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             in_projection_eq_hack: false,
 165         }
 166         .evaluate_goal(goal);
 167
 168         assert!(search_graph.is_empty());
 169         result
 170     }
 171 }
 172
 173 struct EvalCtxt<'a, 'tcx> {
 174     infcx: &'a InferCtxt<'tcx>,
 175     var_values: CanonicalVarValues<'tcx>,
 176
 177     search_graph: &'a mut search_graph::SearchGraph<'tcx>,
 178
 179     /// This field is used by a debug assertion in [`EvalCtxt::evaluate_goal`],
 180     /// see the comment in that method for more details.
 181     in_projection_eq_hack: bool,
 182 }
 183
 184 impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
 185     fn tcx(&self) -> TyCtxt<'tcx> {
 186         self.infcx.tcx
 187     }
 188
 189     /// The entry point of the solver.
 190     ///
 191     /// This function deals with (coinductive) cycles, overflow, and caching
 192     /// and then calls [`EvalCtxt::compute_goal`] which contains the actual
 193     /// logic of the solver.
 194     ///
 195     /// Instead of calling this function directly, use either [EvalCtxt::evaluate_goal]
 196     /// if you're inside of the solver or [InferCtxtEvalExt::evaluate_root_goal] if you're
 197     /// outside of it.
 198     #[instrument(level = "debug", skip(tcx, search_graph), ret)]
 199     fn evaluate_canonical_goal(
 200         tcx: TyCtxt<'tcx>,
 201         search_graph: &'a mut search_graph::SearchGraph<'tcx>,
 202         canonical_goal: CanonicalGoal<'tcx>,
 203     ) -> QueryResult<'tcx> {
 204         match search_graph.try_push_stack(tcx, canonical_goal) {
 205             Ok(()) => {}
 206             // Our goal is already on the stack, eager return.
 207             Err(response) => return response,
 208         }
 209
 210         // We may have to repeatedly recompute the goal in case of coinductive cycles,
 211         // check out the `cache` module for more information.
 212         //
 213         // FIXME: Similar to `evaluate_all`, this has to check for overflow.
 214         loop {
 215             let (ref infcx, goal, var_values) =
 216                 tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &canonical_goal);
 217             let mut ecx =
 218                 EvalCtxt { infcx, var_values, search_graph, in_projection_eq_hack: false };
 219             let result = ecx.compute_goal(goal);
 220
 221             // FIXME: `Response` should be `Copy`
 222             if search_graph.try_finalize_goal(tcx, canonical_goal, result.clone()) {
 223                 return result;
 224             }
 225         }
 226     }
 227
 228     fn make_canonical_response(&self, certainty: Certainty) -> QueryResult<'tcx> {
 229         let external_constraints = take_external_constraints(self.infcx)?;
 230
 231         Ok(self.infcx.canonicalize_response(Response {
 232             var_values: self.var_values,
 233             external_constraints,
 234             certainty,
 235         }))
 236     }
 237
 238     /// Recursively evaluates `goal`, returning whether any inference vars have
 239     /// been constrained and the certainty of the result.
 240     fn evaluate_goal(
 241         &mut self,
 242         goal: Goal<'tcx, ty::Predicate<'tcx>>,
 243     ) -> Result<(bool, Certainty), NoSolution> {
 244         let mut orig_values = OriginalQueryValues::default();
 245         let canonical_goal = self.infcx.canonicalize_query(goal, &mut orig_values);
 246         let canonical_response =
 247             EvalCtxt::evaluate_canonical_goal(self.tcx(), self.search_graph, canonical_goal)?;
 248
 249         let has_changed = !canonical_response.value.var_values.is_identity();
 250         let certainty =
 251             instantiate_canonical_query_response(self.infcx, &orig_values, canonical_response);
 252
 253         // Check that rerunning this query with its inference constraints applied
 254         // doesn't result in new inference constraints and has the same result.
 255         //
 256         // If we have projection goals like `<T as Trait>::Assoc == u32` we recursively
 257         // call `exists<U> <T as Trait>::Assoc == U` to enable better caching. This goal
 258         // could constrain `U` to `u32` which would cause this check to result in a
 259         // solver cycle.
 260         if cfg!(debug_assertions) && has_changed && !self.in_projection_eq_hack {
 261             let mut orig_values = OriginalQueryValues::default();
 262             let canonical_goal = self.infcx.canonicalize_query(goal, &mut orig_values);
 263             let canonical_response =
 264                 EvalCtxt::evaluate_canonical_goal(self.tcx(), self.search_graph, canonical_goal)?;
 265             assert!(canonical_response.value.var_values.is_identity());
 266             assert_eq!(certainty, canonical_response.value.certainty);
 267         }
 268
 269         Ok((has_changed, certainty))
 270     }
 271
 272     fn compute_goal(&mut self, goal: Goal<'tcx, ty::Predicate<'tcx>>) -> QueryResult<'tcx> {
 273         let Goal { param_env, predicate } = goal;
 274         let kind = predicate.kind();
 275         if let Some(kind) = kind.no_bound_vars() {
 276             match kind {
 277                 ty::PredicateKind::Clause(ty::Clause::Trait(predicate)) => {
 278                     self.compute_trait_goal(Goal { param_env, predicate })
 279                 }
 280                 ty::PredicateKind::Clause(ty::Clause::Projection(predicate)) => {
 281                     self.compute_projection_goal(Goal { param_env, predicate })
 282                 }
 283                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(predicate)) => {
 284                     self.compute_type_outlives_goal(Goal { param_env, predicate })
 285                 }
 286                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(predicate)) => {
 287                     self.compute_region_outlives_goal(Goal { param_env, predicate })
 288                 }
 289                 ty::PredicateKind::Subtype(predicate) => {
 290                     self.compute_subtype_goal(Goal { param_env, predicate })
 291                 }
 292                 ty::PredicateKind::Coerce(predicate) => {
 293                     self.compute_coerce_goal(Goal { param_env, predicate })
 294                 }
 295                 ty::PredicateKind::ClosureKind(def_id, substs, kind) => self
 296                     .compute_closure_kind_goal(Goal {
 297                         param_env,
 298                         predicate: (def_id, substs, kind),
 299                     }),
 300                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 301                     self.compute_object_safe_goal(trait_def_id)
 302                 }
 303                 ty::PredicateKind::WellFormed(arg) => {
 304                     self.compute_well_formed_goal(Goal { param_env, predicate: arg })
 305                 }
 306                 ty::PredicateKind::Ambiguous => self.make_canonical_response(Certainty::AMBIGUOUS),
 307                 // FIXME: implement these predicates :)
 308                 ty::PredicateKind::ConstEvaluatable(_) | ty::PredicateKind::ConstEquate(_, _) => {
 309                     self.make_canonical_response(Certainty::Yes)
 310                 }
 311                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 312                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 313                 }
 314             }
 315         } else {
 316             let kind = self.infcx.replace_bound_vars_with_placeholders(kind);
 317             let goal = goal.with(self.tcx(), ty::Binder::dummy(kind));
 318             let (_, certainty) = self.evaluate_goal(goal)?;
 319             self.make_canonical_response(certainty)
 320         }
 321     }
 322
 323     fn compute_type_outlives_goal(
 324         &mut self,
 325         _goal: Goal<'tcx, TypeOutlivesPredicate<'tcx>>,
 326     ) -> QueryResult<'tcx> {
 327         self.make_canonical_response(Certainty::Yes)
 328     }
 329
 330     fn compute_region_outlives_goal(
 331         &mut self,
 332         _goal: Goal<'tcx, RegionOutlivesPredicate<'tcx>>,
 333     ) -> QueryResult<'tcx> {
 334         self.make_canonical_response(Certainty::Yes)
 335     }
 336
 337     fn compute_coerce_goal(
 338         &mut self,
 339         goal: Goal<'tcx, CoercePredicate<'tcx>>,
 340     ) -> QueryResult<'tcx> {
 341         self.compute_subtype_goal(Goal {
 342             param_env: goal.param_env,
 343             predicate: SubtypePredicate {
 344                 a_is_expected: false,
 345                 a: goal.predicate.a,
 346                 b: goal.predicate.b,
 347             },
 348         })
 349     }
 350
 351     fn compute_subtype_goal(
 352         &mut self,
 353         goal: Goal<'tcx, SubtypePredicate<'tcx>>,
 354     ) -> QueryResult<'tcx> {
 355         if goal.predicate.a.is_ty_var() && goal.predicate.b.is_ty_var() {
 356             // FIXME: Do we want to register a subtype relation between these vars?
 357             // That won't actually reflect in the query response, so it seems moot.
 358             self.make_canonical_response(Certainty::AMBIGUOUS)
 359         } else {
 360             let InferOk { value: (), obligations } = self
 361                 .infcx
 362                 .at(&ObligationCause::dummy(), goal.param_env)
 363                 .sub(goal.predicate.a, goal.predicate.b)?;
 364             self.evaluate_all_and_make_canonical_response(
 365                 obligations.into_iter().map(|pred| pred.into()).collect(),
 366             )
 367         }
 368     }
 369
 370     fn compute_closure_kind_goal(
 371         &mut self,
 372         goal: Goal<'tcx, (DefId, ty::SubstsRef<'tcx>, ty::ClosureKind)>,
 373     ) -> QueryResult<'tcx> {
 374         let (_, substs, expected_kind) = goal.predicate;
 375         let found_kind = substs.as_closure().kind_ty().to_opt_closure_kind();
 376
 377         let Some(found_kind) = found_kind else {
 378             return self.make_canonical_response(Certainty::AMBIGUOUS);
 379         };
 380         if found_kind.extends(expected_kind) {
 381             self.make_canonical_response(Certainty::Yes)
 382         } else {
 383             Err(NoSolution)
 384         }
 385     }
 386
 387     fn compute_object_safe_goal(&mut self, trait_def_id: DefId) -> QueryResult<'tcx> {
 388         if self.tcx().check_is_object_safe(trait_def_id) {
 389             self.make_canonical_response(Certainty::Yes)
 390         } else {
 391             Err(NoSolution)
 392         }
 393     }
 394
 395     fn compute_well_formed_goal(
 396         &mut self,
 397         goal: Goal<'tcx, ty::GenericArg<'tcx>>,
 398     ) -> QueryResult<'tcx> {
 399         match crate::traits::wf::unnormalized_obligations(
 400             self.infcx,
 401             goal.param_env,
 402             goal.predicate,
 403         ) {
 404             Some(obligations) => self.evaluate_all_and_make_canonical_response(
 405                 obligations.into_iter().map(|o| o.into()).collect(),
 406             ),
 407             None => self.make_canonical_response(Certainty::AMBIGUOUS),
 408         }
 409     }
 410 }
 411
 412 impl<'tcx> EvalCtxt<'_, 'tcx> {
 413     // Recursively evaluates a list of goals to completion, returning the certainty
 414     // of all of the goals.
 415     fn evaluate_all(
 416         &mut self,
 417         mut goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
 418     ) -> Result<Certainty, NoSolution> {
 419         let mut new_goals = Vec::new();
 420         self.repeat_while_none(|this| {
 421             let mut has_changed = Err(Certainty::Yes);
 422             for goal in goals.drain(..) {
 423                 let (changed, certainty) = match this.evaluate_goal(goal) {
 424                     Ok(result) => result,
 425                     Err(NoSolution) => return Some(Err(NoSolution)),
 426                 };
 427
 428                 if changed {
 429                     has_changed = Ok(());
 430                 }
 431
 432                 match certainty {
 433                     Certainty::Yes => {}
 434                     Certainty::Maybe(_) => {
 435                         new_goals.push(goal);
 436                         has_changed = has_changed.map_err(|c| c.unify_and(certainty));
 437                     }
 438                 }
 439             }
 440
 441             match has_changed {
 442                 Ok(()) => {
 443                     mem::swap(&mut new_goals, &mut goals);
 444                     None
 445                 }
 446                 Err(certainty) => Some(Ok(certainty)),
 447             }
 448         })
 449     }
 450
 451     // Recursively evaluates a list of goals to completion, making a query response.
 452     //
 453     // This is just a convenient way of calling [`EvalCtxt::evaluate_all`],
 454     // then [`EvalCtxt::make_canonical_response`].
 455     fn evaluate_all_and_make_canonical_response(
 456         &mut self,
 457         goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
 458     ) -> QueryResult<'tcx> {
 459         self.evaluate_all(goals).and_then(|certainty| self.make_canonical_response(certainty))
 460     }
 461 }
 462
 463 #[instrument(level = "debug", skip(infcx), ret)]
 464 fn take_external_constraints<'tcx>(
 465     infcx: &InferCtxt<'tcx>,
 466 ) -> Result<ExternalConstraints<'tcx>, NoSolution> {
 467     let region_obligations = infcx.take_registered_region_obligations();
 468     let opaque_types = infcx.take_opaque_types_for_query_response();
 469     Ok(ExternalConstraints {
 470         // FIXME: Now that's definitely wrong :)
 471         //
 472         // Should also do the leak check here I think
 473         regions: drop(region_obligations),
 474         opaque_types,
 475     })
 476 }
 477
 478 fn instantiate_canonical_query_response<'tcx>(
 479     infcx: &InferCtxt<'tcx>,
 480     original_values: &OriginalQueryValues<'tcx>,
 481     response: CanonicalResponse<'tcx>,
 482 ) -> Certainty {
 483     let Ok(InferOk { value, obligations }) = infcx
 484         .instantiate_query_response_and_region_obligations(
 485             &ObligationCause::dummy(),
 486             ty::ParamEnv::empty(),
 487             original_values,
 488             &response.unchecked_map(|resp| QueryResponse {
 489                 var_values: resp.var_values,
 490                 region_constraints: QueryRegionConstraints::default(),
 491                 certainty: match resp.certainty {
 492                     Certainty::Yes => OldCertainty::Proven,
 493                     Certainty::Maybe(_) => OldCertainty::Ambiguous,
 494                 },
 495                 opaque_types: resp.external_constraints.opaque_types,
 496                 value: resp.certainty,
 497             }),
 498         ) else { bug!(); };
 499     assert!(obligations.is_empty());
 500     value
 501 }
 502
 503 pub(super) fn response_no_constraints<'tcx>(
 504     tcx: TyCtxt<'tcx>,
 505     goal: Canonical<'tcx, impl Sized>,
 506     certainty: Certainty,
 507 ) -> QueryResult<'tcx> {
 508     Ok(Canonical {
 509         max_universe: goal.max_universe,
 510         variables: goal.variables,
 511         value: Response {
 512             var_values: CanonicalVarValues::make_identity(tcx, goal.variables),
 513             external_constraints: Default::default(),
 514             certainty,
 515         },
 516     })
 517 }