(&mut self) -> &mut str {
1682 unsafe { mem::transmute(&mut *self.vec) }
1686 /// An error when parsing a `String`.
1688 /// This `enum` is slightly awkward: it will never actually exist. This error is
1689 /// part of the type signature of the implementation of [`FromStr`] on
1690 /// [`String`]. The return type of [`from_str()`], requires that an error be
1691 /// defined, but, given that a [`String`] can always be made into a new
1692 /// [`String`] without error, this type will never actually be returned. As
1693 /// such, it is only here to satisfy said signature, and is useless otherwise.
1695 /// [`FromStr`]: ../../std/str/trait.FromStr.html
1696 /// [`String`]: struct.String.html
1697 /// [`from_str()`]: ../../std/str/trait.FromStr.html#tymethod.from_str
1698 #[stable(feature = "str_parse_error", since = "1.5.0")]
1700 pub enum ParseError {}
1702 #[stable(feature = "rust1", since = "1.0.0")]
1703 impl FromStr for String {
1704 type Err = ParseError;
1706 fn from_str(s: &str) -> Result<String, ParseError> {
1711 #[stable(feature = "str_parse_error", since = "1.5.0")]
1712 impl Clone for ParseError {
1713 fn clone(&self) -> ParseError {
1718 #[stable(feature = "str_parse_error", since = "1.5.0")]
1719 impl fmt::Debug for ParseError {
1720 fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
1725 #[stable(feature = "str_parse_error2", since = "1.8.0")]
1726 impl fmt::Display for ParseError {
1727 fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
1732 #[stable(feature = "str_parse_error", since = "1.5.0")]
1733 impl PartialEq for ParseError {
1734 fn eq(&self, _: &ParseError) -> bool {
1739 #[stable(feature = "str_parse_error", since = "1.5.0")]
1740 impl Eq for ParseError {}
1742 /// A trait for converting a value to a `String`.
1744 /// This trait is automatically implemented for any type which implements the
1745 /// [`Display`] trait. As such, `ToString` shouldn't be implemented directly:
1746 /// [`Display`] should be implemented instead, and you get the `ToString`
1747 /// implementation for free.
1749 /// [`Display`]: ../../std/fmt/trait.Display.html
1750 #[stable(feature = "rust1", since = "1.0.0")]
1751 pub trait ToString {
1752 /// Converts the given value to a `String`.
1760 /// let five = String::from("5");
1762 /// assert_eq!(five, i.to_string());
1764 #[stable(feature = "rust1", since = "1.0.0")]
1765 fn to_string(&self) -> String;
1768 #[stable(feature = "rust1", since = "1.0.0")]
1769 impl<T: fmt::Display + ?Sized> ToString for T {
1771 default fn to_string(&self) -> String {
1772 use core::fmt::Write;
1773 let mut buf = String::new();
1774 let _ = buf.write_fmt(format_args!("{}", self));
1775 buf.shrink_to_fit();
1780 #[stable(feature = "str_to_string_specialization", since = "1.9.0")]
1781 impl ToString for str {
1783 fn to_string(&self) -> String {
1788 #[stable(feature = "rust1", since = "1.0.0")]
1789 impl AsRef<str> for String {
1791 fn as_ref(&self) -> &str {
1796 #[stable(feature = "rust1", since = "1.0.0")]
1797 impl AsRef<[u8]> for String {
1799 fn as_ref(&self) -> &[u8] {
1804 #[stable(feature = "rust1", since = "1.0.0")]
1805 impl<'a> From<&'a str> for String {
1806 fn from(s: &'a str) -> String {
1811 #[stable(feature = "rust1", since = "1.0.0")]
1812 impl<'a> From<&'a str> for Cow<'a, str> {
1814 fn from(s: &'a str) -> Cow<'a, str> {
1819 #[stable(feature = "rust1", since = "1.0.0")]
1820 impl<'a> From<String> for Cow<'a, str> {
1822 fn from(s: String) -> Cow<'a, str> {
1827 #[stable(feature = "rust1", since = "1.0.0")]
1828 impl Into<Vec<u8>> for String {
1829 fn into(self) -> Vec<u8> {
1834 #[stable(feature = "rust1", since = "1.0.0")]
1835 impl fmt::Write for String {
1837 fn write_str(&mut self, s: &str) -> fmt::Result {
1843 fn write_char(&mut self, c: char) -> fmt::Result {
1849 /// A draining iterator for `String`.
1851 /// This struct is created by the [`drain()`] method on [`String`]. See its
1852 /// documentation for more.
1854 /// [`drain()`]: struct.String.html#method.drain
1855 /// [`String`]: struct.String.html
1856 #[stable(feature = "drain", since = "1.6.0")]
1857 pub struct Drain<'a> {
1858 /// Will be used as &'a mut String in the destructor
1859 string: *mut String,
1860 /// Start of part to remove
1862 /// End of part to remove
1864 /// Current remaining range to remove
1868 #[stable(feature = "drain", since = "1.6.0")]
1869 unsafe impl<'a> Sync for Drain<'a> {}
1870 #[stable(feature = "drain", since = "1.6.0")]
1871 unsafe impl<'a> Send for Drain<'a> {}
1873 #[stable(feature = "drain", since = "1.6.0")]
1874 impl<'a> Drop for Drain<'a> {
1875 fn drop(&mut self) {
1877 // Use Vec::drain. "Reaffirm" the bounds checks to avoid
1878 // panic code being inserted again.
1879 let self_vec = (*self.string).as_mut_vec();
1880 if self.start <= self.end && self.end <= self_vec.len() {
1881 self_vec.drain(self.start..self.end);
1887 #[stable(feature = "drain", since = "1.6.0")]
1888 impl<'a> Iterator for Drain<'a> {
1892 fn next(&mut self) -> Option<char> {
1896 fn size_hint(&self) -> (usize, Option<usize>) {
1897 self.iter.size_hint()
1901 #[stable(feature = "drain", since = "1.6.0")]
1902 impl<'a> DoubleEndedIterator for Drain<'a> {
1904 fn next_back(&mut self) -> Option<char> {
1905 self.iter.next_back()