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 TyCtxtExt<'tcx> {
 145     fn evaluate_goal(self, goal: CanonicalGoal<'tcx>) -> QueryResult<'tcx>;
 146 }
 147
 148 impl<'tcx> TyCtxtExt<'tcx> for TyCtxt<'tcx> {
 149     fn evaluate_goal(self, goal: CanonicalGoal<'tcx>) -> QueryResult<'tcx> {
 150         let mut search_graph = search_graph::SearchGraph::new(self);
 151         EvalCtxt::evaluate_canonical_goal(self, &mut search_graph, goal)
 152     }
 153 }
 154
 155 struct EvalCtxt<'a, 'tcx> {
 156     infcx: &'a InferCtxt<'tcx>,
 157     var_values: CanonicalVarValues<'tcx>,
 158
 159     search_graph: &'a mut search_graph::SearchGraph<'tcx>,
 160 }
 161
 162 impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
 163     fn tcx(&self) -> TyCtxt<'tcx> {
 164         self.infcx.tcx
 165     }
 166
 167     /// Creates a new evaluation context outside of the trait solver.
 168     ///
 169     /// With this solver making a canonical response doesn't make much sense.
 170     /// The `search_graph` for this solver has to be completely empty.
 171     fn new_outside_solver(
 172         infcx: &'a InferCtxt<'tcx>,
 173         search_graph: &'a mut search_graph::SearchGraph<'tcx>,
 174     ) -> EvalCtxt<'a, 'tcx> {
 175         assert!(search_graph.is_empty());
 176         EvalCtxt { infcx, var_values: CanonicalVarValues::dummy(), search_graph }
 177     }
 178
 179     #[instrument(level = "debug", skip(tcx, search_graph), ret)]
 180     fn evaluate_canonical_goal(
 181         tcx: TyCtxt<'tcx>,
 182         search_graph: &'a mut search_graph::SearchGraph<'tcx>,
 183         canonical_goal: CanonicalGoal<'tcx>,
 184     ) -> QueryResult<'tcx> {
 185         match search_graph.try_push_stack(tcx, canonical_goal) {
 186             Ok(()) => {}
 187             // Our goal is already on the stack, eager return.
 188             Err(response) => return response,
 189         }
 190
 191         // We may have to repeatedly recompute the goal in case of coinductive cycles,
 192         // check out the `cache` module for more information.
 193         //
 194         // FIXME: Similar to `evaluate_all`, this has to check for overflow.
 195         loop {
 196             let (ref infcx, goal, var_values) =
 197                 tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &canonical_goal);
 198             let mut ecx = EvalCtxt { infcx, var_values, search_graph };
 199             let result = ecx.compute_goal(goal);
 200
 201             // FIXME: `Response` should be `Copy`
 202             if search_graph.try_finalize_goal(tcx, canonical_goal, result.clone()) {
 203                 return result;
 204             }
 205         }
 206     }
 207
 208     fn make_canonical_response(&self, certainty: Certainty) -> QueryResult<'tcx> {
 209         let external_constraints = take_external_constraints(self.infcx)?;
 210
 211         Ok(self.infcx.canonicalize_response(Response {
 212             var_values: self.var_values.clone(),
 213             external_constraints,
 214             certainty,
 215         }))
 216     }
 217
 218     /// Recursively evaluates `goal`, returning whether any inference vars have
 219     /// been constrained and the certainty of the result.
 220     fn evaluate_goal(
 221         &mut self,
 222         goal: Goal<'tcx, ty::Predicate<'tcx>>,
 223     ) -> Result<(bool, Certainty), NoSolution> {
 224         let mut orig_values = OriginalQueryValues::default();
 225         let canonical_goal = self.infcx.canonicalize_query(goal, &mut orig_values);
 226         let canonical_response =
 227             EvalCtxt::evaluate_canonical_goal(self.tcx(), self.search_graph, canonical_goal)?;
 228         Ok((
 229             !canonical_response.value.var_values.is_identity(),
 230             instantiate_canonical_query_response(self.infcx, &orig_values, canonical_response),
 231         ))
 232     }
 233
 234     fn compute_goal(&mut self, goal: Goal<'tcx, ty::Predicate<'tcx>>) -> QueryResult<'tcx> {
 235         let Goal { param_env, predicate } = goal;
 236         let kind = predicate.kind();
 237         if let Some(kind) = kind.no_bound_vars() {
 238             match kind {
 239                 ty::PredicateKind::Clause(ty::Clause::Trait(predicate)) => {
 240                     self.compute_trait_goal(Goal { param_env, predicate })
 241                 }
 242                 ty::PredicateKind::Clause(ty::Clause::Projection(predicate)) => {
 243                     self.compute_projection_goal(Goal { param_env, predicate })
 244                 }
 245                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(predicate)) => {
 246                     self.compute_type_outlives_goal(Goal { param_env, predicate })
 247                 }
 248                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(predicate)) => {
 249                     self.compute_region_outlives_goal(Goal { param_env, predicate })
 250                 }
 251                 ty::PredicateKind::Subtype(predicate) => {
 252                     self.compute_subtype_goal(Goal { param_env, predicate })
 253                 }
 254                 ty::PredicateKind::Coerce(predicate) => {
 255                     self.compute_coerce_goal(Goal { param_env, predicate })
 256                 }
 257                 ty::PredicateKind::ClosureKind(def_id, substs, kind) => self
 258                     .compute_closure_kind_goal(Goal {
 259                         param_env,
 260                         predicate: (def_id, substs, kind),
 261                     }),
 262                 ty::PredicateKind::Ambiguous => self.make_canonical_response(Certainty::AMBIGUOUS),
 263                 // FIXME: implement these predicates :)
 264                 ty::PredicateKind::WellFormed(_)
 265                 | ty::PredicateKind::ObjectSafe(_)
 266                 | ty::PredicateKind::ConstEvaluatable(_)
 267                 | ty::PredicateKind::ConstEquate(_, _) => {
 268                     self.make_canonical_response(Certainty::Yes)
 269                 }
 270                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 271                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 272                 }
 273             }
 274         } else {
 275             let kind = self.infcx.replace_bound_vars_with_placeholders(kind);
 276             let goal = goal.with(self.tcx(), ty::Binder::dummy(kind));
 277             let (_, certainty) = self.evaluate_goal(goal)?;
 278             self.make_canonical_response(certainty)
 279         }
 280     }
 281
 282     fn compute_type_outlives_goal(
 283         &mut self,
 284         _goal: Goal<'tcx, TypeOutlivesPredicate<'tcx>>,
 285     ) -> QueryResult<'tcx> {
 286         self.make_canonical_response(Certainty::Yes)
 287     }
 288
 289     fn compute_region_outlives_goal(
 290         &mut self,
 291         _goal: Goal<'tcx, RegionOutlivesPredicate<'tcx>>,
 292     ) -> QueryResult<'tcx> {
 293         self.make_canonical_response(Certainty::Yes)
 294     }
 295
 296     fn compute_coerce_goal(
 297         &mut self,
 298         goal: Goal<'tcx, CoercePredicate<'tcx>>,
 299     ) -> QueryResult<'tcx> {
 300         self.compute_subtype_goal(Goal {
 301             param_env: goal.param_env,
 302             predicate: SubtypePredicate {
 303                 a_is_expected: false,
 304                 a: goal.predicate.a,
 305                 b: goal.predicate.b,
 306             },
 307         })
 308     }
 309
 310     fn compute_subtype_goal(
 311         &mut self,
 312         goal: Goal<'tcx, SubtypePredicate<'tcx>>,
 313     ) -> QueryResult<'tcx> {
 314         if goal.predicate.a.is_ty_var() && goal.predicate.b.is_ty_var() {
 315             // FIXME: Do we want to register a subtype relation between these vars?
 316             // That won't actually reflect in the query response, so it seems moot.
 317             self.make_canonical_response(Certainty::AMBIGUOUS)
 318         } else {
 319             self.infcx.probe(|_| {
 320                 let InferOk { value: (), obligations } = self
 321                     .infcx
 322                     .at(&ObligationCause::dummy(), goal.param_env)
 323                     .sub(goal.predicate.a, goal.predicate.b)?;
 324                 self.evaluate_all_and_make_canonical_response(
 325                     obligations.into_iter().map(|pred| pred.into()).collect(),
 326                 )
 327             })
 328         }
 329     }
 330
 331     fn compute_closure_kind_goal(
 332         &mut self,
 333         goal: Goal<'tcx, (DefId, ty::SubstsRef<'tcx>, ty::ClosureKind)>,
 334     ) -> QueryResult<'tcx> {
 335         let (_, substs, expected_kind) = goal.predicate;
 336         let found_kind = substs.as_closure().kind_ty().to_opt_closure_kind();
 337
 338         let Some(found_kind) = found_kind else {
 339             return self.make_canonical_response(Certainty::AMBIGUOUS);
 340         };
 341         if found_kind.extends(expected_kind) {
 342             self.make_canonical_response(Certainty::Yes)
 343         } else {
 344             Err(NoSolution)
 345         }
 346     }
 347 }
 348
 349 impl<'tcx> EvalCtxt<'_, 'tcx> {
 350     fn evaluate_all(
 351         &mut self,
 352         mut goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
 353     ) -> Result<Certainty, NoSolution> {
 354         let mut new_goals = Vec::new();
 355         self.repeat_while_none(|this| {
 356             let mut has_changed = Err(Certainty::Yes);
 357             for goal in goals.drain(..) {
 358                 let (changed, certainty) = match this.evaluate_goal(goal) {
 359                     Ok(result) => result,
 360                     Err(NoSolution) => return Some(Err(NoSolution)),
 361                 };
 362
 363                 if changed {
 364                     has_changed = Ok(());
 365                 }
 366
 367                 match certainty {
 368                     Certainty::Yes => {}
 369                     Certainty::Maybe(_) => {
 370                         new_goals.push(goal);
 371                         has_changed = has_changed.map_err(|c| c.unify_and(certainty));
 372                     }
 373                 }
 374             }
 375
 376             match has_changed {
 377                 Ok(()) => {
 378                     mem::swap(&mut new_goals, &mut goals);
 379                     None
 380                 }
 381                 Err(certainty) => Some(Ok(certainty)),
 382             }
 383         })
 384     }
 385
 386     fn evaluate_all_and_make_canonical_response(
 387         &mut self,
 388         goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
 389     ) -> QueryResult<'tcx> {
 390         self.evaluate_all(goals).and_then(|certainty| self.make_canonical_response(certainty))
 391     }
 392 }
 393
 394 #[instrument(level = "debug", skip(infcx), ret)]
 395 fn take_external_constraints<'tcx>(
 396     infcx: &InferCtxt<'tcx>,
 397 ) -> Result<ExternalConstraints<'tcx>, NoSolution> {
 398     let region_obligations = infcx.take_registered_region_obligations();
 399     let opaque_types = infcx.take_opaque_types_for_query_response();
 400     Ok(ExternalConstraints {
 401         // FIXME: Now that's definitely wrong :)
 402         //
 403         // Should also do the leak check here I think
 404         regions: drop(region_obligations),
 405         opaque_types,
 406     })
 407 }
 408
 409 fn instantiate_canonical_query_response<'tcx>(
 410     infcx: &InferCtxt<'tcx>,
 411     original_values: &OriginalQueryValues<'tcx>,
 412     response: CanonicalResponse<'tcx>,
 413 ) -> Certainty {
 414     let Ok(InferOk { value, obligations }) = infcx
 415         .instantiate_query_response_and_region_obligations(
 416             &ObligationCause::dummy(),
 417             ty::ParamEnv::empty(),
 418             original_values,
 419             &response.unchecked_map(|resp| QueryResponse {
 420                 var_values: resp.var_values,
 421                 region_constraints: QueryRegionConstraints::default(),
 422                 certainty: match resp.certainty {
 423                     Certainty::Yes => OldCertainty::Proven,
 424                     Certainty::Maybe(_) => OldCertainty::Ambiguous,
 425                 },
 426                 opaque_types: resp.external_constraints.opaque_types,
 427                 value: resp.certainty,
 428             }),
 429         ) else { bug!(); };
 430     assert!(obligations.is_empty());
 431     value
 432 }
 433
 434 pub(super) fn response_no_constraints<'tcx>(
 435     tcx: TyCtxt<'tcx>,
 436     goal: Canonical<'tcx, impl Sized>,
 437     certainty: Certainty,
 438 ) -> QueryResult<'tcx> {
 439     let var_values = goal
 440         .variables
 441         .iter()
 442         .enumerate()
 443         .map(|(i, info)| match info.kind {
 444             CanonicalVarKind::Ty(_) | CanonicalVarKind::PlaceholderTy(_) => {
 445                 tcx.mk_ty(ty::Bound(ty::INNERMOST, ty::BoundVar::from_usize(i).into())).into()
 446             }
 447             CanonicalVarKind::Region(_) | CanonicalVarKind::PlaceholderRegion(_) => {
 448                 let br = ty::BoundRegion {
 449                     var: ty::BoundVar::from_usize(i),
 450                     kind: ty::BrAnon(i as u32, None),
 451                 };
 452                 tcx.mk_region(ty::ReLateBound(ty::INNERMOST, br)).into()
 453             }
 454             CanonicalVarKind::Const(_, ty) | CanonicalVarKind::PlaceholderConst(_, ty) => tcx
 455                 .mk_const(ty::ConstKind::Bound(ty::INNERMOST, ty::BoundVar::from_usize(i)), ty)
 456                 .into(),
 457         })
 458         .collect();
 459
 460     Ok(Canonical {
 461         max_universe: goal.max_universe,
 462         variables: goal.variables,
 463         value: Response {
 464             var_values: CanonicalVarValues { var_values },
 465             external_constraints: Default::default(),
 466             certainty,
 467         },
 468     })
 469 }