src/librustc/middle/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 middle::ty::{TyVar};
  45 use middle::ty::{IntType, UintType};
  46 use middle::ty::{self, Ty};
  47 use middle::ty_fold;
  48 use middle::ty_fold::{TypeFolder, TypeFoldable};
  49 use middle::ty_relate::{self, Relate, RelateResult, TypeRelation};
  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 }
  61
  62 pub fn super_combine_tys<'a,'tcx:'a,R>(infcx: &InferCtxt<'a, 'tcx>,
  63                                        relation: &mut R,
  64                                        a: Ty<'tcx>,
  65                                        b: Ty<'tcx>)
  66                                        -> RelateResult<'tcx, Ty<'tcx>>
  67     where R: TypeRelation<'a,'tcx>
  68 {
  69     let a_is_expected = relation.a_is_expected();
  70
  71     match (&a.sty, &b.sty) {
  72         // Relate integral variables to other types
  73         (&ty::TyInfer(ty::IntVar(a_id)), &ty::TyInfer(ty::IntVar(b_id))) => {
  74             try!(infcx.int_unification_table
  75                       .borrow_mut()
  76                       .unify_var_var(a_id, b_id)
  77                       .map_err(|e| int_unification_error(a_is_expected, e)));
  78             Ok(a)
  79         }
  80         (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyInt(v)) => {
  81             unify_integral_variable(infcx, a_is_expected, v_id, IntType(v))
  82         }
  83         (&ty::TyInt(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  84             unify_integral_variable(infcx, !a_is_expected, v_id, IntType(v))
  85         }
  86         (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyUint(v)) => {
  87             unify_integral_variable(infcx, a_is_expected, v_id, UintType(v))
  88         }
  89         (&ty::TyUint(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  90             unify_integral_variable(infcx, !a_is_expected, v_id, UintType(v))
  91         }
  92
  93         // Relate floating-point variables to other types
  94         (&ty::TyInfer(ty::FloatVar(a_id)), &ty::TyInfer(ty::FloatVar(b_id))) => {
  95             try!(infcx.float_unification_table
  96                       .borrow_mut()
  97                       .unify_var_var(a_id, b_id)
  98                       .map_err(|e| float_unification_error(relation.a_is_expected(), e)));
  99             Ok(a)
 100         }
 101         (&ty::TyInfer(ty::FloatVar(v_id)), &ty::TyFloat(v)) => {
 102             unify_float_variable(infcx, a_is_expected, v_id, v)
 103         }
 104         (&ty::TyFloat(v), &ty::TyInfer(ty::FloatVar(v_id))) => {
 105             unify_float_variable(infcx, !a_is_expected, v_id, v)
 106         }
 107
 108         // All other cases of inference are errors
 109         (&ty::TyInfer(_), _) |
 110         (_, &ty::TyInfer(_)) => {
 111             Err(ty::terr_sorts(ty_relate::expected_found(relation, &a, &b)))
 112         }
 113
 114
 115         _ => {
 116             ty_relate::super_relate_tys(relation, a, b)
 117         }
 118     }
 119 }
 120
 121 fn unify_integral_variable<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
 122                                     vid_is_expected: bool,
 123                                     vid: ty::IntVid,
 124                                     val: ty::IntVarValue)
 125                                     -> RelateResult<'tcx, Ty<'tcx>>
 126 {
 127     try!(infcx
 128          .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(ty::mk_mach_int(infcx.tcx, v)),
 134         UintType(v) => Ok(ty::mk_mach_uint(infcx.tcx, 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     try!(infcx
 145          .float_unification_table
 146          .borrow_mut()
 147          .unify_var_value(vid, val)
 148          .map_err(|e| float_unification_error(vid_is_expected, e)));
 149     Ok(ty::mk_mach_float(infcx.tcx, val))
 150 }
 151
 152 impl<'a, 'tcx> CombineFields<'a, 'tcx> {
 153     pub fn tcx(&self) -> &'a ty::ctxt<'tcx> {
 154         self.infcx.tcx
 155     }
 156
 157     pub fn switch_expected(&self) -> CombineFields<'a, 'tcx> {
 158         CombineFields {
 159             a_is_expected: !self.a_is_expected,
 160             ..(*self).clone()
 161         }
 162     }
 163
 164     pub fn equate(&self) -> Equate<'a, 'tcx> {
 165         Equate::new(self.clone())
 166     }
 167
 168     pub fn bivariate(&self) -> Bivariate<'a, 'tcx> {
 169         Bivariate::new(self.clone())
 170     }
 171
 172     pub fn sub(&self) -> Sub<'a, 'tcx> {
 173         Sub::new(self.clone())
 174     }
 175
 176     pub fn lub(&self) -> Lub<'a, 'tcx> {
 177         Lub::new(self.clone())
 178     }
 179
 180     pub fn glb(&self) -> Glb<'a, 'tcx> {
 181         Glb::new(self.clone())
 182     }
 183
 184     pub fn instantiate(&self,
 185                        a_ty: Ty<'tcx>,
 186                        dir: RelationDir,
 187                        b_vid: ty::TyVid)
 188                        -> RelateResult<'tcx, ()>
 189     {
 190         let mut stack = Vec::new();
 191         stack.push((a_ty, dir, b_vid));
 192         loop {
 193             // For each turn of the loop, we extract a tuple
 194             //
 195             //     (a_ty, dir, b_vid)
 196             //
 197             // to relate. Here dir is either SubtypeOf or
 198             // SupertypeOf. The idea is that we should ensure that
 199             // the type `a_ty` is a subtype or supertype (respectively) of the
 200             // type to which `b_vid` is bound.
 201             //
 202             // If `b_vid` has not yet been instantiated with a type
 203             // (which is always true on the first iteration, but not
 204             // necessarily true on later iterations), we will first
 205             // instantiate `b_vid` with a *generalized* version of
 206             // `a_ty`. Generalization introduces other inference
 207             // variables wherever subtyping could occur (at time of
 208             // this writing, this means replacing free regions with
 209             // region variables).
 210             let (a_ty, dir, b_vid) = match stack.pop() {
 211                 None => break,
 212                 Some(e) => e,
 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 = self.infcx.type_variables.borrow().probe(b_vid);
 224             let b_ty = match b_ty {
 225                 Some(t) => t, // ...already instantiated.
 226                 None => {     // ...not yet instantiated:
 227                     // Generalize type if necessary.
 228                     let generalized_ty = try!(match dir {
 229                         EqTo => self.generalize(a_ty, b_vid, false),
 230                         BiTo | SupertypeOf | SubtypeOf => self.generalize(a_ty, b_vid, true),
 231                     });
 232                     debug!("instantiate(a_ty={:?}, dir={:?}, \
 233                                         b_vid={:?}, generalized_ty={:?})",
 234                            a_ty, dir, b_vid,
 235                            generalized_ty);
 236                     self.infcx.type_variables
 237                         .borrow_mut()
 238                         .instantiate_and_push(
 239                             b_vid, generalized_ty, &mut stack);
 240                     generalized_ty
 241                 }
 242             };
 243
 244             // The original triple was `(a_ty, dir, b_vid)` -- now we have
 245             // resolved `b_vid` to `b_ty`, so apply `(a_ty, dir, b_ty)`:
 246             //
 247             // FIXME(#16847): This code is non-ideal because all these subtype
 248             // relations wind up attributed to the same spans. We need
 249             // to associate causes/spans with each of the relations in
 250             // the stack to get this right.
 251             try!(match dir {
 252                 BiTo => self.bivariate().relate(&a_ty, &b_ty),
 253                 EqTo => self.equate().relate(&a_ty, &b_ty),
 254                 SubtypeOf => self.sub().relate(&a_ty, &b_ty),
 255                 SupertypeOf => self.sub().relate_with_variance(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(ty::terr_cyclic_ty)
 282         } else {
 283             Ok(u)
 284         }
 285     }
 286 }
 287
 288 struct Generalizer<'cx, 'tcx:'cx> {
 289     infcx: &'cx InferCtxt<'cx, 'tcx>,
 290     span: Span,
 291     for_vid: ty::TyVid,
 292     make_region_vars: bool,
 293     cycle_detected: bool,
 294 }
 295
 296 impl<'cx, 'tcx> ty_fold::TypeFolder<'tcx> for Generalizer<'cx, 'tcx> {
 297     fn tcx(&self) -> &ty::ctxt<'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                 if vid == self.for_vid {
 312                     self.cycle_detected = true;
 313                     self.tcx().types.err
 314                 } else {
 315                     match self.infcx.type_variables.borrow().probe(vid) {
 316                         Some(u) => self.fold_ty(u),
 317                         None => t,
 318                     }
 319                 }
 320             }
 321             _ => {
 322                 ty_fold::super_fold_ty(self, t)
 323             }
 324         }
 325     }
 326
 327     fn fold_region(&mut self, r: ty::Region) -> ty::Region {
 328         match r {
 329             // Never make variables for regions bound within the type itself.
 330             ty::ReLateBound(..) => { return r; }
 331
 332             // Early-bound regions should really have been substituted away before
 333             // we get to this point.
 334             ty::ReEarlyBound(..) => {
 335                 self.tcx().sess.span_bug(
 336                     self.span,
 337                     &format!("Encountered early bound region when generalizing: {:?}",
 338                             r));
 339             }
 340
 341             // Always make a fresh region variable for skolemized regions;
 342             // the higher-ranked decision procedures rely on this.
 343             ty::ReInfer(ty::ReSkolemized(..)) => { }
 344
 345             // For anything else, we make a region variable, unless we
 346             // are *equating*, in which case it's just wasteful.
 347             ty::ReEmpty |
 348             ty::ReStatic |
 349             ty::ReScope(..) |
 350             ty::ReInfer(ty::ReVar(..)) |
 351             ty::ReFree(..) => {
 352                 if !self.make_region_vars {
 353                     return r;
 354                 }
 355             }
 356         }
 357
 358         // FIXME: This is non-ideal because we don't give a
 359         // very descriptive origin for this region variable.
 360         self.infcx.next_region_var(MiscVariable(self.span))
 361     }
 362 }
 363
 364 pub trait RelateResultCompare<'tcx, T> {
 365     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 366         F: FnOnce() -> ty::type_err<'tcx>;
 367 }
 368
 369 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
 370     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 371         F: FnOnce() -> ty::type_err<'tcx>,
 372     {
 373         self.clone().and_then(|s| {
 374             if s == t {
 375                 self.clone()
 376             } else {
 377                 Err(f())
 378             }
 379         })
 380     }
 381 }
 382
 383 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
 384                                -> ty::type_err<'tcx>
 385 {
 386     let (a, b) = v;
 387     ty::terr_int_mismatch(ty_relate::expected_found_bool(a_is_expected, &a, &b))
 388 }
 389
 390 fn float_unification_error<'tcx>(a_is_expected: bool,
 391                                  v: (ast::FloatTy, ast::FloatTy))
 392                                  -> ty::type_err<'tcx>
 393 {
 394     let (a, b) = v;
 395     ty::terr_float_mismatch(ty_relate::expected_found_bool(a_is_expected, &a, &b))
 396 }