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.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Basic functions for dealing with memory.
13 //! This module contains functions for querying the size and alignment of
14 //! types, initializing and manipulating memory.
16 #![stable(feature = "rust1", since = "1.0.0")]
23 use marker::{Copy, PhantomData, Sized};
25 use ops::{Deref, DerefMut};
27 #[stable(feature = "rust1", since = "1.0.0")]
28 pub use intrinsics::transmute;
30 /// Leaks a value: takes ownership and "forgets" about the value **without running
33 /// Any resources the value manages, such as heap memory or a file handle, will linger
34 /// forever in an unreachable state.
36 /// If you want to dispose of a value properly, running its destructor, see
37 /// [`mem::drop`][drop].
41 /// `forget` is not marked as `unsafe`, because Rust's safety guarantees
42 /// do not include a guarantee that destructors will always run. For example,
43 /// a program can create a reference cycle using [`Rc`][rc], or call
44 /// [`process::exit`][exit] to exit without running destructors. Thus, allowing
45 /// `mem::forget` from safe code does not fundamentally change Rust's safety
48 /// That said, leaking resources such as memory or I/O objects is usually undesirable,
49 /// so `forget` is only recommended for specialized use cases like those shown below.
51 /// Because forgetting a value is allowed, any `unsafe` code you write must
52 /// allow for this possibility. You cannot return a value and expect that the
53 /// caller will necessarily run the value's destructor.
55 /// [rc]: ../../std/rc/struct.Rc.html
56 /// [exit]: ../../std/process/fn.exit.html
60 /// Leak some heap memory by never deallocating it:
65 /// let heap_memory = Box::new(3);
66 /// mem::forget(heap_memory);
69 /// Leak an I/O object, never closing the file:
73 /// use std::fs::File;
75 /// let file = File::open("foo.txt").unwrap();
76 /// mem::forget(file);
79 /// The practical use cases for `forget` are rather specialized and mainly come
80 /// up in unsafe or FFI code.
84 /// You have created an uninitialized value using [`mem::uninitialized`][uninit].
85 /// You must either initialize or `forget` it on every computation path before
86 /// Rust drops it automatically, like at the end of a scope or after a panic.
87 /// Running the destructor on an uninitialized value would be [undefined behavior][ub].
93 /// # let some_condition = false;
95 /// let mut uninit_vec: Vec<u32> = mem::uninitialized();
97 /// if some_condition {
98 /// // Initialize the variable.
99 /// ptr::write(&mut uninit_vec, Vec::new());
101 /// // Forget the uninitialized value so its destructor doesn't run.
102 /// mem::forget(uninit_vec);
109 /// You have duplicated the bytes making up a value, without doing a proper
110 /// [`Clone`][clone]. You need the value's destructor to run only once,
111 /// because a double `free` is undefined behavior.
113 /// An example is a possible implementation of [`mem::swap`][swap]:
119 /// # #[allow(dead_code)]
120 /// fn swap<T>(x: &mut T, y: &mut T) {
122 /// // Give ourselves some scratch space to work with
123 /// let mut t: T = mem::uninitialized();
125 /// // Perform the swap, `&mut` pointers never alias
126 /// ptr::copy_nonoverlapping(&*x, &mut t, 1);
127 /// ptr::copy_nonoverlapping(&*y, x, 1);
128 /// ptr::copy_nonoverlapping(&t, y, 1);
130 /// // y and t now point to the same thing, but we need to completely
131 /// // forget `t` because we do not want to run the destructor for `T`
132 /// // on its value, which is still owned somewhere outside this function.
140 /// You are transferring ownership across a [FFI] boundary to code written in
141 /// another language. You need to `forget` the value on the Rust side because Rust
142 /// code is no longer responsible for it.
148 /// fn my_c_function(x: *const u32);
151 /// let x: Box<u32> = Box::new(3);
153 /// // Transfer ownership into C code.
155 /// my_c_function(&*x);
160 /// In this case, C code must call back into Rust to free the object. Calling C's `free`
161 /// function on a [`Box`][box] is *not* safe! Also, `Box` provides an [`into_raw`][into_raw]
162 /// method which is the preferred way to do this in practice.
164 /// [drop]: fn.drop.html
165 /// [uninit]: fn.uninitialized.html
166 /// [clone]: ../clone/trait.Clone.html
167 /// [swap]: fn.swap.html
168 /// [FFI]: ../../book/first-edition/ffi.html
169 /// [box]: ../../std/boxed/struct.Box.html
170 /// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw
171 /// [ub]: ../../reference/behavior-considered-undefined.html
173 #[stable(feature = "rust1", since = "1.0.0")]
174 pub fn forget<T>(t: T) {
175 ManuallyDrop::new(t);
178 /// Returns the size of a type in bytes.
180 /// More specifically, this is the offset in bytes between successive elements
181 /// in an array with that item type including alignment padding. Thus, for any
182 /// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`.
184 /// In general, the size of a type is not stable across compilations, but
185 /// specific types such as primitives are.
187 /// The following table gives the size for primitives.
189 /// Type | size_of::\<Type>()
190 /// ---- | ---------------
204 /// Furthermore, `usize` and `isize` have the same size.
206 /// The types `*const T`, `&T`, `Box<T>`, `Option<&T>`, and `Option<Box<T>>` all have
207 /// the same size. If `T` is Sized, all of those types have the same size as `usize`.
209 /// The mutability of a pointer does not change its size. As such, `&T` and `&mut T`
210 /// have the same size. Likewise for `*const T` and `*mut T`.
217 /// // Some primitives
218 /// assert_eq!(4, mem::size_of::<i32>());
219 /// assert_eq!(8, mem::size_of::<f64>());
220 /// assert_eq!(0, mem::size_of::<()>());
223 /// assert_eq!(8, mem::size_of::<[i32; 2]>());
224 /// assert_eq!(12, mem::size_of::<[i32; 3]>());
225 /// assert_eq!(0, mem::size_of::<[i32; 0]>());
228 /// // Pointer size equality
229 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>());
230 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>());
231 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>());
232 /// assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>());
235 #[stable(feature = "rust1", since = "1.0.0")]
236 #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_size_of"))]
237 pub const fn size_of<T>() -> usize {
238 unsafe { intrinsics::size_of::<T>() }
241 /// Returns the size of the pointed-to value in bytes.
243 /// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
244 /// statically known size, e.g. a slice [`[T]`][slice] or a [trait object],
245 /// then `size_of_val` can be used to get the dynamically-known size.
247 /// [slice]: ../../std/primitive.slice.html
248 /// [trait object]: ../../book/first-edition/trait-objects.html
255 /// assert_eq!(4, mem::size_of_val(&5i32));
257 /// let x: [u8; 13] = [0; 13];
258 /// let y: &[u8] = &x;
259 /// assert_eq!(13, mem::size_of_val(y));
262 #[stable(feature = "rust1", since = "1.0.0")]
263 pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
264 unsafe { intrinsics::size_of_val(val) }
267 /// Returns the [ABI]-required minimum alignment of a type.
269 /// Every reference to a value of the type `T` must be a multiple of this number.
271 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
273 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
278 /// # #![allow(deprecated)]
281 /// assert_eq!(4, mem::min_align_of::<i32>());
284 #[stable(feature = "rust1", since = "1.0.0")]
285 #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
286 pub fn min_align_of<T>() -> usize {
287 unsafe { intrinsics::min_align_of::<T>() }
290 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
292 /// Every reference to a value of the type `T` must be a multiple of this number.
294 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
299 /// # #![allow(deprecated)]
302 /// assert_eq!(4, mem::min_align_of_val(&5i32));
305 #[stable(feature = "rust1", since = "1.0.0")]
306 #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
307 pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
308 unsafe { intrinsics::min_align_of_val(val) }
311 /// Returns the [ABI]-required minimum alignment of a type.
313 /// Every reference to a value of the type `T` must be a multiple of this number.
315 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
317 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
324 /// assert_eq!(4, mem::align_of::<i32>());
327 #[stable(feature = "rust1", since = "1.0.0")]
328 #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_align_of"))]
329 pub const fn align_of<T>() -> usize {
330 unsafe { intrinsics::min_align_of::<T>() }
333 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
335 /// Every reference to a value of the type `T` must be a multiple of this number.
337 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
344 /// assert_eq!(4, mem::align_of_val(&5i32));
347 #[stable(feature = "rust1", since = "1.0.0")]
348 pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
349 unsafe { intrinsics::min_align_of_val(val) }
352 /// Returns whether dropping values of type `T` matters.
354 /// This is purely an optimization hint, and may be implemented conservatively.
355 /// For instance, always returning `true` would be a valid implementation of
358 /// Low level implementations of things like collections, which need to manually
359 /// drop their data, should use this function to avoid unnecessarily
360 /// trying to drop all their contents when they are destroyed. This might not
361 /// make a difference in release builds (where a loop that has no side-effects
362 /// is easily detected and eliminated), but is often a big win for debug builds.
364 /// Note that `ptr::drop_in_place` already performs this check, so if your workload
365 /// can be reduced to some small number of drop_in_place calls, using this is
366 /// unnecessary. In particular note that you can drop_in_place a slice, and that
367 /// will do a single needs_drop check for all the values.
369 /// Types like Vec therefore just `drop_in_place(&mut self[..])` without using
370 /// needs_drop explicitly. Types like HashMap, on the other hand, have to drop
371 /// values one at a time and should use this API.
376 /// Here's an example of how a collection might make use of needs_drop:
379 /// use std::{mem, ptr};
381 /// pub struct MyCollection<T> {
385 /// # impl<T> MyCollection<T> {
386 /// # fn iter_mut(&mut self) -> &mut [T] { &mut self.data }
387 /// # fn free_buffer(&mut self) {}
390 /// impl<T> Drop for MyCollection<T> {
391 /// fn drop(&mut self) {
394 /// if mem::needs_drop::<T>() {
395 /// for x in self.iter_mut() {
396 /// ptr::drop_in_place(x);
399 /// self.free_buffer();
405 #[stable(feature = "needs_drop", since = "1.21.0")]
406 pub fn needs_drop<T>() -> bool {
407 unsafe { intrinsics::needs_drop::<T>() }
410 /// Creates a value whose bytes are all zero.
412 /// This has the same effect as allocating space with
413 /// [`mem::uninitialized`][uninit] and then zeroing it out. It is useful for
414 /// [FFI] sometimes, but should generally be avoided.
416 /// There is no guarantee that an all-zero byte-pattern represents a valid value of
417 /// some type `T`. If `T` has a destructor and the value is destroyed (due to
418 /// a panic or the end of a scope) before being initialized, then the destructor
419 /// will run on zeroed data, likely leading to [undefined behavior][ub].
421 /// See also the documentation for [`mem::uninitialized`][uninit], which has
422 /// many of the same caveats.
424 /// [uninit]: fn.uninitialized.html
425 /// [FFI]: ../../book/first-edition/ffi.html
426 /// [ub]: ../../reference/behavior-considered-undefined.html
433 /// let x: i32 = unsafe { mem::zeroed() };
434 /// assert_eq!(0, x);
437 #[stable(feature = "rust1", since = "1.0.0")]
438 pub unsafe fn zeroed<T>() -> T {
442 /// Bypasses Rust's normal memory-initialization checks by pretending to
443 /// produce a value of type `T`, while doing nothing at all.
445 /// **This is incredibly dangerous and should not be done lightly. Deeply
446 /// consider initializing your memory with a default value instead.**
448 /// This is useful for [FFI] functions and initializing arrays sometimes,
449 /// but should generally be avoided.
451 /// [FFI]: ../../book/first-edition/ffi.html
453 /// # Undefined behavior
455 /// It is [undefined behavior][ub] to read uninitialized memory, even just an
456 /// uninitialized boolean. For instance, if you branch on the value of such
457 /// a boolean, your program may take one, both, or neither of the branches.
459 /// Writing to the uninitialized value is similarly dangerous. Rust believes the
460 /// value is initialized, and will therefore try to [`Drop`] the uninitialized
461 /// value and its fields if you try to overwrite it in a normal manner. The only way
462 /// to safely initialize an uninitialized value is with [`ptr::write`][write],
463 /// [`ptr::copy`][copy], or [`ptr::copy_nonoverlapping`][copy_no].
465 /// If the value does implement [`Drop`], it must be initialized before
466 /// it goes out of scope (and therefore would be dropped). Note that this
467 /// includes a `panic` occurring and unwinding the stack suddenly.
471 /// Here's how to safely initialize an array of [`Vec`]s.
477 /// // Only declare the array. This safely leaves it
478 /// // uninitialized in a way that Rust will track for us.
479 /// // However we can't initialize it element-by-element
480 /// // safely, and we can't use the `[value; 1000]`
481 /// // constructor because it only works with `Copy` data.
482 /// let mut data: [Vec<u32>; 1000];
485 /// // So we need to do this to initialize it.
486 /// data = mem::uninitialized();
488 /// // DANGER ZONE: if anything panics or otherwise
489 /// // incorrectly reads the array here, we will have
490 /// // Undefined Behavior.
492 /// // It's ok to mutably iterate the data, since this
493 /// // doesn't involve reading it at all.
494 /// // (ptr and len are statically known for arrays)
495 /// for elem in &mut data[..] {
496 /// // *elem = Vec::new() would try to drop the
497 /// // uninitialized memory at `elem` -- bad!
499 /// // Vec::new doesn't allocate or do really
500 /// // anything. It's only safe to call here
501 /// // because we know it won't panic.
502 /// ptr::write(elem, Vec::new());
505 /// // SAFE ZONE: everything is initialized.
508 /// println!("{:?}", &data[0]);
511 /// This example emphasizes exactly how delicate and dangerous using `mem::uninitialized`
512 /// can be. Note that the [`vec!`] macro *does* let you initialize every element with a
513 /// value that is only [`Clone`], so the following is semantically equivalent and
514 /// vastly less dangerous, as long as you can live with an extra heap
518 /// let data: Vec<Vec<u32>> = vec![Vec::new(); 1000];
519 /// println!("{:?}", &data[0]);
522 /// [`Vec`]: ../../std/vec/struct.Vec.html
523 /// [`vec!`]: ../../std/macro.vec.html
524 /// [`Clone`]: ../../std/clone/trait.Clone.html
525 /// [ub]: ../../reference/behavior-considered-undefined.html
526 /// [write]: ../ptr/fn.write.html
527 /// [copy]: ../intrinsics/fn.copy.html
528 /// [copy_no]: ../intrinsics/fn.copy_nonoverlapping.html
529 /// [`Drop`]: ../ops/trait.Drop.html
531 #[stable(feature = "rust1", since = "1.0.0")]
532 pub unsafe fn uninitialized<T>() -> T {
536 /// Swaps the values at two mutable locations, without deinitializing either one.
546 /// mem::swap(&mut x, &mut y);
548 /// assert_eq!(42, x);
549 /// assert_eq!(5, y);
552 #[stable(feature = "rust1", since = "1.0.0")]
553 pub fn swap<T>(x: &mut T, y: &mut T) {
555 ptr::swap_nonoverlapping(x, y, 1);
559 /// Replaces the value at a mutable location with a new one, returning the old value, without
560 /// deinitializing either one.
564 /// A simple example:
569 /// let mut v: Vec<i32> = vec![1, 2];
571 /// let old_v = mem::replace(&mut v, vec![3, 4, 5]);
572 /// assert_eq!(2, old_v.len());
573 /// assert_eq!(3, v.len());
576 /// `replace` allows consumption of a struct field by replacing it with another value.
577 /// Without `replace` you can run into issues like these:
579 /// ```compile_fail,E0507
580 /// struct Buffer<T> { buf: Vec<T> }
582 /// impl<T> Buffer<T> {
583 /// fn get_and_reset(&mut self) -> Vec<T> {
584 /// // error: cannot move out of dereference of `&mut`-pointer
585 /// let buf = self.buf;
586 /// self.buf = Vec::new();
592 /// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset
593 /// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from
594 /// `self`, allowing it to be returned:
597 /// # #![allow(dead_code)]
600 /// # struct Buffer<T> { buf: Vec<T> }
601 /// impl<T> Buffer<T> {
602 /// fn get_and_reset(&mut self) -> Vec<T> {
603 /// mem::replace(&mut self.buf, Vec::new())
608 /// [`Clone`]: ../../std/clone/trait.Clone.html
610 #[stable(feature = "rust1", since = "1.0.0")]
611 pub fn replace<T>(dest: &mut T, mut src: T) -> T {
612 swap(dest, &mut src);
616 /// Disposes of a value.
618 /// While this does call the argument's implementation of [`Drop`][drop],
619 /// it will not release any borrows, as borrows are based on lexical scope.
621 /// This effectively does nothing for
622 /// [types which implement `Copy`](../../book/first-edition/ownership.html#copy-types),
623 /// e.g. integers. Such values are copied and _then_ moved into the function,
624 /// so the value persists after this function call.
626 /// This function is not magic; it is literally defined as
629 /// pub fn drop<T>(_x: T) { }
632 /// Because `_x` is moved into the function, it is automatically dropped before
633 /// the function returns.
635 /// [drop]: ../ops/trait.Drop.html
642 /// let v = vec![1, 2, 3];
644 /// drop(v); // explicitly drop the vector
647 /// Borrows are based on lexical scope, so this produces an error:
649 /// ```compile_fail,E0502
650 /// let mut v = vec![1, 2, 3];
653 /// drop(x); // explicitly drop the reference, but the borrow still exists
655 /// v.push(4); // error: cannot borrow `v` as mutable because it is also
656 /// // borrowed as immutable
659 /// An inner scope is needed to fix this:
662 /// let mut v = vec![1, 2, 3];
667 /// drop(x); // this is now redundant, as `x` is going out of scope anyway
670 /// v.push(4); // no problems
673 /// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can
674 /// release a [`RefCell`] borrow:
677 /// use std::cell::RefCell;
679 /// let x = RefCell::new(1);
681 /// let mut mutable_borrow = x.borrow_mut();
682 /// *mutable_borrow = 1;
684 /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
686 /// let borrow = x.borrow();
687 /// println!("{}", *borrow);
690 /// Integers and other types implementing [`Copy`] are unaffected by `drop`.
693 /// #[derive(Copy, Clone)]
698 /// drop(x); // a copy of `x` is moved and dropped
699 /// drop(y); // a copy of `y` is moved and dropped
701 /// println!("x: {}, y: {}", x, y.0); // still available
704 /// [`RefCell`]: ../../std/cell/struct.RefCell.html
705 /// [`Copy`]: ../../std/marker/trait.Copy.html
707 #[stable(feature = "rust1", since = "1.0.0")]
708 pub fn drop<T>(_x: T) { }
710 /// Interprets `src` as having type `&U`, and then reads `src` without moving
711 /// the contained value.
713 /// This function will unsafely assume the pointer `src` is valid for
714 /// [`size_of::<U>`][size_of] bytes by transmuting `&T` to `&U` and then reading
715 /// the `&U`. It will also unsafely create a copy of the contained value instead of
716 /// moving out of `src`.
718 /// It is not a compile-time error if `T` and `U` have different sizes, but it
719 /// is highly encouraged to only invoke this function where `T` and `U` have the
720 /// same size. This function triggers [undefined behavior][ub] if `U` is larger than
723 /// [ub]: ../../reference/behavior-considered-undefined.html
724 /// [size_of]: fn.size_of.html
736 /// let foo_slice = [10u8];
739 /// // Copy the data from 'foo_slice' and treat it as a 'Foo'
740 /// let mut foo_struct: Foo = mem::transmute_copy(&foo_slice);
741 /// assert_eq!(foo_struct.bar, 10);
743 /// // Modify the copied data
744 /// foo_struct.bar = 20;
745 /// assert_eq!(foo_struct.bar, 20);
748 /// // The contents of 'foo_slice' should not have changed
749 /// assert_eq!(foo_slice, [10]);
752 #[stable(feature = "rust1", since = "1.0.0")]
753 pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
754 ptr::read(src as *const T as *const U)
757 /// Opaque type representing the discriminant of an enum.
759 /// See the `discriminant` function in this module for more information.
760 #[stable(feature = "discriminant_value", since = "1.21.0")]
761 pub struct Discriminant<T>(u64, PhantomData<*const T>);
763 // N.B. These trait implementations cannot be derived because we don't want any bounds on T.
765 #[stable(feature = "discriminant_value", since = "1.21.0")]
766 impl<T> Copy for Discriminant<T> {}
768 #[stable(feature = "discriminant_value", since = "1.21.0")]
769 impl<T> clone::Clone for Discriminant<T> {
770 fn clone(&self) -> Self {
775 #[stable(feature = "discriminant_value", since = "1.21.0")]
776 impl<T> cmp::PartialEq for Discriminant<T> {
777 fn eq(&self, rhs: &Self) -> bool {
782 #[stable(feature = "discriminant_value", since = "1.21.0")]
783 impl<T> cmp::Eq for Discriminant<T> {}
785 #[stable(feature = "discriminant_value", since = "1.21.0")]
786 impl<T> hash::Hash for Discriminant<T> {
787 fn hash<H: hash::Hasher>(&self, state: &mut H) {
792 #[stable(feature = "discriminant_value", since = "1.21.0")]
793 impl<T> fmt::Debug for Discriminant<T> {
794 fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
795 fmt.debug_tuple("Discriminant")
801 /// Returns a value uniquely identifying the enum variant in `v`.
803 /// If `T` is not an enum, calling this function will not result in undefined behavior, but the
804 /// return value is unspecified.
808 /// The discriminant of an enum variant may change if the enum definition changes. A discriminant
809 /// of some variant will not change between compilations with the same compiler.
813 /// This can be used to compare enums that carry data, while disregarding
819 /// enum Foo { A(&'static str), B(i32), C(i32) }
821 /// assert!(mem::discriminant(&Foo::A("bar")) == mem::discriminant(&Foo::A("baz")));
822 /// assert!(mem::discriminant(&Foo::B(1)) == mem::discriminant(&Foo::B(2)));
823 /// assert!(mem::discriminant(&Foo::B(3)) != mem::discriminant(&Foo::C(3)));
825 #[stable(feature = "discriminant_value", since = "1.21.0")]
826 pub fn discriminant<T>(v: &T) -> Discriminant<T> {
828 Discriminant(intrinsics::discriminant_value(v), PhantomData)
833 /// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
835 /// This wrapper is 0-cost.
839 /// This wrapper helps with explicitly documenting the drop order dependencies between fields of
843 /// use std::mem::ManuallyDrop;
847 /// struct FruitBox {
848 /// // Immediately clear there’s something non-trivial going on with these fields.
849 /// peach: ManuallyDrop<Peach>,
850 /// melon: Melon, // Field that’s independent of the other two.
851 /// banana: ManuallyDrop<Banana>,
854 /// impl Drop for FruitBox {
855 /// fn drop(&mut self) {
857 /// // Explicit ordering in which field destructors are run specified in the intuitive
858 /// // location – the destructor of the structure containing the fields.
859 /// // Moreover, one can now reorder fields within the struct however much they want.
860 /// ManuallyDrop::drop(&mut self.peach);
861 /// ManuallyDrop::drop(&mut self.banana);
863 /// // After destructor for `FruitBox` runs (this function), the destructor for Melon gets
864 /// // invoked in the usual manner, as it is not wrapped in `ManuallyDrop`.
868 #[stable(feature = "manually_drop", since = "1.20.0")]
869 #[allow(unions_with_drop_fields)]
871 pub union ManuallyDrop<T>{ value: T }
873 impl<T> ManuallyDrop<T> {
874 /// Wrap a value to be manually dropped.
879 /// use std::mem::ManuallyDrop;
880 /// ManuallyDrop::new(Box::new(()));
882 #[stable(feature = "manually_drop", since = "1.20.0")]
884 pub fn new(value: T) -> ManuallyDrop<T> {
885 ManuallyDrop { value: value }
888 /// Extract the value from the ManuallyDrop container.
893 /// use std::mem::ManuallyDrop;
894 /// let x = ManuallyDrop::new(Box::new(()));
895 /// let _: Box<()> = ManuallyDrop::into_inner(x);
897 #[stable(feature = "manually_drop", since = "1.20.0")]
899 pub fn into_inner(slot: ManuallyDrop<T>) -> T {
905 /// Manually drops the contained value.
909 /// This function runs the destructor of the contained value and thus the wrapped value
910 /// now represents uninitialized data. It is up to the user of this method to ensure the
911 /// uninitialized data is not actually used.
912 #[stable(feature = "manually_drop", since = "1.20.0")]
914 pub unsafe fn drop(slot: &mut ManuallyDrop<T>) {
915 ptr::drop_in_place(&mut slot.value)
919 #[stable(feature = "manually_drop", since = "1.20.0")]
920 impl<T> Deref for ManuallyDrop<T> {
923 fn deref(&self) -> &Self::Target {
930 #[stable(feature = "manually_drop", since = "1.20.0")]
931 impl<T> DerefMut for ManuallyDrop<T> {
933 fn deref_mut(&mut self) -> &mut Self::Target {
940 #[stable(feature = "manually_drop", since = "1.20.0")]
941 impl<T: ::fmt::Debug> ::fmt::Debug for ManuallyDrop<T> {
942 fn fmt(&self, fmt: &mut ::fmt::Formatter) -> ::fmt::Result {
944 fmt.debug_tuple("ManuallyDrop").field(&self.value).finish()
949 #[stable(feature = "manually_drop", since = "1.20.0")]
950 impl<T: Clone> Clone for ManuallyDrop<T> {
951 fn clone(&self) -> Self {
952 ManuallyDrop::new(self.deref().clone())
955 fn clone_from(&mut self, source: &Self) {
956 self.deref_mut().clone_from(source);
960 #[stable(feature = "manually_drop", since = "1.20.0")]
961 impl<T: Default> Default for ManuallyDrop<T> {
962 fn default() -> Self {
963 ManuallyDrop::new(Default::default())
967 #[stable(feature = "manually_drop", since = "1.20.0")]
968 impl<T: PartialEq> PartialEq for ManuallyDrop<T> {
969 fn eq(&self, other: &Self) -> bool {
970 self.deref().eq(other)
973 fn ne(&self, other: &Self) -> bool {
974 self.deref().ne(other)
978 #[stable(feature = "manually_drop", since = "1.20.0")]
979 impl<T: Eq> Eq for ManuallyDrop<T> {}
981 #[stable(feature = "manually_drop", since = "1.20.0")]
982 impl<T: PartialOrd> PartialOrd for ManuallyDrop<T> {
983 fn partial_cmp(&self, other: &Self) -> Option<::cmp::Ordering> {
984 self.deref().partial_cmp(other)
987 fn lt(&self, other: &Self) -> bool {
988 self.deref().lt(other)
991 fn le(&self, other: &Self) -> bool {
992 self.deref().le(other)
995 fn gt(&self, other: &Self) -> bool {
996 self.deref().gt(other)
999 fn ge(&self, other: &Self) -> bool {
1000 self.deref().ge(other)
1004 #[stable(feature = "manually_drop", since = "1.20.0")]
1005 impl<T: Ord> Ord for ManuallyDrop<T> {
1006 fn cmp(&self, other: &Self) -> ::cmp::Ordering {
1007 self.deref().cmp(other)
1011 #[stable(feature = "manually_drop", since = "1.20.0")]
1012 impl<T: ::hash::Hash> ::hash::Hash for ManuallyDrop<T> {
1013 fn hash<H: ::hash::Hasher>(&self, state: &mut H) {
1014 self.deref().hash(state);
1018 /// Tells LLVM that this point in the code is not reachable, enabling further
1021 /// NB: This is very different from the `unreachable!()` macro: Unlike the
1022 /// macro, which panics when it is executed, it is *undefined behavior* to
1023 /// reach code marked with this function.
1025 #[unstable(feature = "unreachable", issue = "43751")]
1026 pub unsafe fn unreachable() -> ! {
1027 intrinsics::unreachable()