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::{TypeFolder, TypeFoldable};
  48 use ty::relate::{Relate, RelateResult, TypeRelation};
  49 use traits::PredicateObligations;
  50
  51 use syntax::ast;
  52 use syntax::codemap::Span;
  53
  54 #[derive(Clone)]
  55 pub struct CombineFields<'a, 'tcx: 'a> {
  56     pub infcx: &'a InferCtxt<'a, 'tcx>,
  57     pub a_is_expected: bool,
  58     pub trace: TypeTrace<'tcx>,
  59     pub cause: Option<ty::relate::Cause>,
  60     pub obligations: PredicateObligations<'tcx>,
  61 }
  62
  63 pub fn super_combine_tys<'a,'tcx:'a,R>(infcx: &InferCtxt<'a, 'tcx>,
  64                                        relation: &mut R,
  65                                        a: Ty<'tcx>,
  66                                        b: Ty<'tcx>)
  67                                        -> RelateResult<'tcx, Ty<'tcx>>
  68     where R: TypeRelation<'a,'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             infcx.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             unify_integral_variable(infcx, a_is_expected, v_id, IntType(v))
  83         }
  84         (&ty::TyInt(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  85             unify_integral_variable(infcx, !a_is_expected, v_id, IntType(v))
  86         }
  87         (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyUint(v)) => {
  88             unify_integral_variable(infcx, a_is_expected, v_id, UintType(v))
  89         }
  90         (&ty::TyUint(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  91             unify_integral_variable(infcx, !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             infcx.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             unify_float_variable(infcx, a_is_expected, v_id, v)
 104         }
 105         (&ty::TyFloat(v), &ty::TyInfer(ty::FloatVar(v_id))) => {
 106             unify_float_variable(infcx, !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<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
 123                                     vid_is_expected: bool,
 124                                     vid: ty::IntVid,
 125                                     val: ty::IntVarValue)
 126                                     -> RelateResult<'tcx, Ty<'tcx>>
 127 {
 128     infcx.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(infcx.tcx.mk_mach_int(v)),
 134         UintType(v) => Ok(infcx.tcx.mk_mach_uint(v)),
 135     }
 136 }
 137
 138 fn unify_float_variable<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
 139                                  vid_is_expected: bool,
 140                                  vid: ty::FloatVid,
 141                                  val: ast::FloatTy)
 142                                  -> RelateResult<'tcx, Ty<'tcx>>
 143 {
 144     infcx.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(infcx.tcx.mk_mach_float(val))
 149 }
 150
 151 impl<'a, 'tcx> CombineFields<'a, 'tcx> {
 152     pub fn tcx(&self) -> &'a TyCtxt<'tcx> {
 153         self.infcx.tcx
 154     }
 155
 156     pub fn switch_expected(&self) -> CombineFields<'a, 'tcx> {
 157         CombineFields {
 158             a_is_expected: !self.a_is_expected,
 159             ..(*self).clone()
 160         }
 161     }
 162
 163     pub fn equate(&self) -> Equate<'a, 'tcx> {
 164         Equate::new(self.clone())
 165     }
 166
 167     pub fn bivariate(&self) -> Bivariate<'a, 'tcx> {
 168         Bivariate::new(self.clone())
 169     }
 170
 171     pub fn sub(&self) -> Sub<'a, 'tcx> {
 172         Sub::new(self.clone())
 173     }
 174
 175     pub fn lub(&self) -> Lub<'a, 'tcx> {
 176         Lub::new(self.clone())
 177     }
 178
 179     pub fn glb(&self) -> Glb<'a, 'tcx> {
 180         Glb::new(self.clone())
 181     }
 182
 183     pub fn instantiate(&self,
 184                        a_ty: Ty<'tcx>,
 185                        dir: RelationDir,
 186                        b_vid: ty::TyVid)
 187                        -> RelateResult<'tcx, ()>
 188     {
 189         let mut stack = Vec::new();
 190         stack.push((a_ty, dir, b_vid));
 191         loop {
 192             // For each turn of the loop, we extract a tuple
 193             //
 194             //     (a_ty, dir, b_vid)
 195             //
 196             // to relate. Here dir is either SubtypeOf or
 197             // SupertypeOf. The idea is that we should ensure that
 198             // the type `a_ty` is a subtype or supertype (respectively) of the
 199             // type to which `b_vid` is bound.
 200             //
 201             // If `b_vid` has not yet been instantiated with a type
 202             // (which is always true on the first iteration, but not
 203             // necessarily true on later iterations), we will first
 204             // instantiate `b_vid` with a *generalized* version of
 205             // `a_ty`. Generalization introduces other inference
 206             // variables wherever subtyping could occur (at time of
 207             // this writing, this means replacing free regions with
 208             // region variables).
 209             let (a_ty, dir, b_vid) = match stack.pop() {
 210                 None => break,
 211                 Some(e) => e,
 212             };
 213             // Get the actual variable that b_vid has been inferred to
 214             let (b_vid, b_ty) = {
 215                 let mut variables = self.infcx.type_variables.borrow_mut();
 216                 let b_vid = variables.root_var(b_vid);
 217                 (b_vid, variables.probe_root(b_vid))
 218             };
 219
 220             debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})",
 221                    a_ty,
 222                    dir,
 223                    b_vid);
 224
 225             // Check whether `vid` has been instantiated yet.  If not,
 226             // make a generalized form of `ty` and instantiate with
 227             // that.
 228             let b_ty = match b_ty {
 229                 Some(t) => t, // ...already instantiated.
 230                 None => {     // ...not yet instantiated:
 231                     // Generalize type if necessary.
 232                     let generalized_ty = match dir {
 233                         EqTo => self.generalize(a_ty, b_vid, false),
 234                         BiTo | SupertypeOf | SubtypeOf => self.generalize(a_ty, b_vid, true),
 235                     }?;
 236                     debug!("instantiate(a_ty={:?}, dir={:?}, \
 237                                         b_vid={:?}, generalized_ty={:?})",
 238                            a_ty, dir, b_vid,
 239                            generalized_ty);
 240                     self.infcx.type_variables
 241                         .borrow_mut()
 242                         .instantiate_and_push(
 243                             b_vid, generalized_ty, &mut stack);
 244                     generalized_ty
 245                 }
 246             };
 247
 248             // The original triple was `(a_ty, dir, b_vid)` -- now we have
 249             // resolved `b_vid` to `b_ty`, so apply `(a_ty, dir, b_ty)`:
 250             //
 251             // FIXME(#16847): This code is non-ideal because all these subtype
 252             // relations wind up attributed to the same spans. We need
 253             // to associate causes/spans with each of the relations in
 254             // the stack to get this right.
 255             match dir {
 256                 BiTo => self.bivariate().relate(&a_ty, &b_ty),
 257                 EqTo => self.equate().relate(&a_ty, &b_ty),
 258                 SubtypeOf => self.sub().relate(&a_ty, &b_ty),
 259                 SupertypeOf => self.sub().relate_with_variance(ty::Contravariant, &a_ty, &b_ty),
 260             }?;
 261         }
 262
 263         Ok(())
 264     }
 265
 266     /// Attempts to generalize `ty` for the type variable `for_vid`.  This checks for cycle -- that
 267     /// is, whether the type `ty` references `for_vid`. If `make_region_vars` is true, it will also
 268     /// replace all regions with fresh variables. Returns `TyError` in the case of a cycle, `Ok`
 269     /// otherwise.
 270     fn generalize(&self,
 271                   ty: Ty<'tcx>,
 272                   for_vid: ty::TyVid,
 273                   make_region_vars: bool)
 274                   -> RelateResult<'tcx, Ty<'tcx>>
 275     {
 276         let mut generalize = Generalizer {
 277             infcx: self.infcx,
 278             span: self.trace.origin.span(),
 279             for_vid: for_vid,
 280             make_region_vars: make_region_vars,
 281             cycle_detected: false
 282         };
 283         let u = ty.fold_with(&mut generalize);
 284         if generalize.cycle_detected {
 285             Err(TypeError::CyclicTy)
 286         } else {
 287             Ok(u)
 288         }
 289     }
 290 }
 291
 292 struct Generalizer<'cx, 'tcx:'cx> {
 293     infcx: &'cx InferCtxt<'cx, 'tcx>,
 294     span: Span,
 295     for_vid: ty::TyVid,
 296     make_region_vars: bool,
 297     cycle_detected: bool,
 298 }
 299
 300 impl<'cx, 'tcx> ty::fold::TypeFolder<'tcx> for Generalizer<'cx, 'tcx> {
 301     fn tcx(&self) -> &TyCtxt<'tcx> {
 302         self.infcx.tcx
 303     }
 304
 305     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 306         // Check to see whether the type we are genealizing references
 307         // `vid`. At the same time, also update any type variables to
 308         // the values that they are bound to. This is needed to truly
 309         // check for cycles, but also just makes things readable.
 310         //
 311         // (In particular, you could have something like `$0 = Box<$1>`
 312         //  where `$1` has already been instantiated with `Box<$0>`)
 313         match t.sty {
 314             ty::TyInfer(ty::TyVar(vid)) => {
 315                 let mut variables = self.infcx.type_variables.borrow_mut();
 316                 let vid = variables.root_var(vid);
 317                 if vid == self.for_vid {
 318                     self.cycle_detected = true;
 319                     self.tcx().types.err
 320                 } else {
 321                     match variables.probe_root(vid) {
 322                         Some(u) => {
 323                             drop(variables);
 324                             self.fold_ty(u)
 325                         }
 326                         None => t,
 327                     }
 328                 }
 329             }
 330             _ => {
 331                 t.super_fold_with(self)
 332             }
 333         }
 334     }
 335
 336     fn fold_region(&mut self, r: ty::Region) -> ty::Region {
 337         match r {
 338             // Never make variables for regions bound within the type itself.
 339             ty::ReLateBound(..) => { return r; }
 340
 341             // Early-bound regions should really have been substituted away before
 342             // we get to this point.
 343             ty::ReEarlyBound(..) => {
 344                 span_bug!(
 345                     self.span,
 346                     "Encountered early bound region when generalizing: {:?}",
 347                     r);
 348             }
 349
 350             // Always make a fresh region variable for skolemized regions;
 351             // the higher-ranked decision procedures rely on this.
 352             ty::ReSkolemized(..) => { }
 353
 354             // For anything else, we make a region variable, unless we
 355             // are *equating*, in which case it's just wasteful.
 356             ty::ReEmpty |
 357             ty::ReStatic |
 358             ty::ReScope(..) |
 359             ty::ReVar(..) |
 360             ty::ReFree(..) => {
 361                 if !self.make_region_vars {
 362                     return r;
 363                 }
 364             }
 365         }
 366
 367         // FIXME: This is non-ideal because we don't give a
 368         // very descriptive origin for this region variable.
 369         self.infcx.next_region_var(MiscVariable(self.span))
 370     }
 371 }
 372
 373 pub trait RelateResultCompare<'tcx, T> {
 374     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 375         F: FnOnce() -> TypeError<'tcx>;
 376 }
 377
 378 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
 379     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 380         F: FnOnce() -> TypeError<'tcx>,
 381     {
 382         self.clone().and_then(|s| {
 383             if s == t {
 384                 self.clone()
 385             } else {
 386                 Err(f())
 387             }
 388         })
 389     }
 390 }
 391
 392 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
 393                                -> TypeError<'tcx>
 394 {
 395     let (a, b) = v;
 396     TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 397 }
 398
 399 fn float_unification_error<'tcx>(a_is_expected: bool,
 400                                  v: (ast::FloatTy, ast::FloatTy))
 401                                  -> TypeError<'tcx>
 402 {
 403     let (a, b) = v;
 404     TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 405 }