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
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).

use super::components::VarZeroVecComponents;
use super::*;
use crate::ule::*;
use alloc::boxed::Box;
use alloc::vec::Vec;
use core::cmp::{Ord, Ordering, PartialOrd};
use core::fmt;
use core::marker::PhantomData;
use core::mem;

use core::ops::Index;
use core::ops::Range;

/// A zero-copy "slice", that works for unsized types, i.e. the zero-copy version of `[T]`
/// where `T` is not `Sized`.
///
/// This behaves similarly to [`VarZeroVec<T>`], however [`VarZeroVec<T>`] is allowed to contain
/// owned data and as such is ideal for deserialization since most human readable
/// serialization formats cannot unconditionally deserialize zero-copy.
///
/// This type can be used inside [`VarZeroVec<T>`](crate::VarZeroVec) and [`ZeroMap`](crate::ZeroMap):
/// This essentially allows for the construction of zero-copy types isomorphic to `Vec<Vec<T>>` by instead
/// using `VarZeroVec<ZeroSlice<T>>`.
///
/// The `F` type parameter is a [`VarZeroVecFormat`] (see its docs for more details), which can be used to select the
/// precise format of the backing buffer with various size and performance tradeoffs. It defaults to [`Index16`].
///
/// This type can be nested within itself to allow for multi-level nested `Vec`s.
///
/// # Examples
///
/// ## Nested Slices
///
/// The following code constructs the conceptual zero-copy equivalent of `Vec<Vec<Vec<str>>>`
///
/// ```rust
/// use zerovec::{VarZeroSlice, VarZeroVec};
/// let strings_1: Vec<&str> = vec!["foo", "bar", "baz"];
/// let strings_2: Vec<&str> = vec!["twelve", "seventeen", "forty two"];
/// let strings_3: Vec<&str> = vec!["我", "喜歡", "烏龍茶"];
/// let strings_4: Vec<&str> = vec!["w", "ω", "文", "𑄃"];
/// let strings_12 = vec![&*strings_1, &*strings_2];
/// let strings_34 = vec![&*strings_3, &*strings_4];
/// let all_strings = vec![strings_12, strings_34];
///
/// let vzv_1: VarZeroVec<str> = VarZeroVec::from(&strings_1);
/// let vzv_2: VarZeroVec<str> = VarZeroVec::from(&strings_2);
/// let vzv_3: VarZeroVec<str> = VarZeroVec::from(&strings_3);
/// let vzv_4: VarZeroVec<str> = VarZeroVec::from(&strings_4);
/// let vzv_12 = VarZeroVec::from(&[vzv_1.as_slice(), vzv_2.as_slice()]);
/// let vzv_34 = VarZeroVec::from(&[vzv_3.as_slice(), vzv_4.as_slice()]);
/// let vzv_all = VarZeroVec::from(&[vzv_12.as_slice(), vzv_34.as_slice()]);
///
/// let reconstructed: Vec<Vec<Vec<String>>> = vzv_all
///     .iter()
///     .map(|v: &VarZeroSlice<VarZeroSlice<str>>| {
///         v.iter()
///             .map(|x: &VarZeroSlice<_>| {
///                 x.as_varzerovec()
///                     .iter()
///                     .map(|s| s.to_owned())
///                     .collect::<Vec<String>>()
///             })
///             .collect::<Vec<_>>()
///     })
///     .collect::<Vec<_>>();
/// assert_eq!(reconstructed, all_strings);
///
/// let bytes = vzv_all.as_bytes();
/// let vzv_from_bytes: VarZeroVec<VarZeroSlice<VarZeroSlice<str>>> =
///     VarZeroVec::parse_byte_slice(bytes).unwrap();
/// assert_eq!(vzv_from_bytes, vzv_all);
/// ```
///
/// ## Iterate over Windows
///
/// Although [`VarZeroSlice`] does not itself have a `.windows` iterator like
/// [core::slice::Windows], this behavior can be easily modeled using an iterator:
///
/// ```
/// use zerovec::VarZeroVec;
///
/// let vzv = VarZeroVec::<str>::from(&["a", "b", "c", "d"]);
/// # let mut pairs: Vec<(&str, &str)> = Vec::new();
///
/// let mut it = vzv.iter().peekable();
/// while let (Some(x), Some(y)) = (it.next(), it.peek()) {
///     // Evaluate (x, y) here.
/// #   pairs.push((x, y));
/// }
/// # assert_eq!(pairs, &[("a", "b"), ("b", "c"), ("c", "d")]);
/// ```
//
// safety invariant: The slice MUST be one which parses to
// a valid VarZeroVecComponents<T>
#[repr(transparent)]
pub struct VarZeroSlice<T: ?Sized, F = Index16> {
    marker: PhantomData<(F, T)>,
    /// The original slice this was constructed from
    entire_slice: [u8],
}

impl<T: VarULE + ?Sized, F: VarZeroVecFormat> VarZeroSlice<T, F> {
    /// Construct a new empty VarZeroSlice
    pub const fn new_empty() -> &'static Self {
        // The empty VZV is special-cased to the empty slice
        unsafe { mem::transmute(&[] as &[u8]) }
    }

    /// Obtain a [`VarZeroVecComponents`] borrowing from the internal buffer
    #[inline]
    pub(crate) fn as_components<'a>(&'a self) -> VarZeroVecComponents<'a, T, F> {
        unsafe {
            // safety: VarZeroSlice is guaranteed to parse here
            VarZeroVecComponents::from_bytes_unchecked(&self.entire_slice)
        }
    }

    /// Uses a `&[u8]` buffer as a `VarZeroSlice<T>` without any verification.
    ///
    /// # Safety
    ///
    /// `bytes` need to be an output from [`VarZeroSlice::as_bytes()`].
    pub const unsafe fn from_bytes_unchecked(bytes: &[u8]) -> &Self {
        // self is really just a wrapper around a byte slice
        mem::transmute(bytes)
    }

    /// Get the number of elements in this slice
    ///
    /// # Example
    ///
    /// ```rust
    /// # use zerovec::ule::ZeroVecError;
    /// # use zerovec::VarZeroVec;
    ///
    /// let strings = vec!["foo", "bar", "baz", "quux"];
    /// let vec = VarZeroVec::<str>::from(&strings);
    ///
    /// assert_eq!(vec.len(), 4);
    /// # Ok::<(), ZeroVecError>(())
    /// ```
    pub fn len(&self) -> usize {
        self.as_components().len()
    }

    /// Returns `true` if the slice contains no elements.
    ///
    /// # Examples
    ///
    /// ```
    /// # use zerovec::ule::ZeroVecError;
    /// # use zerovec::VarZeroVec;
    ///
    /// let strings: Vec<String> = vec![];
    /// let vec = VarZeroVec::<str>::from(&strings);
    ///
    /// assert!(vec.is_empty());
    /// # Ok::<(), ZeroVecError>(())
    /// ```
    pub fn is_empty(&self) -> bool {
        self.as_components().is_empty()
    }

    /// Obtain an iterator over this slice's elements
    ///
    /// # Example
    ///
    /// ```rust
    /// # use zerovec::ule::ZeroVecError;
    /// # use zerovec::VarZeroVec;
    ///
    /// let strings = vec!["foo", "bar", "baz", "quux"];
    /// let vec = VarZeroVec::<str>::from(&strings);
    ///
    /// let mut iter_results: Vec<&str> = vec.iter().collect();
    /// assert_eq!(iter_results[0], "foo");
    /// assert_eq!(iter_results[1], "bar");
    /// assert_eq!(iter_results[2], "baz");
    /// assert_eq!(iter_results[3], "quux");
    /// # Ok::<(), ZeroVecError>(())
    /// ```
    pub fn iter<'b>(&'b self) -> impl Iterator<Item = &'b T> {
        self.as_components().iter()
    }

    /// Get one of this slice's elements, returning `None` if the index is out of bounds
    ///
    /// # Example
    ///
    /// ```rust
    /// # use zerovec::ule::ZeroVecError;
    /// # use zerovec::VarZeroVec;
    ///
    /// let strings = vec!["foo", "bar", "baz", "quux"];
    /// let vec = VarZeroVec::<str>::from(&strings);
    ///
    /// let mut iter_results: Vec<&str> = vec.iter().collect();
    /// assert_eq!(vec.get(0), Some("foo"));
    /// assert_eq!(vec.get(1), Some("bar"));
    /// assert_eq!(vec.get(2), Some("baz"));
    /// assert_eq!(vec.get(3), Some("quux"));
    /// assert_eq!(vec.get(4), None);
    /// # Ok::<(), ZeroVecError>(())
    /// ```
    pub fn get(&self, idx: usize) -> Option<&T> {
        self.as_components().get(idx)
    }

    /// Get one of this slice's elements
    ///
    /// # Safety
    ///
    /// `index` must be in range
    ///
    /// # Example
    ///
    /// ```rust
    /// # use zerovec::ule::ZeroVecError;
    /// # use zerovec::VarZeroVec;
    ///
    /// let strings = vec!["foo", "bar", "baz", "quux"];
    /// let vec = VarZeroVec::<str>::from(&strings);
    ///
    /// let mut iter_results: Vec<&str> = vec.iter().collect();
    /// unsafe {
    ///     assert_eq!(vec.get_unchecked(0), "foo");
    ///     assert_eq!(vec.get_unchecked(1), "bar");
    ///     assert_eq!(vec.get_unchecked(2), "baz");
    ///     assert_eq!(vec.get_unchecked(3), "quux");
    /// }
    /// # Ok::<(), ZeroVecError>(())
    /// ```
    pub unsafe fn get_unchecked(&self, idx: usize) -> &T {
        self.as_components().get_unchecked(idx)
    }

    /// Obtain an owned `Vec<Box<T>>` out of this
    pub fn to_vec(&self) -> Vec<Box<T>> {
        self.as_components().to_vec()
    }

    /// Get a reference to the entire encoded backing buffer of this slice
    ///
    /// The bytes can be passed back to [`Self::parse_byte_slice()`].
    ///
    /// To take the bytes as a vector, see [`VarZeroVec::into_bytes()`].
    ///
    /// # Example
    ///
    /// ```rust
    /// # use zerovec::ule::ZeroVecError;
    /// # use zerovec::VarZeroVec;
    ///
    /// let strings = vec!["foo", "bar", "baz"];
    /// let vzv = VarZeroVec::<str>::from(&strings);
    ///
    /// assert_eq!(vzv, VarZeroVec::parse_byte_slice(vzv.as_bytes()).unwrap());
    ///
    /// # Ok::<(), ZeroVecError>(())
    /// ```
    #[inline]
    pub const fn as_bytes(&self) -> &[u8] {
        &self.entire_slice
    }

    /// Get this [`VarZeroSlice`] as a borrowed [`VarZeroVec`]
    ///
    /// If you wish to repeatedly call methods on this [`VarZeroSlice`],
    /// it is more efficient to perform this conversion first
    pub const fn as_varzerovec<'a>(&'a self) -> VarZeroVec<'a, T, F> {
        VarZeroVec::Borrowed(self)
    }

    /// Parse a VarZeroSlice from a slice of the appropriate format
    ///
    /// Slices of the right format can be obtained via [`VarZeroSlice::as_bytes()`]
    pub fn parse_byte_slice<'a>(slice: &'a [u8]) -> Result<&'a Self, ZeroVecError> {
        <Self as VarULE>::parse_byte_slice(slice)
    }

    /// Convert a `bytes` array known to represent a `VarZeroSlice` to a mutable reference to a `VarZeroSlice`
    ///
    /// # Safety
    /// - `bytes` must be a valid sequence of bytes for this VarZeroVec
    pub(crate) unsafe fn from_byte_slice_unchecked_mut(bytes: &mut [u8]) -> &mut Self {
        // self is really just a wrapper around a byte slice
        mem::transmute(bytes)
    }

    pub(crate) unsafe fn get_bytes_at_mut(&mut self, idx: usize) -> &mut [u8] {
        let range = self.as_components().get_range(idx);
        #[allow(clippy::indexing_slicing)] // get_range() is known to return in-bounds ranges
        &mut self.entire_slice[range]
    }
}

impl<T, F> VarZeroSlice<T, F>
where
    T: VarULE,
    T: ?Sized,
    T: Ord,
    F: VarZeroVecFormat,
{
    /// Binary searches a sorted `VarZeroVec<T>` for the given element. For more information, see
    /// the standard library function [`binary_search`].
    ///
    /// # Example
    ///
    /// ```
    /// # use zerovec::ule::ZeroVecError;
    /// # use zerovec::VarZeroVec;
    ///
    /// let strings = vec!["a", "b", "f", "g"];
    /// let vec = VarZeroVec::<str>::from(&strings);
    ///
    /// assert_eq!(vec.binary_search("f"), Ok(2));
    /// assert_eq!(vec.binary_search("e"), Err(2));
    /// # Ok::<(), ZeroVecError>(())
    /// ```
    ///
    /// [`binary_search`]: https://doc.rust-lang.org/std/primitive.slice.html#method.binary_search
    #[inline]
    pub fn binary_search(&self, x: &T) -> Result<usize, usize> {
        self.as_components().binary_search(x)
    }

    /// Binary searches a `VarZeroVec<T>` for the given element within a certain sorted range.
    ///
    /// If the range is out of bounds, returns `None`. Otherwise, returns a `Result` according
    /// to the behavior of the standard library function [`binary_search`].
    ///
    /// The index is returned relative to the start of the range.
    ///
    /// # Example
    ///
    /// ```
    /// # use zerovec::ule::ZeroVecError;
    /// # use zerovec::VarZeroVec;
    ///
    /// let strings = vec!["a", "b", "f", "g", "m", "n", "q"];
    /// let vec = VarZeroVec::<str>::from(&strings);
    ///
    /// // Same behavior as binary_search when the range covers the whole slice:
    /// assert_eq!(vec.binary_search_in_range("g", 0..7), Some(Ok(3)));
    /// assert_eq!(vec.binary_search_in_range("h", 0..7), Some(Err(4)));
    ///
    /// // Will not look outside of the range:
    /// assert_eq!(vec.binary_search_in_range("g", 0..1), Some(Err(1)));
    /// assert_eq!(vec.binary_search_in_range("g", 6..7), Some(Err(0)));
    ///
    /// // Will return indices relative to the start of the range:
    /// assert_eq!(vec.binary_search_in_range("g", 1..6), Some(Ok(2)));
    /// assert_eq!(vec.binary_search_in_range("h", 1..6), Some(Err(3)));
    ///
    /// // Will return `None` if the range is out of bounds:
    /// assert_eq!(vec.binary_search_in_range("x", 100..200), None);
    /// assert_eq!(vec.binary_search_in_range("x", 0..200), None);
    /// # Ok::<(), ZeroVecError>(())
    /// ```
    ///
    /// [`binary_search`]: https://doc.rust-lang.org/std/primitive.slice.html#method.binary_search
    #[inline]
    pub fn binary_search_in_range(
        &self,
        x: &T,
        range: Range<usize>,
    ) -> Option<Result<usize, usize>> {
        self.as_components().binary_search_in_range(x, range)
    }
}

impl<T, F> VarZeroSlice<T, F>
where
    T: VarULE,
    T: ?Sized,
    F: VarZeroVecFormat,
{
    /// Binary searches a sorted `VarZeroVec<T>` for the given predicate. For more information, see
    /// the standard library function [`binary_search_by`].
    ///
    /// # Example
    ///
    /// ```
    /// # use zerovec::ule::ZeroVecError;
    /// # use zerovec::VarZeroVec;
    ///
    /// let strings = vec!["a", "b", "f", "g"];
    /// let vec = VarZeroVec::<str>::from(&strings);
    ///
    /// assert_eq!(vec.binary_search_by(|probe| probe.cmp("f")), Ok(2));
    /// assert_eq!(vec.binary_search_by(|probe| probe.cmp("e")), Err(2));
    /// # Ok::<(), ZeroVecError>(())
    /// ```
    ///
    /// [`binary_search_by`]: https://doc.rust-lang.org/std/primitive.slice.html#method.binary_search_by
    #[inline]
    pub fn binary_search_by(&self, predicate: impl FnMut(&T) -> Ordering) -> Result<usize, usize> {
        self.as_components().binary_search_by(predicate)
    }

    /// Binary searches a `VarZeroVec<T>` for the given predicate within a certain sorted range.
    ///
    /// If the range is out of bounds, returns `None`. Otherwise, returns a `Result` according
    /// to the behavior of the standard library function [`binary_search`].
    ///
    /// The index is returned relative to the start of the range.
    ///
    /// # Example
    ///
    /// ```
    /// # use zerovec::ule::ZeroVecError;
    /// # use zerovec::VarZeroVec;
    ///
    /// let strings = vec!["a", "b", "f", "g", "m", "n", "q"];
    /// let vec = VarZeroVec::<str>::from(&strings);
    ///
    /// // Same behavior as binary_search when the range covers the whole slice:
    /// assert_eq!(
    ///     vec.binary_search_in_range_by(|v| v.cmp("g"), 0..7),
    ///     Some(Ok(3))
    /// );
    /// assert_eq!(
    ///     vec.binary_search_in_range_by(|v| v.cmp("h"), 0..7),
    ///     Some(Err(4))
    /// );
    ///
    /// // Will not look outside of the range:
    /// assert_eq!(
    ///     vec.binary_search_in_range_by(|v| v.cmp("g"), 0..1),
    ///     Some(Err(1))
    /// );
    /// assert_eq!(
    ///     vec.binary_search_in_range_by(|v| v.cmp("g"), 6..7),
    ///     Some(Err(0))
    /// );
    ///
    /// // Will return indices relative to the start of the range:
    /// assert_eq!(
    ///     vec.binary_search_in_range_by(|v| v.cmp("g"), 1..6),
    ///     Some(Ok(2))
    /// );
    /// assert_eq!(
    ///     vec.binary_search_in_range_by(|v| v.cmp("h"), 1..6),
    ///     Some(Err(3))
    /// );
    ///
    /// // Will return `None` if the range is out of bounds:
    /// assert_eq!(
    ///     vec.binary_search_in_range_by(|v| v.cmp("x"), 100..200),
    ///     None
    /// );
    /// assert_eq!(vec.binary_search_in_range_by(|v| v.cmp("x"), 0..200), None);
    /// # Ok::<(), ZeroVecError>(())
    /// ```
    ///
    /// [`binary_search`]: https://doc.rust-lang.org/std/primitive.slice.html#method.binary_search
    pub fn binary_search_in_range_by(
        &self,
        predicate: impl FnMut(&T) -> Ordering,
        range: Range<usize>,
    ) -> Option<Result<usize, usize>> {
        self.as_components()
            .binary_search_in_range_by(predicate, range)
    }
}
// Safety (based on the safety checklist on the VarULE trait):
//  1. VarZeroSlice does not include any uninitialized or padding bytes (achieved by `#[repr(transparent)]` on a
//     `[u8]` slice which satisfies this invariant)
//  2. VarZeroSlice is aligned to 1 byte (achieved by `#[repr(transparent)]` on a
//     `[u8]` slice which satisfies this invariant)
//  3. The impl of `validate_byte_slice()` returns an error if any byte is not valid.
//  4. The impl of `validate_byte_slice()` returns an error if the slice cannot be used in its entirety
//  5. The impl of `from_byte_slice_unchecked()` returns a reference to the same data.
//  6. `as_byte_slice()` is equivalent to a regular transmute of the underlying data
//  7. VarZeroSlice byte equality is semantic equality (relying on the guideline of the underlying VarULE type)
unsafe impl<T: VarULE + ?Sized + 'static, F: VarZeroVecFormat> VarULE for VarZeroSlice<T, F> {
    fn validate_byte_slice(bytes: &[u8]) -> Result<(), ZeroVecError> {
        let _: VarZeroVecComponents<T, F> = VarZeroVecComponents::parse_byte_slice(bytes)?;
        Ok(())
    }

    unsafe fn from_byte_slice_unchecked(bytes: &[u8]) -> &Self {
        // self is really just a wrapper around a byte slice
        mem::transmute(bytes)
    }

    fn as_byte_slice(&self) -> &[u8] {
        &self.entire_slice
    }
}

impl<T: VarULE + ?Sized, F: VarZeroVecFormat> Index<usize> for VarZeroSlice<T, F> {
    type Output = T;
    fn index(&self, index: usize) -> &Self::Output {
        #[allow(clippy::panic)] // documented
        match self.get(index) {
            Some(x) => x,
            None => panic!(
                "index out of bounds: the len is {} but the index is {index}",
                self.len()
            ),
        }
    }
}

impl<T, F> PartialEq<VarZeroSlice<T, F>> for VarZeroSlice<T, F>
where
    T: VarULE,
    T: ?Sized,
    T: PartialEq,
    F: VarZeroVecFormat,
{
    #[inline]
    fn eq(&self, other: &VarZeroSlice<T, F>) -> bool {
        // VarULE has an API guarantee that this is equivalent
        // to `T::VarULE::eq()`
        self.entire_slice.eq(&other.entire_slice)
    }
}

impl<T, F> Eq for VarZeroSlice<T, F>
where
    T: VarULE,
    T: ?Sized,
    T: Eq,
    F: VarZeroVecFormat,
{
}

impl<T: VarULE + ?Sized + PartialOrd, F: VarZeroVecFormat> PartialOrd for VarZeroSlice<T, F> {
    #[inline]
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        self.iter().partial_cmp(other.iter())
    }
}

impl<T: VarULE + ?Sized + Ord, F: VarZeroVecFormat> Ord for VarZeroSlice<T, F> {
    #[inline]
    fn cmp(&self, other: &Self) -> Ordering {
        self.iter().cmp(other.iter())
    }
}

impl<T: VarULE + ?Sized, F: VarZeroVecFormat> fmt::Debug for VarZeroSlice<T, F>
where
    T: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_list().entries(self.iter()).finish()
    }
}

impl<T: ?Sized, F: VarZeroVecFormat> AsRef<VarZeroSlice<T, F>> for VarZeroSlice<T, F> {
    fn as_ref(&self) -> &VarZeroSlice<T, F> {
        self
    }
}