Add `ptr::Alignment` type

Essentially no new code here, just exposing the previously-`pub(crate)` `ValidAlign` type under the name from the ACP.

1 files changed, 295 insertions, 0 deletions

diff --git a/library/core/src/ptr/alignment.rs b/library/core/src/ptr/alignment.rs
new file mode 100644
index 00000000000..5e7628c5a18
--- /dev/null
+++ b/library/core/src/ptr/alignment.rs
@@ -0,0 +1,295 @@
+use crate::convert::{TryFrom, TryInto};
+use crate::intrinsics::assert_unsafe_precondition;
+use crate::num::NonZeroUsize;
+use crate::{cmp, fmt, hash, mem, num};
+
+/// A type storing a `usize` which is a power of two, and thus
+/// represents a possible alignment in the rust abstract machine.
+///
+/// Note that particularly large alignments, while representable in this type,
+/// are likely not to be supported by actual allocators and linkers.
+#[unstable(feature = "ptr_alignment_type", issue = "102070")]
+#[derive(Copy, Clone)]
+#[repr(transparent)]
+pub struct Alignment(AlignmentEnum);
+
+// Alignment is `repr(usize)`, but via extra steps.
+const _: () = assert!(mem::size_of::<Alignment>() == mem::size_of::<usize>());
+const _: () = assert!(mem::align_of::<Alignment>() == mem::align_of::<usize>());
+
+impl Alignment {
+    /// Returns the alignment for a type.
+    ///
+    /// This provides the same numerical value as [`mem::align_of`],
+    /// but in an `Alignment` instead of a `usize.
+    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
+    #[inline]
+    pub(crate) fn of<T>() -> Self {
+        // SAFETY: rustc ensures that type alignment is always a power of two.
+        unsafe { Alignment::new_unchecked(mem::align_of::<T>()) }
+    }
+
+    /// Creates an `Alignment` from a `usize`, or returns `None` if it's
+    /// not a power of two.
+    ///
+    /// Note that `0` is not a power of two, nor a valid alignment.
+    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
+    #[inline]
+    pub const fn new(align: usize) -> Option<Self> {
+        if align.is_power_of_two() {
+            // SAFETY: Just checked it only has one bit set
+            Some(unsafe { Self::new_unchecked(align) })
+        } else {
+            None
+        }
+    }
+
+    /// Creates an `Alignment` from a power-of-two `usize`.
+    ///
+    /// # Safety
+    ///
+    /// `align` must be a power of two.
+    ///
+    /// Equivalently, it must be `1 << exp` for some `exp` in `0..usize::BITS`.
+    /// It must *not* be zero.
+    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
+    #[inline]
+    pub const unsafe fn new_unchecked(align: usize) -> Self {
+        // SAFETY: Precondition passed to the caller.
+        unsafe { assert_unsafe_precondition!((align: usize) => align.is_power_of_two()) };
+
+        // SAFETY: By precondition, this must be a power of two, and
+        // our variants encompass all possible powers of two.
+        unsafe { mem::transmute::<usize, Alignment>(align) }
+    }
+
+    /// Returns the alignment as a [`NonZeroUsize`]
+    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
+    #[inline]
+    pub const fn as_usize(self) -> usize {
+        self.0 as usize
+    }
+
+    /// Returns the alignment as a [`usize`]
+    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
+    #[inline]
+    pub const fn as_nonzero(self) -> NonZeroUsize {
+        // SAFETY: All the discriminants are non-zero.
+        unsafe { NonZeroUsize::new_unchecked(self.as_usize()) }
+    }
+
+    /// Returns the base-2 logarithm of the alignment.
+    ///
+    /// This is always exact, as `self` represents a power of two.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(ptr_alignment_type)]
+    /// use std::ptr::Alignment;
+    ///
+    /// assert_eq!(Alignment::of::<u8>().log2(), 0);
+    /// assert_eq!(Alignment::new(1024).unwrap().log2(), 10);
+    /// ```
+    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
+    #[inline]
+    pub fn log2(self) -> u32 {
+        self.as_nonzero().trailing_zeros()
+    }
+}
+
+#[unstable(feature = "ptr_alignment_type", issue = "102070")]
+impl fmt::Debug for Alignment {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:?} (1 << {:?})", self.as_nonzero(), self.log2())
+    }
+}
+
+#[unstable(feature = "ptr_alignment_type", issue = "102070")]
+impl TryFrom<NonZeroUsize> for Alignment {
+    type Error = num::TryFromIntError;
+
+    #[inline]
+    fn try_from(align: NonZeroUsize) -> Result<Alignment, Self::Error> {
+        align.get().try_into()
+    }
+}
+
+#[unstable(feature = "ptr_alignment_type", issue = "102070")]
+impl TryFrom<usize> for Alignment {
+    type Error = num::TryFromIntError;
+
+    #[inline]
+    fn try_from(align: usize) -> Result<Alignment, Self::Error> {
+        Self::new(align).ok_or(num::TryFromIntError(()))
+    }
+}
+
+#[unstable(feature = "ptr_alignment_type", issue = "102070")]
+impl cmp::Eq for Alignment {}
+
+#[unstable(feature = "ptr_alignment_type", issue = "102070")]
+impl cmp::PartialEq for Alignment {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        self.as_nonzero() == other.as_nonzero()
+    }
+}
+
+#[unstable(feature = "ptr_alignment_type", issue = "102070")]
+impl cmp::Ord for Alignment {
+    #[inline]
+    fn cmp(&self, other: &Self) -> cmp::Ordering {
+        self.as_nonzero().cmp(&other.as_nonzero())
+    }
+}
+
+#[unstable(feature = "ptr_alignment_type", issue = "102070")]
+impl cmp::PartialOrd for Alignment {
+    #[inline]
+    fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+#[unstable(feature = "ptr_alignment_type", issue = "102070")]
+impl hash::Hash for Alignment {
+    #[inline]
+    fn hash<H: hash::Hasher>(&self, state: &mut H) {
+        self.as_nonzero().hash(state)
+    }
+}
+
+#[cfg(target_pointer_width = "16")]
+type AlignmentEnum = AlignmentEnum16;
+#[cfg(target_pointer_width = "32")]
+type AlignmentEnum = AlignmentEnum32;
+#[cfg(target_pointer_width = "64")]
+type AlignmentEnum = AlignmentEnum64;
+
+#[derive(Copy, Clone)]
+#[repr(u16)]
+enum AlignmentEnum16 {
+    _Align1Shl0 = 1 << 0,
+    _Align1Shl1 = 1 << 1,
+    _Align1Shl2 = 1 << 2,
+    _Align1Shl3 = 1 << 3,
+    _Align1Shl4 = 1 << 4,
+    _Align1Shl5 = 1 << 5,
+    _Align1Shl6 = 1 << 6,
+    _Align1Shl7 = 1 << 7,
+    _Align1Shl8 = 1 << 8,
+    _Align1Shl9 = 1 << 9,
+    _Align1Shl10 = 1 << 10,
+    _Align1Shl11 = 1 << 11,
+    _Align1Shl12 = 1 << 12,
+    _Align1Shl13 = 1 << 13,
+    _Align1Shl14 = 1 << 14,
+    _Align1Shl15 = 1 << 15,
+}
+
+#[derive(Copy, Clone)]
+#[repr(u32)]
+enum AlignmentEnum32 {
+    _Align1Shl0 = 1 << 0,
+    _Align1Shl1 = 1 << 1,
+    _Align1Shl2 = 1 << 2,
+    _Align1Shl3 = 1 << 3,
+    _Align1Shl4 = 1 << 4,
+    _Align1Shl5 = 1 << 5,
+    _Align1Shl6 = 1 << 6,
+    _Align1Shl7 = 1 << 7,
+    _Align1Shl8 = 1 << 8,
+    _Align1Shl9 = 1 << 9,
+    _Align1Shl10 = 1 << 10,
+    _Align1Shl11 = 1 << 11,
+    _Align1Shl12 = 1 << 12,
+    _Align1Shl13 = 1 << 13,
+    _Align1Shl14 = 1 << 14,
+    _Align1Shl15 = 1 << 15,
+    _Align1Shl16 = 1 << 16,
+    _Align1Shl17 = 1 << 17,
+    _Align1Shl18 = 1 << 18,
+    _Align1Shl19 = 1 << 19,
+    _Align1Shl20 = 1 << 20,
+    _Align1Shl21 = 1 << 21,
+    _Align1Shl22 = 1 << 22,
+    _Align1Shl23 = 1 << 23,
+    _Align1Shl24 = 1 << 24,
+    _Align1Shl25 = 1 << 25,
+    _Align1Shl26 = 1 << 26,
+    _Align1Shl27 = 1 << 27,
+    _Align1Shl28 = 1 << 28,
+    _Align1Shl29 = 1 << 29,
+    _Align1Shl30 = 1 << 30,
+    _Align1Shl31 = 1 << 31,
+}
+
+#[derive(Copy, Clone)]
+#[repr(u64)]
+enum AlignmentEnum64 {
+    _Align1Shl0 = 1 << 0,
+    _Align1Shl1 = 1 << 1,
+    _Align1Shl2 = 1 << 2,
+    _Align1Shl3 = 1 << 3,
+    _Align1Shl4 = 1 << 4,
+    _Align1Shl5 = 1 << 5,
+    _Align1Shl6 = 1 << 6,
+    _Align1Shl7 = 1 << 7,
+    _Align1Shl8 = 1 << 8,
+    _Align1Shl9 = 1 << 9,
+    _Align1Shl10 = 1 << 10,
+    _Align1Shl11 = 1 << 11,
+    _Align1Shl12 = 1 << 12,
+    _Align1Shl13 = 1 << 13,
+    _Align1Shl14 = 1 << 14,
+    _Align1Shl15 = 1 << 15,
+    _Align1Shl16 = 1 << 16,
+    _Align1Shl17 = 1 << 17,
+    _Align1Shl18 = 1 << 18,
+    _Align1Shl19 = 1 << 19,
+    _Align1Shl20 = 1 << 20,
+    _Align1Shl21 = 1 << 21,
+    _Align1Shl22 = 1 << 22,
+    _Align1Shl23 = 1 << 23,
+    _Align1Shl24 = 1 << 24,
+    _Align1Shl25 = 1 << 25,
+    _Align1Shl26 = 1 << 26,
+    _Align1Shl27 = 1 << 27,
+    _Align1Shl28 = 1 << 28,
+    _Align1Shl29 = 1 << 29,
+    _Align1Shl30 = 1 << 30,
+    _Align1Shl31 = 1 << 31,
+    _Align1Shl32 = 1 << 32,
+    _Align1Shl33 = 1 << 33,
+    _Align1Shl34 = 1 << 34,
+    _Align1Shl35 = 1 << 35,
+    _Align1Shl36 = 1 << 36,
+    _Align1Shl37 = 1 << 37,
+    _Align1Shl38 = 1 << 38,
+    _Align1Shl39 = 1 << 39,
+    _Align1Shl40 = 1 << 40,
+    _Align1Shl41 = 1 << 41,
+    _Align1Shl42 = 1 << 42,
+    _Align1Shl43 = 1 << 43,
+    _Align1Shl44 = 1 << 44,
+    _Align1Shl45 = 1 << 45,
+    _Align1Shl46 = 1 << 46,
+    _Align1Shl47 = 1 << 47,
+    _Align1Shl48 = 1 << 48,
+    _Align1Shl49 = 1 << 49,
+    _Align1Shl50 = 1 << 50,
+    _Align1Shl51 = 1 << 51,
+    _Align1Shl52 = 1 << 52,
+    _Align1Shl53 = 1 << 53,
+    _Align1Shl54 = 1 << 54,
+    _Align1Shl55 = 1 << 55,
+    _Align1Shl56 = 1 << 56,
+    _Align1Shl57 = 1 << 57,
+    _Align1Shl58 = 1 << 58,
+    _Align1Shl59 = 1 << 59,
+    _Align1Shl60 = 1 << 60,
+    _Align1Shl61 = 1 << 61,
+    _Align1Shl62 = 1 << 62,
+    _Align1Shl63 = 1 << 63,
+}