src/librustc/infer/combine.rs

   1 // Copyright 2012 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 ///////////////////////////////////////////////////////////////////////////
  12 // # Type combining
  13 //
  14 // There are four type combiners: equate, sub, lub, and glb.  Each
  15 // implements the trait `Combine` and contains methods for combining
  16 // two instances of various things and yielding a new instance.  These
  17 // combiner methods always yield a `Result<T>`.  There is a lot of
  18 // common code for these operations, implemented as default methods on
  19 // the `Combine` trait.
  20 //
  21 // Each operation may have side-effects on the inference context,
  22 // though these can be unrolled using snapshots. On success, the
  23 // LUB/GLB operations return the appropriate bound. The Eq and Sub
  24 // operations generally return the first operand.
  25 //
  26 // ## Contravariance
  27 //
  28 // When you are relating two things which have a contravariant
  29 // relationship, you should use `contratys()` or `contraregions()`,
  30 // rather than inversing the order of arguments!  This is necessary
  31 // because the order of arguments is not relevant for LUB and GLB.  It
  32 // is also useful to track which value is the "expected" value in
  33 // terms of error reporting.
  34
  35 use super::equate::Equate;
  36 use super::glb::Glb;
  37 use super::{InferCtxt, MiscVariable, TypeTrace};
  38 use super::lub::Lub;
  39 use super::sub::Sub;
  40 use super::type_variable::TypeVariableValue;
  41
  42 use hir::def_id::DefId;
  43 use ty::{IntType, UintType};
  44 use ty::{self, Ty, TyCtxt};
  45 use ty::error::TypeError;
  46 use ty::relate::{self, Relate, RelateResult, TypeRelation};
  47 use ty::subst::Substs;
  48 use traits::{Obligation, PredicateObligations};
  49
  50 use syntax::ast;
  51 use syntax_pos::Span;
  52
  53 #[derive(Clone)]
  54 pub struct CombineFields<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
  55     pub infcx: &'infcx InferCtxt<'infcx, 'gcx, 'tcx>,
  56     pub trace: TypeTrace<'tcx>,
  57     pub cause: Option<ty::relate::Cause>,
  58     pub param_env: ty::ParamEnv<'tcx>,
  59     pub obligations: PredicateObligations<'tcx>,
  60 }
  61
  62 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
  63 pub enum RelationDir {
  64     SubtypeOf, SupertypeOf, EqTo
  65 }
  66
  67 impl<'infcx, 'gcx, 'tcx> InferCtxt<'infcx, 'gcx, 'tcx> {
  68     pub fn super_combine_tys<R>(&self,
  69                                 relation: &mut R,
  70                                 a: Ty<'tcx>,
  71                                 b: Ty<'tcx>)
  72                                 -> RelateResult<'tcx, Ty<'tcx>>
  73         where R: TypeRelation<'infcx, 'gcx, 'tcx>
  74     {
  75         let a_is_expected = relation.a_is_expected();
  76
  77         match (&a.sty, &b.sty) {
  78             // Relate integral variables to other types
  79             (&ty::Infer(ty::IntVar(a_id)), &ty::Infer(ty::IntVar(b_id))) => {
  80                 self.int_unification_table
  81                     .borrow_mut()
  82                     .unify_var_var(a_id, b_id)
  83                     .map_err(|e| int_unification_error(a_is_expected, e))?;
  84                 Ok(a)
  85             }
  86             (&ty::Infer(ty::IntVar(v_id)), &ty::Int(v)) => {
  87                 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
  88             }
  89             (&ty::Int(v), &ty::Infer(ty::IntVar(v_id))) => {
  90                 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
  91             }
  92             (&ty::Infer(ty::IntVar(v_id)), &ty::Uint(v)) => {
  93                 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
  94             }
  95             (&ty::Uint(v), &ty::Infer(ty::IntVar(v_id))) => {
  96                 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
  97             }
  98
  99             // Relate floating-point variables to other types
 100             (&ty::Infer(ty::FloatVar(a_id)), &ty::Infer(ty::FloatVar(b_id))) => {
 101                 self.float_unification_table
 102                     .borrow_mut()
 103                     .unify_var_var(a_id, b_id)
 104                     .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
 105                 Ok(a)
 106             }
 107             (&ty::Infer(ty::FloatVar(v_id)), &ty::Float(v)) => {
 108                 self.unify_float_variable(a_is_expected, v_id, v)
 109             }
 110             (&ty::Float(v), &ty::Infer(ty::FloatVar(v_id))) => {
 111                 self.unify_float_variable(!a_is_expected, v_id, v)
 112             }
 113
 114             // All other cases of inference are errors
 115             (&ty::Infer(_), _) |
 116             (_, &ty::Infer(_)) => {
 117                 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
 118             }
 119
 120
 121             _ => {
 122                 ty::relate::super_relate_tys(relation, a, b)
 123             }
 124         }
 125     }
 126
 127     fn unify_integral_variable(&self,
 128                                vid_is_expected: bool,
 129                                vid: ty::IntVid,
 130                                val: ty::IntVarValue)
 131                                -> RelateResult<'tcx, Ty<'tcx>>
 132     {
 133         self.int_unification_table
 134             .borrow_mut()
 135             .unify_var_value(vid, Some(val))
 136             .map_err(|e| int_unification_error(vid_is_expected, e))?;
 137         match val {
 138             IntType(v) => Ok(self.tcx.mk_mach_int(v)),
 139             UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
 140         }
 141     }
 142
 143     fn unify_float_variable(&self,
 144                             vid_is_expected: bool,
 145                             vid: ty::FloatVid,
 146                             val: ast::FloatTy)
 147                             -> RelateResult<'tcx, Ty<'tcx>>
 148     {
 149         self.float_unification_table
 150             .borrow_mut()
 151             .unify_var_value(vid, Some(ty::FloatVarValue(val)))
 152             .map_err(|e| float_unification_error(vid_is_expected, e))?;
 153         Ok(self.tcx.mk_mach_float(val))
 154     }
 155 }
 156
 157 impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
 158     pub fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> {
 159         self.infcx.tcx
 160     }
 161
 162     pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'gcx, 'tcx> {
 163         Equate::new(self, a_is_expected)
 164     }
 165
 166     pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'gcx, 'tcx> {
 167         Sub::new(self, a_is_expected)
 168     }
 169
 170     pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'gcx, 'tcx> {
 171         Lub::new(self, a_is_expected)
 172     }
 173
 174     pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'gcx, 'tcx> {
 175         Glb::new(self, a_is_expected)
 176     }
 177
 178     /// Here dir is either EqTo, SubtypeOf, or SupertypeOf. The
 179     /// idea is that we should ensure that the type `a_ty` is equal
 180     /// to, a subtype of, or a supertype of (respectively) the type
 181     /// to which `b_vid` is bound.
 182     ///
 183     /// Since `b_vid` has not yet been instantiated with a type, we
 184     /// will first instantiate `b_vid` with a *generalized* version
 185     /// of `a_ty`. Generalization introduces other inference
 186     /// variables wherever subtyping could occur.
 187     pub fn instantiate(&mut self,
 188                        a_ty: Ty<'tcx>,
 189                        dir: RelationDir,
 190                        b_vid: ty::TyVid,
 191                        a_is_expected: bool)
 192                        -> RelateResult<'tcx, ()>
 193     {
 194         use self::RelationDir::*;
 195
 196         // Get the actual variable that b_vid has been inferred to
 197         debug_assert!(self.infcx.type_variables.borrow_mut().probe(b_vid).is_unknown());
 198
 199         debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})", a_ty, dir, b_vid);
 200
 201         // Generalize type of `a_ty` appropriately depending on the
 202         // direction.  As an example, assume:
 203         //
 204         // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
 205         //   inference variable,
 206         // - and `dir` == `SubtypeOf`.
 207         //
 208         // Then the generalized form `b_ty` would be `&'?2 ?3`, where
 209         // `'?2` and `?3` are fresh region/type inference
 210         // variables. (Down below, we will relate `a_ty <: b_ty`,
 211         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
 212         let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
 213         debug!("instantiate(a_ty={:?}, dir={:?}, b_vid={:?}, generalized b_ty={:?})",
 214                a_ty, dir, b_vid, b_ty);
 215         self.infcx.type_variables.borrow_mut().instantiate(b_vid, b_ty);
 216
 217         if needs_wf {
 218             self.obligations.push(Obligation::new(self.trace.cause.clone(),
 219                                                   self.param_env,
 220                                                   ty::Predicate::WellFormed(b_ty)));
 221         }
 222
 223         // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
 224         //
 225         // FIXME(#16847): This code is non-ideal because all these subtype
 226         // relations wind up attributed to the same spans. We need
 227         // to associate causes/spans with each of the relations in
 228         // the stack to get this right.
 229         match dir {
 230             EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
 231             SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
 232             SupertypeOf => self.sub(a_is_expected).relate_with_variance(
 233                 ty::Contravariant, &a_ty, &b_ty),
 234         }?;
 235
 236         Ok(())
 237     }
 238
 239     /// Attempts to generalize `ty` for the type variable `for_vid`.
 240     /// This checks for cycle -- that is, whether the type `ty`
 241     /// references `for_vid`. The `dir` is the "direction" for which we
 242     /// a performing the generalization (i.e., are we producing a type
 243     /// that can be used as a supertype etc).
 244     ///
 245     /// Preconditions:
 246     ///
 247     /// - `for_vid` is a "root vid"
 248     fn generalize(&self,
 249                   ty: Ty<'tcx>,
 250                   for_vid: ty::TyVid,
 251                   dir: RelationDir)
 252                   -> RelateResult<'tcx, Generalization<'tcx>>
 253     {
 254         // Determine the ambient variance within which `ty` appears.
 255         // The surrounding equation is:
 256         //
 257         //     ty [op] ty2
 258         //
 259         // where `op` is either `==`, `<:`, or `:>`. This maps quite
 260         // naturally.
 261         let ambient_variance = match dir {
 262             RelationDir::EqTo => ty::Invariant,
 263             RelationDir::SubtypeOf => ty::Covariant,
 264             RelationDir::SupertypeOf => ty::Contravariant,
 265         };
 266
 267         let mut generalize = Generalizer {
 268             infcx: self.infcx,
 269             span: self.trace.cause.span,
 270             for_vid_sub_root: self.infcx.type_variables.borrow_mut().sub_root_var(for_vid),
 271             ambient_variance,
 272             needs_wf: false,
 273             root_ty: ty,
 274         };
 275
 276         let ty = generalize.relate(&ty, &ty)?;
 277         let needs_wf = generalize.needs_wf;
 278         Ok(Generalization { ty, needs_wf })
 279     }
 280 }
 281
 282 struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
 283     infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
 284
 285     /// Span, used when creating new type variables and things.
 286     span: Span,
 287
 288     /// The vid of the type variable that is in the process of being
 289     /// instantiated; if we find this within the type we are folding,
 290     /// that means we would have created a cyclic type.
 291     for_vid_sub_root: ty::TyVid,
 292
 293     /// Track the variance as we descend into the type.
 294     ambient_variance: ty::Variance,
 295
 296     /// See the field `needs_wf` in `Generalization`.
 297     needs_wf: bool,
 298
 299     /// The root type that we are generalizing. Used when reporting cycles.
 300     root_ty: Ty<'tcx>,
 301 }
 302
 303 /// Result from a generalization operation. This includes
 304 /// not only the generalized type, but also a bool flag
 305 /// indicating whether further WF checks are needed.
 306 struct Generalization<'tcx> {
 307     ty: Ty<'tcx>,
 308
 309     /// If true, then the generalized type may not be well-formed,
 310     /// even if the source type is well-formed, so we should add an
 311     /// additional check to enforce that it is. This arises in
 312     /// particular around 'bivariant' type parameters that are only
 313     /// constrained by a where-clause. As an example, imagine a type:
 314     ///
 315     ///     struct Foo<A, B> where A: Iterator<Item=B> {
 316     ///         data: A
 317     ///     }
 318     ///
 319     /// here, `A` will be covariant, but `B` is
 320     /// unconstrained. However, whatever it is, for `Foo` to be WF, it
 321     /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
 322     /// then after generalization we will wind up with a type like
 323     /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
 324     /// ?D>` (or `>:`), we will wind up with the requirement that `?A
 325     /// <: ?C`, but no particular relationship between `?B` and `?D`
 326     /// (after all, we do not know the variance of the normalized form
 327     /// of `A::Item` with respect to `A`). If we do nothing else, this
 328     /// may mean that `?D` goes unconstrained (as in #41677).  So, in
 329     /// this scenario where we create a new type variable in a
 330     /// bivariant context, we set the `needs_wf` flag to true. This
 331     /// will force the calling code to check that `WF(Foo<?C, ?D>)`
 332     /// holds, which in turn implies that `?C::Item == ?D`. So once
 333     /// `?C` is constrained, that should suffice to restrict `?D`.
 334     needs_wf: bool,
 335 }
 336
 337 impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> {
 338     fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
 339         self.infcx.tcx
 340     }
 341
 342     fn tag(&self) -> &'static str {
 343         "Generalizer"
 344     }
 345
 346     fn a_is_expected(&self) -> bool {
 347         true
 348     }
 349
 350     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
 351                   -> RelateResult<'tcx, ty::Binder<T>>
 352         where T: Relate<'tcx>
 353     {
 354         Ok(ty::Binder::bind(self.relate(a.skip_binder(), b.skip_binder())?))
 355     }
 356
 357     fn relate_item_substs(&mut self,
 358                           item_def_id: DefId,
 359                           a_subst: &'tcx Substs<'tcx>,
 360                           b_subst: &'tcx Substs<'tcx>)
 361                           -> RelateResult<'tcx, &'tcx Substs<'tcx>>
 362     {
 363         if self.ambient_variance == ty::Variance::Invariant {
 364             // Avoid fetching the variance if we are in an invariant
 365             // context; no need, and it can induce dependency cycles
 366             // (e.g. #41849).
 367             relate::relate_substs(self, None, a_subst, b_subst)
 368         } else {
 369             let opt_variances = self.tcx().variances_of(item_def_id);
 370             relate::relate_substs(self, Some(&opt_variances), a_subst, b_subst)
 371         }
 372     }
 373
 374     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
 375                                              variance: ty::Variance,
 376                                              a: &T,
 377                                              b: &T)
 378                                              -> RelateResult<'tcx, T>
 379     {
 380         let old_ambient_variance = self.ambient_variance;
 381         self.ambient_variance = self.ambient_variance.xform(variance);
 382
 383         let result = self.relate(a, b);
 384         self.ambient_variance = old_ambient_variance;
 385         result
 386     }
 387
 388     fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
 389         assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 390
 391         // Check to see whether the type we are genealizing references
 392         // any other type variable related to `vid` via
 393         // subtyping. This is basically our "occurs check", preventing
 394         // us from creating infinitely sized types.
 395         match t.sty {
 396             ty::Infer(ty::TyVar(vid)) => {
 397                 let mut variables = self.infcx.type_variables.borrow_mut();
 398                 let vid = variables.root_var(vid);
 399                 let sub_vid = variables.sub_root_var(vid);
 400                 if sub_vid == self.for_vid_sub_root {
 401                     // If sub-roots are equal, then `for_vid` and
 402                     // `vid` are related via subtyping.
 403                     return Err(TypeError::CyclicTy(self.root_ty));
 404                 } else {
 405                     match variables.probe(vid) {
 406                         TypeVariableValue::Known { value: u } => {
 407                             drop(variables);
 408                             self.relate(&u, &u)
 409                         }
 410                         TypeVariableValue::Unknown { universe } => {
 411                             match self.ambient_variance {
 412                                 // Invariant: no need to make a fresh type variable.
 413                                 ty::Invariant => return Ok(t),
 414
 415                                 // Bivariant: make a fresh var, but we
 416                                 // may need a WF predicate. See
 417                                 // comment on `needs_wf` field for
 418                                 // more info.
 419                                 ty::Bivariant => self.needs_wf = true,
 420
 421                                 // Co/contravariant: this will be
 422                                 // sufficiently constrained later on.
 423                                 ty::Covariant | ty::Contravariant => (),
 424                             }
 425
 426                             let origin = *variables.var_origin(vid);
 427                             let new_var_id = variables.new_var(universe, false, origin);
 428                             let u = self.tcx().mk_var(new_var_id);
 429                             debug!("generalize: replacing original vid={:?} with new={:?}",
 430                                    vid, u);
 431                             return Ok(u);
 432                         }
 433                     }
 434                 }
 435             }
 436             ty::Infer(ty::IntVar(_)) |
 437             ty::Infer(ty::FloatVar(_)) => {
 438                 // No matter what mode we are in,
 439                 // integer/floating-point types must be equal to be
 440                 // relatable.
 441                 Ok(t)
 442             }
 443             _ => {
 444                 relate::super_relate_tys(self, t, t)
 445             }
 446         }
 447     }
 448
 449     fn regions(&mut self, r: ty::Region<'tcx>, r2: ty::Region<'tcx>)
 450                -> RelateResult<'tcx, ty::Region<'tcx>> {
 451         assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 452
 453         match *r {
 454             // Never make variables for regions bound within the type itself,
 455             // nor for erased regions.
 456             ty::ReLateBound(..) |
 457             ty::ReErased => {
 458                 return Ok(r);
 459             }
 460
 461             // Always make a fresh region variable for placeholder
 462             // regions; the higher-ranked decision procedures rely on
 463             // this.
 464             ty::RePlaceholder(..) => { }
 465
 466             // For anything else, we make a region variable, unless we
 467             // are *equating*, in which case it's just wasteful.
 468             ty::ReEmpty |
 469             ty::ReStatic |
 470             ty::ReScope(..) |
 471             ty::ReVar(..) |
 472             ty::ReEarlyBound(..) |
 473             ty::ReFree(..) => {
 474                 match self.ambient_variance {
 475                     ty::Invariant => return Ok(r),
 476                     ty::Bivariant | ty::Covariant | ty::Contravariant => (),
 477                 }
 478             }
 479
 480             ty::ReCanonical(..) |
 481             ty::ReClosureBound(..) => {
 482                 span_bug!(
 483                     self.span,
 484                     "encountered unexpected ReClosureBound: {:?}",
 485                     r,
 486                 );
 487             }
 488         }
 489
 490         // FIXME: This is non-ideal because we don't give a
 491         // very descriptive origin for this region variable.
 492         Ok(self.infcx.next_region_var(MiscVariable(self.span)))
 493     }
 494 }
 495
 496 pub trait RelateResultCompare<'tcx, T> {
 497     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 498         F: FnOnce() -> TypeError<'tcx>;
 499 }
 500
 501 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
 502     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 503         F: FnOnce() -> TypeError<'tcx>,
 504     {
 505         self.clone().and_then(|s| {
 506             if s == t {
 507                 self.clone()
 508             } else {
 509                 Err(f())
 510             }
 511         })
 512     }
 513 }
 514
 515 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
 516                                -> TypeError<'tcx>
 517 {
 518     let (a, b) = v;
 519     TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 520 }
 521
 522 fn float_unification_error<'tcx>(a_is_expected: bool,
 523                                  v: (ty::FloatVarValue, ty::FloatVarValue))
 524                                  -> TypeError<'tcx>
 525 {
 526     let (ty::FloatVarValue(a), ty::FloatVarValue(b)) = v;
 527     TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 528 }