core/ptr/
alignment.rs

1use crate::num::NonZero;
2use crate::ub_checks::assert_unsafe_precondition;
3use crate::{cmp, fmt, hash, mem, num};
4
5/// A type storing a `usize` which is a power of two, and thus
6/// represents a possible alignment in the Rust abstract machine.
7///
8/// Note that particularly large alignments, while representable in this type,
9/// are likely not to be supported by actual allocators and linkers.
10#[unstable(feature = "ptr_alignment_type", issue = "102070")]
11#[derive(Copy, Clone, PartialEq, Eq)]
12#[repr(transparent)]
13pub struct Alignment(AlignmentEnum);
14
15// Alignment is `repr(usize)`, but via extra steps.
16const _: () = assert!(size_of::<Alignment>() == size_of::<usize>());
17const _: () = assert!(align_of::<Alignment>() == align_of::<usize>());
18
19fn _alignment_can_be_structurally_matched(a: Alignment) -> bool {
20    matches!(a, Alignment::MIN)
21}
22
23impl Alignment {
24    /// The smallest possible alignment, 1.
25    ///
26    /// All addresses are always aligned at least this much.
27    ///
28    /// # Examples
29    ///
30    /// ```
31    /// #![feature(ptr_alignment_type)]
32    /// use std::ptr::Alignment;
33    ///
34    /// assert_eq!(Alignment::MIN.as_usize(), 1);
35    /// ```
36    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
37    pub const MIN: Self = Self(AlignmentEnum::_Align1Shl0);
38
39    /// Returns the alignment for a type.
40    ///
41    /// This provides the same numerical value as [`align_of`],
42    /// but in an `Alignment` instead of a `usize`.
43    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
44    #[inline]
45    #[must_use]
46    pub const fn of<T>() -> Self {
47        // This can't actually panic since type alignment is always a power of two.
48        const { Alignment::new(align_of::<T>()).unwrap() }
49    }
50
51    /// Creates an `Alignment` from a `usize`, or returns `None` if it's
52    /// not a power of two.
53    ///
54    /// Note that `0` is not a power of two, nor a valid alignment.
55    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
56    #[inline]
57    pub const fn new(align: usize) -> Option<Self> {
58        if align.is_power_of_two() {
59            // SAFETY: Just checked it only has one bit set
60            Some(unsafe { Self::new_unchecked(align) })
61        } else {
62            None
63        }
64    }
65
66    /// Creates an `Alignment` from a power-of-two `usize`.
67    ///
68    /// # Safety
69    ///
70    /// `align` must be a power of two.
71    ///
72    /// Equivalently, it must be `1 << exp` for some `exp` in `0..usize::BITS`.
73    /// It must *not* be zero.
74    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
75    #[inline]
76    #[track_caller]
77    pub const unsafe fn new_unchecked(align: usize) -> Self {
78        assert_unsafe_precondition!(
79            check_language_ub,
80            "Alignment::new_unchecked requires a power of two",
81            (align: usize = align) => align.is_power_of_two()
82        );
83
84        // SAFETY: By precondition, this must be a power of two, and
85        // our variants encompass all possible powers of two.
86        unsafe { mem::transmute::<usize, Alignment>(align) }
87    }
88
89    /// Returns the alignment as a [`usize`].
90    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
91    #[inline]
92    pub const fn as_usize(self) -> usize {
93        self.0 as usize
94    }
95
96    /// Returns the alignment as a <code>[NonZero]<[usize]></code>.
97    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
98    #[inline]
99    pub const fn as_nonzero(self) -> NonZero<usize> {
100        // This transmutes directly to avoid the UbCheck in `NonZero::new_unchecked`
101        // since there's no way for the user to trip that check anyway -- the
102        // validity invariant of the type would have to have been broken earlier --
103        // and emitting it in an otherwise simple method is bad for compile time.
104
105        // SAFETY: All the discriminants are non-zero.
106        unsafe { mem::transmute::<Alignment, NonZero<usize>>(self) }
107    }
108
109    /// Returns the base-2 logarithm of the alignment.
110    ///
111    /// This is always exact, as `self` represents a power of two.
112    ///
113    /// # Examples
114    ///
115    /// ```
116    /// #![feature(ptr_alignment_type)]
117    /// use std::ptr::Alignment;
118    ///
119    /// assert_eq!(Alignment::of::<u8>().log2(), 0);
120    /// assert_eq!(Alignment::new(1024).unwrap().log2(), 10);
121    /// ```
122    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
123    #[inline]
124    pub const fn log2(self) -> u32 {
125        self.as_nonzero().trailing_zeros()
126    }
127
128    /// Returns a bit mask that can be used to match this alignment.
129    ///
130    /// This is equivalent to `!(self.as_usize() - 1)`.
131    ///
132    /// # Examples
133    ///
134    /// ```
135    /// #![feature(ptr_alignment_type)]
136    /// #![feature(ptr_mask)]
137    /// use std::ptr::{Alignment, NonNull};
138    ///
139    /// #[repr(align(1))] struct Align1(u8);
140    /// #[repr(align(2))] struct Align2(u16);
141    /// #[repr(align(4))] struct Align4(u32);
142    /// let one = <NonNull<Align1>>::dangling().as_ptr();
143    /// let two = <NonNull<Align2>>::dangling().as_ptr();
144    /// let four = <NonNull<Align4>>::dangling().as_ptr();
145    ///
146    /// assert_eq!(four.mask(Alignment::of::<Align1>().mask()), four);
147    /// assert_eq!(four.mask(Alignment::of::<Align2>().mask()), four);
148    /// assert_eq!(four.mask(Alignment::of::<Align4>().mask()), four);
149    /// assert_ne!(one.mask(Alignment::of::<Align4>().mask()), one);
150    /// ```
151    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
152    #[inline]
153    pub const fn mask(self) -> usize {
154        // SAFETY: The alignment is always nonzero, and therefore decrementing won't overflow.
155        !(unsafe { self.as_usize().unchecked_sub(1) })
156    }
157
158    // FIXME(const-hack) Remove me once `Ord::max` is usable in const
159    pub(crate) const fn max(a: Self, b: Self) -> Self {
160        if a.as_usize() > b.as_usize() { a } else { b }
161    }
162}
163
164#[unstable(feature = "ptr_alignment_type", issue = "102070")]
165impl fmt::Debug for Alignment {
166    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
167        write!(f, "{:?} (1 << {:?})", self.as_nonzero(), self.log2())
168    }
169}
170
171#[unstable(feature = "ptr_alignment_type", issue = "102070")]
172impl TryFrom<NonZero<usize>> for Alignment {
173    type Error = num::TryFromIntError;
174
175    #[inline]
176    fn try_from(align: NonZero<usize>) -> Result<Alignment, Self::Error> {
177        align.get().try_into()
178    }
179}
180
181#[unstable(feature = "ptr_alignment_type", issue = "102070")]
182impl TryFrom<usize> for Alignment {
183    type Error = num::TryFromIntError;
184
185    #[inline]
186    fn try_from(align: usize) -> Result<Alignment, Self::Error> {
187        Self::new(align).ok_or(num::TryFromIntError(()))
188    }
189}
190
191#[unstable(feature = "ptr_alignment_type", issue = "102070")]
192impl From<Alignment> for NonZero<usize> {
193    #[inline]
194    fn from(align: Alignment) -> NonZero<usize> {
195        align.as_nonzero()
196    }
197}
198
199#[unstable(feature = "ptr_alignment_type", issue = "102070")]
200impl From<Alignment> for usize {
201    #[inline]
202    fn from(align: Alignment) -> usize {
203        align.as_usize()
204    }
205}
206
207#[unstable(feature = "ptr_alignment_type", issue = "102070")]
208impl cmp::Ord for Alignment {
209    #[inline]
210    fn cmp(&self, other: &Self) -> cmp::Ordering {
211        self.as_nonzero().get().cmp(&other.as_nonzero().get())
212    }
213}
214
215#[unstable(feature = "ptr_alignment_type", issue = "102070")]
216impl cmp::PartialOrd for Alignment {
217    #[inline]
218    fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
219        Some(self.cmp(other))
220    }
221}
222
223#[unstable(feature = "ptr_alignment_type", issue = "102070")]
224impl hash::Hash for Alignment {
225    #[inline]
226    fn hash<H: hash::Hasher>(&self, state: &mut H) {
227        self.as_nonzero().hash(state)
228    }
229}
230
231/// Returns [`Alignment::MIN`], which is valid for any type.
232#[unstable(feature = "ptr_alignment_type", issue = "102070")]
233impl Default for Alignment {
234    fn default() -> Alignment {
235        Alignment::MIN
236    }
237}
238
239#[cfg(target_pointer_width = "16")]
240#[derive(Copy, Clone, PartialEq, Eq)]
241#[repr(u16)]
242enum AlignmentEnum {
243    _Align1Shl0 = 1 << 0,
244    _Align1Shl1 = 1 << 1,
245    _Align1Shl2 = 1 << 2,
246    _Align1Shl3 = 1 << 3,
247    _Align1Shl4 = 1 << 4,
248    _Align1Shl5 = 1 << 5,
249    _Align1Shl6 = 1 << 6,
250    _Align1Shl7 = 1 << 7,
251    _Align1Shl8 = 1 << 8,
252    _Align1Shl9 = 1 << 9,
253    _Align1Shl10 = 1 << 10,
254    _Align1Shl11 = 1 << 11,
255    _Align1Shl12 = 1 << 12,
256    _Align1Shl13 = 1 << 13,
257    _Align1Shl14 = 1 << 14,
258    _Align1Shl15 = 1 << 15,
259}
260
261#[cfg(target_pointer_width = "32")]
262#[derive(Copy, Clone, PartialEq, Eq)]
263#[repr(u32)]
264enum AlignmentEnum {
265    _Align1Shl0 = 1 << 0,
266    _Align1Shl1 = 1 << 1,
267    _Align1Shl2 = 1 << 2,
268    _Align1Shl3 = 1 << 3,
269    _Align1Shl4 = 1 << 4,
270    _Align1Shl5 = 1 << 5,
271    _Align1Shl6 = 1 << 6,
272    _Align1Shl7 = 1 << 7,
273    _Align1Shl8 = 1 << 8,
274    _Align1Shl9 = 1 << 9,
275    _Align1Shl10 = 1 << 10,
276    _Align1Shl11 = 1 << 11,
277    _Align1Shl12 = 1 << 12,
278    _Align1Shl13 = 1 << 13,
279    _Align1Shl14 = 1 << 14,
280    _Align1Shl15 = 1 << 15,
281    _Align1Shl16 = 1 << 16,
282    _Align1Shl17 = 1 << 17,
283    _Align1Shl18 = 1 << 18,
284    _Align1Shl19 = 1 << 19,
285    _Align1Shl20 = 1 << 20,
286    _Align1Shl21 = 1 << 21,
287    _Align1Shl22 = 1 << 22,
288    _Align1Shl23 = 1 << 23,
289    _Align1Shl24 = 1 << 24,
290    _Align1Shl25 = 1 << 25,
291    _Align1Shl26 = 1 << 26,
292    _Align1Shl27 = 1 << 27,
293    _Align1Shl28 = 1 << 28,
294    _Align1Shl29 = 1 << 29,
295    _Align1Shl30 = 1 << 30,
296    _Align1Shl31 = 1 << 31,
297}
298
299#[cfg(target_pointer_width = "64")]
300#[derive(Copy, Clone, PartialEq, Eq)]
301#[repr(u64)]
302enum AlignmentEnum {
303    _Align1Shl0 = 1 << 0,
304    _Align1Shl1 = 1 << 1,
305    _Align1Shl2 = 1 << 2,
306    _Align1Shl3 = 1 << 3,
307    _Align1Shl4 = 1 << 4,
308    _Align1Shl5 = 1 << 5,
309    _Align1Shl6 = 1 << 6,
310    _Align1Shl7 = 1 << 7,
311    _Align1Shl8 = 1 << 8,
312    _Align1Shl9 = 1 << 9,
313    _Align1Shl10 = 1 << 10,
314    _Align1Shl11 = 1 << 11,
315    _Align1Shl12 = 1 << 12,
316    _Align1Shl13 = 1 << 13,
317    _Align1Shl14 = 1 << 14,
318    _Align1Shl15 = 1 << 15,
319    _Align1Shl16 = 1 << 16,
320    _Align1Shl17 = 1 << 17,
321    _Align1Shl18 = 1 << 18,
322    _Align1Shl19 = 1 << 19,
323    _Align1Shl20 = 1 << 20,
324    _Align1Shl21 = 1 << 21,
325    _Align1Shl22 = 1 << 22,
326    _Align1Shl23 = 1 << 23,
327    _Align1Shl24 = 1 << 24,
328    _Align1Shl25 = 1 << 25,
329    _Align1Shl26 = 1 << 26,
330    _Align1Shl27 = 1 << 27,
331    _Align1Shl28 = 1 << 28,
332    _Align1Shl29 = 1 << 29,
333    _Align1Shl30 = 1 << 30,
334    _Align1Shl31 = 1 << 31,
335    _Align1Shl32 = 1 << 32,
336    _Align1Shl33 = 1 << 33,
337    _Align1Shl34 = 1 << 34,
338    _Align1Shl35 = 1 << 35,
339    _Align1Shl36 = 1 << 36,
340    _Align1Shl37 = 1 << 37,
341    _Align1Shl38 = 1 << 38,
342    _Align1Shl39 = 1 << 39,
343    _Align1Shl40 = 1 << 40,
344    _Align1Shl41 = 1 << 41,
345    _Align1Shl42 = 1 << 42,
346    _Align1Shl43 = 1 << 43,
347    _Align1Shl44 = 1 << 44,
348    _Align1Shl45 = 1 << 45,
349    _Align1Shl46 = 1 << 46,
350    _Align1Shl47 = 1 << 47,
351    _Align1Shl48 = 1 << 48,
352    _Align1Shl49 = 1 << 49,
353    _Align1Shl50 = 1 << 50,
354    _Align1Shl51 = 1 << 51,
355    _Align1Shl52 = 1 << 52,
356    _Align1Shl53 = 1 << 53,
357    _Align1Shl54 = 1 << 54,
358    _Align1Shl55 = 1 << 55,
359    _Align1Shl56 = 1 << 56,
360    _Align1Shl57 = 1 << 57,
361    _Align1Shl58 = 1 << 58,
362    _Align1Shl59 = 1 << 59,
363    _Align1Shl60 = 1 << 60,
364    _Align1Shl61 = 1 << 61,
365    _Align1Shl62 = 1 << 62,
366    _Align1Shl63 = 1 << 63,
367}
core/ptr/alignment.rs

core/ptr/
alignment.rs
