1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
//! Checked versions of the casting functions exposed in crate root
//! that support [`CheckedBitPattern`] types.

use crate::{
  internal::{self, something_went_wrong},
  AnyBitPattern, NoUninit,
};

/// A marker trait that allows types that have some invalid bit patterns to be
/// used in places that otherwise require [`AnyBitPattern`] or [`Pod`] types by
/// performing a runtime check on a perticular set of bits. This is particularly
/// useful for types like fieldless ('C-style') enums, [`char`], bool, and
/// structs containing them.
///
/// To do this, we define a `Bits` type which is a type with equivalent layout
/// to `Self` other than the invalid bit patterns which disallow `Self` from
/// being [`AnyBitPattern`]. This `Bits` type must itself implement
/// [`AnyBitPattern`]. Then, we implement a function that checks whether a
/// certain instance of the `Bits` is also a valid bit pattern of `Self`. If
/// this check passes, then we can allow casting from the `Bits` to `Self` (and
/// therefore, any type which is able to be cast to `Bits` is also able to be
/// cast to `Self`).
///
/// [`AnyBitPattern`] is a subset of [`CheckedBitPattern`], meaning that any `T:
/// AnyBitPattern` is also [`CheckedBitPattern`]. This means you can also use
/// any [`AnyBitPattern`] type in the checked versions of casting functions in
/// this module. If it's possible, prefer implementing [`AnyBitPattern`] for
/// your type directly instead of [`CheckedBitPattern`] as it gives greater
/// flexibility.
///
/// # Derive
///
/// A `#[derive(CheckedBitPattern)]` macro is provided under the `derive`
/// feature flag which will automatically validate the requirements of this
/// trait and implement the trait for you for both enums and structs. This is
/// the recommended method for implementing the trait, however it's also
/// possible to do manually.
///
/// # Example
///
/// If manually implementing the trait, we can do something like so:
///
/// ```rust
/// use bytemuck::{CheckedBitPattern, NoUninit};
///
/// #[repr(u32)]
/// #[derive(Copy, Clone)]
/// enum MyEnum {
///     Variant0 = 0,
///     Variant1 = 1,
///     Variant2 = 2,
/// }
///
/// unsafe impl CheckedBitPattern for MyEnum {
///     type Bits = u32;
///
///     fn is_valid_bit_pattern(bits: &u32) -> bool {
///         match *bits {
///             0 | 1 | 2 => true,
///             _ => false,
///         }
///     }
/// }
///
/// // It is often useful to also implement `NoUninit` on our `CheckedBitPattern` types.
/// // This will allow us to do casting of mutable references (and mutable slices).
/// // It is not always possible to do so, but in this case we have no padding so it is.
/// unsafe impl NoUninit for MyEnum {}
/// ```
///
/// We can now use relevant casting functions. For example,
///
/// ```rust
/// # use bytemuck::{CheckedBitPattern, NoUninit};
/// # #[repr(u32)]
/// # #[derive(Copy, Clone, PartialEq, Eq, Debug)]
/// # enum MyEnum {
/// #     Variant0 = 0,
/// #     Variant1 = 1,
/// #     Variant2 = 2,
/// # }
/// # unsafe impl NoUninit for MyEnum {}
/// # unsafe impl CheckedBitPattern for MyEnum {
/// #     type Bits = u32;
/// #     fn is_valid_bit_pattern(bits: &u32) -> bool {
/// #         match *bits {
/// #             0 | 1 | 2 => true,
/// #             _ => false,
/// #         }
/// #     }
/// # }
/// use bytemuck::{bytes_of, bytes_of_mut};
/// use bytemuck::checked;
///
/// let bytes = bytes_of(&2u32);
/// let result = checked::try_from_bytes::<MyEnum>(bytes);
/// assert_eq!(result, Ok(&MyEnum::Variant2));
///
/// // Fails for invalid discriminant
/// let bytes = bytes_of(&100u32);
/// let result = checked::try_from_bytes::<MyEnum>(bytes);
/// assert!(result.is_err());
///
/// // Since we implemented NoUninit, we can also cast mutably from an original type
/// // that is `NoUninit + AnyBitPattern`:
/// let mut my_u32 = 2u32;
/// {
///   let as_enum_mut = checked::cast_mut::<_, MyEnum>(&mut my_u32);
///   assert_eq!(as_enum_mut, &mut MyEnum::Variant2);
///   *as_enum_mut = MyEnum::Variant0;
/// }
/// assert_eq!(my_u32, 0u32);
/// ```
///
/// # Safety
///
/// * `Self` *must* have the same layout as the specified `Bits` except for
/// the possible invalid bit patterns being checked during
/// [`is_valid_bit_pattern`].
///   * This almost certainly means your type must be `#[repr(C)]` or a similar
///   specified repr, but if you think you know better, you probably don't. If
/// you   still think you know better, be careful and have fun. And don't mess
/// it up   (I mean it).
/// * If [`is_valid_bit_pattern`] returns true, then the bit pattern contained
///   in `bits` must also be valid for an instance of `Self`.
/// * Probably more, don't mess it up (I mean it 2.0)
///
/// [`is_valid_bit_pattern`]: CheckedBitPattern::is_valid_bit_pattern
/// [`Pod`]: crate::Pod
pub unsafe trait CheckedBitPattern: Copy {
  /// `Self` *must* have the same layout as the specified `Bits` except for
  /// the possible invalid bit patterns being checked during
  /// [`is_valid_bit_pattern`].
  ///
  /// [`is_valid_bit_pattern`]: CheckedBitPattern::is_valid_bit_pattern
  type Bits: AnyBitPattern;

  /// If this function returns true, then it must be valid to reinterpret `bits`
  /// as `&Self`.
  fn is_valid_bit_pattern(bits: &Self::Bits) -> bool;
}

unsafe impl<T: AnyBitPattern> CheckedBitPattern for T {
  type Bits = T;

  #[inline(always)]
  fn is_valid_bit_pattern(_bits: &T) -> bool {
    true
  }
}

unsafe impl CheckedBitPattern for char {
  type Bits = u32;

  #[inline]
  fn is_valid_bit_pattern(bits: &Self::Bits) -> bool {
    core::char::from_u32(*bits).is_some()
  }
}

unsafe impl CheckedBitPattern for bool {
  type Bits = u8;

  #[inline]
  fn is_valid_bit_pattern(bits: &Self::Bits) -> bool {
    match *bits {
      0 | 1 => true,
      _ => false,
    }
  }
}

macro_rules! impl_checked_for_nonzero {
  ($($nonzero:ty: $primitive:ty),* $(,)?) => {
    $(
      unsafe impl CheckedBitPattern for $nonzero {
        type Bits = $primitive;

        #[inline]
        fn is_valid_bit_pattern(bits: &Self::Bits) -> bool {
          // Note(zachs18): The size and alignment check are almost certainly
          // not necessary, but Rust currently doesn't explicitly document that
          // NonZero[int] has the same layout as [int], so we check it to be safe.
          // In a const to reduce debug-profile overhead.
          const LAYOUT_SAME: bool =
            core::mem::size_of::<$nonzero>() == core::mem::size_of::<$primitive>()
            && core::mem::align_of::<$nonzero>() == core::mem::align_of::<$primitive>();
          LAYOUT_SAME && *bits != 0
        }
      }
    )*
  };
}
impl_checked_for_nonzero! {
  core::num::NonZeroU8: u8,
  core::num::NonZeroI8: i8,
  core::num::NonZeroU16: u16,
  core::num::NonZeroI16: i16,
  core::num::NonZeroU32: u32,
  core::num::NonZeroI32: i32,
  core::num::NonZeroU64: u64,
  core::num::NonZeroI64: i64,
  core::num::NonZeroI128: i128,
  core::num::NonZeroU128: u128,
  core::num::NonZeroUsize: usize,
  core::num::NonZeroIsize: isize,
}

/// The things that can go wrong when casting between [`CheckedBitPattern`] data
/// forms.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum CheckedCastError {
  /// An error occurred during a true-[`Pod`] cast
  ///
  /// [`Pod`]: crate::Pod
  PodCastError(crate::PodCastError),
  /// When casting to a [`CheckedBitPattern`] type, it is possible that the
  /// original data contains an invalid bit pattern. If so, the cast will
  /// fail and this error will be returned. Will never happen on casts
  /// between [`Pod`] types.
  ///
  /// [`Pod`]: crate::Pod
  InvalidBitPattern,
}

#[cfg(not(target_arch = "spirv"))]
impl core::fmt::Display for CheckedCastError {
  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
    write!(f, "{:?}", self)
  }
}
#[cfg(feature = "extern_crate_std")]
impl std::error::Error for CheckedCastError {}

impl From<crate::PodCastError> for CheckedCastError {
  fn from(err: crate::PodCastError) -> CheckedCastError {
    CheckedCastError::PodCastError(err)
  }
}

/// Re-interprets `&[u8]` as `&T`.
///
/// ## Failure
///
/// * If the slice isn't aligned for the new type
/// * If the slice's length isn’t exactly the size of the new type
/// * If the slice contains an invalid bit pattern for `T`
#[inline]
pub fn try_from_bytes<T: CheckedBitPattern>(
  s: &[u8],
) -> Result<&T, CheckedCastError> {
  let pod = unsafe { internal::try_from_bytes(s) }?;

  if <T as CheckedBitPattern>::is_valid_bit_pattern(pod) {
    Ok(unsafe { &*(pod as *const <T as CheckedBitPattern>::Bits as *const T) })
  } else {
    Err(CheckedCastError::InvalidBitPattern)
  }
}

/// Re-interprets `&mut [u8]` as `&mut T`.
///
/// ## Failure
///
/// * If the slice isn't aligned for the new type
/// * If the slice's length isn’t exactly the size of the new type
/// * If the slice contains an invalid bit pattern for `T`
#[inline]
pub fn try_from_bytes_mut<T: CheckedBitPattern + NoUninit>(
  s: &mut [u8],
) -> Result<&mut T, CheckedCastError> {
  let pod = unsafe { internal::try_from_bytes_mut(s) }?;

  if <T as CheckedBitPattern>::is_valid_bit_pattern(pod) {
    Ok(unsafe { &mut *(pod as *mut <T as CheckedBitPattern>::Bits as *mut T) })
  } else {
    Err(CheckedCastError::InvalidBitPattern)
  }
}

/// Reads from the bytes as if they were a `T`.
///
/// ## Failure
/// * If the `bytes` length is not equal to `size_of::<T>()`.
/// * If the slice contains an invalid bit pattern for `T`
#[inline]
pub fn try_pod_read_unaligned<T: CheckedBitPattern>(
  bytes: &[u8],
) -> Result<T, CheckedCastError> {
  let pod = unsafe { internal::try_pod_read_unaligned(bytes) }?;

  if <T as CheckedBitPattern>::is_valid_bit_pattern(&pod) {
    Ok(unsafe { transmute!(pod) })
  } else {
    Err(CheckedCastError::InvalidBitPattern)
  }
}

/// Try to cast `T` into `U`.
///
/// Note that for this particular type of cast, alignment isn't a factor. The
/// input value is semantically copied into the function and then returned to a
/// new memory location which will have whatever the required alignment of the
/// output type is.
///
/// ## Failure
///
/// * If the types don't have the same size this fails.
/// * If `a` contains an invalid bit pattern for `B` this fails.
#[inline]
pub fn try_cast<A: NoUninit, B: CheckedBitPattern>(
  a: A,
) -> Result<B, CheckedCastError> {
  let pod = unsafe { internal::try_cast(a) }?;

  if <B as CheckedBitPattern>::is_valid_bit_pattern(&pod) {
    Ok(unsafe { transmute!(pod) })
  } else {
    Err(CheckedCastError::InvalidBitPattern)
  }
}

/// Try to convert a `&T` into `&U`.
///
/// ## Failure
///
/// * If the reference isn't aligned in the new type
/// * If the source type and target type aren't the same size.
/// * If `a` contains an invalid bit pattern for `B` this fails.
#[inline]
pub fn try_cast_ref<A: NoUninit, B: CheckedBitPattern>(
  a: &A,
) -> Result<&B, CheckedCastError> {
  let pod = unsafe { internal::try_cast_ref(a) }?;

  if <B as CheckedBitPattern>::is_valid_bit_pattern(pod) {
    Ok(unsafe { &*(pod as *const <B as CheckedBitPattern>::Bits as *const B) })
  } else {
    Err(CheckedCastError::InvalidBitPattern)
  }
}

/// Try to convert a `&mut T` into `&mut U`.
///
/// As [`try_cast_ref`], but `mut`.
#[inline]
pub fn try_cast_mut<
  A: NoUninit + AnyBitPattern,
  B: CheckedBitPattern + NoUninit,
>(
  a: &mut A,
) -> Result<&mut B, CheckedCastError> {
  let pod = unsafe { internal::try_cast_mut(a) }?;

  if <B as CheckedBitPattern>::is_valid_bit_pattern(pod) {
    Ok(unsafe { &mut *(pod as *mut <B as CheckedBitPattern>::Bits as *mut B) })
  } else {
    Err(CheckedCastError::InvalidBitPattern)
  }
}

/// Try to convert `&[A]` into `&[B]` (possibly with a change in length).
///
/// * `input.as_ptr() as usize == output.as_ptr() as usize`
/// * `input.len() * size_of::<A>() == output.len() * size_of::<B>()`
///
/// ## Failure
///
/// * If the target type has a greater alignment requirement and the input slice
///   isn't aligned.
/// * If the target element type is a different size from the current element
///   type, and the output slice wouldn't be a whole number of elements when
///   accounting for the size change (eg: 3 `u16` values is 1.5 `u32` values, so
///   that's a failure).
/// * Similarly, you can't convert between a [ZST](https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts)
///   and a non-ZST.
/// * If any element of the converted slice would contain an invalid bit pattern
///   for `B` this fails.
#[inline]
pub fn try_cast_slice<A: NoUninit, B: CheckedBitPattern>(
  a: &[A],
) -> Result<&[B], CheckedCastError> {
  let pod = unsafe { internal::try_cast_slice(a) }?;

  if pod.iter().all(|pod| <B as CheckedBitPattern>::is_valid_bit_pattern(pod)) {
    Ok(unsafe {
      core::slice::from_raw_parts(pod.as_ptr() as *const B, pod.len())
    })
  } else {
    Err(CheckedCastError::InvalidBitPattern)
  }
}

/// Try to convert `&mut [A]` into `&mut [B]` (possibly with a change in
/// length).
///
/// As [`try_cast_slice`], but `&mut`.
#[inline]
pub fn try_cast_slice_mut<
  A: NoUninit + AnyBitPattern,
  B: CheckedBitPattern + NoUninit,
>(
  a: &mut [A],
) -> Result<&mut [B], CheckedCastError> {
  let pod = unsafe { internal::try_cast_slice_mut(a) }?;

  if pod.iter().all(|pod| <B as CheckedBitPattern>::is_valid_bit_pattern(pod)) {
    Ok(unsafe {
      core::slice::from_raw_parts_mut(pod.as_mut_ptr() as *mut B, pod.len())
    })
  } else {
    Err(CheckedCastError::InvalidBitPattern)
  }
}

/// Re-interprets `&[u8]` as `&T`.
///
/// ## Panics
///
/// This is [`try_from_bytes`] but will panic on error.
#[inline]
pub fn from_bytes<T: CheckedBitPattern>(s: &[u8]) -> &T {
  match try_from_bytes(s) {
    Ok(t) => t,
    Err(e) => something_went_wrong("from_bytes", e),
  }
}

/// Re-interprets `&mut [u8]` as `&mut T`.
///
/// ## Panics
///
/// This is [`try_from_bytes_mut`] but will panic on error.
#[inline]
pub fn from_bytes_mut<T: NoUninit + CheckedBitPattern>(s: &mut [u8]) -> &mut T {
  match try_from_bytes_mut(s) {
    Ok(t) => t,
    Err(e) => something_went_wrong("from_bytes_mut", e),
  }
}

/// Reads the slice into a `T` value.
///
/// ## Panics
/// * This is like `try_pod_read_unaligned` but will panic on failure.
#[inline]
pub fn pod_read_unaligned<T: CheckedBitPattern>(bytes: &[u8]) -> T {
  match try_pod_read_unaligned(bytes) {
    Ok(t) => t,
    Err(e) => something_went_wrong("pod_read_unaligned", e),
  }
}

/// Cast `T` into `U`
///
/// ## Panics
///
/// * This is like [`try_cast`](try_cast), but will panic on a size mismatch.
#[inline]
pub fn cast<A: NoUninit, B: CheckedBitPattern>(a: A) -> B {
  match try_cast(a) {
    Ok(t) => t,
    Err(e) => something_went_wrong("cast", e),
  }
}

/// Cast `&mut T` into `&mut U`.
///
/// ## Panics
///
/// This is [`try_cast_mut`] but will panic on error.
#[inline]
pub fn cast_mut<
  A: NoUninit + AnyBitPattern,
  B: NoUninit + CheckedBitPattern,
>(
  a: &mut A,
) -> &mut B {
  match try_cast_mut(a) {
    Ok(t) => t,
    Err(e) => something_went_wrong("cast_mut", e),
  }
}

/// Cast `&T` into `&U`.
///
/// ## Panics
///
/// This is [`try_cast_ref`] but will panic on error.
#[inline]
pub fn cast_ref<A: NoUninit, B: CheckedBitPattern>(a: &A) -> &B {
  match try_cast_ref(a) {
    Ok(t) => t,
    Err(e) => something_went_wrong("cast_ref", e),
  }
}

/// Cast `&[A]` into `&[B]`.
///
/// ## Panics
///
/// This is [`try_cast_slice`] but will panic on error.
#[inline]
pub fn cast_slice<A: NoUninit, B: CheckedBitPattern>(a: &[A]) -> &[B] {
  match try_cast_slice(a) {
    Ok(t) => t,
    Err(e) => something_went_wrong("cast_slice", e),
  }
}

/// Cast `&mut [T]` into `&mut [U]`.
///
/// ## Panics
///
/// This is [`try_cast_slice_mut`] but will panic on error.
#[inline]
pub fn cast_slice_mut<
  A: NoUninit + AnyBitPattern,
  B: NoUninit + CheckedBitPattern,
>(
  a: &mut [A],
) -> &mut [B] {
  match try_cast_slice_mut(a) {
    Ok(t) => t,
    Err(e) => something_went_wrong("cast_slice_mut", e),
  }
}