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
//! Canonicalization is used to separate some goal from its context,
//! throwing away unnecessary information in the process.
//!
//! This is necessary to cache goals containing inference variables
//! and placeholders without restricting them to the current `InferCtxt`.
//!
//! Canonicalization is fairly involved, for more details see the relevant
//! section of the [rustc-dev-guide][c].
//!
//! [c]: https://rustc-dev-guide.rust-lang.org/solve/canonicalization.html

use std::iter;

use rustc_index::IndexVec;
use rustc_type_ir::fold::TypeFoldable;
use rustc_type_ir::inherent::*;
use rustc_type_ir::{self as ty, Canonical, CanonicalVarValues, InferCtxtLike, Interner};
use tracing::{instrument, trace};

use crate::canonicalizer::{CanonicalizeMode, Canonicalizer};
use crate::delegate::SolverDelegate;
use crate::resolve::EagerResolver;
use crate::solve::eval_ctxt::NestedGoals;
use crate::solve::inspect;
use crate::solve::{
    response_no_constraints_raw, CanonicalInput, CanonicalResponse, Certainty, EvalCtxt,
    ExternalConstraintsData, Goal, MaybeCause, NestedNormalizationGoals, NoSolution,
    PredefinedOpaquesData, QueryInput, QueryResult, Response,
};

trait ResponseT<I: Interner> {
    fn var_values(&self) -> CanonicalVarValues<I>;
}

impl<I: Interner> ResponseT<I> for Response<I> {
    fn var_values(&self) -> CanonicalVarValues<I> {
        self.var_values
    }
}

impl<I: Interner, T> ResponseT<I> for inspect::State<I, T> {
    fn var_values(&self) -> CanonicalVarValues<I> {
        self.var_values
    }
}

impl<D, I> EvalCtxt<'_, D>
where
    D: SolverDelegate<Interner = I>,
    I: Interner,
{
    /// Canonicalizes the goal remembering the original values
    /// for each bound variable.
    pub(super) fn canonicalize_goal<T: TypeFoldable<I>>(
        &self,
        goal: Goal<I, T>,
    ) -> (Vec<I::GenericArg>, CanonicalInput<I, T>) {
        let opaque_types = self.delegate.clone_opaque_types_for_query_response();
        let (goal, opaque_types) =
            (goal, opaque_types).fold_with(&mut EagerResolver::new(self.delegate));

        let mut orig_values = Default::default();
        let canonical_goal = Canonicalizer::canonicalize(
            self.delegate,
            CanonicalizeMode::Input,
            &mut orig_values,
            QueryInput {
                goal,
                predefined_opaques_in_body: self
                    .cx()
                    .mk_predefined_opaques_in_body(PredefinedOpaquesData { opaque_types }),
            },
        );
        (orig_values, canonical_goal)
    }

    /// To return the constraints of a canonical query to the caller, we canonicalize:
    ///
    /// - `var_values`: a map from bound variables in the canonical goal to
    ///   the values inferred while solving the instantiated goal.
    /// - `external_constraints`: additional constraints which aren't expressible
    ///   using simple unification of inference variables.
    #[instrument(level = "trace", skip(self), ret)]
    pub(in crate::solve) fn evaluate_added_goals_and_make_canonical_response(
        &mut self,
        certainty: Certainty,
    ) -> QueryResult<I> {
        self.inspect.make_canonical_response(certainty);

        let goals_certainty = self.try_evaluate_added_goals()?;
        assert_eq!(
            self.tainted,
            Ok(()),
            "EvalCtxt is tainted -- nested goals may have been dropped in a \
            previous call to `try_evaluate_added_goals!`"
        );

        // We only check for leaks from universes which were entered inside
        // of the query.
        self.delegate.leak_check(self.max_input_universe).map_err(|NoSolution| {
            trace!("failed the leak check");
            NoSolution
        })?;

        // When normalizing, we've replaced the expected term with an unconstrained
        // inference variable. This means that we dropped information which could
        // have been important. We handle this by instead returning the nested goals
        // to the caller, where they are then handled.
        //
        // As we return all ambiguous nested goals, we can ignore the certainty returned
        // by `try_evaluate_added_goals()`.
        let (certainty, normalization_nested_goals) = if self.is_normalizes_to_goal {
            let NestedGoals { normalizes_to_goals, goals } = std::mem::take(&mut self.nested_goals);
            if cfg!(debug_assertions) {
                assert!(normalizes_to_goals.is_empty());
                if goals.is_empty() {
                    assert!(matches!(goals_certainty, Certainty::Yes));
                }
            }
            (certainty, NestedNormalizationGoals(goals))
        } else {
            let certainty = certainty.unify_with(goals_certainty);
            (certainty, NestedNormalizationGoals::empty())
        };

        let external_constraints =
            self.compute_external_query_constraints(certainty, normalization_nested_goals);
        let (var_values, mut external_constraints) = (self.var_values, external_constraints)
            .fold_with(&mut EagerResolver::new(self.delegate));
        // Remove any trivial region constraints once we've resolved regions
        external_constraints
            .region_constraints
            .retain(|outlives| outlives.0.as_region().map_or(true, |re| re != outlives.1));

        let canonical = Canonicalizer::canonicalize(
            self.delegate,
            CanonicalizeMode::Response { max_input_universe: self.max_input_universe },
            &mut Default::default(),
            Response {
                var_values,
                certainty,
                external_constraints: self.cx().mk_external_constraints(external_constraints),
            },
        );

        Ok(canonical)
    }

    /// Constructs a totally unconstrained, ambiguous response to a goal.
    ///
    /// Take care when using this, since often it's useful to respond with
    /// ambiguity but return constrained variables to guide inference.
    pub(in crate::solve) fn make_ambiguous_response_no_constraints(
        &self,
        maybe_cause: MaybeCause,
    ) -> CanonicalResponse<I> {
        response_no_constraints_raw(
            self.cx(),
            self.max_input_universe,
            self.variables,
            Certainty::Maybe(maybe_cause),
        )
    }

    /// Computes the region constraints and *new* opaque types registered when
    /// proving a goal.
    ///
    /// If an opaque was already constrained before proving this goal, then the
    /// external constraints do not need to record that opaque, since if it is
    /// further constrained by inference, that will be passed back in the var
    /// values.
    #[instrument(level = "trace", skip(self), ret)]
    fn compute_external_query_constraints(
        &self,
        certainty: Certainty,
        normalization_nested_goals: NestedNormalizationGoals<I>,
    ) -> ExternalConstraintsData<I> {
        // We only return region constraints once the certainty is `Yes`. This
        // is necessary as we may drop nested goals on ambiguity, which may result
        // in unconstrained inference variables in the region constraints. It also
        // prevents us from emitting duplicate region constraints, avoiding some
        // unnecessary work. This slightly weakens the leak check in case it uses
        // region constraints from an ambiguous nested goal. This is tested in both
        // `tests/ui/higher-ranked/leak-check/leak-check-in-selection-5-ambig.rs` and
        // `tests/ui/higher-ranked/leak-check/leak-check-in-selection-6-ambig-unify.rs`.
        let region_constraints = if certainty == Certainty::Yes {
            self.delegate.make_deduplicated_outlives_constraints()
        } else {
            Default::default()
        };

        ExternalConstraintsData {
            region_constraints,
            opaque_types: self
                .delegate
                .clone_opaque_types_for_query_response()
                .into_iter()
                // Only return *newly defined* opaque types.
                .filter(|(a, _)| {
                    self.predefined_opaques_in_body.opaque_types.iter().all(|(pa, _)| pa != a)
                })
                .collect(),
            normalization_nested_goals,
        }
    }

    /// After calling a canonical query, we apply the constraints returned
    /// by the query using this function.
    ///
    /// This happens in three steps:
    /// - we instantiate the bound variables of the query response
    /// - we unify the `var_values` of the response with the `original_values`
    /// - we apply the `external_constraints` returned by the query, returning
    ///   the `normalization_nested_goals`
    pub(super) fn instantiate_and_apply_query_response(
        &mut self,
        param_env: I::ParamEnv,
        original_values: Vec<I::GenericArg>,
        response: CanonicalResponse<I>,
    ) -> (NestedNormalizationGoals<I>, Certainty) {
        let instantiation = Self::compute_query_response_instantiation_values(
            self.delegate,
            &original_values,
            &response,
        );

        let Response { var_values, external_constraints, certainty } =
            self.delegate.instantiate_canonical(response, instantiation);

        Self::unify_query_var_values(self.delegate, param_env, &original_values, var_values);

        let ExternalConstraintsData {
            region_constraints,
            opaque_types,
            normalization_nested_goals,
        } = &*external_constraints;
        self.register_region_constraints(region_constraints);
        self.register_new_opaque_types(opaque_types);
        (normalization_nested_goals.clone(), certainty)
    }

    /// This returns the canoncial variable values to instantiate the bound variables of
    /// the canonical response. This depends on the `original_values` for the
    /// bound variables.
    fn compute_query_response_instantiation_values<T: ResponseT<I>>(
        delegate: &D,
        original_values: &[I::GenericArg],
        response: &Canonical<I, T>,
    ) -> CanonicalVarValues<I> {
        // FIXME: Longterm canonical queries should deal with all placeholders
        // created inside of the query directly instead of returning them to the
        // caller.
        let prev_universe = delegate.universe();
        let universes_created_in_query = response.max_universe.index();
        for _ in 0..universes_created_in_query {
            delegate.create_next_universe();
        }

        let var_values = response.value.var_values();
        assert_eq!(original_values.len(), var_values.len());

        // If the query did not make progress with constraining inference variables,
        // we would normally create a new inference variables for bound existential variables
        // only then unify this new inference variable with the inference variable from
        // the input.
        //
        // We therefore instantiate the existential variable in the canonical response with the
        // inference variable of the input right away, which is more performant.
        let mut opt_values = IndexVec::from_elem_n(None, response.variables.len());
        for (original_value, result_value) in
            iter::zip(original_values, var_values.var_values.iter())
        {
            match result_value.kind() {
                ty::GenericArgKind::Type(t) => {
                    if let ty::Bound(debruijn, b) = t.kind() {
                        assert_eq!(debruijn, ty::INNERMOST);
                        opt_values[b.var()] = Some(*original_value);
                    }
                }
                ty::GenericArgKind::Lifetime(r) => {
                    if let ty::ReBound(debruijn, br) = r.kind() {
                        assert_eq!(debruijn, ty::INNERMOST);
                        opt_values[br.var()] = Some(*original_value);
                    }
                }
                ty::GenericArgKind::Const(c) => {
                    if let ty::ConstKind::Bound(debruijn, bv) = c.kind() {
                        assert_eq!(debruijn, ty::INNERMOST);
                        opt_values[bv.var()] = Some(*original_value);
                    }
                }
            }
        }

        let var_values = delegate.cx().mk_args_from_iter(
            response.variables.iter().enumerate().map(|(index, info)| {
                if info.universe() != ty::UniverseIndex::ROOT {
                    // A variable from inside a binder of the query. While ideally these shouldn't
                    // exist at all (see the FIXME at the start of this method), we have to deal with
                    // them for now.
                    delegate.instantiate_canonical_var_with_infer(info, |idx| {
                        ty::UniverseIndex::from(prev_universe.index() + idx.index())
                    })
                } else if info.is_existential() {
                    // As an optimization we sometimes avoid creating a new inference variable here.
                    //
                    // All new inference variables we create start out in the current universe of the caller.
                    // This is conceptually wrong as these inference variables would be able to name
                    // more placeholders then they should be able to. However the inference variables have
                    // to "come from somewhere", so by equating them with the original values of the caller
                    // later on, we pull them down into their correct universe again.
                    if let Some(v) = opt_values[ty::BoundVar::from_usize(index)] {
                        v
                    } else {
                        delegate.instantiate_canonical_var_with_infer(info, |_| prev_universe)
                    }
                } else {
                    // For placeholders which were already part of the input, we simply map this
                    // universal bound variable back the placeholder of the input.
                    original_values[info.expect_placeholder_index()]
                }
            }),
        );

        CanonicalVarValues { var_values }
    }

    /// Unify the `original_values` with the `var_values` returned by the canonical query..
    ///
    /// This assumes that this unification will always succeed. This is the case when
    /// applying a query response right away. However, calling a canonical query, doing any
    /// other kind of trait solving, and only then instantiating the result of the query
    /// can cause the instantiation to fail. This is not supported and we ICE in this case.
    ///
    /// We always structurally instantiate aliases. Relating aliases needs to be different
    /// depending on whether the alias is *rigid* or not. We're only really able to tell
    /// whether an alias is rigid by using the trait solver. When instantiating a response
    /// from the solver we assume that the solver correctly handled aliases and therefore
    /// always relate them structurally here.
    #[instrument(level = "trace", skip(delegate))]
    fn unify_query_var_values(
        delegate: &D,
        param_env: I::ParamEnv,
        original_values: &[I::GenericArg],
        var_values: CanonicalVarValues<I>,
    ) {
        assert_eq!(original_values.len(), var_values.len());

        for (&orig, response) in iter::zip(original_values, var_values.var_values.iter()) {
            let goals =
                delegate.eq_structurally_relating_aliases(param_env, orig, response).unwrap();
            assert!(goals.is_empty());
        }
    }

    fn register_region_constraints(
        &mut self,
        outlives: &[ty::OutlivesPredicate<I, I::GenericArg>],
    ) {
        for &ty::OutlivesPredicate(lhs, rhs) in outlives {
            match lhs.kind() {
                ty::GenericArgKind::Lifetime(lhs) => self.register_region_outlives(lhs, rhs),
                ty::GenericArgKind::Type(lhs) => self.register_ty_outlives(lhs, rhs),
                ty::GenericArgKind::Const(_) => panic!("const outlives: {lhs:?}: {rhs:?}"),
            }
        }
    }

    fn register_new_opaque_types(&mut self, opaque_types: &[(ty::OpaqueTypeKey<I>, I::Ty)]) {
        for &(key, ty) in opaque_types {
            self.delegate.inject_new_hidden_type_unchecked(key, ty);
        }
    }
}

/// Used by proof trees to be able to recompute intermediate actions while
/// evaluating a goal. The `var_values` not only include the bound variables
/// of the query input, but also contain all unconstrained inference vars
/// created while evaluating this goal.
pub(in crate::solve) fn make_canonical_state<D, T, I>(
    delegate: &D,
    var_values: &[I::GenericArg],
    max_input_universe: ty::UniverseIndex,
    data: T,
) -> inspect::CanonicalState<I, T>
where
    D: SolverDelegate<Interner = I>,
    I: Interner,
    T: TypeFoldable<I>,
{
    let var_values = CanonicalVarValues { var_values: delegate.cx().mk_args(var_values) };
    let state = inspect::State { var_values, data };
    let state = state.fold_with(&mut EagerResolver::new(delegate));
    Canonicalizer::canonicalize(
        delegate,
        CanonicalizeMode::Response { max_input_universe },
        &mut vec![],
        state,
    )
}

// FIXME: needs to be pub to be accessed by downstream
// `rustc_trait_selection::solve::inspect::analyse`.
pub fn instantiate_canonical_state<D, I, T: TypeFoldable<I>>(
    delegate: &D,
    span: D::Span,
    param_env: I::ParamEnv,
    orig_values: &mut Vec<I::GenericArg>,
    state: inspect::CanonicalState<I, T>,
) -> T
where
    D: SolverDelegate<Interner = I>,
    I: Interner,
{
    // In case any fresh inference variables have been created between `state`
    // and the previous instantiation, extend `orig_values` for it.
    assert!(orig_values.len() <= state.value.var_values.len());
    for &arg in &state.value.var_values.var_values.as_slice()
        [orig_values.len()..state.value.var_values.len()]
    {
        // FIXME: This is so ugly.
        let unconstrained = delegate.fresh_var_for_kind_with_span(arg, span);
        orig_values.push(unconstrained);
    }

    let instantiation =
        EvalCtxt::compute_query_response_instantiation_values(delegate, orig_values, &state);

    let inspect::State { var_values, data } = delegate.instantiate_canonical(state, instantiation);

    EvalCtxt::unify_query_var_values(delegate, param_env, orig_values, var_values);
    data
}