src/libcore/mem.rs

   1 // Copyright 2012-2014 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 //! Basic functions for dealing with memory.
  12 //!
  13 //! This module contains functions for querying the size and alignment of
  14 //! types, initializing and manipulating memory.
  15
  16 #![stable(feature = "rust1", since = "1.0.0")]
  17
  18 use clone;
  19 use cmp;
  20 use fmt;
  21 use hash;
  22 use intrinsics;
  23 use marker::{Copy, PhantomData, Sized};
  24 use ptr;
  25
  26 #[stable(feature = "rust1", since = "1.0.0")]
  27 pub use intrinsics::transmute;
  28
  29 /// Leaks a value: takes ownership and "forgets" about the value **without running
  30 /// its destructor**.
  31 ///
  32 /// Any resources the value manages, such as heap memory or a file handle, will linger
  33 /// forever in an unreachable state.
  34 ///
  35 /// If you want to dispose of a value properly, running its destructor, see
  36 /// [`mem::drop`][drop].
  37 ///
  38 /// # Safety
  39 ///
  40 /// `forget` is not marked as `unsafe`, because Rust's safety guarantees
  41 /// do not include a guarantee that destructors will always run. For example,
  42 /// a program can create a reference cycle using [`Rc`][rc], or call
  43 /// [`process:exit`][exit] to exit without running destructors. Thus, allowing
  44 /// `mem::forget` from safe code does not fundamentally change Rust's safety
  45 /// guarantees.
  46 ///
  47 /// That said, leaking resources such as memory or I/O objects is usually undesirable,
  48 /// so `forget` is only recommended for specialized use cases like those shown below.
  49 ///
  50 /// Because forgetting a value is allowed, any `unsafe` code you write must
  51 /// allow for this possibility. You cannot return a value and expect that the
  52 /// caller will necessarily run the value's destructor.
  53 ///
  54 /// [rc]: ../../std/rc/struct.Rc.html
  55 /// [exit]: ../../std/process/fn.exit.html
  56 ///
  57 /// # Examples
  58 ///
  59 /// Leak some heap memory by never deallocating it:
  60 ///
  61 /// ```
  62 /// use std::mem;
  63 ///
  64 /// let heap_memory = Box::new(3);
  65 /// mem::forget(heap_memory);
  66 /// ```
  67 ///
  68 /// Leak an I/O object, never closing the file:
  69 ///
  70 /// ```no_run
  71 /// use std::mem;
  72 /// use std::fs::File;
  73 ///
  74 /// let file = File::open("foo.txt").unwrap();
  75 /// mem::forget(file);
  76 /// ```
  77 ///
  78 /// The practical use cases for `forget` are rather specialized and mainly come
  79 /// up in unsafe or FFI code.
  80 ///
  81 /// ## Use case 1
  82 ///
  83 /// You have created an uninitialized value using [`mem::uninitialized`][uninit].
  84 /// You must either initialize or `forget` it on every computation path before
  85 /// Rust drops it automatically, like at the end of a scope or after a panic.
  86 /// Running the destructor on an uninitialized value would be [undefined behavior][ub].
  87 ///
  88 /// ```
  89 /// use std::mem;
  90 /// use std::ptr;
  91 ///
  92 /// # let some_condition = false;
  93 /// unsafe {
  94 ///     let mut uninit_vec: Vec<u32> = mem::uninitialized();
  95 ///
  96 ///     if some_condition {
  97 ///         // Initialize the variable.
  98 ///         ptr::write(&mut uninit_vec, Vec::new());
  99 ///     } else {
 100 ///         // Forget the uninitialized value so its destructor doesn't run.
 101 ///         mem::forget(uninit_vec);
 102 ///     }
 103 /// }
 104 /// ```
 105 ///
 106 /// ## Use case 2
 107 ///
 108 /// You have duplicated the bytes making up a value, without doing a proper
 109 /// [`Clone`][clone]. You need the value's destructor to run only once,
 110 /// because a double `free` is undefined behavior.
 111 ///
 112 /// An example is the definition of [`mem::swap`][swap] in this module:
 113 ///
 114 /// ```
 115 /// use std::mem;
 116 /// use std::ptr;
 117 ///
 118 /// # #[allow(dead_code)]
 119 /// fn swap<T>(x: &mut T, y: &mut T) {
 120 ///     unsafe {
 121 ///         // Give ourselves some scratch space to work with
 122 ///         let mut t: T = mem::uninitialized();
 123 ///
 124 ///         // Perform the swap, `&mut` pointers never alias
 125 ///         ptr::copy_nonoverlapping(&*x, &mut t, 1);
 126 ///         ptr::copy_nonoverlapping(&*y, x, 1);
 127 ///         ptr::copy_nonoverlapping(&t, y, 1);
 128 ///
 129 ///         // y and t now point to the same thing, but we need to completely
 130 ///         // forget `t` because we do not want to run the destructor for `T`
 131 ///         // on its value, which is still owned somewhere outside this function.
 132 ///         mem::forget(t);
 133 ///     }
 134 /// }
 135 /// ```
 136 ///
 137 /// ## Use case 3
 138 ///
 139 /// You are transferring ownership across a [FFI] boundary to code written in
 140 /// another language. You need to `forget` the value on the Rust side because Rust
 141 /// code is no longer responsible for it.
 142 ///
 143 /// ```no_run
 144 /// use std::mem;
 145 ///
 146 /// extern "C" {
 147 ///     fn my_c_function(x: *const u32);
 148 /// }
 149 ///
 150 /// let x: Box<u32> = Box::new(3);
 151 ///
 152 /// // Transfer ownership into C code.
 153 /// unsafe {
 154 ///     my_c_function(&*x);
 155 /// }
 156 /// mem::forget(x);
 157 /// ```
 158 ///
 159 /// In this case, C code must call back into Rust to free the object. Calling C's `free`
 160 /// function on a [`Box`][box] is *not* safe! Also, `Box` provides an [`into_raw`][into_raw]
 161 /// method which is the preferred way to do this in practice.
 162 ///
 163 /// [drop]: fn.drop.html
 164 /// [uninit]: fn.uninitialized.html
 165 /// [clone]: ../clone/trait.Clone.html
 166 /// [swap]: fn.swap.html
 167 /// [FFI]: ../../book/ffi.html
 168 /// [box]: ../../std/boxed/struct.Box.html
 169 /// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw
 170 /// [ub]: ../../reference/behavior-considered-undefined.html
 171 #[inline]
 172 #[stable(feature = "rust1", since = "1.0.0")]
 173 pub fn forget<T>(t: T) {
 174     unsafe { intrinsics::forget(t) }
 175 }
 176
 177 /// Returns the size of a type in bytes.
 178 ///
 179 /// More specifically, this is the offset in bytes between successive
 180 /// items of the same type, including alignment padding.
 181 ///
 182 /// # Examples
 183 ///
 184 /// ```
 185 /// use std::mem;
 186 ///
 187 /// assert_eq!(4, mem::size_of::<i32>());
 188 /// ```
 189 #[inline]
 190 #[stable(feature = "rust1", since = "1.0.0")]
 191 pub fn size_of<T>() -> usize {
 192     unsafe { intrinsics::size_of::<T>() }
 193 }
 194
 195 /// Returns the size of the pointed-to value in bytes.
 196 ///
 197 /// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
 198 /// statically known size, e.g. a slice [`[T]`][slice] or a [trait object],
 199 /// then `size_of_val` can be used to get the dynamically-known size.
 200 ///
 201 /// [slice]: ../../std/primitive.slice.html
 202 /// [trait object]: ../../book/trait-objects.html
 203 ///
 204 /// # Examples
 205 ///
 206 /// ```
 207 /// use std::mem;
 208 ///
 209 /// assert_eq!(4, mem::size_of_val(&5i32));
 210 ///
 211 /// let x: [u8; 13] = [0; 13];
 212 /// let y: &[u8] = &x;
 213 /// assert_eq!(13, mem::size_of_val(y));
 214 /// ```
 215 #[inline]
 216 #[stable(feature = "rust1", since = "1.0.0")]
 217 pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
 218     unsafe { intrinsics::size_of_val(val) }
 219 }
 220
 221 /// Returns the [ABI]-required minimum alignment of a type.
 222 ///
 223 /// Every valid address of a value of the type `T` must be a multiple of this number.
 224 ///
 225 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
 226 ///
 227 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
 228 ///
 229 /// # Examples
 230 ///
 231 /// ```
 232 /// # #![allow(deprecated)]
 233 /// use std::mem;
 234 ///
 235 /// assert_eq!(4, mem::min_align_of::<i32>());
 236 /// ```
 237 #[inline]
 238 #[stable(feature = "rust1", since = "1.0.0")]
 239 #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
 240 pub fn min_align_of<T>() -> usize {
 241     unsafe { intrinsics::min_align_of::<T>() }
 242 }
 243
 244 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
 245 ///
 246 /// Every valid address of a value of the type `T` must be a multiple of this number.
 247 ///
 248 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
 249 ///
 250 /// # Examples
 251 ///
 252 /// ```
 253 /// # #![allow(deprecated)]
 254 /// use std::mem;
 255 ///
 256 /// assert_eq!(4, mem::min_align_of_val(&5i32));
 257 /// ```
 258 #[inline]
 259 #[stable(feature = "rust1", since = "1.0.0")]
 260 #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
 261 pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
 262     unsafe { intrinsics::min_align_of_val(val) }
 263 }
 264
 265 /// Returns the [ABI]-required minimum alignment of a type.
 266 ///
 267 /// Every valid address of a value of the type `T` must be a multiple of this number.
 268 ///
 269 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
 270 ///
 271 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
 272 ///
 273 /// # Examples
 274 ///
 275 /// ```
 276 /// use std::mem;
 277 ///
 278 /// assert_eq!(4, mem::align_of::<i32>());
 279 /// ```
 280 #[inline]
 281 #[stable(feature = "rust1", since = "1.0.0")]
 282 pub fn align_of<T>() -> usize {
 283     unsafe { intrinsics::min_align_of::<T>() }
 284 }
 285
 286 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
 287 ///
 288 /// Every valid address of a value of the type `T` must be a multiple of this number.
 289 ///
 290 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
 291 ///
 292 /// # Examples
 293 ///
 294 /// ```
 295 /// use std::mem;
 296 ///
 297 /// assert_eq!(4, mem::align_of_val(&5i32));
 298 /// ```
 299 #[inline]
 300 #[stable(feature = "rust1", since = "1.0.0")]
 301 pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
 302     unsafe { intrinsics::min_align_of_val(val) }
 303 }
 304
 305 /// Creates a value whose bytes are all zero.
 306 ///
 307 /// This has the same effect as allocating space with
 308 /// [`mem::uninitialized`][uninit] and then zeroing it out. It is useful for
 309 /// [FFI] sometimes, but should generally be avoided.
 310 ///
 311 /// There is no guarantee that an all-zero byte-pattern represents a valid value of
 312 /// some type `T`. If `T` has a destructor and the value is destroyed (due to
 313 /// a panic or the end of a scope) before being initialized, then the destructor
 314 /// will run on zeroed data, likely leading to [undefined behavior][ub].
 315 ///
 316 /// See also the documentation for [`mem::uninitialized`][uninit], which has
 317 /// many of the same caveats.
 318 ///
 319 /// [uninit]: fn.uninitialized.html
 320 /// [FFI]: ../../book/ffi.html
 321 /// [ub]: ../../reference/behavior-considered-undefined.html
 322 ///
 323 /// # Examples
 324 ///
 325 /// ```
 326 /// use std::mem;
 327 ///
 328 /// let x: i32 = unsafe { mem::zeroed() };
 329 /// assert_eq!(0, x);
 330 /// ```
 331 #[inline]
 332 #[stable(feature = "rust1", since = "1.0.0")]
 333 pub unsafe fn zeroed<T>() -> T {
 334     intrinsics::init()
 335 }
 336
 337 /// Bypasses Rust's normal memory-initialization checks by pretending to
 338 /// produce a value of type `T`, while doing nothing at all.
 339 ///
 340 /// **This is incredibly dangerous and should not be done lightly. Deeply
 341 /// consider initializing your memory with a default value instead.**
 342 ///
 343 /// This is useful for [FFI] functions and initializing arrays sometimes,
 344 /// but should generally be avoided.
 345 ///
 346 /// [FFI]: ../../book/ffi.html
 347 ///
 348 /// # Undefined behavior
 349 ///
 350 /// It is [undefined behavior][ub] to read uninitialized memory, even just an
 351 /// uninitialized boolean. For instance, if you branch on the value of such
 352 /// a boolean, your program may take one, both, or neither of the branches.
 353 ///
 354 /// Writing to the uninitialized value is similarly dangerous. Rust believes the
 355 /// value is initialized, and will therefore try to [`Drop`] the uninitialized
 356 /// value and its fields if you try to overwrite it in a normal manner. The only way
 357 /// to safely initialize an uninitialized value is with [`ptr::write`][write],
 358 /// [`ptr::copy`][copy], or [`ptr::copy_nonoverlapping`][copy_no].
 359 ///
 360 /// If the value does implement [`Drop`], it must be initialized before
 361 /// it goes out of scope (and therefore would be dropped). Note that this
 362 /// includes a `panic` occurring and unwinding the stack suddenly.
 363 ///
 364 /// # Examples
 365 ///
 366 /// Here's how to safely initialize an array of [`Vec`]s.
 367 ///
 368 /// ```
 369 /// use std::mem;
 370 /// use std::ptr;
 371 ///
 372 /// // Only declare the array. This safely leaves it
 373 /// // uninitialized in a way that Rust will track for us.
 374 /// // However we can't initialize it element-by-element
 375 /// // safely, and we can't use the `[value; 1000]`
 376 /// // constructor because it only works with `Copy` data.
 377 /// let mut data: [Vec<u32>; 1000];
 378 ///
 379 /// unsafe {
 380 ///     // So we need to do this to initialize it.
 381 ///     data = mem::uninitialized();
 382 ///
 383 ///     // DANGER ZONE: if anything panics or otherwise
 384 ///     // incorrectly reads the array here, we will have
 385 ///     // Undefined Behavior.
 386 ///
 387 ///     // It's ok to mutably iterate the data, since this
 388 ///     // doesn't involve reading it at all.
 389 ///     // (ptr and len are statically known for arrays)
 390 ///     for elem in &mut data[..] {
 391 ///         // *elem = Vec::new() would try to drop the
 392 ///         // uninitialized memory at `elem` -- bad!
 393 ///         //
 394 ///         // Vec::new doesn't allocate or do really
 395 ///         // anything. It's only safe to call here
 396 ///         // because we know it won't panic.
 397 ///         ptr::write(elem, Vec::new());
 398 ///     }
 399 ///
 400 ///     // SAFE ZONE: everything is initialized.
 401 /// }
 402 ///
 403 /// println!("{:?}", &data[0]);
 404 /// ```
 405 ///
 406 /// This example emphasizes exactly how delicate and dangerous using `mem::uninitialized`
 407 /// can be. Note that the [`vec!`] macro *does* let you initialize every element with a
 408 /// value that is only [`Clone`], so the following is semantically equivalent and
 409 /// vastly less dangerous, as long as you can live with an extra heap
 410 /// allocation:
 411 ///
 412 /// ```
 413 /// let data: Vec<Vec<u32>> = vec![Vec::new(); 1000];
 414 /// println!("{:?}", &data[0]);
 415 /// ```
 416 ///
 417 /// [`Vec`]: ../../std/vec/struct.Vec.html
 418 /// [`vec!`]: ../../std/macro.vec.html
 419 /// [`Clone`]: ../../std/clone/trait.Clone.html
 420 /// [ub]: ../../reference/behavior-considered-undefined.html
 421 /// [write]: ../ptr/fn.write.html
 422 /// [copy]: ../intrinsics/fn.copy.html
 423 /// [copy_no]: ../intrinsics/fn.copy_nonoverlapping.html
 424 /// [`Drop`]: ../ops/trait.Drop.html
 425 #[inline]
 426 #[stable(feature = "rust1", since = "1.0.0")]
 427 pub unsafe fn uninitialized<T>() -> T {
 428     intrinsics::uninit()
 429 }
 430
 431 /// Swaps the values at two mutable locations, without deinitializing either one.
 432 ///
 433 /// # Examples
 434 ///
 435 /// ```
 436 /// use std::mem;
 437 ///
 438 /// let mut x = 5;
 439 /// let mut y = 42;
 440 ///
 441 /// mem::swap(&mut x, &mut y);
 442 ///
 443 /// assert_eq!(42, x);
 444 /// assert_eq!(5, y);
 445 /// ```
 446 #[inline]
 447 #[stable(feature = "rust1", since = "1.0.0")]
 448 pub fn swap<T>(x: &mut T, y: &mut T) {
 449     unsafe {
 450         // Give ourselves some scratch space to work with
 451         let mut t: T = uninitialized();
 452
 453         // Perform the swap, `&mut` pointers never alias
 454         ptr::copy_nonoverlapping(&*x, &mut t, 1);
 455         ptr::copy_nonoverlapping(&*y, x, 1);
 456         ptr::copy_nonoverlapping(&t, y, 1);
 457
 458         // y and t now point to the same thing, but we need to completely
 459         // forget `t` because we do not want to run the destructor for `T`
 460         // on its value, which is still owned somewhere outside this function.
 461         forget(t);
 462     }
 463 }
 464
 465 /// Replaces the value at a mutable location with a new one, returning the old value, without
 466 /// deinitializing either one.
 467 ///
 468 /// # Examples
 469 ///
 470 /// A simple example:
 471 ///
 472 /// ```
 473 /// use std::mem;
 474 ///
 475 /// let mut v: Vec<i32> = vec![1, 2];
 476 ///
 477 /// let old_v = mem::replace(&mut v, vec![3, 4, 5]);
 478 /// assert_eq!(2, old_v.len());
 479 /// assert_eq!(3, v.len());
 480 /// ```
 481 ///
 482 /// `replace` allows consumption of a struct field by replacing it with another value.
 483 /// Without `replace` you can run into issues like these:
 484 ///
 485 /// ```ignore
 486 /// struct Buffer<T> { buf: Vec<T> }
 487 ///
 488 /// impl<T> Buffer<T> {
 489 ///     fn get_and_reset(&mut self) -> Vec<T> {
 490 ///         // error: cannot move out of dereference of `&mut`-pointer
 491 ///         let buf = self.buf;
 492 ///         self.buf = Vec::new();
 493 ///         buf
 494 ///     }
 495 /// }
 496 /// ```
 497 ///
 498 /// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset
 499 /// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from
 500 /// `self`, allowing it to be returned:
 501 ///
 502 /// ```
 503 /// # #![allow(dead_code)]
 504 /// use std::mem;
 505 ///
 506 /// # struct Buffer<T> { buf: Vec<T> }
 507 /// impl<T> Buffer<T> {
 508 ///     fn get_and_reset(&mut self) -> Vec<T> {
 509 ///         mem::replace(&mut self.buf, Vec::new())
 510 ///     }
 511 /// }
 512 /// ```
 513 ///
 514 /// [`Clone`]: ../../std/clone/trait.Clone.html
 515 #[inline]
 516 #[stable(feature = "rust1", since = "1.0.0")]
 517 pub fn replace<T>(dest: &mut T, mut src: T) -> T {
 518     swap(dest, &mut src);
 519     src
 520 }
 521
 522 /// Disposes of a value.
 523 ///
 524 /// While this does call the argument's implementation of [`Drop`][drop],
 525 /// it will not release any borrows, as borrows are based on lexical scope.
 526 ///
 527 /// This effectively does nothing for
 528 /// [types which implement `Copy`](../../book/ownership.html#copy-types),
 529 /// e.g. integers. Such values are copied and _then_ moved into the function,
 530 /// so the value persists after this function call.
 531 ///
 532 /// This function is not magic; it is literally defined as
 533 ///
 534 /// ```
 535 /// pub fn drop<T>(_x: T) { }
 536 /// ```
 537 ///
 538 /// Because `_x` is moved into the function, it is automatically dropped before
 539 /// the function returns.
 540 ///
 541 /// [drop]: ../ops/trait.Drop.html
 542 ///
 543 /// # Examples
 544 ///
 545 /// Basic usage:
 546 ///
 547 /// ```
 548 /// let v = vec![1, 2, 3];
 549 ///
 550 /// drop(v); // explicitly drop the vector
 551 /// ```
 552 ///
 553 /// Borrows are based on lexical scope, so this produces an error:
 554 ///
 555 /// ```ignore
 556 /// let mut v = vec![1, 2, 3];
 557 /// let x = &v[0];
 558 ///
 559 /// drop(x); // explicitly drop the reference, but the borrow still exists
 560 ///
 561 /// v.push(4); // error: cannot borrow `v` as mutable because it is also
 562 ///            // borrowed as immutable
 563 /// ```
 564 ///
 565 /// An inner scope is needed to fix this:
 566 ///
 567 /// ```
 568 /// let mut v = vec![1, 2, 3];
 569 ///
 570 /// {
 571 ///     let x = &v[0];
 572 ///
 573 ///     drop(x); // this is now redundant, as `x` is going out of scope anyway
 574 /// }
 575 ///
 576 /// v.push(4); // no problems
 577 /// ```
 578 ///
 579 /// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can
 580 /// release a [`RefCell`] borrow:
 581 ///
 582 /// ```
 583 /// use std::cell::RefCell;
 584 ///
 585 /// let x = RefCell::new(1);
 586 ///
 587 /// let mut mutable_borrow = x.borrow_mut();
 588 /// *mutable_borrow = 1;
 589 ///
 590 /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
 591 ///
 592 /// let borrow = x.borrow();
 593 /// println!("{}", *borrow);
 594 /// ```
 595 ///
 596 /// Integers and other types implementing [`Copy`] are unaffected by `drop`.
 597 ///
 598 /// ```
 599 /// #[derive(Copy, Clone)]
 600 /// struct Foo(u8);
 601 ///
 602 /// let x = 1;
 603 /// let y = Foo(2);
 604 /// drop(x); // a copy of `x` is moved and dropped
 605 /// drop(y); // a copy of `y` is moved and dropped
 606 ///
 607 /// println!("x: {}, y: {}", x, y.0); // still available
 608 /// ```
 609 ///
 610 /// [`RefCell`]: ../../std/cell/struct.RefCell.html
 611 /// [`Copy`]: ../../std/marker/trait.Copy.html
 612 #[inline]
 613 #[stable(feature = "rust1", since = "1.0.0")]
 614 pub fn drop<T>(_x: T) { }
 615
 616 /// Interprets `src` as having type `&U`, and then reads `src` without moving
 617 /// the contained value.
 618 ///
 619 /// This function will unsafely assume the pointer `src` is valid for
 620 /// [`size_of::<U>()`][size_of] bytes by transmuting `&T` to `&U` and then reading
 621 /// the `&U`. It will also unsafely create a copy of the contained value instead of
 622 /// moving out of `src`.
 623 ///
 624 /// It is not a compile-time error if `T` and `U` have different sizes, but it
 625 /// is highly encouraged to only invoke this function where `T` and `U` have the
 626 /// same size. This function triggers [undefined behavior][ub] if `U` is larger than
 627 /// `T`.
 628 ///
 629 /// [ub]: ../../reference/behavior-considered-undefined.html
 630 /// [size_of]: fn.size_of.html
 631 ///
 632 /// # Examples
 633 ///
 634 /// ```
 635 /// use std::mem;
 636 ///
 637 /// #[repr(packed)]
 638 /// struct Foo {
 639 ///     bar: u8,
 640 /// }
 641 ///
 642 /// let foo_slice = [10u8];
 643 ///
 644 /// unsafe {
 645 ///     // Copy the data from 'foo_slice' and treat it as a 'Foo'
 646 ///     let mut foo_struct: Foo = mem::transmute_copy(&foo_slice);
 647 ///     assert_eq!(foo_struct.bar, 10);
 648 ///
 649 ///     // Modify the copied data
 650 ///     foo_struct.bar = 20;
 651 ///     assert_eq!(foo_struct.bar, 20);
 652 /// }
 653 ///
 654 /// // The contents of 'foo_slice' should not have changed
 655 /// assert_eq!(foo_slice, [10]);
 656 /// ```
 657 #[inline]
 658 #[stable(feature = "rust1", since = "1.0.0")]
 659 pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
 660     ptr::read(src as *const T as *const U)
 661 }
 662
 663 /// Opaque type representing the discriminant of an enum.
 664 ///
 665 /// See the `discriminant` function in this module for more information.
 666 #[unstable(feature = "discriminant_value", reason = "recently added, follows RFC", issue = "24263")]
 667 pub struct Discriminant<T>(u64, PhantomData<*const T>);
 668
 669 // N.B. These trait implementations cannot be derived because we don't want any bounds on T.
 670
 671 #[unstable(feature = "discriminant_value", reason = "recently added, follows RFC", issue = "24263")]
 672 impl<T> Copy for Discriminant<T> {}
 673
 674 #[unstable(feature = "discriminant_value", reason = "recently added, follows RFC", issue = "24263")]
 675 impl<T> clone::Clone for Discriminant<T> {
 676     fn clone(&self) -> Self {
 677         *self
 678     }
 679 }
 680
 681 #[unstable(feature = "discriminant_value", reason = "recently added, follows RFC", issue = "24263")]
 682 impl<T> cmp::PartialEq for Discriminant<T> {
 683     fn eq(&self, rhs: &Self) -> bool {
 684         self.0 == rhs.0
 685     }
 686 }
 687
 688 #[unstable(feature = "discriminant_value", reason = "recently added, follows RFC", issue = "24263")]
 689 impl<T> cmp::Eq for Discriminant<T> {}
 690
 691 #[unstable(feature = "discriminant_value", reason = "recently added, follows RFC", issue = "24263")]
 692 impl<T> hash::Hash for Discriminant<T> {
 693     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 694         self.0.hash(state);
 695     }
 696 }
 697
 698 #[unstable(feature = "discriminant_value", reason = "recently added, follows RFC", issue = "24263")]
 699 impl<T> fmt::Debug for Discriminant<T> {
 700     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
 701         fmt.debug_tuple("Discriminant")
 702            .field(&self.0)
 703            .finish()
 704     }
 705 }
 706
 707 /// Returns a value uniquely identifying the enum variant in `v`.
 708 ///
 709 /// If `T` is not an enum, calling this function will not result in undefined behavior, but the
 710 /// return value is unspecified.
 711 ///
 712 /// # Stability
 713 ///
 714 /// The discriminant of an enum variant may change if the enum definition changes. A discriminant
 715 /// of some variant will not change between compilations with the same compiler.
 716 ///
 717 /// # Examples
 718 ///
 719 /// This can be used to compare enums that carry data, while disregarding
 720 /// the actual data:
 721 ///
 722 /// ```
 723 /// #![feature(discriminant_value)]
 724 /// use std::mem;
 725 ///
 726 /// enum Foo { A(&'static str), B(i32), C(i32) }
 727 ///
 728 /// assert!(mem::discriminant(&Foo::A("bar")) == mem::discriminant(&Foo::A("baz")));
 729 /// assert!(mem::discriminant(&Foo::B(1))     == mem::discriminant(&Foo::B(2)));
 730 /// assert!(mem::discriminant(&Foo::B(3))     != mem::discriminant(&Foo::C(3)));
 731 /// ```
 732 #[unstable(feature = "discriminant_value", reason = "recently added, follows RFC", issue = "24263")]
 733 pub fn discriminant<T>(v: &T) -> Discriminant<T> {
 734     unsafe {
 735         Discriminant(intrinsics::discriminant_value(v), PhantomData)
 736     }
 737 }
 738