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: `CanonicalVarValues` should be interned and `Copy`.
  17
  18 // FIXME: uses of `infcx.at` need to enable deferred projection equality once that's implemented.
  19
  20 use std::mem;
  21
  22 use rustc_hir::def_id::DefId;
  23 use rustc_infer::infer::canonical::{Canonical, CanonicalVarKind, CanonicalVarValues};
  24 use rustc_infer::infer::canonical::{OriginalQueryValues, QueryRegionConstraints, QueryResponse};
  25 use rustc_infer::infer::{InferCtxt, InferOk, TyCtxtInferExt};
  26 use rustc_infer::traits::query::NoSolution;
  27 use rustc_infer::traits::Obligation;
  28 use rustc_middle::infer::canonical::Certainty as OldCertainty;
  29 use rustc_middle::ty::{self, Ty, TyCtxt};
  30 use rustc_middle::ty::{
  31     CoercePredicate, RegionOutlivesPredicate, SubtypePredicate, ToPredicate, TypeOutlivesPredicate,
  32 };
  33 use rustc_span::DUMMY_SP;
  34
  35 use crate::traits::ObligationCause;
  36
  37 mod assembly;
  38 mod fulfill;
  39 mod infcx_ext;
  40 mod project_goals;
  41 mod search_graph;
  42 mod trait_goals;
  43
  44 pub use fulfill::FulfillmentCtxt;
  45
  46 /// A goal is a statement, i.e. `predicate`, we want to prove
  47 /// given some assumptions, i.e. `param_env`.
  48 ///
  49 /// Most of the time the `param_env` contains the `where`-bounds of the function
  50 /// we're currently typechecking while the `predicate` is some trait bound.
  51 #[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
  52 pub struct Goal<'tcx, P> {
  53     param_env: ty::ParamEnv<'tcx>,
  54     predicate: P,
  55 }
  56
  57 impl<'tcx, P> Goal<'tcx, P> {
  58     pub fn new(
  59         tcx: TyCtxt<'tcx>,
  60         param_env: ty::ParamEnv<'tcx>,
  61         predicate: impl ToPredicate<'tcx, P>,
  62     ) -> Goal<'tcx, P> {
  63         Goal { param_env, predicate: predicate.to_predicate(tcx) }
  64     }
  65
  66     /// Updates the goal to one with a different `predicate` but the same `param_env`.
  67     fn with<Q>(self, tcx: TyCtxt<'tcx>, predicate: impl ToPredicate<'tcx, Q>) -> Goal<'tcx, Q> {
  68         Goal { param_env: self.param_env, predicate: predicate.to_predicate(tcx) }
  69     }
  70 }
  71
  72 impl<'tcx, P> From<Obligation<'tcx, P>> for Goal<'tcx, P> {
  73     fn from(obligation: Obligation<'tcx, P>) -> Goal<'tcx, P> {
  74         Goal { param_env: obligation.param_env, predicate: obligation.predicate }
  75     }
  76 }
  77
  78 #[derive(Debug, PartialEq, Eq, Clone, Hash, TypeFoldable, TypeVisitable)]
  79 pub struct Response<'tcx> {
  80     pub var_values: CanonicalVarValues<'tcx>,
  81     /// Additional constraints returned by this query.
  82     pub external_constraints: ExternalConstraints<'tcx>,
  83     pub certainty: Certainty,
  84 }
  85
  86 #[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
  87 pub enum Certainty {
  88     Yes,
  89     Maybe(MaybeCause),
  90 }
  91
  92 impl Certainty {
  93     pub const AMBIGUOUS: Certainty = Certainty::Maybe(MaybeCause::Ambiguity);
  94
  95     /// When proving multiple goals using **AND**, e.g. nested obligations for an impl,
  96     /// use this function to unify the certainty of these goals
  97     pub fn unify_and(self, other: Certainty) -> Certainty {
  98         match (self, other) {
  99             (Certainty::Yes, Certainty::Yes) => Certainty::Yes,
 100             (Certainty::Yes, Certainty::Maybe(_)) => other,
 101             (Certainty::Maybe(_), Certainty::Yes) => self,
 102             (Certainty::Maybe(MaybeCause::Overflow), Certainty::Maybe(MaybeCause::Overflow)) => {
 103                 Certainty::Maybe(MaybeCause::Overflow)
 104             }
 105             // If at least one of the goals is ambiguous, hide the overflow as the ambiguous goal
 106             // may still result in failure.
 107             (Certainty::Maybe(MaybeCause::Ambiguity), Certainty::Maybe(_))
 108             | (Certainty::Maybe(_), Certainty::Maybe(MaybeCause::Ambiguity)) => {
 109                 Certainty::Maybe(MaybeCause::Ambiguity)
 110             }
 111         }
 112     }
 113 }
 114
 115 /// Why we failed to evaluate a goal.
 116 #[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
 117 pub enum MaybeCause {
 118     /// We failed due to ambiguity. This ambiguity can either
 119     /// be a true ambiguity, i.e. there are multiple different answers,
 120     /// or we hit a case where we just don't bother, e.g. `?x: Trait` goals.
 121     Ambiguity,
 122     /// We gave up due to an overflow, most often by hitting the recursion limit.
 123     Overflow,
 124 }
 125
 126 /// Additional constraints returned on success.
 127 #[derive(Debug, PartialEq, Eq, Clone, Hash, TypeFoldable, TypeVisitable, Default)]
 128 pub struct ExternalConstraints<'tcx> {
 129     // FIXME: implement this.
 130     regions: (),
 131     opaque_types: Vec<(Ty<'tcx>, Ty<'tcx>)>,
 132 }
 133
 134 type CanonicalGoal<'tcx, T = ty::Predicate<'tcx>> = Canonical<'tcx, Goal<'tcx, T>>;
 135 type CanonicalResponse<'tcx> = Canonical<'tcx, Response<'tcx>>;
 136 /// The result of evaluating a canonical query.
 137 ///
 138 /// FIXME: We use a different type than the existing canonical queries. This is because
 139 /// we need to add a `Certainty` for `overflow` and may want to restructure this code without
 140 /// having to worry about changes to currently used code. Once we've made progress on this
 141 /// solver, merge the two responses again.
 142 pub type QueryResult<'tcx> = Result<CanonicalResponse<'tcx>, NoSolution>;
 143
 144 pub trait InferCtxtEvalExt<'tcx> {
 145     /// Evaluates a goal from **outside** of the trait solver.
 146     ///
 147     /// Using this while inside of the solver is wrong as it uses a new
 148     /// search graph which would break cycle detection.
 149     fn evaluate_root_goal(
 150         &self,
 151         goal: Goal<'tcx, ty::Predicate<'tcx>>,
 152     ) -> Result<(bool, Certainty), NoSolution>;
 153 }
 154
 155 impl<'tcx> InferCtxtEvalExt<'tcx> for InferCtxt<'tcx> {
 156     fn evaluate_root_goal(
 157         &self,
 158         goal: Goal<'tcx, ty::Predicate<'tcx>>,
 159     ) -> Result<(bool, Certainty), NoSolution> {
 160         let mut search_graph = search_graph::SearchGraph::new(self.tcx);
 161
 162         let result = EvalCtxt {
 163             search_graph: &mut search_graph,
 164             infcx: self,
 165             var_values: CanonicalVarValues::dummy(),
 166         }
 167         .evaluate_goal(goal);
 168
 169         assert!(search_graph.is_empty());
 170         result
 171     }
 172 }
 173
 174 struct EvalCtxt<'a, 'tcx> {
 175     infcx: &'a InferCtxt<'tcx>,
 176     var_values: CanonicalVarValues<'tcx>,
 177
 178     search_graph: &'a mut search_graph::SearchGraph<'tcx>,
 179 }
 180
 181 impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
 182     fn tcx(&self) -> TyCtxt<'tcx> {
 183         self.infcx.tcx
 184     }
 185
 186     /// The entry point of the solver.
 187     ///
 188     /// This function deals with (coinductive) cycles, overflow, and caching
 189     /// and then calls [`EvalCtxt::compute_goal`] which contains the actual
 190     /// logic of the solver.
 191     ///
 192     /// Instead of calling this function directly, use either [EvalCtxt::evaluate_goal]
 193     /// if you're inside of the solver or [InferCtxtEvalExt::evaluate_root_goal] if you're
 194     /// outside of it.
 195     #[instrument(level = "debug", skip(tcx, search_graph), ret)]
 196     fn evaluate_canonical_goal(
 197         tcx: TyCtxt<'tcx>,
 198         search_graph: &'a mut search_graph::SearchGraph<'tcx>,
 199         canonical_goal: CanonicalGoal<'tcx>,
 200     ) -> QueryResult<'tcx> {
 201         match search_graph.try_push_stack(tcx, canonical_goal) {
 202             Ok(()) => {}
 203             // Our goal is already on the stack, eager return.
 204             Err(response) => return response,
 205         }
 206
 207         // We may have to repeatedly recompute the goal in case of coinductive cycles,
 208         // check out the `cache` module for more information.
 209         //
 210         // FIXME: Similar to `evaluate_all`, this has to check for overflow.
 211         loop {
 212             let (ref infcx, goal, var_values) =
 213                 tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &canonical_goal);
 214             let mut ecx = EvalCtxt { infcx, var_values, search_graph };
 215             let result = ecx.compute_goal(goal);
 216
 217             // FIXME: `Response` should be `Copy`
 218             if search_graph.try_finalize_goal(tcx, canonical_goal, result.clone()) {
 219                 return result;
 220             }
 221         }
 222     }
 223
 224     fn make_canonical_response(&self, certainty: Certainty) -> QueryResult<'tcx> {
 225         let external_constraints = take_external_constraints(self.infcx)?;
 226
 227         Ok(self.infcx.canonicalize_response(Response {
 228             var_values: self.var_values.clone(),
 229             external_constraints,
 230             certainty,
 231         }))
 232     }
 233
 234     /// Recursively evaluates `goal`, returning whether any inference vars have
 235     /// been constrained and the certainty of the result.
 236     fn evaluate_goal(
 237         &mut self,
 238         goal: Goal<'tcx, ty::Predicate<'tcx>>,
 239     ) -> Result<(bool, Certainty), NoSolution> {
 240         let mut orig_values = OriginalQueryValues::default();
 241         let canonical_goal = self.infcx.canonicalize_query(goal, &mut orig_values);
 242         let canonical_response =
 243             EvalCtxt::evaluate_canonical_goal(self.tcx(), self.search_graph, canonical_goal)?;
 244         Ok((
 245             !canonical_response.value.var_values.is_identity(),
 246             instantiate_canonical_query_response(self.infcx, &orig_values, canonical_response),
 247         ))
 248     }
 249
 250     fn compute_goal(&mut self, goal: Goal<'tcx, ty::Predicate<'tcx>>) -> QueryResult<'tcx> {
 251         let Goal { param_env, predicate } = goal;
 252         let kind = predicate.kind();
 253         if let Some(kind) = kind.no_bound_vars() {
 254             match kind {
 255                 ty::PredicateKind::Clause(ty::Clause::Trait(predicate)) => {
 256                     self.compute_trait_goal(Goal { param_env, predicate })
 257                 }
 258                 ty::PredicateKind::Clause(ty::Clause::Projection(predicate)) => {
 259                     self.compute_projection_goal(Goal { param_env, predicate })
 260                 }
 261                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(predicate)) => {
 262                     self.compute_type_outlives_goal(Goal { param_env, predicate })
 263                 }
 264                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(predicate)) => {
 265                     self.compute_region_outlives_goal(Goal { param_env, predicate })
 266                 }
 267                 ty::PredicateKind::Subtype(predicate) => {
 268                     self.compute_subtype_goal(Goal { param_env, predicate })
 269                 }
 270                 ty::PredicateKind::Coerce(predicate) => {
 271                     self.compute_coerce_goal(Goal { param_env, predicate })
 272                 }
 273                 ty::PredicateKind::ClosureKind(def_id, substs, kind) => self
 274                     .compute_closure_kind_goal(Goal {
 275                         param_env,
 276                         predicate: (def_id, substs, kind),
 277                     }),
 278                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 279                     self.compute_object_safe_goal(trait_def_id)
 280                 }
 281                 ty::PredicateKind::WellFormed(arg) => {
 282                     self.compute_well_formed_goal(Goal { param_env, predicate: arg })
 283                 }
 284                 ty::PredicateKind::Ambiguous => self.make_canonical_response(Certainty::AMBIGUOUS),
 285                 // FIXME: implement these predicates :)
 286                 ty::PredicateKind::ConstEvaluatable(_) | ty::PredicateKind::ConstEquate(_, _) => {
 287                     self.make_canonical_response(Certainty::Yes)
 288                 }
 289                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 290                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 291                 }
 292             }
 293         } else {
 294             let kind = self.infcx.replace_bound_vars_with_placeholders(kind);
 295             let goal = goal.with(self.tcx(), ty::Binder::dummy(kind));
 296             let (_, certainty) = self.evaluate_goal(goal)?;
 297             self.make_canonical_response(certainty)
 298         }
 299     }
 300
 301     fn compute_type_outlives_goal(
 302         &mut self,
 303         _goal: Goal<'tcx, TypeOutlivesPredicate<'tcx>>,
 304     ) -> QueryResult<'tcx> {
 305         self.make_canonical_response(Certainty::Yes)
 306     }
 307
 308     fn compute_region_outlives_goal(
 309         &mut self,
 310         _goal: Goal<'tcx, RegionOutlivesPredicate<'tcx>>,
 311     ) -> QueryResult<'tcx> {
 312         self.make_canonical_response(Certainty::Yes)
 313     }
 314
 315     fn compute_coerce_goal(
 316         &mut self,
 317         goal: Goal<'tcx, CoercePredicate<'tcx>>,
 318     ) -> QueryResult<'tcx> {
 319         self.compute_subtype_goal(Goal {
 320             param_env: goal.param_env,
 321             predicate: SubtypePredicate {
 322                 a_is_expected: false,
 323                 a: goal.predicate.a,
 324                 b: goal.predicate.b,
 325             },
 326         })
 327     }
 328
 329     fn compute_subtype_goal(
 330         &mut self,
 331         goal: Goal<'tcx, SubtypePredicate<'tcx>>,
 332     ) -> QueryResult<'tcx> {
 333         if goal.predicate.a.is_ty_var() && goal.predicate.b.is_ty_var() {
 334             // FIXME: Do we want to register a subtype relation between these vars?
 335             // That won't actually reflect in the query response, so it seems moot.
 336             self.make_canonical_response(Certainty::AMBIGUOUS)
 337         } else {
 338             self.infcx.probe(|_| {
 339                 let InferOk { value: (), obligations } = self
 340                     .infcx
 341                     .at(&ObligationCause::dummy(), goal.param_env)
 342                     .sub(goal.predicate.a, goal.predicate.b)?;
 343                 self.evaluate_all_and_make_canonical_response(
 344                     obligations.into_iter().map(|pred| pred.into()).collect(),
 345                 )
 346             })
 347         }
 348     }
 349
 350     fn compute_closure_kind_goal(
 351         &mut self,
 352         goal: Goal<'tcx, (DefId, ty::SubstsRef<'tcx>, ty::ClosureKind)>,
 353     ) -> QueryResult<'tcx> {
 354         let (_, substs, expected_kind) = goal.predicate;
 355         let found_kind = substs.as_closure().kind_ty().to_opt_closure_kind();
 356
 357         let Some(found_kind) = found_kind else {
 358             return self.make_canonical_response(Certainty::AMBIGUOUS);
 359         };
 360         if found_kind.extends(expected_kind) {
 361             self.make_canonical_response(Certainty::Yes)
 362         } else {
 363             Err(NoSolution)
 364         }
 365     }
 366
 367     fn compute_object_safe_goal(&mut self, trait_def_id: DefId) -> QueryResult<'tcx> {
 368         if self.tcx().is_object_safe(trait_def_id) {
 369             self.make_canonical_response(Certainty::Yes)
 370         } else {
 371             Err(NoSolution)
 372         }
 373     }
 374
 375     fn compute_well_formed_goal(
 376         &mut self,
 377         goal: Goal<'tcx, ty::GenericArg<'tcx>>,
 378     ) -> QueryResult<'tcx> {
 379         self.infcx.probe(|_| {
 380             match crate::traits::wf::unnormalized_obligations(
 381                 self.infcx,
 382                 goal.param_env,
 383                 goal.predicate,
 384             ) {
 385                 Some(obligations) => self.evaluate_all_and_make_canonical_response(
 386                     obligations.into_iter().map(|o| o.into()).collect(),
 387                 ),
 388                 None => self.make_canonical_response(Certainty::AMBIGUOUS),
 389             }
 390         })
 391     }
 392 }
 393
 394 impl<'tcx> EvalCtxt<'_, 'tcx> {
 395     fn evaluate_all(
 396         &mut self,
 397         mut goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
 398     ) -> Result<Certainty, NoSolution> {
 399         let mut new_goals = Vec::new();
 400         self.repeat_while_none(|this| {
 401             let mut has_changed = Err(Certainty::Yes);
 402             for goal in goals.drain(..) {
 403                 let (changed, certainty) = match this.evaluate_goal(goal) {
 404                     Ok(result) => result,
 405                     Err(NoSolution) => return Some(Err(NoSolution)),
 406                 };
 407
 408                 if changed {
 409                     has_changed = Ok(());
 410                 }
 411
 412                 match certainty {
 413                     Certainty::Yes => {}
 414                     Certainty::Maybe(_) => {
 415                         new_goals.push(goal);
 416                         has_changed = has_changed.map_err(|c| c.unify_and(certainty));
 417                     }
 418                 }
 419             }
 420
 421             match has_changed {
 422                 Ok(()) => {
 423                     mem::swap(&mut new_goals, &mut goals);
 424                     None
 425                 }
 426                 Err(certainty) => Some(Ok(certainty)),
 427             }
 428         })
 429     }
 430
 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     let var_values = goal
 485         .variables
 486         .iter()
 487         .enumerate()
 488         .map(|(i, info)| match info.kind {
 489             CanonicalVarKind::Ty(_) | CanonicalVarKind::PlaceholderTy(_) => {
 490                 tcx.mk_ty(ty::Bound(ty::INNERMOST, ty::BoundVar::from_usize(i).into())).into()
 491             }
 492             CanonicalVarKind::Region(_) | CanonicalVarKind::PlaceholderRegion(_) => {
 493                 let br = ty::BoundRegion {
 494                     var: ty::BoundVar::from_usize(i),
 495                     kind: ty::BrAnon(i as u32, None),
 496                 };
 497                 tcx.mk_region(ty::ReLateBound(ty::INNERMOST, br)).into()
 498             }
 499             CanonicalVarKind::Const(_, ty) | CanonicalVarKind::PlaceholderConst(_, ty) => tcx
 500                 .mk_const(ty::ConstKind::Bound(ty::INNERMOST, ty::BoundVar::from_usize(i)), ty)
 501                 .into(),
 502         })
 503         .collect();
 504
 505     Ok(Canonical {
 506         max_universe: goal.max_universe,
 507         variables: goal.variables,
 508         value: Response {
 509             var_values: CanonicalVarValues { var_values },
 510             external_constraints: Default::default(),
 511             certainty,
 512         },
 513     })
 514 }