1 //! Basic functions for dealing with memory.
3 //! This module contains functions for querying the size and alignment of
4 //! types, initializing and manipulating memory.
6 #![stable(feature = "rust1", since = "1.0.0")]
12 use crate::intrinsics;
13 use crate::marker::{Copy, DiscriminantKind, Sized};
17 #[stable(feature = "manually_drop", since = "1.20.0")]
18 pub use manually_drop::ManuallyDrop;
21 #[stable(feature = "maybe_uninit", since = "1.36.0")]
22 pub use maybe_uninit::MaybeUninit;
24 // FIXME: This is left here for now to avoid complications around pending reverts.
25 // Once <https://github.com/rust-lang/rust/issues/101899> is fully resolved,
26 // this should be removed and the references in `alloc::Layout` updated.
27 pub(crate) use ptr::Alignment as ValidAlign;
30 #[unstable(feature = "transmutability", issue = "99571")]
31 pub use transmutability::{Assume, BikeshedIntrinsicFrom};
33 #[stable(feature = "rust1", since = "1.0.0")]
35 pub use crate::intrinsics::transmute;
37 /// Takes ownership and "forgets" about the value **without running its destructor**.
39 /// Any resources the value manages, such as heap memory or a file handle, will linger
40 /// forever in an unreachable state. However, it does not guarantee that pointers
41 /// to this memory will remain valid.
43 /// * If you want to leak memory, see [`Box::leak`].
44 /// * If you want to obtain a raw pointer to the memory, see [`Box::into_raw`].
45 /// * If you want to dispose of a value properly, running its destructor, see
50 /// `forget` is not marked as `unsafe`, because Rust's safety guarantees
51 /// do not include a guarantee that destructors will always run. For example,
52 /// a program can create a reference cycle using [`Rc`][rc], or call
53 /// [`process::exit`][exit] to exit without running destructors. Thus, allowing
54 /// `mem::forget` from safe code does not fundamentally change Rust's safety
57 /// That said, leaking resources such as memory or I/O objects is usually undesirable.
58 /// The need comes up in some specialized use cases for FFI or unsafe code, but even
59 /// then, [`ManuallyDrop`] is typically preferred.
61 /// Because forgetting a value is allowed, any `unsafe` code you write must
62 /// allow for this possibility. You cannot return a value and expect that the
63 /// caller will necessarily run the value's destructor.
65 /// [rc]: ../../std/rc/struct.Rc.html
66 /// [exit]: ../../std/process/fn.exit.html
70 /// The canonical safe use of `mem::forget` is to circumvent a value's destructor
71 /// implemented by the `Drop` trait. For example, this will leak a `File`, i.e. reclaim
72 /// the space taken by the variable but never close the underlying system resource:
76 /// use std::fs::File;
78 /// let file = File::open("foo.txt").unwrap();
79 /// mem::forget(file);
82 /// This is useful when the ownership of the underlying resource was previously
83 /// transferred to code outside of Rust, for example by transmitting the raw
84 /// file descriptor to C code.
86 /// # Relationship with `ManuallyDrop`
88 /// While `mem::forget` can also be used to transfer *memory* ownership, doing so is error-prone.
89 /// [`ManuallyDrop`] should be used instead. Consider, for example, this code:
94 /// let mut v = vec![65, 122];
95 /// // Build a `String` using the contents of `v`
96 /// let s = unsafe { String::from_raw_parts(v.as_mut_ptr(), v.len(), v.capacity()) };
97 /// // leak `v` because its memory is now managed by `s`
98 /// mem::forget(v); // ERROR - v is invalid and must not be passed to a function
99 /// assert_eq!(s, "Az");
100 /// // `s` is implicitly dropped and its memory deallocated.
103 /// There are two issues with the above example:
105 /// * If more code were added between the construction of `String` and the invocation of
106 /// `mem::forget()`, a panic within it would cause a double free because the same memory
107 /// is handled by both `v` and `s`.
108 /// * After calling `v.as_mut_ptr()` and transmitting the ownership of the data to `s`,
109 /// the `v` value is invalid. Even when a value is just moved to `mem::forget` (which won't
110 /// inspect it), some types have strict requirements on their values that
111 /// make them invalid when dangling or no longer owned. Using invalid values in any
112 /// way, including passing them to or returning them from functions, constitutes
113 /// undefined behavior and may break the assumptions made by the compiler.
115 /// Switching to `ManuallyDrop` avoids both issues:
118 /// use std::mem::ManuallyDrop;
120 /// let v = vec![65, 122];
121 /// // Before we disassemble `v` into its raw parts, make sure it
122 /// // does not get dropped!
123 /// let mut v = ManuallyDrop::new(v);
124 /// // Now disassemble `v`. These operations cannot panic, so there cannot be a leak.
125 /// let (ptr, len, cap) = (v.as_mut_ptr(), v.len(), v.capacity());
126 /// // Finally, build a `String`.
127 /// let s = unsafe { String::from_raw_parts(ptr, len, cap) };
128 /// assert_eq!(s, "Az");
129 /// // `s` is implicitly dropped and its memory deallocated.
132 /// `ManuallyDrop` robustly prevents double-free because we disable `v`'s destructor
133 /// before doing anything else. `mem::forget()` doesn't allow this because it consumes its
134 /// argument, forcing us to call it only after extracting anything we need from `v`. Even
135 /// if a panic were introduced between construction of `ManuallyDrop` and building the
136 /// string (which cannot happen in the code as shown), it would result in a leak and not a
137 /// double free. In other words, `ManuallyDrop` errs on the side of leaking instead of
138 /// erring on the side of (double-)dropping.
140 /// Also, `ManuallyDrop` prevents us from having to "touch" `v` after transferring the
141 /// ownership to `s` — the final step of interacting with `v` to dispose of it without
142 /// running its destructor is entirely avoided.
144 /// [`Box`]: ../../std/boxed/struct.Box.html
145 /// [`Box::leak`]: ../../std/boxed/struct.Box.html#method.leak
146 /// [`Box::into_raw`]: ../../std/boxed/struct.Box.html#method.into_raw
147 /// [`mem::drop`]: drop
148 /// [ub]: ../../reference/behavior-considered-undefined.html
150 #[rustc_const_stable(feature = "const_forget", since = "1.46.0")]
151 #[stable(feature = "rust1", since = "1.0.0")]
152 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_forget")]
153 pub const fn forget<T>(t: T) {
154 let _ = ManuallyDrop::new(t);
157 /// Like [`forget`], but also accepts unsized values.
159 /// This function is just a shim intended to be removed when the `unsized_locals` feature gets
162 #[unstable(feature = "forget_unsized", issue = "none")]
163 pub fn forget_unsized<T: ?Sized>(t: T) {
164 intrinsics::forget(t)
167 /// Returns the size of a type in bytes.
169 /// More specifically, this is the offset in bytes between successive elements
170 /// in an array with that item type including alignment padding. Thus, for any
171 /// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`.
173 /// In general, the size of a type is not stable across compilations, but
174 /// specific types such as primitives are.
176 /// The following table gives the size for primitives.
178 /// Type | size_of::\<Type>()
179 /// ---- | ---------------
196 /// Furthermore, `usize` and `isize` have the same size.
198 /// The types `*const T`, `&T`, `Box<T>`, `Option<&T>`, and `Option<Box<T>>` all have
199 /// the same size. If `T` is Sized, all of those types have the same size as `usize`.
201 /// The mutability of a pointer does not change its size. As such, `&T` and `&mut T`
202 /// have the same size. Likewise for `*const T` and `*mut T`.
204 /// # Size of `#[repr(C)]` items
206 /// The `C` representation for items has a defined layout. With this layout,
207 /// the size of items is also stable as long as all fields have a stable size.
209 /// ## Size of Structs
211 /// For `structs`, the size is determined by the following algorithm.
213 /// For each field in the struct ordered by declaration order:
215 /// 1. Add the size of the field.
216 /// 2. Round up the current size to the nearest multiple of the next field's [alignment].
218 /// Finally, round the size of the struct to the nearest multiple of its [alignment].
219 /// The alignment of the struct is usually the largest alignment of all its
220 /// fields; this can be changed with the use of `repr(align(N))`.
222 /// Unlike `C`, zero sized structs are not rounded up to one byte in size.
226 /// Enums that carry no data other than the discriminant have the same size as C enums
227 /// on the platform they are compiled for.
229 /// ## Size of Unions
231 /// The size of a union is the size of its largest field.
233 /// Unlike `C`, zero sized unions are not rounded up to one byte in size.
240 /// // Some primitives
241 /// assert_eq!(4, mem::size_of::<i32>());
242 /// assert_eq!(8, mem::size_of::<f64>());
243 /// assert_eq!(0, mem::size_of::<()>());
246 /// assert_eq!(8, mem::size_of::<[i32; 2]>());
247 /// assert_eq!(12, mem::size_of::<[i32; 3]>());
248 /// assert_eq!(0, mem::size_of::<[i32; 0]>());
251 /// // Pointer size equality
252 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>());
253 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>());
254 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>());
255 /// assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>());
258 /// Using `#[repr(C)]`.
264 /// struct FieldStruct {
270 /// // The size of the first field is 1, so add 1 to the size. Size is 1.
271 /// // The alignment of the second field is 2, so add 1 to the size for padding. Size is 2.
272 /// // The size of the second field is 2, so add 2 to the size. Size is 4.
273 /// // The alignment of the third field is 1, so add 0 to the size for padding. Size is 4.
274 /// // The size of the third field is 1, so add 1 to the size. Size is 5.
275 /// // Finally, the alignment of the struct is 2 (because the largest alignment amongst its
276 /// // fields is 2), so add 1 to the size for padding. Size is 6.
277 /// assert_eq!(6, mem::size_of::<FieldStruct>());
280 /// struct TupleStruct(u8, u16, u8);
282 /// // Tuple structs follow the same rules.
283 /// assert_eq!(6, mem::size_of::<TupleStruct>());
285 /// // Note that reordering the fields can lower the size. We can remove both padding bytes
286 /// // by putting `third` before `second`.
288 /// struct FieldStructOptimized {
294 /// assert_eq!(4, mem::size_of::<FieldStructOptimized>());
296 /// // Union size is the size of the largest field.
298 /// union ExampleUnion {
303 /// assert_eq!(2, mem::size_of::<ExampleUnion>());
306 /// [alignment]: align_of
309 #[stable(feature = "rust1", since = "1.0.0")]
311 #[rustc_const_stable(feature = "const_mem_size_of", since = "1.24.0")]
312 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_size_of")]
313 pub const fn size_of<T>() -> usize {
314 intrinsics::size_of::<T>()
317 /// Returns the size of the pointed-to value in bytes.
319 /// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
320 /// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],
321 /// then `size_of_val` can be used to get the dynamically-known size.
323 /// [trait object]: ../../book/ch17-02-trait-objects.html
330 /// assert_eq!(4, mem::size_of_val(&5i32));
332 /// let x: [u8; 13] = [0; 13];
333 /// let y: &[u8] = &x;
334 /// assert_eq!(13, mem::size_of_val(y));
338 #[stable(feature = "rust1", since = "1.0.0")]
339 #[rustc_const_unstable(feature = "const_size_of_val", issue = "46571")]
340 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_size_of_val")]
341 pub const fn size_of_val<T: ?Sized>(val: &T) -> usize {
342 // SAFETY: `val` is a reference, so it's a valid raw pointer
343 unsafe { intrinsics::size_of_val(val) }
346 /// Returns the size of the pointed-to value in bytes.
348 /// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
349 /// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],
350 /// then `size_of_val_raw` can be used to get the dynamically-known size.
354 /// This function is only safe to call if the following conditions hold:
356 /// - If `T` is `Sized`, this function is always safe to call.
357 /// - If the unsized tail of `T` is:
358 /// - a [slice], then the length of the slice tail must be an initialized
359 /// integer, and the size of the *entire value*
360 /// (dynamic tail length + statically sized prefix) must fit in `isize`.
361 /// - a [trait object], then the vtable part of the pointer must point
362 /// to a valid vtable acquired by an unsizing coercion, and the size
363 /// of the *entire value* (dynamic tail length + statically sized prefix)
364 /// must fit in `isize`.
365 /// - an (unstable) [extern type], then this function is always safe to
366 /// call, but may panic or otherwise return the wrong value, as the
367 /// extern type's layout is not known. This is the same behavior as
368 /// [`size_of_val`] on a reference to a type with an extern type tail.
369 /// - otherwise, it is conservatively not allowed to call this function.
371 /// [trait object]: ../../book/ch17-02-trait-objects.html
372 /// [extern type]: ../../unstable-book/language-features/extern-types.html
377 /// #![feature(layout_for_ptr)]
380 /// assert_eq!(4, mem::size_of_val(&5i32));
382 /// let x: [u8; 13] = [0; 13];
383 /// let y: &[u8] = &x;
384 /// assert_eq!(13, unsafe { mem::size_of_val_raw(y) });
388 #[unstable(feature = "layout_for_ptr", issue = "69835")]
389 #[rustc_const_unstable(feature = "const_size_of_val_raw", issue = "46571")]
390 pub const unsafe fn size_of_val_raw<T: ?Sized>(val: *const T) -> usize {
391 // SAFETY: the caller must provide a valid raw pointer
392 unsafe { intrinsics::size_of_val(val) }
395 /// Returns the [ABI]-required minimum alignment of a type in bytes.
397 /// Every reference to a value of the type `T` must be a multiple of this number.
399 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
401 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
406 /// # #![allow(deprecated)]
409 /// assert_eq!(4, mem::min_align_of::<i32>());
413 #[stable(feature = "rust1", since = "1.0.0")]
414 #[deprecated(note = "use `align_of` instead", since = "1.2.0")]
415 pub fn min_align_of<T>() -> usize {
416 intrinsics::min_align_of::<T>()
419 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to in
422 /// Every reference to a value of the type `T` must be a multiple of this number.
424 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
429 /// # #![allow(deprecated)]
432 /// assert_eq!(4, mem::min_align_of_val(&5i32));
436 #[stable(feature = "rust1", since = "1.0.0")]
437 #[deprecated(note = "use `align_of_val` instead", since = "1.2.0")]
438 pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
439 // SAFETY: val is a reference, so it's a valid raw pointer
440 unsafe { intrinsics::min_align_of_val(val) }
443 /// Returns the [ABI]-required minimum alignment of a type in bytes.
445 /// Every reference to a value of the type `T` must be a multiple of this number.
447 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
449 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
456 /// assert_eq!(4, mem::align_of::<i32>());
460 #[stable(feature = "rust1", since = "1.0.0")]
462 #[rustc_const_stable(feature = "const_align_of", since = "1.24.0")]
463 pub const fn align_of<T>() -> usize {
464 intrinsics::min_align_of::<T>()
467 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to in
470 /// Every reference to a value of the type `T` must be a multiple of this number.
472 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
479 /// assert_eq!(4, mem::align_of_val(&5i32));
483 #[stable(feature = "rust1", since = "1.0.0")]
484 #[rustc_const_unstable(feature = "const_align_of_val", issue = "46571")]
486 pub const fn align_of_val<T: ?Sized>(val: &T) -> usize {
487 // SAFETY: val is a reference, so it's a valid raw pointer
488 unsafe { intrinsics::min_align_of_val(val) }
491 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to in
494 /// Every reference to a value of the type `T` must be a multiple of this number.
496 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
500 /// This function is only safe to call if the following conditions hold:
502 /// - If `T` is `Sized`, this function is always safe to call.
503 /// - If the unsized tail of `T` is:
504 /// - a [slice], then the length of the slice tail must be an initialized
505 /// integer, and the size of the *entire value*
506 /// (dynamic tail length + statically sized prefix) must fit in `isize`.
507 /// - a [trait object], then the vtable part of the pointer must point
508 /// to a valid vtable acquired by an unsizing coercion, and the size
509 /// of the *entire value* (dynamic tail length + statically sized prefix)
510 /// must fit in `isize`.
511 /// - an (unstable) [extern type], then this function is always safe to
512 /// call, but may panic or otherwise return the wrong value, as the
513 /// extern type's layout is not known. This is the same behavior as
514 /// [`align_of_val`] on a reference to a type with an extern type tail.
515 /// - otherwise, it is conservatively not allowed to call this function.
517 /// [trait object]: ../../book/ch17-02-trait-objects.html
518 /// [extern type]: ../../unstable-book/language-features/extern-types.html
523 /// #![feature(layout_for_ptr)]
526 /// assert_eq!(4, unsafe { mem::align_of_val_raw(&5i32) });
530 #[unstable(feature = "layout_for_ptr", issue = "69835")]
531 #[rustc_const_unstable(feature = "const_align_of_val_raw", issue = "46571")]
532 pub const unsafe fn align_of_val_raw<T: ?Sized>(val: *const T) -> usize {
533 // SAFETY: the caller must provide a valid raw pointer
534 unsafe { intrinsics::min_align_of_val(val) }
537 /// Returns `true` if dropping values of type `T` matters.
539 /// This is purely an optimization hint, and may be implemented conservatively:
540 /// it may return `true` for types that don't actually need to be dropped.
541 /// As such always returning `true` would be a valid implementation of
542 /// this function. However if this function actually returns `false`, then you
543 /// can be certain dropping `T` has no side effect.
545 /// Low level implementations of things like collections, which need to manually
546 /// drop their data, should use this function to avoid unnecessarily
547 /// trying to drop all their contents when they are destroyed. This might not
548 /// make a difference in release builds (where a loop that has no side-effects
549 /// is easily detected and eliminated), but is often a big win for debug builds.
551 /// Note that [`drop_in_place`] already performs this check, so if your workload
552 /// can be reduced to some small number of [`drop_in_place`] calls, using this is
553 /// unnecessary. In particular note that you can [`drop_in_place`] a slice, and that
554 /// will do a single needs_drop check for all the values.
556 /// Types like Vec therefore just `drop_in_place(&mut self[..])` without using
557 /// `needs_drop` explicitly. Types like [`HashMap`], on the other hand, have to drop
558 /// values one at a time and should use this API.
560 /// [`drop_in_place`]: crate::ptr::drop_in_place
561 /// [`HashMap`]: ../../std/collections/struct.HashMap.html
565 /// Here's an example of how a collection might make use of `needs_drop`:
568 /// use std::{mem, ptr};
570 /// pub struct MyCollection<T> {
574 /// # impl<T> MyCollection<T> {
575 /// # fn iter_mut(&mut self) -> &mut [T] { &mut self.data }
576 /// # fn free_buffer(&mut self) {}
579 /// impl<T> Drop for MyCollection<T> {
580 /// fn drop(&mut self) {
583 /// if mem::needs_drop::<T>() {
584 /// for x in self.iter_mut() {
585 /// ptr::drop_in_place(x);
588 /// self.free_buffer();
595 #[stable(feature = "needs_drop", since = "1.21.0")]
596 #[rustc_const_stable(feature = "const_mem_needs_drop", since = "1.36.0")]
597 #[rustc_diagnostic_item = "needs_drop"]
598 pub const fn needs_drop<T: ?Sized>() -> bool {
599 intrinsics::needs_drop::<T>()
602 /// Returns the value of type `T` represented by the all-zero byte-pattern.
604 /// This means that, for example, the padding byte in `(u8, u16)` is not
605 /// necessarily zeroed.
607 /// There is no guarantee that an all-zero byte-pattern represents a valid value
608 /// of some type `T`. For example, the all-zero byte-pattern is not a valid value
609 /// for reference types (`&T`, `&mut T`) and functions pointers. Using `zeroed`
610 /// on such types causes immediate [undefined behavior][ub] because [the Rust
611 /// compiler assumes][inv] that there always is a valid value in a variable it
612 /// considers initialized.
614 /// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed].
615 /// It is useful for FFI sometimes, but should generally be avoided.
617 /// [zeroed]: MaybeUninit::zeroed
618 /// [ub]: ../../reference/behavior-considered-undefined.html
619 /// [inv]: MaybeUninit#initialization-invariant
623 /// Correct usage of this function: initializing an integer with zero.
628 /// let x: i32 = unsafe { mem::zeroed() };
629 /// assert_eq!(0, x);
632 /// *Incorrect* usage of this function: initializing a reference with zero.
635 /// # #![allow(invalid_value)]
638 /// let _x: &i32 = unsafe { mem::zeroed() }; // Undefined behavior!
639 /// let _y: fn() = unsafe { mem::zeroed() }; // And again!
643 #[stable(feature = "rust1", since = "1.0.0")]
644 #[allow(deprecated_in_future)]
646 #[rustc_diagnostic_item = "mem_zeroed"]
648 pub unsafe fn zeroed<T>() -> T {
649 // SAFETY: the caller must guarantee that an all-zero value is valid for `T`.
651 intrinsics::assert_zero_valid::<T>();
652 MaybeUninit::zeroed().assume_init()
656 /// Bypasses Rust's normal memory-initialization checks by pretending to
657 /// produce a value of type `T`, while doing nothing at all.
659 /// **This function is deprecated.** Use [`MaybeUninit<T>`] instead.
660 /// It also might be slower than using `MaybeUninit<T>` due to mitigations that were put in place to
661 /// limit the potential harm caused by incorrect use of this function in legacy code.
663 /// The reason for deprecation is that the function basically cannot be used
664 /// correctly: it has the same effect as [`MaybeUninit::uninit().assume_init()`][uninit].
665 /// As the [`assume_init` documentation][assume_init] explains,
666 /// [the Rust compiler assumes][inv] that values are properly initialized.
668 /// Truly uninitialized memory like what gets returned here
669 /// is special in that the compiler knows that it does not have a fixed value.
670 /// This makes it undefined behavior to have uninitialized data in a variable even
671 /// if that variable has an integer type.
673 /// Therefore, it is immediate undefined behavior to call this function on nearly all types,
674 /// including integer types and arrays of integer types, and even if the result is unused.
676 /// [uninit]: MaybeUninit::uninit
677 /// [assume_init]: MaybeUninit::assume_init
678 /// [inv]: MaybeUninit#initialization-invariant
681 #[deprecated(since = "1.39.0", note = "use `mem::MaybeUninit` instead")]
682 #[stable(feature = "rust1", since = "1.0.0")]
683 #[allow(deprecated_in_future)]
685 #[rustc_diagnostic_item = "mem_uninitialized"]
687 pub unsafe fn uninitialized<T>() -> T {
688 // SAFETY: the caller must guarantee that an uninitialized value is valid for `T`.
690 intrinsics::assert_uninit_valid::<T>();
691 let mut val = MaybeUninit::<T>::uninit();
693 // Fill memory with 0x01, as an imperfect mitigation for old code that uses this function on
694 // bool, nonnull, and noundef types. But don't do this if we actively want to detect UB.
695 if !cfg!(any(miri, sanitize = "memory")) {
696 val.as_mut_ptr().write_bytes(0x01, 1);
703 /// Swaps the values at two mutable locations, without deinitializing either one.
705 /// * If you want to swap with a default or dummy value, see [`take`].
706 /// * If you want to swap with a passed value, returning the old value, see [`replace`].
716 /// mem::swap(&mut x, &mut y);
718 /// assert_eq!(42, x);
719 /// assert_eq!(5, y);
722 #[stable(feature = "rust1", since = "1.0.0")]
723 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
724 pub const fn swap<T>(x: &mut T, y: &mut T) {
725 // NOTE(eddyb) SPIR-V's Logical addressing model doesn't allow for arbitrary
726 // reinterpretation of values as (chunkable) byte arrays, and the loop in the
727 // block optimization in `swap_slice` is hard to rewrite back
728 // into the (unoptimized) direct swapping implementation, so we disable it.
729 // FIXME(eddyb) the block optimization also prevents MIR optimizations from
730 // understanding `mem::replace`, `Option::take`, etc. - a better overall
731 // solution might be to make `ptr::swap_nonoverlapping` into an intrinsic, which
732 // a backend can choose to implement using the block optimization, or not.
733 // NOTE(scottmcm) MIRI is disabled here as reading in smaller units is a
734 // pessimization for it. Also, if the type contains any unaligned pointers,
735 // copying those over multiple reads is difficult to support.
736 #[cfg(not(any(target_arch = "spirv", miri)))]
738 // For types that are larger multiples of their alignment, the simple way
739 // tends to copy the whole thing to stack rather than doing it one part
740 // at a time, so instead treat them as one-element slices and piggy-back
741 // the slice optimizations that will split up the swaps.
742 if size_of::<T>() / align_of::<T>() > 4 {
743 // SAFETY: exclusive references always point to one non-overlapping
744 // element and are non-null and properly aligned.
745 return unsafe { ptr::swap_nonoverlapping(x, y, 1) };
749 // If a scalar consists of just a small number of alignment units, let
750 // the codegen just swap those pieces directly, as it's likely just a
751 // few instructions and anything else is probably overcomplicated.
753 // Most importantly, this covers primitives and simd types that tend to
754 // have size=align where doing anything else can be a pessimization.
755 // (This will also be used for ZSTs, though any solution works for them.)
759 /// Same as [`swap`] semantically, but always uses the simple implementation.
761 /// Used elsewhere in `mem` and `ptr` at the bottom layer of calls.
762 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
764 pub(crate) const fn swap_simple<T>(x: &mut T, y: &mut T) {
765 // We arrange for this to typically be called with small types,
766 // so this reads-and-writes approach is actually better than using
767 // copy_nonoverlapping as it easily puts things in LLVM registers
768 // directly and doesn't end up inlining allocas.
769 // And LLVM actually optimizes it to 3×memcpy if called with
770 // a type larger than it's willing to keep in a register.
771 // Having typed reads and writes in MIR here is also good as
772 // it lets MIRI and CTFE understand them better, including things
773 // like enforcing type validity for them.
774 // Importantly, read+copy_nonoverlapping+write introduces confusing
775 // asymmetry to the behaviour where one value went through read+write
776 // whereas the other was copied over by the intrinsic (see #94371).
778 // SAFETY: exclusive references are always valid to read/write,
779 // including being aligned, and nothing here panics so it's drop-safe.
781 let a = ptr::read(x);
782 let b = ptr::read(y);
788 /// Replaces `dest` with the default value of `T`, returning the previous `dest` value.
790 /// * If you want to replace the values of two variables, see [`swap`].
791 /// * If you want to replace with a passed value instead of the default value, see [`replace`].
795 /// A simple example:
800 /// let mut v: Vec<i32> = vec![1, 2];
802 /// let old_v = mem::take(&mut v);
803 /// assert_eq!(vec![1, 2], old_v);
804 /// assert!(v.is_empty());
807 /// `take` allows taking ownership of a struct field by replacing it with an "empty" value.
808 /// Without `take` you can run into issues like these:
810 /// ```compile_fail,E0507
811 /// struct Buffer<T> { buf: Vec<T> }
813 /// impl<T> Buffer<T> {
814 /// fn get_and_reset(&mut self) -> Vec<T> {
815 /// // error: cannot move out of dereference of `&mut`-pointer
816 /// let buf = self.buf;
817 /// self.buf = Vec::new();
823 /// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset
824 /// `self.buf`. But `take` can be used to disassociate the original value of `self.buf` from
825 /// `self`, allowing it to be returned:
830 /// # struct Buffer<T> { buf: Vec<T> }
831 /// impl<T> Buffer<T> {
832 /// fn get_and_reset(&mut self) -> Vec<T> {
833 /// mem::take(&mut self.buf)
837 /// let mut buffer = Buffer { buf: vec![0, 1] };
838 /// assert_eq!(buffer.buf.len(), 2);
840 /// assert_eq!(buffer.get_and_reset(), vec![0, 1]);
841 /// assert_eq!(buffer.buf.len(), 0);
844 #[stable(feature = "mem_take", since = "1.40.0")]
845 pub fn take<T: Default>(dest: &mut T) -> T {
846 replace(dest, T::default())
849 /// Moves `src` into the referenced `dest`, returning the previous `dest` value.
851 /// Neither value is dropped.
853 /// * If you want to replace the values of two variables, see [`swap`].
854 /// * If you want to replace with a default value, see [`take`].
858 /// A simple example:
863 /// let mut v: Vec<i32> = vec![1, 2];
865 /// let old_v = mem::replace(&mut v, vec![3, 4, 5]);
866 /// assert_eq!(vec![1, 2], old_v);
867 /// assert_eq!(vec![3, 4, 5], v);
870 /// `replace` allows consumption of a struct field by replacing it with another value.
871 /// Without `replace` you can run into issues like these:
873 /// ```compile_fail,E0507
874 /// struct Buffer<T> { buf: Vec<T> }
876 /// impl<T> Buffer<T> {
877 /// fn replace_index(&mut self, i: usize, v: T) -> T {
878 /// // error: cannot move out of dereference of `&mut`-pointer
879 /// let t = self.buf[i];
886 /// Note that `T` does not necessarily implement [`Clone`], so we can't even clone `self.buf[i]` to
887 /// avoid the move. But `replace` can be used to disassociate the original value at that index from
888 /// `self`, allowing it to be returned:
891 /// # #![allow(dead_code)]
894 /// # struct Buffer<T> { buf: Vec<T> }
895 /// impl<T> Buffer<T> {
896 /// fn replace_index(&mut self, i: usize, v: T) -> T {
897 /// mem::replace(&mut self.buf[i], v)
901 /// let mut buffer = Buffer { buf: vec![0, 1] };
902 /// assert_eq!(buffer.buf[0], 0);
904 /// assert_eq!(buffer.replace_index(0, 2), 0);
905 /// assert_eq!(buffer.buf[0], 2);
908 #[stable(feature = "rust1", since = "1.0.0")]
909 #[must_use = "if you don't need the old value, you can just assign the new value directly"]
910 #[rustc_const_unstable(feature = "const_replace", issue = "83164")]
911 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_replace")]
912 pub const fn replace<T>(dest: &mut T, src: T) -> T {
913 // SAFETY: We read from `dest` but directly write `src` into it afterwards,
914 // such that the old value is not duplicated. Nothing is dropped and
915 // nothing here can panic.
917 let result = ptr::read(dest);
918 ptr::write(dest, src);
923 /// Disposes of a value.
925 /// This does so by calling the argument's implementation of [`Drop`][drop].
927 /// This effectively does nothing for types which implement `Copy`, e.g.
928 /// integers. Such values are copied and _then_ moved into the function, so the
929 /// value persists after this function call.
931 /// This function is not magic; it is literally defined as
934 /// pub fn drop<T>(_x: T) { }
937 /// Because `_x` is moved into the function, it is automatically dropped before
938 /// the function returns.
947 /// let v = vec![1, 2, 3];
949 /// drop(v); // explicitly drop the vector
952 /// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can
953 /// release a [`RefCell`] borrow:
956 /// use std::cell::RefCell;
958 /// let x = RefCell::new(1);
960 /// let mut mutable_borrow = x.borrow_mut();
961 /// *mutable_borrow = 1;
963 /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
965 /// let borrow = x.borrow();
966 /// println!("{}", *borrow);
969 /// Integers and other types implementing [`Copy`] are unaffected by `drop`.
972 /// #[derive(Copy, Clone)]
977 /// drop(x); // a copy of `x` is moved and dropped
978 /// drop(y); // a copy of `y` is moved and dropped
980 /// println!("x: {}, y: {}", x, y.0); // still available
983 /// [`RefCell`]: crate::cell::RefCell
985 #[stable(feature = "rust1", since = "1.0.0")]
986 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_drop")]
987 pub fn drop<T>(_x: T) {}
989 /// Bitwise-copies a value.
991 /// This function is not magic; it is literally defined as
993 /// pub fn copy<T: Copy>(x: &T) -> T { *x }
996 /// It is useful when you want to pass a function pointer to a combinator, rather than defining a new closure.
1000 /// #![feature(mem_copy_fn)]
1001 /// use core::mem::copy;
1002 /// let result_from_ffi_function: Result<(), &i32> = Err(&1);
1003 /// let result_copied: Result<(), i32> = result_from_ffi_function.map_err(copy);
1006 #[unstable(feature = "mem_copy_fn", issue = "98262")]
1007 pub fn copy<T: Copy>(x: &T) -> T {
1011 /// Interprets `src` as having type `&Dst`, and then reads `src` without moving
1012 /// the contained value.
1014 /// This function will unsafely assume the pointer `src` is valid for [`size_of::<Dst>`][size_of]
1015 /// bytes by transmuting `&Src` to `&Dst` and then reading the `&Dst` (except that this is done
1016 /// in a way that is correct even when `&Dst` has stricter alignment requirements than `&Src`).
1017 /// It will also unsafely create a copy of the contained value instead of moving out of `src`.
1019 /// It is not a compile-time error if `Src` and `Dst` have different sizes, but it
1020 /// is highly encouraged to only invoke this function where `Src` and `Dst` have the
1021 /// same size. This function triggers [undefined behavior][ub] if `Dst` is larger than
1024 /// [ub]: ../../reference/behavior-considered-undefined.html
1036 /// let foo_array = [10u8];
1039 /// // Copy the data from 'foo_array' and treat it as a 'Foo'
1040 /// let mut foo_struct: Foo = mem::transmute_copy(&foo_array);
1041 /// assert_eq!(foo_struct.bar, 10);
1043 /// // Modify the copied data
1044 /// foo_struct.bar = 20;
1045 /// assert_eq!(foo_struct.bar, 20);
1048 /// // The contents of 'foo_array' should not have changed
1049 /// assert_eq!(foo_array, [10]);
1053 #[stable(feature = "rust1", since = "1.0.0")]
1054 #[rustc_const_unstable(feature = "const_transmute_copy", issue = "83165")]
1055 pub const unsafe fn transmute_copy<Src, Dst>(src: &Src) -> Dst {
1057 size_of::<Src>() >= size_of::<Dst>(),
1058 "cannot transmute_copy if Dst is larger than Src"
1061 // If Dst has a higher alignment requirement, src might not be suitably aligned.
1062 if align_of::<Dst>() > align_of::<Src>() {
1063 // SAFETY: `src` is a reference which is guaranteed to be valid for reads.
1064 // The caller must guarantee that the actual transmutation is safe.
1065 unsafe { ptr::read_unaligned(src as *const Src as *const Dst) }
1067 // SAFETY: `src` is a reference which is guaranteed to be valid for reads.
1068 // We just checked that `src as *const Dst` was properly aligned.
1069 // The caller must guarantee that the actual transmutation is safe.
1070 unsafe { ptr::read(src as *const Src as *const Dst) }
1074 /// Opaque type representing the discriminant of an enum.
1076 /// See the [`discriminant`] function in this module for more information.
1077 #[stable(feature = "discriminant_value", since = "1.21.0")]
1078 pub struct Discriminant<T>(<T as DiscriminantKind>::Discriminant);
1080 // N.B. These trait implementations cannot be derived because we don't want any bounds on T.
1082 #[stable(feature = "discriminant_value", since = "1.21.0")]
1083 impl<T> Copy for Discriminant<T> {}
1085 #[stable(feature = "discriminant_value", since = "1.21.0")]
1086 impl<T> clone::Clone for Discriminant<T> {
1087 fn clone(&self) -> Self {
1092 #[stable(feature = "discriminant_value", since = "1.21.0")]
1093 impl<T> cmp::PartialEq for Discriminant<T> {
1094 fn eq(&self, rhs: &Self) -> bool {
1099 #[stable(feature = "discriminant_value", since = "1.21.0")]
1100 impl<T> cmp::Eq for Discriminant<T> {}
1102 #[stable(feature = "discriminant_value", since = "1.21.0")]
1103 impl<T> hash::Hash for Discriminant<T> {
1104 fn hash<H: hash::Hasher>(&self, state: &mut H) {
1109 #[stable(feature = "discriminant_value", since = "1.21.0")]
1110 impl<T> fmt::Debug for Discriminant<T> {
1111 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
1112 fmt.debug_tuple("Discriminant").field(&self.0).finish()
1116 /// Returns a value uniquely identifying the enum variant in `v`.
1118 /// If `T` is not an enum, calling this function will not result in undefined behavior, but the
1119 /// return value is unspecified.
1123 /// The discriminant of an enum variant may change if the enum definition changes. A discriminant
1124 /// of some variant will not change between compilations with the same compiler.
1128 /// This can be used to compare enums that carry data, while disregarding
1129 /// the actual data:
1134 /// enum Foo { A(&'static str), B(i32), C(i32) }
1136 /// assert_eq!(mem::discriminant(&Foo::A("bar")), mem::discriminant(&Foo::A("baz")));
1137 /// assert_eq!(mem::discriminant(&Foo::B(1)), mem::discriminant(&Foo::B(2)));
1138 /// assert_ne!(mem::discriminant(&Foo::B(3)), mem::discriminant(&Foo::C(3)));
1140 #[stable(feature = "discriminant_value", since = "1.21.0")]
1141 #[rustc_const_unstable(feature = "const_discriminant", issue = "69821")]
1142 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_discriminant")]
1143 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
1144 pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
1145 Discriminant(intrinsics::discriminant_value(v))
1148 /// Returns the number of variants in the enum type `T`.
1150 /// If `T` is not an enum, calling this function will not result in undefined behavior, but the
1151 /// return value is unspecified. Equally, if `T` is an enum with more variants than `usize::MAX`
1152 /// the return value is unspecified. Uninhabited variants will be counted.
1154 /// Note that an enum may be expanded with additional variants in the future
1155 /// as a non-breaking change, for example if it is marked `#[non_exhaustive]`,
1156 /// which will change the result of this function.
1161 /// # #![feature(never_type)]
1162 /// # #![feature(variant_count)]
1167 /// enum Foo { A(&'static str), B(i32), C(i32) }
1169 /// assert_eq!(mem::variant_count::<Void>(), 0);
1170 /// assert_eq!(mem::variant_count::<Foo>(), 3);
1172 /// assert_eq!(mem::variant_count::<Option<!>>(), 2);
1173 /// assert_eq!(mem::variant_count::<Result<!, !>>(), 2);
1177 #[unstable(feature = "variant_count", issue = "73662")]
1178 #[rustc_const_unstable(feature = "variant_count", issue = "73662")]
1179 #[rustc_diagnostic_item = "mem_variant_count"]
1180 pub const fn variant_count<T>() -> usize {
1181 intrinsics::variant_count::<T>()
1184 /// Provides associated constants for various useful properties of types,
1185 /// to give them a canonical form in our code and make them easier to read.
1187 /// This is here only to simplify all the ZST checks we need in the library.
1188 /// It's not on a stabilization track right now.
1190 #[unstable(feature = "sized_type_properties", issue = "none")]
1191 pub trait SizedTypeProperties: Sized {
1192 /// `true` if this type requires no storage.
1193 /// `false` if its [size](size_of) is greater than zero.
1198 /// #![feature(sized_type_properties)]
1199 /// use core::mem::SizedTypeProperties;
1201 /// fn do_something_with<T>() {
1203 /// // ... special approach ...
1205 /// // ... the normal thing ...
1210 /// assert!(MyUnit::IS_ZST);
1212 /// // For negative checks, consider using UFCS to emphasize the negation
1213 /// assert!(!<i32>::IS_ZST);
1214 /// // As it can sometimes hide in the type otherwise
1215 /// assert!(!String::IS_ZST);
1218 #[unstable(feature = "sized_type_properties", issue = "none")]
1219 const IS_ZST: bool = size_of::<Self>() == 0;
1222 #[unstable(feature = "sized_type_properties", issue = "none")]
1223 impl<T> SizedTypeProperties for T {}