src/liballoc/string.rs

   1 //! A UTF-8 encoded, growable string.
   2 //!
   3 //! This module contains the [`String`] type, a trait for converting
   4 //! [`ToString`]s, and several error types that may result from working with
   5 //! [`String`]s.
   6 //!
   7 //! [`ToString`]: trait.ToString.html
   8 //!
   9 //! # Examples
  10 //!
  11 //! There are multiple ways to create a new [`String`] from a string literal:
  12 //!
  13 //! ```
  14 //! let s = "Hello".to_string();
  15 //!
  16 //! let s = String::from("world");
  17 //! let s: String = "also this".into();
  18 //! ```
  19 //!
  20 //! You can create a new [`String`] from an existing one by concatenating with
  21 //! `+`:
  22 //!
  23 //! [`String`]: struct.String.html
  24 //!
  25 //! ```
  26 //! let s = "Hello".to_string();
  27 //!
  28 //! let message = s + " world!";
  29 //! ```
  30 //!
  31 //! If you have a vector of valid UTF-8 bytes, you can make a [`String`] out of
  32 //! it. You can do the reverse too.
  33 //!
  34 //! ```
  35 //! let sparkle_heart = vec![240, 159, 146, 150];
  36 //!
  37 //! // We know these bytes are valid, so we'll use `unwrap()`.
  38 //! let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
  39 //!
  40 //! assert_eq!("💖", sparkle_heart);
  41 //!
  42 //! let bytes = sparkle_heart.into_bytes();
  43 //!
  44 //! assert_eq!(bytes, [240, 159, 146, 150]);
  45 //! ```
  46
  47 #![stable(feature = "rust1", since = "1.0.0")]
  48
  49 use core::char::{decode_utf16, REPLACEMENT_CHARACTER};
  50 use core::fmt;
  51 use core::hash;
  52 use core::iter::{FromIterator, FusedIterator};
  53 use core::ops::Bound::{Excluded, Included, Unbounded};
  54 use core::ops::{self, Add, AddAssign, Index, IndexMut, RangeBounds};
  55 use core::ptr;
  56 use core::str::{lossy, pattern::Pattern};
  57
  58 use crate::borrow::{Cow, ToOwned};
  59 use crate::boxed::Box;
  60 use crate::collections::TryReserveError;
  61 use crate::str::{self, from_boxed_utf8_unchecked, Chars, FromStr, Utf8Error};
  62 use crate::vec::Vec;
  63
  64 /// A UTF-8 encoded, growable string.
  65 ///
  66 /// The `String` type is the most common string type that has ownership over the
  67 /// contents of the string. It has a close relationship with its borrowed
  68 /// counterpart, the primitive [`str`].
  69 ///
  70 /// [`str`]: ../../std/primitive.str.html
  71 ///
  72 /// # Examples
  73 ///
  74 /// You can create a `String` from a literal string with [`String::from`]:
  75 ///
  76 /// ```
  77 /// let hello = String::from("Hello, world!");
  78 /// ```
  79 ///
  80 /// You can append a [`char`] to a `String` with the [`push`] method, and
  81 /// append a [`&str`] with the [`push_str`] method:
  82 ///
  83 /// ```
  84 /// let mut hello = String::from("Hello, ");
  85 ///
  86 /// hello.push('w');
  87 /// hello.push_str("orld!");
  88 /// ```
  89 ///
  90 /// [`String::from`]: #method.from
  91 /// [`char`]: ../../std/primitive.char.html
  92 /// [`push`]: #method.push
  93 /// [`push_str`]: #method.push_str
  94 ///
  95 /// If you have a vector of UTF-8 bytes, you can create a `String` from it with
  96 /// the [`from_utf8`] method:
  97 ///
  98 /// ```
  99 /// // some bytes, in a vector
 100 /// let sparkle_heart = vec![240, 159, 146, 150];
 101 ///
 102 /// // We know these bytes are valid, so we'll use `unwrap()`.
 103 /// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
 104 ///
 105 /// assert_eq!("💖", sparkle_heart);
 106 /// ```
 107 ///
 108 /// [`from_utf8`]: #method.from_utf8
 109 ///
 110 /// # UTF-8
 111 ///
 112 /// `String`s are always valid UTF-8. This has a few implications, the first of
 113 /// which is that if you need a non-UTF-8 string, consider [`OsString`]. It is
 114 /// similar, but without the UTF-8 constraint. The second implication is that
 115 /// you cannot index into a `String`:
 116 ///
 117 /// ```compile_fail,E0277
 118 /// let s = "hello";
 119 ///
 120 /// println!("The first letter of s is {}", s[0]); // ERROR!!!
 121 /// ```
 122 ///
 123 /// [`OsString`]: ../../std/ffi/struct.OsString.html
 124 ///
 125 /// Indexing is intended to be a constant-time operation, but UTF-8 encoding
 126 /// does not allow us to do this. Furthermore, it's not clear what sort of
 127 /// thing the index should return: a byte, a codepoint, or a grapheme cluster.
 128 /// The [`bytes`] and [`chars`] methods return iterators over the first
 129 /// two, respectively.
 130 ///
 131 /// [`bytes`]: #method.bytes
 132 /// [`chars`]: #method.chars
 133 ///
 134 /// # Deref
 135 ///
 136 /// `String`s implement [`Deref`]`<Target=str>`, and so inherit all of [`str`]'s
 137 /// methods. In addition, this means that you can pass a `String` to a
 138 /// function which takes a [`&str`] by using an ampersand (`&`):
 139 ///
 140 /// ```
 141 /// fn takes_str(s: &str) { }
 142 ///
 143 /// let s = String::from("Hello");
 144 ///
 145 /// takes_str(&s);
 146 /// ```
 147 ///
 148 /// This will create a [`&str`] from the `String` and pass it in. This
 149 /// conversion is very inexpensive, and so generally, functions will accept
 150 /// [`&str`]s as arguments unless they need a `String` for some specific
 151 /// reason.
 152 ///
 153 /// In certain cases Rust doesn't have enough information to make this
 154 /// conversion, known as [`Deref`] coercion. In the following example a string
 155 /// slice [`&'a str`][`&str`] implements the trait `TraitExample`, and the function
 156 /// `example_func` takes anything that implements the trait. In this case Rust
 157 /// would need to make two implicit conversions, which Rust doesn't have the
 158 /// means to do. For that reason, the following example will not compile.
 159 ///
 160 /// ```compile_fail,E0277
 161 /// trait TraitExample {}
 162 ///
 163 /// impl<'a> TraitExample for &'a str {}
 164 ///
 165 /// fn example_func<A: TraitExample>(example_arg: A) {}
 166 ///
 167 /// let example_string = String::from("example_string");
 168 /// example_func(&example_string);
 169 /// ```
 170 ///
 171 /// There are two options that would work instead. The first would be to
 172 /// change the line `example_func(&example_string);` to
 173 /// `example_func(example_string.as_str());`, using the method [`as_str()`]
 174 /// to explicitly extract the string slice containing the string. The second
 175 /// way changes `example_func(&example_string);` to
 176 /// `example_func(&*example_string);`. In this case we are dereferencing a
 177 /// `String` to a [`str`][`&str`], then referencing the [`str`][`&str`] back to
 178 /// [`&str`]. The second way is more idiomatic, however both work to do the
 179 /// conversion explicitly rather than relying on the implicit conversion.
 180 ///
 181 /// # Representation
 182 ///
 183 /// A `String` is made up of three components: a pointer to some bytes, a
 184 /// length, and a capacity. The pointer points to an internal buffer `String`
 185 /// uses to store its data. The length is the number of bytes currently stored
 186 /// in the buffer, and the capacity is the size of the buffer in bytes. As such,
 187 /// the length will always be less than or equal to the capacity.
 188 ///
 189 /// This buffer is always stored on the heap.
 190 ///
 191 /// You can look at these with the [`as_ptr`], [`len`], and [`capacity`]
 192 /// methods:
 193 ///
 194 /// ```
 195 /// use std::mem;
 196 ///
 197 /// let story = String::from("Once upon a time...");
 198 ///
 199 // FIXME Update this when vec_into_raw_parts is stabilized
 200 /// // Prevent automatically dropping the String's data
 201 /// let mut story = mem::ManuallyDrop::new(story);
 202 ///
 203 /// let ptr = story.as_mut_ptr();
 204 /// let len = story.len();
 205 /// let capacity = story.capacity();
 206 ///
 207 /// // story has nineteen bytes
 208 /// assert_eq!(19, len);
 209 ///
 210 /// // We can re-build a String out of ptr, len, and capacity. This is all
 211 /// // unsafe because we are responsible for making sure the components are
 212 /// // valid:
 213 /// let s = unsafe { String::from_raw_parts(ptr, len, capacity) } ;
 214 ///
 215 /// assert_eq!(String::from("Once upon a time..."), s);
 216 /// ```
 217 ///
 218 /// [`as_ptr`]: #method.as_ptr
 219 /// [`len`]: #method.len
 220 /// [`capacity`]: #method.capacity
 221 ///
 222 /// If a `String` has enough capacity, adding elements to it will not
 223 /// re-allocate. For example, consider this program:
 224 ///
 225 /// ```
 226 /// let mut s = String::new();
 227 ///
 228 /// println!("{}", s.capacity());
 229 ///
 230 /// for _ in 0..5 {
 231 ///     s.push_str("hello");
 232 ///     println!("{}", s.capacity());
 233 /// }
 234 /// ```
 235 ///
 236 /// This will output the following:
 237 ///
 238 /// ```text
 239 /// 0
 240 /// 5
 241 /// 10
 242 /// 20
 243 /// 20
 244 /// 40
 245 /// ```
 246 ///
 247 /// At first, we have no memory allocated at all, but as we append to the
 248 /// string, it increases its capacity appropriately. If we instead use the
 249 /// [`with_capacity`] method to allocate the correct capacity initially:
 250 ///
 251 /// ```
 252 /// let mut s = String::with_capacity(25);
 253 ///
 254 /// println!("{}", s.capacity());
 255 ///
 256 /// for _ in 0..5 {
 257 ///     s.push_str("hello");
 258 ///     println!("{}", s.capacity());
 259 /// }
 260 /// ```
 261 ///
 262 /// [`with_capacity`]: #method.with_capacity
 263 ///
 264 /// We end up with a different output:
 265 ///
 266 /// ```text
 267 /// 25
 268 /// 25
 269 /// 25
 270 /// 25
 271 /// 25
 272 /// 25
 273 /// ```
 274 ///
 275 /// Here, there's no need to allocate more memory inside the loop.
 276 ///
 277 /// [`&str`]: ../../std/primitive.str.html
 278 /// [`Deref`]: ../../std/ops/trait.Deref.html
 279 /// [`as_str()`]: struct.String.html#method.as_str
 280 #[derive(PartialOrd, Eq, Ord)]
 281 #[cfg_attr(not(test), rustc_diagnostic_item = "string_type")]
 282 #[stable(feature = "rust1", since = "1.0.0")]
 283 pub struct String {
 284     vec: Vec<u8>,
 285 }
 286
 287 /// A possible error value when converting a `String` from a UTF-8 byte vector.
 288 ///
 289 /// This type is the error type for the [`from_utf8`] method on [`String`]. It
 290 /// is designed in such a way to carefully avoid reallocations: the
 291 /// [`into_bytes`] method will give back the byte vector that was used in the
 292 /// conversion attempt.
 293 ///
 294 /// [`from_utf8`]: struct.String.html#method.from_utf8
 295 /// [`String`]: struct.String.html
 296 /// [`into_bytes`]: struct.FromUtf8Error.html#method.into_bytes
 297 ///
 298 /// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
 299 /// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
 300 /// an analogue to `FromUtf8Error`, and you can get one from a `FromUtf8Error`
 301 /// through the [`utf8_error`] method.
 302 ///
 303 /// [`Utf8Error`]: ../../std/str/struct.Utf8Error.html
 304 /// [`std::str`]: ../../std/str/index.html
 305 /// [`u8`]: ../../std/primitive.u8.html
 306 /// [`&str`]: ../../std/primitive.str.html
 307 /// [`utf8_error`]: #method.utf8_error
 308 ///
 309 /// # Examples
 310 ///
 311 /// Basic usage:
 312 ///
 313 /// ```
 314 /// // some invalid bytes, in a vector
 315 /// let bytes = vec![0, 159];
 316 ///
 317 /// let value = String::from_utf8(bytes);
 318 ///
 319 /// assert!(value.is_err());
 320 /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
 321 /// ```
 322 #[stable(feature = "rust1", since = "1.0.0")]
 323 #[derive(Debug, Clone, PartialEq, Eq)]
 324 pub struct FromUtf8Error {
 325     bytes: Vec<u8>,
 326     error: Utf8Error,
 327 }
 328
 329 /// A possible error value when converting a `String` from a UTF-16 byte slice.
 330 ///
 331 /// This type is the error type for the [`from_utf16`] method on [`String`].
 332 ///
 333 /// [`from_utf16`]: struct.String.html#method.from_utf16
 334 /// [`String`]: struct.String.html
 335 ///
 336 /// # Examples
 337 ///
 338 /// Basic usage:
 339 ///
 340 /// ```
 341 /// // 𝄞mu<invalid>ic
 342 /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
 343 ///           0xD800, 0x0069, 0x0063];
 344 ///
 345 /// assert!(String::from_utf16(v).is_err());
 346 /// ```
 347 #[stable(feature = "rust1", since = "1.0.0")]
 348 #[derive(Debug)]
 349 pub struct FromUtf16Error(());
 350
 351 impl String {
 352     /// Creates a new empty `String`.
 353     ///
 354     /// Given that the `String` is empty, this will not allocate any initial
 355     /// buffer. While that means that this initial operation is very
 356     /// inexpensive, it may cause excessive allocation later when you add
 357     /// data. If you have an idea of how much data the `String` will hold,
 358     /// consider the [`with_capacity`] method to prevent excessive
 359     /// re-allocation.
 360     ///
 361     /// [`with_capacity`]: #method.with_capacity
 362     ///
 363     /// # Examples
 364     ///
 365     /// Basic usage:
 366     ///
 367     /// ```
 368     /// let s = String::new();
 369     /// ```
 370     #[inline]
 371     #[rustc_const_stable(feature = "const_string_new", since = "1.32.0")]
 372     #[stable(feature = "rust1", since = "1.0.0")]
 373     pub const fn new() -> String {
 374         String { vec: Vec::new() }
 375     }
 376
 377     /// Creates a new empty `String` with a particular capacity.
 378     ///
 379     /// `String`s have an internal buffer to hold their data. The capacity is
 380     /// the length of that buffer, and can be queried with the [`capacity`]
 381     /// method. This method creates an empty `String`, but one with an initial
 382     /// buffer that can hold `capacity` bytes. This is useful when you may be
 383     /// appending a bunch of data to the `String`, reducing the number of
 384     /// reallocations it needs to do.
 385     ///
 386     /// [`capacity`]: #method.capacity
 387     ///
 388     /// If the given capacity is `0`, no allocation will occur, and this method
 389     /// is identical to the [`new`] method.
 390     ///
 391     /// [`new`]: #method.new
 392     ///
 393     /// # Examples
 394     ///
 395     /// Basic usage:
 396     ///
 397     /// ```
 398     /// let mut s = String::with_capacity(10);
 399     ///
 400     /// // The String contains no chars, even though it has capacity for more
 401     /// assert_eq!(s.len(), 0);
 402     ///
 403     /// // These are all done without reallocating...
 404     /// let cap = s.capacity();
 405     /// for _ in 0..10 {
 406     ///     s.push('a');
 407     /// }
 408     ///
 409     /// assert_eq!(s.capacity(), cap);
 410     ///
 411     /// // ...but this may make the string reallocate
 412     /// s.push('a');
 413     /// ```
 414     #[inline]
 415     #[stable(feature = "rust1", since = "1.0.0")]
 416     pub fn with_capacity(capacity: usize) -> String {
 417         String { vec: Vec::with_capacity(capacity) }
 418     }
 419
 420     // HACK(japaric): with cfg(test) the inherent `[T]::to_vec` method, which is
 421     // required for this method definition, is not available. Since we don't
 422     // require this method for testing purposes, I'll just stub it
 423     // NB see the slice::hack module in slice.rs for more information
 424     #[inline]
 425     #[cfg(test)]
 426     pub fn from_str(_: &str) -> String {
 427         panic!("not available with cfg(test)");
 428     }
 429
 430     /// Converts a vector of bytes to a `String`.
 431     ///
 432     /// A string ([`String`]) is made of bytes ([`u8`]), and a vector of bytes
 433     /// ([`Vec<u8>`]) is made of bytes, so this function converts between the
 434     /// two. Not all byte slices are valid `String`s, however: `String`
 435     /// requires that it is valid UTF-8. `from_utf8()` checks to ensure that
 436     /// the bytes are valid UTF-8, and then does the conversion.
 437     ///
 438     /// If you are sure that the byte slice is valid UTF-8, and you don't want
 439     /// to incur the overhead of the validity check, there is an unsafe version
 440     /// of this function, [`from_utf8_unchecked`], which has the same behavior
 441     /// but skips the check.
 442     ///
 443     /// This method will take care to not copy the vector, for efficiency's
 444     /// sake.
 445     ///
 446     /// If you need a [`&str`] instead of a `String`, consider
 447     /// [`str::from_utf8`].
 448     ///
 449     /// The inverse of this method is [`into_bytes`].
 450     ///
 451     /// # Errors
 452     ///
 453     /// Returns [`Err`] if the slice is not UTF-8 with a description as to why the
 454     /// provided bytes are not UTF-8. The vector you moved in is also included.
 455     ///
 456     /// # Examples
 457     ///
 458     /// Basic usage:
 459     ///
 460     /// ```
 461     /// // some bytes, in a vector
 462     /// let sparkle_heart = vec![240, 159, 146, 150];
 463     ///
 464     /// // We know these bytes are valid, so we'll use `unwrap()`.
 465     /// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
 466     ///
 467     /// assert_eq!("💖", sparkle_heart);
 468     /// ```
 469     ///
 470     /// Incorrect bytes:
 471     ///
 472     /// ```
 473     /// // some invalid bytes, in a vector
 474     /// let sparkle_heart = vec![0, 159, 146, 150];
 475     ///
 476     /// assert!(String::from_utf8(sparkle_heart).is_err());
 477     /// ```
 478     ///
 479     /// See the docs for [`FromUtf8Error`] for more details on what you can do
 480     /// with this error.
 481     ///
 482     /// [`from_utf8_unchecked`]: struct.String.html#method.from_utf8_unchecked
 483     /// [`String`]: struct.String.html
 484     /// [`u8`]: ../../std/primitive.u8.html
 485     /// [`Vec<u8>`]: ../../std/vec/struct.Vec.html
 486     /// [`&str`]: ../../std/primitive.str.html
 487     /// [`str::from_utf8`]: ../../std/str/fn.from_utf8.html
 488     /// [`into_bytes`]: struct.String.html#method.into_bytes
 489     /// [`FromUtf8Error`]: struct.FromUtf8Error.html
 490     /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
 491     #[inline]
 492     #[stable(feature = "rust1", since = "1.0.0")]
 493     pub fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error> {
 494         match str::from_utf8(&vec) {
 495             Ok(..) => Ok(String { vec }),
 496             Err(e) => Err(FromUtf8Error { bytes: vec, error: e }),
 497         }
 498     }
 499
 500     /// Converts a slice of bytes to a string, including invalid characters.
 501     ///
 502     /// Strings are made of bytes ([`u8`]), and a slice of bytes
 503     /// ([`&[u8]`][byteslice]) is made of bytes, so this function converts
 504     /// between the two. Not all byte slices are valid strings, however: strings
 505     /// are required to be valid UTF-8. During this conversion,
 506     /// `from_utf8_lossy()` will replace any invalid UTF-8 sequences with
 507     /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD], which looks like this: �
 508     ///
 509     /// [`u8`]: ../../std/primitive.u8.html
 510     /// [byteslice]: ../../std/primitive.slice.html
 511     /// [U+FFFD]: ../char/constant.REPLACEMENT_CHARACTER.html
 512     ///
 513     /// If you are sure that the byte slice is valid UTF-8, and you don't want
 514     /// to incur the overhead of the conversion, there is an unsafe version
 515     /// of this function, [`from_utf8_unchecked`], which has the same behavior
 516     /// but skips the checks.
 517     ///
 518     /// [`from_utf8_unchecked`]: struct.String.html#method.from_utf8_unchecked
 519     ///
 520     /// This function returns a [`Cow<'a, str>`]. If our byte slice is invalid
 521     /// UTF-8, then we need to insert the replacement characters, which will
 522     /// change the size of the string, and hence, require a `String`. But if
 523     /// it's already valid UTF-8, we don't need a new allocation. This return
 524     /// type allows us to handle both cases.
 525     ///
 526     /// [`Cow<'a, str>`]: ../../std/borrow/enum.Cow.html
 527     ///
 528     /// # Examples
 529     ///
 530     /// Basic usage:
 531     ///
 532     /// ```
 533     /// // some bytes, in a vector
 534     /// let sparkle_heart = vec![240, 159, 146, 150];
 535     ///
 536     /// let sparkle_heart = String::from_utf8_lossy(&sparkle_heart);
 537     ///
 538     /// assert_eq!("💖", sparkle_heart);
 539     /// ```
 540     ///
 541     /// Incorrect bytes:
 542     ///
 543     /// ```
 544     /// // some invalid bytes
 545     /// let input = b"Hello \xF0\x90\x80World";
 546     /// let output = String::from_utf8_lossy(input);
 547     ///
 548     /// assert_eq!("Hello �World", output);
 549     /// ```
 550     #[stable(feature = "rust1", since = "1.0.0")]
 551     pub fn from_utf8_lossy(v: &[u8]) -> Cow<'_, str> {
 552         let mut iter = lossy::Utf8Lossy::from_bytes(v).chunks();
 553
 554         let (first_valid, first_broken) = if let Some(chunk) = iter.next() {
 555             let lossy::Utf8LossyChunk { valid, broken } = chunk;
 556             if valid.len() == v.len() {
 557                 debug_assert!(broken.is_empty());
 558                 return Cow::Borrowed(valid);
 559             }
 560             (valid, broken)
 561         } else {
 562             return Cow::Borrowed("");
 563         };
 564
 565         const REPLACEMENT: &str = "\u{FFFD}";
 566
 567         let mut res = String::with_capacity(v.len());
 568         res.push_str(first_valid);
 569         if !first_broken.is_empty() {
 570             res.push_str(REPLACEMENT);
 571         }
 572
 573         for lossy::Utf8LossyChunk { valid, broken } in iter {
 574             res.push_str(valid);
 575             if !broken.is_empty() {
 576                 res.push_str(REPLACEMENT);
 577             }
 578         }
 579
 580         Cow::Owned(res)
 581     }
 582
 583     /// Decode a UTF-16 encoded vector `v` into a `String`, returning [`Err`]
 584     /// if `v` contains any invalid data.
 585     ///
 586     /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
 587     ///
 588     /// # Examples
 589     ///
 590     /// Basic usage:
 591     ///
 592     /// ```
 593     /// // 𝄞music
 594     /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
 595     ///           0x0073, 0x0069, 0x0063];
 596     /// assert_eq!(String::from("𝄞music"),
 597     ///            String::from_utf16(v).unwrap());
 598     ///
 599     /// // 𝄞mu<invalid>ic
 600     /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
 601     ///           0xD800, 0x0069, 0x0063];
 602     /// assert!(String::from_utf16(v).is_err());
 603     /// ```
 604     #[stable(feature = "rust1", since = "1.0.0")]
 605     pub fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error> {
 606         // This isn't done via collect::<Result<_, _>>() for performance reasons.
 607         // FIXME: the function can be simplified again when #48994 is closed.
 608         let mut ret = String::with_capacity(v.len());
 609         for c in decode_utf16(v.iter().cloned()) {
 610             if let Ok(c) = c {
 611                 ret.push(c);
 612             } else {
 613                 return Err(FromUtf16Error(()));
 614             }
 615         }
 616         Ok(ret)
 617     }
 618
 619     /// Decode a UTF-16 encoded slice `v` into a `String`, replacing
 620     /// invalid data with [the replacement character (`U+FFFD`)][U+FFFD].
 621     ///
 622     /// Unlike [`from_utf8_lossy`] which returns a [`Cow<'a, str>`],
 623     /// `from_utf16_lossy` returns a `String` since the UTF-16 to UTF-8
 624     /// conversion requires a memory allocation.
 625     ///
 626     /// [`from_utf8_lossy`]: #method.from_utf8_lossy
 627     /// [`Cow<'a, str>`]: ../borrow/enum.Cow.html
 628     /// [U+FFFD]: ../char/constant.REPLACEMENT_CHARACTER.html
 629     ///
 630     /// # Examples
 631     ///
 632     /// Basic usage:
 633     ///
 634     /// ```
 635     /// // 𝄞mus<invalid>ic<invalid>
 636     /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
 637     ///           0x0073, 0xDD1E, 0x0069, 0x0063,
 638     ///           0xD834];
 639     ///
 640     /// assert_eq!(String::from("𝄞mus\u{FFFD}ic\u{FFFD}"),
 641     ///            String::from_utf16_lossy(v));
 642     /// ```
 643     #[inline]
 644     #[stable(feature = "rust1", since = "1.0.0")]
 645     pub fn from_utf16_lossy(v: &[u16]) -> String {
 646         decode_utf16(v.iter().cloned()).map(|r| r.unwrap_or(REPLACEMENT_CHARACTER)).collect()
 647     }
 648
 649     /// Decomposes a `String` into its raw components.
 650     ///
 651     /// Returns the raw pointer to the underlying data, the length of
 652     /// the string (in bytes), and the allocated capacity of the data
 653     /// (in bytes). These are the same arguments in the same order as
 654     /// the arguments to [`from_raw_parts`].
 655     ///
 656     /// After calling this function, the caller is responsible for the
 657     /// memory previously managed by the `String`. The only way to do
 658     /// this is to convert the raw pointer, length, and capacity back
 659     /// into a `String` with the [`from_raw_parts`] function, allowing
 660     /// the destructor to perform the cleanup.
 661     ///
 662     /// [`from_raw_parts`]: #method.from_raw_parts
 663     ///
 664     /// # Examples
 665     ///
 666     /// ```
 667     /// #![feature(vec_into_raw_parts)]
 668     /// let s = String::from("hello");
 669     ///
 670     /// let (ptr, len, cap) = s.into_raw_parts();
 671     ///
 672     /// let rebuilt = unsafe { String::from_raw_parts(ptr, len, cap) };
 673     /// assert_eq!(rebuilt, "hello");
 674     /// ```
 675     #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
 676     pub fn into_raw_parts(self) -> (*mut u8, usize, usize) {
 677         self.vec.into_raw_parts()
 678     }
 679
 680     /// Creates a new `String` from a length, capacity, and pointer.
 681     ///
 682     /// # Safety
 683     ///
 684     /// This is highly unsafe, due to the number of invariants that aren't
 685     /// checked:
 686     ///
 687     /// * The memory at `ptr` needs to have been previously allocated by the
 688     ///   same allocator the standard library uses, with a required alignment of exactly 1.
 689     /// * `length` needs to be less than or equal to `capacity`.
 690     /// * `capacity` needs to be the correct value.
 691     ///
 692     /// Violating these may cause problems like corrupting the allocator's
 693     /// internal data structures.
 694     ///
 695     /// The ownership of `ptr` is effectively transferred to the
 696     /// `String` which may then deallocate, reallocate or change the
 697     /// contents of memory pointed to by the pointer at will. Ensure
 698     /// that nothing else uses the pointer after calling this
 699     /// function.
 700     ///
 701     /// # Examples
 702     ///
 703     /// Basic usage:
 704     ///
 705     /// ```
 706     /// use std::mem;
 707     ///
 708     /// unsafe {
 709     ///     let s = String::from("hello");
 710     ///
 711     // FIXME Update this when vec_into_raw_parts is stabilized
 712     ///     // Prevent automatically dropping the String's data
 713     ///     let mut s = mem::ManuallyDrop::new(s);
 714     ///
 715     ///     let ptr = s.as_mut_ptr();
 716     ///     let len = s.len();
 717     ///     let capacity = s.capacity();
 718     ///
 719     ///     let s = String::from_raw_parts(ptr, len, capacity);
 720     ///
 721     ///     assert_eq!(String::from("hello"), s);
 722     /// }
 723     /// ```
 724     #[inline]
 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) }
 728     }
 729
 730     /// Converts a vector of bytes to a `String` without checking that the
 731     /// string contains valid UTF-8.
 732     ///
 733     /// See the safe version, [`from_utf8`], for more details.
 734     ///
 735     /// [`from_utf8`]: struct.String.html#method.from_utf8
 736     ///
 737     /// # Safety
 738     ///
 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.
 743     ///
 744     /// # Examples
 745     ///
 746     /// Basic usage:
 747     ///
 748     /// ```
 749     /// // some bytes, in a vector
 750     /// let sparkle_heart = vec![240, 159, 146, 150];
 751     ///
 752     /// let sparkle_heart = unsafe {
 753     ///     String::from_utf8_unchecked(sparkle_heart)
 754     /// };
 755     ///
 756     /// assert_eq!("💖", sparkle_heart);
 757     /// ```
 758     #[inline]
 759     #[stable(feature = "rust1", since = "1.0.0")]
 760     pub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String {
 761         String { vec: bytes }
 762     }
 763
 764     /// Converts a `String` into a byte vector.
 765     ///
 766     /// This consumes the `String`, so we do not need to copy its contents.
 767     ///
 768     /// # Examples
 769     ///
 770     /// Basic usage:
 771     ///
 772     /// ```
 773     /// let s = String::from("hello");
 774     /// let bytes = s.into_bytes();
 775     ///
 776     /// assert_eq!(&[104, 101, 108, 108, 111][..], &bytes[..]);
 777     /// ```
 778     #[inline]
 779     #[stable(feature = "rust1", since = "1.0.0")]
 780     pub fn into_bytes(self) -> Vec<u8> {
 781         self.vec
 782     }
 783
 784     /// Extracts a string slice containing the entire `String`.
 785     ///
 786     /// # Examples
 787     ///
 788     /// Basic usage:
 789     ///
 790     /// ```
 791     /// let s = String::from("foo");
 792     ///
 793     /// assert_eq!("foo", s.as_str());
 794     /// ```
 795     #[inline]
 796     #[stable(feature = "string_as_str", since = "1.7.0")]
 797     pub fn as_str(&self) -> &str {
 798         self
 799     }
 800
 801     /// Converts a `String` into a mutable string slice.
 802     ///
 803     /// # Examples
 804     ///
 805     /// Basic usage:
 806     ///
 807     /// ```
 808     /// let mut s = String::from("foobar");
 809     /// let s_mut_str = s.as_mut_str();
 810     ///
 811     /// s_mut_str.make_ascii_uppercase();
 812     ///
 813     /// assert_eq!("FOOBAR", s_mut_str);
 814     /// ```
 815     #[inline]
 816     #[stable(feature = "string_as_str", since = "1.7.0")]
 817     pub fn as_mut_str(&mut self) -> &mut str {
 818         self
 819     }
 820
 821     /// Appends a given string slice onto the end of this `String`.
 822     ///
 823     /// # Examples
 824     ///
 825     /// Basic usage:
 826     ///
 827     /// ```
 828     /// let mut s = String::from("foo");
 829     ///
 830     /// s.push_str("bar");
 831     ///
 832     /// assert_eq!("foobar", s);
 833     /// ```
 834     #[inline]
 835     #[stable(feature = "rust1", since = "1.0.0")]
 836     pub fn push_str(&mut self, string: &str) {
 837         self.vec.extend_from_slice(string.as_bytes())
 838     }
 839
 840     /// Returns this `String`'s capacity, in bytes.
 841     ///
 842     /// # Examples
 843     ///
 844     /// Basic usage:
 845     ///
 846     /// ```
 847     /// let s = String::with_capacity(10);
 848     ///
 849     /// assert!(s.capacity() >= 10);
 850     /// ```
 851     #[inline]
 852     #[stable(feature = "rust1", since = "1.0.0")]
 853     pub fn capacity(&self) -> usize {
 854         self.vec.capacity()
 855     }
 856
 857     /// Ensures that this `String`'s capacity is at least `additional` bytes
 858     /// larger than its length.
 859     ///
 860     /// The capacity may be increased by more than `additional` bytes if it
 861     /// chooses, to prevent frequent reallocations.
 862     ///
 863     /// If you do not want this "at least" behavior, see the [`reserve_exact`]
 864     /// method.
 865     ///
 866     /// # Panics
 867     ///
 868     /// Panics if the new capacity overflows [`usize`].
 869     ///
 870     /// [`reserve_exact`]: struct.String.html#method.reserve_exact
 871     /// [`usize`]: ../../std/primitive.usize.html
 872     ///
 873     /// # Examples
 874     ///
 875     /// Basic usage:
 876     ///
 877     /// ```
 878     /// let mut s = String::new();
 879     ///
 880     /// s.reserve(10);
 881     ///
 882     /// assert!(s.capacity() >= 10);
 883     /// ```
 884     ///
 885     /// This may not actually increase the capacity:
 886     ///
 887     /// ```
 888     /// let mut s = String::with_capacity(10);
 889     /// s.push('a');
 890     /// s.push('b');
 891     ///
 892     /// // s now has a length of 2 and a capacity of 10
 893     /// assert_eq!(2, s.len());
 894     /// assert_eq!(10, s.capacity());
 895     ///
 896     /// // Since we already have an extra 8 capacity, calling this...
 897     /// s.reserve(8);
 898     ///
 899     /// // ... doesn't actually increase.
 900     /// assert_eq!(10, s.capacity());
 901     /// ```
 902     #[inline]
 903     #[stable(feature = "rust1", since = "1.0.0")]
 904     pub fn reserve(&mut self, additional: usize) {
 905         self.vec.reserve(additional)
 906     }
 907
 908     /// Ensures that this `String`'s capacity is `additional` bytes
 909     /// larger than its length.
 910     ///
 911     /// Consider using the [`reserve`] method unless you absolutely know
 912     /// better than the allocator.
 913     ///
 914     /// [`reserve`]: #method.reserve
 915     ///
 916     /// # Panics
 917     ///
 918     /// Panics if the new capacity overflows `usize`.
 919     ///
 920     /// # Examples
 921     ///
 922     /// Basic usage:
 923     ///
 924     /// ```
 925     /// let mut s = String::new();
 926     ///
 927     /// s.reserve_exact(10);
 928     ///
 929     /// assert!(s.capacity() >= 10);
 930     /// ```
 931     ///
 932     /// This may not actually increase the capacity:
 933     ///
 934     /// ```
 935     /// let mut s = String::with_capacity(10);
 936     /// s.push('a');
 937     /// s.push('b');
 938     ///
 939     /// // s now has a length of 2 and a capacity of 10
 940     /// assert_eq!(2, s.len());
 941     /// assert_eq!(10, s.capacity());
 942     ///
 943     /// // Since we already have an extra 8 capacity, calling this...
 944     /// s.reserve_exact(8);
 945     ///
 946     /// // ... doesn't actually increase.
 947     /// assert_eq!(10, s.capacity());
 948     /// ```
 949     #[inline]
 950     #[stable(feature = "rust1", since = "1.0.0")]
 951     pub fn reserve_exact(&mut self, additional: usize) {
 952         self.vec.reserve_exact(additional)
 953     }
 954
 955     /// Tries to reserve capacity for at least `additional` more elements to be inserted
 956     /// in the given `String`. The collection may reserve more space to avoid
 957     /// frequent reallocations. After calling `reserve`, capacity will be
 958     /// greater than or equal to `self.len() + additional`. Does nothing if
 959     /// capacity is already sufficient.
 960     ///
 961     /// # Errors
 962     ///
 963     /// If the capacity overflows, or the allocator reports a failure, then an error
 964     /// is returned.
 965     ///
 966     /// # Examples
 967     ///
 968     /// ```
 969     /// #![feature(try_reserve)]
 970     /// use std::collections::TryReserveError;
 971     ///
 972     /// fn process_data(data: &str) -> Result<String, TryReserveError> {
 973     ///     let mut output = String::new();
 974     ///
 975     ///     // Pre-reserve the memory, exiting if we can't
 976     ///     output.try_reserve(data.len())?;
 977     ///
 978     ///     // Now we know this can't OOM in the middle of our complex work
 979     ///     output.push_str(data);
 980     ///
 981     ///     Ok(output)
 982     /// }
 983     /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
 984     /// ```
 985     #[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]
 986     pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
 987         self.vec.try_reserve(additional)
 988     }
 989
 990     /// Tries to reserves the minimum capacity for exactly `additional` more elements to
 991     /// be inserted in the given `String`. After calling `reserve_exact`,
 992     /// capacity will be greater than or equal to `self.len() + additional`.
 993     /// Does nothing if the capacity is already sufficient.
 994     ///
 995     /// Note that the allocator may give the collection more space than it
 996     /// requests. Therefore, capacity can not be relied upon to be precisely
 997     /// minimal. Prefer `reserve` if future insertions are expected.
 998     ///
 999     /// # Errors
1000     ///
1001     /// If the capacity overflows, or the allocator reports a failure, then an error
1002     /// is returned.
1003     ///
1004     /// # Examples
1005     ///
1006     /// ```
1007     /// #![feature(try_reserve)]
1008     /// use std::collections::TryReserveError;
1009     ///
1010     /// fn process_data(data: &str) -> Result<String, TryReserveError> {
1011     ///     let mut output = String::new();
1012     ///
1013     ///     // Pre-reserve the memory, exiting if we can't
1014     ///     output.try_reserve(data.len())?;
1015     ///
1016     ///     // Now we know this can't OOM in the middle of our complex work
1017     ///     output.push_str(data);
1018     ///
1019     ///     Ok(output)
1020     /// }
1021     /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
1022     /// ```
1023     #[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]
1024     pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
1025         self.vec.try_reserve_exact(additional)
1026     }
1027
1028     /// Shrinks the capacity of this `String` to match its length.
1029     ///
1030     /// # Examples
1031     ///
1032     /// Basic usage:
1033     ///
1034     /// ```
1035     /// let mut s = String::from("foo");
1036     ///
1037     /// s.reserve(100);
1038     /// assert!(s.capacity() >= 100);
1039     ///
1040     /// s.shrink_to_fit();
1041     /// assert_eq!(3, s.capacity());
1042     /// ```
1043     #[inline]
1044     #[stable(feature = "rust1", since = "1.0.0")]
1045     pub fn shrink_to_fit(&mut self) {
1046         self.vec.shrink_to_fit()
1047     }
1048
1049     /// Shrinks the capacity of this `String` with a lower bound.
1050     ///
1051     /// The capacity will remain at least as large as both the length
1052     /// and the supplied value.
1053     ///
1054     /// Panics if the current capacity is smaller than the supplied
1055     /// minimum capacity.
1056     ///
1057     /// # Examples
1058     ///
1059     /// ```
1060     /// #![feature(shrink_to)]
1061     /// let mut s = String::from("foo");
1062     ///
1063     /// s.reserve(100);
1064     /// assert!(s.capacity() >= 100);
1065     ///
1066     /// s.shrink_to(10);
1067     /// assert!(s.capacity() >= 10);
1068     /// s.shrink_to(0);
1069     /// assert!(s.capacity() >= 3);
1070     /// ```
1071     #[inline]
1072     #[unstable(feature = "shrink_to", reason = "new API", issue = "56431")]
1073     pub fn shrink_to(&mut self, min_capacity: usize) {
1074         self.vec.shrink_to(min_capacity)
1075     }
1076
1077     /// Appends the given [`char`] to the end of this `String`.
1078     ///
1079     /// [`char`]: ../../std/primitive.char.html
1080     ///
1081     /// # Examples
1082     ///
1083     /// Basic usage:
1084     ///
1085     /// ```
1086     /// let mut s = String::from("abc");
1087     ///
1088     /// s.push('1');
1089     /// s.push('2');
1090     /// s.push('3');
1091     ///
1092     /// assert_eq!("abc123", s);
1093     /// ```
1094     #[inline]
1095     #[stable(feature = "rust1", since = "1.0.0")]
1096     pub fn push(&mut self, ch: char) {
1097         match ch.len_utf8() {
1098             1 => self.vec.push(ch as u8),
1099             _ => self.vec.extend_from_slice(ch.encode_utf8(&mut [0; 4]).as_bytes()),
1100         }
1101     }
1102
1103     /// Returns a byte slice of this `String`'s contents.
1104     ///
1105     /// The inverse of this method is [`from_utf8`].
1106     ///
1107     /// [`from_utf8`]: #method.from_utf8
1108     ///
1109     /// # Examples
1110     ///
1111     /// Basic usage:
1112     ///
1113     /// ```
1114     /// let s = String::from("hello");
1115     ///
1116     /// assert_eq!(&[104, 101, 108, 108, 111], s.as_bytes());
1117     /// ```
1118     #[inline]
1119     #[stable(feature = "rust1", since = "1.0.0")]
1120     pub fn as_bytes(&self) -> &[u8] {
1121         &self.vec
1122     }
1123
1124     /// Shortens this `String` to the specified length.
1125     ///
1126     /// If `new_len` is greater than the string's current length, this has no
1127     /// effect.
1128     ///
1129     /// Note that this method has no effect on the allocated capacity
1130     /// of the string
1131     ///
1132     /// # Panics
1133     ///
1134     /// Panics if `new_len` does not lie on a [`char`] boundary.
1135     ///
1136     /// [`char`]: ../../std/primitive.char.html
1137     ///
1138     /// # Examples
1139     ///
1140     /// Basic usage:
1141     ///
1142     /// ```
1143     /// let mut s = String::from("hello");
1144     ///
1145     /// s.truncate(2);
1146     ///
1147     /// assert_eq!("he", s);
1148     /// ```
1149     #[inline]
1150     #[stable(feature = "rust1", since = "1.0.0")]
1151     pub fn truncate(&mut self, new_len: usize) {
1152         if new_len <= self.len() {
1153             assert!(self.is_char_boundary(new_len));
1154             self.vec.truncate(new_len)
1155         }
1156     }
1157
1158     /// Removes the last character from the string buffer and returns it.
1159     ///
1160     /// Returns [`None`] if this `String` is empty.
1161     ///
1162     /// [`None`]: ../../std/option/enum.Option.html#variant.None
1163     ///
1164     /// # Examples
1165     ///
1166     /// Basic usage:
1167     ///
1168     /// ```
1169     /// let mut s = String::from("foo");
1170     ///
1171     /// assert_eq!(s.pop(), Some('o'));
1172     /// assert_eq!(s.pop(), Some('o'));
1173     /// assert_eq!(s.pop(), Some('f'));
1174     ///
1175     /// assert_eq!(s.pop(), None);
1176     /// ```
1177     #[inline]
1178     #[stable(feature = "rust1", since = "1.0.0")]
1179     pub fn pop(&mut self) -> Option<char> {
1180         let ch = self.chars().rev().next()?;
1181         let newlen = self.len() - ch.len_utf8();
1182         unsafe {
1183             self.vec.set_len(newlen);
1184         }
1185         Some(ch)
1186     }
1187
1188     /// Removes a [`char`] from this `String` at a byte position and returns it.
1189     ///
1190     /// This is an `O(n)` operation, as it requires copying every element in the
1191     /// buffer.
1192     ///
1193     /// # Panics
1194     ///
1195     /// Panics if `idx` is larger than or equal to the `String`'s length,
1196     /// or if it does not lie on a [`char`] boundary.
1197     ///
1198     /// [`char`]: ../../std/primitive.char.html
1199     ///
1200     /// # Examples
1201     ///
1202     /// Basic usage:
1203     ///
1204     /// ```
1205     /// let mut s = String::from("foo");
1206     ///
1207     /// assert_eq!(s.remove(0), 'f');
1208     /// assert_eq!(s.remove(1), 'o');
1209     /// assert_eq!(s.remove(0), 'o');
1210     /// ```
1211     #[inline]
1212     #[stable(feature = "rust1", since = "1.0.0")]
1213     pub fn remove(&mut self, idx: usize) -> char {
1214         let ch = match self[idx..].chars().next() {
1215             Some(ch) => ch,
1216             None => panic!("cannot remove a char from the end of a string"),
1217         };
1218
1219         let next = idx + ch.len_utf8();
1220         let len = self.len();
1221         unsafe {
1222             ptr::copy(self.vec.as_ptr().add(next), self.vec.as_mut_ptr().add(idx), len - next);
1223             self.vec.set_len(len - (next - idx));
1224         }
1225         ch
1226     }
1227
1228     /// Retains only the characters specified by the predicate.
1229     ///
1230     /// In other words, remove all characters `c` such that `f(c)` returns `false`.
1231     /// This method operates in place, visiting each character exactly once in the
1232     /// original order, and preserves the order of the retained characters.
1233     ///
1234     /// # Examples
1235     ///
1236     /// ```
1237     /// let mut s = String::from("f_o_ob_ar");
1238     ///
1239     /// s.retain(|c| c != '_');
1240     ///
1241     /// assert_eq!(s, "foobar");
1242     /// ```
1243     ///
1244     /// The exact order may be useful for tracking external state, like an index.
1245     ///
1246     /// ```
1247     /// let mut s = String::from("abcde");
1248     /// let keep = [false, true, true, false, true];
1249     /// let mut i = 0;
1250     /// s.retain(|_| (keep[i], i += 1).0);
1251     /// assert_eq!(s, "bce");
1252     /// ```
1253     #[inline]
1254     #[stable(feature = "string_retain", since = "1.26.0")]
1255     pub fn retain<F>(&mut self, mut f: F)
1256     where
1257         F: FnMut(char) -> bool,
1258     {
1259         let len = self.len();
1260         let mut del_bytes = 0;
1261         let mut idx = 0;
1262
1263         while idx < len {
1264             let ch = unsafe { self.get_unchecked(idx..len).chars().next().unwrap() };
1265             let ch_len = ch.len_utf8();
1266
1267             if !f(ch) {
1268                 del_bytes += ch_len;
1269             } else if del_bytes > 0 {
1270                 unsafe {
1271                     ptr::copy(
1272                         self.vec.as_ptr().add(idx),
1273                         self.vec.as_mut_ptr().add(idx - del_bytes),
1274                         ch_len,
1275                     );
1276                 }
1277             }
1278
1279             // Point idx to the next char
1280             idx += ch_len;
1281         }
1282
1283         if del_bytes > 0 {
1284             unsafe {
1285                 self.vec.set_len(len - del_bytes);
1286             }
1287         }
1288     }
1289
1290     /// Inserts a character into this `String` at a byte position.
1291     ///
1292     /// This is an `O(n)` operation as it requires copying every element in the
1293     /// buffer.
1294     ///
1295     /// # Panics
1296     ///
1297     /// Panics if `idx` is larger than the `String`'s length, or if it does not
1298     /// lie on a [`char`] boundary.
1299     ///
1300     /// [`char`]: ../../std/primitive.char.html
1301     ///
1302     /// # Examples
1303     ///
1304     /// Basic usage:
1305     ///
1306     /// ```
1307     /// let mut s = String::with_capacity(3);
1308     ///
1309     /// s.insert(0, 'f');
1310     /// s.insert(1, 'o');
1311     /// s.insert(2, 'o');
1312     ///
1313     /// assert_eq!("foo", s);
1314     /// ```
1315     #[inline]
1316     #[stable(feature = "rust1", since = "1.0.0")]
1317     pub fn insert(&mut self, idx: usize, ch: char) {
1318         assert!(self.is_char_boundary(idx));
1319         let mut bits = [0; 4];
1320         let bits = ch.encode_utf8(&mut bits).as_bytes();
1321
1322         unsafe {
1323             self.insert_bytes(idx, bits);
1324         }
1325     }
1326
1327     unsafe fn insert_bytes(&mut self, idx: usize, bytes: &[u8]) {
1328         let len = self.len();
1329         let amt = bytes.len();
1330         self.vec.reserve(amt);
1331
1332         ptr::copy(self.vec.as_ptr().add(idx), self.vec.as_mut_ptr().add(idx + amt), len - idx);
1333         ptr::copy(bytes.as_ptr(), self.vec.as_mut_ptr().add(idx), amt);
1334         self.vec.set_len(len + amt);
1335     }
1336
1337     /// Inserts a string slice into this `String` at a byte position.
1338     ///
1339     /// This is an `O(n)` operation as it requires copying every element in the
1340     /// buffer.
1341     ///
1342     /// # Panics
1343     ///
1344     /// Panics if `idx` is larger than the `String`'s length, or if it does not
1345     /// lie on a [`char`] boundary.
1346     ///
1347     /// [`char`]: ../../std/primitive.char.html
1348     ///
1349     /// # Examples
1350     ///
1351     /// Basic usage:
1352     ///
1353     /// ```
1354     /// let mut s = String::from("bar");
1355     ///
1356     /// s.insert_str(0, "foo");
1357     ///
1358     /// assert_eq!("foobar", s);
1359     /// ```
1360     #[inline]
1361     #[stable(feature = "insert_str", since = "1.16.0")]
1362     pub fn insert_str(&mut self, idx: usize, string: &str) {
1363         assert!(self.is_char_boundary(idx));
1364
1365         unsafe {
1366             self.insert_bytes(idx, string.as_bytes());
1367         }
1368     }
1369
1370     /// Returns a mutable reference to the contents of this `String`.
1371     ///
1372     /// # Safety
1373     ///
1374     /// This function is unsafe because it does not check that the bytes passed
1375     /// to it are valid UTF-8. If this constraint is violated, it may cause
1376     /// memory unsafety issues with future users of the `String`, as the rest of
1377     /// the standard library assumes that `String`s are valid UTF-8.
1378     ///
1379     /// # Examples
1380     ///
1381     /// Basic usage:
1382     ///
1383     /// ```
1384     /// let mut s = String::from("hello");
1385     ///
1386     /// unsafe {
1387     ///     let vec = s.as_mut_vec();
1388     ///     assert_eq!(&[104, 101, 108, 108, 111][..], &vec[..]);
1389     ///
1390     ///     vec.reverse();
1391     /// }
1392     /// assert_eq!(s, "olleh");
1393     /// ```
1394     #[inline]
1395     #[stable(feature = "rust1", since = "1.0.0")]
1396     pub unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8> {
1397         &mut self.vec
1398     }
1399
1400     /// Returns the length of this `String`, in bytes, not [`char`]s or
1401     /// graphemes. In other words, it may not be what a human considers the
1402     /// length of the string.
1403     ///
1404     /// # Examples
1405     ///
1406     /// Basic usage:
1407     ///
1408     /// ```
1409     /// let a = String::from("foo");
1410     /// assert_eq!(a.len(), 3);
1411     ///
1412     /// let fancy_f = String::from("ƒoo");
1413     /// assert_eq!(fancy_f.len(), 4);
1414     /// assert_eq!(fancy_f.chars().count(), 3);
1415     /// ```
1416     #[inline]
1417     #[stable(feature = "rust1", since = "1.0.0")]
1418     pub fn len(&self) -> usize {
1419         self.vec.len()
1420     }
1421
1422     /// Returns `true` if this `String` has a length of zero, and `false` otherwise.
1423     ///
1424     /// # Examples
1425     ///
1426     /// Basic usage:
1427     ///
1428     /// ```
1429     /// let mut v = String::new();
1430     /// assert!(v.is_empty());
1431     ///
1432     /// v.push('a');
1433     /// assert!(!v.is_empty());
1434     /// ```
1435     #[inline]
1436     #[stable(feature = "rust1", since = "1.0.0")]
1437     pub fn is_empty(&self) -> bool {
1438         self.len() == 0
1439     }
1440
1441     /// Splits the string into two at the given index.
1442     ///
1443     /// Returns a newly allocated `String`. `self` contains bytes `[0, at)`, and
1444     /// the returned `String` contains bytes `[at, len)`. `at` must be on the
1445     /// boundary of a UTF-8 code point.
1446     ///
1447     /// Note that the capacity of `self` does not change.
1448     ///
1449     /// # Panics
1450     ///
1451     /// Panics if `at` is not on a `UTF-8` code point boundary, or if it is beyond the last
1452     /// code point of the string.
1453     ///
1454     /// # Examples
1455     ///
1456     /// ```
1457     /// # fn main() {
1458     /// let mut hello = String::from("Hello, World!");
1459     /// let world = hello.split_off(7);
1460     /// assert_eq!(hello, "Hello, ");
1461     /// assert_eq!(world, "World!");
1462     /// # }
1463     /// ```
1464     #[inline]
1465     #[stable(feature = "string_split_off", since = "1.16.0")]
1466     #[must_use = "use `.truncate()` if you don't need the other half"]
1467     pub fn split_off(&mut self, at: usize) -> String {
1468         assert!(self.is_char_boundary(at));
1469         let other = self.vec.split_off(at);
1470         unsafe { String::from_utf8_unchecked(other) }
1471     }
1472
1473     /// Truncates this `String`, removing all contents.
1474     ///
1475     /// While this means the `String` will have a length of zero, it does not
1476     /// touch its capacity.
1477     ///
1478     /// # Examples
1479     ///
1480     /// Basic usage:
1481     ///
1482     /// ```
1483     /// let mut s = String::from("foo");
1484     ///
1485     /// s.clear();
1486     ///
1487     /// assert!(s.is_empty());
1488     /// assert_eq!(0, s.len());
1489     /// assert_eq!(3, s.capacity());
1490     /// ```
1491     #[inline]
1492     #[stable(feature = "rust1", since = "1.0.0")]
1493     pub fn clear(&mut self) {
1494         self.vec.clear()
1495     }
1496
1497     /// Creates a draining iterator that removes the specified range in the `String`
1498     /// and yields the removed `chars`.
1499     ///
1500     /// Note: The element range is removed even if the iterator is not
1501     /// consumed until the end.
1502     ///
1503     /// # Panics
1504     ///
1505     /// Panics if the starting point or end point do not lie on a [`char`]
1506     /// boundary, or if they're out of bounds.
1507     ///
1508     /// [`char`]: ../../std/primitive.char.html
1509     ///
1510     /// # Examples
1511     ///
1512     /// Basic usage:
1513     ///
1514     /// ```
1515     /// let mut s = String::from("α is alpha, β is beta");
1516     /// let beta_offset = s.find('β').unwrap_or(s.len());
1517     ///
1518     /// // Remove the range up until the β from the string
1519     /// let t: String = s.drain(..beta_offset).collect();
1520     /// assert_eq!(t, "α is alpha, ");
1521     /// assert_eq!(s, "β is beta");
1522     ///
1523     /// // A full range clears the string
1524     /// s.drain(..);
1525     /// assert_eq!(s, "");
1526     /// ```
1527     #[stable(feature = "drain", since = "1.6.0")]
1528     pub fn drain<R>(&mut self, range: R) -> Drain<'_>
1529     where
1530         R: RangeBounds<usize>,
1531     {
1532         // Memory safety
1533         //
1534         // The String version of Drain does not have the memory safety issues
1535         // of the vector version. The data is just plain bytes.
1536         // Because the range removal happens in Drop, if the Drain iterator is leaked,
1537         // the removal will not happen.
1538         let len = self.len();
1539         let start = match range.start_bound() {
1540             Included(&n) => n,
1541             Excluded(&n) => n + 1,
1542             Unbounded => 0,
1543         };
1544         let end = match range.end_bound() {
1545             Included(&n) => n + 1,
1546             Excluded(&n) => n,
1547             Unbounded => len,
1548         };
1549
1550         // Take out two simultaneous borrows. The &mut String won't be accessed
1551         // until iteration is over, in Drop.
1552         let self_ptr = self as *mut _;
1553         // slicing does the appropriate bounds checks
1554         let chars_iter = self[start..end].chars();
1555
1556         Drain { start, end, iter: chars_iter, string: self_ptr }
1557     }
1558
1559     /// Removes the specified range in the string,
1560     /// and replaces it with the given string.
1561     /// The given string doesn't need to be the same length as the range.
1562     ///
1563     /// # Panics
1564     ///
1565     /// Panics if the starting point or end point do not lie on a [`char`]
1566     /// boundary, or if they're out of bounds.
1567     ///
1568     /// [`char`]: ../../std/primitive.char.html
1569     /// [`Vec::splice`]: ../../std/vec/struct.Vec.html#method.splice
1570     ///
1571     /// # Examples
1572     ///
1573     /// Basic usage:
1574     ///
1575     /// ```
1576     /// let mut s = String::from("α is alpha, β is beta");
1577     /// let beta_offset = s.find('β').unwrap_or(s.len());
1578     ///
1579     /// // Replace the range up until the β from the string
1580     /// s.replace_range(..beta_offset, "Α is capital alpha; ");
1581     /// assert_eq!(s, "Α is capital alpha; β is beta");
1582     /// ```
1583     #[stable(feature = "splice", since = "1.27.0")]
1584     pub fn replace_range<R>(&mut self, range: R, replace_with: &str)
1585     where
1586         R: RangeBounds<usize>,
1587     {
1588         // Memory safety
1589         //
1590         // Replace_range does not have the memory safety issues of a vector Splice.
1591         // of the vector version. The data is just plain bytes.
1592
1593         match range.start_bound() {
1594             Included(&n) => assert!(self.is_char_boundary(n)),
1595             Excluded(&n) => assert!(self.is_char_boundary(n + 1)),
1596             Unbounded => {}
1597         };
1598         match range.end_bound() {
1599             Included(&n) => assert!(self.is_char_boundary(n + 1)),
1600             Excluded(&n) => assert!(self.is_char_boundary(n)),
1601             Unbounded => {}
1602         };
1603
1604         unsafe { self.as_mut_vec() }.splice(range, replace_with.bytes());
1605     }
1606
1607     /// Converts this `String` into a [`Box`]`<`[`str`]`>`.
1608     ///
1609     /// This will drop any excess capacity.
1610     ///
1611     /// [`Box`]: ../../std/boxed/struct.Box.html
1612     /// [`str`]: ../../std/primitive.str.html
1613     ///
1614     /// # Examples
1615     ///
1616     /// Basic usage:
1617     ///
1618     /// ```
1619     /// let s = String::from("hello");
1620     ///
1621     /// let b = s.into_boxed_str();
1622     /// ```
1623     #[stable(feature = "box_str", since = "1.4.0")]
1624     #[inline]
1625     pub fn into_boxed_str(self) -> Box<str> {
1626         let slice = self.vec.into_boxed_slice();
1627         unsafe { from_boxed_utf8_unchecked(slice) }
1628     }
1629 }
1630
1631 impl FromUtf8Error {
1632     /// Returns a slice of [`u8`]s bytes that were attempted to convert to a `String`.
1633     ///
1634     /// # Examples
1635     ///
1636     /// Basic usage:
1637     ///
1638     /// ```
1639     /// // some invalid bytes, in a vector
1640     /// let bytes = vec![0, 159];
1641     ///
1642     /// let value = String::from_utf8(bytes);
1643     ///
1644     /// assert_eq!(&[0, 159], value.unwrap_err().as_bytes());
1645     /// ```
1646     #[stable(feature = "from_utf8_error_as_bytes", since = "1.26.0")]
1647     pub fn as_bytes(&self) -> &[u8] {
1648         &self.bytes[..]
1649     }
1650
1651     /// Returns the bytes that were attempted to convert to a `String`.
1652     ///
1653     /// This method is carefully constructed to avoid allocation. It will
1654     /// consume the error, moving out the bytes, so that a copy of the bytes
1655     /// does not need to be made.
1656     ///
1657     /// # Examples
1658     ///
1659     /// Basic usage:
1660     ///
1661     /// ```
1662     /// // some invalid bytes, in a vector
1663     /// let bytes = vec![0, 159];
1664     ///
1665     /// let value = String::from_utf8(bytes);
1666     ///
1667     /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
1668     /// ```
1669     #[stable(feature = "rust1", since = "1.0.0")]
1670     pub fn into_bytes(self) -> Vec<u8> {
1671         self.bytes
1672     }
1673
1674     /// Fetch a `Utf8Error` to get more details about the conversion failure.
1675     ///
1676     /// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
1677     /// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
1678     /// an analogue to `FromUtf8Error`. See its documentation for more details
1679     /// on using it.
1680     ///
1681     /// [`Utf8Error`]: ../../std/str/struct.Utf8Error.html
1682     /// [`std::str`]: ../../std/str/index.html
1683     /// [`u8`]: ../../std/primitive.u8.html
1684     /// [`&str`]: ../../std/primitive.str.html
1685     ///
1686     /// # Examples
1687     ///
1688     /// Basic usage:
1689     ///
1690     /// ```
1691     /// // some invalid bytes, in a vector
1692     /// let bytes = vec![0, 159];
1693     ///
1694     /// let error = String::from_utf8(bytes).unwrap_err().utf8_error();
1695     ///
1696     /// // the first byte is invalid here
1697     /// assert_eq!(1, error.valid_up_to());
1698     /// ```
1699     #[stable(feature = "rust1", since = "1.0.0")]
1700     pub fn utf8_error(&self) -> Utf8Error {
1701         self.error
1702     }
1703 }
1704
1705 #[stable(feature = "rust1", since = "1.0.0")]
1706 impl fmt::Display for FromUtf8Error {
1707     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1708         fmt::Display::fmt(&self.error, f)
1709     }
1710 }
1711
1712 #[stable(feature = "rust1", since = "1.0.0")]
1713 impl fmt::Display for FromUtf16Error {
1714     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1715         fmt::Display::fmt("invalid utf-16: lone surrogate found", f)
1716     }
1717 }
1718
1719 #[stable(feature = "rust1", since = "1.0.0")]
1720 impl Clone for String {
1721     fn clone(&self) -> Self {
1722         String { vec: self.vec.clone() }
1723     }
1724
1725     fn clone_from(&mut self, source: &Self) {
1726         self.vec.clone_from(&source.vec);
1727     }
1728 }
1729
1730 #[stable(feature = "rust1", since = "1.0.0")]
1731 impl FromIterator<char> for String {
1732     fn from_iter<I: IntoIterator<Item = char>>(iter: I) -> String {
1733         let mut buf = String::new();
1734         buf.extend(iter);
1735         buf
1736     }
1737 }
1738
1739 #[stable(feature = "string_from_iter_by_ref", since = "1.17.0")]
1740 impl<'a> FromIterator<&'a char> for String {
1741     fn from_iter<I: IntoIterator<Item = &'a char>>(iter: I) -> String {
1742         let mut buf = String::new();
1743         buf.extend(iter);
1744         buf
1745     }
1746 }
1747
1748 #[stable(feature = "rust1", since = "1.0.0")]
1749 impl<'a> FromIterator<&'a str> for String {
1750     fn from_iter<I: IntoIterator<Item = &'a str>>(iter: I) -> String {
1751         let mut buf = String::new();
1752         buf.extend(iter);
1753         buf
1754     }
1755 }
1756
1757 #[stable(feature = "extend_string", since = "1.4.0")]
1758 impl FromIterator<String> for String {
1759     fn from_iter<I: IntoIterator<Item = String>>(iter: I) -> String {
1760         let mut iterator = iter.into_iter();
1761
1762         // Because we're iterating over `String`s, we can avoid at least
1763         // one allocation by getting the first string from the iterator
1764         // and appending to it all the subsequent strings.
1765         match iterator.next() {
1766             None => String::new(),
1767             Some(mut buf) => {
1768                 buf.extend(iterator);
1769                 buf
1770             }
1771         }
1772     }
1773 }
1774
1775 #[stable(feature = "herd_cows", since = "1.19.0")]
1776 impl<'a> FromIterator<Cow<'a, str>> for String {
1777     fn from_iter<I: IntoIterator<Item = Cow<'a, str>>>(iter: I) -> String {
1778         let mut iterator = iter.into_iter();
1779
1780         // Because we're iterating over CoWs, we can (potentially) avoid at least
1781         // one allocation by getting the first item and appending to it all the
1782         // subsequent items.
1783         match iterator.next() {
1784             None => String::new(),
1785             Some(cow) => {
1786                 let mut buf = cow.into_owned();
1787                 buf.extend(iterator);
1788                 buf
1789             }
1790         }
1791     }
1792 }
1793
1794 #[stable(feature = "rust1", since = "1.0.0")]
1795 impl Extend<char> for String {
1796     fn extend<I: IntoIterator<Item = char>>(&mut self, iter: I) {
1797         let iterator = iter.into_iter();
1798         let (lower_bound, _) = iterator.size_hint();
1799         self.reserve(lower_bound);
1800         iterator.for_each(move |c| self.push(c));
1801     }
1802
1803     #[inline]
1804     fn extend_one(&mut self, c: char) {
1805         self.push(c);
1806     }
1807
1808     #[inline]
1809     fn extend_reserve(&mut self, additional: usize) {
1810         self.reserve(additional);
1811     }
1812 }
1813
1814 #[stable(feature = "extend_ref", since = "1.2.0")]
1815 impl<'a> Extend<&'a char> for String {
1816     fn extend<I: IntoIterator<Item = &'a char>>(&mut self, iter: I) {
1817         self.extend(iter.into_iter().cloned());
1818     }
1819
1820     #[inline]
1821     fn extend_one(&mut self, &c: &'a char) {
1822         self.push(c);
1823     }
1824
1825     #[inline]
1826     fn extend_reserve(&mut self, additional: usize) {
1827         self.reserve(additional);
1828     }
1829 }
1830
1831 #[stable(feature = "rust1", since = "1.0.0")]
1832 impl<'a> Extend<&'a str> for String {
1833     fn extend<I: IntoIterator<Item = &'a str>>(&mut self, iter: I) {
1834         iter.into_iter().for_each(move |s| self.push_str(s));
1835     }
1836
1837     #[inline]
1838     fn extend_one(&mut self, s: &'a str) {
1839         self.push_str(s);
1840     }
1841 }
1842
1843 #[stable(feature = "extend_string", since = "1.4.0")]
1844 impl Extend<String> for String {
1845     fn extend<I: IntoIterator<Item = String>>(&mut self, iter: I) {
1846         iter.into_iter().for_each(move |s| self.push_str(&s));
1847     }
1848
1849     #[inline]
1850     fn extend_one(&mut self, s: String) {
1851         self.push_str(&s);
1852     }
1853 }
1854
1855 #[stable(feature = "herd_cows", since = "1.19.0")]
1856 impl<'a> Extend<Cow<'a, str>> for String {
1857     fn extend<I: IntoIterator<Item = Cow<'a, str>>>(&mut self, iter: I) {
1858         iter.into_iter().for_each(move |s| self.push_str(&s));
1859     }
1860
1861     #[inline]
1862     fn extend_one(&mut self, s: Cow<'a, str>) {
1863         self.push_str(&s);
1864     }
1865 }
1866
1867 /// A convenience impl that delegates to the impl for `&str`.
1868 ///
1869 /// # Examples
1870 ///
1871 /// ```
1872 /// assert_eq!(String::from("Hello world").find("world"), Some(6));
1873 /// ```
1874 #[unstable(
1875     feature = "pattern",
1876     reason = "API not fully fleshed out and ready to be stabilized",
1877     issue = "27721"
1878 )]
1879 impl<'a, 'b> Pattern<'a> for &'b String {
1880     type Searcher = <&'b str as Pattern<'a>>::Searcher;
1881
1882     fn into_searcher(self, haystack: &'a str) -> <&'b str as Pattern<'a>>::Searcher {
1883         self[..].into_searcher(haystack)
1884     }
1885
1886     #[inline]
1887     fn is_contained_in(self, haystack: &'a str) -> bool {
1888         self[..].is_contained_in(haystack)
1889     }
1890
1891     #[inline]
1892     fn is_prefix_of(self, haystack: &'a str) -> bool {
1893         self[..].is_prefix_of(haystack)
1894     }
1895
1896     #[inline]
1897     fn strip_prefix_of(self, haystack: &'a str) -> Option<&'a str> {
1898         self[..].strip_prefix_of(haystack)
1899     }
1900
1901     #[inline]
1902     fn is_suffix_of(self, haystack: &'a str) -> bool {
1903         self[..].is_suffix_of(haystack)
1904     }
1905
1906     #[inline]
1907     fn strip_suffix_of(self, haystack: &'a str) -> Option<&'a str> {
1908         self[..].strip_suffix_of(haystack)
1909     }
1910 }
1911
1912 #[stable(feature = "rust1", since = "1.0.0")]
1913 impl PartialEq for String {
1914     #[inline]
1915     fn eq(&self, other: &String) -> bool {
1916         PartialEq::eq(&self[..], &other[..])
1917     }
1918     #[inline]
1919     fn ne(&self, other: &String) -> bool {
1920         PartialEq::ne(&self[..], &other[..])
1921     }
1922 }
1923
1924 macro_rules! impl_eq {
1925     ($lhs:ty, $rhs: ty) => {
1926         #[stable(feature = "rust1", since = "1.0.0")]
1927         #[allow(unused_lifetimes)]
1928         impl<'a, 'b> PartialEq<$rhs> for $lhs {
1929             #[inline]
1930             fn eq(&self, other: &$rhs) -> bool {
1931                 PartialEq::eq(&self[..], &other[..])
1932             }
1933             #[inline]
1934             fn ne(&self, other: &$rhs) -> bool {
1935                 PartialEq::ne(&self[..], &other[..])
1936             }
1937         }
1938
1939         #[stable(feature = "rust1", since = "1.0.0")]
1940         #[allow(unused_lifetimes)]
1941         impl<'a, 'b> PartialEq<$lhs> for $rhs {
1942             #[inline]
1943             fn eq(&self, other: &$lhs) -> bool {
1944                 PartialEq::eq(&self[..], &other[..])
1945             }
1946             #[inline]
1947             fn ne(&self, other: &$lhs) -> bool {
1948                 PartialEq::ne(&self[..], &other[..])
1949             }
1950         }
1951     };
1952 }
1953
1954 impl_eq! { String, str }
1955 impl_eq! { String, &'a str }
1956 impl_eq! { Cow<'a, str>, str }
1957 impl_eq! { Cow<'a, str>, &'b str }
1958 impl_eq! { Cow<'a, str>, String }
1959
1960 #[stable(feature = "rust1", since = "1.0.0")]
1961 impl Default for String {
1962     /// Creates an empty `String`.
1963     #[inline]
1964     fn default() -> String {
1965         String::new()
1966     }
1967 }
1968
1969 #[stable(feature = "rust1", since = "1.0.0")]
1970 impl fmt::Display for String {
1971     #[inline]
1972     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1973         fmt::Display::fmt(&**self, f)
1974     }
1975 }
1976
1977 #[stable(feature = "rust1", since = "1.0.0")]
1978 impl fmt::Debug for String {
1979     #[inline]
1980     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1981         fmt::Debug::fmt(&**self, f)
1982     }
1983 }
1984
1985 #[stable(feature = "rust1", since = "1.0.0")]
1986 impl hash::Hash for String {
1987     #[inline]
1988     fn hash<H: hash::Hasher>(&self, hasher: &mut H) {
1989         (**self).hash(hasher)
1990     }
1991 }
1992
1993 /// Implements the `+` operator for concatenating two strings.
1994 ///
1995 /// This consumes the `String` on the left-hand side and re-uses its buffer (growing it if
1996 /// necessary). This is done to avoid allocating a new `String` and copying the entire contents on
1997 /// every operation, which would lead to `O(n^2)` running time when building an `n`-byte string by
1998 /// repeated concatenation.
1999 ///
2000 /// The string on the right-hand side is only borrowed; its contents are copied into the returned
2001 /// `String`.
2002 ///
2003 /// # Examples
2004 ///
2005 /// Concatenating two `String`s takes the first by value and borrows the second:
2006 ///
2007 /// ```
2008 /// let a = String::from("hello");
2009 /// let b = String::from(" world");
2010 /// let c = a + &b;
2011 /// // `a` is moved and can no longer be used here.
2012 /// ```
2013 ///
2014 /// If you want to keep using the first `String`, you can clone it and append to the clone instead:
2015 ///
2016 /// ```
2017 /// let a = String::from("hello");
2018 /// let b = String::from(" world");
2019 /// let c = a.clone() + &b;
2020 /// // `a` is still valid here.
2021 /// ```
2022 ///
2023 /// Concatenating `&str` slices can be done by converting the first to a `String`:
2024 ///
2025 /// ```
2026 /// let a = "hello";
2027 /// let b = " world";
2028 /// let c = a.to_string() + b;
2029 /// ```
2030 #[stable(feature = "rust1", since = "1.0.0")]
2031 impl Add<&str> for String {
2032     type Output = String;
2033
2034     #[inline]
2035     fn add(mut self, other: &str) -> String {
2036         self.push_str(other);
2037         self
2038     }
2039 }
2040
2041 /// Implements the `+=` operator for appending to a `String`.
2042 ///
2043 /// This has the same behavior as the [`push_str`][String::push_str] method.
2044 #[stable(feature = "stringaddassign", since = "1.12.0")]
2045 impl AddAssign<&str> for String {
2046     #[inline]
2047     fn add_assign(&mut self, other: &str) {
2048         self.push_str(other);
2049     }
2050 }
2051
2052 #[stable(feature = "rust1", since = "1.0.0")]
2053 impl ops::Index<ops::Range<usize>> for String {
2054     type Output = str;
2055
2056     #[inline]
2057     fn index(&self, index: ops::Range<usize>) -> &str {
2058         &self[..][index]
2059     }
2060 }
2061 #[stable(feature = "rust1", since = "1.0.0")]
2062 impl ops::Index<ops::RangeTo<usize>> for String {
2063     type Output = str;
2064
2065     #[inline]
2066     fn index(&self, index: ops::RangeTo<usize>) -> &str {
2067         &self[..][index]
2068     }
2069 }
2070 #[stable(feature = "rust1", since = "1.0.0")]
2071 impl ops::Index<ops::RangeFrom<usize>> for String {
2072     type Output = str;
2073
2074     #[inline]
2075     fn index(&self, index: ops::RangeFrom<usize>) -> &str {
2076         &self[..][index]
2077     }
2078 }
2079 #[stable(feature = "rust1", since = "1.0.0")]
2080 impl ops::Index<ops::RangeFull> for String {
2081     type Output = str;
2082
2083     #[inline]
2084     fn index(&self, _index: ops::RangeFull) -> &str {
2085         unsafe { str::from_utf8_unchecked(&self.vec) }
2086     }
2087 }
2088 #[stable(feature = "inclusive_range", since = "1.26.0")]
2089 impl ops::Index<ops::RangeInclusive<usize>> for String {
2090     type Output = str;
2091
2092     #[inline]
2093     fn index(&self, index: ops::RangeInclusive<usize>) -> &str {
2094         Index::index(&**self, index)
2095     }
2096 }
2097 #[stable(feature = "inclusive_range", since = "1.26.0")]
2098 impl ops::Index<ops::RangeToInclusive<usize>> for String {
2099     type Output = str;
2100
2101     #[inline]
2102     fn index(&self, index: ops::RangeToInclusive<usize>) -> &str {
2103         Index::index(&**self, index)
2104     }
2105 }
2106
2107 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2108 impl ops::IndexMut<ops::Range<usize>> for String {
2109     #[inline]
2110     fn index_mut(&mut self, index: ops::Range<usize>) -> &mut str {
2111         &mut self[..][index]
2112     }
2113 }
2114 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2115 impl ops::IndexMut<ops::RangeTo<usize>> for String {
2116     #[inline]
2117     fn index_mut(&mut self, index: ops::RangeTo<usize>) -> &mut str {
2118         &mut self[..][index]
2119     }
2120 }
2121 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2122 impl ops::IndexMut<ops::RangeFrom<usize>> for String {
2123     #[inline]
2124     fn index_mut(&mut self, index: ops::RangeFrom<usize>) -> &mut str {
2125         &mut self[..][index]
2126     }
2127 }
2128 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2129 impl ops::IndexMut<ops::RangeFull> for String {
2130     #[inline]
2131     fn index_mut(&mut self, _index: ops::RangeFull) -> &mut str {
2132         unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
2133     }
2134 }
2135 #[stable(feature = "inclusive_range", since = "1.26.0")]
2136 impl ops::IndexMut<ops::RangeInclusive<usize>> for String {
2137     #[inline]
2138     fn index_mut(&mut self, index: ops::RangeInclusive<usize>) -> &mut str {
2139         IndexMut::index_mut(&mut **self, index)
2140     }
2141 }
2142 #[stable(feature = "inclusive_range", since = "1.26.0")]
2143 impl ops::IndexMut<ops::RangeToInclusive<usize>> for String {
2144     #[inline]
2145     fn index_mut(&mut self, index: ops::RangeToInclusive<usize>) -> &mut str {
2146         IndexMut::index_mut(&mut **self, index)
2147     }
2148 }
2149
2150 #[stable(feature = "rust1", since = "1.0.0")]
2151 impl ops::Deref for String {
2152     type Target = str;
2153
2154     #[inline]
2155     fn deref(&self) -> &str {
2156         unsafe { str::from_utf8_unchecked(&self.vec) }
2157     }
2158 }
2159
2160 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2161 impl ops::DerefMut for String {
2162     #[inline]
2163     fn deref_mut(&mut self) -> &mut str {
2164         unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
2165     }
2166 }
2167
2168 /// A type alias for [`Infallible`].
2169 ///
2170 /// This alias exists for backwards compatibility, and may be eventually deprecated.
2171 ///
2172 /// [`Infallible`]: ../../core/convert/enum.Infallible.html
2173 #[stable(feature = "str_parse_error", since = "1.5.0")]
2174 pub type ParseError = core::convert::Infallible;
2175
2176 #[stable(feature = "rust1", since = "1.0.0")]
2177 impl FromStr for String {
2178     type Err = core::convert::Infallible;
2179     #[inline]
2180     fn from_str(s: &str) -> Result<String, Self::Err> {
2181         Ok(String::from(s))
2182     }
2183 }
2184
2185 /// A trait for converting a value to a `String`.
2186 ///
2187 /// This trait is automatically implemented for any type which implements the
2188 /// [`Display`] trait. As such, `ToString` shouldn't be implemented directly:
2189 /// [`Display`] should be implemented instead, and you get the `ToString`
2190 /// implementation for free.
2191 ///
2192 /// [`Display`]: ../../std/fmt/trait.Display.html
2193 #[stable(feature = "rust1", since = "1.0.0")]
2194 pub trait ToString {
2195     /// Converts the given value to a `String`.
2196     ///
2197     /// # Examples
2198     ///
2199     /// Basic usage:
2200     ///
2201     /// ```
2202     /// let i = 5;
2203     /// let five = String::from("5");
2204     ///
2205     /// assert_eq!(five, i.to_string());
2206     /// ```
2207     #[rustc_conversion_suggestion]
2208     #[stable(feature = "rust1", since = "1.0.0")]
2209     fn to_string(&self) -> String;
2210 }
2211
2212 /// # Panics
2213 ///
2214 /// In this implementation, the `to_string` method panics
2215 /// if the `Display` implementation returns an error.
2216 /// This indicates an incorrect `Display` implementation
2217 /// since `fmt::Write for String` never returns an error itself.
2218 #[stable(feature = "rust1", since = "1.0.0")]
2219 impl<T: fmt::Display + ?Sized> ToString for T {
2220     #[inline]
2221     default fn to_string(&self) -> String {
2222         use fmt::Write;
2223         let mut buf = String::new();
2224         buf.write_fmt(format_args!("{}", self))
2225             .expect("a Display implementation returned an error unexpectedly");
2226         buf.shrink_to_fit();
2227         buf
2228     }
2229 }
2230
2231 #[stable(feature = "str_to_string_specialization", since = "1.9.0")]
2232 impl ToString for str {
2233     #[inline]
2234     fn to_string(&self) -> String {
2235         String::from(self)
2236     }
2237 }
2238
2239 #[stable(feature = "cow_str_to_string_specialization", since = "1.17.0")]
2240 impl ToString for Cow<'_, str> {
2241     #[inline]
2242     fn to_string(&self) -> String {
2243         self[..].to_owned()
2244     }
2245 }
2246
2247 #[stable(feature = "string_to_string_specialization", since = "1.17.0")]
2248 impl ToString for String {
2249     #[inline]
2250     fn to_string(&self) -> String {
2251         self.to_owned()
2252     }
2253 }
2254
2255 #[stable(feature = "rust1", since = "1.0.0")]
2256 impl AsRef<str> for String {
2257     #[inline]
2258     fn as_ref(&self) -> &str {
2259         self
2260     }
2261 }
2262
2263 #[stable(feature = "string_as_mut", since = "1.43.0")]
2264 impl AsMut<str> for String {
2265     #[inline]
2266     fn as_mut(&mut self) -> &mut str {
2267         self
2268     }
2269 }
2270
2271 #[stable(feature = "rust1", since = "1.0.0")]
2272 impl AsRef<[u8]> for String {
2273     #[inline]
2274     fn as_ref(&self) -> &[u8] {
2275         self.as_bytes()
2276     }
2277 }
2278
2279 #[stable(feature = "rust1", since = "1.0.0")]
2280 impl From<&str> for String {
2281     #[inline]
2282     fn from(s: &str) -> String {
2283         s.to_owned()
2284     }
2285 }
2286
2287 #[stable(feature = "from_mut_str_for_string", since = "1.44.0")]
2288 impl From<&mut str> for String {
2289     /// Converts a `&mut str` into a `String`.
2290     ///
2291     /// The result is allocated on the heap.
2292     #[inline]
2293     fn from(s: &mut str) -> String {
2294         s.to_owned()
2295     }
2296 }
2297
2298 #[stable(feature = "from_ref_string", since = "1.35.0")]
2299 impl From<&String> for String {
2300     #[inline]
2301     fn from(s: &String) -> String {
2302         s.clone()
2303     }
2304 }
2305
2306 // note: test pulls in libstd, which causes errors here
2307 #[cfg(not(test))]
2308 #[stable(feature = "string_from_box", since = "1.18.0")]
2309 impl From<Box<str>> for String {
2310     /// Converts the given boxed `str` slice to a `String`.
2311     /// It is notable that the `str` slice is owned.
2312     ///
2313     /// # Examples
2314     ///
2315     /// Basic usage:
2316     ///
2317     /// ```
2318     /// let s1: String = String::from("hello world");
2319     /// let s2: Box<str> = s1.into_boxed_str();
2320     /// let s3: String = String::from(s2);
2321     ///
2322     /// assert_eq!("hello world", s3)
2323     /// ```
2324     fn from(s: Box<str>) -> String {
2325         s.into_string()
2326     }
2327 }
2328
2329 #[stable(feature = "box_from_str", since = "1.20.0")]
2330 impl From<String> for Box<str> {
2331     /// Converts the given `String` to a boxed `str` slice that is owned.
2332     ///
2333     /// # Examples
2334     ///
2335     /// Basic usage:
2336     ///
2337     /// ```
2338     /// let s1: String = String::from("hello world");
2339     /// let s2: Box<str> = Box::from(s1);
2340     /// let s3: String = String::from(s2);
2341     ///
2342     /// assert_eq!("hello world", s3)
2343     /// ```
2344     fn from(s: String) -> Box<str> {
2345         s.into_boxed_str()
2346     }
2347 }
2348
2349 #[stable(feature = "string_from_cow_str", since = "1.14.0")]
2350 impl<'a> From<Cow<'a, str>> for String {
2351     fn from(s: Cow<'a, str>) -> String {
2352         s.into_owned()
2353     }
2354 }
2355
2356 #[stable(feature = "rust1", since = "1.0.0")]
2357 impl<'a> From<&'a str> for Cow<'a, str> {
2358     #[inline]
2359     fn from(s: &'a str) -> Cow<'a, str> {
2360         Cow::Borrowed(s)
2361     }
2362 }
2363
2364 #[stable(feature = "rust1", since = "1.0.0")]
2365 impl<'a> From<String> for Cow<'a, str> {
2366     #[inline]
2367     fn from(s: String) -> Cow<'a, str> {
2368         Cow::Owned(s)
2369     }
2370 }
2371
2372 #[stable(feature = "cow_from_string_ref", since = "1.28.0")]
2373 impl<'a> From<&'a String> for Cow<'a, str> {
2374     #[inline]
2375     fn from(s: &'a String) -> Cow<'a, str> {
2376         Cow::Borrowed(s.as_str())
2377     }
2378 }
2379
2380 #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
2381 impl<'a> FromIterator<char> for Cow<'a, str> {
2382     fn from_iter<I: IntoIterator<Item = char>>(it: I) -> Cow<'a, str> {
2383         Cow::Owned(FromIterator::from_iter(it))
2384     }
2385 }
2386
2387 #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
2388 impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str> {
2389     fn from_iter<I: IntoIterator<Item = &'b str>>(it: I) -> Cow<'a, str> {
2390         Cow::Owned(FromIterator::from_iter(it))
2391     }
2392 }
2393
2394 #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
2395 impl<'a> FromIterator<String> for Cow<'a, str> {
2396     fn from_iter<I: IntoIterator<Item = String>>(it: I) -> Cow<'a, str> {
2397         Cow::Owned(FromIterator::from_iter(it))
2398     }
2399 }
2400
2401 #[stable(feature = "from_string_for_vec_u8", since = "1.14.0")]
2402 impl From<String> for Vec<u8> {
2403     /// Converts the given `String` to a vector `Vec` that holds values of type `u8`.
2404     ///
2405     /// # Examples
2406     ///
2407     /// Basic usage:
2408     ///
2409     /// ```
2410     /// let s1 = String::from("hello world");
2411     /// let v1 = Vec::from(s1);
2412     ///
2413     /// for b in v1 {
2414     ///     println!("{}", b);
2415     /// }
2416     /// ```
2417     fn from(string: String) -> Vec<u8> {
2418         string.into_bytes()
2419     }
2420 }
2421
2422 #[stable(feature = "rust1", since = "1.0.0")]
2423 impl fmt::Write for String {
2424     #[inline]
2425     fn write_str(&mut self, s: &str) -> fmt::Result {
2426         self.push_str(s);
2427         Ok(())
2428     }
2429
2430     #[inline]
2431     fn write_char(&mut self, c: char) -> fmt::Result {
2432         self.push(c);
2433         Ok(())
2434     }
2435 }
2436
2437 /// A draining iterator for `String`.
2438 ///
2439 /// This struct is created by the [`drain`] method on [`String`]. See its
2440 /// documentation for more.
2441 ///
2442 /// [`drain`]: struct.String.html#method.drain
2443 /// [`String`]: struct.String.html
2444 #[stable(feature = "drain", since = "1.6.0")]
2445 pub struct Drain<'a> {
2446     /// Will be used as &'a mut String in the destructor
2447     string: *mut String,
2448     /// Start of part to remove
2449     start: usize,
2450     /// End of part to remove
2451     end: usize,
2452     /// Current remaining range to remove
2453     iter: Chars<'a>,
2454 }
2455
2456 #[stable(feature = "collection_debug", since = "1.17.0")]
2457 impl fmt::Debug for Drain<'_> {
2458     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2459         f.pad("Drain { .. }")
2460     }
2461 }
2462
2463 #[stable(feature = "drain", since = "1.6.0")]
2464 unsafe impl Sync for Drain<'_> {}
2465 #[stable(feature = "drain", since = "1.6.0")]
2466 unsafe impl Send for Drain<'_> {}
2467
2468 #[stable(feature = "drain", since = "1.6.0")]
2469 impl Drop for Drain<'_> {
2470     fn drop(&mut self) {
2471         unsafe {
2472             // Use Vec::drain. "Reaffirm" the bounds checks to avoid
2473             // panic code being inserted again.
2474             let self_vec = (*self.string).as_mut_vec();
2475             if self.start <= self.end && self.end <= self_vec.len() {
2476                 self_vec.drain(self.start..self.end);
2477             }
2478         }
2479     }
2480 }
2481
2482 #[stable(feature = "drain", since = "1.6.0")]
2483 impl Iterator for Drain<'_> {
2484     type Item = char;
2485
2486     #[inline]
2487     fn next(&mut self) -> Option<char> {
2488         self.iter.next()
2489     }
2490
2491     fn size_hint(&self) -> (usize, Option<usize>) {
2492         self.iter.size_hint()
2493     }
2494
2495     #[inline]
2496     fn last(mut self) -> Option<char> {
2497         self.next_back()
2498     }
2499 }
2500
2501 #[stable(feature = "drain", since = "1.6.0")]
2502 impl DoubleEndedIterator for Drain<'_> {
2503     #[inline]
2504     fn next_back(&mut self) -> Option<char> {
2505         self.iter.next_back()
2506     }
2507 }
2508
2509 #[stable(feature = "fused", since = "1.26.0")]
2510 impl FusedIterator for Drain<'_> {}