compiler/rustc_trait_selection/src/solve/mod.rs

   1 //! The new trait solver, currently still WIP.
   2 //!
   3 //! As a user of the trait system, you can use `TyCtxt::evaluate_goal` to
   4 //! interact with this solver.
   5 //!
   6 //! For a high-level overview of how this solver works, check out the relevant
   7 //! section of the rustc-dev-guide.
   8 //!
   9 //! FIXME(@lcnr): Write that section. If you read this before then ask me
  10 //! about it on zulip.
  11
  12 // FIXME: Instead of using `infcx.canonicalize_query` we have to add a new routine which
  13 // preserves universes and creates a unique var (in the highest universe) for each
  14 // appearance of a region.
  15
  16 // FIXME: uses of `infcx.at` need to enable deferred projection equality once that's implemented.
  17
  18 use std::mem;
  19
  20 use rustc_hir::def_id::DefId;
  21 use rustc_infer::infer::canonical::{Canonical, CanonicalVarValues};
  22 use rustc_infer::infer::canonical::{OriginalQueryValues, QueryRegionConstraints, QueryResponse};
  23 use rustc_infer::infer::{InferCtxt, InferOk, TyCtxtInferExt};
  24 use rustc_infer::traits::query::NoSolution;
  25 use rustc_infer::traits::Obligation;
  26 use rustc_middle::infer::canonical::Certainty as OldCertainty;
  27 use rustc_middle::ty::{self, Ty, TyCtxt};
  28 use rustc_middle::ty::{
  29     CoercePredicate, RegionOutlivesPredicate, SubtypePredicate, ToPredicate, TypeOutlivesPredicate,
  30 };
  31 use rustc_span::DUMMY_SP;
  32
  33 use crate::traits::ObligationCause;
  34
  35 mod assembly;
  36 mod fulfill;
  37 mod infcx_ext;
  38 mod project_goals;
  39 mod search_graph;
  40 mod trait_goals;
  41
  42 pub use fulfill::FulfillmentCtxt;
  43
  44 /// A goal is a statement, i.e. `predicate`, we want to prove
  45 /// given some assumptions, i.e. `param_env`.
  46 ///
  47 /// Most of the time the `param_env` contains the `where`-bounds of the function
  48 /// we're currently typechecking while the `predicate` is some trait bound.
  49 #[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
  50 pub struct Goal<'tcx, P> {
  51     param_env: ty::ParamEnv<'tcx>,
  52     predicate: P,
  53 }
  54
  55 impl<'tcx, P> Goal<'tcx, P> {
  56     pub fn new(
  57         tcx: TyCtxt<'tcx>,
  58         param_env: ty::ParamEnv<'tcx>,
  59         predicate: impl ToPredicate<'tcx, P>,
  60     ) -> Goal<'tcx, P> {
  61         Goal { param_env, predicate: predicate.to_predicate(tcx) }
  62     }
  63
  64     /// Updates the goal to one with a different `predicate` but the same `param_env`.
  65     fn with<Q>(self, tcx: TyCtxt<'tcx>, predicate: impl ToPredicate<'tcx, Q>) -> Goal<'tcx, Q> {
  66         Goal { param_env: self.param_env, predicate: predicate.to_predicate(tcx) }
  67     }
  68 }
  69
  70 impl<'tcx, P> From<Obligation<'tcx, P>> for Goal<'tcx, P> {
  71     fn from(obligation: Obligation<'tcx, P>) -> Goal<'tcx, P> {
  72         Goal { param_env: obligation.param_env, predicate: obligation.predicate }
  73     }
  74 }
  75
  76 #[derive(Debug, PartialEq, Eq, Clone, Hash, TypeFoldable, TypeVisitable)]
  77 pub struct Response<'tcx> {
  78     pub var_values: CanonicalVarValues<'tcx>,
  79     /// Additional constraints returned by this query.
  80     pub external_constraints: ExternalConstraints<'tcx>,
  81     pub certainty: Certainty,
  82 }
  83
  84 #[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
  85 pub enum Certainty {
  86     Yes,
  87     Maybe(MaybeCause),
  88 }
  89
  90 impl Certainty {
  91     pub const AMBIGUOUS: Certainty = Certainty::Maybe(MaybeCause::Ambiguity);
  92
  93     /// When proving multiple goals using **AND**, e.g. nested obligations for an impl,
  94     /// use this function to unify the certainty of these goals
  95     pub fn unify_and(self, other: Certainty) -> Certainty {
  96         match (self, other) {
  97             (Certainty::Yes, Certainty::Yes) => Certainty::Yes,
  98             (Certainty::Yes, Certainty::Maybe(_)) => other,
  99             (Certainty::Maybe(_), Certainty::Yes) => self,
 100             (Certainty::Maybe(MaybeCause::Overflow), Certainty::Maybe(MaybeCause::Overflow)) => {
 101                 Certainty::Maybe(MaybeCause::Overflow)
 102             }
 103             // If at least one of the goals is ambiguous, hide the overflow as the ambiguous goal
 104             // may still result in failure.
 105             (Certainty::Maybe(MaybeCause::Ambiguity), Certainty::Maybe(_))
 106             | (Certainty::Maybe(_), Certainty::Maybe(MaybeCause::Ambiguity)) => {
 107                 Certainty::Maybe(MaybeCause::Ambiguity)
 108             }
 109         }
 110     }
 111 }
 112
 113 /// Why we failed to evaluate a goal.
 114 #[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
 115 pub enum MaybeCause {
 116     /// We failed due to ambiguity. This ambiguity can either
 117     /// be a true ambiguity, i.e. there are multiple different answers,
 118     /// or we hit a case where we just don't bother, e.g. `?x: Trait` goals.
 119     Ambiguity,
 120     /// We gave up due to an overflow, most often by hitting the recursion limit.
 121     Overflow,
 122 }
 123
 124 /// Additional constraints returned on success.
 125 #[derive(Debug, PartialEq, Eq, Clone, Hash, TypeFoldable, TypeVisitable, Default)]
 126 pub struct ExternalConstraints<'tcx> {
 127     // FIXME: implement this.
 128     regions: (),
 129     opaque_types: Vec<(Ty<'tcx>, Ty<'tcx>)>,
 130 }
 131
 132 type CanonicalGoal<'tcx, T = ty::Predicate<'tcx>> = Canonical<'tcx, Goal<'tcx, T>>;
 133 type CanonicalResponse<'tcx> = Canonical<'tcx, Response<'tcx>>;
 134 /// The result of evaluating a canonical query.
 135 ///
 136 /// FIXME: We use a different type than the existing canonical queries. This is because
 137 /// we need to add a `Certainty` for `overflow` and may want to restructure this code without
 138 /// having to worry about changes to currently used code. Once we've made progress on this
 139 /// solver, merge the two responses again.
 140 pub type QueryResult<'tcx> = Result<CanonicalResponse<'tcx>, NoSolution>;
 141
 142 pub trait InferCtxtEvalExt<'tcx> {
 143     /// Evaluates a goal from **outside** of the trait solver.
 144     ///
 145     /// Using this while inside of the solver is wrong as it uses a new
 146     /// search graph which would break cycle detection.
 147     fn evaluate_root_goal(
 148         &self,
 149         goal: Goal<'tcx, ty::Predicate<'tcx>>,
 150     ) -> Result<(bool, Certainty), NoSolution>;
 151 }
 152
 153 impl<'tcx> InferCtxtEvalExt<'tcx> for InferCtxt<'tcx> {
 154     fn evaluate_root_goal(
 155         &self,
 156         goal: Goal<'tcx, ty::Predicate<'tcx>>,
 157     ) -> Result<(bool, Certainty), NoSolution> {
 158         let mut search_graph = search_graph::SearchGraph::new(self.tcx);
 159
 160         let result = EvalCtxt {
 161             search_graph: &mut search_graph,
 162             infcx: self,
 163             var_values: CanonicalVarValues::dummy(),
 164         }
 165         .evaluate_goal(goal);
 166
 167         assert!(search_graph.is_empty());
 168         result
 169     }
 170 }
 171
 172 struct EvalCtxt<'a, 'tcx> {
 173     infcx: &'a InferCtxt<'tcx>,
 174     var_values: CanonicalVarValues<'tcx>,
 175
 176     search_graph: &'a mut search_graph::SearchGraph<'tcx>,
 177 }
 178
 179 impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
 180     fn tcx(&self) -> TyCtxt<'tcx> {
 181         self.infcx.tcx
 182     }
 183
 184     /// The entry point of the solver.
 185     ///
 186     /// This function deals with (coinductive) cycles, overflow, and caching
 187     /// and then calls [`EvalCtxt::compute_goal`] which contains the actual
 188     /// logic of the solver.
 189     ///
 190     /// Instead of calling this function directly, use either [EvalCtxt::evaluate_goal]
 191     /// if you're inside of the solver or [InferCtxtEvalExt::evaluate_root_goal] if you're
 192     /// outside of it.
 193     #[instrument(level = "debug", skip(tcx, search_graph), ret)]
 194     fn evaluate_canonical_goal(
 195         tcx: TyCtxt<'tcx>,
 196         search_graph: &'a mut search_graph::SearchGraph<'tcx>,
 197         canonical_goal: CanonicalGoal<'tcx>,
 198     ) -> QueryResult<'tcx> {
 199         match search_graph.try_push_stack(tcx, canonical_goal) {
 200             Ok(()) => {}
 201             // Our goal is already on the stack, eager return.
 202             Err(response) => return response,
 203         }
 204
 205         // We may have to repeatedly recompute the goal in case of coinductive cycles,
 206         // check out the `cache` module for more information.
 207         //
 208         // FIXME: Similar to `evaluate_all`, this has to check for overflow.
 209         loop {
 210             let (ref infcx, goal, var_values) =
 211                 tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &canonical_goal);
 212             let mut ecx = EvalCtxt { infcx, var_values, search_graph };
 213             let result = ecx.compute_goal(goal);
 214
 215             // FIXME: `Response` should be `Copy`
 216             if search_graph.try_finalize_goal(tcx, canonical_goal, result.clone()) {
 217                 return result;
 218             }
 219         }
 220     }
 221
 222     fn make_canonical_response(&self, certainty: Certainty) -> QueryResult<'tcx> {
 223         let external_constraints = take_external_constraints(self.infcx)?;
 224
 225         Ok(self.infcx.canonicalize_response(Response {
 226             var_values: self.var_values,
 227             external_constraints,
 228             certainty,
 229         }))
 230     }
 231
 232     /// Recursively evaluates `goal`, returning whether any inference vars have
 233     /// been constrained and the certainty of the result.
 234     fn evaluate_goal(
 235         &mut self,
 236         goal: Goal<'tcx, ty::Predicate<'tcx>>,
 237     ) -> Result<(bool, Certainty), NoSolution> {
 238         let mut orig_values = OriginalQueryValues::default();
 239         let canonical_goal = self.infcx.canonicalize_query(goal, &mut orig_values);
 240         let canonical_response =
 241             EvalCtxt::evaluate_canonical_goal(self.tcx(), self.search_graph, canonical_goal)?;
 242         Ok((
 243             !canonical_response.value.var_values.is_identity(),
 244             instantiate_canonical_query_response(self.infcx, &orig_values, canonical_response),
 245         ))
 246     }
 247
 248     fn compute_goal(&mut self, goal: Goal<'tcx, ty::Predicate<'tcx>>) -> QueryResult<'tcx> {
 249         let Goal { param_env, predicate } = goal;
 250         let kind = predicate.kind();
 251         if let Some(kind) = kind.no_bound_vars() {
 252             match kind {
 253                 ty::PredicateKind::Clause(ty::Clause::Trait(predicate)) => {
 254                     self.compute_trait_goal(Goal { param_env, predicate })
 255                 }
 256                 ty::PredicateKind::Clause(ty::Clause::Projection(predicate)) => {
 257                     self.compute_projection_goal(Goal { param_env, predicate })
 258                 }
 259                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(predicate)) => {
 260                     self.compute_type_outlives_goal(Goal { param_env, predicate })
 261                 }
 262                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(predicate)) => {
 263                     self.compute_region_outlives_goal(Goal { param_env, predicate })
 264                 }
 265                 ty::PredicateKind::Subtype(predicate) => {
 266                     self.compute_subtype_goal(Goal { param_env, predicate })
 267                 }
 268                 ty::PredicateKind::Coerce(predicate) => {
 269                     self.compute_coerce_goal(Goal { param_env, predicate })
 270                 }
 271                 ty::PredicateKind::ClosureKind(def_id, substs, kind) => self
 272                     .compute_closure_kind_goal(Goal {
 273                         param_env,
 274                         predicate: (def_id, substs, kind),
 275                     }),
 276                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 277                     self.compute_object_safe_goal(trait_def_id)
 278                 }
 279                 ty::PredicateKind::WellFormed(arg) => {
 280                     self.compute_well_formed_goal(Goal { param_env, predicate: arg })
 281                 }
 282                 ty::PredicateKind::Ambiguous => self.make_canonical_response(Certainty::AMBIGUOUS),
 283                 // FIXME: implement these predicates :)
 284                 ty::PredicateKind::ConstEvaluatable(_) | ty::PredicateKind::ConstEquate(_, _) => {
 285                     self.make_canonical_response(Certainty::Yes)
 286                 }
 287                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 288                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 289                 }
 290             }
 291         } else {
 292             let kind = self.infcx.replace_bound_vars_with_placeholders(kind);
 293             let goal = goal.with(self.tcx(), ty::Binder::dummy(kind));
 294             let (_, certainty) = self.evaluate_goal(goal)?;
 295             self.make_canonical_response(certainty)
 296         }
 297     }
 298
 299     fn compute_type_outlives_goal(
 300         &mut self,
 301         _goal: Goal<'tcx, TypeOutlivesPredicate<'tcx>>,
 302     ) -> QueryResult<'tcx> {
 303         self.make_canonical_response(Certainty::Yes)
 304     }
 305
 306     fn compute_region_outlives_goal(
 307         &mut self,
 308         _goal: Goal<'tcx, RegionOutlivesPredicate<'tcx>>,
 309     ) -> QueryResult<'tcx> {
 310         self.make_canonical_response(Certainty::Yes)
 311     }
 312
 313     fn compute_coerce_goal(
 314         &mut self,
 315         goal: Goal<'tcx, CoercePredicate<'tcx>>,
 316     ) -> QueryResult<'tcx> {
 317         self.compute_subtype_goal(Goal {
 318             param_env: goal.param_env,
 319             predicate: SubtypePredicate {
 320                 a_is_expected: false,
 321                 a: goal.predicate.a,
 322                 b: goal.predicate.b,
 323             },
 324         })
 325     }
 326
 327     fn compute_subtype_goal(
 328         &mut self,
 329         goal: Goal<'tcx, SubtypePredicate<'tcx>>,
 330     ) -> QueryResult<'tcx> {
 331         if goal.predicate.a.is_ty_var() && goal.predicate.b.is_ty_var() {
 332             // FIXME: Do we want to register a subtype relation between these vars?
 333             // That won't actually reflect in the query response, so it seems moot.
 334             self.make_canonical_response(Certainty::AMBIGUOUS)
 335         } else {
 336             let InferOk { value: (), obligations } = self
 337                 .infcx
 338                 .at(&ObligationCause::dummy(), goal.param_env)
 339                 .sub(goal.predicate.a, goal.predicate.b)?;
 340             self.evaluate_all_and_make_canonical_response(
 341                 obligations.into_iter().map(|pred| pred.into()).collect(),
 342             )
 343         }
 344     }
 345
 346     fn compute_closure_kind_goal(
 347         &mut self,
 348         goal: Goal<'tcx, (DefId, ty::SubstsRef<'tcx>, ty::ClosureKind)>,
 349     ) -> QueryResult<'tcx> {
 350         let (_, substs, expected_kind) = goal.predicate;
 351         let found_kind = substs.as_closure().kind_ty().to_opt_closure_kind();
 352
 353         let Some(found_kind) = found_kind else {
 354             return self.make_canonical_response(Certainty::AMBIGUOUS);
 355         };
 356         if found_kind.extends(expected_kind) {
 357             self.make_canonical_response(Certainty::Yes)
 358         } else {
 359             Err(NoSolution)
 360         }
 361     }
 362
 363     fn compute_object_safe_goal(&mut self, trait_def_id: DefId) -> QueryResult<'tcx> {
 364         if self.tcx().check_is_object_safe(trait_def_id) {
 365             self.make_canonical_response(Certainty::Yes)
 366         } else {
 367             Err(NoSolution)
 368         }
 369     }
 370
 371     fn compute_well_formed_goal(
 372         &mut self,
 373         goal: Goal<'tcx, ty::GenericArg<'tcx>>,
 374     ) -> QueryResult<'tcx> {
 375         match crate::traits::wf::unnormalized_obligations(
 376             self.infcx,
 377             goal.param_env,
 378             goal.predicate,
 379         ) {
 380             Some(obligations) => self.evaluate_all_and_make_canonical_response(
 381                 obligations.into_iter().map(|o| o.into()).collect(),
 382             ),
 383             None => self.make_canonical_response(Certainty::AMBIGUOUS),
 384         }
 385     }
 386 }
 387
 388 impl<'tcx> EvalCtxt<'_, 'tcx> {
 389     // Recursively evaluates a list of goals to completion, returning the certainty
 390     // of all of the goals.
 391     fn evaluate_all(
 392         &mut self,
 393         mut goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
 394     ) -> Result<Certainty, NoSolution> {
 395         let mut new_goals = Vec::new();
 396         self.repeat_while_none(|this| {
 397             let mut has_changed = Err(Certainty::Yes);
 398             for goal in goals.drain(..) {
 399                 let (changed, certainty) = match this.evaluate_goal(goal) {
 400                     Ok(result) => result,
 401                     Err(NoSolution) => return Some(Err(NoSolution)),
 402                 };
 403
 404                 if changed {
 405                     has_changed = Ok(());
 406                 }
 407
 408                 match certainty {
 409                     Certainty::Yes => {}
 410                     Certainty::Maybe(_) => {
 411                         new_goals.push(goal);
 412                         has_changed = has_changed.map_err(|c| c.unify_and(certainty));
 413                     }
 414                 }
 415             }
 416
 417             match has_changed {
 418                 Ok(()) => {
 419                     mem::swap(&mut new_goals, &mut goals);
 420                     None
 421                 }
 422                 Err(certainty) => Some(Ok(certainty)),
 423             }
 424         })
 425     }
 426
 427     // Recursively evaluates a list of goals to completion, making a query response.
 428     //
 429     // This is just a convenient way of calling [`EvalCtxt::evaluate_all`],
 430     // then [`EvalCtxt::make_canonical_response`].
 431     fn evaluate_all_and_make_canonical_response(
 432         &mut self,
 433         goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
 434     ) -> QueryResult<'tcx> {
 435         self.evaluate_all(goals).and_then(|certainty| self.make_canonical_response(certainty))
 436     }
 437 }
 438
 439 #[instrument(level = "debug", skip(infcx), ret)]
 440 fn take_external_constraints<'tcx>(
 441     infcx: &InferCtxt<'tcx>,
 442 ) -> Result<ExternalConstraints<'tcx>, NoSolution> {
 443     let region_obligations = infcx.take_registered_region_obligations();
 444     let opaque_types = infcx.take_opaque_types_for_query_response();
 445     Ok(ExternalConstraints {
 446         // FIXME: Now that's definitely wrong :)
 447         //
 448         // Should also do the leak check here I think
 449         regions: drop(region_obligations),
 450         opaque_types,
 451     })
 452 }
 453
 454 fn instantiate_canonical_query_response<'tcx>(
 455     infcx: &InferCtxt<'tcx>,
 456     original_values: &OriginalQueryValues<'tcx>,
 457     response: CanonicalResponse<'tcx>,
 458 ) -> Certainty {
 459     let Ok(InferOk { value, obligations }) = infcx
 460         .instantiate_query_response_and_region_obligations(
 461             &ObligationCause::dummy(),
 462             ty::ParamEnv::empty(),
 463             original_values,
 464             &response.unchecked_map(|resp| QueryResponse {
 465                 var_values: resp.var_values,
 466                 region_constraints: QueryRegionConstraints::default(),
 467                 certainty: match resp.certainty {
 468                     Certainty::Yes => OldCertainty::Proven,
 469                     Certainty::Maybe(_) => OldCertainty::Ambiguous,
 470                 },
 471                 opaque_types: resp.external_constraints.opaque_types,
 472                 value: resp.certainty,
 473             }),
 474         ) else { bug!(); };
 475     assert!(obligations.is_empty());
 476     value
 477 }
 478
 479 pub(super) fn response_no_constraints<'tcx>(
 480     tcx: TyCtxt<'tcx>,
 481     goal: Canonical<'tcx, impl Sized>,
 482     certainty: Certainty,
 483 ) -> QueryResult<'tcx> {
 484     Ok(Canonical {
 485         max_universe: goal.max_universe,
 486         variables: goal.variables,
 487         value: Response {
 488             var_values: CanonicalVarValues::make_identity(tcx, goal.variables),
 489             external_constraints: Default::default(),
 490             certainty,
 491         },
 492     })
 493 }