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
//! The next-generation trait solver, currently still WIP.
//!
//! As a user of rust, you can use `-Znext-solver` to enable the new trait solver.
//!
//! As a developer of rustc, you shouldn't be using the new trait
//! solver without asking the trait-system-refactor-initiative, but it can
//! be enabled with `InferCtxtBuilder::with_next_trait_solver`. This will
//! ensure that trait solving using that inference context will be routed
//! to the new trait solver.
//!
//! For a high-level overview of how this solver works, check out the relevant
//! section of the rustc-dev-guide.
//!
//! FIXME(@lcnr): Write that section. If you read this before then ask me
//! about it on zulip.
use rustc_hir::def_id::DefId;
use rustc_infer::infer::canonical::{Canonical, CanonicalVarValues};
use rustc_infer::traits::query::NoSolution;
use rustc_middle::infer::canonical::CanonicalVarInfos;
use rustc_middle::traits::solve::{
    CanonicalResponse, Certainty, ExternalConstraintsData, Goal, GoalSource, QueryResult, Response,
};
use rustc_middle::ty::{self, AliasRelationDirection, Ty, TyCtxt, UniverseIndex};
use rustc_middle::ty::{
    CoercePredicate, RegionOutlivesPredicate, SubtypePredicate, TypeOutlivesPredicate,
};

mod alias_relate;
mod assembly;
mod eval_ctxt;
mod fulfill;
pub mod inspect;
mod normalize;
mod normalizes_to;
mod project_goals;
mod search_graph;
mod trait_goals;

pub use eval_ctxt::{EvalCtxt, GenerateProofTree, InferCtxtEvalExt, InferCtxtSelectExt};
pub use fulfill::FulfillmentCtxt;
pub(crate) use normalize::deeply_normalize_for_diagnostics;
pub use normalize::{deeply_normalize, deeply_normalize_with_skipped_universes};

/// How many fixpoint iterations we should attempt inside of the solver before bailing
/// with overflow.
///
/// We previously used  `tcx.recursion_limit().0.checked_ilog2().unwrap_or(0)` for this.
/// However, it feels unlikely that uncreasing the recursion limit by a power of two
/// to get one more itereation is every useful or desirable. We now instead used a constant
/// here. If there ever ends up some use-cases where a bigger number of fixpoint iterations
/// is required, we can add a new attribute for that or revert this to be dependant on the
/// recursion limit again. However, this feels very unlikely.
const FIXPOINT_STEP_LIMIT: usize = 8;

#[derive(Debug, Clone, Copy)]
enum SolverMode {
    /// Ordinary trait solving, using everywhere except for coherence.
    Normal,
    /// Trait solving during coherence. There are a few notable differences
    /// between coherence and ordinary trait solving.
    ///
    /// Most importantly, trait solving during coherence must not be incomplete,
    /// i.e. return `Err(NoSolution)` for goals for which a solution exists.
    /// This means that we must not make any guesses or arbitrary choices.
    Coherence,
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
enum GoalEvaluationKind {
    Root,
    Nested,
}

#[extension(trait CanonicalResponseExt)]
impl<'tcx> Canonical<'tcx, Response<'tcx>> {
    fn has_no_inference_or_external_constraints(&self) -> bool {
        self.value.external_constraints.region_constraints.is_empty()
            && self.value.var_values.is_identity()
            && self.value.external_constraints.opaque_types.is_empty()
    }
}

impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
    #[instrument(level = "debug", skip(self))]
    fn compute_type_outlives_goal(
        &mut self,
        goal: Goal<'tcx, TypeOutlivesPredicate<'tcx>>,
    ) -> QueryResult<'tcx> {
        let ty::OutlivesPredicate(ty, lt) = goal.predicate;
        self.register_ty_outlives(ty, lt);
        self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
    }

    #[instrument(level = "debug", skip(self))]
    fn compute_region_outlives_goal(
        &mut self,
        goal: Goal<'tcx, RegionOutlivesPredicate<'tcx>>,
    ) -> QueryResult<'tcx> {
        let ty::OutlivesPredicate(a, b) = goal.predicate;
        self.register_region_outlives(a, b);
        self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
    }

    #[instrument(level = "debug", skip(self))]
    fn compute_coerce_goal(
        &mut self,
        goal: Goal<'tcx, CoercePredicate<'tcx>>,
    ) -> QueryResult<'tcx> {
        self.compute_subtype_goal(Goal {
            param_env: goal.param_env,
            predicate: SubtypePredicate {
                a_is_expected: false,
                a: goal.predicate.a,
                b: goal.predicate.b,
            },
        })
    }

    #[instrument(level = "debug", skip(self))]
    fn compute_subtype_goal(
        &mut self,
        goal: Goal<'tcx, SubtypePredicate<'tcx>>,
    ) -> QueryResult<'tcx> {
        if goal.predicate.a.is_ty_var() && goal.predicate.b.is_ty_var() {
            self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
        } else {
            self.sub(goal.param_env, goal.predicate.a, goal.predicate.b)?;
            self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
        }
    }

    fn compute_object_safe_goal(&mut self, trait_def_id: DefId) -> QueryResult<'tcx> {
        if self.tcx().check_is_object_safe(trait_def_id) {
            self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
        } else {
            Err(NoSolution)
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn compute_well_formed_goal(
        &mut self,
        goal: Goal<'tcx, ty::GenericArg<'tcx>>,
    ) -> QueryResult<'tcx> {
        match self.well_formed_goals(goal.param_env, goal.predicate) {
            Some(goals) => {
                self.add_goals(GoalSource::Misc, goals);
                self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
            }
            None => self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS),
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn compute_const_evaluatable_goal(
        &mut self,
        Goal { param_env, predicate: ct }: Goal<'tcx, ty::Const<'tcx>>,
    ) -> QueryResult<'tcx> {
        match ct.kind() {
            ty::ConstKind::Unevaluated(uv) => {
                // We never return `NoSolution` here as `try_const_eval_resolve` emits an
                // error itself when failing to evaluate, so emitting an additional fulfillment
                // error in that case is unnecessary noise. This may change in the future once
                // evaluation failures are allowed to impact selection, e.g. generic const
                // expressions in impl headers or `where`-clauses.

                // FIXME(generic_const_exprs): Implement handling for generic
                // const expressions here.
                if let Some(_normalized) = self.try_const_eval_resolve(param_env, uv, ct.ty()) {
                    self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
                } else {
                    self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
                }
            }
            ty::ConstKind::Infer(_) => {
                self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
            }
            ty::ConstKind::Placeholder(_) | ty::ConstKind::Value(_) | ty::ConstKind::Error(_) => {
                self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
            }
            // We can freely ICE here as:
            // - `Param` gets replaced with a placeholder during canonicalization
            // - `Bound` cannot exist as we don't have a binder around the self Type
            // - `Expr` is part of `feature(generic_const_exprs)` and is not implemented yet
            ty::ConstKind::Param(_) | ty::ConstKind::Bound(_, _) | ty::ConstKind::Expr(_) => {
                bug!("unexpect const kind: {:?}", ct)
            }
        }
    }

    #[instrument(level = "debug", skip(self), ret)]
    fn compute_const_arg_has_type_goal(
        &mut self,
        goal: Goal<'tcx, (ty::Const<'tcx>, Ty<'tcx>)>,
    ) -> QueryResult<'tcx> {
        let (ct, ty) = goal.predicate;
        self.eq(goal.param_env, ct.ty(), ty)?;
        self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
    }
}

impl<'tcx> EvalCtxt<'_, 'tcx> {
    #[instrument(level = "debug", skip(self))]
    fn add_normalizes_to_goal(&mut self, goal: Goal<'tcx, ty::NormalizesTo<'tcx>>) {
        inspect::ProofTreeBuilder::add_normalizes_to_goal(self, goal);
        self.nested_goals.normalizes_to_goals.push(goal);
    }

    #[instrument(level = "debug", skip(self))]
    fn add_goal(&mut self, source: GoalSource, goal: Goal<'tcx, ty::Predicate<'tcx>>) {
        inspect::ProofTreeBuilder::add_goal(self, source, goal);
        self.nested_goals.goals.push((source, goal));
    }

    #[instrument(level = "debug", skip(self, goals))]
    fn add_goals(
        &mut self,
        source: GoalSource,
        goals: impl IntoIterator<Item = Goal<'tcx, ty::Predicate<'tcx>>>,
    ) {
        for goal in goals {
            self.add_goal(source, goal);
        }
    }

    /// Try to merge multiple possible ways to prove a goal, if that is not possible returns `None`.
    ///
    /// In this case we tend to flounder and return ambiguity by calling `[EvalCtxt::flounder]`.
    #[instrument(level = "debug", skip(self), ret)]
    fn try_merge_responses(
        &mut self,
        responses: &[CanonicalResponse<'tcx>],
    ) -> Option<CanonicalResponse<'tcx>> {
        if responses.is_empty() {
            return None;
        }

        // FIXME(-Znext-solver): We should instead try to find a `Certainty::Yes` response with
        // a subset of the constraints that all the other responses have.
        let one = responses[0];
        if responses[1..].iter().all(|&resp| resp == one) {
            return Some(one);
        }

        responses
            .iter()
            .find(|response| {
                response.value.certainty == Certainty::Yes
                    && response.has_no_inference_or_external_constraints()
            })
            .copied()
    }

    /// If we fail to merge responses we flounder and return overflow or ambiguity.
    #[instrument(level = "debug", skip(self), ret)]
    fn flounder(&mut self, responses: &[CanonicalResponse<'tcx>]) -> QueryResult<'tcx> {
        if responses.is_empty() {
            return Err(NoSolution);
        }

        let Certainty::Maybe(maybe_cause) =
            responses.iter().fold(Certainty::AMBIGUOUS, |certainty, response| {
                certainty.unify_with(response.value.certainty)
            })
        else {
            bug!("expected flounder response to be ambiguous")
        };

        Ok(self.make_ambiguous_response_no_constraints(maybe_cause))
    }

    /// Normalize a type for when it is structurally matched on.
    ///
    /// This function is necessary in nearly all cases before matching on a type.
    /// Not doing so is likely to be incomplete and therefore unsound during
    /// coherence.
    #[instrument(level = "debug", skip(self, param_env), ret)]
    fn structurally_normalize_ty(
        &mut self,
        param_env: ty::ParamEnv<'tcx>,
        ty: Ty<'tcx>,
    ) -> Result<Ty<'tcx>, NoSolution> {
        if let ty::Alias(..) = ty.kind() {
            let normalized_ty = self.next_ty_infer();
            let alias_relate_goal = Goal::new(
                self.tcx(),
                param_env,
                ty::PredicateKind::AliasRelate(
                    ty.into(),
                    normalized_ty.into(),
                    AliasRelationDirection::Equate,
                ),
            );
            self.add_goal(GoalSource::Misc, alias_relate_goal);
            self.try_evaluate_added_goals()?;
            Ok(self.resolve_vars_if_possible(normalized_ty))
        } else {
            Ok(ty)
        }
    }
}

fn response_no_constraints_raw<'tcx>(
    tcx: TyCtxt<'tcx>,
    max_universe: UniverseIndex,
    variables: CanonicalVarInfos<'tcx>,
    certainty: Certainty,
) -> CanonicalResponse<'tcx> {
    Canonical {
        max_universe,
        variables,
        value: Response {
            var_values: CanonicalVarValues::make_identity(tcx, variables),
            // FIXME: maybe we should store the "no response" version in tcx, like
            // we do for tcx.types and stuff.
            external_constraints: tcx.mk_external_constraints(ExternalConstraintsData::default()),
            certainty,
        },
        defining_opaque_types: Default::default(),
    }
}