core/ptr/
alignment.rs

1#![allow(clippy::enum_clike_unportable_variant)]
2
3use crate::num::NonZero;
4use crate::ub_checks::assert_unsafe_precondition;
5use crate::{cmp, fmt, hash, mem, num};
6
7/// A type storing a `usize` which is a power of two, and thus
8/// represents a possible alignment in the Rust abstract machine.
9///
10/// Note that particularly large alignments, while representable in this type,
11/// are likely not to be supported by actual allocators and linkers.
12#[unstable(feature = "ptr_alignment_type", issue = "102070")]
13#[derive(Copy, Clone, PartialEq, Eq)]
14#[repr(transparent)]
15pub struct Alignment(AlignmentEnum);
16
17// Alignment is `repr(usize)`, but via extra steps.
18const _: () = assert!(size_of::<Alignment>() == size_of::<usize>());
19const _: () = assert!(align_of::<Alignment>() == align_of::<usize>());
20
21fn _alignment_can_be_structurally_matched(a: Alignment) -> bool {
22    matches!(a, Alignment::MIN)
23}
24
25impl Alignment {
26    /// The smallest possible alignment, 1.
27    ///
28    /// All addresses are always aligned at least this much.
29    ///
30    /// # Examples
31    ///
32    /// ```
33    /// #![feature(ptr_alignment_type)]
34    /// use std::ptr::Alignment;
35    ///
36    /// assert_eq!(Alignment::MIN.as_usize(), 1);
37    /// ```
38    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
39    pub const MIN: Self = Self(AlignmentEnum::_Align1Shl0);
40
41    /// Returns the alignment for a type.
42    ///
43    /// This provides the same numerical value as [`align_of`],
44    /// but in an `Alignment` instead of a `usize`.
45    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
46    #[inline]
47    #[must_use]
48    pub const fn of<T>() -> Self {
49        // This can't actually panic since type alignment is always a power of two.
50        const { Alignment::new(align_of::<T>()).unwrap() }
51    }
52
53    /// Creates an `Alignment` from a `usize`, or returns `None` if it's
54    /// not a power of two.
55    ///
56    /// Note that `0` is not a power of two, nor a valid alignment.
57    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
58    #[inline]
59    pub const fn new(align: usize) -> Option<Self> {
60        if align.is_power_of_two() {
61            // SAFETY: Just checked it only has one bit set
62            Some(unsafe { Self::new_unchecked(align) })
63        } else {
64            None
65        }
66    }
67
68    /// Creates an `Alignment` from a power-of-two `usize`.
69    ///
70    /// # Safety
71    ///
72    /// `align` must be a power of two.
73    ///
74    /// Equivalently, it must be `1 << exp` for some `exp` in `0..usize::BITS`.
75    /// It must *not* be zero.
76    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
77    #[inline]
78    #[track_caller]
79    pub const unsafe fn new_unchecked(align: usize) -> Self {
80        assert_unsafe_precondition!(
81            check_language_ub,
82            "Alignment::new_unchecked requires a power of two",
83            (align: usize = align) => align.is_power_of_two()
84        );
85
86        // SAFETY: By precondition, this must be a power of two, and
87        // our variants encompass all possible powers of two.
88        unsafe { mem::transmute::<usize, Alignment>(align) }
89    }
90
91    /// Returns the alignment as a [`usize`].
92    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
93    #[inline]
94    pub const fn as_usize(self) -> usize {
95        self.0 as usize
96    }
97
98    /// Returns the alignment as a <code>[NonZero]<[usize]></code>.
99    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
100    #[inline]
101    pub const fn as_nonzero(self) -> NonZero<usize> {
102        // This transmutes directly to avoid the UbCheck in `NonZero::new_unchecked`
103        // since there's no way for the user to trip that check anyway -- the
104        // validity invariant of the type would have to have been broken earlier --
105        // and emitting it in an otherwise simple method is bad for compile time.
106
107        // SAFETY: All the discriminants are non-zero.
108        unsafe { mem::transmute::<Alignment, NonZero<usize>>(self) }
109    }
110
111    /// Returns the base-2 logarithm of the alignment.
112    ///
113    /// This is always exact, as `self` represents a power of two.
114    ///
115    /// # Examples
116    ///
117    /// ```
118    /// #![feature(ptr_alignment_type)]
119    /// use std::ptr::Alignment;
120    ///
121    /// assert_eq!(Alignment::of::<u8>().log2(), 0);
122    /// assert_eq!(Alignment::new(1024).unwrap().log2(), 10);
123    /// ```
124    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
125    #[inline]
126    pub const fn log2(self) -> u32 {
127        self.as_nonzero().trailing_zeros()
128    }
129
130    /// Returns a bit mask that can be used to match this alignment.
131    ///
132    /// This is equivalent to `!(self.as_usize() - 1)`.
133    ///
134    /// # Examples
135    ///
136    /// ```
137    /// #![feature(ptr_alignment_type)]
138    /// #![feature(ptr_mask)]
139    /// use std::ptr::{Alignment, NonNull};
140    ///
141    /// #[repr(align(1))] struct Align1(u8);
142    /// #[repr(align(2))] struct Align2(u16);
143    /// #[repr(align(4))] struct Align4(u32);
144    /// let one = <NonNull<Align1>>::dangling().as_ptr();
145    /// let two = <NonNull<Align2>>::dangling().as_ptr();
146    /// let four = <NonNull<Align4>>::dangling().as_ptr();
147    ///
148    /// assert_eq!(four.mask(Alignment::of::<Align1>().mask()), four);
149    /// assert_eq!(four.mask(Alignment::of::<Align2>().mask()), four);
150    /// assert_eq!(four.mask(Alignment::of::<Align4>().mask()), four);
151    /// assert_ne!(one.mask(Alignment::of::<Align4>().mask()), one);
152    /// ```
153    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
154    #[inline]
155    pub const fn mask(self) -> usize {
156        // SAFETY: The alignment is always nonzero, and therefore decrementing won't overflow.
157        !(unsafe { self.as_usize().unchecked_sub(1) })
158    }
159
160    // FIXME(const-hack) Remove me once `Ord::max` is usable in const
161    pub(crate) const fn max(a: Self, b: Self) -> Self {
162        if a.as_usize() > b.as_usize() { a } else { b }
163    }
164}
165
166#[unstable(feature = "ptr_alignment_type", issue = "102070")]
167impl fmt::Debug for Alignment {
168    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
169        write!(f, "{:?} (1 << {:?})", self.as_nonzero(), self.log2())
170    }
171}
172
173#[unstable(feature = "ptr_alignment_type", issue = "102070")]
174#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
175impl const TryFrom<NonZero<usize>> for Alignment {
176    type Error = num::TryFromIntError;
177
178    #[inline]
179    fn try_from(align: NonZero<usize>) -> Result<Alignment, Self::Error> {
180        align.get().try_into()
181    }
182}
183
184#[unstable(feature = "ptr_alignment_type", issue = "102070")]
185#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
186impl const TryFrom<usize> for Alignment {
187    type Error = num::TryFromIntError;
188
189    #[inline]
190    fn try_from(align: usize) -> Result<Alignment, Self::Error> {
191        Self::new(align).ok_or(num::TryFromIntError(()))
192    }
193}
194
195#[unstable(feature = "ptr_alignment_type", issue = "102070")]
196#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
197impl const From<Alignment> for NonZero<usize> {
198    #[inline]
199    fn from(align: Alignment) -> NonZero<usize> {
200        align.as_nonzero()
201    }
202}
203
204#[unstable(feature = "ptr_alignment_type", issue = "102070")]
205#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
206impl const From<Alignment> for usize {
207    #[inline]
208    fn from(align: Alignment) -> usize {
209        align.as_usize()
210    }
211}
212
213#[unstable(feature = "ptr_alignment_type", issue = "102070")]
214impl cmp::Ord for Alignment {
215    #[inline]
216    fn cmp(&self, other: &Self) -> cmp::Ordering {
217        self.as_nonzero().get().cmp(&other.as_nonzero().get())
218    }
219}
220
221#[unstable(feature = "ptr_alignment_type", issue = "102070")]
222impl cmp::PartialOrd for Alignment {
223    #[inline]
224    fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
225        Some(self.cmp(other))
226    }
227}
228
229#[unstable(feature = "ptr_alignment_type", issue = "102070")]
230impl hash::Hash for
core/ptr/alignment.rs

core/ptr/
alignment.rs
