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, ToPredicate, 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.trace.cause.clone(),
 351                 self.param_env,
 352                 ty::Binder::dummy(ty::PredicateKind::WellFormed(b_ty.into()))
 353                     .to_predicate(self.infcx.tcx),
 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.trace.cause.clone(),
 448             self.param_env,
 449             ty::Binder::dummy(predicate).to_predicate(self.tcx()),
 450         ));
 451     }
 452 }
 453
 454 struct Generalizer<'cx, 'tcx> {
 455     infcx: &'cx InferCtxt<'tcx>,
 456
 457     /// The span, used when creating new type variables and things.
 458     cause: &'cx ObligationCause<'tcx>,
 459
 460     /// The vid of the type variable that is in the process of being
 461     /// instantiated; if we find this within the type we are folding,
 462     /// that means we would have created a cyclic type.
 463     for_vid_sub_root: ty::TyVid,
 464
 465     /// The universe of the type variable that is in the process of
 466     /// being instantiated. Any fresh variables that we create in this
 467     /// process should be in that same universe.
 468     for_universe: ty::UniverseIndex,
 469
 470     /// Track the variance as we descend into the type.
 471     ambient_variance: ty::Variance,
 472
 473     /// See the field `needs_wf` in `Generalization`.
 474     needs_wf: bool,
 475
 476     /// The root type that we are generalizing. Used when reporting cycles.
 477     root_ty: Ty<'tcx>,
 478
 479     param_env: ty::ParamEnv<'tcx>,
 480
 481     cache: SsoHashMap<Ty<'tcx>, Ty<'tcx>>,
 482 }
 483
 484 /// Result from a generalization operation. This includes
 485 /// not only the generalized type, but also a bool flag
 486 /// indicating whether further WF checks are needed.
 487 #[derive(Debug)]
 488 struct Generalization<'tcx> {
 489     ty: Ty<'tcx>,
 490
 491     /// If true, then the generalized type may not be well-formed,
 492     /// even if the source type is well-formed, so we should add an
 493     /// additional check to enforce that it is. This arises in
 494     /// particular around 'bivariant' type parameters that are only
 495     /// constrained by a where-clause. As an example, imagine a type:
 496     ///
 497     ///     struct Foo<A, B> where A: Iterator<Item = B> {
 498     ///         data: A
 499     ///     }
 500     ///
 501     /// here, `A` will be covariant, but `B` is
 502     /// unconstrained. However, whatever it is, for `Foo` to be WF, it
 503     /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
 504     /// then after generalization we will wind up with a type like
 505     /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
 506     /// ?D>` (or `>:`), we will wind up with the requirement that `?A
 507     /// <: ?C`, but no particular relationship between `?B` and `?D`
 508     /// (after all, we do not know the variance of the normalized form
 509     /// of `A::Item` with respect to `A`). If we do nothing else, this
 510     /// may mean that `?D` goes unconstrained (as in #41677). So, in
 511     /// this scenario where we create a new type variable in a
 512     /// bivariant context, we set the `needs_wf` flag to true. This
 513     /// will force the calling code to check that `WF(Foo<?C, ?D>)`
 514     /// holds, which in turn implies that `?C::Item == ?D`. So once
 515     /// `?C` is constrained, that should suffice to restrict `?D`.
 516     needs_wf: bool,
 517 }
 518
 519 impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
 520     fn tcx(&self) -> TyCtxt<'tcx> {
 521         self.infcx.tcx
 522     }
 523     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 524         self.param_env
 525     }
 526
 527     fn tag(&self) -> &'static str {
 528         "Generalizer"
 529     }
 530
 531     fn a_is_expected(&self) -> bool {
 532         true
 533     }
 534
 535     fn binders<T>(
 536         &mut self,
 537         a: ty::Binder<'tcx, T>,
 538         b: ty::Binder<'tcx, T>,
 539     ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
 540     where
 541         T: Relate<'tcx>,
 542     {
 543         Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
 544     }
 545
 546     fn relate_item_substs(
 547         &mut self,
 548         item_def_id: DefId,
 549         a_subst: SubstsRef<'tcx>,
 550         b_subst: SubstsRef<'tcx>,
 551     ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
 552         if self.ambient_variance == ty::Variance::Invariant {
 553             // Avoid fetching the variance if we are in an invariant
 554             // context; no need, and it can induce dependency cycles
 555             // (e.g., #41849).
 556             relate::relate_substs(self, a_subst, b_subst)
 557         } else {
 558             let tcx = self.tcx();
 559             let opt_variances = tcx.variances_of(item_def_id);
 560             relate::relate_substs_with_variances(
 561                 self,
 562                 item_def_id,
 563                 &opt_variances,
 564                 a_subst,
 565                 b_subst,
 566             )
 567         }
 568     }
 569
 570     fn relate_with_variance<T: Relate<'tcx>>(
 571         &mut self,
 572         variance: ty::Variance,
 573         _info: ty::VarianceDiagInfo<'tcx>,
 574         a: T,
 575         b: T,
 576     ) -> RelateResult<'tcx, T> {
 577         let old_ambient_variance = self.ambient_variance;
 578         self.ambient_variance = self.ambient_variance.xform(variance);
 579
 580         let result = self.relate(a, b);
 581         self.ambient_variance = old_ambient_variance;
 582         result
 583     }
 584
 585     fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
 586         assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 587
 588         if let Some(&result) = self.cache.get(&t) {
 589             return Ok(result);
 590         }
 591         debug!("generalize: t={:?}", t);
 592
 593         // Check to see whether the type we are generalizing references
 594         // any other type variable related to `vid` via
 595         // subtyping. This is basically our "occurs check", preventing
 596         // us from creating infinitely sized types.
 597         let result = match *t.kind() {
 598             ty::Infer(ty::TyVar(vid)) => {
 599                 let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
 600                 let sub_vid = self.infcx.inner.borrow_mut().type_variables().sub_root_var(vid);
 601                 if sub_vid == self.for_vid_sub_root {
 602                     // If sub-roots are equal, then `for_vid` and
 603                     // `vid` are related via subtyping.
 604                     Err(TypeError::CyclicTy(self.root_ty))
 605                 } else {
 606                     let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
 607                     match probe {
 608                         TypeVariableValue::Known { value: u } => {
 609                             debug!("generalize: known value {:?}", u);
 610                             self.relate(u, u)
 611                         }
 612                         TypeVariableValue::Unknown { universe } => {
 613                             match self.ambient_variance {
 614                                 // Invariant: no need to make a fresh type variable.
 615                                 ty::Invariant => {
 616                                     if self.for_universe.can_name(universe) {
 617                                         return Ok(t);
 618                                     }
 619                                 }
 620
 621                                 // Bivariant: make a fresh var, but we
 622                                 // may need a WF predicate. See
 623                                 // comment on `needs_wf` field for
 624                                 // more info.
 625                                 ty::Bivariant => self.needs_wf = true,
 626
 627                                 // Co/contravariant: this will be
 628                                 // sufficiently constrained later on.
 629                                 ty::Covariant | ty::Contravariant => (),
 630                             }
 631
 632                             let origin =
 633                                 *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
 634                             let new_var_id = self
 635                                 .infcx
 636                                 .inner
 637                                 .borrow_mut()
 638                                 .type_variables()
 639                                 .new_var(self.for_universe, origin);
 640                             let u = self.tcx().mk_ty_var(new_var_id);
 641
 642                             // Record that we replaced `vid` with `new_var_id` as part of a generalization
 643                             // operation. This is needed to detect cyclic types. To see why, see the
 644                             // docs in the `type_variables` module.
 645                             self.infcx.inner.borrow_mut().type_variables().sub(vid, new_var_id);
 646                             debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
 647                             Ok(u)
 648                         }
 649                     }
 650                 }
 651             }
 652             ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
 653                 // No matter what mode we are in,
 654                 // integer/floating-point types must be equal to be
 655                 // relatable.
 656                 Ok(t)
 657             }
 658             _ => relate::super_relate_tys(self, t, t),
 659         }?;
 660
 661         self.cache.insert(t, result);
 662         Ok(result)
 663     }
 664
 665     fn regions(
 666         &mut self,
 667         r: ty::Region<'tcx>,
 668         r2: ty::Region<'tcx>,
 669     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
 670         assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 671
 672         debug!("generalize: regions r={:?}", r);
 673
 674         match *r {
 675             // Never make variables for regions bound within the type itself,
 676             // nor for erased regions.
 677             ty::ReLateBound(..) | ty::ReErased => {
 678                 return Ok(r);
 679             }
 680
 681             ty::RePlaceholder(..)
 682             | ty::ReVar(..)
 683             | ty::ReStatic
 684             | ty::ReEarlyBound(..)
 685             | ty::ReFree(..) => {
 686                 // see common code below
 687             }
 688         }
 689
 690         // If we are in an invariant context, we can re-use the region
 691         // as is, unless it happens to be in some universe that we
 692         // can't name. (In the case of a region *variable*, we could
 693         // use it if we promoted it into our universe, but we don't
 694         // bother.)
 695         if let ty::Invariant = self.ambient_variance {
 696             let r_universe = self.infcx.universe_of_region(r);
 697             if self.for_universe.can_name(r_universe) {
 698                 return Ok(r);
 699             }
 700         }
 701
 702         // FIXME: This is non-ideal because we don't give a
 703         // very descriptive origin for this region variable.
 704         Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.cause.span), self.for_universe))
 705     }
 706
 707     fn consts(
 708         &mut self,
 709         c: ty::Const<'tcx>,
 710         c2: ty::Const<'tcx>,
 711     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
 712         assert_eq!(c, c2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 713
 714         match c.kind() {
 715             ty::ConstKind::Infer(InferConst::Var(vid)) => {
 716                 let mut inner = self.infcx.inner.borrow_mut();
 717                 let variable_table = &mut inner.const_unification_table();
 718                 let var_value = variable_table.probe_value(vid);
 719                 match var_value.val {
 720                     ConstVariableValue::Known { value: u } => {
 721                         drop(inner);
 722                         self.relate(u, u)
 723                     }
 724                     ConstVariableValue::Unknown { universe } => {
 725                         if self.for_universe.can_name(universe) {
 726                             Ok(c)
 727                         } else {
 728                             let new_var_id = variable_table.new_key(ConstVarValue {
 729                                 origin: var_value.origin,
 730                                 val: ConstVariableValue::Unknown { universe: self.for_universe },
 731                             });
 732                             Ok(self.tcx().mk_const_var(new_var_id, c.ty()))
 733                         }
 734                     }
 735                 }
 736             }
 737             ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs }) => {
 738                 let substs = self.relate_with_variance(
 739                     ty::Variance::Invariant,
 740                     ty::VarianceDiagInfo::default(),
 741                     substs,
 742                     substs,
 743                 )?;
 744                 Ok(self.tcx().mk_const(ty::ConstS {
 745                     ty: c.ty(),
 746                     kind: ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs }),
 747                 }))
 748             }
 749             _ => relate::super_relate_consts(self, c, c),
 750         }
 751     }
 752 }
 753
 754 pub trait ConstEquateRelation<'tcx>: TypeRelation<'tcx> {
 755     /// Register an obligation that both constants must be equal to each other.
 756     ///
 757     /// If they aren't equal then the relation doesn't hold.
 758     fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>);
 759 }
 760
 761 fn int_unification_error<'tcx>(
 762     a_is_expected: bool,
 763     v: (ty::IntVarValue, ty::IntVarValue),
 764 ) -> TypeError<'tcx> {
 765     let (a, b) = v;
 766     TypeError::IntMismatch(ExpectedFound::new(a_is_expected, a, b))
 767 }
 768
 769 fn float_unification_error<'tcx>(
 770     a_is_expected: bool,
 771     v: (ty::FloatVarValue, ty::FloatVarValue),
 772 ) -> TypeError<'tcx> {
 773     let (ty::FloatVarValue(a), ty::FloatVarValue(b)) = v;
 774     TypeError::FloatMismatch(ExpectedFound::new(a_is_expected, a, b))
 775 }
 776
 777 struct ConstInferUnifier<'cx, 'tcx> {
 778     infcx: &'cx InferCtxt<'tcx>,
 779
 780     span: Span,
 781
 782     param_env: ty::ParamEnv<'tcx>,
 783
 784     for_universe: ty::UniverseIndex,
 785
 786     /// The vid of the const variable that is in the process of being
 787     /// instantiated; if we find this within the const we are folding,
 788     /// that means we would have created a cyclic const.
 789     target_vid: ty::ConstVid<'tcx>,
 790 }
 791
 792 // We use `TypeRelation` here to propagate `RelateResult` upwards.
 793 //
 794 // Both inputs are expected to be the same.
 795 impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
 796     fn tcx(&self) -> TyCtxt<'tcx> {
 797         self.infcx.tcx
 798     }
 799
 800     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 801         self.param_env
 802     }
 803
 804     fn tag(&self) -> &'static str {
 805         "ConstInferUnifier"
 806     }
 807
 808     fn a_is_expected(&self) -> bool {
 809         true
 810     }
 811
 812     fn relate_with_variance<T: Relate<'tcx>>(
 813         &mut self,
 814         _variance: ty::Variance,
 815         _info: ty::VarianceDiagInfo<'tcx>,
 816         a: T,
 817         b: T,
 818     ) -> RelateResult<'tcx, T> {
 819         // We don't care about variance here.
 820         self.relate(a, b)
 821     }
 822
 823     fn binders<T>(
 824         &mut self,
 825         a: ty::Binder<'tcx, T>,
 826         b: ty::Binder<'tcx, T>,
 827     ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
 828     where
 829         T: Relate<'tcx>,
 830     {
 831         Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
 832     }
 833
 834     #[instrument(level = "debug", skip(self), ret)]
 835     fn tys(&mut self, t: Ty<'tcx>, _t: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
 836         debug_assert_eq!(t, _t);
 837
 838         match t.kind() {
 839             &ty::Infer(ty::TyVar(vid)) => {
 840                 let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
 841                 let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
 842                 match probe {
 843                     TypeVariableValue::Known { value: u } => {
 844                         debug!("ConstOccursChecker: known value {:?}", u);
 845                         self.tys(u, u)
 846                     }
 847                     TypeVariableValue::Unknown { universe } => {
 848                         if self.for_universe.can_name(universe) {
 849                             return Ok(t);
 850                         }
 851
 852                         let origin =
 853                             *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
 854                         let new_var_id = self
 855                             .infcx
 856                             .inner
 857                             .borrow_mut()
 858                             .type_variables()
 859                             .new_var(self.for_universe, origin);
 860                         Ok(self.tcx().mk_ty_var(new_var_id))
 861                     }
 862                 }
 863             }
 864             ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => Ok(t),
 865             _ => relate::super_relate_tys(self, t, t),
 866         }
 867     }
 868
 869     fn regions(
 870         &mut self,
 871         r: ty::Region<'tcx>,
 872         _r: ty::Region<'tcx>,
 873     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
 874         debug_assert_eq!(r, _r);
 875         debug!("ConstInferUnifier: r={:?}", r);
 876
 877         match *r {
 878             // Never make variables for regions bound within the type itself,
 879             // nor for erased regions.
 880             ty::ReLateBound(..) | ty::ReErased => {
 881                 return Ok(r);
 882             }
 883
 884             ty::RePlaceholder(..)
 885             | ty::ReVar(..)
 886             | ty::ReStatic
 887             | ty::ReEarlyBound(..)
 888             | ty::ReFree(..) => {
 889                 // see common code below
 890             }
 891         }
 892
 893         let r_universe = self.infcx.universe_of_region(r);
 894         if self.for_universe.can_name(r_universe) {
 895             return Ok(r);
 896         } else {
 897             // FIXME: This is non-ideal because we don't give a
 898             // very descriptive origin for this region variable.
 899             Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.span), self.for_universe))
 900         }
 901     }
 902
 903     #[instrument(level = "debug", skip(self))]
 904     fn consts(
 905         &mut self,
 906         c: ty::Const<'tcx>,
 907         _c: ty::Const<'tcx>,
 908     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
 909         debug_assert_eq!(c, _c);
 910
 911         match c.kind() {
 912             ty::ConstKind::Infer(InferConst::Var(vid)) => {
 913                 // Check if the current unification would end up
 914                 // unifying `target_vid` with a const which contains
 915                 // an inference variable which is unioned with `target_vid`.
 916                 //
 917                 // Not doing so can easily result in stack overflows.
 918                 if self
 919                     .infcx
 920                     .inner
 921                     .borrow_mut()
 922                     .const_unification_table()
 923                     .unioned(self.target_vid, vid)
 924                 {
 925                     return Err(TypeError::CyclicConst(c));
 926                 }
 927
 928                 let var_value =
 929                     self.infcx.inner.borrow_mut().const_unification_table().probe_value(vid);
 930                 match var_value.val {
 931                     ConstVariableValue::Known { value: u } => self.consts(u, u),
 932                     ConstVariableValue::Unknown { universe } => {
 933                         if self.for_universe.can_name(universe) {
 934                             Ok(c)
 935                         } else {
 936                             let new_var_id =
 937                                 self.infcx.inner.borrow_mut().const_unification_table().new_key(
 938                                     ConstVarValue {
 939                                         origin: var_value.origin,
 940                                         val: ConstVariableValue::Unknown {
 941                                             universe: self.for_universe,
 942                                         },
 943                                     },
 944                                 );
 945                             Ok(self.tcx().mk_const_var(new_var_id, c.ty()))
 946                         }
 947                     }
 948                 }
 949             }
 950             ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs }) => {
 951                 let substs = self.relate_with_variance(
 952                     ty::Variance::Invariant,
 953                     ty::VarianceDiagInfo::default(),
 954                     substs,
 955                     substs,
 956                 )?;
 957
 958                 Ok(self.tcx().mk_const(ty::ConstS {
 959                     ty: c.ty(),
 960                     kind: ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs }),
 961                 }))
 962             }
 963             _ => relate::super_relate_consts(self, c, c),
 964         }
 965     }
 966 }