src/liballoc/string.rs

   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.
   4 //
   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.
  10
  11 //! A UTF-8 encoded, growable string.
  12 //!
  13 //! This module contains the [`String`] type, a trait for converting
  14 //! [`ToString`]s, and several error types that may result from working with
  15 //! [`String`]s.
  16 //!
  17 //! [`ToString`]: trait.ToString.html
  18 //!
  19 //! # Examples
  20 //!
  21 //! There are multiple ways to create a new [`String`] from a string literal:
  22 //!
  23 //! ```
  24 //! let s = "Hello".to_string();
  25 //!
  26 //! let s = String::from("world");
  27 //! let s: String = "also this".into();
  28 //! ```
  29 //!
  30 //! You can create a new [`String`] from an existing one by concatenating with
  31 //! `+`:
  32 //!
  33 //! [`String`]: struct.String.html
  34 //!
  35 //! ```
  36 //! let s = "Hello".to_string();
  37 //!
  38 //! let message = s + " world!";
  39 //! ```
  40 //!
  41 //! If you have a vector of valid UTF-8 bytes, you can make a [`String`] out of
  42 //! it. You can do the reverse too.
  43 //!
  44 //! ```
  45 //! let sparkle_heart = vec![240, 159, 146, 150];
  46 //!
  47 //! // We know these bytes are valid, so we'll use `unwrap()`.
  48 //! let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
  49 //!
  50 //! assert_eq!("💖", sparkle_heart);
  51 //!
  52 //! let bytes = sparkle_heart.into_bytes();
  53 //!
  54 //! assert_eq!(bytes, [240, 159, 146, 150]);
  55 //! ```
  56
  57 #![stable(feature = "rust1", since = "1.0.0")]
  58
  59 use core::char::{decode_utf16, REPLACEMENT_CHARACTER};
  60 use core::fmt;
  61 use core::hash;
  62 use core::iter::{FromIterator, FusedIterator};
  63 use core::ops::Bound::{Excluded, Included, Unbounded};
  64 use core::ops::{self, Add, AddAssign, Index, IndexMut, RangeBounds};
  65 use core::ptr;
  66 use core::str::pattern::Pattern;
  67 use core::str::lossy;
  68
  69 use collections::CollectionAllocErr;
  70 use borrow::{Cow, ToOwned};
  71 use boxed::Box;
  72 use str::{self, from_boxed_utf8_unchecked, FromStr, Utf8Error, Chars};
  73 use vec::Vec;
  74
  75 /// A UTF-8 encoded, growable string.
  76 ///
  77 /// The `String` type is the most common string type that has ownership over the
  78 /// contents of the string. It has a close relationship with its borrowed
  79 /// counterpart, the primitive [`str`].
  80 ///
  81 /// [`str`]: ../../std/primitive.str.html
  82 ///
  83 /// # Examples
  84 ///
  85 /// You can create a `String` from a literal string with [`String::from`]:
  86 ///
  87 /// ```
  88 /// let hello = String::from("Hello, world!");
  89 /// ```
  90 ///
  91 /// You can append a [`char`] to a `String` with the [`push`] method, and
  92 /// append a [`&str`] with the [`push_str`] method:
  93 ///
  94 /// ```
  95 /// let mut hello = String::from("Hello, ");
  96 ///
  97 /// hello.push('w');
  98 /// hello.push_str("orld!");
  99 /// ```
 100 ///
 101 /// [`String::from`]: #method.from
 102 /// [`char`]: ../../std/primitive.char.html
 103 /// [`push`]: #method.push
 104 /// [`push_str`]: #method.push_str
 105 ///
 106 /// If you have a vector of UTF-8 bytes, you can create a `String` from it with
 107 /// the [`from_utf8`] method:
 108 ///
 109 /// ```
 110 /// // some bytes, in a vector
 111 /// let sparkle_heart = vec![240, 159, 146, 150];
 112 ///
 113 /// // We know these bytes are valid, so we'll use `unwrap()`.
 114 /// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
 115 ///
 116 /// assert_eq!("💖", sparkle_heart);
 117 /// ```
 118 ///
 119 /// [`from_utf8`]: #method.from_utf8
 120 ///
 121 /// # UTF-8
 122 ///
 123 /// `String`s are always valid UTF-8. This has a few implications, the first of
 124 /// which is that if you need a non-UTF-8 string, consider [`OsString`]. It is
 125 /// similar, but without the UTF-8 constraint. The second implication is that
 126 /// you cannot index into a `String`:
 127 ///
 128 /// ```compile_fail,E0277
 129 /// let s = "hello";
 130 ///
 131 /// println!("The first letter of s is {}", s[0]); // ERROR!!!
 132 /// ```
 133 ///
 134 /// [`OsString`]: ../../std/ffi/struct.OsString.html
 135 ///
 136 /// Indexing is intended to be a constant-time operation, but UTF-8 encoding
 137 /// does not allow us to do this. Furthermore, it's not clear what sort of
 138 /// thing the index should return: a byte, a codepoint, or a grapheme cluster.
 139 /// The [`bytes`] and [`chars`] methods return iterators over the first
 140 /// two, respectively.
 141 ///
 142 /// [`bytes`]: #method.bytes
 143 /// [`chars`]: #method.chars
 144 ///
 145 /// # Deref
 146 ///
 147 /// `String`s implement [`Deref`]`<Target=str>`, and so inherit all of [`str`]'s
 148 /// methods. In addition, this means that you can pass a `String` to a
 149 /// function which takes a [`&str`] by using an ampersand (`&`):
 150 ///
 151 /// ```
 152 /// fn takes_str(s: &str) { }
 153 ///
 154 /// let s = String::from("Hello");
 155 ///
 156 /// takes_str(&s);
 157 /// ```
 158 ///
 159 /// This will create a [`&str`] from the `String` and pass it in. This
 160 /// conversion is very inexpensive, and so generally, functions will accept
 161 /// [`&str`]s as arguments unless they need a `String` for some specific
 162 /// reason.
 163 ///
 164 /// In certain cases Rust doesn't have enough information to make this
 165 /// conversion, known as [`Deref`] coercion. In the following example a string
 166 /// slice [`&'a str`][`&str`] implements the trait `TraitExample`, and the function
 167 /// `example_func` takes anything that implements the trait. In this case Rust
 168 /// would need to make two implicit conversions, which Rust doesn't have the
 169 /// means to do. For that reason, the following example will not compile.
 170 ///
 171 /// ```compile_fail,E0277
 172 /// trait TraitExample {}
 173 ///
 174 /// impl<'a> TraitExample for &'a str {}
 175 ///
 176 /// fn example_func<A: TraitExample>(example_arg: A) {}
 177 ///
 178 /// fn main() {
 179 ///     let example_string = String::from("example_string");
 180 ///     example_func(&example_string);
 181 /// }
 182 /// ```
 183 ///
 184 /// There are two options that would work instead. The first would be to
 185 /// change the line `example_func(&example_string);` to
 186 /// `example_func(example_string.as_str());`, using the method [`as_str()`]
 187 /// to explicitly extract the string slice containing the string. The second
 188 /// way changes `example_func(&example_string);` to
 189 /// `example_func(&*example_string);`. In this case we are dereferencing a
 190 /// `String` to a [`str`][`&str`], then referencing the [`str`][`&str`] back to
 191 /// [`&str`]. The second way is more idiomatic, however both work to do the
 192 /// conversion explicitly rather than relying on the implicit conversion.
 193 ///
 194 /// # Representation
 195 ///
 196 /// A `String` is made up of three components: a pointer to some bytes, a
 197 /// length, and a capacity. The pointer points to an internal buffer `String`
 198 /// uses to store its data. The length is the number of bytes currently stored
 199 /// in the buffer, and the capacity is the size of the buffer in bytes. As such,
 200 /// the length will always be less than or equal to the capacity.
 201 ///
 202 /// This buffer is always stored on the heap.
 203 ///
 204 /// You can look at these with the [`as_ptr`], [`len`], and [`capacity`]
 205 /// methods:
 206 ///
 207 /// ```
 208 /// use std::mem;
 209 ///
 210 /// let story = String::from("Once upon a time...");
 211 ///
 212 /// let ptr = story.as_ptr();
 213 /// let len = story.len();
 214 /// let capacity = story.capacity();
 215 ///
 216 /// // story has nineteen bytes
 217 /// assert_eq!(19, len);
 218 ///
 219 /// // Now that we have our parts, we throw the story away.
 220 /// mem::forget(story);
 221 ///
 222 /// // We can re-build a String out of ptr, len, and capacity. This is all
 223 /// // unsafe because we are responsible for making sure the components are
 224 /// // valid:
 225 /// let s = unsafe { String::from_raw_parts(ptr as *mut _, len, capacity) } ;
 226 ///
 227 /// assert_eq!(String::from("Once upon a time..."), s);
 228 /// ```
 229 ///
 230 /// [`as_ptr`]: #method.as_ptr
 231 /// [`len`]: #method.len
 232 /// [`capacity`]: #method.capacity
 233 ///
 234 /// If a `String` has enough capacity, adding elements to it will not
 235 /// re-allocate. For example, consider this program:
 236 ///
 237 /// ```
 238 /// let mut s = String::new();
 239 ///
 240 /// println!("{}", s.capacity());
 241 ///
 242 /// for _ in 0..5 {
 243 ///     s.push_str("hello");
 244 ///     println!("{}", s.capacity());
 245 /// }
 246 /// ```
 247 ///
 248 /// This will output the following:
 249 ///
 250 /// ```text
 251 /// 0
 252 /// 5
 253 /// 10
 254 /// 20
 255 /// 20
 256 /// 40
 257 /// ```
 258 ///
 259 /// At first, we have no memory allocated at all, but as we append to the
 260 /// string, it increases its capacity appropriately. If we instead use the
 261 /// [`with_capacity`] method to allocate the correct capacity initially:
 262 ///
 263 /// ```
 264 /// let mut s = String::with_capacity(25);
 265 ///
 266 /// println!("{}", s.capacity());
 267 ///
 268 /// for _ in 0..5 {
 269 ///     s.push_str("hello");
 270 ///     println!("{}", s.capacity());
 271 /// }
 272 /// ```
 273 ///
 274 /// [`with_capacity`]: #method.with_capacity
 275 ///
 276 /// We end up with a different output:
 277 ///
 278 /// ```text
 279 /// 25
 280 /// 25
 281 /// 25
 282 /// 25
 283 /// 25
 284 /// 25
 285 /// ```
 286 ///
 287 /// Here, there's no need to allocate more memory inside the loop.
 288 ///
 289 /// [`&str`]: ../../std/primitive.str.html
 290 /// [`Deref`]: ../../std/ops/trait.Deref.html
 291 /// [`as_str()`]: struct.String.html#method.as_str
 292 #[derive(PartialOrd, Eq, Ord)]
 293 #[stable(feature = "rust1", since = "1.0.0")]
 294 pub struct String {
 295     vec: Vec<u8>,
 296 }
 297
 298 /// A possible error value when converting a `String` from a UTF-8 byte vector.
 299 ///
 300 /// This type is the error type for the [`from_utf8`] method on [`String`]. It
 301 /// is designed in such a way to carefully avoid reallocations: the
 302 /// [`into_bytes`] method will give back the byte vector that was used in the
 303 /// conversion attempt.
 304 ///
 305 /// [`from_utf8`]: struct.String.html#method.from_utf8
 306 /// [`String`]: struct.String.html
 307 /// [`into_bytes`]: struct.FromUtf8Error.html#method.into_bytes
 308 ///
 309 /// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
 310 /// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
 311 /// an analogue to `FromUtf8Error`, and you can get one from a `FromUtf8Error`
 312 /// through the [`utf8_error`] method.
 313 ///
 314 /// [`Utf8Error`]: ../../std/str/struct.Utf8Error.html
 315 /// [`std::str`]: ../../std/str/index.html
 316 /// [`u8`]: ../../std/primitive.u8.html
 317 /// [`&str`]: ../../std/primitive.str.html
 318 /// [`utf8_error`]: #method.utf8_error
 319 ///
 320 /// # Examples
 321 ///
 322 /// Basic usage:
 323 ///
 324 /// ```
 325 /// // some invalid bytes, in a vector
 326 /// let bytes = vec![0, 159];
 327 ///
 328 /// let value = String::from_utf8(bytes);
 329 ///
 330 /// assert!(value.is_err());
 331 /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
 332 /// ```
 333 #[stable(feature = "rust1", since = "1.0.0")]
 334 #[derive(Debug)]
 335 pub struct FromUtf8Error {
 336     bytes: Vec<u8>,
 337     error: Utf8Error,
 338 }
 339
 340 /// A possible error value when converting a `String` from a UTF-16 byte slice.
 341 ///
 342 /// This type is the error type for the [`from_utf16`] method on [`String`].
 343 ///
 344 /// [`from_utf16`]: struct.String.html#method.from_utf16
 345 /// [`String`]: struct.String.html
 346 ///
 347 /// # Examples
 348 ///
 349 /// Basic usage:
 350 ///
 351 /// ```
 352 /// // 𝄞mu<invalid>ic
 353 /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
 354 ///           0xD800, 0x0069, 0x0063];
 355 ///
 356 /// assert!(String::from_utf16(v).is_err());
 357 /// ```
 358 #[stable(feature = "rust1", since = "1.0.0")]
 359 #[derive(Debug)]
 360 pub struct FromUtf16Error(());
 361
 362 impl String {
 363     /// Creates a new empty `String`.
 364     ///
 365     /// Given that the `String` is empty, this will not allocate any initial
 366     /// buffer. While that means that this initial operation is very
 367     /// inexpensive, it may cause excessive allocation later when you add
 368     /// data. If you have an idea of how much data the `String` will hold,
 369     /// consider the [`with_capacity`] method to prevent excessive
 370     /// re-allocation.
 371     ///
 372     /// [`with_capacity`]: #method.with_capacity
 373     ///
 374     /// # Examples
 375     ///
 376     /// Basic usage:
 377     ///
 378     /// ```
 379     /// let s = String::new();
 380     /// ```
 381     #[inline]
 382     #[stable(feature = "rust1", since = "1.0.0")]
 383     #[rustc_const_unstable(feature = "const_string_new")]
 384     pub const fn new() -> String {
 385         String { vec: Vec::new() }
 386     }
 387
 388     /// Creates a new empty `String` with a particular capacity.
 389     ///
 390     /// `String`s have an internal buffer to hold their data. The capacity is
 391     /// the length of that buffer, and can be queried with the [`capacity`]
 392     /// method. This method creates an empty `String`, but one with an initial
 393     /// buffer that can hold `capacity` bytes. This is useful when you may be
 394     /// appending a bunch of data to the `String`, reducing the number of
 395     /// reallocations it needs to do.
 396     ///
 397     /// [`capacity`]: #method.capacity
 398     ///
 399     /// If the given capacity is `0`, no allocation will occur, and this method
 400     /// is identical to the [`new`] method.
 401     ///
 402     /// [`new`]: #method.new
 403     ///
 404     /// # Examples
 405     ///
 406     /// Basic usage:
 407     ///
 408     /// ```
 409     /// let mut s = String::with_capacity(10);
 410     ///
 411     /// // The String contains no chars, even though it has capacity for more
 412     /// assert_eq!(s.len(), 0);
 413     ///
 414     /// // These are all done without reallocating...
 415     /// let cap = s.capacity();
 416     /// for i in 0..10 {
 417     ///     s.push('a');
 418     /// }
 419     ///
 420     /// assert_eq!(s.capacity(), cap);
 421     ///
 422     /// // ...but this may make the vector reallocate
 423     /// s.push('a');
 424     /// ```
 425     #[inline]
 426     #[stable(feature = "rust1", since = "1.0.0")]
 427     pub fn with_capacity(capacity: usize) -> String {
 428         String { vec: Vec::with_capacity(capacity) }
 429     }
 430
 431     // HACK(japaric): with cfg(test) the inherent `[T]::to_vec` method, which is
 432     // required for this method definition, is not available. Since we don't
 433     // require this method for testing purposes, I'll just stub it
 434     // NB see the slice::hack module in slice.rs for more information
 435     #[inline]
 436     #[cfg(test)]
 437     pub fn from_str(_: &str) -> String {
 438         panic!("not available with cfg(test)");
 439     }
 440
 441     /// Converts a vector of bytes to a `String`.
 442     ///
 443     /// A string slice ([`&str`]) is made of bytes ([`u8`]), and a vector of bytes
 444     /// ([`Vec<u8>`]) is made of bytes, so this function converts between the
 445     /// two. Not all byte slices are valid `String`s, however: `String`
 446     /// requires that it is valid UTF-8. `from_utf8()` checks to ensure that
 447     /// the bytes are valid UTF-8, and then does the conversion.
 448     ///
 449     /// If you are sure that the byte slice is valid UTF-8, and you don't want
 450     /// to incur the overhead of the validity check, there is an unsafe version
 451     /// of this function, [`from_utf8_unchecked`], which has the same behavior
 452     /// but skips the check.
 453     ///
 454     /// This method will take care to not copy the vector, for efficiency's
 455     /// sake.
 456     ///
 457     /// If you need a [`&str`] instead of a `String`, consider
 458     /// [`str::from_utf8`].
 459     ///
 460     /// The inverse of this method is [`as_bytes`].
 461     ///
 462     /// # Errors
 463     ///
 464     /// Returns [`Err`] if the slice is not UTF-8 with a description as to why the
 465     /// provided bytes are not UTF-8. The vector you moved in is also included.
 466     ///
 467     /// # Examples
 468     ///
 469     /// Basic usage:
 470     ///
 471     /// ```
 472     /// // some bytes, in a vector
 473     /// let sparkle_heart = vec![240, 159, 146, 150];
 474     ///
 475     /// // We know these bytes are valid, so we'll use `unwrap()`.
 476     /// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
 477     ///
 478     /// assert_eq!("💖", sparkle_heart);
 479     /// ```
 480     ///
 481     /// Incorrect bytes:
 482     ///
 483     /// ```
 484     /// // some invalid bytes, in a vector
 485     /// let sparkle_heart = vec![0, 159, 146, 150];
 486     ///
 487     /// assert!(String::from_utf8(sparkle_heart).is_err());
 488     /// ```
 489     ///
 490     /// See the docs for [`FromUtf8Error`] for more details on what you can do
 491     /// with this error.
 492     ///
 493     /// [`from_utf8_unchecked`]: struct.String.html#method.from_utf8_unchecked
 494     /// [`&str`]: ../../std/primitive.str.html
 495     /// [`u8`]: ../../std/primitive.u8.html
 496     /// [`Vec<u8>`]: ../../std/vec/struct.Vec.html
 497     /// [`str::from_utf8`]: ../../std/str/fn.from_utf8.html
 498     /// [`as_bytes`]: struct.String.html#method.as_bytes
 499     /// [`FromUtf8Error`]: struct.FromUtf8Error.html
 500     /// [`Err`]: ../../stdresult/enum.Result.html#variant.Err
 501     #[inline]
 502     #[stable(feature = "rust1", since = "1.0.0")]
 503     pub fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error> {
 504         match str::from_utf8(&vec) {
 505             Ok(..) => Ok(String { vec: vec }),
 506             Err(e) => {
 507                 Err(FromUtf8Error {
 508                     bytes: vec,
 509                     error: e,
 510                 })
 511             }
 512         }
 513     }
 514
 515     /// Converts a slice of bytes to a string, including invalid characters.
 516     ///
 517     /// Strings are made of bytes ([`u8`]), and a slice of bytes
 518     /// ([`&[u8]`][byteslice]) is made of bytes, so this function converts
 519     /// between the two. Not all byte slices are valid strings, however: strings
 520     /// are required to be valid UTF-8. During this conversion,
 521     /// `from_utf8_lossy()` will replace any invalid UTF-8 sequences with
 522     /// `U+FFFD REPLACEMENT CHARACTER`, which looks like this: �
 523     ///
 524     /// [`u8`]: ../../std/primitive.u8.html
 525     /// [byteslice]: ../../std/primitive.slice.html
 526     ///
 527     /// If you are sure that the byte slice is valid UTF-8, and you don't want
 528     /// to incur the overhead of the conversion, there is an unsafe version
 529     /// of this function, [`from_utf8_unchecked`], which has the same behavior
 530     /// but skips the checks.
 531     ///
 532     /// [`from_utf8_unchecked`]: struct.String.html#method.from_utf8_unchecked
 533     ///
 534     /// This function returns a [`Cow<'a, str>`]. If our byte slice is invalid
 535     /// UTF-8, then we need to insert the replacement characters, which will
 536     /// change the size of the string, and hence, require a `String`. But if
 537     /// it's already valid UTF-8, we don't need a new allocation. This return
 538     /// type allows us to handle both cases.
 539     ///
 540     /// [`Cow<'a, str>`]: ../../std/borrow/enum.Cow.html
 541     ///
 542     /// # Examples
 543     ///
 544     /// Basic usage:
 545     ///
 546     /// ```
 547     /// // some bytes, in a vector
 548     /// let sparkle_heart = vec![240, 159, 146, 150];
 549     ///
 550     /// let sparkle_heart = String::from_utf8_lossy(&sparkle_heart);
 551     ///
 552     /// assert_eq!("💖", sparkle_heart);
 553     /// ```
 554     ///
 555     /// Incorrect bytes:
 556     ///
 557     /// ```
 558     /// // some invalid bytes
 559     /// let input = b"Hello \xF0\x90\x80World";
 560     /// let output = String::from_utf8_lossy(input);
 561     ///
 562     /// assert_eq!("Hello �World", output);
 563     /// ```
 564     #[stable(feature = "rust1", since = "1.0.0")]
 565     pub fn from_utf8_lossy<'a>(v: &'a [u8]) -> Cow<'a, str> {
 566         let mut iter = lossy::Utf8Lossy::from_bytes(v).chunks();
 567
 568         let (first_valid, first_broken) = if let Some(chunk) = iter.next() {
 569             let lossy::Utf8LossyChunk { valid, broken } = chunk;
 570             if valid.len() == v.len() {
 571                 debug_assert!(broken.is_empty());
 572                 return Cow::Borrowed(valid);
 573             }
 574             (valid, broken)
 575         } else {
 576             return Cow::Borrowed("");
 577         };
 578
 579         const REPLACEMENT: &'static str = "\u{FFFD}";
 580
 581         let mut res = String::with_capacity(v.len());
 582         res.push_str(first_valid);
 583         if !first_broken.is_empty() {
 584             res.push_str(REPLACEMENT);
 585         }
 586
 587         for lossy::Utf8LossyChunk { valid, broken } in iter {
 588             res.push_str(valid);
 589             if !broken.is_empty() {
 590                 res.push_str(REPLACEMENT);
 591             }
 592         }
 593
 594         Cow::Owned(res)
 595     }
 596
 597     /// Decode a UTF-16 encoded vector `v` into a `String`, returning [`Err`]
 598     /// if `v` contains any invalid data.
 599     ///
 600     /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
 601     ///
 602     /// # Examples
 603     ///
 604     /// Basic usage:
 605     ///
 606     /// ```
 607     /// // 𝄞music
 608     /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
 609     ///           0x0073, 0x0069, 0x0063];
 610     /// assert_eq!(String::from("𝄞music"),
 611     ///            String::from_utf16(v).unwrap());
 612     ///
 613     /// // 𝄞mu<invalid>ic
 614     /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
 615     ///           0xD800, 0x0069, 0x0063];
 616     /// assert!(String::from_utf16(v).is_err());
 617     /// ```
 618     #[stable(feature = "rust1", since = "1.0.0")]
 619     pub fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error> {
 620         decode_utf16(v.iter().cloned()).collect::<Result<_, _>>().map_err(|_| FromUtf16Error(()))
 621     }
 622
 623     /// Decode a UTF-16 encoded slice `v` into a `String`, replacing
 624     /// invalid data with the replacement character (U+FFFD).
 625     ///
 626     /// Unlike [`from_utf8_lossy`] which returns a [`Cow<'a, str>`],
 627     /// `from_utf16_lossy` returns a `String` since the UTF-16 to UTF-8
 628     /// conversion requires a memory allocation.
 629     ///
 630     /// [`from_utf8_lossy`]: #method.from_utf8_lossy
 631     /// [`Cow<'a, str>`]: ../borrow/enum.Cow.html
 632     ///
 633     /// # Examples
 634     ///
 635     /// Basic usage:
 636     ///
 637     /// ```
 638     /// // 𝄞mus<invalid>ic<invalid>
 639     /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
 640     ///           0x0073, 0xDD1E, 0x0069, 0x0063,
 641     ///           0xD834];
 642     ///
 643     /// assert_eq!(String::from("𝄞mus\u{FFFD}ic\u{FFFD}"),
 644     ///            String::from_utf16_lossy(v));
 645     /// ```
 646     #[inline]
 647     #[stable(feature = "rust1", since = "1.0.0")]
 648     pub fn from_utf16_lossy(v: &[u16]) -> String {
 649         decode_utf16(v.iter().cloned()).map(|r| r.unwrap_or(REPLACEMENT_CHARACTER)).collect()
 650     }
 651
 652     /// Creates a new `String` from a length, capacity, and pointer.
 653     ///
 654     /// # Safety
 655     ///
 656     /// This is highly unsafe, due to the number of invariants that aren't
 657     /// checked:
 658     ///
 659     /// * The memory at `ptr` needs to have been previously allocated by the
 660     ///   same allocator the standard library uses.
 661     /// * `length` needs to be less than or equal to `capacity`.
 662     /// * `capacity` needs to be the correct value.
 663     ///
 664     /// Violating these may cause problems like corrupting the allocator's
 665     /// internal data structures.
 666     ///
 667     /// The ownership of `ptr` is effectively transferred to the
 668     /// `String` which may then deallocate, reallocate or change the
 669     /// contents of memory pointed to by the pointer at will. Ensure
 670     /// that nothing else uses the pointer after calling this
 671     /// function.
 672     ///
 673     /// # Examples
 674     ///
 675     /// Basic usage:
 676     ///
 677     /// ```
 678     /// use std::mem;
 679     ///
 680     /// unsafe {
 681     ///     let s = String::from("hello");
 682     ///     let ptr = s.as_ptr();
 683     ///     let len = s.len();
 684     ///     let capacity = s.capacity();
 685     ///
 686     ///     mem::forget(s);
 687     ///
 688     ///     let s = String::from_raw_parts(ptr as *mut _, len, capacity);
 689     ///
 690     ///     assert_eq!(String::from("hello"), s);
 691     /// }
 692     /// ```
 693     #[inline]
 694     #[stable(feature = "rust1", since = "1.0.0")]
 695     pub unsafe fn from_raw_parts(buf: *mut u8, length: usize, capacity: usize) -> String {
 696         String { vec: Vec::from_raw_parts(buf, length, capacity) }
 697     }
 698
 699     /// Converts a vector of bytes to a `String` without checking that the
 700     /// string contains valid UTF-8.
 701     ///
 702     /// See the safe version, [`from_utf8`], for more details.
 703     ///
 704     /// [`from_utf8`]: struct.String.html#method.from_utf8
 705     ///
 706     /// # Safety
 707     ///
 708     /// This function is unsafe because it does not check that the bytes passed
 709     /// to it are valid UTF-8. If this constraint is violated, it may cause
 710     /// memory unsafety issues with future users of the `String`, as the rest of
 711     /// the standard library assumes that `String`s are valid UTF-8.
 712     ///
 713     /// # Examples
 714     ///
 715     /// Basic usage:
 716     ///
 717     /// ```
 718     /// // some bytes, in a vector
 719     /// let sparkle_heart = vec![240, 159, 146, 150];
 720     ///
 721     /// let sparkle_heart = unsafe {
 722     ///     String::from_utf8_unchecked(sparkle_heart)
 723     /// };
 724     ///
 725     /// assert_eq!("💖", sparkle_heart);
 726     /// ```
 727     #[inline]
 728     #[stable(feature = "rust1", since = "1.0.0")]
 729     pub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String {
 730         String { vec: bytes }
 731     }
 732
 733     /// Converts a `String` into a byte vector.
 734     ///
 735     /// This consumes the `String`, so we do not need to copy its contents.
 736     ///
 737     /// # Examples
 738     ///
 739     /// Basic usage:
 740     ///
 741     /// ```
 742     /// let s = String::from("hello");
 743     /// let bytes = s.into_bytes();
 744     ///
 745     /// assert_eq!(&[104, 101, 108, 108, 111][..], &bytes[..]);
 746     /// ```
 747     #[inline]
 748     #[stable(feature = "rust1", since = "1.0.0")]
 749     pub fn into_bytes(self) -> Vec<u8> {
 750         self.vec
 751     }
 752
 753     /// Extracts a string slice containing the entire string.
 754     ///
 755     /// # Examples
 756     ///
 757     /// Basic usage:
 758     ///
 759     /// ```
 760     /// let s = String::from("foo");
 761     ///
 762     /// assert_eq!("foo", s.as_str());
 763     /// ```
 764     #[inline]
 765     #[stable(feature = "string_as_str", since = "1.7.0")]
 766     pub fn as_str(&self) -> &str {
 767         self
 768     }
 769
 770     /// Converts a `String` into a mutable string slice.
 771     ///
 772     /// # Examples
 773     ///
 774     /// Basic usage:
 775     ///
 776     /// ```
 777     /// let mut s = String::from("foobar");
 778     /// let s_mut_str = s.as_mut_str();
 779     ///
 780     /// s_mut_str.make_ascii_uppercase();
 781     ///
 782     /// assert_eq!("FOOBAR", s_mut_str);
 783     /// ```
 784     #[inline]
 785     #[stable(feature = "string_as_str", since = "1.7.0")]
 786     pub fn as_mut_str(&mut self) -> &mut str {
 787         self
 788     }
 789
 790     /// Appends a given string slice onto the end of this `String`.
 791     ///
 792     /// # Examples
 793     ///
 794     /// Basic usage:
 795     ///
 796     /// ```
 797     /// let mut s = String::from("foo");
 798     ///
 799     /// s.push_str("bar");
 800     ///
 801     /// assert_eq!("foobar", s);
 802     /// ```
 803     #[inline]
 804     #[stable(feature = "rust1", since = "1.0.0")]
 805     pub fn push_str(&mut self, string: &str) {
 806         self.vec.extend_from_slice(string.as_bytes())
 807     }
 808
 809     /// Returns this `String`'s capacity, in bytes.
 810     ///
 811     /// # Examples
 812     ///
 813     /// Basic usage:
 814     ///
 815     /// ```
 816     /// let s = String::with_capacity(10);
 817     ///
 818     /// assert!(s.capacity() >= 10);
 819     /// ```
 820     #[inline]
 821     #[stable(feature = "rust1", since = "1.0.0")]
 822     pub fn capacity(&self) -> usize {
 823         self.vec.capacity()
 824     }
 825
 826     /// Ensures that this `String`'s capacity is at least `additional` bytes
 827     /// larger than its length.
 828     ///
 829     /// The capacity may be increased by more than `additional` bytes if it
 830     /// chooses, to prevent frequent reallocations.
 831     ///
 832     /// If you do not want this "at least" behavior, see the [`reserve_exact`]
 833     /// method.
 834     ///
 835     /// # Panics
 836     ///
 837     /// Panics if the new capacity overflows [`usize`].
 838     ///
 839     /// [`reserve_exact`]: struct.String.html#method.reserve_exact
 840     /// [`usize`]: ../../std/primitive.usize.html
 841     ///
 842     /// # Examples
 843     ///
 844     /// Basic usage:
 845     ///
 846     /// ```
 847     /// let mut s = String::new();
 848     ///
 849     /// s.reserve(10);
 850     ///
 851     /// assert!(s.capacity() >= 10);
 852     /// ```
 853     ///
 854     /// This may not actually increase the capacity:
 855     ///
 856     /// ```
 857     /// let mut s = String::with_capacity(10);
 858     /// s.push('a');
 859     /// s.push('b');
 860     ///
 861     /// // s now has a length of 2 and a capacity of 10
 862     /// assert_eq!(2, s.len());
 863     /// assert_eq!(10, s.capacity());
 864     ///
 865     /// // Since we already have an extra 8 capacity, calling this...
 866     /// s.reserve(8);
 867     ///
 868     /// // ... doesn't actually increase.
 869     /// assert_eq!(10, s.capacity());
 870     /// ```
 871     #[inline]
 872     #[stable(feature = "rust1", since = "1.0.0")]
 873     pub fn reserve(&mut self, additional: usize) {
 874         self.vec.reserve(additional)
 875     }
 876
 877     /// Ensures that this `String`'s capacity is `additional` bytes
 878     /// larger than its length.
 879     ///
 880     /// Consider using the [`reserve`] method unless you absolutely know
 881     /// better than the allocator.
 882     ///
 883     /// [`reserve`]: #method.reserve
 884     ///
 885     /// # Panics
 886     ///
 887     /// Panics if the new capacity overflows `usize`.
 888     ///
 889     /// # Examples
 890     ///
 891     /// Basic usage:
 892     ///
 893     /// ```
 894     /// let mut s = String::new();
 895     ///
 896     /// s.reserve_exact(10);
 897     ///
 898     /// assert!(s.capacity() >= 10);
 899     /// ```
 900     ///
 901     /// This may not actually increase the capacity:
 902     ///
 903     /// ```
 904     /// let mut s = String::with_capacity(10);
 905     /// s.push('a');
 906     /// s.push('b');
 907     ///
 908     /// // s now has a length of 2 and a capacity of 10
 909     /// assert_eq!(2, s.len());
 910     /// assert_eq!(10, s.capacity());
 911     ///
 912     /// // Since we already have an extra 8 capacity, calling this...
 913     /// s.reserve_exact(8);
 914     ///
 915     /// // ... doesn't actually increase.
 916     /// assert_eq!(10, s.capacity());
 917     /// ```
 918     #[inline]
 919     #[stable(feature = "rust1", since = "1.0.0")]
 920     pub fn reserve_exact(&mut self, additional: usize) {
 921         self.vec.reserve_exact(additional)
 922     }
 923
 924     /// Tries to reserve capacity for at least `additional` more elements to be inserted
 925     /// in the given `String`. The collection may reserve more space to avoid
 926     /// frequent reallocations. After calling `reserve`, capacity will be
 927     /// greater than or equal to `self.len() + additional`. Does nothing if
 928     /// capacity is already sufficient.
 929     ///
 930     /// # Errors
 931     ///
 932     /// If the capacity overflows, or the allocator reports a failure, then an error
 933     /// is returned.
 934     ///
 935     /// # Examples
 936     ///
 937     /// ```
 938     /// #![feature(try_reserve)]
 939     /// use std::collections::CollectionAllocErr;
 940     ///
 941     /// fn process_data(data: &str) -> Result<String, CollectionAllocErr> {
 942     ///     let mut output = String::new();
 943     ///
 944     ///     // Pre-reserve the memory, exiting if we can't
 945     ///     output.try_reserve(data.len())?;
 946     ///
 947     ///     // Now we know this can't OOM in the middle of our complex work
 948     ///     output.push_str(data);
 949     ///
 950     ///     Ok(output)
 951     /// }
 952     /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
 953     /// ```
 954     #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
 955     pub fn try_reserve(&mut self, additional: usize) -> Result<(), CollectionAllocErr> {
 956         self.vec.try_reserve(additional)
 957     }
 958
 959     /// Tries to reserves the minimum capacity for exactly `additional` more elements to
 960     /// be inserted in the given `String`. After calling `reserve_exact`,
 961     /// capacity will be greater than or equal to `self.len() + additional`.
 962     /// Does nothing if the capacity is already sufficient.
 963     ///
 964     /// Note that the allocator may give the collection more space than it
 965     /// requests. Therefore capacity can not be relied upon to be precisely
 966     /// minimal. Prefer `reserve` if future insertions are expected.
 967     ///
 968     /// # Errors
 969     ///
 970     /// If the capacity overflows, or the allocator reports a failure, then an error
 971     /// is returned.
 972     ///
 973     /// # Examples
 974     ///
 975     /// ```
 976     /// #![feature(try_reserve)]
 977     /// use std::collections::CollectionAllocErr;
 978     ///
 979     /// fn process_data(data: &str) -> Result<String, CollectionAllocErr> {
 980     ///     let mut output = String::new();
 981     ///
 982     ///     // Pre-reserve the memory, exiting if we can't
 983     ///     output.try_reserve(data.len())?;
 984     ///
 985     ///     // Now we know this can't OOM in the middle of our complex work
 986     ///     output.push_str(data);
 987     ///
 988     ///     Ok(output)
 989     /// }
 990     /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
 991     /// ```
 992     #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
 993     pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), CollectionAllocErr>  {
 994         self.vec.try_reserve_exact(additional)
 995     }
 996
 997     /// Shrinks the capacity of this `String` to match its length.
 998     ///
 999     /// # Examples
1000     ///
1001     /// Basic usage:
1002     ///
1003     /// ```
1004     /// let mut s = String::from("foo");
1005     ///
1006     /// s.reserve(100);
1007     /// assert!(s.capacity() >= 100);
1008     ///
1009     /// s.shrink_to_fit();
1010     /// assert_eq!(3, s.capacity());
1011     /// ```
1012     #[inline]
1013     #[stable(feature = "rust1", since = "1.0.0")]
1014     pub fn shrink_to_fit(&mut self) {
1015         self.vec.shrink_to_fit()
1016     }
1017
1018     /// Shrinks the capacity of this `String` with a lower bound.
1019     ///
1020     /// The capacity will remain at least as large as both the length
1021     /// and the supplied value.
1022     ///
1023     /// Panics if the current capacity is smaller than the supplied
1024     /// minimum capacity.
1025     ///
1026     /// # Examples
1027     ///
1028     /// ```
1029     /// #![feature(shrink_to)]
1030     /// let mut s = String::from("foo");
1031     ///
1032     /// s.reserve(100);
1033     /// assert!(s.capacity() >= 100);
1034     ///
1035     /// s.shrink_to(10);
1036     /// assert!(s.capacity() >= 10);
1037     /// s.shrink_to(0);
1038     /// assert!(s.capacity() >= 3);
1039     /// ```
1040     #[inline]
1041     #[unstable(feature = "shrink_to", reason = "new API", issue="0")]
1042     pub fn shrink_to(&mut self, min_capacity: usize) {
1043         self.vec.shrink_to(min_capacity)
1044     }
1045
1046     /// Appends the given [`char`] to the end of this `String`.
1047     ///
1048     /// [`char`]: ../../std/primitive.char.html
1049     ///
1050     /// # Examples
1051     ///
1052     /// Basic usage:
1053     ///
1054     /// ```
1055     /// let mut s = String::from("abc");
1056     ///
1057     /// s.push('1');
1058     /// s.push('2');
1059     /// s.push('3');
1060     ///
1061     /// assert_eq!("abc123", s);
1062     /// ```
1063     #[inline]
1064     #[stable(feature = "rust1", since = "1.0.0")]
1065     pub fn push(&mut self, ch: char) {
1066         match ch.len_utf8() {
1067             1 => self.vec.push(ch as u8),
1068             _ => self.vec.extend_from_slice(ch.encode_utf8(&mut [0; 4]).as_bytes()),
1069         }
1070     }
1071
1072     /// Returns a byte slice of this `String`'s contents.
1073     ///
1074     /// The inverse of this method is [`from_utf8`].
1075     ///
1076     /// [`from_utf8`]: #method.from_utf8
1077     ///
1078     /// # Examples
1079     ///
1080     /// Basic usage:
1081     ///
1082     /// ```
1083     /// let s = String::from("hello");
1084     ///
1085     /// assert_eq!(&[104, 101, 108, 108, 111], s.as_bytes());
1086     /// ```
1087     #[inline]
1088     #[stable(feature = "rust1", since = "1.0.0")]
1089     pub fn as_bytes(&self) -> &[u8] {
1090         &self.vec
1091     }
1092
1093     /// Shortens this `String` to the specified length.
1094     ///
1095     /// If `new_len` is greater than the string's current length, this has no
1096     /// effect.
1097     ///
1098     /// Note that this method has no effect on the allocated capacity
1099     /// of the string
1100     ///
1101     /// # Panics
1102     ///
1103     /// Panics if `new_len` does not lie on a [`char`] boundary.
1104     ///
1105     /// [`char`]: ../../std/primitive.char.html
1106     ///
1107     /// # Examples
1108     ///
1109     /// Basic usage:
1110     ///
1111     /// ```
1112     /// let mut s = String::from("hello");
1113     ///
1114     /// s.truncate(2);
1115     ///
1116     /// assert_eq!("he", s);
1117     /// ```
1118     #[inline]
1119     #[stable(feature = "rust1", since = "1.0.0")]
1120     pub fn truncate(&mut self, new_len: usize) {
1121         if new_len <= self.len() {
1122             assert!(self.is_char_boundary(new_len));
1123             self.vec.truncate(new_len)
1124         }
1125     }
1126
1127     /// Removes the last character from the string buffer and returns it.
1128     ///
1129     /// Returns [`None`] if this `String` is empty.
1130     ///
1131     /// [`None`]: ../../std/option/enum.Option.html#variant.None
1132     ///
1133     /// # Examples
1134     ///
1135     /// Basic usage:
1136     ///
1137     /// ```
1138     /// let mut s = String::from("foo");
1139     ///
1140     /// assert_eq!(s.pop(), Some('o'));
1141     /// assert_eq!(s.pop(), Some('o'));
1142     /// assert_eq!(s.pop(), Some('f'));
1143     ///
1144     /// assert_eq!(s.pop(), None);
1145     /// ```
1146     #[inline]
1147     #[stable(feature = "rust1", since = "1.0.0")]
1148     pub fn pop(&mut self) -> Option<char> {
1149         let ch = self.chars().rev().next()?;
1150         let newlen = self.len() - ch.len_utf8();
1151         unsafe {
1152             self.vec.set_len(newlen);
1153         }
1154         Some(ch)
1155     }
1156
1157     /// Removes a [`char`] from this `String` at a byte position and returns it.
1158     ///
1159     /// This is an `O(n)` operation, as it requires copying every element in the
1160     /// buffer.
1161     ///
1162     /// # Panics
1163     ///
1164     /// Panics if `idx` is larger than or equal to the `String`'s length,
1165     /// or if it does not lie on a [`char`] boundary.
1166     ///
1167     /// [`char`]: ../../std/primitive.char.html
1168     ///
1169     /// # Examples
1170     ///
1171     /// Basic usage:
1172     ///
1173     /// ```
1174     /// let mut s = String::from("foo");
1175     ///
1176     /// assert_eq!(s.remove(0), 'f');
1177     /// assert_eq!(s.remove(1), 'o');
1178     /// assert_eq!(s.remove(0), 'o');
1179     /// ```
1180     #[inline]
1181     #[stable(feature = "rust1", since = "1.0.0")]
1182     pub fn remove(&mut self, idx: usize) -> char {
1183         let ch = match self[idx..].chars().next() {
1184             Some(ch) => ch,
1185             None => panic!("cannot remove a char from the end of a string"),
1186         };
1187
1188         let next = idx + ch.len_utf8();
1189         let len = self.len();
1190         unsafe {
1191             ptr::copy(self.vec.as_ptr().offset(next as isize),
1192                       self.vec.as_mut_ptr().offset(idx as isize),
1193                       len - next);
1194             self.vec.set_len(len - (next - idx));
1195         }
1196         ch
1197     }
1198
1199     /// Retains only the characters specified by the predicate.
1200     ///
1201     /// In other words, remove all characters `c` such that `f(c)` returns `false`.
1202     /// This method operates in place and preserves the order of the retained
1203     /// characters.
1204     ///
1205     /// # Examples
1206     ///
1207     /// ```
1208     /// let mut s = String::from("f_o_ob_ar");
1209     ///
1210     /// s.retain(|c| c != '_');
1211     ///
1212     /// assert_eq!(s, "foobar");
1213     /// ```
1214     #[inline]
1215     #[stable(feature = "string_retain", since = "1.26.0")]
1216     pub fn retain<F>(&mut self, mut f: F)
1217         where F: FnMut(char) -> bool
1218     {
1219         let len = self.len();
1220         let mut del_bytes = 0;
1221         let mut idx = 0;
1222
1223         while idx < len {
1224             let ch = unsafe {
1225                 self.get_unchecked(idx..len).chars().next().unwrap()
1226             };
1227             let ch_len = ch.len_utf8();
1228
1229             if !f(ch) {
1230                 del_bytes += ch_len;
1231             } else if del_bytes > 0 {
1232                 unsafe {
1233                     ptr::copy(self.vec.as_ptr().offset(idx as isize),
1234                               self.vec.as_mut_ptr().offset((idx - del_bytes) as isize),
1235                               ch_len);
1236                 }
1237             }
1238
1239             // Point idx to the next char
1240             idx += ch_len;
1241         }
1242
1243         if del_bytes > 0 {
1244             unsafe { self.vec.set_len(len - del_bytes); }
1245         }
1246     }
1247
1248     /// Inserts a character into this `String` at a byte position.
1249     ///
1250     /// This is an `O(n)` operation as it requires copying every element in the
1251     /// buffer.
1252     ///
1253     /// # Panics
1254     ///
1255     /// Panics if `idx` is larger than the `String`'s length, or if it does not
1256     /// lie on a [`char`] boundary.
1257     ///
1258     /// [`char`]: ../../std/primitive.char.html
1259     ///
1260     /// # Examples
1261     ///
1262     /// Basic usage:
1263     ///
1264     /// ```
1265     /// let mut s = String::with_capacity(3);
1266     ///
1267     /// s.insert(0, 'f');
1268     /// s.insert(1, 'o');
1269     /// s.insert(2, 'o');
1270     ///
1271     /// assert_eq!("foo", s);
1272     /// ```
1273     #[inline]
1274     #[stable(feature = "rust1", since = "1.0.0")]
1275     pub fn insert(&mut self, idx: usize, ch: char) {
1276         assert!(self.is_char_boundary(idx));
1277         let mut bits = [0; 4];
1278         let bits = ch.encode_utf8(&mut bits).as_bytes();
1279
1280         unsafe {
1281             self.insert_bytes(idx, bits);
1282         }
1283     }
1284
1285     unsafe fn insert_bytes(&mut self, idx: usize, bytes: &[u8]) {
1286         let len = self.len();
1287         let amt = bytes.len();
1288         self.vec.reserve(amt);
1289
1290         ptr::copy(self.vec.as_ptr().offset(idx as isize),
1291                   self.vec.as_mut_ptr().offset((idx + amt) as isize),
1292                   len - idx);
1293         ptr::copy(bytes.as_ptr(),
1294                   self.vec.as_mut_ptr().offset(idx as isize),
1295                   amt);
1296         self.vec.set_len(len + amt);
1297     }
1298
1299     /// Inserts a string slice into this `String` at a byte position.
1300     ///
1301     /// This is an `O(n)` operation as it requires copying every element in the
1302     /// buffer.
1303     ///
1304     /// # Panics
1305     ///
1306     /// Panics if `idx` is larger than the `String`'s length, or if it does not
1307     /// lie on a [`char`] boundary.
1308     ///
1309     /// [`char`]: ../../std/primitive.char.html
1310     ///
1311     /// # Examples
1312     ///
1313     /// Basic usage:
1314     ///
1315     /// ```
1316     /// let mut s = String::from("bar");
1317     ///
1318     /// s.insert_str(0, "foo");
1319     ///
1320     /// assert_eq!("foobar", s);
1321     /// ```
1322     #[inline]
1323     #[stable(feature = "insert_str", since = "1.16.0")]
1324     pub fn insert_str(&mut self, idx: usize, string: &str) {
1325         assert!(self.is_char_boundary(idx));
1326
1327         unsafe {
1328             self.insert_bytes(idx, string.as_bytes());
1329         }
1330     }
1331
1332     /// Returns a mutable reference to the contents of this `String`.
1333     ///
1334     /// # Safety
1335     ///
1336     /// This function is unsafe because it does not check that the bytes passed
1337     /// to it are valid UTF-8. If this constraint is violated, it may cause
1338     /// memory unsafety issues with future users of the `String`, as the rest of
1339     /// the standard library assumes that `String`s are valid UTF-8.
1340     ///
1341     /// # Examples
1342     ///
1343     /// Basic usage:
1344     ///
1345     /// ```
1346     /// let mut s = String::from("hello");
1347     ///
1348     /// unsafe {
1349     ///     let vec = s.as_mut_vec();
1350     ///     assert_eq!(&[104, 101, 108, 108, 111][..], &vec[..]);
1351     ///
1352     ///     vec.reverse();
1353     /// }
1354     /// assert_eq!(s, "olleh");
1355     /// ```
1356     #[inline]
1357     #[stable(feature = "rust1", since = "1.0.0")]
1358     pub unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8> {
1359         &mut self.vec
1360     }
1361
1362     /// Returns the length of this `String`, in bytes.
1363     ///
1364     /// # Examples
1365     ///
1366     /// Basic usage:
1367     ///
1368     /// ```
1369     /// let a = String::from("foo");
1370     ///
1371     /// assert_eq!(a.len(), 3);
1372     /// ```
1373     #[inline]
1374     #[stable(feature = "rust1", since = "1.0.0")]
1375     pub fn len(&self) -> usize {
1376         self.vec.len()
1377     }
1378
1379     /// Returns `true` if this `String` has a length of zero.
1380     ///
1381     /// Returns `false` otherwise.
1382     ///
1383     /// # Examples
1384     ///
1385     /// Basic usage:
1386     ///
1387     /// ```
1388     /// let mut v = String::new();
1389     /// assert!(v.is_empty());
1390     ///
1391     /// v.push('a');
1392     /// assert!(!v.is_empty());
1393     /// ```
1394     #[inline]
1395     #[stable(feature = "rust1", since = "1.0.0")]
1396     pub fn is_empty(&self) -> bool {
1397         self.len() == 0
1398     }
1399
1400     /// Splits the string into two at the given index.
1401     ///
1402     /// Returns a newly allocated `String`. `self` contains bytes `[0, at)`, and
1403     /// the returned `String` contains bytes `[at, len)`. `at` must be on the
1404     /// boundary of a UTF-8 code point.
1405     ///
1406     /// Note that the capacity of `self` does not change.
1407     ///
1408     /// # Panics
1409     ///
1410     /// Panics if `at` is not on a `UTF-8` code point boundary, or if it is beyond the last
1411     /// code point of the string.
1412     ///
1413     /// # Examples
1414     ///
1415     /// ```
1416     /// # fn main() {
1417     /// let mut hello = String::from("Hello, World!");
1418     /// let world = hello.split_off(7);
1419     /// assert_eq!(hello, "Hello, ");
1420     /// assert_eq!(world, "World!");
1421     /// # }
1422     /// ```
1423     #[inline]
1424     #[stable(feature = "string_split_off", since = "1.16.0")]
1425     pub fn split_off(&mut self, at: usize) -> String {
1426         assert!(self.is_char_boundary(at));
1427         let other = self.vec.split_off(at);
1428         unsafe { String::from_utf8_unchecked(other) }
1429     }
1430
1431     /// Truncates this `String`, removing all contents.
1432     ///
1433     /// While this means the `String` will have a length of zero, it does not
1434     /// touch its capacity.
1435     ///
1436     /// # Examples
1437     ///
1438     /// Basic usage:
1439     ///
1440     /// ```
1441     /// let mut s = String::from("foo");
1442     ///
1443     /// s.clear();
1444     ///
1445     /// assert!(s.is_empty());
1446     /// assert_eq!(0, s.len());
1447     /// assert_eq!(3, s.capacity());
1448     /// ```
1449     #[inline]
1450     #[stable(feature = "rust1", since = "1.0.0")]
1451     pub fn clear(&mut self) {
1452         self.vec.clear()
1453     }
1454
1455     /// Creates a draining iterator that removes the specified range in the string
1456     /// and yields the removed chars.
1457     ///
1458     /// Note: The element range is removed even if the iterator is not
1459     /// consumed until the end.
1460     ///
1461     /// # Panics
1462     ///
1463     /// Panics if the starting point or end point do not lie on a [`char`]
1464     /// boundary, or if they're out of bounds.
1465     ///
1466     /// [`char`]: ../../std/primitive.char.html
1467     ///
1468     /// # Examples
1469     ///
1470     /// Basic usage:
1471     ///
1472     /// ```
1473     /// let mut s = String::from("α is alpha, β is beta");
1474     /// let beta_offset = s.find('β').unwrap_or(s.len());
1475     ///
1476     /// // Remove the range up until the β from the string
1477     /// let t: String = s.drain(..beta_offset).collect();
1478     /// assert_eq!(t, "α is alpha, ");
1479     /// assert_eq!(s, "β is beta");
1480     ///
1481     /// // A full range clears the string
1482     /// s.drain(..);
1483     /// assert_eq!(s, "");
1484     /// ```
1485     #[stable(feature = "drain", since = "1.6.0")]
1486     pub fn drain<R>(&mut self, range: R) -> Drain
1487         where R: RangeBounds<usize>
1488     {
1489         // Memory safety
1490         //
1491         // The String version of Drain does not have the memory safety issues
1492         // of the vector version. The data is just plain bytes.
1493         // Because the range removal happens in Drop, if the Drain iterator is leaked,
1494         // the removal will not happen.
1495         let len = self.len();
1496         let start = match range.start_bound() {
1497             Included(&n) => n,
1498             Excluded(&n) => n + 1,
1499             Unbounded => 0,
1500         };
1501         let end = match range.end_bound() {
1502             Included(&n) => n + 1,
1503             Excluded(&n) => n,
1504             Unbounded => len,
1505         };
1506
1507         // Take out two simultaneous borrows. The &mut String won't be accessed
1508         // until iteration is over, in Drop.
1509         let self_ptr = self as *mut _;
1510         // slicing does the appropriate bounds checks
1511         let chars_iter = self[start..end].chars();
1512
1513         Drain {
1514             start,
1515             end,
1516             iter: chars_iter,
1517             string: self_ptr,
1518         }
1519     }
1520
1521     /// Removes the specified range in the string,
1522     /// and replaces it with the given string.
1523     /// The given string doesn't need to be the same length as the range.
1524     ///
1525     /// # Panics
1526     ///
1527     /// Panics if the starting point or end point do not lie on a [`char`]
1528     /// boundary, or if they're out of bounds.
1529     ///
1530     /// [`char`]: ../../std/primitive.char.html
1531     /// [`Vec::splice`]: ../../std/vec/struct.Vec.html#method.splice
1532     ///
1533     /// # Examples
1534     ///
1535     /// Basic usage:
1536     ///
1537     /// ```
1538     /// let mut s = String::from("α is alpha, β is beta");
1539     /// let beta_offset = s.find('β').unwrap_or(s.len());
1540     ///
1541     /// // Replace the range up until the β from the string
1542     /// s.replace_range(..beta_offset, "Α is capital alpha; ");
1543     /// assert_eq!(s, "Α is capital alpha; β is beta");
1544     /// ```
1545     #[stable(feature = "splice", since = "1.27.0")]
1546     pub fn replace_range<R>(&mut self, range: R, replace_with: &str)
1547         where R: RangeBounds<usize>
1548     {
1549         // Memory safety
1550         //
1551         // Replace_range does not have the memory safety issues of a vector Splice.
1552         // of the vector version. The data is just plain bytes.
1553
1554         match range.start_bound() {
1555              Included(&n) => assert!(self.is_char_boundary(n)),
1556              Excluded(&n) => assert!(self.is_char_boundary(n + 1)),
1557              Unbounded => {},
1558         };
1559         match range.end_bound() {
1560              Included(&n) => assert!(self.is_char_boundary(n + 1)),
1561              Excluded(&n) => assert!(self.is_char_boundary(n)),
1562              Unbounded => {},
1563         };
1564
1565         unsafe {
1566             self.as_mut_vec()
1567         }.splice(range, replace_with.bytes());
1568     }
1569
1570     /// Converts this `String` into a [`Box`]`<`[`str`]`>`.
1571     ///
1572     /// This will drop any excess capacity.
1573     ///
1574     /// [`Box`]: ../../std/boxed/struct.Box.html
1575     /// [`str`]: ../../std/primitive.str.html
1576     ///
1577     /// # Examples
1578     ///
1579     /// Basic usage:
1580     ///
1581     /// ```
1582     /// let s = String::from("hello");
1583     ///
1584     /// let b = s.into_boxed_str();
1585     /// ```
1586     #[stable(feature = "box_str", since = "1.4.0")]
1587     #[inline]
1588     pub fn into_boxed_str(self) -> Box<str> {
1589         let slice = self.vec.into_boxed_slice();
1590         unsafe { from_boxed_utf8_unchecked(slice) }
1591     }
1592 }
1593
1594 impl FromUtf8Error {
1595     /// Returns a slice of [`u8`]s bytes that were attempted to convert to a `String`.
1596     ///
1597     /// # Examples
1598     ///
1599     /// Basic usage:
1600     ///
1601     /// ```
1602     /// // some invalid bytes, in a vector
1603     /// let bytes = vec![0, 159];
1604     ///
1605     /// let value = String::from_utf8(bytes);
1606     ///
1607     /// assert_eq!(&[0, 159], value.unwrap_err().as_bytes());
1608     /// ```
1609     #[stable(feature = "from_utf8_error_as_bytes", since = "1.26.0")]
1610     pub fn as_bytes(&self) -> &[u8] {
1611         &self.bytes[..]
1612     }
1613
1614     /// Returns the bytes that were attempted to convert to a `String`.
1615     ///
1616     /// This method is carefully constructed to avoid allocation. It will
1617     /// consume the error, moving out the bytes, so that a copy of the bytes
1618     /// does not need to be made.
1619     ///
1620     /// # Examples
1621     ///
1622     /// Basic usage:
1623     ///
1624     /// ```
1625     /// // some invalid bytes, in a vector
1626     /// let bytes = vec![0, 159];
1627     ///
1628     /// let value = String::from_utf8(bytes);
1629     ///
1630     /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
1631     /// ```
1632     #[stable(feature = "rust1", since = "1.0.0")]
1633     pub fn into_bytes(self) -> Vec<u8> {
1634         self.bytes
1635     }
1636
1637     /// Fetch a `Utf8Error` to get more details about the conversion failure.
1638     ///
1639     /// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
1640     /// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
1641     /// an analogue to `FromUtf8Error`. See its documentation for more details
1642     /// on using it.
1643     ///
1644     /// [`Utf8Error`]: ../../std/str/struct.Utf8Error.html
1645     /// [`std::str`]: ../../std/str/index.html
1646     /// [`u8`]: ../../std/primitive.u8.html
1647     /// [`&str`]: ../../std/primitive.str.html
1648     ///
1649     /// # Examples
1650     ///
1651     /// Basic usage:
1652     ///
1653     /// ```
1654     /// // some invalid bytes, in a vector
1655     /// let bytes = vec![0, 159];
1656     ///
1657     /// let error = String::from_utf8(bytes).unwrap_err().utf8_error();
1658     ///
1659     /// // the first byte is invalid here
1660     /// assert_eq!(1, error.valid_up_to());
1661     /// ```
1662     #[stable(feature = "rust1", since = "1.0.0")]
1663     pub fn utf8_error(&self) -> Utf8Error {
1664         self.error
1665     }
1666 }
1667
1668 #[stable(feature = "rust1", since = "1.0.0")]
1669 impl fmt::Display for FromUtf8Error {
1670     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1671         fmt::Display::fmt(&self.error, f)
1672     }
1673 }
1674
1675 #[stable(feature = "rust1", since = "1.0.0")]
1676 impl fmt::Display for FromUtf16Error {
1677     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1678         fmt::Display::fmt("invalid utf-16: lone surrogate found", f)
1679     }
1680 }
1681
1682 #[stable(feature = "rust1", since = "1.0.0")]
1683 impl Clone for String {
1684     fn clone(&self) -> Self {
1685         String { vec: self.vec.clone() }
1686     }
1687
1688     fn clone_from(&mut self, source: &Self) {
1689         self.vec.clone_from(&source.vec);
1690     }
1691 }
1692
1693 #[stable(feature = "rust1", since = "1.0.0")]
1694 impl FromIterator<char> for String {
1695     fn from_iter<I: IntoIterator<Item = char>>(iter: I) -> String {
1696         let mut buf = String::new();
1697         buf.extend(iter);
1698         buf
1699     }
1700 }
1701
1702 #[stable(feature = "string_from_iter_by_ref", since = "1.17.0")]
1703 impl<'a> FromIterator<&'a char> for String {
1704     fn from_iter<I: IntoIterator<Item = &'a char>>(iter: I) -> String {
1705         let mut buf = String::new();
1706         buf.extend(iter);
1707         buf
1708     }
1709 }
1710
1711 #[stable(feature = "rust1", since = "1.0.0")]
1712 impl<'a> FromIterator<&'a str> for String {
1713     fn from_iter<I: IntoIterator<Item = &'a str>>(iter: I) -> String {
1714         let mut buf = String::new();
1715         buf.extend(iter);
1716         buf
1717     }
1718 }
1719
1720 #[stable(feature = "extend_string", since = "1.4.0")]
1721 impl FromIterator<String> for String {
1722     fn from_iter<I: IntoIterator<Item = String>>(iter: I) -> String {
1723         let mut buf = String::new();
1724         buf.extend(iter);
1725         buf
1726     }
1727 }
1728
1729 #[stable(feature = "herd_cows", since = "1.19.0")]
1730 impl<'a> FromIterator<Cow<'a, str>> for String {
1731     fn from_iter<I: IntoIterator<Item = Cow<'a, str>>>(iter: I) -> String {
1732         let mut buf = String::new();
1733         buf.extend(iter);
1734         buf
1735     }
1736 }
1737
1738 #[stable(feature = "rust1", since = "1.0.0")]
1739 impl Extend<char> for String {
1740     fn extend<I: IntoIterator<Item = char>>(&mut self, iter: I) {
1741         let iterator = iter.into_iter();
1742         let (lower_bound, _) = iterator.size_hint();
1743         self.reserve(lower_bound);
1744         for ch in iterator {
1745             self.push(ch)
1746         }
1747     }
1748 }
1749
1750 #[stable(feature = "extend_ref", since = "1.2.0")]
1751 impl<'a> Extend<&'a char> for String {
1752     fn extend<I: IntoIterator<Item = &'a char>>(&mut self, iter: I) {
1753         self.extend(iter.into_iter().cloned());
1754     }
1755 }
1756
1757 #[stable(feature = "rust1", since = "1.0.0")]
1758 impl<'a> Extend<&'a str> for String {
1759     fn extend<I: IntoIterator<Item = &'a str>>(&mut self, iter: I) {
1760         for s in iter {
1761             self.push_str(s)
1762         }
1763     }
1764 }
1765
1766 #[stable(feature = "extend_string", since = "1.4.0")]
1767 impl Extend<String> for String {
1768     fn extend<I: IntoIterator<Item = String>>(&mut self, iter: I) {
1769         for s in iter {
1770             self.push_str(&s)
1771         }
1772     }
1773 }
1774
1775 #[stable(feature = "herd_cows", since = "1.19.0")]
1776 impl<'a> Extend<Cow<'a, str>> for String {
1777     fn extend<I: IntoIterator<Item = Cow<'a, str>>>(&mut self, iter: I) {
1778         for s in iter {
1779             self.push_str(&s)
1780         }
1781     }
1782 }
1783
1784 /// A convenience impl that delegates to the impl for `&str`
1785 #[unstable(feature = "pattern",
1786            reason = "API not fully fleshed out and ready to be stabilized",
1787            issue = "27721")]
1788 impl<'a, 'b> Pattern<'a> for &'b String {
1789     type Searcher = <&'b str as Pattern<'a>>::Searcher;
1790
1791     fn into_searcher(self, haystack: &'a str) -> <&'b str as Pattern<'a>>::Searcher {
1792         self[..].into_searcher(haystack)
1793     }
1794
1795     #[inline]
1796     fn is_contained_in(self, haystack: &'a str) -> bool {
1797         self[..].is_contained_in(haystack)
1798     }
1799
1800     #[inline]
1801     fn is_prefix_of(self, haystack: &'a str) -> bool {
1802         self[..].is_prefix_of(haystack)
1803     }
1804 }
1805
1806 #[stable(feature = "rust1", since = "1.0.0")]
1807 impl PartialEq for String {
1808     #[inline]
1809     fn eq(&self, other: &String) -> bool {
1810         PartialEq::eq(&self[..], &other[..])
1811     }
1812     #[inline]
1813     fn ne(&self, other: &String) -> bool {
1814         PartialEq::ne(&self[..], &other[..])
1815     }
1816 }
1817
1818 macro_rules! impl_eq {
1819     ($lhs:ty, $rhs: ty) => {
1820         #[stable(feature = "rust1", since = "1.0.0")]
1821         impl<'a, 'b> PartialEq<$rhs> for $lhs {
1822             #[inline]
1823             fn eq(&self, other: &$rhs) -> bool { PartialEq::eq(&self[..], &other[..]) }
1824             #[inline]
1825             fn ne(&self, other: &$rhs) -> bool { PartialEq::ne(&self[..], &other[..]) }
1826         }
1827
1828         #[stable(feature = "rust1", since = "1.0.0")]
1829         impl<'a, 'b> PartialEq<$lhs> for $rhs {
1830             #[inline]
1831             fn eq(&self, other: &$lhs) -> bool { PartialEq::eq(&self[..], &other[..]) }
1832             #[inline]
1833             fn ne(&self, other: &$lhs) -> bool { PartialEq::ne(&self[..], &other[..]) }
1834         }
1835
1836     }
1837 }
1838
1839 impl_eq! { String, str }
1840 impl_eq! { String, &'a str }
1841 impl_eq! { Cow<'a, str>, str }
1842 impl_eq! { Cow<'a, str>, &'b str }
1843 impl_eq! { Cow<'a, str>, String }
1844
1845 #[stable(feature = "rust1", since = "1.0.0")]
1846 impl Default for String {
1847     /// Creates an empty `String`.
1848     #[inline]
1849     fn default() -> String {
1850         String::new()
1851     }
1852 }
1853
1854 #[stable(feature = "rust1", since = "1.0.0")]
1855 impl fmt::Display for String {
1856     #[inline]
1857     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1858         fmt::Display::fmt(&**self, f)
1859     }
1860 }
1861
1862 #[stable(feature = "rust1", since = "1.0.0")]
1863 impl fmt::Debug for String {
1864     #[inline]
1865     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1866         fmt::Debug::fmt(&**self, f)
1867     }
1868 }
1869
1870 #[stable(feature = "rust1", since = "1.0.0")]
1871 impl hash::Hash for String {
1872     #[inline]
1873     fn hash<H: hash::Hasher>(&self, hasher: &mut H) {
1874         (**self).hash(hasher)
1875     }
1876 }
1877
1878 /// Implements the `+` operator for concatenating two strings.
1879 ///
1880 /// This consumes the `String` on the left-hand side and re-uses its buffer (growing it if
1881 /// necessary). This is done to avoid allocating a new `String` and copying the entire contents on
1882 /// every operation, which would lead to `O(n^2)` running time when building an `n`-byte string by
1883 /// repeated concatenation.
1884 ///
1885 /// The string on the right-hand side is only borrowed; its contents are copied into the returned
1886 /// `String`.
1887 ///
1888 /// # Examples
1889 ///
1890 /// Concatenating two `String`s takes the first by value and borrows the second:
1891 ///
1892 /// ```
1893 /// let a = String::from("hello");
1894 /// let b = String::from(" world");
1895 /// let c = a + &b;
1896 /// // `a` is moved and can no longer be used here.
1897 /// ```
1898 ///
1899 /// If you want to keep using the first `String`, you can clone it and append to the clone instead:
1900 ///
1901 /// ```
1902 /// let a = String::from("hello");
1903 /// let b = String::from(" world");
1904 /// let c = a.clone() + &b;
1905 /// // `a` is still valid here.
1906 /// ```
1907 ///
1908 /// Concatenating `&str` slices can be done by converting the first to a `String`:
1909 ///
1910 /// ```
1911 /// let a = "hello";
1912 /// let b = " world";
1913 /// let c = a.to_string() + b;
1914 /// ```
1915 #[stable(feature = "rust1", since = "1.0.0")]
1916 impl<'a> Add<&'a str> for String {
1917     type Output = String;
1918
1919     #[inline]
1920     fn add(mut self, other: &str) -> String {
1921         self.push_str(other);
1922         self
1923     }
1924 }
1925
1926 /// Implements the `+=` operator for appending to a `String`.
1927 ///
1928 /// This has the same behavior as the [`push_str`] method.
1929 ///
1930 /// [`push_str`]: struct.String.html#method.push_str
1931 #[stable(feature = "stringaddassign", since = "1.12.0")]
1932 impl<'a> AddAssign<&'a str> for String {
1933     #[inline]
1934     fn add_assign(&mut self, other: &str) {
1935         self.push_str(other);
1936     }
1937 }
1938
1939 #[stable(feature = "rust1", since = "1.0.0")]
1940 impl ops::Index<ops::Range<usize>> for String {
1941     type Output = str;
1942
1943     #[inline]
1944     fn index(&self, index: ops::Range<usize>) -> &str {
1945         &self[..][index]
1946     }
1947 }
1948 #[stable(feature = "rust1", since = "1.0.0")]
1949 impl ops::Index<ops::RangeTo<usize>> for String {
1950     type Output = str;
1951
1952     #[inline]
1953     fn index(&self, index: ops::RangeTo<usize>) -> &str {
1954         &self[..][index]
1955     }
1956 }
1957 #[stable(feature = "rust1", since = "1.0.0")]
1958 impl ops::Index<ops::RangeFrom<usize>> for String {
1959     type Output = str;
1960
1961     #[inline]
1962     fn index(&self, index: ops::RangeFrom<usize>) -> &str {
1963         &self[..][index]
1964     }
1965 }
1966 #[stable(feature = "rust1", since = "1.0.0")]
1967 impl ops::Index<ops::RangeFull> for String {
1968     type Output = str;
1969
1970     #[inline]
1971     fn index(&self, _index: ops::RangeFull) -> &str {
1972         unsafe { str::from_utf8_unchecked(&self.vec) }
1973     }
1974 }
1975 #[stable(feature = "inclusive_range", since = "1.26.0")]
1976 impl ops::Index<ops::RangeInclusive<usize>> for String {
1977     type Output = str;
1978
1979     #[inline]
1980     fn index(&self, index: ops::RangeInclusive<usize>) -> &str {
1981         Index::index(&**self, index)
1982     }
1983 }
1984 #[stable(feature = "inclusive_range", since = "1.26.0")]
1985 impl ops::Index<ops::RangeToInclusive<usize>> for String {
1986     type Output = str;
1987
1988     #[inline]
1989     fn index(&self, index: ops::RangeToInclusive<usize>) -> &str {
1990         Index::index(&**self, index)
1991     }
1992 }
1993
1994 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
1995 impl ops::IndexMut<ops::Range<usize>> for String {
1996     #[inline]
1997     fn index_mut(&mut self, index: ops::Range<usize>) -> &mut str {
1998         &mut self[..][index]
1999     }
2000 }
2001 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2002 impl ops::IndexMut<ops::RangeTo<usize>> for String {
2003     #[inline]
2004     fn index_mut(&mut self, index: ops::RangeTo<usize>) -> &mut str {
2005         &mut self[..][index]
2006     }
2007 }
2008 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2009 impl ops::IndexMut<ops::RangeFrom<usize>> for String {
2010     #[inline]
2011     fn index_mut(&mut self, index: ops::RangeFrom<usize>) -> &mut str {
2012         &mut self[..][index]
2013     }
2014 }
2015 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2016 impl ops::IndexMut<ops::RangeFull> for String {
2017     #[inline]
2018     fn index_mut(&mut self, _index: ops::RangeFull) -> &mut str {
2019         unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
2020     }
2021 }
2022 #[stable(feature = "inclusive_range", since = "1.26.0")]
2023 impl ops::IndexMut<ops::RangeInclusive<usize>> for String {
2024     #[inline]
2025     fn index_mut(&mut self, index: ops::RangeInclusive<usize>) -> &mut str {
2026         IndexMut::index_mut(&mut **self, index)
2027     }
2028 }
2029 #[stable(feature = "inclusive_range", since = "1.26.0")]
2030 impl ops::IndexMut<ops::RangeToInclusive<usize>> for String {
2031     #[inline]
2032     fn index_mut(&mut self, index: ops::RangeToInclusive<usize>) -> &mut str {
2033         IndexMut::index_mut(&mut **self, index)
2034     }
2035 }
2036
2037 #[stable(feature = "rust1", since = "1.0.0")]
2038 impl ops::Deref for String {
2039     type Target = str;
2040
2041     #[inline]
2042     fn deref(&self) -> &str {
2043         unsafe { str::from_utf8_unchecked(&self.vec) }
2044     }
2045 }
2046
2047 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2048 impl ops::DerefMut for String {
2049     #[inline]
2050     fn deref_mut(&mut self) -> &mut str {
2051         unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
2052     }
2053 }
2054
2055 /// An error when parsing a `String`.
2056 ///
2057 /// This `enum` is slightly awkward: it will never actually exist. This error is
2058 /// part of the type signature of the implementation of [`FromStr`] on
2059 /// [`String`]. The return type of [`from_str`], requires that an error be
2060 /// defined, but, given that a [`String`] can always be made into a new
2061 /// [`String`] without error, this type will never actually be returned. As
2062 /// such, it is only here to satisfy said signature, and is useless otherwise.
2063 ///
2064 /// [`FromStr`]: ../../std/str/trait.FromStr.html
2065 /// [`String`]: struct.String.html
2066 /// [`from_str`]: ../../std/str/trait.FromStr.html#tymethod.from_str
2067 #[stable(feature = "str_parse_error", since = "1.5.0")]
2068 #[derive(Copy)]
2069 pub enum ParseError {}
2070
2071 #[stable(feature = "rust1", since = "1.0.0")]
2072 impl FromStr for String {
2073     type Err = ParseError;
2074     #[inline]
2075     fn from_str(s: &str) -> Result<String, ParseError> {
2076         Ok(String::from(s))
2077     }
2078 }
2079
2080 #[stable(feature = "str_parse_error", since = "1.5.0")]
2081 impl Clone for ParseError {
2082     fn clone(&self) -> ParseError {
2083         match *self {}
2084     }
2085 }
2086
2087 #[stable(feature = "str_parse_error", since = "1.5.0")]
2088 impl fmt::Debug for ParseError {
2089     fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
2090         match *self {}
2091     }
2092 }
2093
2094 #[stable(feature = "str_parse_error2", since = "1.8.0")]
2095 impl fmt::Display for ParseError {
2096     fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
2097         match *self {}
2098     }
2099 }
2100
2101 #[stable(feature = "str_parse_error", since = "1.5.0")]
2102 impl PartialEq for ParseError {
2103     fn eq(&self, _: &ParseError) -> bool {
2104         match *self {}
2105     }
2106 }
2107
2108 #[stable(feature = "str_parse_error", since = "1.5.0")]
2109 impl Eq for ParseError {}
2110
2111 /// A trait for converting a value to a `String`.
2112 ///
2113 /// This trait is automatically implemented for any type which implements the
2114 /// [`Display`] trait. As such, `ToString` shouldn't be implemented directly:
2115 /// [`Display`] should be implemented instead, and you get the `ToString`
2116 /// implementation for free.
2117 ///
2118 /// [`Display`]: ../../std/fmt/trait.Display.html
2119 #[stable(feature = "rust1", since = "1.0.0")]
2120 pub trait ToString {
2121     /// Converts the given value to a `String`.
2122     ///
2123     /// # Examples
2124     ///
2125     /// Basic usage:
2126     ///
2127     /// ```
2128     /// let i = 5;
2129     /// let five = String::from("5");
2130     ///
2131     /// assert_eq!(five, i.to_string());
2132     /// ```
2133     #[rustc_conversion_suggestion]
2134     #[stable(feature = "rust1", since = "1.0.0")]
2135     fn to_string(&self) -> String;
2136 }
2137
2138 /// # Panics
2139 ///
2140 /// In this implementation, the `to_string` method panics
2141 /// if the `Display` implementation returns an error.
2142 /// This indicates an incorrect `Display` implementation
2143 /// since `fmt::Write for String` never returns an error itself.
2144 #[stable(feature = "rust1", since = "1.0.0")]
2145 impl<T: fmt::Display + ?Sized> ToString for T {
2146     #[inline]
2147     default fn to_string(&self) -> String {
2148         use core::fmt::Write;
2149         let mut buf = String::new();
2150         buf.write_fmt(format_args!("{}", self))
2151            .expect("a Display implementation return an error unexpectedly");
2152         buf.shrink_to_fit();
2153         buf
2154     }
2155 }
2156
2157 #[stable(feature = "str_to_string_specialization", since = "1.9.0")]
2158 impl ToString for str {
2159     #[inline]
2160     fn to_string(&self) -> String {
2161         String::from(self)
2162     }
2163 }
2164
2165 #[stable(feature = "cow_str_to_string_specialization", since = "1.17.0")]
2166 impl<'a> ToString for Cow<'a, str> {
2167     #[inline]
2168     fn to_string(&self) -> String {
2169         self[..].to_owned()
2170     }
2171 }
2172
2173 #[stable(feature = "string_to_string_specialization", since = "1.17.0")]
2174 impl ToString for String {
2175     #[inline]
2176     fn to_string(&self) -> String {
2177         self.to_owned()
2178     }
2179 }
2180
2181 #[stable(feature = "rust1", since = "1.0.0")]
2182 impl AsRef<str> for String {
2183     #[inline]
2184     fn as_ref(&self) -> &str {
2185         self
2186     }
2187 }
2188
2189 #[stable(feature = "rust1", since = "1.0.0")]
2190 impl AsRef<[u8]> for String {
2191     #[inline]
2192     fn as_ref(&self) -> &[u8] {
2193         self.as_bytes()
2194     }
2195 }
2196
2197 #[stable(feature = "rust1", since = "1.0.0")]
2198 impl<'a> From<&'a str> for String {
2199     fn from(s: &'a str) -> String {
2200         s.to_owned()
2201     }
2202 }
2203
2204 // note: test pulls in libstd, which causes errors here
2205 #[cfg(not(test))]
2206 #[stable(feature = "string_from_box", since = "1.18.0")]
2207 impl From<Box<str>> for String {
2208     fn from(s: Box<str>) -> String {
2209         s.into_string()
2210     }
2211 }
2212
2213 #[stable(feature = "box_from_str", since = "1.20.0")]
2214 impl From<String> for Box<str> {
2215     fn from(s: String) -> Box<str> {
2216         s.into_boxed_str()
2217     }
2218 }
2219
2220 #[stable(feature = "string_from_cow_str", since = "1.14.0")]
2221 impl<'a> From<Cow<'a, str>> for String {
2222     fn from(s: Cow<'a, str>) -> String {
2223         s.into_owned()
2224     }
2225 }
2226
2227 #[stable(feature = "rust1", since = "1.0.0")]
2228 impl<'a> From<&'a str> for Cow<'a, str> {
2229     #[inline]
2230     fn from(s: &'a str) -> Cow<'a, str> {
2231         Cow::Borrowed(s)
2232     }
2233 }
2234
2235 #[stable(feature = "rust1", since = "1.0.0")]
2236 impl<'a> From<String> for Cow<'a, str> {
2237     #[inline]
2238     fn from(s: String) -> Cow<'a, str> {
2239         Cow::Owned(s)
2240     }
2241 }
2242
2243 #[stable(feature = "cow_from_string_ref", since = "1.28.0")]
2244 impl<'a> From<&'a String> for Cow<'a, str> {
2245     #[inline]
2246     fn from(s: &'a String) -> Cow<'a, str> {
2247         Cow::Borrowed(s.as_str())
2248     }
2249 }
2250
2251 #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
2252 impl<'a> FromIterator<char> for Cow<'a, str> {
2253     fn from_iter<I: IntoIterator<Item = char>>(it: I) -> Cow<'a, str> {
2254         Cow::Owned(FromIterator::from_iter(it))
2255     }
2256 }
2257
2258 #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
2259 impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str> {
2260     fn from_iter<I: IntoIterator<Item = &'b str>>(it: I) -> Cow<'a, str> {
2261         Cow::Owned(FromIterator::from_iter(it))
2262     }
2263 }
2264
2265 #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
2266 impl<'a> FromIterator<String> for Cow<'a, str> {
2267     fn from_iter<I: IntoIterator<Item = String>>(it: I) -> Cow<'a, str> {
2268         Cow::Owned(FromIterator::from_iter(it))
2269     }
2270 }
2271
2272 #[stable(feature = "from_string_for_vec_u8", since = "1.14.0")]
2273 impl From<String> for Vec<u8> {
2274     fn from(string: String) -> Vec<u8> {
2275         string.into_bytes()
2276     }
2277 }
2278
2279 #[stable(feature = "rust1", since = "1.0.0")]
2280 impl fmt::Write for String {
2281     #[inline]
2282     fn write_str(&mut self, s: &str) -> fmt::Result {
2283         self.push_str(s);
2284         Ok(())
2285     }
2286
2287     #[inline]
2288     fn write_char(&mut self, c: char) -> fmt::Result {
2289         self.push(c);
2290         Ok(())
2291     }
2292 }
2293
2294 /// A draining iterator for `String`.
2295 ///
2296 /// This struct is created by the [`drain`] method on [`String`]. See its
2297 /// documentation for more.
2298 ///
2299 /// [`drain`]: struct.String.html#method.drain
2300 /// [`String`]: struct.String.html
2301 #[stable(feature = "drain", since = "1.6.0")]
2302 pub struct Drain<'a> {
2303     /// Will be used as &'a mut String in the destructor
2304     string: *mut String,
2305     /// Start of part to remove
2306     start: usize,
2307     /// End of part to remove
2308     end: usize,
2309     /// Current remaining range to remove
2310     iter: Chars<'a>,
2311 }
2312
2313 #[stable(feature = "collection_debug", since = "1.17.0")]
2314 impl<'a> fmt::Debug for Drain<'a> {
2315     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
2316         f.pad("Drain { .. }")
2317     }
2318 }
2319
2320 #[stable(feature = "drain", since = "1.6.0")]
2321 unsafe impl<'a> Sync for Drain<'a> {}
2322 #[stable(feature = "drain", since = "1.6.0")]
2323 unsafe impl<'a> Send for Drain<'a> {}
2324
2325 #[stable(feature = "drain", since = "1.6.0")]
2326 impl<'a> Drop for Drain<'a> {
2327     fn drop(&mut self) {
2328         unsafe {
2329             // Use Vec::drain. "Reaffirm" the bounds checks to avoid
2330             // panic code being inserted again.
2331             let self_vec = (*self.string).as_mut_vec();
2332             if self.start <= self.end && self.end <= self_vec.len() {
2333                 self_vec.drain(self.start..self.end);
2334             }
2335         }
2336     }
2337 }
2338
2339 #[stable(feature = "drain", since = "1.6.0")]
2340 impl<'a> Iterator for Drain<'a> {
2341     type Item = char;
2342
2343     #[inline]
2344     fn next(&mut self) -> Option<char> {
2345         self.iter.next()
2346     }
2347
2348     fn size_hint(&self) -> (usize, Option<usize>) {
2349         self.iter.size_hint()
2350     }
2351 }
2352
2353 #[stable(feature = "drain", since = "1.6.0")]
2354 impl<'a> DoubleEndedIterator for Drain<'a> {
2355     #[inline]
2356     fn next_back(&mut self) -> Option<char> {
2357         self.iter.next_back()
2358     }
2359 }
2360
2361 #[stable(feature = "fused", since = "1.26.0")]
2362 impl<'a> FusedIterator for Drain<'a> {}