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::bivariate::Bivariate;
  36 use super::equate::Equate;
  37 use super::glb::Glb;
  38 use super::lub::Lub;
  39 use super::sub::Sub;
  40 use super::InferCtxt;
  41 use super::{MiscVariable, TypeTrace};
  42 use super::type_variable::{RelationDir, BiTo, EqTo, SubtypeOf, SupertypeOf};
  43
  44 use ty::{IntType, UintType};
  45 use ty::{self, Ty, TyCtxt};
  46 use ty::error::TypeError;
  47 use ty::fold::TypeFoldable;
  48 use ty::relate::{RelateResult, TypeRelation};
  49 use traits::PredicateObligations;
  50
  51 use syntax::ast;
  52 use syntax_pos::Span;
  53
  54 #[derive(Clone)]
  55 pub struct CombineFields<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
  56     pub infcx: &'infcx InferCtxt<'infcx, 'gcx, 'tcx>,
  57     pub trace: TypeTrace<'tcx>,
  58     pub cause: Option<ty::relate::Cause>,
  59     pub obligations: PredicateObligations<'tcx>,
  60 }
  61
  62 impl<'infcx, 'gcx, 'tcx> InferCtxt<'infcx, 'gcx, 'tcx> {
  63     pub fn super_combine_tys<R>(&self,
  64                                 relation: &mut R,
  65                                 a: Ty<'tcx>,
  66                                 b: Ty<'tcx>)
  67                                 -> RelateResult<'tcx, Ty<'tcx>>
  68         where R: TypeRelation<'infcx, 'gcx, '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::TyInfer(ty::IntVar(a_id)), &ty::TyInfer(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::TyInfer(ty::IntVar(v_id)), &ty::TyInt(v)) => {
  82                 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
  83             }
  84             (&ty::TyInt(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  85                 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
  86             }
  87             (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyUint(v)) => {
  88                 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
  89             }
  90             (&ty::TyUint(v), &ty::TyInfer(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::TyInfer(ty::FloatVar(a_id)), &ty::TyInfer(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::TyInfer(ty::FloatVar(v_id)), &ty::TyFloat(v)) => {
 103                 self.unify_float_variable(a_is_expected, v_id, v)
 104             }
 105             (&ty::TyFloat(v), &ty::TyInfer(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::TyInfer(_), _) |
 111             (_, &ty::TyInfer(_)) => {
 112                 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
 113             }
 114
 115
 116             _ => {
 117                 ty::relate::super_relate_tys(relation, a, b)
 118             }
 119         }
 120     }
 121
 122     fn unify_integral_variable(&self,
 123                                vid_is_expected: bool,
 124                                vid: ty::IntVid,
 125                                val: ty::IntVarValue)
 126                                -> RelateResult<'tcx, Ty<'tcx>>
 127     {
 128         self.int_unification_table
 129             .borrow_mut()
 130             .unify_var_value(vid, val)
 131             .map_err(|e| int_unification_error(vid_is_expected, e))?;
 132         match val {
 133             IntType(v) => Ok(self.tcx.mk_mach_int(v)),
 134             UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
 135         }
 136     }
 137
 138     fn unify_float_variable(&self,
 139                             vid_is_expected: bool,
 140                             vid: ty::FloatVid,
 141                             val: ast::FloatTy)
 142                             -> RelateResult<'tcx, Ty<'tcx>>
 143     {
 144         self.float_unification_table
 145             .borrow_mut()
 146             .unify_var_value(vid, val)
 147             .map_err(|e| float_unification_error(vid_is_expected, e))?;
 148         Ok(self.tcx.mk_mach_float(val))
 149     }
 150 }
 151
 152 impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
 153     pub fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> {
 154         self.infcx.tcx
 155     }
 156
 157     pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'gcx, 'tcx> {
 158         Equate::new(self, a_is_expected)
 159     }
 160
 161     pub fn bivariate<'a>(&'a mut self, a_is_expected: bool) -> Bivariate<'a, 'infcx, 'gcx, 'tcx> {
 162         Bivariate::new(self, a_is_expected)
 163     }
 164
 165     pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'gcx, 'tcx> {
 166         Sub::new(self, a_is_expected)
 167     }
 168
 169     pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'gcx, 'tcx> {
 170         Lub::new(self, a_is_expected)
 171     }
 172
 173     pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'gcx, 'tcx> {
 174         Glb::new(self, a_is_expected)
 175     }
 176
 177     pub fn instantiate(&mut self,
 178                        a_ty: Ty<'tcx>,
 179                        dir: RelationDir,
 180                        b_vid: ty::TyVid,
 181                        a_is_expected: bool)
 182                        -> RelateResult<'tcx, ()>
 183     {
 184         let mut stack = Vec::new();
 185         stack.push((a_ty, dir, b_vid));
 186         loop {
 187             // For each turn of the loop, we extract a tuple
 188             //
 189             //     (a_ty, dir, b_vid)
 190             //
 191             // to relate. Here dir is either SubtypeOf or
 192             // SupertypeOf. The idea is that we should ensure that
 193             // the type `a_ty` is a subtype or supertype (respectively) of the
 194             // type to which `b_vid` is bound.
 195             //
 196             // If `b_vid` has not yet been instantiated with a type
 197             // (which is always true on the first iteration, but not
 198             // necessarily true on later iterations), we will first
 199             // instantiate `b_vid` with a *generalized* version of
 200             // `a_ty`. Generalization introduces other inference
 201             // variables wherever subtyping could occur (at time of
 202             // this writing, this means replacing free regions with
 203             // region variables).
 204             let (a_ty, dir, b_vid) = match stack.pop() {
 205                 None => break,
 206                 Some(e) => e,
 207             };
 208             // Get the actual variable that b_vid has been inferred to
 209             let (b_vid, b_ty) = {
 210                 let mut variables = self.infcx.type_variables.borrow_mut();
 211                 let b_vid = variables.root_var(b_vid);
 212                 (b_vid, variables.probe_root(b_vid))
 213             };
 214
 215             debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})",
 216                    a_ty,
 217                    dir,
 218                    b_vid);
 219
 220             // Check whether `vid` has been instantiated yet.  If not,
 221             // make a generalized form of `ty` and instantiate with
 222             // that.
 223             let b_ty = match b_ty {
 224                 Some(t) => t, // ...already instantiated.
 225                 None => {     // ...not yet instantiated:
 226                     // Generalize type if necessary.
 227                     let generalized_ty = match dir {
 228                         EqTo => self.generalize(a_ty, b_vid, false),
 229                         BiTo | SupertypeOf | SubtypeOf => self.generalize(a_ty, b_vid, true),
 230                     }?;
 231                     debug!("instantiate(a_ty={:?}, dir={:?}, \
 232                                         b_vid={:?}, generalized_ty={:?})",
 233                            a_ty, dir, b_vid,
 234                            generalized_ty);
 235                     self.infcx.type_variables
 236                         .borrow_mut()
 237                         .instantiate_and_push(
 238                             b_vid, generalized_ty, &mut stack);
 239                     generalized_ty
 240                 }
 241             };
 242
 243             // The original triple was `(a_ty, dir, b_vid)` -- now we have
 244             // resolved `b_vid` to `b_ty`, so apply `(a_ty, dir, b_ty)`:
 245             //
 246             // FIXME(#16847): This code is non-ideal because all these subtype
 247             // relations wind up attributed to the same spans. We need
 248             // to associate causes/spans with each of the relations in
 249             // the stack to get this right.
 250             match dir {
 251                 BiTo => self.bivariate(a_is_expected).relate(&a_ty, &b_ty),
 252                 EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
 253                 SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
 254                 SupertypeOf => self.sub(a_is_expected).relate_with_variance(
 255                     ty::Contravariant, &a_ty, &b_ty),
 256             }?;
 257         }
 258
 259         Ok(())
 260     }
 261
 262     /// Attempts to generalize `ty` for the type variable `for_vid`.  This checks for cycle -- that
 263     /// is, whether the type `ty` references `for_vid`. If `make_region_vars` is true, it will also
 264     /// replace all regions with fresh variables. Returns `TyError` in the case of a cycle, `Ok`
 265     /// otherwise.
 266     fn generalize(&self,
 267                   ty: Ty<'tcx>,
 268                   for_vid: ty::TyVid,
 269                   make_region_vars: bool)
 270                   -> RelateResult<'tcx, Ty<'tcx>>
 271     {
 272         let mut generalize = Generalizer {
 273             infcx: self.infcx,
 274             span: self.trace.origin.span(),
 275             for_vid: for_vid,
 276             make_region_vars: make_region_vars,
 277             cycle_detected: false
 278         };
 279         let u = ty.fold_with(&mut generalize);
 280         if generalize.cycle_detected {
 281             Err(TypeError::CyclicTy)
 282         } else {
 283             Ok(u)
 284         }
 285     }
 286 }
 287
 288 struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
 289     infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
 290     span: Span,
 291     for_vid: ty::TyVid,
 292     make_region_vars: bool,
 293     cycle_detected: bool,
 294 }
 295
 296 impl<'cx, 'gcx, 'tcx> ty::fold::TypeFolder<'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> {
 297     fn tcx<'a>(&'a self) -> TyCtxt<'a, 'gcx, 'tcx> {
 298         self.infcx.tcx
 299     }
 300
 301     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 302         // Check to see whether the type we are genealizing references
 303         // `vid`. At the same time, also update any type variables to
 304         // the values that they are bound to. This is needed to truly
 305         // check for cycles, but also just makes things readable.
 306         //
 307         // (In particular, you could have something like `$0 = Box<$1>`
 308         //  where `$1` has already been instantiated with `Box<$0>`)
 309         match t.sty {
 310             ty::TyInfer(ty::TyVar(vid)) => {
 311                 let mut variables = self.infcx.type_variables.borrow_mut();
 312                 let vid = variables.root_var(vid);
 313                 if vid == self.for_vid {
 314                     self.cycle_detected = true;
 315                     self.tcx().types.err
 316                 } else {
 317                     match variables.probe_root(vid) {
 318                         Some(u) => {
 319                             drop(variables);
 320                             self.fold_ty(u)
 321                         }
 322                         None => t,
 323                     }
 324                 }
 325             }
 326             _ => {
 327                 t.super_fold_with(self)
 328             }
 329         }
 330     }
 331
 332     fn fold_region(&mut self, r: &'tcx ty::Region) -> &'tcx ty::Region {
 333         match *r {
 334             // Never make variables for regions bound within the type itself,
 335             // nor for erased regions.
 336             ty::ReLateBound(..) |
 337             ty::ReErased => { return r; }
 338
 339             // Early-bound regions should really have been substituted away before
 340             // we get to this point.
 341             ty::ReEarlyBound(..) => {
 342                 span_bug!(
 343                     self.span,
 344                     "Encountered early bound region when generalizing: {:?}",
 345                     r);
 346             }
 347
 348             // Always make a fresh region variable for skolemized regions;
 349             // the higher-ranked decision procedures rely on this.
 350             ty::ReSkolemized(..) => { }
 351
 352             // For anything else, we make a region variable, unless we
 353             // are *equating*, in which case it's just wasteful.
 354             ty::ReEmpty |
 355             ty::ReStatic |
 356             ty::ReScope(..) |
 357             ty::ReVar(..) |
 358             ty::ReFree(..) => {
 359                 if !self.make_region_vars {
 360                     return r;
 361                 }
 362             }
 363         }
 364
 365         // FIXME: This is non-ideal because we don't give a
 366         // very descriptive origin for this region variable.
 367         self.infcx.next_region_var(MiscVariable(self.span))
 368     }
 369 }
 370
 371 pub trait RelateResultCompare<'tcx, T> {
 372     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 373         F: FnOnce() -> TypeError<'tcx>;
 374 }
 375
 376 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
 377     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 378         F: FnOnce() -> TypeError<'tcx>,
 379     {
 380         self.clone().and_then(|s| {
 381             if s == t {
 382                 self.clone()
 383             } else {
 384                 Err(f())
 385             }
 386         })
 387     }
 388 }
 389
 390 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
 391                                -> TypeError<'tcx>
 392 {
 393     let (a, b) = v;
 394     TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 395 }
 396
 397 fn float_unification_error<'tcx>(a_is_expected: bool,
 398                                  v: (ast::FloatTy, ast::FloatTy))
 399                                  -> TypeError<'tcx>
 400 {
 401     let (a, b) = v;
 402     TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 403 }