}
}
-/// A newtype to construct uninitialized instances of `T`
+/// A newtype to construct uninitialized instances of `T`.
+///
+/// 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, 0-initializing a variable of reference
+/// type causes instantaneous undefined behavior, no matter whether that reference
+/// ever gets used to access memory:
+/// ```rust,no_run
+/// use std::mem;
+///
+/// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior!
+/// ```
+/// This is exploited by the compiler for various optimizations, such as eliding
+/// run-time checks and optimizing `enum` layout.
+///
+/// Not initializing memory at all (instead of 0-initializing it) causes the same
+/// issue: after all, the initial value of the variable might just happen to be
+/// one that violates the invariant.
+///
+/// `MaybeUninit` 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
+/// #![feature(maybe_uninit)]
+/// use std::mem::MaybeUninit;
+///
+/// // Create an explicitly uninitialized reference.
+/// let mut x = MaybeUninit::<&i32>::uninitialized();
+/// // Set it to a valid value.
+/// x.set(&0);
+/// // Extract the initialized data -- this is only allowed *after* properly
+/// // initializing `x`!
+/// let x = unsafe { x.into_initialized() };
+/// ```
+/// The compiler then knows to not optimize this code.
#[allow(missing_debug_implementations)]
#[unstable(feature = "maybe_uninit", issue = "53491")]
// NOTE after stabilizing `MaybeUninit` proceed to deprecate `mem::{uninitialized,zeroed}`
}
/// Set the value of the `MaybeUninit`. This overwrites any previous value without dropping it.
+ /// For your convenience, this also returns a mutable reference to the (now
+ /// safely initialized) content of `self`.
#[unstable(feature = "maybe_uninit", issue = "53491")]
#[inline(always)]
- pub fn set(&mut self, val: T) {
+ pub fn set(&mut self, val: T) -> &mut T {
unsafe {
self.value = ManuallyDrop::new(val);
+ self.get_mut()
}
}
/// state, otherwise this will immediately cause undefined behavior.
#[unstable(feature = "maybe_uninit", issue = "53491")]
#[inline(always)]
- pub unsafe fn into_inner(self) -> T {
+ pub unsafe fn into_initialized(self) -> T {
intrinsics::panic_if_uninhabited::<T>();
ManuallyDrop::into_inner(self.value)
}
+ /// Deprecated alternative to `into_initialized`. Will never get stabilized.
+ /// Exists only to transition stdsimd to `into_initialized`.
+ #[inline(always)]
+ #[allow(unused)]
+ pub(crate) unsafe fn into_inner(self) -> T {
+ self.into_initialized()
+ }
+
/// Get a reference to the contained value.
///
/// # Unsafety
&mut *self.value
}
- /// Get a pointer to the contained value. Reading from this pointer will be undefined
- /// behavior unless the `MaybeUninit` is initialized.
+ /// Get a pointer to the contained value. Reading from this pointer or turning it
+ /// into a reference will be undefined behavior unless the `MaybeUninit` is initialized.
#[unstable(feature = "maybe_uninit", issue = "53491")]
#[inline(always)]
pub fn as_ptr(&self) -> *const T {
unsafe { &*self.value as *const T }
}
- /// Get a mutable pointer to the contained value. Reading from this pointer will be undefined
- /// behavior unless the `MaybeUninit` is initialized.
+ /// Get a mutable pointer to the contained value. Reading from this pointer or turning it
+ /// into a reference will be undefined behavior unless the `MaybeUninit` is initialized.
#[unstable(feature = "maybe_uninit", issue = "53491")]
#[inline(always)]
pub fn as_mut_ptr(&mut self) -> *mut T {