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
#![allow(deprecated)]
#![allow(unused_imports)]

use core::cell::UnsafeCell;
use core::marker;
use core::{mem, mem::MaybeUninit};

use super::invariant::*;
use super::modes::*;
use super::pervasive::*;
use super::prelude::*;
pub use super::raw_ptr::MemContents;
use super::set::*;
use super::*;

verus! {

broadcast use super::map::group_map_axioms, super::set::group_set_axioms;
// TODO implement: borrow_mut; figure out Drop, see if we can avoid leaking?

/// `PCell<V>` (which stands for "permissioned call") is the primitive Verus `Cell` type.
///
/// Technically, it is a wrapper around
/// `core::cell::UnsafeCell<core::mem::MaybeUninit<V>>`, and thus has the same runtime
/// properties: there are no runtime checks (as there would be for Rust's traditional
/// [`core::cell::RefCell`](https://doc.rust-lang.org/core/cell/struct.RefCell.html)).
/// Its data may be uninitialized.
///
/// Furthermore (and unlike both
/// [`core::cell::Cell`](https://doc.rust-lang.org/core/cell/struct.Cell.html) and
/// [`core::cell::RefCell`](https://doc.rust-lang.org/core/cell/struct.RefCell.html)),
/// a `PCell<V>` may be `Sync` (depending on `V`).
/// Thanks to verification, Verus ensures that access to the cell is data-race-free.
///
/// `PCell` uses a _ghost permission token_ similar to [`simple_pptr::PPtr`] -- see the [`simple_pptr::PPtr`]
/// documentation for the basics.
/// For `PCell`, the associated type of the permission token is [`cell::PointsTo`].
///
/// ### Differences from `PPtr`.
///
/// The key difference is that, whereas [`simple_pptr::PPtr`] represents a fixed address in memory,
/// a `PCell` has _no_ fixed address because a `PCell` might be moved.
/// As such, the [`pcell.id()`](PCell::id) does not correspond to a memory address; rather,
/// it is a unique identifier that is fixed for a given cell, even when it is moved.
///
/// The arbitrary ID given by [`pcell.id()`](PCell::id) is of type [`CellId`].
/// Despite the fact that it is, in some ways, "like a pointer", note that
/// `CellId` does not support any meangingful arithmetic,
/// has no concept of a "null ID",
/// and has no runtime representation.
///
/// Also note that the `PCell` might be dropped before the `PointsTo` token is dropped,
/// although in that case it will no longer be possible to use the `PointsTo` in `exec` code
/// to extract data from the cell.
///
/// ### Example (TODO)
#[verifier::external_body]
#[verifier::accept_recursive_types(V)]
pub struct PCell<V> {
    ucell: UnsafeCell<MaybeUninit<V>>,
}

/// `PCell` is _always_ safe to `Send` or `Sync`. Rather, it is the [`PointsTo`] object where `Send` and `Sync` matter.
/// (It doesn't matter if you move the bytes to another thread if you can't access them.)
#[verifier::external]
unsafe impl<T> Sync for PCell<T> {

}

#[verifier::external]
unsafe impl<T> Send for PCell<T> {

}

/// Permission object associated with a [`PCell<V>`].
///
/// See the documentation of [`PCell<V>`] for more information.
// PointsTo<V>, on the other hand, needs to inherit both Send and Sync from the V,
// which it does by default in the given definition.
// (Note: this depends on the current behavior that #[verifier::spec] fields are still counted for marker traits)
#[verifier::external_body]
#[verifier::reject_recursive_types_in_ground_variants(V)]
pub tracked struct PointsTo<V> {
    phantom: marker::PhantomData<V>,
    no_copy: NoCopy,
}

pub ghost struct PointsToData<V> {
    pub pcell: CellId,
    #[cfg_attr(not(verus_verify_core), deprecated = "use `pcell_points!`, or `mem_contents()` instead")]
    pub value: Option<V>,
}

#[doc(hidden)]
pub open spec fn option_from_mem_contents<V>(val: MemContents<V>) -> Option<V> {
    match val {
        MemContents::Init(v) => Some(v),
        MemContents::Uninit => None,
    }
}

#[doc(hidden)]
#[macro_export]
macro_rules! pcell_opt_internal {
    [$pcell:expr => $val:expr] => {
        $crate::vstd::cell::PointsToData {
            pcell: $pcell,
            value: $val,
        }
    };
}

#[cfg_attr(not(verus_verify_core), deprecated = "use pcell_points! instead")]
#[macro_export]
macro_rules! pcell_opt {
    [$($tail:tt)*] => {
        ::builtin_macros::verus_proof_macro_exprs!(
            $crate::vstd::cell::pcell_opt_internal!($($tail)*)
        )
    }
}

pub use pcell_opt_internal;
pub use pcell_opt;

#[doc(hidden)]
#[macro_export]
macro_rules! pcell_points_internal {
    [$pcell:expr => $val:expr] => {
        $crate::vstd::cell::PointsToData {
            pcell: $pcell,
            value: $crate::vstd::cell::option_from_mem_contents($val),
        }
    };
}

#[macro_export]
macro_rules! pcell_points {
    [$($tail:tt)*] => {
        ::builtin_macros::verus_proof_macro_exprs!(
            $crate::vstd::cell::pcell_points_internal!($($tail)*)
        )
    }
}

pub use pcell_points_internal;
pub use pcell_points;

#[verifier::external_body]
pub struct CellId {
    id: int,
}

impl<V> PointsTo<V> {
    /// The [`CellId`] of the [`PCell`] this permission is associated with.
    pub uninterp spec fn id(&self) -> CellId;

    /// The contents of the cell, either unitialized or initialized to some `V`.
    pub uninterp spec fn mem_contents(&self) -> MemContents<V>;

    pub open spec fn view(self) -> PointsToData<V> {
        PointsToData { pcell: self.id(), value: option_from_mem_contents(self.mem_contents()) }
    }

    #[cfg_attr(not(verus_verify_core), deprecated = "use mem_contents() instead")]
    pub open spec fn opt_value(&self) -> Option<V> {
        match self.mem_contents() {
            MemContents::Init(value) => Some(value),
            MemContents::Uninit => None,
        }
    }

    /// Is this cell initialized?
    #[verifier::inline]
    pub open spec fn is_init(&self) -> bool {
        self.mem_contents().is_init()
    }

    /// Is this cell uninitialized?
    #[verifier::inline]
    pub open spec fn is_uninit(&self) -> bool {
        self.mem_contents().is_uninit()
    }

    /// Value of the cell (if initialized)
    #[verifier::inline]
    pub open spec fn value(&self) -> V
        recommends
            self.is_init(),
    {
        self.mem_contents().value()
    }
}

impl<V> PCell<V> {
    /// A unique ID for the cell.
    pub uninterp spec fn id(&self) -> CellId;

    /// Return an empty ("uninitialized") cell.
    #[inline(always)]
    #[verifier::external_body]
    pub const fn empty() -> (pt: (PCell<V>, Tracked<PointsTo<V>>))
        ensures
            pt.1@@ === pcell_points![ pt.0.id() => MemContents::Uninit ],
    {
        let p = PCell { ucell: UnsafeCell::new(MaybeUninit::uninit()) };
        (p, Tracked::assume_new())
    }

    #[inline(always)]
    #[verifier::external_body]
    pub const fn new(v: V) -> (pt: (PCell<V>, Tracked<PointsTo<V>>))
        ensures
            pt.1@@ === pcell_points! [ pt.0.id() => MemContents::Init(v) ],
    {
        let p = PCell { ucell: UnsafeCell::new(MaybeUninit::new(v)) };
        (p, Tracked::assume_new())
    }

    #[inline(always)]
    #[verifier::external_body]
    pub fn put(&self, Tracked(perm): Tracked<&mut PointsTo<V>>, v: V)
        requires
            old(perm)@ === pcell_points![ self.id() => MemContents::Uninit ],
        ensures
            perm@ === pcell_points![ self.id() => MemContents::Init(v) ],
        opens_invariants none
        no_unwind
    {
        unsafe {
            *(self.ucell.get()) = MaybeUninit::new(v);
        }
    }

    #[inline(always)]
    #[verifier::external_body]
    pub fn take(&self, Tracked(perm): Tracked<&mut PointsTo<V>>) -> (v: V)
        requires
            self.id() === old(perm)@.pcell,
            old(perm).is_init(),
        ensures
            perm.id() === old(perm)@.pcell,
            perm.mem_contents() === MemContents::Uninit,
            v === old(perm).value(),
        opens_invariants none
        no_unwind
    {
        unsafe {
            let mut m = MaybeUninit::uninit();
            mem::swap(&mut m, &mut *self.ucell.get());
            m.assume_init()
        }
    }

    #[inline(always)]
    #[verifier::external_body]
    pub fn replace(&self, Tracked(perm): Tracked<&mut PointsTo<V>>, in_v: V) -> (out_v: V)
        requires
            self.id() === old(perm)@.pcell,
            old(perm).is_init(),
        ensures
            perm.id() === old(perm)@.pcell,
            perm.mem_contents() === MemContents::Init(in_v),
            out_v === old(perm).value(),
        opens_invariants none
        no_unwind
    {
        unsafe {
            let mut m = MaybeUninit::new(in_v);
            mem::swap(&mut m, &mut *self.ucell.get());
            m.assume_init()
        }
    }

    // The reason for the the lifetime parameter 'a is
    // that `self` actually contains the data in its interior, so it needs
    // to outlive the returned borrow.
    #[inline(always)]
    #[verifier::external_body]
    pub fn borrow<'a>(&'a self, Tracked(perm): Tracked<&'a PointsTo<V>>) -> (v: &'a V)
        requires
            self.id() === perm@.pcell,
            perm.is_init(),
        ensures
            *v === perm.value(),
        opens_invariants none
        no_unwind
    {
        unsafe { (*self.ucell.get()).assume_init_ref() }
    }

    //////////////////////////////////
    // Untrusted functions below here
    #[inline(always)]
    pub fn into_inner(self, Tracked(perm): Tracked<PointsTo<V>>) -> (v: V)
        requires
            self.id() === perm@.pcell,
            perm.is_init(),
        ensures
            v === perm.value(),
        opens_invariants none
        no_unwind
    {
        let tracked mut perm = perm;
        self.take(Tracked(&mut perm))
    }
    // TODO this should replace the external_body implementation of `new` above;
    // however it requires unstable features: const_mut_refs and const_refs_to_cell
    //#[inline(always)]
    //pub const fn new(v: V) -> (pt: (PCell<V>, Tracked<PointsTo<V>>))
    //    ensures (pt.1@@ === PointsToData{ pcell: pt.0.id(), value: MemContents::Init(v) }),
    //{
    //    let (p, Tracked(mut t)) = Self::empty();
    //    p.put(Tracked(&mut t), v);
    //    (p, Tracked(t))
    //}

}

impl<V: Copy> PCell<V> {
    #[inline(always)]
    #[verifier::external_body]
    pub fn write(&self, Tracked(perm): Tracked<&mut PointsTo<V>>, in_v: V)
        requires
            self.id() === old(perm)@.pcell,
            old(perm).is_init(),
        ensures
            perm.id() === old(perm)@.pcell,
            perm.mem_contents() === MemContents::Init(in_v),
        opens_invariants none
        no_unwind
    {
        let _out = self.replace(Tracked(&mut *perm), in_v);
    }
}

struct InvCellPred {}

impl<T> InvariantPredicate<(Set<T>, PCell<T>), PointsTo<T>> for InvCellPred {
    closed spec fn inv(k: (Set<T>, PCell<T>), perm: PointsTo<T>) -> bool {
        let (possible_values, pcell) = k;
        {
            &&& perm.is_init()
            &&& possible_values.contains(perm.value())
            &&& pcell.id() === perm@.pcell
        }
    }
}

#[verifier::reject_recursive_types(T)]
pub struct InvCell<T> {
    possible_values: Ghost<Set<T>>,
    pcell: PCell<T>,
    perm_inv: Tracked<LocalInvariant<(Set<T>, PCell<T>), PointsTo<T>, InvCellPred>>,
}

impl<T> InvCell<T> {
    #[verifier::type_invariant]
    closed spec fn wf(&self) -> bool {
        &&& self.perm_inv@.constant() === (self.possible_values@, self.pcell)
    }

    pub closed spec fn inv(&self, val: T) -> bool {
        &&& self.possible_values@.contains(val)
    }

    pub fn new(val: T, Ghost(f): Ghost<spec_fn(T) -> bool>) -> (cell: Self)
        requires
            f(val),
        ensures
            forall|v| f(v) <==> cell.inv(v),
    {
        let (pcell, Tracked(perm)) = PCell::new(val);
        let ghost possible_values = Set::new(f);
        let tracked perm_inv = LocalInvariant::new((possible_values, pcell), perm, 0);
        InvCell { possible_values: Ghost(possible_values), pcell, perm_inv: Tracked(perm_inv) }
    }
}

impl<T> InvCell<T> {
    pub fn replace(&self, val: T) -> (old_val: T)
        requires
            self.inv(val),
        ensures
            self.inv(old_val),
    {
        proof {
            use_type_invariant(self);
        }
        let r;
        open_local_invariant!(self.perm_inv.borrow() => perm => {
            r = self.pcell.replace(Tracked(&mut perm), val);
        });
        r
    }
}

impl<T: Copy> InvCell<T> {
    pub fn get(&self) -> (val: T)
        ensures
            self.inv(val),
    {
        proof {
            use_type_invariant(self);
        }
        let r;
        open_local_invariant!(self.perm_inv.borrow() => perm => {
            r = *self.pcell.borrow(Tracked(&perm));
        });
        r
    }
}

} // verus!