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::lub::Lub;
  38 use super::sub::Sub;
  39 use super::InferCtxt;
  40 use super::{MiscVariable, TypeTrace};
  41
  42 use ty::{IntType, UintType};
  43 use ty::{self, Ty, TyCtxt};
  44 use ty::error::TypeError;
  45 use ty::relate::{self, Relate, RelateResult, TypeRelation};
  46 use traits::PredicateObligations;
  47
  48 use syntax::ast;
  49 use syntax_pos::Span;
  50
  51 #[derive(Clone)]
  52 pub struct CombineFields<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
  53     pub infcx: &'infcx InferCtxt<'infcx, 'gcx, 'tcx>,
  54     pub trace: TypeTrace<'tcx>,
  55     pub cause: Option<ty::relate::Cause>,
  56     pub obligations: PredicateObligations<'tcx>,
  57 }
  58
  59 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
  60 pub enum RelationDir {
  61     SubtypeOf, SupertypeOf, EqTo
  62 }
  63
  64 impl<'infcx, 'gcx, 'tcx> InferCtxt<'infcx, 'gcx, 'tcx> {
  65     pub fn super_combine_tys<R>(&self,
  66                                 relation: &mut R,
  67                                 a: Ty<'tcx>,
  68                                 b: Ty<'tcx>)
  69                                 -> RelateResult<'tcx, Ty<'tcx>>
  70         where R: TypeRelation<'infcx, 'gcx, 'tcx>
  71     {
  72         let a_is_expected = relation.a_is_expected();
  73
  74         match (&a.sty, &b.sty) {
  75             // Relate integral variables to other types
  76             (&ty::TyInfer(ty::IntVar(a_id)), &ty::TyInfer(ty::IntVar(b_id))) => {
  77                 self.int_unification_table
  78                     .borrow_mut()
  79                     .unify_var_var(a_id, b_id)
  80                     .map_err(|e| int_unification_error(a_is_expected, e))?;
  81                 Ok(a)
  82             }
  83             (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyInt(v)) => {
  84                 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
  85             }
  86             (&ty::TyInt(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  87                 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
  88             }
  89             (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyUint(v)) => {
  90                 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
  91             }
  92             (&ty::TyUint(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  93                 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
  94             }
  95
  96             // Relate floating-point variables to other types
  97             (&ty::TyInfer(ty::FloatVar(a_id)), &ty::TyInfer(ty::FloatVar(b_id))) => {
  98                 self.float_unification_table
  99                     .borrow_mut()
 100                     .unify_var_var(a_id, b_id)
 101                     .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
 102                 Ok(a)
 103             }
 104             (&ty::TyInfer(ty::FloatVar(v_id)), &ty::TyFloat(v)) => {
 105                 self.unify_float_variable(a_is_expected, v_id, v)
 106             }
 107             (&ty::TyFloat(v), &ty::TyInfer(ty::FloatVar(v_id))) => {
 108                 self.unify_float_variable(!a_is_expected, v_id, v)
 109             }
 110
 111             // All other cases of inference are errors
 112             (&ty::TyInfer(_), _) |
 113             (_, &ty::TyInfer(_)) => {
 114                 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
 115             }
 116
 117
 118             _ => {
 119                 ty::relate::super_relate_tys(relation, a, b)
 120             }
 121         }
 122     }
 123
 124     fn unify_integral_variable(&self,
 125                                vid_is_expected: bool,
 126                                vid: ty::IntVid,
 127                                val: ty::IntVarValue)
 128                                -> RelateResult<'tcx, Ty<'tcx>>
 129     {
 130         self.int_unification_table
 131             .borrow_mut()
 132             .unify_var_value(vid, val)
 133             .map_err(|e| int_unification_error(vid_is_expected, e))?;
 134         match val {
 135             IntType(v) => Ok(self.tcx.mk_mach_int(v)),
 136             UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
 137         }
 138     }
 139
 140     fn unify_float_variable(&self,
 141                             vid_is_expected: bool,
 142                             vid: ty::FloatVid,
 143                             val: ast::FloatTy)
 144                             -> RelateResult<'tcx, Ty<'tcx>>
 145     {
 146         self.float_unification_table
 147             .borrow_mut()
 148             .unify_var_value(vid, val)
 149             .map_err(|e| float_unification_error(vid_is_expected, e))?;
 150         Ok(self.tcx.mk_mach_float(val))
 151     }
 152 }
 153
 154 impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
 155     pub fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> {
 156         self.infcx.tcx
 157     }
 158
 159     pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'gcx, 'tcx> {
 160         Equate::new(self, a_is_expected)
 161     }
 162
 163     pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'gcx, 'tcx> {
 164         Sub::new(self, a_is_expected)
 165     }
 166
 167     pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'gcx, 'tcx> {
 168         Lub::new(self, a_is_expected)
 169     }
 170
 171     pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'gcx, 'tcx> {
 172         Glb::new(self, a_is_expected)
 173     }
 174
 175     /// Here dir is either EqTo, SubtypeOf, or SupertypeOf. The
 176     /// idea is that we should ensure that the type `a_ty` is equal
 177     /// to, a subtype of, or a supertype of (respectively) the type
 178     /// to which `b_vid` is bound.
 179     ///
 180     /// Since `b_vid` has not yet been instantiated with a type, we
 181     /// will first instantiate `b_vid` with a *generalized* version
 182     /// of `a_ty`. Generalization introduces other inference
 183     /// variables wherever subtyping could occur.
 184     pub fn instantiate(&mut self,
 185                        a_ty: Ty<'tcx>,
 186                        dir: RelationDir,
 187                        b_vid: ty::TyVid,
 188                        a_is_expected: bool)
 189                        -> RelateResult<'tcx, ()>
 190     {
 191         use self::RelationDir::*;
 192
 193         // Get the actual variable that b_vid has been inferred to
 194         debug_assert!(self.infcx.type_variables.borrow_mut().probe(b_vid).is_none());
 195
 196         debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})", a_ty, dir, b_vid);
 197
 198         // Generalize type of `a_ty` appropriately depending on the
 199         // direction.  As an example, assume:
 200         //
 201         // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
 202         //   inference variable,
 203         // - and `dir` == `SubtypeOf`.
 204         //
 205         // Then the generalized form `b_ty` would be `&'?2 ?3`, where
 206         // `'?2` and `?3` are fresh region/type inference
 207         // variables. (Down below, we will relate `a_ty <: b_ty`,
 208         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
 209         let b_ty = self.generalize(a_ty, b_vid, dir)?;
 210         debug!("instantiate(a_ty={:?}, dir={:?}, b_vid={:?}, generalized b_ty={:?})",
 211                a_ty, dir, b_vid, b_ty);
 212         self.infcx.type_variables.borrow_mut().instantiate(b_vid, b_ty);
 213
 214         // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
 215         //
 216         // FIXME(#16847): This code is non-ideal because all these subtype
 217         // relations wind up attributed to the same spans. We need
 218         // to associate causes/spans with each of the relations in
 219         // the stack to get this right.
 220         match dir {
 221             EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
 222             SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
 223             SupertypeOf => self.sub(a_is_expected).relate_with_variance(
 224                 ty::Contravariant, &a_ty, &b_ty),
 225         }?;
 226
 227         Ok(())
 228     }
 229
 230     /// Attempts to generalize `ty` for the type variable `for_vid`.
 231     /// This checks for cycle -- that is, whether the type `ty`
 232     /// references `for_vid`. If `is_eq_relation` is false, it will
 233     /// also replace all regions/unbound-type-variables with fresh
 234     /// variables. Returns `TyError` in the case of a cycle, `Ok`
 235     /// otherwise.
 236     ///
 237     /// Preconditions:
 238     ///
 239     /// - `for_vid` is a "root vid"
 240     fn generalize(&self,
 241                   ty: Ty<'tcx>,
 242                   for_vid: ty::TyVid,
 243                   dir: RelationDir)
 244                   -> RelateResult<'tcx, Ty<'tcx>>
 245     {
 246         // Determine the ambient variance within which `ty` appears.
 247         // The surrounding equation is:
 248         //
 249         //     ty [op] ty2
 250         //
 251         // where `op` is either `==`, `<:`, or `:>`. This maps quite
 252         // naturally.
 253         let ambient_variance = match dir {
 254             RelationDir::EqTo => ty::Invariant,
 255             RelationDir::SubtypeOf => ty::Covariant,
 256             RelationDir::SupertypeOf => ty::Contravariant,
 257         };
 258
 259         let mut generalize = Generalizer {
 260             infcx: self.infcx,
 261             span: self.trace.cause.span,
 262             for_vid_sub_root: self.infcx.type_variables.borrow_mut().sub_root_var(for_vid),
 263             ambient_variance: ambient_variance,
 264         };
 265
 266         generalize.relate(&ty, &ty)
 267     }
 268 }
 269
 270 struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
 271     infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
 272     span: Span,
 273     for_vid_sub_root: ty::TyVid,
 274     ambient_variance: ty::Variance,
 275 }
 276
 277 impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> {
 278     fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
 279         self.infcx.tcx
 280     }
 281
 282     fn tag(&self) -> &'static str {
 283         "Generalizer"
 284     }
 285
 286     fn a_is_expected(&self) -> bool {
 287         true
 288     }
 289
 290     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
 291                   -> RelateResult<'tcx, ty::Binder<T>>
 292         where T: Relate<'tcx>
 293     {
 294         Ok(ty::Binder(self.relate(a.skip_binder(), b.skip_binder())?))
 295     }
 296
 297     fn relate_with_variance<T: Relate<'tcx>>(&mut self,
 298                                              variance: ty::Variance,
 299                                              a: &T,
 300                                              b: &T)
 301                                              -> RelateResult<'tcx, T>
 302     {
 303         let old_ambient_variance = self.ambient_variance;
 304         self.ambient_variance = self.ambient_variance.xform(variance);
 305
 306         let result = self.relate(a, b);
 307         self.ambient_variance = old_ambient_variance;
 308         result
 309     }
 310
 311     fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
 312         assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 313
 314         // Check to see whether the type we are genealizing references
 315         // any other type variable related to `vid` via
 316         // subtyping. This is basically our "occurs check", preventing
 317         // us from creating infinitely sized types.
 318         match t.sty {
 319             ty::TyInfer(ty::TyVar(vid)) => {
 320                 let mut variables = self.infcx.type_variables.borrow_mut();
 321                 let vid = variables.root_var(vid);
 322                 let sub_vid = variables.sub_root_var(vid);
 323                 if sub_vid == self.for_vid_sub_root {
 324                     // If sub-roots are equal, then `for_vid` and
 325                     // `vid` are related via subtyping.
 326                     return Err(TypeError::CyclicTy);
 327                 } else {
 328                     match variables.probe_root(vid) {
 329                         Some(u) => {
 330                             drop(variables);
 331                             self.relate(&u, &u)
 332                         }
 333                         None => {
 334                             match self.ambient_variance {
 335                                 ty::Invariant => Ok(t),
 336
 337                                 ty::Bivariant | ty::Covariant | ty::Contravariant => {
 338                                     let origin = variables.origin(vid);
 339                                     let new_var_id = variables.new_var(false, origin, None);
 340                                     let u = self.tcx().mk_var(new_var_id);
 341                                     debug!("generalize: replacing original vid={:?} with new={:?}",
 342                                            vid, u);
 343                                     Ok(u)
 344                                 }
 345                             }
 346                         }
 347                     }
 348                 }
 349             }
 350             ty::TyInfer(ty::IntVar(_)) |
 351             ty::TyInfer(ty::FloatVar(_)) => {
 352                 // No matter what mode we are in,
 353                 // integer/floating-point types must be equal to be
 354                 // relatable.
 355                 Ok(t)
 356             }
 357             _ => {
 358                 relate::super_relate_tys(self, t, t)
 359             }
 360         }
 361     }
 362
 363     fn regions(&mut self, r: ty::Region<'tcx>, r2: ty::Region<'tcx>)
 364                -> RelateResult<'tcx, ty::Region<'tcx>> {
 365         assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 366
 367         match *r {
 368             // Never make variables for regions bound within the type itself,
 369             // nor for erased regions.
 370             ty::ReLateBound(..) |
 371             ty::ReErased => {
 372                 return Ok(r);
 373             }
 374
 375             // Early-bound regions should really have been substituted away before
 376             // we get to this point.
 377             ty::ReEarlyBound(..) => {
 378                 span_bug!(
 379                     self.span,
 380                     "Encountered early bound region when generalizing: {:?}",
 381                     r);
 382             }
 383
 384             // Always make a fresh region variable for skolemized regions;
 385             // the higher-ranked decision procedures rely on this.
 386             ty::ReSkolemized(..) => { }
 387
 388             // For anything else, we make a region variable, unless we
 389             // are *equating*, in which case it's just wasteful.
 390             ty::ReEmpty |
 391             ty::ReStatic |
 392             ty::ReScope(..) |
 393             ty::ReVar(..) |
 394             ty::ReFree(..) => {
 395                 match self.ambient_variance {
 396                     ty::Invariant => return Ok(r),
 397                     ty::Bivariant | ty::Covariant | ty::Contravariant => (),
 398                 }
 399             }
 400         }
 401
 402         // FIXME: This is non-ideal because we don't give a
 403         // very descriptive origin for this region variable.
 404         Ok(self.infcx.next_region_var(MiscVariable(self.span)))
 405     }
 406 }
 407
 408 pub trait RelateResultCompare<'tcx, T> {
 409     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 410         F: FnOnce() -> TypeError<'tcx>;
 411 }
 412
 413 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
 414     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 415         F: FnOnce() -> TypeError<'tcx>,
 416     {
 417         self.clone().and_then(|s| {
 418             if s == t {
 419                 self.clone()
 420             } else {
 421                 Err(f())
 422             }
 423         })
 424     }
 425 }
 426
 427 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
 428                                -> TypeError<'tcx>
 429 {
 430     let (a, b) = v;
 431     TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 432 }
 433
 434 fn float_unification_error<'tcx>(a_is_expected: bool,
 435                                  v: (ast::FloatTy, ast::FloatTy))
 436                                  -> TypeError<'tcx>
 437 {
 438     let (a, b) = v;
 439     TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 440 }