bytemuck/
internal.rs

1//! Internal implementation of casting functions not bound by marker traits
2//! and therefore marked as unsafe. This is used so that we don't need to
3//! duplicate the business logic contained in these functions between the
4//! versions exported in the crate root, `checked`, and `relaxed` modules.
5#![allow(unused_unsafe)]
6
7use crate::PodCastError;
8use core::{marker::*, mem::*};
9
10/*
11
12Note(Lokathor): We've switched all of the `unwrap` to `match` because there is
13apparently a bug: https://github.com/rust-lang/rust/issues/68667
14and it doesn't seem to show up in simple godbolt examples but has been reported
15as having an impact when there's a cast mixed in with other more complicated
16code around it. Rustc/LLVM ends up missing that the `Err` can't ever happen for
17particular type combinations, and then it doesn't fully eliminated the panic
18possibility code branch.
19
20*/
21
22/// Immediately panics.
23#[cfg(not(target_arch = "spirv"))]
24#[cold]
25#[inline(never)]
26#[cfg_attr(feature = "track_caller", track_caller)]
27pub(crate) fn something_went_wrong<D: core::fmt::Display>(
28  _src: &str, _err: D,
29) -> ! {
30  // Note(Lokathor): Keeping the panic here makes the panic _formatting_ go
31  // here too, which helps assembly readability and also helps keep down
32  // the inline pressure.
33  panic!("{src}>{err}", src = _src, err = _err);
34}
35
36/// Immediately panics.
37#[cfg(target_arch = "spirv")]
38#[cold]
39#[inline(never)]
40pub(crate) fn something_went_wrong<D>(_src: &str, _err: D) -> ! {
41  // Note: On the spirv targets from [rust-gpu](https://github.com/EmbarkStudios/rust-gpu)
42  // panic formatting cannot be used. We we just give a generic error message
43  // The chance that the panicking version of these functions will ever get
44  // called on spir-v targets with invalid inputs is small, but giving a
45  // simple error message is better than no error message at all.
46  panic!("Called a panicing helper from bytemuck which paniced");
47}
48
49/// Re-interprets `&T` as `&[u8]`.
50///
51/// Any ZST becomes an empty slice, and in that case the pointer value of that
52/// empty slice might not match the pointer value of the input reference.
53#[inline(always)]
54pub(crate) unsafe fn bytes_of<T: Copy>(t: &T) -> &[u8] {
55  match try_cast_slice::<T, u8>(core::slice::from_ref(t)) {
56    Ok(s) => s,
57    Err(_) => unreachable!(),
58  }
59}
60
61/// Re-interprets `&mut T` as `&mut [u8]`.
62///
63/// Any ZST becomes an empty slice, and in that case the pointer value of that
64/// empty slice might not match the pointer value of the input reference.
65#[inline]
66pub(crate) unsafe fn bytes_of_mut<T: Copy>(t: &mut T) -> &mut [u8] {
67  match try_cast_slice_mut::<T, u8>(core::slice::from_mut(t)) {
68    Ok(s) => s,
69    Err(_) => unreachable!(),
70  }
71}
72
73/// Re-interprets `&[u8]` as `&T`.
74///
75/// ## Panics
76///
77/// This is [`try_from_bytes`] but will panic on error.
78#[inline]
79#[cfg_attr(feature = "track_caller", track_caller)]
80pub(crate) unsafe fn from_bytes<T: Copy>(s: &[u8]) -> &T {
81  match try_from_bytes(s) {
82    Ok(t) => t,
83    Err(e) => something_went_wrong("from_bytes", e),
84  }
85}
86
87/// Re-interprets `&mut [u8]` as `&mut T`.
88///
89/// ## Panics
90///
91/// This is [`try_from_bytes_mut`] but will panic on error.
92#[inline]
93#[cfg_attr(feature = "track_caller", track_caller)]
94pub(crate) unsafe fn from_bytes_mut<T: Copy>(s: &mut [u8]) -> &mut T {
95  match try_from_bytes_mut(s) {
96    Ok(t) => t,
97    Err(e) => something_went_wrong("from_bytes_mut", e),
98  }
99}
100
101/// Reads from the bytes as if they were a `T`.
102///
103/// ## Failure
104/// * If the `bytes` length is not equal to `size_of::<T>()`.
105#[inline]
106pub(crate) unsafe fn try_pod_read_unaligned<T: Copy>(
107  bytes: &[u8],
108) -> Result<T, PodCastError> {
109  if bytes.len() != size_of::<T>() {
110    Err(PodCastError::SizeMismatch)
111  } else {
112    Ok(unsafe { (bytes.as_ptr() as *const T).read_unaligned() })
113  }
114}
115
116/// Reads the slice into a `T` value.
117///
118/// ## Panics
119/// * This is like `try_pod_read_unaligned` but will panic on failure.
120#[inline]
121#[cfg_attr(feature = "track_caller", track_caller)]
122pub(crate) unsafe fn pod_read_unaligned<T: Copy>(bytes: &[u8]) -> T {
123  match try_pod_read_unaligned(bytes) {
124    Ok(t) => t,
125    Err(e) => something_went_wrong("pod_read_unaligned", e),
126  }
127}
128
129/// Checks if `ptr` is aligned to an `align` memory boundary.
130///
131/// ## Panics
132/// * If `align` is not a power of two. This includes when `align` is zero.
133#[inline]
134#[cfg_attr(feature = "track_caller", track_caller)]
135pub(crate) fn is_aligned_to(ptr: *const (), align: usize) -> bool {
136  #[cfg(feature = "align_offset")]
137  {
138    // This is in a way better than `ptr as usize % align == 0`,
139    // because casting a pointer to an integer has the side effect that it
140    // exposes the pointer's provenance, which may theoretically inhibit
141    // some compiler optimizations.
142    ptr.align_offset(align) == 0
143  }
144  #[cfg(not(feature = "align_offset"))]
145  {
146    ((ptr as usize) % align) == 0
147  }
148}
149
150/// Re-interprets `&[u8]` as `&T`.
151///
152/// ## Failure
153///
154/// * If the slice isn't aligned for the new type
155/// * If the slice's length isn’t exactly the size of the new type
156#[inline]
157pub(crate) unsafe fn try_from_bytes<T: Copy>(
158  s: &[u8],
159) -> Result<&T, PodCastError> {
160  if s.len() != size_of::<T>() {
161    Err(PodCastError::SizeMismatch)
162  } else if !is_aligned_to(s.as_ptr() as *const (), align_of::<T>()) {
163    Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
164  } else {
165    Ok(unsafe { &*(s.as_ptr() as *const T) })
166  }
167}
168
169/// Re-interprets `&mut [u8]` as `&mut T`.
170///
171/// ## Failure
172///
173/// * If the slice isn't aligned for the new type
174/// * If the slice's length isn’t exactly the size of the new type
175#[inline]
176pub(crate) unsafe fn try_from_bytes_mut<T: Copy>(
177  s: &mut [u8],
178) -> Result<&mut T, PodCastError> {
179  if s.len() != size_of::<T>() {
180    Err(PodCastError::SizeMismatch)
181  } else if !is_aligned_to(s.as_ptr() as *const (), align_of::<T>()) {
182    Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
183  } else {
184    Ok(unsafe { &mut *(s.as_mut_ptr() as *mut T) })
185  }
186}
187
188/// Cast `A` into `B`
189///
190/// ## Panics
191///
192/// * This is like [`try_cast`](try_cast), but will panic on a size mismatch.
193#[inline]
194#[cfg_attr(feature = "track_caller", track_caller)]
195pub(crate) unsafe fn cast<A: Copy, B: Copy>(a: A) -> B {
196  if size_of::<A>() == size_of::<B>() {
197    unsafe { transmute!(a) }
198  } else {
199    something_went_wrong("cast", PodCastError::SizeMismatch)
200  }
201}
202
203/// Cast `&mut A` into `&mut B`.
204///
205/// ## Panics
206///
207/// This is [`try_cast_mut`] but will panic on error.
208#[inline]
209#[cfg_attr(feature = "track_caller", track_caller)]
210pub(crate) unsafe fn cast_mut<A: Copy, B: Copy>(a: &mut A) -> &mut B {
211  if size_of::<A>() == size_of::<B>() && align_of::<A>() >= align_of::<B>() {
212    // Plz mr compiler, just notice that we can't ever hit Err in this case.
213    match try_cast_mut(a) {
214      Ok(b) => b,
215      Err(_) => unreachable!(),
216    }
217  } else {
218    match try_cast_mut(a) {
219      Ok(b) => b,
220      Err(e) => something_went_wrong("cast_mut", e),
221    }
222  }
223}
224
225/// Cast `&A` into `&B`.
226///
227/// ## Panics
228///
229/// This is [`try_cast_ref`] but will panic on error.
230#[inline]
231#[cfg_attr(feature = "track_caller", track_caller)]
232pub(crate) unsafe fn cast_ref<A: Copy, B: Copy>(a: &A) -> &B {
233  if size_of::<A>() == size_of::<B>() && align_of::<A>() >= align_of::<B>() {
234    // Plz mr compiler, just notice that we can't ever hit Err in this case.
235    match try_cast_ref(a) {
236      Ok(b) => b,
237      Err(_) => unreachable!(),
238    }
239  } else {
240    match try_cast_ref(a) {
241      Ok(b) => b,
242      Err(e) => something_went_wrong("cast_ref", e),
243    }
244  }
245}
246
247/// Cast `&[A]` into `&[B]`.
248///
249/// ## Panics
250///
251/// This is [`try_cast_slice`] but will panic on error.
252#[inline]
253#[cfg_attr(feature = "track_caller", track_caller)]
254pub(crate) unsafe fn cast_slice<A: Copy, B: Copy>(a: &[A]) -> &[B] {
255  match try_cast_slice(a) {
256    Ok(b) => b,
257    Err(e) => something_went_wrong("cast_slice", e),
258  }
259}
260
261/// Cast `&mut [A]` into `&mut [B]`.
262///
263/// ## Panics
264///
265/// This is [`try_cast_slice_mut`] but will panic on error.
266#[inline]
267#[cfg_attr(feature = "track_caller", track_caller)]
268pub(crate) unsafe fn cast_slice_mut<A: Copy, B: Copy>(a: &mut [A]) -> &mut [B] {
269  match try_cast_slice_mut(a) {
270    Ok(b) => b,
271    Err(e) => something_went_wrong("cast_slice_mut", e),
272  }
273}
274
275/// Try to cast `A` into `B`.
276///
277/// Note that for this particular type of cast, alignment isn't a factor. The
278/// input value is semantically copied into the function and then returned to a
279/// new memory location which will have whatever the required alignment of the
280/// output type is.
281///
282/// ## Failure
283///
284/// * If the types don't have the same size this fails.
285#[inline]
286pub(crate) unsafe fn try_cast<A: Copy, B: Copy>(
287  a: A,
288) -> Result<B, PodCastError> {
289  if size_of::<A>() == size_of::<B>() {
290    Ok(unsafe { transmute!(a) })
291  } else {
292    Err(PodCastError::SizeMismatch)
293  }
294}
295
296/// Try to convert a `&A` into `&B`.
297///
298/// ## Failure
299///
300/// * If the reference isn't aligned in the new type
301/// * If the source type and target type aren't the same size.
302#[inline]
303pub(crate) unsafe fn try_cast_ref<A: Copy, B: Copy>(
304  a: &A,
305) -> Result<&B, PodCastError> {
306  // Note(Lokathor): everything with `align_of` and `size_of` will optimize away
307  // after monomorphization.
308  if align_of::<B>() > align_of::<A>()
309    && !is_aligned_to(a as *const A as *const (), align_of::<B>())
310  {
311    Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
312  } else if size_of::<B>() == size_of::<A>() {
313    Ok(unsafe { &*(a as *const A as *const B) })
314  } else {
315    Err(PodCastError::SizeMismatch)
316  }
317}
318
319/// Try to convert a `&mut A` into `&mut B`.
320///
321/// As [`try_cast_ref`], but `mut`.
322#[inline]
323pub(crate) unsafe fn try_cast_mut<A: Copy, B: Copy>(
324  a: &mut A,
325) -> Result<&mut B, PodCastError> {
326  // Note(Lokathor): everything with `align_of` and `size_of` will optimize away
327  // after monomorphization.
328  if align_of::<B>() > align_of::<A>()
329    && !is_aligned_to(a as *const A as *const (), align_of::<B>())
330  {
331    Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
332  } else if size_of::<B>() == size_of::<A>() {
333    Ok(unsafe { &mut *(a as *mut A as *mut B) })
334  } else {
335    Err(PodCastError::SizeMismatch)
336  }
337}
338
339/// Try to convert `&[A]` into `&[B]` (possibly with a change in length).
340///
341/// * `input.as_ptr() as usize == output.as_ptr() as usize`
342/// * `input.len() * size_of::<A>() == output.len() * size_of::<B>()`
343///
344/// ## Failure
345///
346/// * If the target type has a greater alignment requirement and the input slice
347///   isn't aligned.
348/// * If the target element type is a different size from the current element
349///   type, and the output slice wouldn't be a whole number of elements when
350///   accounting for the size change (eg: 3 `u16` values is 1.5 `u32` values, so
351///   that's a failure).
352#[inline]
353pub(crate) unsafe fn try_cast_slice<A: Copy, B: Copy>(
354  a: &[A],
355) -> Result<&[B], PodCastError> {
356  let input_bytes = core::mem::size_of_val::<[A]>(a);
357  // Note(Lokathor): everything with `align_of` and `size_of` will optimize away
358  // after monomorphization.
359  if align_of::<B>() > align_of::<A>()
360    && !is_aligned_to(a.as_ptr() as *const (), align_of::<B>())
361  {
362    Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
363  } else if size_of::<B>() == size_of::<A>() {
364    Ok(unsafe { core::slice::from_raw_parts(a.as_ptr() as *const B, a.len()) })
365  } else if (size_of::<B>() != 0 && input_bytes % size_of::<B>() == 0)
366    || (size_of::<B>() == 0 && input_bytes == 0)
367  {
368    let new_len =
369      if size_of::<B>() != 0 { input_bytes / size_of::<B>() } else { 0 };
370    Ok(unsafe { core::slice::from_raw_parts(a.as_ptr() as *const B, new_len) })
371  } else {
372    Err(PodCastError::OutputSliceWouldHaveSlop)
373  }
374}
375
376/// Try to convert `&mut [A]` into `&mut [B]` (possibly with a change in
377/// length).
378///
379/// As [`try_cast_slice`], but `&mut`.
380#[inline]
381pub(crate) unsafe fn try_cast_slice_mut<A: Copy, B: Copy>(
382  a: &mut [A],
383) -> Result<&mut [B], PodCastError> {
384  let input_bytes = core::mem::size_of_val::<[A]>(a);
385  // Note(Lokathor): everything with `align_of` and `size_of` will optimize away
386  // after monomorphization.
387  if align_of::<B>() > align_of::<A>()
388    && !is_aligned_to(a.as_ptr() as *const (), align_of::<B>())
389  {
390    Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
391  } else if size_of::<B>() == size_of::<A>() {
392    Ok(unsafe {
393      core::slice::from_raw_parts_mut(a.as_mut_ptr() as *mut B, a.len())
394    })
395  } else if (size_of::<B>() != 0 && input_bytes % size_of::<B>() == 0)
396    || (size_of::<B>() == 0 && input_bytes == 0)
397  {
398    let new_len =
399      if size_of::<B>() != 0 { input_bytes / size_of::<B>() } else { 0 };
400    Ok(unsafe {
401      core::slice::from_raw_parts_mut(a.as_mut_ptr() as *mut B, new_len)
402    })
403  } else {
404    Err(PodCastError::OutputSliceWouldHaveSlop)
405  }
406}
bytemuck/internal.rs

bytemuck/
internal.rs
