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//! Basic functions for dealing with memory. //! //! This module contains functions for querying the size and alignment of //! types, initializing and manipulating memory. #![stable(feature = "rust1", since = "1.0.0")] use crate::clone; use crate::cmp; use crate::fmt; use crate::hash; use crate::intrinsics; use crate::marker::{Copy, PhantomData, Sized}; use crate::ptr; mod manually_drop; #[stable(feature = "manually_drop", since = "1.20.0")] pub use manually_drop::ManuallyDrop; mod maybe_uninit; #[stable(feature = "maybe_uninit", since = "1.36.0")] pub use maybe_uninit::MaybeUninit; #[stable(feature = "rust1", since = "1.0.0")] #[doc(inline)] pub use crate::intrinsics::transmute; /// Takes ownership and "forgets" about the value **without running its destructor**. /// /// Any resources the value manages, such as heap memory or a file handle, will linger /// forever in an unreachable state. However, it does not guarantee that pointers /// to this memory will remain valid. /// /// * If you want to leak memory, see [`Box::leak`][leak]. /// * If you want to obtain a raw pointer to the memory, see [`Box::into_raw`][into_raw]. /// * If you want to dispose of a value properly, running its destructor, see /// [`mem::drop`][drop]. /// /// # Safety /// /// `forget` is not marked as `unsafe`, because Rust's safety guarantees /// do not include a guarantee that destructors will always run. For example, /// a program can create a reference cycle using [`Rc`][rc], or call /// [`process::exit`][exit] to exit without running destructors. Thus, allowing /// `mem::forget` from safe code does not fundamentally change Rust's safety /// guarantees. /// /// That said, leaking resources such as memory or I/O objects is usually undesirable, /// so `forget` is only recommended for specialized use cases like those shown below. /// /// Because forgetting a value is allowed, any `unsafe` code you write must /// allow for this possibility. You cannot return a value and expect that the /// caller will necessarily run the value's destructor. /// /// [rc]: ../../std/rc/struct.Rc.html /// [exit]: ../../std/process/fn.exit.html /// /// # Examples /// /// Leak an I/O object, never closing the file: /// /// ```no_run /// use std::mem; /// use std::fs::File; /// /// let file = File::open("foo.txt").unwrap(); /// mem::forget(file); /// ``` /// /// The practical use cases for `forget` are rather specialized and mainly come /// up in unsafe or FFI code. /// /// [drop]: fn.drop.html /// [uninit]: fn.uninitialized.html /// [clone]: ../clone/trait.Clone.html /// [swap]: fn.swap.html /// [box]: ../../std/boxed/struct.Box.html /// [leak]: ../../std/boxed/struct.Box.html#method.leak /// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw /// [ub]: ../../reference/behavior-considered-undefined.html #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn forget<T>(t: T) { ManuallyDrop::new(t); } /// Like [`forget`], but also accepts unsized values. /// /// This function is just a shim intended to be removed when the `unsized_locals` feature gets /// stabilized. /// /// [`forget`]: fn.forget.html #[inline] #[unstable(feature = "forget_unsized", issue = "0")] pub fn forget_unsized<T: ?Sized>(t: T) { unsafe { intrinsics::forget(t) } } /// Returns the size of a type in bytes. /// /// More specifically, this is the offset in bytes between successive elements /// in an array with that item type including alignment padding. Thus, for any /// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`. /// /// In general, the size of a type is not stable across compilations, but /// specific types such as primitives are. /// /// The following table gives the size for primitives. /// /// Type | size_of::\<Type>() /// ---- | --------------- /// () | 0 /// bool | 1 /// u8 | 1 /// u16 | 2 /// u32 | 4 /// u64 | 8 /// u128 | 16 /// i8 | 1 /// i16 | 2 /// i32 | 4 /// i64 | 8 /// i128 | 16 /// f32 | 4 /// f64 | 8 /// char | 4 /// /// Furthermore, `usize` and `isize` have the same size. /// /// The types `*const T`, `&T`, `Box<T>`, `Option<&T>`, and `Option<Box<T>>` all have /// the same size. If `T` is Sized, all of those types have the same size as `usize`. /// /// The mutability of a pointer does not change its size. As such, `&T` and `&mut T` /// have the same size. Likewise for `*const T` and `*mut T`. /// /// # Size of `#[repr(C)]` items /// /// The `C` representation for items has a defined layout. With this layout, /// the size of items is also stable as long as all fields have a stable size. /// /// ## Size of Structs /// /// For `structs`, the size is determined by the following algorithm. /// /// For each field in the struct ordered by declaration order: /// /// 1. Add the size of the field. /// 2. Round up the current size to the nearest multiple of the next field's [alignment]. /// /// Finally, round the size of the struct to the nearest multiple of its [alignment]. /// The alignment of the struct is usually the largest alignment of all its /// fields; this can be changed with the use of `repr(align(N))`. /// /// Unlike `C`, zero sized structs are not rounded up to one byte in size. /// /// ## Size of Enums /// /// Enums that carry no data other than the discriminant have the same size as C enums /// on the platform they are compiled for. /// /// ## Size of Unions /// /// The size of a union is the size of its largest field. /// /// Unlike `C`, zero sized unions are not rounded up to one byte in size. /// /// # Examples /// /// ``` /// use std::mem; /// /// // Some primitives /// assert_eq!(4, mem::size_of::<i32>()); /// assert_eq!(8, mem::size_of::<f64>()); /// assert_eq!(0, mem::size_of::<()>()); /// /// // Some arrays /// assert_eq!(8, mem::size_of::<[i32; 2]>()); /// assert_eq!(12, mem::size_of::<[i32; 3]>()); /// assert_eq!(0, mem::size_of::<[i32; 0]>()); /// /// /// // Pointer size equality /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>()); /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>()); /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>()); /// assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>()); /// ``` /// /// Using `#[repr(C)]`. /// /// ``` /// use std::mem; /// /// #[repr(C)] /// struct FieldStruct { /// first: u8, /// second: u16, /// third: u8 /// } /// /// // The size of the first field is 1, so add 1 to the size. Size is 1. /// // The alignment of the second field is 2, so add 1 to the size for padding. Size is 2. /// // The size of the second field is 2, so add 2 to the size. Size is 4. /// // The alignment of the third field is 1, so add 0 to the size for padding. Size is 4. /// // The size of the third field is 1, so add 1 to the size. Size is 5. /// // Finally, the alignment of the struct is 2 (because the largest alignment amongst its /// // fields is 2), so add 1 to the size for padding. Size is 6. /// assert_eq!(6, mem::size_of::<FieldStruct>()); /// /// #[repr(C)] /// struct TupleStruct(u8, u16, u8); /// /// // Tuple structs follow the same rules. /// assert_eq!(6, mem::size_of::<TupleStruct>()); /// /// // Note that reordering the fields can lower the size. We can remove both padding bytes /// // by putting `third` before `second`. /// #[repr(C)] /// struct FieldStructOptimized { /// first: u8, /// third: u8, /// second: u16 /// } /// /// assert_eq!(4, mem::size_of::<FieldStructOptimized>()); /// /// // Union size is the size of the largest field. /// #[repr(C)] /// union ExampleUnion { /// smaller: u8, /// larger: u16 /// } /// /// assert_eq!(2, mem::size_of::<ExampleUnion>()); /// ``` /// /// [alignment]: ./fn.align_of.html #[inline] #[stable(feature = "rust1", since = "1.0.0")] #[rustc_promotable] pub const fn size_of<T>() -> usize { intrinsics::size_of::<T>() } /// Returns the size of the pointed-to value in bytes. /// /// This is usually the same as `size_of::<T>()`. However, when `T` *has* no /// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object], /// then `size_of_val` can be used to get the dynamically-known size. /// /// [slice]: ../../std/primitive.slice.html /// [trait object]: ../../book/ch17-02-trait-objects.html /// /// # Examples /// /// ``` /// use std::mem; /// /// assert_eq!(4, mem::size_of_val(&5i32)); /// /// let x: [u8; 13] = [0; 13]; /// let y: &[u8] = &x; /// assert_eq!(13, mem::size_of_val(y)); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn size_of_val<T: ?Sized>(val: &T) -> usize { unsafe { intrinsics::size_of_val(val) } } /// Returns the [ABI]-required minimum alignment of a type. /// /// Every reference to a value of the type `T` must be a multiple of this number. /// /// This is the alignment used for struct fields. It may be smaller than the preferred alignment. /// /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface /// /// # Examples /// /// ``` /// # #![allow(deprecated)] /// use std::mem; /// /// assert_eq!(4, mem::min_align_of::<i32>()); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")] pub fn min_align_of<T>() -> usize { intrinsics::min_align_of::<T>() } /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to. /// /// Every reference to a value of the type `T` must be a multiple of this number. /// /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface /// /// # Examples /// /// ``` /// # #![allow(deprecated)] /// use std::mem; /// /// assert_eq!(4, mem::min_align_of_val(&5i32)); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")] pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize { unsafe { intrinsics::min_align_of_val(val) } } /// Returns the [ABI]-required minimum alignment of a type. /// /// Every reference to a value of the type `T` must be a multiple of this number. /// /// This is the alignment used for struct fields. It may be smaller than the preferred alignment. /// /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface /// /// # Examples /// /// ``` /// use std::mem; /// /// assert_eq!(4, mem::align_of::<i32>()); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] #[rustc_promotable] pub const fn align_of<T>() -> usize { intrinsics::min_align_of::<T>() } /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to. /// /// Every reference to a value of the type `T` must be a multiple of this number. /// /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface /// /// # Examples /// /// ``` /// use std::mem; /// /// assert_eq!(4, mem::align_of_val(&5i32)); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn align_of_val<T: ?Sized>(val: &T) -> usize { unsafe { intrinsics::min_align_of_val(val) } } /// Returns `true` if dropping values of type `T` matters. /// /// This is purely an optimization hint, and may be implemented conservatively: /// it may return `true` for types that don't actually need to be dropped. /// As such always returning `true` would be a valid implementation of /// this function. However if this function actually returns `false`, then you /// can be certain dropping `T` has no side effect. /// /// Low level implementations of things like collections, which need to manually /// drop their data, should use this function to avoid unnecessarily /// trying to drop all their contents when they are destroyed. This might not /// make a difference in release builds (where a loop that has no side-effects /// is easily detected and eliminated), but is often a big win for debug builds. /// /// Note that `ptr::drop_in_place` already performs this check, so if your workload /// can be reduced to some small number of drop_in_place calls, using this is /// unnecessary. In particular note that you can drop_in_place a slice, and that /// will do a single needs_drop check for all the values. /// /// Types like Vec therefore just `drop_in_place(&mut self[..])` without using /// needs_drop explicitly. Types like HashMap, on the other hand, have to drop /// values one at a time and should use this API. /// /// /// # Examples /// /// Here's an example of how a collection might make use of needs_drop: /// /// ``` /// use std::{mem, ptr}; /// /// pub struct MyCollection<T> { /// # data: [T; 1], /// /* ... */ /// } /// # impl<T> MyCollection<T> { /// # fn iter_mut(&mut self) -> &mut [T] { &mut self.data } /// # fn free_buffer(&mut self) {} /// # } /// /// impl<T> Drop for MyCollection<T> { /// fn drop(&mut self) { /// unsafe { /// // drop the data /// if mem::needs_drop::<T>() { /// for x in self.iter_mut() { /// ptr::drop_in_place(x); /// } /// } /// self.free_buffer(); /// } /// } /// } /// ``` #[inline] #[stable(feature = "needs_drop", since = "1.21.0")] pub const fn needs_drop<T>() -> bool { intrinsics::needs_drop::<T>() } /// Creates a value whose bytes are all zero. /// /// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed]. /// It is useful for FFI sometimes, but should generally be avoided. /// /// There is no guarantee that an all-zero byte-pattern represents a valid value of /// some type `T`. For example, the all-zero byte-pattern is not a valid value /// for reference types (`&T` and `&mut T`). Using `zeroed` on such types /// causes immediate [undefined behavior][ub] because [the Rust compiler assumes][inv] /// that there always is a valid value in a variable it considers initialized. /// /// [zeroed]: union.MaybeUninit.html#method.zeroed /// [ub]: ../../reference/behavior-considered-undefined.html /// [inv]: union.MaybeUninit.html#initialization-invariant /// /// # Examples /// /// Correct usage of this function: initializing an integer with zero. /// /// ``` /// use std::mem; /// /// let x: i32 = unsafe { mem::zeroed() }; /// assert_eq!(0, x); /// ``` /// /// *Incorrect* usage of this function: initializing a reference with zero. /// /// ```no_run /// use std::mem; /// /// let _x: &i32 = unsafe { mem::zeroed() }; // Undefined behavior! /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub unsafe fn zeroed<T>() -> T { intrinsics::panic_if_uninhabited::<T>(); intrinsics::init() } /// Bypasses Rust's normal memory-initialization checks by pretending to /// produce a value of type `T`, while doing nothing at all. /// /// **This functon is deprecated.** Use [`MaybeUninit<T>`] instead. /// /// The reason for deprecation is that the function basically cannot be used /// correctly: [the Rust compiler assumes][inv] that values are properly initialized. /// As a consequence, calling e.g. `mem::uninitialized::<bool>()` causes immediate /// undefined behavior for returning a `bool` that is not definitely either `true` /// or `false`. Worse, truly uninitialized memory like what gets returned here /// is special in that the compiler knows that it does not have a fixed value. /// This makes it undefined behavior to have uninitialized data in a variable even /// if that variable has an integer type. /// (Notice that the rules around uninitialized integers are not finalized yet, but /// until they are, it is advisable to avoid them.) /// /// [`MaybeUninit<T>`]: union.MaybeUninit.html /// [inv]: union.MaybeUninit.html#initialization-invariant #[inline] #[rustc_deprecated(since = "1.38.0", reason = "use `mem::MaybeUninit` instead")] #[stable(feature = "rust1", since = "1.0.0")] pub unsafe fn uninitialized<T>() -> T { intrinsics::panic_if_uninhabited::<T>(); intrinsics::uninit() } /// Swaps the values at two mutable locations, without deinitializing either one. /// /// # Examples /// /// ``` /// use std::mem; /// /// let mut x = 5; /// let mut y = 42; /// /// mem::swap(&mut x, &mut y); /// /// assert_eq!(42, x); /// assert_eq!(5, y); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn swap<T>(x: &mut T, y: &mut T) { unsafe { ptr::swap_nonoverlapping_one(x, y); } } /// Moves `src` into the referenced `dest`, returning the previous `dest` value. /// /// Neither value is dropped. /// /// # Examples /// /// A simple example: /// /// ``` /// use std::mem; /// /// let mut v: Vec<i32> = vec![1, 2]; /// /// let old_v = mem::replace(&mut v, vec![3, 4, 5]); /// assert_eq!(vec![1, 2], old_v); /// assert_eq!(vec![3, 4, 5], v); /// ``` /// /// `replace` allows consumption of a struct field by replacing it with another value. /// Without `replace` you can run into issues like these: /// /// ```compile_fail,E0507 /// struct Buffer<T> { buf: Vec<T> } /// /// impl<T> Buffer<T> { /// fn get_and_reset(&mut self) -> Vec<T> { /// // error: cannot move out of dereference of `&mut`-pointer /// let buf = self.buf; /// self.buf = Vec::new(); /// buf /// } /// } /// ``` /// /// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset /// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from /// `self`, allowing it to be returned: /// /// ``` /// # #![allow(dead_code)] /// use std::mem; /// /// # struct Buffer<T> { buf: Vec<T> } /// impl<T> Buffer<T> { /// fn get_and_reset(&mut self) -> Vec<T> { /// mem::replace(&mut self.buf, Vec::new()) /// } /// } /// ``` /// /// [`Clone`]: ../../std/clone/trait.Clone.html #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn replace<T>(dest: &mut T, mut src: T) -> T { swap(dest, &mut src); src } /// Disposes of a value. /// /// This does call the argument's implementation of [`Drop`][drop]. /// /// This effectively does nothing for types which implement `Copy`, e.g. /// integers. Such values are copied and _then_ moved into the function, so the /// value persists after this function call. /// /// This function is not magic; it is literally defined as /// /// ``` /// pub fn drop<T>(_x: T) { } /// ``` /// /// Because `_x` is moved into the function, it is automatically dropped before /// the function returns. /// /// [drop]: ../ops/trait.Drop.html /// /// # Examples /// /// Basic usage: /// /// ``` /// let v = vec![1, 2, 3]; /// /// drop(v); // explicitly drop the vector /// ``` /// /// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can /// release a [`RefCell`] borrow: /// /// ``` /// use std::cell::RefCell; /// /// let x = RefCell::new(1); /// /// let mut mutable_borrow = x.borrow_mut(); /// *mutable_borrow = 1; /// /// drop(mutable_borrow); // relinquish the mutable borrow on this slot /// /// let borrow = x.borrow(); /// println!("{}", *borrow); /// ``` /// /// Integers and other types implementing [`Copy`] are unaffected by `drop`. /// /// ``` /// #[derive(Copy, Clone)] /// struct Foo(u8); /// /// let x = 1; /// let y = Foo(2); /// drop(x); // a copy of `x` is moved and dropped /// drop(y); // a copy of `y` is moved and dropped /// /// println!("x: {}, y: {}", x, y.0); // still available /// ``` /// /// [`RefCell`]: ../../std/cell/struct.RefCell.html /// [`Copy`]: ../../std/marker/trait.Copy.html #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn drop<T>(_x: T) { } /// Interprets `src` as having type `&U`, and then reads `src` without moving /// the contained value. /// /// This function will unsafely assume the pointer `src` is valid for /// [`size_of::<U>`][size_of] bytes by transmuting `&T` to `&U` and then reading /// the `&U`. It will also unsafely create a copy of the contained value instead of /// moving out of `src`. /// /// It is not a compile-time error if `T` and `U` have different sizes, but it /// is highly encouraged to only invoke this function where `T` and `U` have the /// same size. This function triggers [undefined behavior][ub] if `U` is larger than /// `T`. /// /// [ub]: ../../reference/behavior-considered-undefined.html /// [size_of]: fn.size_of.html /// /// # Examples /// /// ``` /// use std::mem; /// /// #[repr(packed)] /// struct Foo { /// bar: u8, /// } /// /// let foo_slice = [10u8]; /// /// unsafe { /// // Copy the data from 'foo_slice' and treat it as a 'Foo' /// let mut foo_struct: Foo = mem::transmute_copy(&foo_slice); /// assert_eq!(foo_struct.bar, 10); /// /// // Modify the copied data /// foo_struct.bar = 20; /// assert_eq!(foo_struct.bar, 20); /// } /// /// // The contents of 'foo_slice' should not have changed /// assert_eq!(foo_slice, [10]); /// ``` #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub unsafe fn transmute_copy<T, U>(src: &T) -> U { ptr::read_unaligned(src as *const T as *const U) } /// Opaque type representing the discriminant of an enum. /// /// See the [`discriminant`] function in this module for more information. /// /// [`discriminant`]: fn.discriminant.html #[stable(feature = "discriminant_value", since = "1.21.0")] pub struct Discriminant<T>(u64, PhantomData<fn() -> T>); // N.B. These trait implementations cannot be derived because we don't want any bounds on T. #[stable(feature = "discriminant_value", since = "1.21.0")] impl<T> Copy for Discriminant<T> {} #[stable(feature = "discriminant_value", since = "1.21.0")] impl<T> clone::Clone for Discriminant<T> { fn clone(&self) -> Self { *self } } #[stable(feature = "discriminant_value", since = "1.21.0")] impl<T> cmp::PartialEq for Discriminant<T> { fn eq(&self, rhs: &Self) -> bool { self.0 == rhs.0 } } #[stable(feature = "discriminant_value", since = "1.21.0")] impl<T> cmp::Eq for Discriminant<T> {} #[stable(feature = "discriminant_value", since = "1.21.0")] impl<T> hash::Hash for Discriminant<T> { fn hash<H: hash::Hasher>(&self, state: &mut H) { self.0.hash(state); } } #[stable(feature = "discriminant_value", since = "1.21.0")] impl<T> fmt::Debug for Discriminant<T> { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { fmt.debug_tuple("Discriminant") .field(&self.0) .finish() } } /// Returns a value uniquely identifying the enum variant in `v`. /// /// If `T` is not an enum, calling this function will not result in undefined behavior, but the /// return value is unspecified. /// /// # Stability /// /// The discriminant of an enum variant may change if the enum definition changes. A discriminant /// of some variant will not change between compilations with the same compiler. /// /// # Examples /// /// This can be used to compare enums that carry data, while disregarding /// the actual data: /// /// ``` /// use std::mem; /// /// enum Foo { A(&'static str), B(i32), C(i32) } /// /// assert!(mem::discriminant(&Foo::A("bar")) == mem::discriminant(&Foo::A("baz"))); /// assert!(mem::discriminant(&Foo::B(1)) == mem::discriminant(&Foo::B(2))); /// assert!(mem::discriminant(&Foo::B(3)) != mem::discriminant(&Foo::C(3))); /// ``` #[stable(feature = "discriminant_value", since = "1.21.0")] pub fn discriminant<T>(v: &T) -> Discriminant<T> { unsafe { Discriminant(intrinsics::discriminant_value(v), PhantomData) } }