| 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 |
| /// 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 |
| /// 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 |
| /// 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 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}; |
| /// use std::ptr; |
| /// |
| /// 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, so 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[..] { |
| /// unsafe { ptr::write(elem.as_mut_ptr(), 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] { |
| /// unsafe { ptr::write(elem.as_mut_ptr(), 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")] |
| #[derive(Copy)] |
| #[cfg_attr(not(bootstrap), 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: () } |
| } |
| |
| /// 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 = "53491")] |
| #[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 |
| /// |
| /// # 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 preferrable 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 = "53491")] |
| #[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 = "53491")] |
| #[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 = "53491")] |
| #[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 = "53491")] |
| #[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 = "53491")] |
| #[inline(always)] |
| pub fn first_ptr_mut(this: &mut [MaybeUninit<T>]) -> *mut T { |
| this as *mut [MaybeUninit<T>] as *mut T |
| } |
| } |