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
use rustc_hir::def_id::DefId;
use rustc_middle::ty::{self, Ty, TyVid};
use rustc_span::symbol::Symbol;
use rustc_span::Span;

use crate::infer::InferCtxtUndoLogs;

use rustc_data_structures::snapshot_vec as sv;
use rustc_data_structures::unify as ut;
use std::cmp;
use std::marker::PhantomData;
use std::ops::Range;

use rustc_data_structures::undo_log::{Rollback, UndoLogs};

/// Represents a single undo-able action that affects a type inference variable.
#[derive(Clone)]
pub(crate) enum UndoLog<'tcx> {
    EqRelation(sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>),
    SubRelation(sv::UndoLog<ut::Delegate<ty::TyVid>>),
    Values(sv::UndoLog<Delegate>),
}

/// Convert from a specific kind of undo to the more general UndoLog
impl<'tcx> From<sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>> for UndoLog<'tcx> {
    fn from(l: sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>) -> Self {
        UndoLog::EqRelation(l)
    }
}

/// Convert from a specific kind of undo to the more general UndoLog
impl<'tcx> From<sv::UndoLog<ut::Delegate<ty::TyVid>>> for UndoLog<'tcx> {
    fn from(l: sv::UndoLog<ut::Delegate<ty::TyVid>>) -> Self {
        UndoLog::SubRelation(l)
    }
}

/// Convert from a specific kind of undo to the more general UndoLog
impl<'tcx> From<sv::UndoLog<Delegate>> for UndoLog<'tcx> {
    fn from(l: sv::UndoLog<Delegate>) -> Self {
        UndoLog::Values(l)
    }
}

/// Convert from a specific kind of undo to the more general UndoLog
impl<'tcx> From<Instantiate> for UndoLog<'tcx> {
    fn from(l: Instantiate) -> Self {
        UndoLog::Values(sv::UndoLog::Other(l))
    }
}

impl<'tcx> Rollback<UndoLog<'tcx>> for TypeVariableStorage<'tcx> {
    fn reverse(&mut self, undo: UndoLog<'tcx>) {
        match undo {
            UndoLog::EqRelation(undo) => self.eq_relations.reverse(undo),
            UndoLog::SubRelation(undo) => self.sub_relations.reverse(undo),
            UndoLog::Values(undo) => self.values.reverse(undo),
        }
    }
}

#[derive(Clone)]
pub struct TypeVariableStorage<'tcx> {
    values: sv::SnapshotVecStorage<Delegate>,

    /// Two variables are unified in `eq_relations` when we have a
    /// constraint `?X == ?Y`. This table also stores, for each key,
    /// the known value.
    eq_relations: ut::UnificationTableStorage<TyVidEqKey<'tcx>>,

    /// Two variables are unified in `sub_relations` when we have a
    /// constraint `?X <: ?Y` *or* a constraint `?Y <: ?X`. This second
    /// table exists only to help with the occurs check. In particular,
    /// we want to report constraints like these as an occurs check
    /// violation:
    /// ``` text
    /// ?1 <: ?3
    /// Box<?3> <: ?1
    /// ```
    /// Without this second table, what would happen in a case like
    /// this is that we would instantiate `?1` with a generalized
    /// type like `Box<?6>`. We would then relate `Box<?3> <: Box<?6>`
    /// and infer that `?3 <: ?6`. Next, since `?1` was instantiated,
    /// we would process `?1 <: ?3`, generalize `?1 = Box<?6>` to `Box<?9>`,
    /// and instantiate `?3` with `Box<?9>`. Finally, we would relate
    /// `?6 <: ?9`. But now that we instantiated `?3`, we can process
    /// `?3 <: ?6`, which gives us `Box<?9> <: ?6`... and the cycle
    /// continues. (This is `occurs-check-2.rs`.)
    ///
    /// What prevents this cycle is that when we generalize
    /// `Box<?3>` to `Box<?6>`, we also sub-unify `?3` and `?6`
    /// (in the generalizer). When we then process `Box<?6> <: ?3`,
    /// the occurs check then fails because `?6` and `?3` are sub-unified,
    /// and hence generalization fails.
    ///
    /// This is reasonable because, in Rust, subtypes have the same
    /// "skeleton" and hence there is no possible type such that
    /// (e.g.)  `Box<?3> <: ?3` for any `?3`.
    ///
    /// In practice, we sometimes sub-unify variables in other spots, such
    /// as when processing subtype predicates. This is not necessary but is
    /// done to aid diagnostics, as it allows us to be more effective when
    /// we guide the user towards where they should insert type hints.
    sub_relations: ut::UnificationTableStorage<ty::TyVid>,
}

pub struct TypeVariableTable<'a, 'tcx> {
    storage: &'a mut TypeVariableStorage<'tcx>,

    undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
}

#[derive(Copy, Clone, Debug)]
pub struct TypeVariableOrigin {
    pub kind: TypeVariableOriginKind,
    pub span: Span,
}

/// Reasons to create a type inference variable
#[derive(Copy, Clone, Debug)]
pub enum TypeVariableOriginKind {
    MiscVariable,
    NormalizeProjectionType,
    TypeInference,
    TypeParameterDefinition(Symbol, Option<DefId>),

    /// One of the upvars or closure kind parameters in a `ClosureSubsts`
    /// (before it has been determined).
    // FIXME(eddyb) distinguish upvar inference variables from the rest.
    ClosureSynthetic,
    SubstitutionPlaceholder,
    AutoDeref,
    AdjustmentType,

    /// In type check, when we are type checking a function that
    /// returns `-> dyn Foo`, we substitute a type variable for the
    /// return type for diagnostic purposes.
    DynReturnFn,
    LatticeVariable,
}

#[derive(Clone)]
pub(crate) struct TypeVariableData {
    origin: TypeVariableOrigin,
}

#[derive(Copy, Clone, Debug)]
pub enum TypeVariableValue<'tcx> {
    Known { value: Ty<'tcx> },
    Unknown { universe: ty::UniverseIndex },
}

impl<'tcx> TypeVariableValue<'tcx> {
    /// If this value is known, returns the type it is known to be.
    /// Otherwise, `None`.
    pub fn known(&self) -> Option<Ty<'tcx>> {
        match *self {
            TypeVariableValue::Unknown { .. } => None,
            TypeVariableValue::Known { value } => Some(value),
        }
    }

    pub fn is_unknown(&self) -> bool {
        match *self {
            TypeVariableValue::Unknown { .. } => true,
            TypeVariableValue::Known { .. } => false,
        }
    }
}

#[derive(Clone)]
pub(crate) struct Instantiate;

pub(crate) struct Delegate;

impl<'tcx> TypeVariableStorage<'tcx> {
    pub fn new() -> TypeVariableStorage<'tcx> {
        TypeVariableStorage {
            values: sv::SnapshotVecStorage::new(),
            eq_relations: ut::UnificationTableStorage::new(),
            sub_relations: ut::UnificationTableStorage::new(),
        }
    }

    #[inline]
    pub(crate) fn with_log<'a>(
        &'a mut self,
        undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
    ) -> TypeVariableTable<'a, 'tcx> {
        TypeVariableTable { storage: self, undo_log }
    }
}

impl<'tcx> TypeVariableTable<'_, 'tcx> {
    /// Returns the origin that was given when `vid` was created.
    ///
    /// Note that this function does not return care whether
    /// `vid` has been unified with something else or not.
    pub fn var_origin(&self, vid: ty::TyVid) -> &TypeVariableOrigin {
        &self.storage.values.get(vid.as_usize()).origin
    }

    /// Records that `a == b`, depending on `dir`.
    ///
    /// Precondition: neither `a` nor `b` are known.
    pub fn equate(&mut self, a: ty::TyVid, b: ty::TyVid) {
        debug_assert!(self.probe(a).is_unknown());
        debug_assert!(self.probe(b).is_unknown());
        self.eq_relations().union(a, b);
        self.sub_relations().union(a, b);
    }

    /// Records that `a <: b`, depending on `dir`.
    ///
    /// Precondition: neither `a` nor `b` are known.
    pub fn sub(&mut self, a: ty::TyVid, b: ty::TyVid) {
        debug_assert!(self.probe(a).is_unknown());
        debug_assert!(self.probe(b).is_unknown());
        self.sub_relations().union(a, b);
    }

    /// Instantiates `vid` with the type `ty`.
    ///
    /// Precondition: `vid` must not have been previously instantiated.
    pub fn instantiate(&mut self, vid: ty::TyVid, ty: Ty<'tcx>) {
        let vid = self.root_var(vid);
        debug_assert!(self.probe(vid).is_unknown());
        debug_assert!(
            self.eq_relations().probe_value(vid).is_unknown(),
            "instantiating type variable `{:?}` twice: new-value = {:?}, old-value={:?}",
            vid,
            ty,
            self.eq_relations().probe_value(vid)
        );
        self.eq_relations().union_value(vid, TypeVariableValue::Known { value: ty });

        // Hack: we only need this so that `types_escaping_snapshot`
        // can see what has been unified; see the Delegate impl for
        // more details.
        self.undo_log.push(Instantiate);
    }

    /// Creates a new type variable.
    ///
    /// - `diverging`: indicates if this is a "diverging" type
    ///   variable, e.g.,  one created as the type of a `return`
    ///   expression. The code in this module doesn't care if a
    ///   variable is diverging, but the main Rust type-checker will
    ///   sometimes "unify" such variables with the `!` or `()` types.
    /// - `origin`: indicates *why* the type variable was created.
    ///   The code in this module doesn't care, but it can be useful
    ///   for improving error messages.
    pub fn new_var(
        &mut self,
        universe: ty::UniverseIndex,
        origin: TypeVariableOrigin,
    ) -> ty::TyVid {
        let eq_key = self.eq_relations().new_key(TypeVariableValue::Unknown { universe });

        let sub_key = self.sub_relations().new_key(());
        assert_eq!(eq_key.vid, sub_key);

        let index = self.values().push(TypeVariableData { origin });
        assert_eq!(eq_key.vid.as_u32(), index as u32);

        debug!("new_var(index={:?}, universe={:?}, origin={:?})", eq_key.vid, universe, origin);

        eq_key.vid
    }

    /// Returns the number of type variables created thus far.
    pub fn num_vars(&self) -> usize {
        self.storage.values.len()
    }

    /// Returns the "root" variable of `vid` in the `eq_relations`
    /// equivalence table. All type variables that have been equated
    /// will yield the same root variable (per the union-find
    /// algorithm), so `root_var(a) == root_var(b)` implies that `a ==
    /// b` (transitively).
    pub fn root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
        self.eq_relations().find(vid).vid
    }

    /// Returns the "root" variable of `vid` in the `sub_relations`
    /// equivalence table. All type variables that have been are
    /// related via equality or subtyping will yield the same root
    /// variable (per the union-find algorithm), so `sub_root_var(a)
    /// == sub_root_var(b)` implies that:
    /// ```text
    /// exists X. (a <: X || X <: a) && (b <: X || X <: b)
    /// ```
    pub fn sub_root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
        self.sub_relations().find(vid)
    }

    /// Returns `true` if `a` and `b` have same "sub-root" (i.e., exists some
    /// type X such that `forall i in {a, b}. (i <: X || X <: i)`.
    pub fn sub_unified(&mut self, a: ty::TyVid, b: ty::TyVid) -> bool {
        self.sub_root_var(a) == self.sub_root_var(b)
    }

    /// Retrieves the type to which `vid` has been instantiated, if
    /// any.
    pub fn probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
        self.inlined_probe(vid)
    }

    /// An always-inlined variant of `probe`, for very hot call sites.
    #[inline(always)]
    pub fn inlined_probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
        self.eq_relations().inlined_probe_value(vid)
    }

    /// If `t` is a type-inference variable, and it has been
    /// instantiated, then return the with which it was
    /// instantiated. Otherwise, returns `t`.
    pub fn replace_if_possible(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
        match *t.kind() {
            ty::Infer(ty::TyVar(v)) => match self.probe(v) {
                TypeVariableValue::Unknown { .. } => t,
                TypeVariableValue::Known { value } => value,
            },
            _ => t,
        }
    }

    #[inline]
    fn values(
        &mut self,
    ) -> sv::SnapshotVec<Delegate, &mut Vec<TypeVariableData>, &mut InferCtxtUndoLogs<'tcx>> {
        self.storage.values.with_log(self.undo_log)
    }

    #[inline]
    fn eq_relations(&mut self) -> super::UnificationTable<'_, 'tcx, TyVidEqKey<'tcx>> {
        self.storage.eq_relations.with_log(self.undo_log)
    }

    #[inline]
    fn sub_relations(&mut self) -> super::UnificationTable<'_, 'tcx, ty::TyVid> {
        self.storage.sub_relations.with_log(self.undo_log)
    }

    /// Returns a range of the type variables created during the snapshot.
    pub fn vars_since_snapshot(
        &mut self,
        value_count: usize,
    ) -> (Range<TyVid>, Vec<TypeVariableOrigin>) {
        let range = TyVid::from_usize(value_count)..TyVid::from_usize(self.num_vars());
        (
            range.start..range.end,
            (range.start.as_usize()..range.end.as_usize())
                .map(|index| self.storage.values.get(index).origin)
                .collect(),
        )
    }

    /// Returns indices of all variables that are not yet
    /// instantiated.
    pub fn unsolved_variables(&mut self) -> Vec<ty::TyVid> {
        (0..self.storage.values.len())
            .filter_map(|i| {
                let vid = ty::TyVid::from_usize(i);
                match self.probe(vid) {
                    TypeVariableValue::Unknown { .. } => Some(vid),
                    TypeVariableValue::Known { .. } => None,
                }
            })
            .collect()
    }
}

impl sv::SnapshotVecDelegate for Delegate {
    type Value = TypeVariableData;
    type Undo = Instantiate;

    fn reverse(_values: &mut Vec<TypeVariableData>, _action: Instantiate) {
        // We don't actually have to *do* anything to reverse an
        // instantiation; the value for a variable is stored in the
        // `eq_relations` and hence its rollback code will handle
        // it. In fact, we could *almost* just remove the
        // `SnapshotVec` entirely, except that we would have to
        // reproduce *some* of its logic, since we want to know which
        // type variables have been instantiated since the snapshot
        // was started, so we can implement `types_escaping_snapshot`.
        //
        // (If we extended the `UnificationTable` to let us see which
        // values have been unified and so forth, that might also
        // suffice.)
    }
}

///////////////////////////////////////////////////////////////////////////

/// These structs (a newtyped TyVid) are used as the unification key
/// for the `eq_relations`; they carry a `TypeVariableValue` along
/// with them.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub(crate) struct TyVidEqKey<'tcx> {
    vid: ty::TyVid,

    // in the table, we map each ty-vid to one of these:
    phantom: PhantomData<TypeVariableValue<'tcx>>,
}

impl<'tcx> From<ty::TyVid> for TyVidEqKey<'tcx> {
    #[inline] // make this function eligible for inlining - it is quite hot.
    fn from(vid: ty::TyVid) -> Self {
        TyVidEqKey { vid, phantom: PhantomData }
    }
}

impl<'tcx> ut::UnifyKey for TyVidEqKey<'tcx> {
    type Value = TypeVariableValue<'tcx>;
    #[inline(always)]
    fn index(&self) -> u32 {
        self.vid.as_u32()
    }
    #[inline]
    fn from_index(i: u32) -> Self {
        TyVidEqKey::from(ty::TyVid::from_u32(i))
    }
    fn tag() -> &'static str {
        "TyVidEqKey"
    }
}

impl<'tcx> ut::UnifyValue for TypeVariableValue<'tcx> {
    type Error = ut::NoError;

    fn unify_values(value1: &Self, value2: &Self) -> Result<Self, ut::NoError> {
        match (value1, value2) {
            // We never equate two type variables, both of which
            // have known types.  Instead, we recursively equate
            // those types.
            (&TypeVariableValue::Known { .. }, &TypeVariableValue::Known { .. }) => {
                bug!("equating two type variables, both of which have known types")
            }

            // If one side is known, prefer that one.
            (&TypeVariableValue::Known { .. }, &TypeVariableValue::Unknown { .. }) => Ok(*value1),
            (&TypeVariableValue::Unknown { .. }, &TypeVariableValue::Known { .. }) => Ok(*value2),

            // If both sides are *unknown*, it hardly matters, does it?
            (
                &TypeVariableValue::Unknown { universe: universe1 },
                &TypeVariableValue::Unknown { universe: universe2 },
            ) => {
                // If we unify two unbound variables, ?T and ?U, then whatever
                // value they wind up taking (which must be the same value) must
                // be nameable by both universes. Therefore, the resulting
                // universe is the minimum of the two universes, because that is
                // the one which contains the fewest names in scope.
                let universe = cmp::min(universe1, universe2);
                Ok(TypeVariableValue::Unknown { universe })
            }
        }
    }
}