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use crate::intrinsics; use crate::mem::ManuallyDrop; /// A wrapper type to construct uninitialized instances of `T`. /// /// # Initialization invariant /// /// The compiler, in general, assumes that variables are properly initialized /// at their respective type. For example, a variable of reference type must /// be aligned and non-NULL. This is an invariant that must *always* be upheld, /// even in unsafe code. As a consequence, zero-initializing a variable of reference /// type causes instantaneous [undefined behavior][ub], no matter whether that reference /// ever gets used to access memory: /// /// ```rust,no_run /// # #![allow(invalid_value)] /// use std::mem::{self, MaybeUninit}; /// /// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior! /// // The equivalent code with `MaybeUninit<&i32>`: /// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior! /// ``` /// /// This is exploited by the compiler for various optimizations, such as eliding /// run-time checks and optimizing `enum` layout. /// /// Similarly, entirely uninitialized memory may have any content, while a `bool` must /// always be `true` or `false`. Hence, creating an uninitialized `bool` is undefined behavior: /// /// ```rust,no_run /// # #![allow(invalid_value)] /// use std::mem::{self, MaybeUninit}; /// /// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior! /// // The equivalent code with `MaybeUninit<bool>`: /// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! /// ``` /// /// Moreover, uninitialized memory 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, /// which otherwise can hold any *fixed* bit pattern: /// /// ```rust,no_run /// # #![allow(invalid_value)] /// use std::mem::{self, MaybeUninit}; /// /// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior! /// // The equivalent code with `MaybeUninit<i32>`: /// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! /// ``` /// (Notice that the rules around uninitialized integers are not finalized yet, but /// until they are, it is advisable to avoid them.) /// /// On top of that, remember that most types have additional invariants beyond merely /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`] /// is considered initialized (under the current implementation; this does not constitute /// a stable guarantee) because the only requirement the compiler knows about it /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause /// *immediate* undefined behavior, but will cause undefined behavior with most /// safe operations (including dropping it). /// /// [`Vec<T>`]: ../../std/vec/struct.Vec.html /// /// # Examples /// /// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data. /// It is a signal to the compiler indicating that the data here might *not* /// be initialized: /// /// ```rust /// use std::mem::MaybeUninit; /// /// // Create an explicitly uninitialized reference. The compiler knows that data inside /// // a `MaybeUninit<T>` may be invalid, and hence this is not UB: /// let mut x = MaybeUninit::<&i32>::uninit(); /// // Set it to a valid value. /// unsafe { x.as_mut_ptr().write(&0); } /// // Extract the initialized data -- this is only allowed *after* properly /// // initializing `x`! /// let x = unsafe { x.assume_init() }; /// ``` /// /// The compiler then knows to not make any incorrect assumptions or optimizations on this code. /// /// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without /// any of the run-time tracking and without any of the safety checks. /// /// ## out-pointers /// /// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data /// from a function, pass it a pointer to some (uninitialized) memory to put the /// result into. This can be useful when it is important for the caller to control /// how the memory the result is stored in gets allocated, and you want to avoid /// unnecessary moves. /// /// ``` /// use std::mem::MaybeUninit; /// /// unsafe fn make_vec(out: *mut Vec<i32>) { /// // `write` does not drop the old contents, which is important. /// out.write(vec![1, 2, 3]); /// } /// /// let mut v = MaybeUninit::uninit(); /// unsafe { make_vec(v.as_mut_ptr()); } /// // Now we know `v` is initialized! This also makes sure the vector gets /// // properly dropped. /// let v = unsafe { v.assume_init() }; /// assert_eq!(&v, &[1, 2, 3]); /// ``` /// /// ## Initializing an array element-by-element /// /// `MaybeUninit<T>` can be used to initialize a large array element-by-element: /// /// ``` /// use std::mem::{self, MaybeUninit}; /// /// let data = { /// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is /// // safe because the type we are claiming to have initialized here is a /// // bunch of `MaybeUninit`s, which do not require initialization. /// let mut data: [MaybeUninit<Vec<u32>>; 1000] = unsafe { /// MaybeUninit::uninit().assume_init() /// }; /// /// // Dropping a `MaybeUninit` does nothing. Thus using raw pointer /// // assignment instead of `ptr::write` does not cause the old /// // uninitialized value to be dropped. Also if there is a panic during /// // this loop, we have a memory leak, but there is no memory safety /// // issue. /// for elem in &mut data[..] { /// *elem = MaybeUninit::new(vec![42]); /// } /// /// // Everything is initialized. Transmute the array to the /// // initialized type. /// unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) } /// }; /// /// assert_eq!(&data[0], &[42]); /// ``` /// /// You can also work with partially initialized arrays, which could /// be found in low-level datastructures. /// /// ``` /// use std::mem::MaybeUninit; /// use std::ptr; /// /// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is /// // safe because the type we are claiming to have initialized here is a /// // bunch of `MaybeUninit`s, which do not require initialization. /// let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() }; /// // Count the number of elements we have assigned. /// let mut data_len: usize = 0; /// /// for elem in &mut data[0..500] { /// *elem = MaybeUninit::new(String::from("hello")); /// data_len += 1; /// } /// /// // For each item in the array, drop if we allocated it. /// for elem in &mut data[0..data_len] { /// unsafe { ptr::drop_in_place(elem.as_mut_ptr()); } /// } /// ``` /// /// ## Initializing a struct field-by-field /// /// There is currently no supported way to create a raw pointer or reference /// to a field of a struct inside `MaybeUninit<Struct>`. That means it is not possible /// to create a struct by calling `MaybeUninit::uninit::<Struct>()` and then writing /// to its fields. /// /// [ub]: ../../reference/behavior-considered-undefined.html /// /// # Layout /// /// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`: /// /// ```rust /// use std::mem::{MaybeUninit, size_of, align_of}; /// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>()); /// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>()); /// ``` /// /// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same /// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as /// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit /// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling /// optimizations, potentially resulting in a larger size: /// /// ```rust /// # use std::mem::{MaybeUninit, size_of}; /// assert_eq!(size_of::<Option<bool>>(), 1); /// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2); /// ``` /// /// If `T` is FFI-safe, then so is `MaybeUninit<T>`. /// /// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size, /// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and /// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type /// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`. /// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the /// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact /// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not /// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has /// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that /// guarantee may evolve. #[allow(missing_debug_implementations)] #[stable(feature = "maybe_uninit", since = "1.36.0")] // Lang item so we can wrap other types in it. This is useful for generators. #[lang = "maybe_uninit"] #[derive(Copy)] #[repr(transparent)] pub union MaybeUninit<T> { uninit: (), value: ManuallyDrop<T>, } #[stable(feature = "maybe_uninit", since = "1.36.0")] impl<T: Copy> Clone for MaybeUninit<T> { #[inline(always)] fn clone(&self) -> Self { // Not calling `T::clone()`, we cannot know if we are initialized enough for that. *self } } impl<T> MaybeUninit<T> { /// Creates a new `MaybeUninit<T>` initialized with the given value. /// It is safe to call [`assume_init`] on the return value of this function. /// /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. /// It is your responsibility to make sure `T` gets dropped if it got initialized. /// /// [`assume_init`]: #method.assume_init #[stable(feature = "maybe_uninit", since = "1.36.0")] #[inline(always)] pub const fn new(val: T) -> MaybeUninit<T> { MaybeUninit { value: ManuallyDrop::new(val) } } /// Creates a new `MaybeUninit<T>` in an uninitialized state. /// /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. /// It is your responsibility to make sure `T` gets dropped if it got initialized. /// /// See the [type-level documentation][type] for some examples. /// /// [type]: union.MaybeUninit.html #[stable(feature = "maybe_uninit", since = "1.36.0")] #[inline(always)] pub const fn uninit() -> MaybeUninit<T> { MaybeUninit { uninit: () } } /// A promotable constant, equivalent to `uninit()`. #[unstable(feature = "internal_uninit_const", issue = "0", reason = "hack to work around promotability")] pub const UNINIT: Self = Self::uninit(); /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being /// filled with `0` bytes. It depends on `T` whether that already makes for /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized, /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not /// be null. /// /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. /// It is your responsibility to make sure `T` gets dropped if it got initialized. /// /// # Example /// /// Correct usage of this function: initializing a struct with zero, where all /// fields of the struct can hold the bit-pattern 0 as a valid value. /// /// ```rust /// use std::mem::MaybeUninit; /// /// let x = MaybeUninit::<(u8, bool)>::zeroed(); /// let x = unsafe { x.assume_init() }; /// assert_eq!(x, (0, false)); /// ``` /// /// *Incorrect* usage of this function: initializing a struct with zero, where some fields /// cannot hold 0 as a valid value. /// /// ```rust,no_run /// use std::mem::MaybeUninit; /// /// enum NotZero { One = 1, Two = 2 }; /// /// let x = MaybeUninit::<(u8, NotZero)>::zeroed(); /// let x = unsafe { x.assume_init() }; /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant. /// // This is undefined behavior. /// ``` #[stable(feature = "maybe_uninit", since = "1.36.0")] #[inline] pub fn zeroed() -> MaybeUninit<T> { let mut u = MaybeUninit::<T>::uninit(); unsafe { u.as_mut_ptr().write_bytes(0u8, 1); } u } /// Sets the value of the `MaybeUninit<T>`. This overwrites any previous value /// without dropping it, so be careful not to use this twice unless you want to /// skip running the destructor. For your convenience, this also returns a mutable /// reference to the (now safely initialized) contents of `self`. #[unstable(feature = "maybe_uninit_extra", issue = "63567")] #[inline(always)] pub fn write(&mut self, val: T) -> &mut T { unsafe { self.value = ManuallyDrop::new(val); self.get_mut() } } /// Gets a pointer to the contained value. Reading from this pointer or turning it /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized. /// Writing to memory that this pointer (non-transitively) points to is undefined behavior /// (except inside an `UnsafeCell<T>`). /// /// # Examples /// /// Correct usage of this method: /// /// ```rust /// use std::mem::MaybeUninit; /// /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); /// unsafe { x.as_mut_ptr().write(vec![0,1,2]); } /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it. /// let x_vec = unsafe { &*x.as_ptr() }; /// assert_eq!(x_vec.len(), 3); /// ``` /// /// *Incorrect* usage of this method: /// /// ```rust,no_run /// use std::mem::MaybeUninit; /// /// let x = MaybeUninit::<Vec<u32>>::uninit(); /// let x_vec = unsafe { &*x.as_ptr() }; /// // We have created a reference to an uninitialized vector! This is undefined behavior. /// ``` /// /// (Notice that the rules around references to uninitialized data are not finalized yet, but /// until they are, it is advisable to avoid them.) #[stable(feature = "maybe_uninit", since = "1.36.0")] #[inline(always)] pub fn as_ptr(&self) -> *const T { unsafe { &*self.value as *const T } } /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized. /// /// # Examples /// /// Correct usage of this method: /// /// ```rust /// use std::mem::MaybeUninit; /// /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); /// unsafe { x.as_mut_ptr().write(vec![0,1,2]); } /// // Create a reference into the `MaybeUninit<Vec<u32>>`. /// // This is okay because we initialized it. /// let x_vec = unsafe { &mut *x.as_mut_ptr() }; /// x_vec.push(3); /// assert_eq!(x_vec.len(), 4); /// ``` /// /// *Incorrect* usage of this method: /// /// ```rust,no_run /// use std::mem::MaybeUninit; /// /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); /// let x_vec = unsafe { &mut *x.as_mut_ptr() }; /// // We have created a reference to an uninitialized vector! This is undefined behavior. /// ``` /// /// (Notice that the rules around references to uninitialized data are not finalized yet, but /// until they are, it is advisable to avoid them.) #[stable(feature = "maybe_uninit", since = "1.36.0")] #[inline(always)] pub fn as_mut_ptr(&mut self) -> *mut T { unsafe { &mut *self.value as *mut T } } /// Extracts the value from the `MaybeUninit<T>` container. This is a great way /// to ensure that the data will get dropped, because the resulting `T` is /// subject to the usual drop handling. /// /// # Safety /// /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized /// state. Calling this when the content is not yet fully initialized causes immediate undefined /// behavior. The [type-level documentation][inv] contains more information about /// this initialization invariant. /// /// [inv]: #initialization-invariant /// /// On top of that, remember that most types have additional invariants beyond merely /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`] /// is considered initialized (under the current implementation; this does not constitute /// a stable guarantee) because the only requirement the compiler knows about it /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause /// *immediate* undefined behavior, but will cause undefined behavior with most /// safe operations (including dropping it). /// /// # Examples /// /// Correct usage of this method: /// /// ```rust /// use std::mem::MaybeUninit; /// /// let mut x = MaybeUninit::<bool>::uninit(); /// unsafe { x.as_mut_ptr().write(true); } /// let x_init = unsafe { x.assume_init() }; /// assert_eq!(x_init, true); /// ``` /// /// *Incorrect* usage of this method: /// /// ```rust,no_run /// use std::mem::MaybeUninit; /// /// let x = MaybeUninit::<Vec<u32>>::uninit(); /// let x_init = unsafe { x.assume_init() }; /// // `x` had not been initialized yet, so this last line caused undefined behavior. /// ``` #[stable(feature = "maybe_uninit", since = "1.36.0")] #[inline(always)] pub unsafe fn assume_init(self) -> T { intrinsics::panic_if_uninhabited::<T>(); ManuallyDrop::into_inner(self.value) } /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject /// to the usual drop handling. /// /// Whenever possible, it is preferable to use [`assume_init`] instead, which /// prevents duplicating the content of the `MaybeUninit<T>`. /// /// # Safety /// /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized /// state. Calling this when the content is not yet fully initialized causes undefined /// behavior. The [type-level documentation][inv] contains more information about /// this initialization invariant. /// /// Moreover, this leaves a copy of the same data behind in the `MaybeUninit<T>`. When using /// multiple copies of the data (by calling `read` multiple times, or first /// calling `read` and then [`assume_init`]), it is your responsibility /// to ensure that that data may indeed be duplicated. /// /// [inv]: #initialization-invariant /// [`assume_init`]: #method.assume_init /// /// # Examples /// /// Correct usage of this method: /// /// ```rust /// #![feature(maybe_uninit_extra)] /// use std::mem::MaybeUninit; /// /// let mut x = MaybeUninit::<u32>::uninit(); /// x.write(13); /// let x1 = unsafe { x.read() }; /// // `u32` is `Copy`, so we may read multiple times. /// let x2 = unsafe { x.read() }; /// assert_eq!(x1, x2); /// /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit(); /// x.write(None); /// let x1 = unsafe { x.read() }; /// // Duplicating a `None` value is okay, so we may read multiple times. /// let x2 = unsafe { x.read() }; /// assert_eq!(x1, x2); /// ``` /// /// *Incorrect* usage of this method: /// /// ```rust,no_run /// #![feature(maybe_uninit_extra)] /// use std::mem::MaybeUninit; /// /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit(); /// x.write(Some(vec![0,1,2])); /// let x1 = unsafe { x.read() }; /// let x2 = unsafe { x.read() }; /// // We now created two copies of the same vector, leading to a double-free when /// // they both get dropped! /// ``` #[unstable(feature = "maybe_uninit_extra", issue = "63567")] #[inline(always)] pub unsafe fn read(&self) -> T { intrinsics::panic_if_uninhabited::<T>(); self.as_ptr().read() } /// Gets a reference to the contained value. /// /// # Safety /// /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized /// state. Calling this when the content is not yet fully initialized causes undefined /// behavior. #[unstable(feature = "maybe_uninit_ref", issue = "63568")] #[inline(always)] pub unsafe fn get_ref(&self) -> &T { &*self.value } /// Gets a mutable reference to the contained value. /// /// # Safety /// /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized /// state. Calling this when the content is not yet fully initialized causes undefined /// behavior. // FIXME(#53491): We currently rely on the above being incorrect, i.e., we have references // to uninitialized data (e.g., in `libcore/fmt/float.rs`). We should make // a final decision about the rules before stabilization. #[unstable(feature = "maybe_uninit_ref", issue = "63568")] #[inline(always)] pub unsafe fn get_mut(&mut self) -> &mut T { &mut *self.value } /// Gets a pointer to the first element of the array. #[unstable(feature = "maybe_uninit_slice", issue = "63569")] #[inline(always)] pub fn first_ptr(this: &[MaybeUninit<T>]) -> *const T { this as *const [MaybeUninit<T>] as *const T } /// Gets a mutable pointer to the first element of the array. #[unstable(feature = "maybe_uninit_slice", issue = "63569")] #[inline(always)] pub fn first_ptr_mut(this: &mut [MaybeUninit<T>]) -> *mut T { this as *mut [MaybeUninit<T>] as *mut T } }