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//! Safe abstractions around pointing at uninitialized memory without references. //! //! It is potentially **UB** to have references to uninitialized memory even if such a reference is //! not 'used' in any particular manner. See [the discussion of the unsafe working group][wg-ref]. //! //! [wg-ref]: https://github.com/rust-lang/unsafe-code-guidelines/issues/77 use core::{fmt, mem, ptr}; use core::alloc::Layout; use core::marker::PhantomData; use crate::boxed::Box; /// Points to an uninitialized place but would otherwise be a valid reference. /// /// This is a `&mut`-like struct that is somewhat of a pendant to `MaybeUninit`. It makes it /// possible to deal with uninitialized allocations without requiring an `unsafe` block /// initializing them and offers a much safer interface for partial initialization and layout /// calculations than raw pointers. /// /// Note that it also supports slices which means it does not use `MaybeUninit` internally but /// offers conversion where necessary. /// /// ## Usage /// /// The basic usage is also interacting with `MaybeUninit`: /// /// ``` /// # #[derive(Default)] /// # struct MyStruct { }; /// use core::mem::MaybeUninit; /// use static_alloc::Uninit; /// /// let mut alloc: MaybeUninit<MyStruct> = MaybeUninit::uninit(); /// let uninit = Uninit::from_maybe_uninit(&mut alloc); /// /// // notice: no unsafe /// let instance: &mut MyStruct = uninit.init(MyStruct::default()); /// ``` /// /// But since we are working on arbitrary uninitialized memory it is also possible to reuse the /// structure for completely arbitrary other types. Just note that there is no integrated mechanis /// for calling `Drop`. /// /// ``` /// use core::mem::MaybeUninit; /// use static_alloc::Uninit; /// /// // Just a generic buffer. /// let mut alloc: MaybeUninit<[u32; 1024]> = MaybeUninit::uninit(); /// let uninit = Uninit::from_maybe_uninit(&mut alloc); /// /// // Now use the first `u32` for a counter: /// let mut counter = uninit.cast().unwrap(); /// let mut tail = counter.split_to_fit(); /// let counter: &mut u32 = counter.init(0); /// /// // And some more for a few `u64`. /// // Note that these are not trivially aligned, but `Uninit` does that for us. /// let mut values = tail.split_cast().unwrap(); /// // No more use, so don't bother with `split_to_fit` and just `init`. /// let values: &mut [u64; 2] = values.init([0xdead, 0xbeef]); /// ``` #[must_use = "This is a pointer-like type that has no effect on its own. Use `init` to insert a value."] pub struct Uninit<'a, T: ?Sized> { /// The underlying view. /// /// Uninit additional imposes on it that the underlying memory is mutable. view: UninitView<'a, T>, /// Reminder for every construction. mutable: PhantomData<&'a mut ()>, } /// A non-mutable view on a region used in an [`Uninit`]. /// /// Makes it possible to utilize the traversal methods (`split*`, `cast*`, ..) without requiring a /// mutable reference to the original `Uninit`. It will also never expose mutable pointers or /// accidentally offer an aliased mutable reference. Prefer this to instead avoiding the borrow of /// the `Uninit` and manually managing pointers to the region. /// /// [`Uninit`]: ./struct.Uninit.html #[must_use = "This is a pointer-like type that has no effect on its own."] pub struct UninitView<'a, T: ?Sized> { /// Pointer to the start of the region. /// /// Note that `len` is always at least as large as the (minimum) size of `T`. Furthermore, the /// pointer is always correctly aligned to a `T`. ptr: ptr::NonNull<u8>, /// The actual length *in bytes*. /// /// May be larger than required. len: usize, /// Virtual lifetime to make this behave more similar to references. /// /// This borrows structures that hand out `Uninit` allocations. lifetime: PhantomData<&'a ()>, /// We'll be holding an actual `NonNull<T>` in the future (when dynamically sized pointers to /// slices are more ergonomic). For now, just type ourselves. typed: PhantomData<*mut T>, } impl Uninit<'_, ()> { /// Create a uninit pointer from raw memory. /// /// ## Safety /// A valid allocation must exist at the pointer with length at least `len`. There must be *no* /// references aliasing the memory location, and it must be valid to write uninitialized bytes /// into arbitrary locations of the region. /// /// In particular, it is **UB** to create this from a reference to a variable of a type for /// which a completely uninitialized content is not valid. The standard type for avoiding the /// UB is `core::mem::MaybeUninit`. /// /// When in doubt, refactor code such that utilization of `from_maybe_uninit` is possible. pub unsafe fn from_memory(ptr: ptr::NonNull<u8>, len: usize) -> Self { Uninit::from_presumed_mutable_view(UninitView { ptr, len, lifetime: PhantomData, typed: PhantomData, }) } /// Split so that the second part fits the layout. /// /// Return `Ok` if this is possible in-bounds and `Err` if it is not. pub fn split_layout(&mut self, layout: Layout) -> Option<Self> { self.view.split_layout(layout) .map(Self::from_presumed_mutable_view) } } impl<'a> Uninit<'a, ()> { fn decast<T: ?Sized>(uninit: Uninit<'a, T>) -> Self { Uninit::from_presumed_mutable_view(UninitView { ptr: uninit.view.ptr.cast(), len: uninit.view.len, lifetime: PhantomData, typed: PhantomData, }) } /// Split so that the tail is aligned and valid for a `U`. /// /// Return `Ok` if this is possible in-bounds (aligned and enough room for at least one `U`) /// and `Err` if it is not. The first tuple element is the `Uninit` pointing to the skipped /// memory. pub fn split_cast<U>(&mut self) -> Option<Uninit<'a, U>> { let split = self.split_layout(Layout::new::<U>())?; let cast = split.cast::<U>().unwrap(); Some(cast) } /// Split so that the tail is aligned for a slice `[U]`. /// /// Return `Ok` if this is possible in-bounds and `Err` if it is not. The first tuple element /// is the `Uninit` pointing to the skipped memory. /// /// The length of the slice is the arbitrary amount that fits into the tail of the allocation. /// Note that the length always fulfills the safety requirements for `slice::from_raw_parts` /// since the `Uninit` must be contained in a single allocation. pub fn split_slice<U>(&mut self) -> Option<Uninit<'a, [U]>> { let layout = Layout::for_value::<[U]>(&[]); let split = self.split_layout(layout)?; let cast = split.cast_slice::<U>().unwrap(); Some(cast) } } impl<T> Uninit<'_, T> { /// Invent a new uninit allocation for a zero-sized type (ZST). /// /// # Panics /// This method panics when the type parameter is not a zero sized type. pub fn invent_for_zst() -> Self { // SAFETY: zst is always unaliased. unsafe { Uninit::from_view(UninitView::invent_for_zst()) } } } impl<'a, T> Uninit<'a, T> { /// Create an `uninit` from a view. /// /// ## Safety /// The caller must prove that the pointed-to memory is mutable and that it is unaliased. pub unsafe fn from_view(view: UninitView<'a, T>) -> Self { Self::from_presumed_mutable_view(view) } /// Create an initializable pointer to the inner bytes of a `MaybeUninit`. pub fn from_maybe_uninit(mem: &'a mut mem::MaybeUninit<T>) -> Self { let ptr = ptr::NonNull::new(mem.as_mut_ptr()).unwrap(); let raw = unsafe { // SAFETY: // * unaliased as we had a mutable reference // * can write uninitialized bytes as much as we want Uninit::from_memory(ptr.cast(), mem::size_of_val(mem)) }; raw.cast().unwrap() } /// Split the uninit slice at a byte boundary. /// /// Return `Ok` if the location is in-bounds and `Err` if it is out of bounds. pub fn split_at_byte(&mut self, at: usize) -> Option<Uninit<'a, ()>> { self.view.split_at_byte(at) .map(Uninit::from_presumed_mutable_view) } /// Try to cast to an `Uninit` for another type. /// /// Return `Ok` if the current `Uninit` is suitably aligned and large enough to hold at least /// one `U` and `Err` if it is not. Note that the successful result points to unused remaining /// memory behind where the instance can be placed. /// /// Use [`split_to_fit`] to get rid of surplus memory at the end. /// /// [`split_to_fit`]: #method.split_to_fit pub fn cast<U>(self) -> Result<Uninit<'a, U>, Self> { self.view.cast() .map(Uninit::from_presumed_mutable_view) .map_err(Self::from_presumed_mutable_view) } /// Try to cast to an `Uninit` for a slice type. /// /// Return `Ok` if the current `Uninit` is suitably aligned and large enough to hold at least /// one `U` and `Err` if it is not. Note that the successful result points to unused remaining /// memory behind where the instances can be placed. pub fn cast_slice<U>(self) -> Result<Uninit<'a, [U]>, Self> { self.view.cast_slice::<U>() .map(Uninit::from_presumed_mutable_view) .map_err(Self::from_presumed_mutable_view) } /// Split off the tail that is not required for holding an instance of `T`. /// /// This operation is idempotent. pub fn split_to_fit(&mut self) -> Uninit<'a, ()> { self.split_at_byte(mem::size_of::<T>()).unwrap() } /// Initialize the place and return a reference to the value. pub fn init(self, val: T) -> &'a mut T { let ptr = self.as_ptr(); unsafe { // SAFETY: // * can only create instances where layout of `T` 'fits' // * valid for lifetime `'a` (as per `UninitView`). // * unaliased for lifetime `'a` (as per own invariant from unsafe constructor). No // other method duplicates the pointer or allows a second `Uninit` without borrowing // the first. ptr::write(ptr, val); &mut *ptr } } /// Acquires the underlying *mut pointer. pub const fn as_ptr(&self) -> *mut T { self.view.ptr.cast().as_ptr() } /// Acquires the underlying pointer as a `NonNull`. pub const fn as_non_null(&self) -> ptr::NonNull<T> { self.view.ptr.cast() } /// Dereferences the content. /// /// The resulting lifetime is bound to self so this behaves "as if" it were actually an /// instance of T that is getting borrowed. If a longer lifetime is needed, use `into_ref`. pub unsafe fn as_ref(&self) -> &T { self.view.as_ref() } /// Mutably dereferences the content. /// /// The resulting lifetime is bound to self so this behaves "as if" it were actually an /// instance of T that is getting borrowed. If a longer lifetime is needed, use `into_mut`. pub unsafe fn as_mut(&mut self) -> &mut T { &mut *self.as_ptr() } /// Turn this into a reference to the content. pub unsafe fn into_ref(self) -> &'a T { &*self.as_ptr() } /// Turn this into a mutable reference to the content. pub unsafe fn into_mut(self) -> &'a mut T { &mut *self.as_ptr() } /// Utilize this `Uninit` allocation for a boxed value. /// /// Stores the value at the pointed-to location and utilizes the `Box` as a RAII-guard to /// properly drop the value when the box itself is dropped. pub fn into_box(self, val: T) -> Box<'a, T> { Box::new(val, self) } /// Read a value from the uninit place without moving it. /// /// The `Uninit` ensures that the inner pointer is correctly aligned, non-null, and points to a /// large enough region for reading a `T`. /// /// ## Safety /// Caller must ensure that the memory is initialized as a valid `T`. It must also avoid double /// `Drop`. Basically, a new instance is created. pub unsafe fn read(&self) -> T { ptr::read(self.as_ptr()) } } impl<'a, T> Uninit<'a, [T]> { /// Creates a pointer to an empty slice. pub fn empty() -> Self { Uninit::from_presumed_mutable_view(UninitView { ptr: ptr::NonNull::<T>::dangling().cast(), len: 0, lifetime: PhantomData, typed: PhantomData, }) } /// Get the pointer to the first element of the slice. /// /// If the slice would be empty then the pointer may be the past-the-end pointer as well. pub const fn as_begin_ptr(&self) -> *mut T { self.view.ptr.as_ptr() as *mut T } /// Calculate the theoretical capacity of a slice in the pointed-to allocation. pub fn capacity(&self) -> usize { self.view.capacity() } /// Split the slice at an index. /// /// This is the pointer equivalent of `slice::split_at`. pub fn split_at(&mut self, at: usize) -> Option<Self> { self.view.split_at(at) .map(Self::from_presumed_mutable_view) } /// Get the trailing bytes behind the slice. /// /// The underlying allocation need not be a multiple of the slice element size which may leave /// unusable bytes. This splits these unusable bytes into an untyped `Uninit` which can be /// reused arbitrarily. /// /// This operation is idempotent. pub fn shrink_to_fit(&mut self) -> Uninit<'a, ()> { Uninit::decast(self.split_at(self.capacity()).unwrap()) } /// Split the first element from the slice. /// /// This is the pointer equivalent of `slice::split_first`. pub fn split_first(&mut self) -> Option<Uninit<'a, T>> { let mut part = self.split_at(1)?; // Now we are the first part, but we wanted the first to be split off. mem::swap(self, &mut part); // If it is a valid slice of length 1 it is a valid `T`. Some(Uninit::decast(part).cast().unwrap()) } /// Split the last element from the slice. /// /// This is the pointer equivalent of `slice::split_last`. pub fn split_last(&mut self) -> Option<Uninit<'a, T>> { // Explicitely wrap here: If capacity is 0 then `0 < size_of::<T> ` and the split will fail. let split = self.capacity().wrapping_sub(1); let part = self.split_at(split)?; // If it is a valid slice of length 1 it is a valid `T`. Some(Uninit::decast(part).cast().unwrap()) } } impl<'a, T: ?Sized> Uninit<'a, T> { /// Check if the view fits some layout. /// /// The `cast` to a type of the provided layout will work without error. pub fn fits(&self, layout: Layout) -> bool { self.view.fits(layout) } /// View the same uninit as untyped memory. pub fn as_memory(self) -> Uninit<'a, ()> { Uninit::decast(self) } /// A private version of the unsafe `from_view`. /// /// This must never be exposed. fn from_presumed_mutable_view(view: UninitView<'a, T>) -> Self { Uninit { view, mutable: PhantomData, } } /// Borrow a view of the `Uninit` region. /// /// This is the equivalent of `&*mut_ref as *const _` but never runs afoul of accidentally /// creating an actual reference. pub fn borrow(&self) -> UninitView<'_, T> { self.view } /// Borrow the `Uninit` region for a shorter duration. /// /// This is the equivalent of `&mut *mut_ref as *mut _` but never runs afoul of accidentally /// creating an actual reference. pub fn borrow_mut(&mut self) -> Uninit<'_, T> { Uninit::from_presumed_mutable_view(self.view) } /// Get the byte size of the total allocation. pub const fn size(&self) -> usize { self.view.size() } } impl UninitView<'_, ()> { /// Create a uninit view from raw memory. /// /// ## Safety /// A valid allocation must exist at the pointer with length at least `len`. /// /// In particular, it is **UB** to create this from a reference to a variable of a type for /// which a completely uninitialized content is not valid. The standard type for avoiding the /// UB is `core::mem::MaybeUninit`. /// /// When in doubt, refactor code such that utilization of `from_maybe_uninit` is possible. pub unsafe fn from_memory(ptr: ptr::NonNull<u8>, len: usize) -> Self { UninitView { ptr, len, lifetime: PhantomData, typed: PhantomData, } } /// Split so that the second part fits the layout. /// /// See [`Uninit::split_layout`] for more details. /// /// [`Uninit::split_layout`]: ./struct.Uninit.html#method.split_layout pub fn split_layout(&mut self, layout: Layout) -> Option<Self> { let align = self.ptr.as_ptr() .align_offset(layout.align()); let aligned_len = self.len .checked_sub(align) .and_then(|len| len.checked_sub(layout.size())); if aligned_len.is_none() { return None; } let aligned = self.split_at_byte(align)?; assert!(aligned.fits(layout)); Some(aligned) } } impl<'a> UninitView<'a, ()> { fn decast<T: ?Sized>(view: UninitView<'a, T>) -> Self { UninitView { ptr: view.ptr.cast(), len: view.len, lifetime: PhantomData, typed: PhantomData, } } /// Split so that the tail is aligned and valid for a `U`. pub fn split_cast<U>(&mut self) -> Option<UninitView<'a, U>> { let split = self.split_layout(Layout::new::<U>())?; let cast = split.cast::<U>().unwrap(); Some(cast) } /// Split so that the tail is aligned for a slice `[U]`. pub fn split_slice<U>(&mut self) -> Option<UninitView<'a, [U]>> { let layout = Layout::for_value::<[U]>(&[]); let split = self.split_layout(layout)?; let cast = split.cast_slice::<U>().unwrap(); Some(cast) } } impl<T> UninitView<'_, T> { /// Invent a new uninit allocation for a zero-sized type (ZST). /// /// # Panics /// This method panics when the type parameter is not a zero sized type. pub fn invent_for_zst() -> Self { assert_eq!(mem::size_of::<T>(), 0, "Invented ZST uninit invoked with non-ZST"); let dangling = ptr::NonNull::<T>::dangling(); // SAFETY: all bytes are within the allocation. let raw = unsafe { UninitView::from_memory(dangling.cast(), 0) }; raw.cast().unwrap() } } impl<'a, T> UninitView<'a, T> { /// Split the uninit view at a byte boundary. /// /// See [`Uninit::split_at_byte`] for more details. /// /// [`Uninit::split_at_byte`]: ./struct.Uninit.html#method.split_at_byte pub fn split_at_byte(&mut self, at: usize) -> Option<UninitView<'a, ()>> { if self.len < at || at < mem::size_of::<T>() { return None; } let base = self.ptr.as_ptr(); // SAFETY: by `from_memory`, all offsets `< len` are within the allocation. // In particular, no pointer within or one-past-the-end is null. let next_base = unsafe { ptr::NonNull::new_unchecked(base.add(at)) }; let next_len = self.len - at; self.len = at; // SAFETY: within one allocation, namely the one we are in. let other = unsafe { UninitView::from_memory(next_base.cast(), next_len) }; Some(other) } /// Create an view to the inner bytes of a `MaybeUninit`. /// /// This is hardly useful on its own but since `UninitView` mirrors the traversal methods of /// `Uninit` it can be used to get pointers to already initialized elements in an immutable /// context. pub fn from_maybe_uninit(mem: &'a mem::MaybeUninit<T>) -> Self { let ptr = ptr::NonNull::new(mem.as_ptr() as *mut T).unwrap(); let raw = unsafe { // SAFETY: // * unaliased as we had a mutable reference // * we will not write through the pointer created UninitView::from_memory(ptr.cast(), mem::size_of_val(mem)) }; raw.cast().unwrap() } /// Try to cast to an `UninitView` for another type. pub fn cast<U>(self) -> Result<UninitView<'a, U>, Self> { if !self.fits(Layout::new::<U>()) { return Err(self); } Ok(UninitView { ptr: self.ptr.cast(), len: self.len, lifetime: PhantomData, typed: PhantomData, }) } /// Try to cast to an `UninitView` for a slice type. pub fn cast_slice<U>(self) -> Result<UninitView<'a, [U]>, Self> { let empty = Layout::for_value::<[U]>(&[]); if !self.fits(empty) { return Err(self) } Ok(UninitView { ptr: self.ptr, len: self.len, lifetime: PhantomData, typed: PhantomData, }) } /// Split off the tail that is not required for holding an instance of `T`. pub fn split_to_fit(&mut self) -> UninitView<'a, ()> { self.split_at_byte(mem::size_of::<T>()).unwrap() } /// Acquires the underlying `*const T` pointer. pub const fn as_ptr(&self) -> *const T { self.ptr.as_ptr() as *const T } /// Acquires the underlying pointer as a `NonNull`. pub fn as_non_null(&self) -> ptr::NonNull<T> { self.ptr.cast() } /// Dereferences the content. /// /// The resulting lifetime is bound to self so this behaves "as if" it were actually an /// instance of T that is getting borrowed. If a longer lifetime is needed, use `into_ref`. /// /// ## Safety /// The caller must ensure that the content has already been initialized. pub unsafe fn as_ref(&self) -> &T { self.into_ref() } /// Turn this into a reference to the content. /// /// ## Safety /// The caller must ensure that the content has already been initialized. pub unsafe fn into_ref(self) -> &'a T { &*self.as_ptr() } } impl<'a, T> UninitView<'a, [T]> { /// Creates a pointer to an empty slice. /// /// Note that it will **not** be a mutable empty slice which means that it would be **UB** to /// use it as an `Uninit`. pub fn empty() -> Self { UninitView { ptr: ptr::NonNull::<T>::dangling().cast(), len: 0, lifetime: PhantomData, typed: PhantomData, } } /// Get the pointer to the first element of the slice. pub fn as_begin_ptr(&self) -> *const T { self.ptr.as_ptr() as *const T } /// Calculate the theoretical capacity of a slice in the pointed-to allocation. pub fn capacity(&self) -> usize { self.size() .checked_div(mem::size_of::<T>()) .unwrap_or_else(usize::max_value) } /// Split the slice at an index. pub fn split_at(&mut self, at: usize) -> Option<Self> { // NOTE: Slice pointers are blocked by Rust stabilization we can not create one from a real // reference to slice as that would restrict us to the memory covered by the reference. // NOTE: Tracked here https://github.com/rust-lang/rust/issues/36925 let bytes = match at.checked_mul(mem::size_of::<T>()) { None => return None, Some(byte) if byte > self.len => return None, Some(byte) => byte, }; let next_len = self.len - bytes; self.len = bytes; let base = self.ptr.as_ptr(); // SAFETY: was previously in bounds. let next_base = unsafe { ptr::NonNull::new_unchecked(base.add(bytes)) }; // SAFETY: total allocation length at least `self.len + next_len`. let other = unsafe { UninitView::from_memory(next_base, next_len) }; Some(other.cast_slice().unwrap()) } /// Get the trailing bytes behind the slice. /// /// The underlying allocation need not be a multiple of the slice element size which may leave /// unusable bytes. This splits these unusable bytes into an untyped `Uninit` which can be /// reused arbitrarily. /// /// This operation is idempotent. pub fn shrink_to_fit(&mut self) -> UninitView<'a, ()> { UninitView::decast(self.split_at(self.capacity()).unwrap()) } /// Split the first element from the slice. pub fn split_first(&mut self) -> Option<UninitView<'a, T>> { let mut part = self.split_at(1)?; // Now we are the first part, but we wanted the first to be split off. mem::swap(self, &mut part); // If it is a valid slice of length 1 it is a valid `T`. Some(UninitView::decast(part).cast().unwrap()) } /// Split the last element from the slice. pub fn split_last(&mut self) -> Option<UninitView<'a, T>> { // Explicitely wrap here: If capacity is 0 then `0 < size_of::<T> ` and the split will fail. let split = self.capacity().wrapping_sub(1); let part = self.split_at(split)?; // If it is a valid slice of length 1 it is a valid `T`. Some(UninitView::decast(part).cast().unwrap()) } } impl<'a, T: ?Sized> UninitView<'a, T> { /// Check if the view fits some layout. /// /// The `cast` to a type of the provided layout will work without error. pub fn fits(&self, layout: Layout) -> bool { self.ptr.as_ptr().align_offset(layout.align()) == 0 && layout.size() <= self.len } /// Borrow another view of the `Uninit` region. pub fn borrow(&self) -> UninitView<'_, T> { *self } /// Get the byte size of the total allocation. pub const fn size(&self) -> usize { self.len } } impl<'a, T> From<&'a mut mem::MaybeUninit<T>> for Uninit<'a, T> { fn from(mem: &'a mut mem::MaybeUninit<T>) -> Self { Uninit::from_maybe_uninit(mem) } } impl<'a, T> From<&'a mem::MaybeUninit<T>> for UninitView<'a, T> { fn from(mem: &'a mem::MaybeUninit<T>) -> Self { UninitView::from_maybe_uninit(mem) } } impl<T: ?Sized> fmt::Debug for Uninit<'_, T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.debug_tuple("Uninit") .field(&self.view.ptr) .field(&self.view.len) .finish() } } impl<T: ?Sized> fmt::Debug for UninitView<'_, T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.debug_tuple("UninitView") .field(&self.ptr) .field(&self.len) .finish() } } impl<'a, T> From<Uninit<'a, T>> for UninitView<'a, T> { fn from(uninit: Uninit<'a, T>) -> Self { uninit.view } } impl<T> Default for Uninit<'_, [T]> { fn default() -> Self { Uninit::empty() } } impl<T> Default for UninitView<'_, [T]> { fn default() -> Self { UninitView::empty() } } impl<T: ?Sized> Clone for UninitView<'_, T> { fn clone(&self) -> Self { *self } } impl<T: ?Sized> Copy for UninitView<'_, T> { } #[cfg(test)] mod tests { use super::Uninit; #[test] fn lifetime_longer() { fn _long<'a, T>(_: Uninit<'a, &'static T>) { } } #[test] fn lifetime_shorter() { fn _short<'a, T>(_: Uninit<'static, &'a T>) { } } #[test] fn in_a_struct() { enum _List<T> { Nil, Cons(T, Uninit<'static, T>), } } }