Make ZST checks in core/alloc more readable

There's a bunch of these checks because of special handing for ZSTs in various unsafe implementations of stuff.

This lets them be `T::IS_ZST` instead of `mem::size_of::<T>() == 0` every time, making them both more readable and more terse.

*Not* proposed for stabilization at this time.  Would be `pub(crate)` except `alloc` wants to use it too.

(And while it doesn't matter now, if we ever get something like 85836 making it a const can help codegen be simpler.)

Author: scottmcm

library/alloc/src/collections/vec_deque/mod.rs

@@ -12,7 +12,7 @@ use core::fmt;
 use core::hash::{Hash, Hasher};
 use core::iter::{repeat_with, FromIterator};
 use core::marker::PhantomData;
-use core::mem::{self, ManuallyDrop, MaybeUninit};
+use core::mem::{ManuallyDrop, MaybeUninit, SizedTypeProperties};
 use core::ops::{Index, IndexMut, Range, RangeBounds};
 use core::ptr::{self, NonNull};
 use core::slice;
@@ -177,7 +177,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// Marginally more convenient
     #[inline]
     fn cap(&self) -> usize {
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // For zero sized types, we are always at maximum capacity
             MAXIMUM_ZST_CAPACITY
         } else {
@@ -3038,7 +3038,7 @@ impl<T, A: Allocator> From<Vec<T, A>> for VecDeque<T, A> {
     /// `Vec<T>` came from `From<VecDeque<T>>` and hasn't been reallocated.
     fn from(mut other: Vec<T, A>) -> Self {
         let len = other.len();
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // There's no actual allocation for ZSTs to worry about capacity,
             // but `VecDeque` can't handle as much length as `Vec`.
             assert!(len < MAXIMUM_ZST_CAPACITY, "capacity overflow");
@@ -3124,7 +3124,7 @@ impl<T, const N: usize> From<[T; N]> for VecDeque<T> {
     fn from(arr: [T; N]) -> Self {
         let mut deq = VecDeque::with_capacity(N);
         let arr = ManuallyDrop::new(arr);
-        if mem::size_of::<T>() != 0 {
+        if !<T>::IS_ZST {
             // SAFETY: VecDeque::with_capacity ensures that there is enough capacity.
             unsafe {
                 ptr::copy_nonoverlapping(arr.as_ptr(), deq.ptr(), N);

library/alloc/src/lib.rs

@@ -136,6 +136,7 @@
 #![feature(receiver_trait)]
 #![feature(saturating_int_impl)]
 #![feature(set_ptr_value)]
+#![feature(sized_type_properties)]
 #![feature(slice_from_ptr_range)]
 #![feature(slice_group_by)]
 #![feature(slice_ptr_get)]

library/alloc/src/raw_vec.rs

@@ -3,7 +3,7 @@
 use core::alloc::LayoutError;
 use core::cmp;
 use core::intrinsics;
-use core::mem::{self, ManuallyDrop, MaybeUninit};
+use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
 use core::ops::Drop;
 use core::ptr::{self, NonNull, Unique};
 use core::slice;
@@ -168,7 +168,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     #[cfg(not(no_global_oom_handling))]
     fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
         // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
-        if mem::size_of::<T>() == 0 || capacity == 0 {
+        if T::IS_ZST || capacity == 0 {
             Self::new_in(alloc)
         } else {
             // We avoid `unwrap_or_else` here because it bloats the amount of
@@ -229,7 +229,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     /// This will always be `usize::MAX` if `T` is zero-sized.
     #[inline(always)]
     pub fn capacity(&self) -> usize {
-        if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap }
+        if T::IS_ZST { usize::MAX } else { self.cap }
     }
 
     /// Returns a shared reference to the allocator backing this `RawVec`.
@@ -238,7 +238,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     }
 
     fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
-        if mem::size_of::<T>() == 0 || self.cap == 0 {
+        if T::IS_ZST || self.cap == 0 {
             None
         } else {
             // We have an allocated chunk of memory, so we can bypass runtime
@@ -380,7 +380,7 @@ impl<T, A: Allocator> RawVec<T, A> {
         // This is ensured by the calling contexts.
         debug_assert!(additional > 0);
 
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // Since we return a capacity of `usize::MAX` when `elem_size` is
             // 0, getting to here necessarily means the `RawVec` is overfull.
             return Err(CapacityOverflow.into());
@@ -406,7 +406,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     // `grow_amortized`, but this method is usually instantiated less often so
     // it's less critical.
     fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // Since we return a capacity of `usize::MAX` when the type size is
             // 0, getting to here necessarily means the `RawVec` is overfull.
             return Err(CapacityOverflow.into());

library/alloc/src/slice.rs

@@ -16,9 +16,7 @@ use core::borrow::{Borrow, BorrowMut};
 #[cfg(not(no_global_oom_handling))]
 use core::cmp::Ordering::{self, Less};
 #[cfg(not(no_global_oom_handling))]
-use core::mem;
-#[cfg(not(no_global_oom_handling))]
-use core::mem::size_of;
+use core::mem::{self, SizedTypeProperties};
 #[cfg(not(no_global_oom_handling))]
 use core::ptr;
 
@@ -1018,7 +1016,7 @@ where
     const MIN_RUN: usize = 10;
 
     // Sorting has no meaningful behavior on zero-sized types.
-    if size_of::<T>() == 0 {
+    if T::IS_ZST {
         return;
     }
 

library/alloc/src/vec/drain.rs

@@ -1,7 +1,7 @@
 use crate::alloc::{Allocator, Global};
 use core::fmt;
 use core::iter::{FusedIterator, TrustedLen};
-use core::mem::{self, ManuallyDrop};
+use core::mem::{self, ManuallyDrop, SizedTypeProperties};
 use core::ptr::{self, NonNull};
 use core::slice::{self};
 
@@ -202,7 +202,7 @@ impl<T, A: Allocator> Drop for Drain<'_, T, A> {
 
         let mut vec = self.vec;
 
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // ZSTs have no identity, so we don't need to move them around, we only need to drop the correct amount.
             // this can be achieved by manipulating the Vec length instead of moving values out from `iter`.
             unsafe {

library/alloc/src/vec/in_place_collect.rs

@@ -135,7 +135,7 @@
 //! vec.truncate(write_idx);
 //! ```
 use core::iter::{InPlaceIterable, SourceIter, TrustedRandomAccessNoCoerce};
-use core::mem::{self, ManuallyDrop};
+use core::mem::{self, ManuallyDrop, SizedTypeProperties};
 use core::ptr::{self};
 
 use super::{InPlaceDrop, SpecFromIter, SpecFromIterNested, Vec};
@@ -154,7 +154,7 @@ where
     default fn from_iter(mut iterator: I) -> Self {
         // See "Layout constraints" section in the module documentation. We rely on const
         // optimization here since these conditions currently cannot be expressed as trait bounds
-        if mem::size_of::<T>() == 0
+        if T::IS_ZST
             || mem::size_of::<T>()
                 != mem::size_of::<<<I as SourceIter>::Source as AsVecIntoIter>::Item>()
             || mem::align_of::<T>()

library/alloc/src/vec/into_iter.rs

@@ -8,7 +8,7 @@ use core::iter::{
     FusedIterator, InPlaceIterable, SourceIter, TrustedLen, TrustedRandomAccessNoCoerce,
 };
 use core::marker::PhantomData;
-use core::mem::{self, ManuallyDrop, MaybeUninit};
+use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
 #[cfg(not(no_global_oom_handling))]
 use core::ops::Deref;
 use core::ptr::{self, NonNull};
@@ -149,7 +149,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     fn next(&mut self) -> Option<T> {
         if self.ptr == self.end {
             None
-        } else if mem::size_of::<T>() == 0 {
+        } else if T::IS_ZST {
             // purposefully don't use 'ptr.offset' because for
             // vectors with 0-size elements this would return the
             // same pointer.
@@ -167,7 +167,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let exact = if mem::size_of::<T>() == 0 {
+        let exact = if T::IS_ZST {
             self.end.addr().wrapping_sub(self.ptr.addr())
         } else {
             unsafe { self.end.sub_ptr(self.ptr) }
@@ -179,7 +179,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     fn advance_by(&mut self, n: usize) -> Result<(), usize> {
         let step_size = self.len().min(n);
         let to_drop = ptr::slice_from_raw_parts_mut(self.ptr as *mut T, step_size);
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // SAFETY: due to unchecked casts of unsigned amounts to signed offsets the wraparound
             // effectively results in unsigned pointers representing positions 0..usize::MAX,
             // which is valid for ZSTs.
@@ -209,7 +209,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
 
         let len = self.len();
 
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             if len < N {
                 self.forget_remaining_elements();
                 // Safety: ZSTs can be conjured ex nihilo, only the amount has to be correct
@@ -253,7 +253,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         // that `T: Copy` so reading elements from the buffer doesn't invalidate
         // them for `Drop`.
         unsafe {
-            if mem::size_of::<T>() == 0 { mem::zeroed() } else { ptr::read(self.ptr.add(i)) }
+            if T::IS_ZST { mem::zeroed() } else { ptr::read(self.ptr.add(i)) }
         }
     }
 }
@@ -264,7 +264,7 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
     fn next_back(&mut self) -> Option<T> {
         if self.end == self.ptr {
             None
-        } else if mem::size_of::<T>() == 0 {
+        } else if T::IS_ZST {
             // See above for why 'ptr.offset' isn't used
             self.end = self.end.wrapping_byte_sub(1);
 
@@ -280,7 +280,7 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
     #[inline]
     fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
         let step_size = self.len().min(n);
-        if mem::size_of::<T>() == 0 {
+        if T::IS_ZST {
             // SAFETY: same as for advance_by()
             self.end = self.end.wrapping_byte_sub(step_size);
         } else {

library/alloc/src/vec/mod.rs

@@ -64,7 +64,7 @@ use core::iter;
 #[cfg(not(no_global_oom_handling))]
 use core::iter::FromIterator;
 use core::marker::PhantomData;
-use core::mem::{self, ManuallyDrop, MaybeUninit};
+use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
 use core::ops::{self, Index, IndexMut, Range, RangeBounds};
 use core::ptr::{self, NonNull};
 use core::slice::{self, SliceIndex};
@@ -2347,7 +2347,7 @@ impl<T, A: Allocator, const N: usize> Vec<[T; N], A> {
     #[unstable(feature = "slice_flatten", issue = "95629")]
     pub fn into_flattened(self) -> Vec<T, A> {
         let (ptr, len, cap, alloc) = self.into_raw_parts_with_alloc();
-        let (new_len, new_cap) = if mem::size_of::<T>() == 0 {
+        let (new_len, new_cap) = if T::IS_ZST {
             (len.checked_mul(N).expect("vec len overflow"), usize::MAX)
         } else {
             // SAFETY:
@@ -2677,7 +2677,7 @@ impl<T, A: Allocator> IntoIterator for Vec<T, A> {
             let mut me = ManuallyDrop::new(self);
             let alloc = ManuallyDrop::new(ptr::read(me.allocator()));
             let begin = me.as_mut_ptr();
-            let end = if mem::size_of::<T>() == 0 {
+            let end = if T::IS_ZST {
                 begin.wrapping_byte_add(me.len())
             } else {
                 begin.add(me.len()) as *const T

library/core/src/mem/mod.rs

@@ -1178,3 +1178,18 @@ pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
 pub const fn variant_count<T>() -> usize {
     intrinsics::variant_count::<T>()
 }
+
+/// This is here only to simplify all the ZST checks we need in the library.
+/// It's not on a stabilization track right now.
+#[doc(hidden)]
+#[unstable(feature = "sized_type_properties", issue = "none")]
+pub trait SizedTypeProperties: Sized {
+    /// `true` if this type requires no storage.
+    /// `false` if its [size](size_of) is greater than zero.
+    #[doc(hidden)]
+    #[unstable(feature = "sized_type_properties", issue = "none")]
+    const IS_ZST: bool = size_of::<Self>() == 0;
+}
+#[doc(hidden)]
+#[unstable(feature = "sized_type_properties", issue = "none")]
+impl<T> SizedTypeProperties for T {}

library/core/src/slice/iter.rs

@@ -9,7 +9,7 @@ use crate::fmt;
 use crate::intrinsics::{assume, exact_div, unchecked_sub};
 use crate::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
 use crate::marker::{PhantomData, Send, Sized, Sync};
-use crate::mem;
+use crate::mem::{self, SizedTypeProperties};
 use crate::num::NonZeroUsize;
 use crate::ptr::NonNull;
 
@@ -91,7 +91,7 @@ impl<'a, T> Iter<'a, T> {
         unsafe {
             assume(!ptr.is_null());
 
-            let end = if mem::size_of::<T>() == 0 {
+            let end = if T::IS_ZST {
                 ptr.wrapping_byte_add(slice.len())
             } else {
                 ptr.add(slice.len())
@@ -227,7 +227,7 @@ impl<'a, T> IterMut<'a, T> {
         unsafe {
             assume(!ptr.is_null());
 
-            let end = if mem::size_of::<T>() == 0 {
+            let end = if T::IS_ZST {
                 ptr.wrapping_byte_add(slice.len())
             } else {
                 ptr.add(slice.len())