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