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(err: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}
368