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, 'tcx> {
  48     pub infcx: &'infcx InferCtxt<'infcx, '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, 'tcx> InferCtxt<'infcx, 'tcx> {
  61     pub fn super_combine_tys<R>(
  62         &self,
  63         relation: &mut R,
  64         a: Ty<'tcx>,
  65         b: Ty<'tcx>,
  66     ) -> RelateResult<'tcx, Ty<'tcx>>
  67     where
  68         R: TypeRelation<'tcx>,
  69     {
  70         let a_is_expected = relation.a_is_expected();
  71
  72         match (&a.sty, &b.sty) {
  73             // Relate integral variables to other types
  74             (&ty::Infer(ty::IntVar(a_id)), &ty::Infer(ty::IntVar(b_id))) => {
  75                 self.int_unification_table
  76                     .borrow_mut()
  77                     .unify_var_var(a_id, b_id)
  78                     .map_err(|e| int_unification_error(a_is_expected, e))?;
  79                 Ok(a)
  80             }
  81             (&ty::Infer(ty::IntVar(v_id)), &ty::Int(v)) => {
  82                 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
  83             }
  84             (&ty::Int(v), &ty::Infer(ty::IntVar(v_id))) => {
  85                 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
  86             }
  87             (&ty::Infer(ty::IntVar(v_id)), &ty::Uint(v)) => {
  88                 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
  89             }
  90             (&ty::Uint(v), &ty::Infer(ty::IntVar(v_id))) => {
  91                 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
  92             }
  93
  94             // Relate floating-point variables to other types
  95             (&ty::Infer(ty::FloatVar(a_id)), &ty::Infer(ty::FloatVar(b_id))) => {
  96                 self.float_unification_table
  97                     .borrow_mut()
  98                     .unify_var_var(a_id, b_id)
  99                     .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
 100                 Ok(a)
 101             }
 102             (&ty::Infer(ty::FloatVar(v_id)), &ty::Float(v)) => {
 103                 self.unify_float_variable(a_is_expected, v_id, v)
 104             }
 105             (&ty::Float(v), &ty::Infer(ty::FloatVar(v_id))) => {
 106                 self.unify_float_variable(!a_is_expected, v_id, v)
 107             }
 108
 109             // All other cases of inference are errors
 110             (&ty::Infer(_), _) |
 111             (_, &ty::Infer(_)) => {
 112                 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
 113             }
 114
 115             _ => {
 116                 ty::relate::super_relate_tys(relation, a, b)
 117             }
 118         }
 119     }
 120
 121     pub fn super_combine_consts<R>(
 122         &self,
 123         relation: &mut R,
 124         a: &'tcx ty::Const<'tcx>,
 125         b: &'tcx ty::Const<'tcx>,
 126     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>>
 127     where
 128         R: TypeRelation<'tcx>,
 129     {
 130         let a_is_expected = relation.a_is_expected();
 131
 132         match (a.val, b.val) {
 133             (ConstValue::Infer(InferConst::Var(a_vid)),
 134                 ConstValue::Infer(InferConst::Var(b_vid))) => {
 135                 self.const_unification_table
 136                     .borrow_mut()
 137                     .unify_var_var(a_vid, b_vid)
 138                     .map_err(|e| const_unification_error(a_is_expected, e))?;
 139                 return Ok(a);
 140             }
 141
 142             // All other cases of inference with other variables are errors.
 143             (ConstValue::Infer(InferConst::Var(_)), ConstValue::Infer(_)) |
 144             (ConstValue::Infer(_), ConstValue::Infer(InferConst::Var(_))) => {
 145                 bug!("tried to combine ConstValue::Infer/ConstValue::Infer(InferConst::Var)")
 146             }
 147
 148             (ConstValue::Infer(InferConst::Var(vid)), _) => {
 149                 return self.unify_const_variable(a_is_expected, vid, b);
 150             }
 151
 152             (_, ConstValue::Infer(InferConst::Var(vid))) => {
 153                 return self.unify_const_variable(!a_is_expected, vid, a);
 154             }
 155
 156             _ => {}
 157         }
 158
 159         ty::relate::super_relate_consts(relation, a, b)
 160     }
 161
 162     pub fn unify_const_variable(
 163         &self,
 164         vid_is_expected: bool,
 165         vid: ty::ConstVid<'tcx>,
 166         value: &'tcx ty::Const<'tcx>,
 167     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
 168         self.const_unification_table
 169             .borrow_mut()
 170             .unify_var_value(vid, ConstVarValue {
 171                 origin: ConstVariableOrigin {
 172                     kind: ConstVariableOriginKind::ConstInference,
 173                     span: DUMMY_SP,
 174                 },
 175                 val: ConstVariableValue::Known { value },
 176             })
 177             .map_err(|e| const_unification_error(vid_is_expected, e))?;
 178         Ok(value)
 179     }
 180
 181     fn unify_integral_variable(&self,
 182                                vid_is_expected: bool,
 183                                vid: ty::IntVid,
 184                                val: ty::IntVarValue)
 185                                -> RelateResult<'tcx, Ty<'tcx>>
 186     {
 187         self.int_unification_table
 188             .borrow_mut()
 189             .unify_var_value(vid, Some(val))
 190             .map_err(|e| int_unification_error(vid_is_expected, e))?;
 191         match val {
 192             IntType(v) => Ok(self.tcx.mk_mach_int(v)),
 193             UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
 194         }
 195     }
 196
 197     fn unify_float_variable(&self,
 198                             vid_is_expected: bool,
 199                             vid: ty::FloatVid,
 200                             val: ast::FloatTy)
 201                             -> RelateResult<'tcx, Ty<'tcx>>
 202     {
 203         self.float_unification_table
 204             .borrow_mut()
 205             .unify_var_value(vid, Some(ty::FloatVarValue(val)))
 206             .map_err(|e| float_unification_error(vid_is_expected, e))?;
 207         Ok(self.tcx.mk_mach_float(val))
 208     }
 209 }
 210
 211 impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
 212     pub fn tcx(&self) -> TyCtxt<'tcx> {
 213         self.infcx.tcx
 214     }
 215
 216     pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'tcx> {
 217         Equate::new(self, a_is_expected)
 218     }
 219
 220     pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'tcx> {
 221         Sub::new(self, a_is_expected)
 222     }
 223
 224     pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'tcx> {
 225         Lub::new(self, a_is_expected)
 226     }
 227
 228     pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'tcx> {
 229         Glb::new(self, a_is_expected)
 230     }
 231
 232     /// Here, `dir` is either `EqTo`, `SubtypeOf`, or `SupertypeOf`.
 233     /// The idea is that we should ensure that the type `a_ty` is equal
 234     /// to, a subtype of, or a supertype of (respectively) the type
 235     /// to which `b_vid` is bound.
 236     ///
 237     /// Since `b_vid` has not yet been instantiated with a type, we
 238     /// will first instantiate `b_vid` with a *generalized* version
 239     /// of `a_ty`. Generalization introduces other inference
 240     /// variables wherever subtyping could occur.
 241     pub fn instantiate(&mut self,
 242                        a_ty: Ty<'tcx>,
 243                        dir: RelationDir,
 244                        b_vid: ty::TyVid,
 245                        a_is_expected: bool)
 246                        -> RelateResult<'tcx, ()>
 247     {
 248         use self::RelationDir::*;
 249
 250         // Get the actual variable that b_vid has been inferred to
 251         debug_assert!(self.infcx.type_variables.borrow_mut().probe(b_vid).is_unknown());
 252
 253         debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})", a_ty, dir, b_vid);
 254
 255         // Generalize type of `a_ty` appropriately depending on the
 256         // direction.  As an example, assume:
 257         //
 258         // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
 259         //   inference variable,
 260         // - and `dir` == `SubtypeOf`.
 261         //
 262         // Then the generalized form `b_ty` would be `&'?2 ?3`, where
 263         // `'?2` and `?3` are fresh region/type inference
 264         // variables. (Down below, we will relate `a_ty <: b_ty`,
 265         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
 266         let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
 267         debug!("instantiate(a_ty={:?}, dir={:?}, b_vid={:?}, generalized b_ty={:?})",
 268                a_ty, dir, b_vid, b_ty);
 269         self.infcx.type_variables.borrow_mut().instantiate(b_vid, b_ty);
 270
 271         if needs_wf {
 272             self.obligations.push(Obligation::new(self.trace.cause.clone(),
 273                                                   self.param_env,
 274                                                   ty::Predicate::WellFormed(b_ty)));
 275         }
 276
 277         // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
 278         //
 279         // FIXME(#16847): This code is non-ideal because all these subtype
 280         // relations wind up attributed to the same spans. We need
 281         // to associate causes/spans with each of the relations in
 282         // the stack to get this right.
 283         match dir {
 284             EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
 285             SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
 286             SupertypeOf => self.sub(a_is_expected).relate_with_variance(
 287                 ty::Contravariant, &a_ty, &b_ty),
 288         }?;
 289
 290         Ok(())
 291     }
 292
 293     /// Attempts to generalize `ty` for the type variable `for_vid`.
 294     /// This checks for cycle -- that is, whether the type `ty`
 295     /// references `for_vid`. The `dir` is the "direction" for which we
 296     /// a performing the generalization (i.e., are we producing a type
 297     /// that can be used as a supertype etc).
 298     ///
 299     /// Preconditions:
 300     ///
 301     /// - `for_vid` is a "root vid"
 302     fn generalize(&self,
 303                   ty: Ty<'tcx>,
 304                   for_vid: ty::TyVid,
 305                   dir: RelationDir)
 306                   -> RelateResult<'tcx, Generalization<'tcx>>
 307     {
 308         debug!("generalize(ty={:?}, for_vid={:?}, dir={:?}", ty, for_vid, dir);
 309         // Determine the ambient variance within which `ty` appears.
 310         // The surrounding equation is:
 311         //
 312         //     ty [op] ty2
 313         //
 314         // where `op` is either `==`, `<:`, or `:>`. This maps quite
 315         // naturally.
 316         let ambient_variance = match dir {
 317             RelationDir::EqTo => ty::Invariant,
 318             RelationDir::SubtypeOf => ty::Covariant,
 319             RelationDir::SupertypeOf => ty::Contravariant,
 320         };
 321
 322         debug!("generalize: ambient_variance = {:?}", ambient_variance);
 323
 324         let for_universe = match self.infcx.type_variables.borrow_mut().probe(for_vid) {
 325             v @ TypeVariableValue::Known { .. } => panic!(
 326                 "instantiating {:?} which has a known value {:?}",
 327                 for_vid,
 328                 v,
 329             ),
 330             TypeVariableValue::Unknown { universe } => universe,
 331         };
 332
 333         debug!("generalize: for_universe = {:?}", for_universe);
 334
 335         let mut generalize = Generalizer {
 336             infcx: self.infcx,
 337             span: self.trace.cause.span,
 338             for_vid_sub_root: self.infcx.type_variables.borrow_mut().sub_root_var(for_vid),
 339             for_universe,
 340             ambient_variance,
 341             needs_wf: false,
 342             root_ty: ty,
 343             param_env: self.param_env,
 344         };
 345
 346         let ty = match generalize.relate(&ty, &ty) {
 347             Ok(ty) => ty,
 348             Err(e) => {
 349                 debug!("generalize: failure {:?}", e);
 350                 return Err(e);
 351             }
 352         };
 353         let needs_wf = generalize.needs_wf;
 354         debug!("generalize: success {{ {:?}, {:?} }}", ty, needs_wf);
 355         Ok(Generalization { ty, needs_wf })
 356     }
 357 }
 358
 359 struct Generalizer<'cx, 'tcx> {
 360     infcx: &'cx InferCtxt<'cx, 'tcx>,
 361
 362     /// The span, used when creating new type variables and things.
 363     span: Span,
 364
 365     /// The vid of the type variable that is in the process of being
 366     /// instantiated; if we find this within the type we are folding,
 367     /// that means we would have created a cyclic type.
 368     for_vid_sub_root: ty::TyVid,
 369
 370     /// The universe of the type variable that is in the process of
 371     /// being instantiated. Any fresh variables that we create in this
 372     /// process should be in that same universe.
 373     for_universe: ty::UniverseIndex,
 374
 375     /// Track the variance as we descend into the type.
 376     ambient_variance: ty::Variance,
 377
 378     /// See the field `needs_wf` in `Generalization`.
 379     needs_wf: bool,
 380
 381     /// The root type that we are generalizing. Used when reporting cycles.
 382     root_ty: Ty<'tcx>,
 383
 384     param_env: ty::ParamEnv<'tcx>,
 385 }
 386
 387 /// Result from a generalization operation. This includes
 388 /// not only the generalized type, but also a bool flag
 389 /// indicating whether further WF checks are needed.
 390 struct Generalization<'tcx> {
 391     ty: Ty<'tcx>,
 392
 393     /// If true, then the generalized type may not be well-formed,
 394     /// even if the source type is well-formed, so we should add an
 395     /// additional check to enforce that it is. This arises in
 396     /// particular around 'bivariant' type parameters that are only
 397     /// constrained by a where-clause. As an example, imagine a type:
 398     ///
 399     ///     struct Foo<A, B> where A: Iterator<Item = B> {
 400     ///         data: A
 401     ///     }
 402     ///
 403     /// here, `A` will be covariant, but `B` is
 404     /// unconstrained. However, whatever it is, for `Foo` to be WF, it
 405     /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
 406     /// then after generalization we will wind up with a type like
 407     /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
 408     /// ?D>` (or `>:`), we will wind up with the requirement that `?A
 409     /// <: ?C`, but no particular relationship between `?B` and `?D`
 410     /// (after all, we do not know the variance of the normalized form
 411     /// of `A::Item` with respect to `A`). If we do nothing else, this
 412     /// may mean that `?D` goes unconstrained (as in #41677). So, in
 413     /// this scenario where we create a new type variable in a
 414     /// bivariant context, we set the `needs_wf` flag to true. This
 415     /// will force the calling code to check that `WF(Foo<?C, ?D>)`
 416     /// holds, which in turn implies that `?C::Item == ?D`. So once
 417     /// `?C` is constrained, that should suffice to restrict `?D`.
 418     needs_wf: bool,
 419 }
 420
 421 impl TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
 422     fn tcx(&self) -> TyCtxt<'tcx> {
 423         self.infcx.tcx
 424     }
 425     fn param_env(&self) -> ty::ParamEnv<'tcx> { self.param_env }
 426
 427     fn tag(&self) -> &'static str {
 428         "Generalizer"
 429     }
 430
 431     fn a_is_expected(&self) -> bool {
 432         true
 433     }
 434
 435     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
 436                   -> RelateResult<'tcx, ty::Binder<T>>
 437         where T: Relate<'tcx>
 438     {
 439         Ok(ty::Binder::bind(self.relate(a.skip_binder(), b.skip_binder())?))
 440     }
 441
 442     fn relate_item_substs(&mut self,
 443                           item_def_id: DefId,
 444                           a_subst: SubstsRef<'tcx>,
 445                           b_subst: SubstsRef<'tcx>)
 446                           -> RelateResult<'tcx, SubstsRef<'tcx>>
 447     {
 448         if self.ambient_variance == ty::Variance::Invariant {
 449             // Avoid fetching the variance if we are in an invariant
 450             // context; no need, and it can induce dependency cycles
 451             // (e.g., #41849).
 452             relate::relate_substs(self, None, a_subst, b_subst)
 453         } else {
 454             let opt_variances = self.tcx().variances_of(item_def_id);
 455             relate::relate_substs(self, Some(&opt_variances), a_subst, b_subst)
 456         }
 457     }
 458
 459     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
 460                                              variance: ty::Variance,
 461                                              a: &T,
 462                                              b: &T)
 463                                              -> RelateResult<'tcx, T>
 464     {
 465         let old_ambient_variance = self.ambient_variance;
 466         self.ambient_variance = self.ambient_variance.xform(variance);
 467
 468         let result = self.relate(a, b);
 469         self.ambient_variance = old_ambient_variance;
 470         result
 471     }
 472
 473     fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
 474         assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 475
 476         debug!("generalize: t={:?}", t);
 477
 478         // Check to see whether the type we are generalizing references
 479         // any other type variable related to `vid` via
 480         // subtyping. This is basically our "occurs check", preventing
 481         // us from creating infinitely sized types.
 482         match t.sty {
 483             ty::Infer(ty::TyVar(vid)) => {
 484                 let mut variables = self.infcx.type_variables.borrow_mut();
 485                 let vid = variables.root_var(vid);
 486                 let sub_vid = variables.sub_root_var(vid);
 487                 if sub_vid == self.for_vid_sub_root {
 488                     // If sub-roots are equal, then `for_vid` and
 489                     // `vid` are related via subtyping.
 490                     return Err(TypeError::CyclicTy(self.root_ty));
 491                 } else {
 492                     match variables.probe(vid) {
 493                         TypeVariableValue::Known { value: u } => {
 494                             drop(variables);
 495                             debug!("generalize: known value {:?}", u);
 496                             self.relate(&u, &u)
 497                         }
 498                         TypeVariableValue::Unknown { universe } => {
 499                             match self.ambient_variance {
 500                                 // Invariant: no need to make a fresh type variable.
 501                                 ty::Invariant => {
 502                                     if self.for_universe.can_name(universe) {
 503                                         return Ok(t);
 504                                     }
 505                                 }
 506
 507                                 // Bivariant: make a fresh var, but we
 508                                 // may need a WF predicate. See
 509                                 // comment on `needs_wf` field for
 510                                 // more info.
 511                                 ty::Bivariant => self.needs_wf = true,
 512
 513                                 // Co/contravariant: this will be
 514                                 // sufficiently constrained later on.
 515                                 ty::Covariant | ty::Contravariant => (),
 516                             }
 517
 518                             let origin = *variables.var_origin(vid);
 519                             let new_var_id = variables.new_var(self.for_universe, false, origin);
 520                             let u = self.tcx().mk_ty_var(new_var_id);
 521                             debug!("generalize: replacing original vid={:?} with new={:?}",
 522                                    vid, u);
 523                             return Ok(u);
 524                         }
 525                     }
 526                 }
 527             }
 528             ty::Infer(ty::IntVar(_)) |
 529             ty::Infer(ty::FloatVar(_)) => {
 530                 // No matter what mode we are in,
 531                 // integer/floating-point types must be equal to be
 532                 // relatable.
 533                 Ok(t)
 534             }
 535             _ => {
 536                 relate::super_relate_tys(self, t, t)
 537             }
 538         }
 539     }
 540
 541     fn regions(&mut self, r: ty::Region<'tcx>, r2: ty::Region<'tcx>)
 542                -> RelateResult<'tcx, ty::Region<'tcx>> {
 543         assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 544
 545         debug!("generalize: regions r={:?}", r);
 546
 547         match *r {
 548             // Never make variables for regions bound within the type itself,
 549             // nor for erased regions.
 550             ty::ReLateBound(..) |
 551             ty::ReErased => {
 552                 return Ok(r);
 553             }
 554
 555             ty::ReClosureBound(..) => {
 556                 span_bug!(
 557                     self.span,
 558                     "encountered unexpected ReClosureBound: {:?}",
 559                     r,
 560                 );
 561             }
 562
 563             ty::RePlaceholder(..) |
 564             ty::ReVar(..) |
 565             ty::ReEmpty |
 566             ty::ReStatic |
 567             ty::ReScope(..) |
 568             ty::ReEarlyBound(..) |
 569             ty::ReFree(..) => {
 570                 // see common code below
 571             }
 572         }
 573
 574         // If we are in an invariant context, we can re-use the region
 575         // as is, unless it happens to be in some universe that we
 576         // can't name. (In the case of a region *variable*, we could
 577         // use it if we promoted it into our universe, but we don't
 578         // bother.)
 579         if let ty::Invariant = self.ambient_variance {
 580             let r_universe = self.infcx.universe_of_region(r);
 581             if self.for_universe.can_name(r_universe) {
 582                 return Ok(r);
 583             }
 584         }
 585
 586         // FIXME: This is non-ideal because we don't give a
 587         // very descriptive origin for this region variable.
 588         Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.span), self.for_universe))
 589     }
 590
 591     fn consts(
 592         &mut self,
 593         c: &'tcx ty::Const<'tcx>,
 594         c2: &'tcx ty::Const<'tcx>
 595     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
 596         assert_eq!(c, c2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 597
 598         match c {
 599             ty::Const { val: ConstValue::Infer(InferConst::Var(vid)), .. } => {
 600                 let mut variable_table = self.infcx.const_unification_table.borrow_mut();
 601                 match variable_table.probe_value(*vid).val.known() {
 602                     Some(u) => {
 603                         self.relate(&u, &u)
 604                     }
 605                     None => Ok(c),
 606                 }
 607             }
 608             _ => {
 609                 relate::super_relate_consts(self, c, c)
 610             }
 611         }
 612     }
 613 }
 614
 615 pub trait RelateResultCompare<'tcx, T> {
 616     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 617         F: FnOnce() -> TypeError<'tcx>;
 618 }
 619
 620 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
 621     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 622         F: FnOnce() -> TypeError<'tcx>,
 623     {
 624         self.clone().and_then(|s| {
 625             if s == t {
 626                 self.clone()
 627             } else {
 628                 Err(f())
 629             }
 630         })
 631     }
 632 }
 633
 634 pub fn const_unification_error<'tcx>(
 635     a_is_expected: bool,
 636     (a, b): (&'tcx ty::Const<'tcx>, &'tcx ty::Const<'tcx>),
 637 ) -> TypeError<'tcx> {
 638     TypeError::ConstMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 639 }
 640
 641 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
 642                                -> TypeError<'tcx>
 643 {
 644     let (a, b) = v;
 645     TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 646 }
 647
 648 fn float_unification_error<'tcx>(a_is_expected: bool,
 649                                  v: (ty::FloatVarValue, ty::FloatVarValue))
 650                                  -> TypeError<'tcx>
 651 {
 652     let (ty::FloatVarValue(a), ty::FloatVarValue(b)) = v;
 653     TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 654 }