src/librustc/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::{InferCtxt, MiscVariable, TypeTrace};
  28 use super::lub::Lub;
  29 use super::sub::Sub;
  30 use super::type_variable::TypeVariableValue;
  31 use super::unify_key::{ConstVarValue, ConstVariableValue};
  32 use super::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
  33
  34 use crate::hir::def_id::DefId;
  35 use crate::mir::interpret::ConstValue;
  36 use crate::ty::{IntType, UintType};
  37 use crate::ty::{self, Ty, TyCtxt, InferConst};
  38 use crate::ty::error::TypeError;
  39 use crate::ty::relate::{self, Relate, RelateResult, TypeRelation};
  40 use crate::ty::subst::SubstsRef;
  41 use crate::traits::{Obligation, PredicateObligations};
  42
  43 use syntax::ast;
  44 use syntax_pos::{Span, DUMMY_SP};
  45
  46 #[derive(Clone)]
  47 pub struct CombineFields<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
  48     pub infcx: &'infcx InferCtxt<'infcx, 'gcx, 'tcx>,
  49     pub trace: TypeTrace<'tcx>,
  50     pub cause: Option<ty::relate::Cause>,
  51     pub param_env: ty::ParamEnv<'tcx>,
  52     pub obligations: PredicateObligations<'tcx>,
  53 }
  54
  55 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
  56 pub enum RelationDir {
  57     SubtypeOf, SupertypeOf, EqTo
  58 }
  59
  60 impl<'infcx, 'gcx, 'tcx> InferCtxt<'infcx, 'gcx, 'tcx> {
  61     pub fn super_combine_tys<R>(&self,
  62                                 relation: &mut R,
  63                                 a: Ty<'tcx>,
  64                                 b: Ty<'tcx>)
  65                                 -> RelateResult<'tcx, Ty<'tcx>>
  66         where R: TypeRelation<'infcx, 'gcx, 'tcx>
  67     {
  68         let a_is_expected = relation.a_is_expected();
  69
  70         match (&a.sty, &b.sty) {
  71             // Relate integral variables to other types
  72             (&ty::Infer(ty::IntVar(a_id)), &ty::Infer(ty::IntVar(b_id))) => {
  73                 self.int_unification_table
  74                     .borrow_mut()
  75                     .unify_var_var(a_id, b_id)
  76                     .map_err(|e| int_unification_error(a_is_expected, e))?;
  77                 Ok(a)
  78             }
  79             (&ty::Infer(ty::IntVar(v_id)), &ty::Int(v)) => {
  80                 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
  81             }
  82             (&ty::Int(v), &ty::Infer(ty::IntVar(v_id))) => {
  83                 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
  84             }
  85             (&ty::Infer(ty::IntVar(v_id)), &ty::Uint(v)) => {
  86                 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
  87             }
  88             (&ty::Uint(v), &ty::Infer(ty::IntVar(v_id))) => {
  89                 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
  90             }
  91
  92             // Relate floating-point variables to other types
  93             (&ty::Infer(ty::FloatVar(a_id)), &ty::Infer(ty::FloatVar(b_id))) => {
  94                 self.float_unification_table
  95                     .borrow_mut()
  96                     .unify_var_var(a_id, b_id)
  97                     .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
  98                 Ok(a)
  99             }
 100             (&ty::Infer(ty::FloatVar(v_id)), &ty::Float(v)) => {
 101                 self.unify_float_variable(a_is_expected, v_id, v)
 102             }
 103             (&ty::Float(v), &ty::Infer(ty::FloatVar(v_id))) => {
 104                 self.unify_float_variable(!a_is_expected, v_id, v)
 105             }
 106
 107             // All other cases of inference are errors
 108             (&ty::Infer(_), _) |
 109             (_, &ty::Infer(_)) => {
 110                 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
 111             }
 112
 113             _ => {
 114                 ty::relate::super_relate_tys(relation, a, b)
 115             }
 116         }
 117     }
 118
 119     pub fn super_combine_consts<R>(
 120         &self,
 121         relation: &mut R,
 122         a: &'tcx ty::Const<'tcx>,
 123         b: &'tcx ty::Const<'tcx>,
 124     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>>
 125     where
 126         R: TypeRelation<'infcx, 'gcx, 'tcx>,
 127     {
 128         let a_is_expected = relation.a_is_expected();
 129
 130         match (a.val, b.val) {
 131             (ConstValue::Infer(InferConst::Var(a_vid)),
 132                 ConstValue::Infer(InferConst::Var(b_vid))) => {
 133                 self.const_unification_table
 134                     .borrow_mut()
 135                     .unify_var_var(a_vid, b_vid)
 136                     .map_err(|e| const_unification_error(a_is_expected, e))?;
 137                 return Ok(a);
 138             }
 139
 140             // All other cases of inference with other variables are errors.
 141             (ConstValue::Infer(InferConst::Var(_)), ConstValue::Infer(_)) |
 142             (ConstValue::Infer(_), ConstValue::Infer(InferConst::Var(_))) => {
 143                 bug!("tried to combine ConstValue::Infer/ConstValue::Infer(InferConst::Var)")
 144             }
 145
 146             (ConstValue::Infer(InferConst::Var(vid)), _) => {
 147                 return self.unify_const_variable(a_is_expected, vid, b);
 148             }
 149
 150             (_, ConstValue::Infer(InferConst::Var(vid))) => {
 151                 return self.unify_const_variable(!a_is_expected, vid, a);
 152             }
 153
 154             _ => {}
 155         }
 156
 157         ty::relate::super_relate_consts(relation, a, b)
 158     }
 159
 160     pub fn unify_const_variable(
 161         &self,
 162         vid_is_expected: bool,
 163         vid: ty::ConstVid<'tcx>,
 164         value: &'tcx ty::Const<'tcx>,
 165     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
 166         self.const_unification_table
 167             .borrow_mut()
 168             .unify_var_value(vid, ConstVarValue {
 169                 origin: ConstVariableOrigin {
 170                     kind: ConstVariableOriginKind::ConstInference,
 171                     span: DUMMY_SP,
 172                 },
 173                 val: ConstVariableValue::Known { value },
 174             })
 175             .map_err(|e| const_unification_error(vid_is_expected, e))?;
 176         Ok(value)
 177     }
 178
 179     fn unify_integral_variable(&self,
 180                                vid_is_expected: bool,
 181                                vid: ty::IntVid,
 182                                val: ty::IntVarValue)
 183                                -> RelateResult<'tcx, Ty<'tcx>>
 184     {
 185         self.int_unification_table
 186             .borrow_mut()
 187             .unify_var_value(vid, Some(val))
 188             .map_err(|e| int_unification_error(vid_is_expected, e))?;
 189         match val {
 190             IntType(v) => Ok(self.tcx.mk_mach_int(v)),
 191             UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
 192         }
 193     }
 194
 195     fn unify_float_variable(&self,
 196                             vid_is_expected: bool,
 197                             vid: ty::FloatVid,
 198                             val: ast::FloatTy)
 199                             -> RelateResult<'tcx, Ty<'tcx>>
 200     {
 201         self.float_unification_table
 202             .borrow_mut()
 203             .unify_var_value(vid, Some(ty::FloatVarValue(val)))
 204             .map_err(|e| float_unification_error(vid_is_expected, e))?;
 205         Ok(self.tcx.mk_mach_float(val))
 206     }
 207 }
 208
 209 impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
 210     pub fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> {
 211         self.infcx.tcx
 212     }
 213
 214     pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'gcx, 'tcx> {
 215         Equate::new(self, a_is_expected)
 216     }
 217
 218     pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'gcx, 'tcx> {
 219         Sub::new(self, a_is_expected)
 220     }
 221
 222     pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'gcx, 'tcx> {
 223         Lub::new(self, a_is_expected)
 224     }
 225
 226     pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'gcx, 'tcx> {
 227         Glb::new(self, a_is_expected)
 228     }
 229
 230     /// Here, `dir` is either `EqTo`, `SubtypeOf`, or `SupertypeOf`.
 231     /// The idea is that we should ensure that the type `a_ty` is equal
 232     /// to, a subtype of, or a supertype of (respectively) the type
 233     /// to which `b_vid` is bound.
 234     ///
 235     /// Since `b_vid` has not yet been instantiated with a type, we
 236     /// will first instantiate `b_vid` with a *generalized* version
 237     /// of `a_ty`. Generalization introduces other inference
 238     /// variables wherever subtyping could occur.
 239     pub fn instantiate(&mut self,
 240                        a_ty: Ty<'tcx>,
 241                        dir: RelationDir,
 242                        b_vid: ty::TyVid,
 243                        a_is_expected: bool)
 244                        -> RelateResult<'tcx, ()>
 245     {
 246         use self::RelationDir::*;
 247
 248         // Get the actual variable that b_vid has been inferred to
 249         debug_assert!(self.infcx.type_variables.borrow_mut().probe(b_vid).is_unknown());
 250
 251         debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})", a_ty, dir, b_vid);
 252
 253         // Generalize type of `a_ty` appropriately depending on the
 254         // direction.  As an example, assume:
 255         //
 256         // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
 257         //   inference variable,
 258         // - and `dir` == `SubtypeOf`.
 259         //
 260         // Then the generalized form `b_ty` would be `&'?2 ?3`, where
 261         // `'?2` and `?3` are fresh region/type inference
 262         // variables. (Down below, we will relate `a_ty <: b_ty`,
 263         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
 264         let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
 265         debug!("instantiate(a_ty={:?}, dir={:?}, b_vid={:?}, generalized b_ty={:?})",
 266                a_ty, dir, b_vid, b_ty);
 267         self.infcx.type_variables.borrow_mut().instantiate(b_vid, b_ty);
 268
 269         if needs_wf {
 270             self.obligations.push(Obligation::new(self.trace.cause.clone(),
 271                                                   self.param_env,
 272                                                   ty::Predicate::WellFormed(b_ty)));
 273         }
 274
 275         // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
 276         //
 277         // FIXME(#16847): This code is non-ideal because all these subtype
 278         // relations wind up attributed to the same spans. We need
 279         // to associate causes/spans with each of the relations in
 280         // the stack to get this right.
 281         match dir {
 282             EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
 283             SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
 284             SupertypeOf => self.sub(a_is_expected).relate_with_variance(
 285                 ty::Contravariant, &a_ty, &b_ty),
 286         }?;
 287
 288         Ok(())
 289     }
 290
 291     /// Attempts to generalize `ty` for the type variable `for_vid`.
 292     /// This checks for cycle -- that is, whether the type `ty`
 293     /// references `for_vid`. The `dir` is the "direction" for which we
 294     /// a performing the generalization (i.e., are we producing a type
 295     /// that can be used as a supertype etc).
 296     ///
 297     /// Preconditions:
 298     ///
 299     /// - `for_vid` is a "root vid"
 300     fn generalize(&self,
 301                   ty: Ty<'tcx>,
 302                   for_vid: ty::TyVid,
 303                   dir: RelationDir)
 304                   -> RelateResult<'tcx, Generalization<'tcx>>
 305     {
 306         debug!("generalize(ty={:?}, for_vid={:?}, dir={:?}", ty, for_vid, dir);
 307         // Determine the ambient variance within which `ty` appears.
 308         // The surrounding equation is:
 309         //
 310         //     ty [op] ty2
 311         //
 312         // where `op` is either `==`, `<:`, or `:>`. This maps quite
 313         // naturally.
 314         let ambient_variance = match dir {
 315             RelationDir::EqTo => ty::Invariant,
 316             RelationDir::SubtypeOf => ty::Covariant,
 317             RelationDir::SupertypeOf => ty::Contravariant,
 318         };
 319
 320         debug!("generalize: ambient_variance = {:?}", ambient_variance);
 321
 322         let for_universe = match self.infcx.type_variables.borrow_mut().probe(for_vid) {
 323             v @ TypeVariableValue::Known { .. } => panic!(
 324                 "instantiating {:?} which has a known value {:?}",
 325                 for_vid,
 326                 v,
 327             ),
 328             TypeVariableValue::Unknown { universe } => universe,
 329         };
 330
 331         debug!("generalize: for_universe = {:?}", for_universe);
 332
 333         let mut generalize = Generalizer {
 334             infcx: self.infcx,
 335             span: self.trace.cause.span,
 336             for_vid_sub_root: self.infcx.type_variables.borrow_mut().sub_root_var(for_vid),
 337             for_universe,
 338             ambient_variance,
 339             needs_wf: false,
 340             root_ty: ty,
 341         };
 342
 343         let ty = match generalize.relate(&ty, &ty) {
 344             Ok(ty) => ty,
 345             Err(e) => {
 346                 debug!("generalize: failure {:?}", e);
 347                 return Err(e);
 348             }
 349         };
 350         let needs_wf = generalize.needs_wf;
 351         debug!("generalize: success {{ {:?}, {:?} }}", ty, needs_wf);
 352         Ok(Generalization { ty, needs_wf })
 353     }
 354 }
 355
 356 struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
 357     infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
 358
 359     /// The span, used when creating new type variables and things.
 360     span: Span,
 361
 362     /// The vid of the type variable that is in the process of being
 363     /// instantiated; if we find this within the type we are folding,
 364     /// that means we would have created a cyclic type.
 365     for_vid_sub_root: ty::TyVid,
 366
 367     /// The universe of the type variable that is in the process of
 368     /// being instantiated. Any fresh variables that we create in this
 369     /// process should be in that same universe.
 370     for_universe: ty::UniverseIndex,
 371
 372     /// Track the variance as we descend into the type.
 373     ambient_variance: ty::Variance,
 374
 375     /// See the field `needs_wf` in `Generalization`.
 376     needs_wf: bool,
 377
 378     /// The root type that we are generalizing. Used when reporting cycles.
 379     root_ty: Ty<'tcx>,
 380 }
 381
 382 /// Result from a generalization operation. This includes
 383 /// not only the generalized type, but also a bool flag
 384 /// indicating whether further WF checks are needed.
 385 struct Generalization<'tcx> {
 386     ty: Ty<'tcx>,
 387
 388     /// If true, then the generalized type may not be well-formed,
 389     /// even if the source type is well-formed, so we should add an
 390     /// additional check to enforce that it is. This arises in
 391     /// particular around 'bivariant' type parameters that are only
 392     /// constrained by a where-clause. As an example, imagine a type:
 393     ///
 394     ///     struct Foo<A, B> where A: Iterator<Item = B> {
 395     ///         data: A
 396     ///     }
 397     ///
 398     /// here, `A` will be covariant, but `B` is
 399     /// unconstrained. However, whatever it is, for `Foo` to be WF, it
 400     /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
 401     /// then after generalization we will wind up with a type like
 402     /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
 403     /// ?D>` (or `>:`), we will wind up with the requirement that `?A
 404     /// <: ?C`, but no particular relationship between `?B` and `?D`
 405     /// (after all, we do not know the variance of the normalized form
 406     /// of `A::Item` with respect to `A`). If we do nothing else, this
 407     /// may mean that `?D` goes unconstrained (as in #41677). So, in
 408     /// this scenario where we create a new type variable in a
 409     /// bivariant context, we set the `needs_wf` flag to true. This
 410     /// will force the calling code to check that `WF(Foo<?C, ?D>)`
 411     /// holds, which in turn implies that `?C::Item == ?D`. So once
 412     /// `?C` is constrained, that should suffice to restrict `?D`.
 413     needs_wf: bool,
 414 }
 415
 416 impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> {
 417     fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
 418         self.infcx.tcx
 419     }
 420
 421     fn tag(&self) -> &'static str {
 422         "Generalizer"
 423     }
 424
 425     fn a_is_expected(&self) -> bool {
 426         true
 427     }
 428
 429     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
 430                   -> RelateResult<'tcx, ty::Binder<T>>
 431         where T: Relate<'tcx>
 432     {
 433         Ok(ty::Binder::bind(self.relate(a.skip_binder(), b.skip_binder())?))
 434     }
 435
 436     fn relate_item_substs(&mut self,
 437                           item_def_id: DefId,
 438                           a_subst: SubstsRef<'tcx>,
 439                           b_subst: SubstsRef<'tcx>)
 440                           -> RelateResult<'tcx, SubstsRef<'tcx>>
 441     {
 442         if self.ambient_variance == ty::Variance::Invariant {
 443             // Avoid fetching the variance if we are in an invariant
 444             // context; no need, and it can induce dependency cycles
 445             // (e.g., #41849).
 446             relate::relate_substs(self, None, a_subst, b_subst)
 447         } else {
 448             let opt_variances = self.tcx().variances_of(item_def_id);
 449             relate::relate_substs(self, Some(&opt_variances), a_subst, b_subst)
 450         }
 451     }
 452
 453     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
 454                                              variance: ty::Variance,
 455                                              a: &T,
 456                                              b: &T)
 457                                              -> RelateResult<'tcx, T>
 458     {
 459         let old_ambient_variance = self.ambient_variance;
 460         self.ambient_variance = self.ambient_variance.xform(variance);
 461
 462         let result = self.relate(a, b);
 463         self.ambient_variance = old_ambient_variance;
 464         result
 465     }
 466
 467     fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
 468         assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 469
 470         debug!("generalize: t={:?}", t);
 471
 472         // Check to see whether the type we are generalizing references
 473         // any other type variable related to `vid` via
 474         // subtyping. This is basically our "occurs check", preventing
 475         // us from creating infinitely sized types.
 476         match t.sty {
 477             ty::Infer(ty::TyVar(vid)) => {
 478                 let mut variables = self.infcx.type_variables.borrow_mut();
 479                 let vid = variables.root_var(vid);
 480                 let sub_vid = variables.sub_root_var(vid);
 481                 if sub_vid == self.for_vid_sub_root {
 482                     // If sub-roots are equal, then `for_vid` and
 483                     // `vid` are related via subtyping.
 484                     return Err(TypeError::CyclicTy(self.root_ty));
 485                 } else {
 486                     match variables.probe(vid) {
 487                         TypeVariableValue::Known { value: u } => {
 488                             drop(variables);
 489                             debug!("generalize: known value {:?}", u);
 490                             self.relate(&u, &u)
 491                         }
 492                         TypeVariableValue::Unknown { universe } => {
 493                             match self.ambient_variance {
 494                                 // Invariant: no need to make a fresh type variable.
 495                                 ty::Invariant => {
 496                                     if self.for_universe.can_name(universe) {
 497                                         return Ok(t);
 498                                     }
 499                                 }
 500
 501                                 // Bivariant: make a fresh var, but we
 502                                 // may need a WF predicate. See
 503                                 // comment on `needs_wf` field for
 504                                 // more info.
 505                                 ty::Bivariant => self.needs_wf = true,
 506
 507                                 // Co/contravariant: this will be
 508                                 // sufficiently constrained later on.
 509                                 ty::Covariant | ty::Contravariant => (),
 510                             }
 511
 512                             let origin = *variables.var_origin(vid);
 513                             let new_var_id = variables.new_var(self.for_universe, false, origin);
 514                             let u = self.tcx().mk_ty_var(new_var_id);
 515                             debug!("generalize: replacing original vid={:?} with new={:?}",
 516                                    vid, u);
 517                             return Ok(u);
 518                         }
 519                     }
 520                 }
 521             }
 522             ty::Infer(ty::IntVar(_)) |
 523             ty::Infer(ty::FloatVar(_)) => {
 524                 // No matter what mode we are in,
 525                 // integer/floating-point types must be equal to be
 526                 // relatable.
 527                 Ok(t)
 528             }
 529             _ => {
 530                 relate::super_relate_tys(self, t, t)
 531             }
 532         }
 533     }
 534
 535     fn regions(&mut self, r: ty::Region<'tcx>, r2: ty::Region<'tcx>)
 536                -> RelateResult<'tcx, ty::Region<'tcx>> {
 537         assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 538
 539         debug!("generalize: regions r={:?}", r);
 540
 541         match *r {
 542             // Never make variables for regions bound within the type itself,
 543             // nor for erased regions.
 544             ty::ReLateBound(..) |
 545             ty::ReErased => {
 546                 return Ok(r);
 547             }
 548
 549             ty::ReClosureBound(..) => {
 550                 span_bug!(
 551                     self.span,
 552                     "encountered unexpected ReClosureBound: {:?}",
 553                     r,
 554                 );
 555             }
 556
 557             ty::RePlaceholder(..) |
 558             ty::ReVar(..) |
 559             ty::ReEmpty |
 560             ty::ReStatic |
 561             ty::ReScope(..) |
 562             ty::ReEarlyBound(..) |
 563             ty::ReFree(..) => {
 564                 // see common code below
 565             }
 566         }
 567
 568         // If we are in an invariant context, we can re-use the region
 569         // as is, unless it happens to be in some universe that we
 570         // can't name. (In the case of a region *variable*, we could
 571         // use it if we promoted it into our universe, but we don't
 572         // bother.)
 573         if let ty::Invariant = self.ambient_variance {
 574             let r_universe = self.infcx.universe_of_region(r);
 575             if self.for_universe.can_name(r_universe) {
 576                 return Ok(r);
 577             }
 578         }
 579
 580         // FIXME: This is non-ideal because we don't give a
 581         // very descriptive origin for this region variable.
 582         Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.span), self.for_universe))
 583     }
 584
 585     fn consts(
 586         &mut self,
 587         c: &'tcx ty::Const<'tcx>,
 588         c2: &'tcx ty::Const<'tcx>
 589     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
 590         assert_eq!(c, c2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 591
 592         match c {
 593             ty::Const { val: ConstValue::Infer(InferConst::Var(vid)), .. } => {
 594                 let mut variable_table = self.infcx.const_unification_table.borrow_mut();
 595                 match variable_table.probe_value(*vid).val.known() {
 596                     Some(u) => {
 597                         self.relate(&u, &u)
 598                     }
 599                     None => Ok(c),
 600                 }
 601             }
 602             _ => {
 603                 relate::super_relate_consts(self, c, c)
 604             }
 605         }
 606     }
 607 }
 608
 609 pub trait RelateResultCompare<'tcx, T> {
 610     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 611         F: FnOnce() -> TypeError<'tcx>;
 612 }
 613
 614 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
 615     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 616         F: FnOnce() -> TypeError<'tcx>,
 617     {
 618         self.clone().and_then(|s| {
 619             if s == t {
 620                 self.clone()
 621             } else {
 622                 Err(f())
 623             }
 624         })
 625     }
 626 }
 627
 628 pub fn const_unification_error<'tcx>(
 629     a_is_expected: bool,
 630     (a, b): (&'tcx ty::Const<'tcx>, &'tcx ty::Const<'tcx>),
 631 ) -> TypeError<'tcx> {
 632     TypeError::ConstMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 633 }
 634
 635 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
 636                                -> TypeError<'tcx>
 637 {
 638     let (a, b) = v;
 639     TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 640 }
 641
 642 fn float_unification_error<'tcx>(a_is_expected: bool,
 643                                  v: (ty::FloatVarValue, ty::FloatVarValue))
 644                                  -> TypeError<'tcx>
 645 {
 646     let (ty::FloatVarValue(a), ty::FloatVarValue(b)) = v;
 647     TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 648 }