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
//! Code for projecting associated types out of trait references.

use super::PredicateObligation;

use crate::infer::InferCtxtUndoLogs;

use rustc_data_structures::{
    snapshot_map::{self, SnapshotMapRef, SnapshotMapStorage},
    undo_log::Rollback,
};
use rustc_middle::ty::{self, Ty};

pub use rustc_middle::traits::{EvaluationResult, Reveal};

pub(crate) type UndoLog<'tcx> =
    snapshot_map::UndoLog<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>;

#[derive(Clone)]
pub struct MismatchedProjectionTypes<'tcx> {
    pub err: ty::error::TypeError<'tcx>,
}

#[derive(Clone)]
pub struct Normalized<'tcx, T> {
    pub value: T,
    pub obligations: Vec<PredicateObligation<'tcx>>,
}

pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;

impl<'tcx, T> Normalized<'tcx, T> {
    pub fn with<U>(self, value: U) -> Normalized<'tcx, U> {
        Normalized { value, obligations: self.obligations }
    }
}

// # Cache

/// The projection cache. Unlike the standard caches, this can include
/// infcx-dependent type variables, therefore we have to roll the
/// cache back each time we roll a snapshot back, to avoid assumptions
/// on yet-unresolved inference variables. Types with placeholder
/// regions also have to be removed when the respective snapshot ends.
///
/// Because of that, projection cache entries can be "stranded" and left
/// inaccessible when type variables inside the key are resolved. We make no
/// attempt to recover or remove "stranded" entries, but rather let them be
/// (for the lifetime of the infcx).
///
/// Entries in the projection cache might contain inference variables
/// that will be resolved by obligations on the projection cache entry (e.g.,
/// when a type parameter in the associated type is constrained through
/// an "RFC 447" projection on the impl).
///
/// When working with a fulfillment context, the derived obligations of each
/// projection cache entry will be registered on the fulfillcx, so any users
/// that can wait for a fulfillcx fixed point need not care about this. However,
/// users that don't wait for a fixed point (e.g., trait evaluation) have to
/// resolve the obligations themselves to make sure the projected result is
/// ok and avoid issues like #43132.
///
/// If that is done, after evaluation the obligations, it is a good idea to
/// call `ProjectionCache::complete` to make sure the obligations won't be
/// re-evaluated and avoid an exponential worst-case.
//
// FIXME: we probably also want some sort of cross-infcx cache here to
// reduce the amount of duplication. Let's see what we get with the Chalk reforms.
pub struct ProjectionCache<'a, 'tcx> {
    map: &'a mut SnapshotMapStorage<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
    undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
}

#[derive(Clone, Default)]
pub struct ProjectionCacheStorage<'tcx> {
    map: SnapshotMapStorage<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
}

#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
pub struct ProjectionCacheKey<'tcx> {
    ty: ty::AliasTy<'tcx>,
}

impl<'tcx> ProjectionCacheKey<'tcx> {
    pub fn new(ty: ty::AliasTy<'tcx>) -> Self {
        Self { ty }
    }
}

#[derive(Clone, Debug)]
pub enum ProjectionCacheEntry<'tcx> {
    InProgress,
    Ambiguous,
    Recur,
    Error,
    NormalizedTy {
        ty: Normalized<'tcx, ty::Term<'tcx>>,
        /// If we were able to successfully evaluate the
        /// corresponding cache entry key during predicate
        /// evaluation, then this field stores the final
        /// result obtained from evaluating all of the projection
        /// sub-obligations. During evaluation, we will skip
        /// evaluating the cached sub-obligations in `ty`
        /// if this field is set. Evaluation only
        /// cares about the final result, so we don't
        /// care about any region constraint side-effects
        /// produced by evaluating the sub-obligations.
        ///
        /// Additionally, we will clear out the sub-obligations
        /// entirely if we ever evaluate the cache entry (along
        /// with all its sub obligations) to `EvaluatedToOk`.
        /// This affects all users of the cache, not just evaluation.
        /// Since a result of `EvaluatedToOk` means that there were
        /// no region obligations that need to be tracked, it's
        /// fine to forget about the sub-obligations - they
        /// don't provide any additional information. However,
        /// we do *not* discard any obligations when we see
        /// `EvaluatedToOkModuloRegions` - we don't know
        /// which sub-obligations may introduce region constraints,
        /// so we keep them all to be safe.
        ///
        /// When we are not performing evaluation
        /// (e.g. in `FulfillmentContext`), we ignore this field,
        /// and always re-process the cached sub-obligations
        /// (which may have been cleared out - see the above
        /// paragraph).
        /// This ensures that we do not lose any regions
        /// constraints that arise from processing the
        /// sub-obligations.
        complete: Option<EvaluationResult>,
    },
}

impl<'tcx> ProjectionCacheStorage<'tcx> {
    #[inline]
    pub(crate) fn with_log<'a>(
        &'a mut self,
        undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
    ) -> ProjectionCache<'a, 'tcx> {
        ProjectionCache { map: &mut self.map, undo_log }
    }
}

impl<'tcx> ProjectionCache<'_, 'tcx> {
    #[inline]
    fn map(
        &mut self,
    ) -> SnapshotMapRef<
        '_,
        ProjectionCacheKey<'tcx>,
        ProjectionCacheEntry<'tcx>,
        InferCtxtUndoLogs<'tcx>,
    > {
        self.map.with_log(self.undo_log)
    }

    pub fn clear(&mut self) {
        self.map().clear();
    }

    /// Try to start normalize `key`; returns an error if
    /// normalization already occurred (this error corresponds to a
    /// cache hit, so it's actually a good thing).
    pub fn try_start(
        &mut self,
        key: ProjectionCacheKey<'tcx>,
    ) -> Result<(), ProjectionCacheEntry<'tcx>> {
        let mut map = self.map();
        if let Some(entry) = map.get(&key) {
            return Err(entry.clone());
        }

        map.insert(key, ProjectionCacheEntry::InProgress);
        Ok(())
    }

    /// Indicates that `key` was normalized to `value`.
    pub fn insert_term(
        &mut self,
        key: ProjectionCacheKey<'tcx>,
        value: Normalized<'tcx, ty::Term<'tcx>>,
    ) {
        debug!(
            "ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
            key, value
        );
        let mut map = self.map();
        if let Some(ProjectionCacheEntry::Recur) = map.get(&key) {
            debug!("Not overwriting Recur");
            return;
        }
        let fresh_key =
            map.insert(key, ProjectionCacheEntry::NormalizedTy { ty: value, complete: None });
        assert!(!fresh_key, "never started projecting `{key:?}`");
    }

    /// Mark the relevant projection cache key as having its derived obligations
    /// complete, so they won't have to be re-computed (this is OK to do in a
    /// snapshot - if the snapshot is rolled back, the obligations will be
    /// marked as incomplete again).
    pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>, result: EvaluationResult) {
        let mut map = self.map();
        match map.get(&key) {
            Some(ProjectionCacheEntry::NormalizedTy { ty, complete: _ }) => {
                info!("ProjectionCacheEntry::complete({:?}) - completing {:?}", key, ty);
                let mut ty = ty.clone();
                if result.must_apply_considering_regions() {
                    ty.obligations = vec![];
                }
                map.insert(key, ProjectionCacheEntry::NormalizedTy { ty, complete: Some(result) });
            }
            ref value => {
                // Type inference could "strand behind" old cache entries. Leave
                // them alone for now.
                info!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}", key, value);
            }
        };
    }

    pub fn is_complete(&mut self, key: ProjectionCacheKey<'tcx>) -> Option<EvaluationResult> {
        self.map().get(&key).and_then(|res| match res {
            ProjectionCacheEntry::NormalizedTy { ty: _, complete } => *complete,
            _ => None,
        })
    }

    /// Indicates that trying to normalize `key` resulted in
    /// ambiguity. No point in trying it again then until we gain more
    /// type information (in which case, the "fully resolved" key will
    /// be different).
    pub fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
        let fresh = self.map().insert(key, ProjectionCacheEntry::Ambiguous);
        assert!(!fresh, "never started projecting `{key:?}`");
    }

    /// Indicates that while trying to normalize `key`, `key` was required to
    /// be normalized again. Selection or evaluation should eventually report
    /// an error here.
    pub fn recur(&mut self, key: ProjectionCacheKey<'tcx>) {
        let fresh = self.map().insert(key, ProjectionCacheEntry::Recur);
        assert!(!fresh, "never started projecting `{key:?}`");
    }

    /// Indicates that trying to normalize `key` resulted in
    /// error.
    pub fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
        let fresh = self.map().insert(key, ProjectionCacheEntry::Error);
        assert!(!fresh, "never started projecting `{key:?}`");
    }
}

impl<'tcx> Rollback<UndoLog<'tcx>> for ProjectionCacheStorage<'tcx> {
    fn reverse(&mut self, undo: UndoLog<'tcx>) {
        self.map.reverse(undo);
    }
}