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 Alignment {
231    #[inline]
232    fn hash<H: hash::Hasher>(&self, state: &mut H) {
233        self.as_nonzero().hash(state)
234    }
235}
236
237/// Returns [`Alignment::MIN`], which is valid for any type.
238#[unstable(feature = "ptr_alignment_type", issue = "102070")]
239#[rustc_const_unstable(feature = "const_default", issue = "143894")]
240impl const Default for Alignment {
241    fn default() -> Alignment {
242        Alignment::MIN
243    }
244}
245
246#[cfg(target_pointer_width = "16")]
247#[derive(Copy, Clone, PartialEq, Eq)]
248#[repr(usize)]
249enum AlignmentEnum {
250    _Align1Shl0 = 1 << 0,
251    _Align1Shl1 = 1 << 1,
252    _Align1Shl2 = 1 << 2,
253    _Align1Shl3 = 1 << 3,
254    _Align1Shl4 = 1 << 4,
255    _Align1Shl5 = 1 << 5,
256    _Align1Shl6 = 1 << 6,
257    _Align1Shl7 = 1 << 7,
258    _Align1Shl8 = 1 << 8,
259    _Align1Shl9 = 1 << 9,
260    _Align1Shl10 = 1 << 10,
261    _Align1Shl11 = 1 << 11,
262    _Align1Shl12 = 1 << 12,
263    _Align1Shl13 = 1 << 13,
264    _Align1Shl14 = 1 << 14,
265    _Align1Shl15 = 1 << 15,
266}
267
268#[cfg(target_pointer_width = "32")]
269#[derive(Copy, Clone, PartialEq, Eq)]
270#[repr(usize)]
271enum AlignmentEnum {
272    _Align1Shl0 = 1 << 0,
273    _Align1Shl1 = 1 << 1,
274    _Align1Shl2 = 1 << 2,
275    _Align1Shl3 = 1 << 3,
276    _Align1Shl4 = 1 << 4,
277    _Align1Shl5 = 1 << 5,
278    _Align1Shl6 = 1 << 6,
279    _Align1Shl7 = 1 << 7,
280    _Align1Shl8 = 1 << 8,
281    _Align1Shl9 = 1 << 9,
282    _Align1Shl10 = 1 << 10,
283    _Align1Shl11 = 1 << 11,
284    _Align1Shl12 = 1 << 12,
285    _Align1Shl13 = 1 << 13,
286    _Align1Shl14 = 1 << 14,
287    _Align1Shl15 = 1 << 15,
288    _Align1Shl16 = 1 << 16,
289    _Align1Shl17 = 1 << 17,
290    _Align1Shl18 = 1 << 18,
291    _Align1Shl19 = 1 << 19,
292    _Align1Shl20 = 1 << 20,
293    _Align1Shl21 = 1 << 21,
294    _Align1Shl22 = 1 << 22,
295    _Align1Shl23 = 1 << 23,
296    _Align1Shl24 = 1 << 24,
297    _Align1Shl25 = 1 << 25,
298    _Align1Shl26 = 1 << 26,
299    _Align1Shl27 = 1 << 27,
300    _Align1Shl28 = 1 << 28,
301    _Align1Shl29 = 1 << 29,
302    _Align1Shl30 = 1 << 30,
303    _Align1Shl31 = 1 << 31,
304}
305
306#[cfg(target_pointer_width = "64")]
307#[derive(Copy, Clone, PartialEq, Eq)]
308#[repr(usize)]
309enum AlignmentEnum {
310    _Align1Shl0 = 1 << 0,
311    _Align1Shl1 = 1 << 1,
312    _Align1Shl2 = 1 << 2,
313    _Align1Shl3 = 1 << 3,
314    _Align1Shl4 = 1 << 4,
315    _Align1Shl5 = 1 << 5,
316    _Align1Shl6 = 1 << 6,
317    _Align1Shl7 = 1 << 7,
318    _Align1Shl8 = 1 << 8,
319    _Align1Shl9 = 1 << 9,
320    _Align1Shl10 = 1 << 10,
321    _Align1Shl11 = 1 << 11,
322    _Align1Shl12 = 1 << 12,
323    _Align1Shl13 = 1 << 13,
324    _Align1Shl14 = 1 << 14,
325    _Align1Shl15 = 1 << 15,
326    _Align1Shl16 = 1 << 16,
327    _Align1Shl17 = 1 << 17,
328    _Align1Shl18 = 1 << 18,
329    _Align1Shl19 = 1 << 19,
330    _Align1Shl20 = 1 << 20,
331    _Align1Shl21 = 1 << 21,
332    _Align1Shl22 = 1 << 22,
333    _Align1Shl23 = 1 << 23,
334    _Align1Shl24 = 1 << 24,
335    _Align1Shl25 = 1 << 25,
336    _Align1Shl26 = 1 << 26,
337    _Align1Shl27 = 1 << 27,
338    _Align1Shl28 = 1 << 28,
339    _Align1Shl29 = 1 << 29,
340    _Align1Shl30 = 1 << 30,
341    _Align1Shl31 = 1 << 31,
342    _Align1Shl32 = 1 << 32,
343    _Align1Shl33 = 1 << 33,
344    _Align1Shl34 = 1 << 34,
345    _Align1Shl35 = 1 << 35,
346    _Align1Shl36 = 1 << 36,
347    _Align1Shl37 = 1 << 37,
348    _Align1Shl38 = 1 << 38,
349    _Align1Shl39 = 1 << 39,
350    _Align1Shl40 = 1 << 40,
351    _Align1Shl41 = 1 << 41,
352    _Align1Shl42 = 1 << 42,
353    _Align1Shl43 = 1 << 43,
354    _Align1Shl44 = 1 << 44,
355    _Align1Shl45 = 1 << 45,
356    _Align1Shl46 = 1 << 46,
357    _Align1Shl47 = 1 << 47,
358    _Align1Shl48 = 1 << 48,
359    _Align1Shl49 = 1 << 49,
360    _Align1Shl50 = 1 << 50,
361    _Align1Shl51 = 1 << 51,
362    _Align1Shl52 = 1 << 52,
363    _Align1Shl53 = 1 << 53,
364    _Align1Shl54 = 1 << 54,
365    _Align1Shl55 = 1 << 55,
366    _Align1Shl56 = 1 << 56,
367    _Align1Shl57 = 1 << 57,
368    _Align1Shl58 = 1 << 58,
369    _Align1Shl59 = 1 << 59,
370    _Align1Shl60 = 1 << 60,
371    _Align1Shl61 = 1 << 61,
372    _Align1Shl62 = 1 << 62,
373    _Align1Shl63 = 1 << 63,
374}
core/ptr/alignment.rs

core/ptr/
alignment.rs
