compiler/rustc_infer/src/infer/combine.rs

   1 ///////////////////////////////////////////////////////////////////////////
   2 // # Type combining
   3 //
   4 // There are four type combiners: equate, sub, lub, and glb.  Each
   5 // implements the trait `Combine` and contains methods for combining
   6 // two instances of various things and yielding a new instance.  These
   7 // combiner methods always yield a `Result<T>`.  There is a lot of
   8 // common code for these operations, implemented as default methods on
   9 // the `Combine` trait.
  10 //
  11 // Each operation may have side-effects on the inference context,
  12 // though these can be unrolled using snapshots. On success, the
  13 // LUB/GLB operations return the appropriate bound. The Eq and Sub
  14 // operations generally return the first operand.
  15 //
  16 // ## Contravariance
  17 //
  18 // When you are relating two things which have a contravariant
  19 // relationship, you should use `contratys()` or `contraregions()`,
  20 // rather than inversing the order of arguments!  This is necessary
  21 // because the order of arguments is not relevant for LUB and GLB.  It
  22 // is also useful to track which value is the "expected" value in
  23 // terms of error reporting.
  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, TypeFoldable};
  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<'infcx, '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<'infcx, 'tcx> InferCtxt<'infcx, '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.val(), b.val()) {
 146             (
 147                 ty::ConstKind::Infer(InferConst::Var(a_vid)),
 148                 ty::ConstKind::Infer(InferConst::Var(b_vid)),
 149             ) => {
 150                 self.inner
 151                     .borrow_mut()
 152                     .const_unification_table()
 153                     .unify_var_var(a_vid, b_vid)
 154                     .map_err(|e| const_unification_error(a_is_expected, e))?;
 155                 return Ok(a);
 156             }
 157
 158             // All other cases of inference with other variables are errors.
 159             (ty::ConstKind::Infer(InferConst::Var(_)), ty::ConstKind::Infer(_))
 160             | (ty::ConstKind::Infer(_), ty::ConstKind::Infer(InferConst::Var(_))) => {
 161                 bug!("tried to combine ConstKind::Infer/ConstKind::Infer(InferConst::Var)")
 162             }
 163
 164             (ty::ConstKind::Infer(InferConst::Var(vid)), _) => {
 165                 return self.unify_const_variable(relation.param_env(), vid, b, a_is_expected);
 166             }
 167
 168             (_, ty::ConstKind::Infer(InferConst::Var(vid))) => {
 169                 return self.unify_const_variable(relation.param_env(), vid, a, !a_is_expected);
 170             }
 171             (ty::ConstKind::Unevaluated(..), _) if self.tcx.lazy_normalization() => {
 172                 // FIXME(#59490): Need to remove the leak check to accommodate
 173                 // escaping bound variables here.
 174                 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
 175                     relation.const_equate_obligation(a, b);
 176                 }
 177                 return Ok(b);
 178             }
 179             (_, ty::ConstKind::Unevaluated(..)) if self.tcx.lazy_normalization() => {
 180                 // FIXME(#59490): Need to remove the leak check to accommodate
 181                 // escaping bound variables here.
 182                 if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
 183                     relation.const_equate_obligation(a, b);
 184                 }
 185                 return Ok(a);
 186             }
 187             _ => {}
 188         }
 189
 190         ty::relate::super_relate_consts(relation, a, b)
 191     }
 192
 193     /// Unifies the const variable `target_vid` with the given constant.
 194     ///
 195     /// This also tests if the given const `ct` contains an inference variable which was previously
 196     /// unioned with `target_vid`. If this is the case, inferring `target_vid` to `ct`
 197     /// would result in an infinite type as we continuously replace an inference variable
 198     /// in `ct` with `ct` itself.
 199     ///
 200     /// This is especially important as unevaluated consts use their parents generics.
 201     /// They therefore often contain unused substs, making these errors far more likely.
 202     ///
 203     /// A good example of this is the following:
 204     ///
 205     /// ```compile_fail,E0308
 206     /// #![feature(generic_const_exprs)]
 207     ///
 208     /// fn bind<const N: usize>(value: [u8; N]) -> [u8; 3 + 4] {
 209     ///     todo!()
 210     /// }
 211     ///
 212     /// fn main() {
 213     ///     let mut arr = Default::default();
 214     ///     arr = bind(arr);
 215     /// }
 216     /// ```
 217     ///
 218     /// Here `3 + 4` ends up as `ConstKind::Unevaluated` which uses the generics
 219     /// of `fn bind` (meaning that its substs contain `N`).
 220     ///
 221     /// `bind(arr)` now infers that the type of `arr` must be `[u8; N]`.
 222     /// The assignment `arr = bind(arr)` now tries to equate `N` with `3 + 4`.
 223     ///
 224     /// As `3 + 4` contains `N` in its substs, this must not succeed.
 225     ///
 226     /// See `src/test/ui/const-generics/occurs-check/` for more examples where this is relevant.
 227     #[instrument(level = "debug", skip(self))]
 228     fn unify_const_variable(
 229         &self,
 230         param_env: ty::ParamEnv<'tcx>,
 231         target_vid: ty::ConstVid<'tcx>,
 232         ct: ty::Const<'tcx>,
 233         vid_is_expected: bool,
 234     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
 235         let (for_universe, span) = {
 236             let mut inner = self.inner.borrow_mut();
 237             let variable_table = &mut inner.const_unification_table();
 238             let var_value = variable_table.probe_value(target_vid);
 239             match var_value.val {
 240                 ConstVariableValue::Known { value } => {
 241                     bug!("instantiating {:?} which has a known value {:?}", target_vid, value)
 242                 }
 243                 ConstVariableValue::Unknown { universe } => (universe, var_value.origin.span),
 244             }
 245         };
 246         let value = ConstInferUnifier { infcx: self, span, param_env, for_universe, target_vid }
 247             .relate(ct, ct)?;
 248
 249         self.inner
 250             .borrow_mut()
 251             .const_unification_table()
 252             .unify_var_value(
 253                 target_vid,
 254                 ConstVarValue {
 255                     origin: ConstVariableOrigin {
 256                         kind: ConstVariableOriginKind::ConstInference,
 257                         span: DUMMY_SP,
 258                     },
 259                     val: ConstVariableValue::Known { value },
 260                 },
 261             )
 262             .map(|()| value)
 263             .map_err(|e| const_unification_error(vid_is_expected, e))
 264     }
 265
 266     fn unify_integral_variable(
 267         &self,
 268         vid_is_expected: bool,
 269         vid: ty::IntVid,
 270         val: ty::IntVarValue,
 271     ) -> RelateResult<'tcx, Ty<'tcx>> {
 272         self.inner
 273             .borrow_mut()
 274             .int_unification_table()
 275             .unify_var_value(vid, Some(val))
 276             .map_err(|e| int_unification_error(vid_is_expected, e))?;
 277         match val {
 278             IntType(v) => Ok(self.tcx.mk_mach_int(v)),
 279             UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
 280         }
 281     }
 282
 283     fn unify_float_variable(
 284         &self,
 285         vid_is_expected: bool,
 286         vid: ty::FloatVid,
 287         val: ty::FloatTy,
 288     ) -> RelateResult<'tcx, Ty<'tcx>> {
 289         self.inner
 290             .borrow_mut()
 291             .float_unification_table()
 292             .unify_var_value(vid, Some(ty::FloatVarValue(val)))
 293             .map_err(|e| float_unification_error(vid_is_expected, e))?;
 294         Ok(self.tcx.mk_mach_float(val))
 295     }
 296 }
 297
 298 impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
 299     pub fn tcx(&self) -> TyCtxt<'tcx> {
 300         self.infcx.tcx
 301     }
 302
 303     pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'tcx> {
 304         Equate::new(self, a_is_expected)
 305     }
 306
 307     pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'tcx> {
 308         Sub::new(self, a_is_expected)
 309     }
 310
 311     pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'tcx> {
 312         Lub::new(self, a_is_expected)
 313     }
 314
 315     pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'tcx> {
 316         Glb::new(self, a_is_expected)
 317     }
 318
 319     /// Here, `dir` is either `EqTo`, `SubtypeOf`, or `SupertypeOf`.
 320     /// The idea is that we should ensure that the type `a_ty` is equal
 321     /// to, a subtype of, or a supertype of (respectively) the type
 322     /// to which `b_vid` is bound.
 323     ///
 324     /// Since `b_vid` has not yet been instantiated with a type, we
 325     /// will first instantiate `b_vid` with a *generalized* version
 326     /// of `a_ty`. Generalization introduces other inference
 327     /// variables wherever subtyping could occur.
 328     #[instrument(skip(self), level = "debug")]
 329     pub fn instantiate(
 330         &mut self,
 331         a_ty: Ty<'tcx>,
 332         dir: RelationDir,
 333         b_vid: ty::TyVid,
 334         a_is_expected: bool,
 335     ) -> RelateResult<'tcx, ()> {
 336         use self::RelationDir::*;
 337
 338         // Get the actual variable that b_vid has been inferred to
 339         debug_assert!(self.infcx.inner.borrow_mut().type_variables().probe(b_vid).is_unknown());
 340
 341         // Generalize type of `a_ty` appropriately depending on the
 342         // direction.  As an example, assume:
 343         //
 344         // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
 345         //   inference variable,
 346         // - and `dir` == `SubtypeOf`.
 347         //
 348         // Then the generalized form `b_ty` would be `&'?2 ?3`, where
 349         // `'?2` and `?3` are fresh region/type inference
 350         // variables. (Down below, we will relate `a_ty <: b_ty`,
 351         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
 352         let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
 353         debug!(?b_ty);
 354         self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
 355
 356         if needs_wf {
 357             self.obligations.push(Obligation::new(
 358                 self.trace.cause.clone(),
 359                 self.param_env,
 360                 ty::Binder::dummy(ty::PredicateKind::WellFormed(b_ty.into()))
 361                     .to_predicate(self.infcx.tcx),
 362             ));
 363         }
 364
 365         // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
 366         //
 367         // FIXME(#16847): This code is non-ideal because all these subtype
 368         // relations wind up attributed to the same spans. We need
 369         // to associate causes/spans with each of the relations in
 370         // the stack to get this right.
 371         match dir {
 372             EqTo => self.equate(a_is_expected).relate(a_ty, b_ty),
 373             SubtypeOf => self.sub(a_is_expected).relate(a_ty, b_ty),
 374             SupertypeOf => self.sub(a_is_expected).relate_with_variance(
 375                 ty::Contravariant,
 376                 ty::VarianceDiagInfo::default(),
 377                 a_ty,
 378                 b_ty,
 379             ),
 380         }?;
 381
 382         Ok(())
 383     }
 384
 385     /// Attempts to generalize `ty` for the type variable `for_vid`.
 386     /// This checks for cycle -- that is, whether the type `ty`
 387     /// references `for_vid`. The `dir` is the "direction" for which we
 388     /// a performing the generalization (i.e., are we producing a type
 389     /// that can be used as a supertype etc).
 390     ///
 391     /// Preconditions:
 392     ///
 393     /// - `for_vid` is a "root vid"
 394     #[instrument(skip(self), level = "trace")]
 395     fn generalize(
 396         &self,
 397         ty: Ty<'tcx>,
 398         for_vid: ty::TyVid,
 399         dir: RelationDir,
 400     ) -> RelateResult<'tcx, Generalization<'tcx>> {
 401         // Determine the ambient variance within which `ty` appears.
 402         // The surrounding equation is:
 403         //
 404         //     ty [op] ty2
 405         //
 406         // where `op` is either `==`, `<:`, or `:>`. This maps quite
 407         // naturally.
 408         let ambient_variance = match dir {
 409             RelationDir::EqTo => ty::Invariant,
 410             RelationDir::SubtypeOf => ty::Covariant,
 411             RelationDir::SupertypeOf => ty::Contravariant,
 412         };
 413
 414         trace!(?ambient_variance);
 415
 416         let for_universe = match self.infcx.inner.borrow_mut().type_variables().probe(for_vid) {
 417             v @ TypeVariableValue::Known { .. } => {
 418                 bug!("instantiating {:?} which has a known value {:?}", for_vid, v,)
 419             }
 420             TypeVariableValue::Unknown { universe } => universe,
 421         };
 422
 423         trace!(?for_universe);
 424         trace!(?self.trace);
 425
 426         let mut generalize = Generalizer {
 427             infcx: self.infcx,
 428             cause: &self.trace.cause,
 429             for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
 430             for_universe,
 431             ambient_variance,
 432             needs_wf: false,
 433             root_ty: ty,
 434             param_env: self.param_env,
 435             cache: SsoHashMap::new(),
 436         };
 437
 438         let ty = match generalize.relate(ty, ty) {
 439             Ok(ty) => ty,
 440             Err(e) => {
 441                 debug!(?e, "failure");
 442                 return Err(e);
 443             }
 444         };
 445         let needs_wf = generalize.needs_wf;
 446         trace!(?ty, ?needs_wf, "success");
 447         Ok(Generalization { ty, needs_wf })
 448     }
 449
 450     pub fn add_const_equate_obligation(
 451         &mut self,
 452         a_is_expected: bool,
 453         a: ty::Const<'tcx>,
 454         b: ty::Const<'tcx>,
 455     ) {
 456         let predicate = if a_is_expected {
 457             ty::PredicateKind::ConstEquate(a, b)
 458         } else {
 459             ty::PredicateKind::ConstEquate(b, a)
 460         };
 461         self.obligations.push(Obligation::new(
 462             self.trace.cause.clone(),
 463             self.param_env,
 464             ty::Binder::dummy(predicate).to_predicate(self.tcx()),
 465         ));
 466     }
 467 }
 468
 469 struct Generalizer<'cx, 'tcx> {
 470     infcx: &'cx InferCtxt<'cx, 'tcx>,
 471
 472     /// The span, used when creating new type variables and things.
 473     cause: &'cx ObligationCause<'tcx>,
 474
 475     /// The vid of the type variable that is in the process of being
 476     /// instantiated; if we find this within the type we are folding,
 477     /// that means we would have created a cyclic type.
 478     for_vid_sub_root: ty::TyVid,
 479
 480     /// The universe of the type variable that is in the process of
 481     /// being instantiated. Any fresh variables that we create in this
 482     /// process should be in that same universe.
 483     for_universe: ty::UniverseIndex,
 484
 485     /// Track the variance as we descend into the type.
 486     ambient_variance: ty::Variance,
 487
 488     /// See the field `needs_wf` in `Generalization`.
 489     needs_wf: bool,
 490
 491     /// The root type that we are generalizing. Used when reporting cycles.
 492     root_ty: Ty<'tcx>,
 493
 494     param_env: ty::ParamEnv<'tcx>,
 495
 496     cache: SsoHashMap<Ty<'tcx>, RelateResult<'tcx, Ty<'tcx>>>,
 497 }
 498
 499 /// Result from a generalization operation. This includes
 500 /// not only the generalized type, but also a bool flag
 501 /// indicating whether further WF checks are needed.
 502 struct Generalization<'tcx> {
 503     ty: Ty<'tcx>,
 504
 505     /// If true, then the generalized type may not be well-formed,
 506     /// even if the source type is well-formed, so we should add an
 507     /// additional check to enforce that it is. This arises in
 508     /// particular around 'bivariant' type parameters that are only
 509     /// constrained by a where-clause. As an example, imagine a type:
 510     ///
 511     ///     struct Foo<A, B> where A: Iterator<Item = B> {
 512     ///         data: A
 513     ///     }
 514     ///
 515     /// here, `A` will be covariant, but `B` is
 516     /// unconstrained. However, whatever it is, for `Foo` to be WF, it
 517     /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
 518     /// then after generalization we will wind up with a type like
 519     /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
 520     /// ?D>` (or `>:`), we will wind up with the requirement that `?A
 521     /// <: ?C`, but no particular relationship between `?B` and `?D`
 522     /// (after all, we do not know the variance of the normalized form
 523     /// of `A::Item` with respect to `A`). If we do nothing else, this
 524     /// may mean that `?D` goes unconstrained (as in #41677). So, in
 525     /// this scenario where we create a new type variable in a
 526     /// bivariant context, we set the `needs_wf` flag to true. This
 527     /// will force the calling code to check that `WF(Foo<?C, ?D>)`
 528     /// holds, which in turn implies that `?C::Item == ?D`. So once
 529     /// `?C` is constrained, that should suffice to restrict `?D`.
 530     needs_wf: bool,
 531 }
 532
 533 impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
 534     fn tcx(&self) -> TyCtxt<'tcx> {
 535         self.infcx.tcx
 536     }
 537     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 538         self.param_env
 539     }
 540
 541     fn tag(&self) -> &'static str {
 542         "Generalizer"
 543     }
 544
 545     fn a_is_expected(&self) -> bool {
 546         true
 547     }
 548
 549     fn binders<T>(
 550         &mut self,
 551         a: ty::Binder<'tcx, T>,
 552         b: ty::Binder<'tcx, T>,
 553     ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
 554     where
 555         T: Relate<'tcx>,
 556     {
 557         Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
 558     }
 559
 560     fn relate_item_substs(
 561         &mut self,
 562         item_def_id: DefId,
 563         a_subst: SubstsRef<'tcx>,
 564         b_subst: SubstsRef<'tcx>,
 565     ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
 566         if self.ambient_variance == ty::Variance::Invariant {
 567             // Avoid fetching the variance if we are in an invariant
 568             // context; no need, and it can induce dependency cycles
 569             // (e.g., #41849).
 570             relate::relate_substs(self, a_subst, b_subst)
 571         } else {
 572             let tcx = self.tcx();
 573             let opt_variances = tcx.variances_of(item_def_id);
 574             relate::relate_substs_with_variances(
 575                 self,
 576                 item_def_id,
 577                 &opt_variances,
 578                 a_subst,
 579                 b_subst,
 580             )
 581         }
 582     }
 583
 584     fn relate_with_variance<T: Relate<'tcx>>(
 585         &mut self,
 586         variance: ty::Variance,
 587         _info: ty::VarianceDiagInfo<'tcx>,
 588         a: T,
 589         b: T,
 590     ) -> RelateResult<'tcx, T> {
 591         let old_ambient_variance = self.ambient_variance;
 592         self.ambient_variance = self.ambient_variance.xform(variance);
 593
 594         let result = self.relate(a, b);
 595         self.ambient_variance = old_ambient_variance;
 596         result
 597     }
 598
 599     fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
 600         assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 601
 602         if let Some(result) = self.cache.get(&t) {
 603             return result.clone();
 604         }
 605         debug!("generalize: t={:?}", t);
 606
 607         // Check to see whether the type we are generalizing references
 608         // any other type variable related to `vid` via
 609         // subtyping. This is basically our "occurs check", preventing
 610         // us from creating infinitely sized types.
 611         let result = match *t.kind() {
 612             ty::Infer(ty::TyVar(vid)) => {
 613                 let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
 614                 let sub_vid = self.infcx.inner.borrow_mut().type_variables().sub_root_var(vid);
 615                 if sub_vid == self.for_vid_sub_root {
 616                     // If sub-roots are equal, then `for_vid` and
 617                     // `vid` are related via subtyping.
 618                     Err(TypeError::CyclicTy(self.root_ty))
 619                 } else {
 620                     let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
 621                     match probe {
 622                         TypeVariableValue::Known { value: u } => {
 623                             debug!("generalize: known value {:?}", u);
 624                             self.relate(u, u)
 625                         }
 626                         TypeVariableValue::Unknown { universe } => {
 627                             match self.ambient_variance {
 628                                 // Invariant: no need to make a fresh type variable.
 629                                 ty::Invariant => {
 630                                     if self.for_universe.can_name(universe) {
 631                                         return Ok(t);
 632                                     }
 633                                 }
 634
 635                                 // Bivariant: make a fresh var, but we
 636                                 // may need a WF predicate. See
 637                                 // comment on `needs_wf` field for
 638                                 // more info.
 639                                 ty::Bivariant => self.needs_wf = true,
 640
 641                                 // Co/contravariant: this will be
 642                                 // sufficiently constrained later on.
 643                                 ty::Covariant | ty::Contravariant => (),
 644                             }
 645
 646                             let origin =
 647                                 *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
 648                             let new_var_id = self
 649                                 .infcx
 650                                 .inner
 651                                 .borrow_mut()
 652                                 .type_variables()
 653                                 .new_var(self.for_universe, origin);
 654                             let u = self.tcx().mk_ty_var(new_var_id);
 655
 656                             // Record that we replaced `vid` with `new_var_id` as part of a generalization
 657                             // operation. This is needed to detect cyclic types. To see why, see the
 658                             // docs in the `type_variables` module.
 659                             self.infcx.inner.borrow_mut().type_variables().sub(vid, new_var_id);
 660                             debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
 661                             Ok(u)
 662                         }
 663                     }
 664                 }
 665             }
 666             ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
 667                 // No matter what mode we are in,
 668                 // integer/floating-point types must be equal to be
 669                 // relatable.
 670                 Ok(t)
 671             }
 672             _ => relate::super_relate_tys(self, t, t),
 673         };
 674
 675         self.cache.insert(t, result.clone());
 676         return result;
 677     }
 678
 679     fn regions(
 680         &mut self,
 681         r: ty::Region<'tcx>,
 682         r2: ty::Region<'tcx>,
 683     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
 684         assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 685
 686         debug!("generalize: regions r={:?}", r);
 687
 688         match *r {
 689             // Never make variables for regions bound within the type itself,
 690             // nor for erased regions.
 691             ty::ReLateBound(..) | ty::ReErased => {
 692                 return Ok(r);
 693             }
 694
 695             ty::RePlaceholder(..)
 696             | ty::ReVar(..)
 697             | ty::ReEmpty(_)
 698             | ty::ReStatic
 699             | ty::ReEarlyBound(..)
 700             | ty::ReFree(..) => {
 701                 // see common code below
 702             }
 703         }
 704
 705         // If we are in an invariant context, we can re-use the region
 706         // as is, unless it happens to be in some universe that we
 707         // can't name. (In the case of a region *variable*, we could
 708         // use it if we promoted it into our universe, but we don't
 709         // bother.)
 710         if let ty::Invariant = self.ambient_variance {
 711             let r_universe = self.infcx.universe_of_region(r);
 712             if self.for_universe.can_name(r_universe) {
 713                 return Ok(r);
 714             }
 715         }
 716
 717         // FIXME: This is non-ideal because we don't give a
 718         // very descriptive origin for this region variable.
 719         Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.cause.span), self.for_universe))
 720     }
 721
 722     fn consts(
 723         &mut self,
 724         c: ty::Const<'tcx>,
 725         c2: ty::Const<'tcx>,
 726     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
 727         assert_eq!(c, c2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 728
 729         match c.val() {
 730             ty::ConstKind::Infer(InferConst::Var(vid)) => {
 731                 let mut inner = self.infcx.inner.borrow_mut();
 732                 let variable_table = &mut inner.const_unification_table();
 733                 let var_value = variable_table.probe_value(vid);
 734                 match var_value.val {
 735                     ConstVariableValue::Known { value: u } => {
 736                         drop(inner);
 737                         self.relate(u, u)
 738                     }
 739                     ConstVariableValue::Unknown { universe } => {
 740                         if self.for_universe.can_name(universe) {
 741                             Ok(c)
 742                         } else {
 743                             let new_var_id = variable_table.new_key(ConstVarValue {
 744                                 origin: var_value.origin,
 745                                 val: ConstVariableValue::Unknown { universe: self.for_universe },
 746                             });
 747                             Ok(self.tcx().mk_const_var(new_var_id, c.ty()))
 748                         }
 749                     }
 750                 }
 751             }
 752             ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted })
 753                 if self.tcx().lazy_normalization() =>
 754             {
 755                 assert_eq!(promoted, None);
 756                 let substs = self.relate_with_variance(
 757                     ty::Variance::Invariant,
 758                     ty::VarianceDiagInfo::default(),
 759                     substs,
 760                     substs,
 761                 )?;
 762                 Ok(self.tcx().mk_const(ty::ConstS {
 763                     ty: c.ty(),
 764                     val: ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted }),
 765                 }))
 766             }
 767             _ => relate::super_relate_consts(self, c, c),
 768         }
 769     }
 770 }
 771
 772 pub trait ConstEquateRelation<'tcx>: TypeRelation<'tcx> {
 773     /// Register an obligation that both constants must be equal to each other.
 774     ///
 775     /// If they aren't equal then the relation doesn't hold.
 776     fn const_equate_obligation(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>);
 777 }
 778
 779 pub trait RelateResultCompare<'tcx, T> {
 780     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T>
 781     where
 782         F: FnOnce() -> TypeError<'tcx>;
 783 }
 784
 785 impl<'tcx, T: Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
 786     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T>
 787     where
 788         F: FnOnce() -> TypeError<'tcx>,
 789     {
 790         self.clone().and_then(|s| if s == t { self.clone() } else { Err(f()) })
 791     }
 792 }
 793
 794 pub fn const_unification_error<'tcx>(
 795     a_is_expected: bool,
 796     (a, b): (ty::Const<'tcx>, ty::Const<'tcx>),
 797 ) -> TypeError<'tcx> {
 798     TypeError::ConstMismatch(ExpectedFound::new(a_is_expected, a, b))
 799 }
 800
 801 fn int_unification_error<'tcx>(
 802     a_is_expected: bool,
 803     v: (ty::IntVarValue, ty::IntVarValue),
 804 ) -> TypeError<'tcx> {
 805     let (a, b) = v;
 806     TypeError::IntMismatch(ExpectedFound::new(a_is_expected, a, b))
 807 }
 808
 809 fn float_unification_error<'tcx>(
 810     a_is_expected: bool,
 811     v: (ty::FloatVarValue, ty::FloatVarValue),
 812 ) -> TypeError<'tcx> {
 813     let (ty::FloatVarValue(a), ty::FloatVarValue(b)) = v;
 814     TypeError::FloatMismatch(ExpectedFound::new(a_is_expected, a, b))
 815 }
 816
 817 struct ConstInferUnifier<'cx, 'tcx> {
 818     infcx: &'cx InferCtxt<'cx, 'tcx>,
 819
 820     span: Span,
 821
 822     param_env: ty::ParamEnv<'tcx>,
 823
 824     for_universe: ty::UniverseIndex,
 825
 826     /// The vid of the const variable that is in the process of being
 827     /// instantiated; if we find this within the const we are folding,
 828     /// that means we would have created a cyclic const.
 829     target_vid: ty::ConstVid<'tcx>,
 830 }
 831
 832 // We use `TypeRelation` here to propagate `RelateResult` upwards.
 833 //
 834 // Both inputs are expected to be the same.
 835 impl<'tcx> TypeRelation<'tcx> for ConstInferUnifier<'_, 'tcx> {
 836     fn tcx(&self) -> TyCtxt<'tcx> {
 837         self.infcx.tcx
 838     }
 839
 840     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 841         self.param_env
 842     }
 843
 844     fn tag(&self) -> &'static str {
 845         "ConstInferUnifier"
 846     }
 847
 848     fn a_is_expected(&self) -> bool {
 849         true
 850     }
 851
 852     fn relate_with_variance<T: Relate<'tcx>>(
 853         &mut self,
 854         _variance: ty::Variance,
 855         _info: ty::VarianceDiagInfo<'tcx>,
 856         a: T,
 857         b: T,
 858     ) -> RelateResult<'tcx, T> {
 859         // We don't care about variance here.
 860         self.relate(a, b)
 861     }
 862
 863     fn binders<T>(
 864         &mut self,
 865         a: ty::Binder<'tcx, T>,
 866         b: ty::Binder<'tcx, T>,
 867     ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
 868     where
 869         T: Relate<'tcx>,
 870     {
 871         Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
 872     }
 873
 874     #[tracing::instrument(level = "debug", skip(self))]
 875     fn tys(&mut self, t: Ty<'tcx>, _t: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
 876         debug_assert_eq!(t, _t);
 877         debug!("ConstInferUnifier: t={:?}", t);
 878
 879         match t.kind() {
 880             &ty::Infer(ty::TyVar(vid)) => {
 881                 let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
 882                 let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
 883                 match probe {
 884                     TypeVariableValue::Known { value: u } => {
 885                         debug!("ConstOccursChecker: known value {:?}", u);
 886                         self.tys(u, u)
 887                     }
 888                     TypeVariableValue::Unknown { universe } => {
 889                         if self.for_universe.can_name(universe) {
 890                             return Ok(t);
 891                         }
 892
 893                         let origin =
 894                             *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
 895                         let new_var_id = self
 896                             .infcx
 897                             .inner
 898                             .borrow_mut()
 899                             .type_variables()
 900                             .new_var(self.for_universe, origin);
 901                         let u = self.tcx().mk_ty_var(new_var_id);
 902                         debug!(
 903                             "ConstInferUnifier: replacing original vid={:?} with new={:?}",
 904                             vid, u
 905                         );
 906                         Ok(u)
 907                     }
 908                 }
 909             }
 910             ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => Ok(t),
 911             _ => relate::super_relate_tys(self, t, t),
 912         }
 913     }
 914
 915     fn regions(
 916         &mut self,
 917         r: ty::Region<'tcx>,
 918         _r: ty::Region<'tcx>,
 919     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
 920         debug_assert_eq!(r, _r);
 921         debug!("ConstInferUnifier: r={:?}", r);
 922
 923         match *r {
 924             // Never make variables for regions bound within the type itself,
 925             // nor for erased regions.
 926             ty::ReLateBound(..) | ty::ReErased => {
 927                 return Ok(r);
 928             }
 929
 930             ty::RePlaceholder(..)
 931             | ty::ReVar(..)
 932             | ty::ReEmpty(_)
 933             | ty::ReStatic
 934             | ty::ReEarlyBound(..)
 935             | ty::ReFree(..) => {
 936                 // see common code below
 937             }
 938         }
 939
 940         let r_universe = self.infcx.universe_of_region(r);
 941         if self.for_universe.can_name(r_universe) {
 942             return Ok(r);
 943         } else {
 944             // FIXME: This is non-ideal because we don't give a
 945             // very descriptive origin for this region variable.
 946             Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.span), self.for_universe))
 947         }
 948     }
 949
 950     #[tracing::instrument(level = "debug", skip(self))]
 951     fn consts(
 952         &mut self,
 953         c: ty::Const<'tcx>,
 954         _c: ty::Const<'tcx>,
 955     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
 956         debug_assert_eq!(c, _c);
 957         debug!("ConstInferUnifier: c={:?}", c);
 958
 959         match c.val() {
 960             ty::ConstKind::Infer(InferConst::Var(vid)) => {
 961                 // Check if the current unification would end up
 962                 // unifying `target_vid` with a const which contains
 963                 // an inference variable which is unioned with `target_vid`.
 964                 //
 965                 // Not doing so can easily result in stack overflows.
 966                 if self
 967                     .infcx
 968                     .inner
 969                     .borrow_mut()
 970                     .const_unification_table()
 971                     .unioned(self.target_vid, vid)
 972                 {
 973                     return Err(TypeError::CyclicConst(c));
 974                 }
 975
 976                 let var_value =
 977                     self.infcx.inner.borrow_mut().const_unification_table().probe_value(vid);
 978                 match var_value.val {
 979                     ConstVariableValue::Known { value: u } => self.consts(u, u),
 980                     ConstVariableValue::Unknown { universe } => {
 981                         if self.for_universe.can_name(universe) {
 982                             Ok(c)
 983                         } else {
 984                             let new_var_id =
 985                                 self.infcx.inner.borrow_mut().const_unification_table().new_key(
 986                                     ConstVarValue {
 987                                         origin: var_value.origin,
 988                                         val: ConstVariableValue::Unknown {
 989                                             universe: self.for_universe,
 990                                         },
 991                                     },
 992                                 );
 993                             Ok(self.tcx().mk_const_var(new_var_id, c.ty()))
 994                         }
 995                     }
 996                 }
 997             }
 998             ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted })
 999                 if self.tcx().lazy_normalization() =>
1000             {
1001                 assert_eq!(promoted, None);
1002                 let substs = self.relate_with_variance(
1003                     ty::Variance::Invariant,
1004                     ty::VarianceDiagInfo::default(),
1005                     substs,
1006                     substs,
1007                 )?;
1008                 Ok(self.tcx().mk_const(ty::ConstS {
1009                     ty: c.ty(),
1010                     val: ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted }),
1011                 }))
1012             }
1013             _ => relate::super_relate_consts(self, c, c),
1014         }
1015     }
1016 }