compiler/rustc_infer/src/infer/combine.rs

   1 //! There are four type combiners: [Equate], [Sub], [Lub], and [Glb].
   2 //! Each implements the trait [TypeRelation] and contains methods for
   3 //! combining two instances of various things and yielding a new instance.
   4 //! These combiner methods always yield a `Result<T>`. To relate two
   5 //! types, you can use `infcx.at(cause, param_env)` which then allows
   6 //! you to use the relevant methods of [At](super::at::At).
   7 //!
   8 //! Combiners mostly do their specific behavior and then hand off the
   9 //! bulk of the work to [InferCtxt::super_combine_tys] and
  10 //! [InferCtxt::super_combine_consts].
  11 //!
  12 //! Combining two types may have side-effects on the inference contexts
  13 //! which can be undone by using snapshots. You probably want to use
  14 //! either [InferCtxt::commit_if_ok] or [InferCtxt::probe].
  15 //!
  16 //! On success, the  LUB/GLB operations return the appropriate bound. The
  17 //! return value of `Equate` or `Sub` shouldn't really be used.
  18 //!
  19 //! ## Contravariance
  20 //!
  21 //! We explicitly track which argument is expected using
  22 //! [TypeRelation::a_is_expected], so when dealing with contravariance
  23 //! this should be correctly updated.
  24
  25 use super::equate::Equate;
  26 use super::glb::Glb;
  27 use super::lub::Lub;
  28 use super::sub::Sub;
  29 use super::type_variable::TypeVariableValue;
  30 use super::{InferCtxt, MiscVariable, TypeTrace};
  31 use crate::traits::{Obligation, PredicateObligations};
  32 use rustc_data_structures::sso::SsoHashMap;
  33 use rustc_hir::def_id::DefId;
  34 use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
  35 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
  36 use rustc_middle::traits::ObligationCause;
  37 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  38 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
  39 use rustc_middle::ty::subst::SubstsRef;
  40 use rustc_middle::ty::{self, InferConst, Ty, TyCtxt, TypeVisitable};
  41 use rustc_middle::ty::{IntType, UintType};
  42 use rustc_span::{Span, DUMMY_SP};
  43
  44 #[derive(Clone)]
  45 pub struct CombineFields<'infcx, 'tcx> {
  46     pub infcx: &'infcx InferCtxt<'tcx>,
  47     pub trace: TypeTrace<'tcx>,
  48     pub cause: Option<ty::relate::Cause>,
  49     pub param_env: ty::ParamEnv<'tcx>,
  50     pub obligations: PredicateObligations<'tcx>,
  51     /// Whether we should define opaque types
  52     /// or just treat them opaquely.
  53     /// Currently only used to prevent predicate
  54     /// matching from matching anything against opaque
  55     /// types.
  56     pub define_opaque_types: bool,
  57 }
  58
  59 #[derive(Copy, Clone, Debug)]
  60 pub enum RelationDir {
  61     SubtypeOf,
  62     SupertypeOf,
  63     EqTo,
  64 }
  65
  66 impl<'tcx> InferCtxt<'tcx> {
  67     pub fn super_combine_tys<R>(
  68         &self,
  69         relation: &mut R,
  70         a: Ty<'tcx>,
  71         b: Ty<'tcx>,
  72     ) -> RelateResult<'tcx, Ty<'tcx>>
  73     where
  74         R: TypeRelation<'tcx>,
  75     {
  76         let a_is_expected = relation.a_is_expected();
  77
  78         match (a.kind(), b.kind()) {
  79             // Relate integral variables to other types
  80             (&ty::Infer(ty::IntVar(a_id)), &ty::Infer(ty::IntVar(b_id))) => {
  81                 self.inner
  82                     .borrow_mut()
  83                     .int_unification_table()
  84                     .unify_var_var(a_id, b_id)
  85                     .map_err(|e| int_unification_error(a_is_expected, e))?;
  86                 Ok(a)
  87             }
  88             (&ty::Infer(ty::IntVar(v_id)), &ty::Int(v)) => {
  89                 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
  90             }
  91             (&ty::Int(v), &ty::Infer(ty::IntVar(v_id))) => {
  92                 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
  93             }
  94             (&ty::Infer(ty::IntVar(v_id)), &ty::Uint(v)) => {
  95                 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
  96             }
  97             (&ty::Uint(v), &ty::Infer(ty::IntVar(v_id))) => {
  98                 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
  99             }
 100
 101             // Relate floating-point variables to other types
 102             (&ty::Infer(ty::FloatVar(a_id)), &ty::Infer(ty::FloatVar(b_id))) => {
 103                 self.inner
 104                     .borrow_mut()
 105                     .float_unification_table()
 106                     .unify_var_var(a_id, b_id)
 107                     .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
 108                 Ok(a)
 109             }
 110             (&ty::Infer(ty::FloatVar(v_id)), &ty::Float(v)) => {
 111                 self.unify_float_variable(a_is_expected, v_id, v)
 112             }
 113             (&ty::Float(v), &ty::Infer(ty::FloatVar(v_id))) => {
 114                 self.unify_float_variable(!a_is_expected, v_id, v)
 115             }
 116
 117             // All other cases of inference are errors
 118             (&ty::Infer(_), _) | (_, &ty::Infer(_)) => {
 119                 Err(TypeError::Sorts(ty::relate::expected_found(relation, a, b)))
 120             }
 121
 122             _ => ty::relate::super_relate_tys(relation, a, b),
 123         }
 124     }
 125
 126     pub fn super_combine_consts<R>(
 127         &self,
 128         relation: &mut R,
 129         a: ty::Const<'tcx>,
 130         b: ty::Const<'tcx>,
 131     ) -> RelateResult<'tcx, ty::Const<'tcx>>
 132     where
 133         R: ConstEquateRelation<'tcx>,
 134     {
 135         debug!("{}.consts({:?}, {:?})", relation.tag(), a, b);
 136         if a == b {
 137             return Ok(a);
 138         }
 139
 140         let a = self.shallow_resolve(a);
 141         let b = self.shallow_resolve(b);
 142
 143         let a_is_expected = relation.a_is_expected();
 144
 145         match (a.kind(), b.kind()) {
 146             (
 147                 ty::ConstKind::Infer(InferConst::Var(a_vid)),
 148                 ty::ConstKind::Infer(InferConst::Var(b_vid)),
 149             ) => {
 150                 self.inner.borrow_mut().const_unification_table().union(a_vid, b_vid);
 151                 return Ok(a);
 152             }
 153
 154             // All other cases of inference with other variables are errors.
 155             (ty::ConstKind::Infer(InferConst::Var(_)), ty::ConstKind::Infer(_))
 156             | (ty::ConstKind::Infer(_), ty::ConstKind::Infer(InferConst::Var(_))) => {
 157                 bug!("tried to combine ConstKind::Infer/ConstKind::Infer(InferConst::Var)")
 158             }
 159
 160             (ty::ConstKind::Infer(InferConst::Var(vid)), _) => {
 161                 return self.unify_const_variable(relation.param_env(), vid, b, a_is_expected);
 162             }
 163
 164             (_, ty::ConstKind::Infer(InferConst::Var(vid))) => {
 165                 return self.unify_const_variable(relation.param_env(), vid, a, !a_is_expected);
 166             }
 167             (ty::ConstKind::Unevaluated(..), _) if self.tcx.lazy_normalization() => {
 168                 // FIXME(#59490): Need to remove the leak check to accommodate
 169                 // escaping bound variables here.
 170                 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
 171                     relation.const_equate_obligation(a, b);
 172                 }
 173                 return Ok(b);
 174             }
 175             (_, ty::ConstKind::Unevaluated(..)) if self.tcx.lazy_normalization() => {
 176                 // FIXME(#59490): Need to remove the leak check to accommodate
 177                 // escaping bound variables here.
 178                 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
 179                     relation.const_equate_obligation(a, b);
 180                 }
 181                 return Ok(a);
 182             }
 183             _ => {}
 184         }
 185
 186         ty::relate::super_relate_consts(relation, a, b)
 187     }
 188
 189     /// Unifies the const variable `target_vid` with the given constant.
 190     ///
 191     /// This also tests if the given const `ct` contains an inference variable which was previously
 192     /// unioned with `target_vid`. If this is the case, inferring `target_vid` to `ct`
 193     /// would result in an infinite type as we continuously replace an inference variable
 194     /// in `ct` with `ct` itself.
 195     ///
 196     /// This is especially important as unevaluated consts use their parents generics.
 197     /// They therefore often contain unused substs, making these errors far more likely.
 198     ///
 199     /// A good example of this is the following:
 200     ///
 201     /// ```compile_fail,E0308
 202     /// #![feature(generic_const_exprs)]
 203     ///
 204     /// fn bind<const N: usize>(value: [u8; N]) -> [u8; 3 + 4] {
 205     ///     todo!()
 206     /// }
 207     ///
 208     /// fn main() {
 209     ///     let mut arr = Default::default();
 210     ///     arr = bind(arr);
 211     /// }
 212     /// ```
 213     ///
 214     /// Here `3 + 4` ends up as `ConstKind::Unevaluated` which uses the generics
 215     /// of `fn bind` (meaning that its substs contain `N`).
 216     ///
 217     /// `bind(arr)` now infers that the type of `arr` must be `[u8; N]`.
 218     /// The assignment `arr = bind(arr)` now tries to equate `N` with `3 + 4`.
 219     ///
 220     /// As `3 + 4` contains `N` in its substs, this must not succeed.
 221     ///
 222     /// See `src/test/ui/const-generics/occurs-check/` for more examples where this is relevant.
 223     #[instrument(level = "debug", skip(self))]
 224     fn unify_const_variable(
 225         &self,
 226         param_env: ty::ParamEnv<'tcx>,
 227         target_vid: ty::ConstVid<'tcx>,
 228         ct: ty::Const<'tcx>,
 229         vid_is_expected: bool,
 230     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
 231         let (for_universe, span) = {
 232             let mut inner = self.inner.borrow_mut();
 233             let variable_table = &mut inner.const_unification_table();
 234             let var_value = variable_table.probe_value(target_vid);
 235             match var_value.val {
 236                 ConstVariableValue::Known { value } => {
 237                     bug!("instantiating {:?} which has a known value {:?}", target_vid, value)
 238                 }
 239                 ConstVariableValue::Unknown { universe } => (universe, var_value.origin.span),
 240             }
 241         };
 242         let value = ConstInferUnifier { infcx: self, span, param_env, for_universe, target_vid }
 243             .relate(ct, ct)?;
 244
 245         self.inner.borrow_mut().const_unification_table().union_value(
 246             target_vid,
 247             ConstVarValue {
 248                 origin: ConstVariableOrigin {
 249                     kind: ConstVariableOriginKind::ConstInference,
 250                     span: DUMMY_SP,
 251                 },
 252                 val: ConstVariableValue::Known { value },
 253             },
 254         );
 255         Ok(value)
 256     }
 257
 258     fn unify_integral_variable(
 259         &self,
 260         vid_is_expected: bool,
 261         vid: ty::IntVid,
 262         val: ty::IntVarValue,
 263     ) -> RelateResult<'tcx, Ty<'tcx>> {
 264         self.inner
 265             .borrow_mut()
 266             .int_unification_table()
 267             .unify_var_value(vid, Some(val))
 268             .map_err(|e| int_unification_error(vid_is_expected, e))?;
 269         match val {
 270             IntType(v) => Ok(self.tcx.mk_mach_int(v)),
 271             UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
 272         }
 273     }
 274
 275     fn unify_float_variable(
 276         &self,
 277         vid_is_expected: bool,
 278         vid: ty::FloatVid,
 279         val: ty::FloatTy,
 280     ) -> RelateResult<'tcx, Ty<'tcx>> {
 281         self.inner
 282             .borrow_mut()
 283             .float_unification_table()
 284             .unify_var_value(vid, Some(ty::FloatVarValue(val)))
 285             .map_err(|e| float_unification_error(vid_is_expected, e))?;
 286         Ok(self.tcx.mk_mach_float(val))
 287     }
 288 }
 289
 290 impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
 291     pub fn tcx(&self) -> TyCtxt<'tcx> {
 292         self.infcx.tcx
 293     }
 294
 295     pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'tcx> {
 296         Equate::new(self, a_is_expected)
 297     }
 298
 299     pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'tcx> {
 300         Sub::new(self, a_is_expected)
 301     }
 302
 303     pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'tcx> {
 304         Lub::new(self, a_is_expected)
 305     }
 306
 307     pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'tcx> {
 308         Glb::new(self, a_is_expected)
 309     }
 310
 311     /// Here, `dir` is either `EqTo`, `SubtypeOf`, or `SupertypeOf`.
 312     /// The idea is that we should ensure that the type `a_ty` is equal
 313     /// to, a subtype of, or a supertype of (respectively) the type
 314     /// to which `b_vid` is bound.
 315     ///
 316     /// Since `b_vid` has not yet been instantiated with a type, we
 317     /// will first instantiate `b_vid` with a *generalized* version
 318     /// of `a_ty`. Generalization introduces other inference
 319     /// variables wherever subtyping could occur.
 320     #[instrument(skip(self), level = "debug")]
 321     pub fn instantiate(
 322         &mut self,
 323         a_ty: Ty<'tcx>,
 324         dir: RelationDir,
 325         b_vid: ty::TyVid,
 326         a_is_expected: bool,
 327     ) -> RelateResult<'tcx, ()> {
 328         use self::RelationDir::*;
 329
 330         // Get the actual variable that b_vid has been inferred to
 331         debug_assert!(self.infcx.inner.borrow_mut().type_variables().probe(b_vid).is_unknown());
 332
 333         // Generalize type of `a_ty` appropriately depending on the
 334         // direction.  As an example, assume:
 335         //
 336         // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
 337         //   inference variable,
 338         // - and `dir` == `SubtypeOf`.
 339         //
 340         // Then the generalized form `b_ty` would be `&'?2 ?3`, where
 341         // `'?2` and `?3` are fresh region/type inference
 342         // variables. (Down below, we will relate `a_ty <: b_ty`,
 343         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
 344         let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
 345         debug!(?b_ty);
 346         self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
 347
 348         if needs_wf {
 349             self.obligations.push(Obligation::new(
 350                 self.tcx(),
 351                 self.trace.cause.clone(),
 352                 self.param_env,
 353                 ty::Binder::dummy(ty::PredicateKind::WellFormed(b_ty.into())),
 354             ));
 355         }
 356
 357         // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
 358         //
 359         // FIXME(#16847): This code is non-ideal because all these subtype
 360         // relations wind up attributed to the same spans. We need
 361         // to associate causes/spans with each of the relations in
 362         // the stack to get this right.
 363         match dir {
 364             EqTo => self.equate(a_is_expected).relate(a_ty, b_ty),
 365             SubtypeOf => self.sub(a_is_expected).relate(a_ty, b_ty),
 366             SupertypeOf => self.sub(a_is_expected).relate_with_variance(
 367                 ty::Contravariant,
 368                 ty::VarianceDiagInfo::default(),
 369                 a_ty,
 370                 b_ty,
 371             ),
 372         }?;
 373
 374         Ok(())
 375     }
 376
 377     /// Attempts to generalize `ty` for the type variable `for_vid`.
 378     /// This checks for cycle -- that is, whether the type `ty`
 379     /// references `for_vid`. The `dir` is the "direction" for which we
 380     /// a performing the generalization (i.e., are we producing a type
 381     /// that can be used as a supertype etc).
 382     ///
 383     /// Preconditions:
 384     ///
 385     /// - `for_vid` is a "root vid"
 386     #[instrument(skip(self), level = "trace", ret)]
 387     fn generalize(
 388         &self,
 389         ty: Ty<'tcx>,
 390         for_vid: ty::TyVid,
 391         dir: RelationDir,
 392     ) -> RelateResult<'tcx, Generalization<'tcx>> {
 393         // Determine the ambient variance within which `ty` appears.
 394         // The surrounding equation is:
 395         //
 396         //     ty [op] ty2
 397         //
 398         // where `op` is either `==`, `<:`, or `:>`. This maps quite
 399         // naturally.
 400         let ambient_variance = match dir {
 401             RelationDir::EqTo => ty::Invariant,
 402             RelationDir::SubtypeOf => ty::Covariant,
 403             RelationDir::SupertypeOf => ty::Contravariant,
 404         };
 405
 406         trace!(?ambient_variance);
 407
 408         let for_universe = match self.infcx.inner.borrow_mut().type_variables().probe(for_vid) {
 409             v @ TypeVariableValue::Known { .. } => {
 410                 bug!("instantiating {:?} which has a known value {:?}", for_vid, v,)
 411             }
 412             TypeVariableValue::Unknown { universe } => universe,
 413         };
 414
 415         trace!(?for_universe);
 416         trace!(?self.trace);
 417
 418         let mut generalize = Generalizer {
 419             infcx: self.infcx,
 420             cause: &self.trace.cause,
 421             for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
 422             for_universe,
 423             ambient_variance,
 424             needs_wf: false,
 425             root_ty: ty,
 426             param_env: self.param_env,
 427             cache: SsoHashMap::new(),
 428         };
 429
 430         let ty = generalize.relate(ty, ty)?;
 431         let needs_wf = generalize.needs_wf;
 432         Ok(Generalization { ty, needs_wf })
 433     }
 434
 435     pub fn add_const_equate_obligation(
 436         &mut self,
 437         a_is_expected: bool,
 438         a: ty::Const<'tcx>,
 439         b: ty::Const<'tcx>,
 440     ) {
 441         let predicate = if a_is_expected {
 442             ty::PredicateKind::ConstEquate(a, b)
 443         } else {
 444             ty::PredicateKind::ConstEquate(b, a)
 445         };
 446         self.obligations.push(Obligation::new(
 447             self.tcx(),
 448             self.trace.cause.clone(),
 449             self.param_env,
 450             ty::Binder::dummy(predicate),
 451         ));
 452     }
 453 }
 454
 455 struct Generalizer<'cx, 'tcx> {
 456     infcx: &'cx InferCtxt<'tcx>,
 457
 458     /// The span, used when creating new type variables and things.
 459     cause: &'cx ObligationCause<'tcx>,
 460
 461     /// The vid of the type variable that is in the process of being
 462     /// instantiated; if we find this within the type we are folding,
 463     /// that means we would have created a cyclic type.
 464     for_vid_sub_root: ty::TyVid,
 465
 466     /// The universe of the type variable that is in the process of
 467     /// being instantiated. Any fresh variables that we create in this
 468     /// process should be in that same universe.
 469     for_universe: ty::UniverseIndex,
 470
 471     /// Track the variance as we descend into the type.
 472     ambient_variance: ty::Variance,
 473
 474     /// See the field `needs_wf` in `Generalization`.
 475     needs_wf: bool,
 476
 477     /// The root type that we are generalizing. Used when reporting cycles.
 478     root_ty: Ty<'tcx>,
 479
 480     param_env: ty::ParamEnv<'tcx>,
 481
 482     cache: SsoHashMap<Ty<'tcx>, Ty<'tcx>>,
 483 }
 484
 485 /// Result from a generalization operation. This includes
 486 /// not only the generalized type, but also a bool flag
 487 /// indicating whether further WF checks are needed.
 488 #[derive(Debug)]
 489 struct Generalization<'tcx> {
 490     ty: Ty<'tcx>,
 491
 492     /// If true, then the generalized type may not be well-formed,
 493     /// even if the source type is well-formed, so we should add an
 494     /// additional check to enforce that it is. This arises in
 495     /// particular around 'bivariant' type parameters that are only
 496     /// constrained by a where-clause. As an example, imagine a type:
 497     ///
 498     ///     struct Foo<A, B> where A: Iterator<Item = B> {
 499     ///         data: A
 500     ///     }
 501     ///
 502     /// here, `A` will be covariant, but `B` is
 503     /// unconstrained. However, whatever it is, for `Foo` to be WF, it
 504     /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
 505     /// then after generalization we will wind up with a type like
 506     /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
 507     /// ?D>` (or `>:`), we will wind up with the requirement that `?A
 508     /// <: ?C`, but no particular relationship between `?B` and `?D`
 509     /// (after all, we do not know the variance of the normalized form
 510     /// of `A::Item` with respect to `A`). If we do nothing else, this
 511     /// may mean that `?D` goes unconstrained (as in #41677). So, in
 512     /// this scenario where we create a new type variable in a
 513     /// bivariant context, we set the `needs_wf` flag to true. This
 514     /// will force the calling code to check that `WF(Foo<?C, ?D>)`
 515     /// holds, which in turn implies that `?C::Item == ?D`. So once
 516     /// `?C` is constrained, that should suffice to restrict `?D`.
 517     needs_wf: bool,
 518 }
 519
 520 impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
 521     fn tcx(&self) -> TyCtxt<'tcx> {
 522         self.infcx.tcx
 523     }
 524     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 525         self.param_env
 526     }
 527
 528     fn tag(&self) -> &'static str {
 529         "Generalizer"
 530     }
 531
 532     fn a_is_expected(&self) -> bool {
 533         true
 534     }
 535
 536     fn binders<T>(
 537         &mut self,
 538         a: ty::Binder<'tcx, T>,
 539         b: ty::Binder<'tcx, T>,
 540     ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
 541     where
 542         T: Relate<'tcx>,
 543     {
 544         Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
 545     }
 546
 547     fn relate_item_substs(
 548         &mut self,
 549         item_def_id: DefId,
 550         a_subst: SubstsRef<'tcx>,
 551         b_subst: SubstsRef<'tcx>,
 552     ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
 553         if self.ambient_variance == ty::Variance::Invariant {
 554             // Avoid fetching the variance if we are in an invariant
 555             // context; no need, and it can induce dependency cycles
 556             // (e.g., #41849).
 557             relate::relate_substs(self, a_subst, b_subst)
 558         } else {
 559             let tcx = self.tcx();
 560             let opt_variances = tcx.variances_of(item_def_id);
 561             relate::relate_substs_with_variances(
 562                 self,
 563                 item_def_id,
 564                 &opt_variances,
 565                 a_subst,
 566                 b_subst,
 567                 true,
 568             )
 569         }
 570     }
 571
 572     fn relate_with_variance<T: Relate<'tcx>>(
 573         &mut self,
 574         variance: ty::Variance,
 575         _info: ty::VarianceDiagInfo<'tcx>,
 576         a: T,
 577         b: T,
 578     ) -> RelateResult<'tcx, T> {
 579         let old_ambient_variance = self.ambient_variance;
 580         self.ambient_variance = self.ambient_variance.xform(variance);
 581
 582         let result = self.relate(a, b);
 583         self.ambient_variance = old_ambient_variance;
 584         result
 585     }
 586
 587     fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
 588         assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 589
 590         if let Some(&result) = self.cache.get(&t) {
 591             return Ok(result);
 592         }
 593         debug!("generalize: t={:?}", t);
 594
 595         // Check to see whether the type we are generalizing references
 596         // any other type variable related to `vid` via
 597         // subtyping. This is basically our "occurs check", preventing
 598         // us from creating infinitely sized types.
 599         let result = match *t.kind() {
 600             ty::Infer(ty::TyVar(vid)) => {
 601                 let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
 602                 let sub_vid = self.infcx.inner.borrow_mut().type_variables().sub_root_var(vid);
 603                 if sub_vid == self.for_vid_sub_root {
 604                     // If sub-roots are equal, then `for_vid` and
 605                     // `vid` are related via subtyping.
 606                     Err(TypeError::CyclicTy(self.root_ty))
 607                 } else {
 608                     let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
 609                     match probe {
 610                         TypeVariableValue::Known { value: u } => {
 611                             debug!("generalize: known value {:?}", u);
 612                             self.relate(u, u)
 613                         }
 614                         TypeVariableValue::Unknown { universe } => {
 615                             match self.ambient_variance {
 616                                 // Invariant: no need to make a fresh type variable.
 617                                 ty::Invariant => {
 618                                     if self.for_universe.can_name(universe) {
 619                                         return Ok(t);
 620                                     }
 621                                 }
 622
 623                                 // Bivariant: make a fresh var, but we
 624                                 // may need a WF predicate. See
 625                                 // comment on `needs_wf` field for
 626                                 // more info.
 627                                 ty::Bivariant => self.needs_wf = true,
 628
 629                                 // Co/contravariant: this will be
 630                                 // sufficiently constrained later on.
 631                                 ty::Covariant | ty::Contravariant => (),
 632                             }
 633
 634                             let origin =
 635                                 *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
 636                             let new_var_id = self
 637                                 .infcx
 638                                 .inner
 639                                 .borrow_mut()
 640                                 .type_variables()
 641                                 .new_var(self.for_universe, origin);
 642                             let u = self.tcx().mk_ty_var(new_var_id);
 643
 644                             // Record that we replaced `vid` with `new_var_id` as part of a generalization
 645                             // operation. This is needed to detect cyclic types. To see why, see the
 646                             // docs in the `type_variables` module.
 647                             self.infcx.inner.borrow_mut().type_variables().sub(vid, new_var_id);
 648                             debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
 649                             Ok(u)
 650                         }
 651                     }
 652                 }
 653             }
 654             ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
 655                 // No matter what mode we are in,
 656                 // integer/floating-point types must be equal to be
 657                 // relatable.
 658                 Ok(t)
 659             }
 660             _ => relate::super_relate_tys(self, t, t),
 661         }?;
 662
 663         self.cache.insert(t, result);
 664         Ok(result)
 665     }
 666
 667     fn regions(
 668         &mut self,
 669         r: ty::Region<'tcx>,
 670         r2: ty::Region<'tcx>,
 671     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
 672         assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 673
 674         debug!("generalize: regions r={:?}", r);
 675
 676         match *r {
 677             // Never make variables for regions bound within the type itself,
 678             // nor for erased regions.
 679             ty::ReLateBound(..) | ty::ReErased => {
 680                 return Ok(r);
 681             }
 682
 683             ty::RePlaceholder(..)
 684             | ty::ReVar(..)
 685             | ty::ReStatic
 686             | ty::ReEarlyBound(..)
 687             | ty::ReFree(..) => {
 688                 // see common code below
 689             }
 690         }
 691
 692         // If we are in an invariant context, we can re-use the region
 693         // as is, unless it happens to be in some universe that we
 694         // can't name. (In the case of a region *variable*, we could
 695         // use it if we promoted it into our universe, but we don't
 696         // bother.)
 697         if let ty::Invariant = self.ambient_variance {
 698             let r_universe = self.infcx.universe_of_region(r);
 699             if self.for_universe.can_name(r_universe) {
 700                 return Ok(r);
 701             }
 702         }
 703
 704         // FIXME: This is non-ideal because we don't give a
 705         // very descriptive origin for this region variable.
 706         Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.cause.span), self.for_universe))
 707     }
 708
 709     fn consts(
 710         &mut self,
 711         c: ty::Const<'tcx>,
 712         c2: ty::Const<'tcx>,
 713     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
 714         assert_eq!(c, c2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 715
 716         match c.kind() {
 717             ty::ConstKind::Infer(InferConst::Var(vid)) => {
 718                 let mut inner = self.infcx.inner.borrow_mut();
 719                 let variable_table = &mut inner.const_unification_table();
 720                 let var_value = variable_table.probe_value(vid);
 721                 match var_value.val {
 722                     ConstVariableValue::Known { value: u } => {
 723                         drop(inner);
 724                         self.relate(u, u)
 725                     }
 726                     ConstVariableValue::Unknown { universe } => {
 727                         if self.for_universe.can_name(universe) {
 728                             Ok(c)
 729                         } else {
 730                             let new_var_id = variable_table.new_key(ConstVarValue {
 731                                 origin: var_value.origin,
 732                                 val: ConstVariableValue::Unknown { universe: self.for_universe },
 733                             });
 734                             Ok(self.tcx().mk_const_var(new_var_id, c.ty()))
 735                         }
 736                     }
 737                 }
 738             }
 739             ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs }) => {
 740                 let substs = self.relate_with_variance(
 741                     ty::Variance::Invariant,
 742                     ty::VarianceDiagInfo::default(),
 743                     substs,
 744                     substs,
 745                 )?;
 746                 Ok(self.tcx().mk_const(
 747                     ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs }),
 748                     c.ty(),
 749                 ))
 750             }
 751             _ => relate::super_relate_consts(self, c, c),
 752         }
 753     }
 754 }
 755
 756 pub trait ConstEquateRelation<'tcx>: TypeRelation<'tcx> {
 757     /// Register an obligation that both constants must be equal to each other.
 758     ///
 759     /// If they aren't equal then the relation doesn't hold.
 760     fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>);
 761 }
 762
 763 fn int_unification_error<'tcx>(
 764     a_is_expected: bool,
 765     v: (ty::IntVarValue, ty::IntVarValue),
 766 ) -> TypeError<'tcx> {
 767     let (a, b) = v;
 768     TypeError::IntMismatch(ExpectedFound::new(a_is_expected, a, b))
 769 }
 770
 771 fn float_unification_error<'tcx>(
 772     a_is_expected: bool,
 773     v: (ty::FloatVarValue, ty::FloatVarValue),
 774 ) -> TypeError<'tcx> {
 775     let (ty::FloatVarValue(a), ty::FloatVarValue(b)) = v;
 776     TypeError::FloatMismatch(ExpectedFound::new(a_is_expected, a, b))
 777 }
 778
 779 struct ConstInferUnifier<'cx, 'tcx> {
 780     infcx: &'cx InferCtxt<'tcx>,
 781
 782     span: Span,
 783
 784     param_env: ty::ParamEnv<'tcx>,
 785
 786     for_universe: ty::UniverseIndex,
 787
 788     /// The vid of the const variable that is in the process of being
 789     /// instantiated; if we find this within the const we are folding,
 790     /// that means we would have created a cyclic const.
 791     target_vid: ty::ConstVid<'tcx>,
 792 }
 793
 794 // We use `TypeRelation` here to propagate `RelateResult` upwards.
 795 //
 796 // Both inputs are expected to be the same.
 797 impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
 798     fn tcx(&self) -> TyCtxt<'tcx> {
 799         self.infcx.tcx
 800     }
 801
 802     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 803         self.param_env
 804     }
 805
 806     fn tag(&self) -> &'static str {
 807         "ConstInferUnifier"
 808     }
 809
 810     fn a_is_expected(&self) -> bool {
 811         true
 812     }
 813
 814     fn relate_with_variance<T: Relate<'tcx>>(
 815         &mut self,
 816         _variance: ty::Variance,
 817         _info: ty::VarianceDiagInfo<'tcx>,
 818         a: T,
 819         b: T,
 820     ) -> RelateResult<'tcx, T> {
 821         // We don't care about variance here.
 822         self.relate(a, b)
 823     }
 824
 825     fn binders<T>(
 826         &mut self,
 827         a: ty::Binder<'tcx, T>,
 828         b: ty::Binder<'tcx, T>,
 829     ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
 830     where
 831         T: Relate<'tcx>,
 832     {
 833         Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
 834     }
 835
 836     #[instrument(level = "debug", skip(self), ret)]
 837     fn tys(&mut self, t: Ty<'tcx>, _t: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
 838         debug_assert_eq!(t, _t);
 839
 840         match t.kind() {
 841             &ty::Infer(ty::TyVar(vid)) => {
 842                 let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
 843                 let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
 844                 match probe {
 845                     TypeVariableValue::Known { value: u } => {
 846                         debug!("ConstOccursChecker: known value {:?}", u);
 847                         self.tys(u, u)
 848                     }
 849                     TypeVariableValue::Unknown { universe } => {
 850                         if self.for_universe.can_name(universe) {
 851                             return Ok(t);
 852                         }
 853
 854                         let origin =
 855                             *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
 856                         let new_var_id = self
 857                             .infcx
 858                             .inner
 859                             .borrow_mut()
 860                             .type_variables()
 861                             .new_var(self.for_universe, origin);
 862                         Ok(self.tcx().mk_ty_var(new_var_id))
 863                     }
 864                 }
 865             }
 866             ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => Ok(t),
 867             _ => relate::super_relate_tys(self, t, t),
 868         }
 869     }
 870
 871     fn regions(
 872         &mut self,
 873         r: ty::Region<'tcx>,
 874         _r: ty::Region<'tcx>,
 875     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
 876         debug_assert_eq!(r, _r);
 877         debug!("ConstInferUnifier: r={:?}", r);
 878
 879         match *r {
 880             // Never make variables for regions bound within the type itself,
 881             // nor for erased regions.
 882             ty::ReLateBound(..) | ty::ReErased => {
 883                 return Ok(r);
 884             }
 885
 886             ty::RePlaceholder(..)
 887             | ty::ReVar(..)
 888             | ty::ReStatic
 889             | ty::ReEarlyBound(..)
 890             | ty::ReFree(..) => {
 891                 // see common code below
 892             }
 893         }
 894
 895         let r_universe = self.infcx.universe_of_region(r);
 896         if self.for_universe.can_name(r_universe) {
 897             return Ok(r);
 898         } else {
 899             // FIXME: This is non-ideal because we don't give a
 900             // very descriptive origin for this region variable.
 901             Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.span), self.for_universe))
 902         }
 903     }
 904
 905     #[instrument(level = "debug", skip(self))]
 906     fn consts(
 907         &mut self,
 908         c: ty::Const<'tcx>,
 909         _c: ty::Const<'tcx>,
 910     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
 911         debug_assert_eq!(c, _c);
 912
 913         match c.kind() {
 914             ty::ConstKind::Infer(InferConst::Var(vid)) => {
 915                 // Check if the current unification would end up
 916                 // unifying `target_vid` with a const which contains
 917                 // an inference variable which is unioned with `target_vid`.
 918                 //
 919                 // Not doing so can easily result in stack overflows.
 920                 if self
 921                     .infcx
 922                     .inner
 923                     .borrow_mut()
 924                     .const_unification_table()
 925                     .unioned(self.target_vid, vid)
 926                 {
 927                     return Err(TypeError::CyclicConst(c));
 928                 }
 929
 930                 let var_value =
 931                     self.infcx.inner.borrow_mut().const_unification_table().probe_value(vid);
 932                 match var_value.val {
 933                     ConstVariableValue::Known { value: u } => self.consts(u, u),
 934                     ConstVariableValue::Unknown { universe } => {
 935                         if self.for_universe.can_name(universe) {
 936                             Ok(c)
 937                         } else {
 938                             let new_var_id =
 939                                 self.infcx.inner.borrow_mut().const_unification_table().new_key(
 940                                     ConstVarValue {
 941                                         origin: var_value.origin,
 942                                         val: ConstVariableValue::Unknown {
 943                                             universe: self.for_universe,
 944                                         },
 945                                     },
 946                                 );
 947                             Ok(self.tcx().mk_const_var(new_var_id, c.ty()))
 948                         }
 949                     }
 950                 }
 951             }
 952             ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs }) => {
 953                 let substs = self.relate_with_variance(
 954                     ty::Variance::Invariant,
 955                     ty::VarianceDiagInfo::default(),
 956                     substs,
 957                     substs,
 958                 )?;
 959
 960                 Ok(self.tcx().mk_const(
 961                     ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs }),
 962                     c.ty(),
 963                 ))
 964             }
 965             _ => relate::super_relate_consts(self, c, c),
 966         }
 967     }
 968 }