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::fold::TypeFoldable;
  46 use ty::relate::{RelateResult, TypeRelation};
  47 use traits::PredicateObligations;
  48
  49 use syntax::ast;
  50 use syntax::util::small_vector::SmallVector;
  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 obligations: PredicateObligations<'tcx>,
  59 }
  60
  61 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
  62 pub enum RelationDir {
  63     SubtypeOf, SupertypeOf, EqTo
  64 }
  65
  66 impl<'infcx, 'gcx, 'tcx> InferCtxt<'infcx, 'gcx, 'tcx> {
  67     pub fn super_combine_tys<R>(&self,
  68                                 relation: &mut R,
  69                                 a: Ty<'tcx>,
  70                                 b: Ty<'tcx>)
  71                                 -> RelateResult<'tcx, Ty<'tcx>>
  72         where R: TypeRelation<'infcx, 'gcx, 'tcx>
  73     {
  74         let a_is_expected = relation.a_is_expected();
  75
  76         match (&a.sty, &b.sty) {
  77             // Relate integral variables to other types
  78             (&ty::TyInfer(ty::IntVar(a_id)), &ty::TyInfer(ty::IntVar(b_id))) => {
  79                 self.int_unification_table
  80                     .borrow_mut()
  81                     .unify_var_var(a_id, b_id)
  82                     .map_err(|e| int_unification_error(a_is_expected, e))?;
  83                 Ok(a)
  84             }
  85             (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyInt(v)) => {
  86                 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
  87             }
  88             (&ty::TyInt(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  89                 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
  90             }
  91             (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyUint(v)) => {
  92                 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
  93             }
  94             (&ty::TyUint(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  95                 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
  96             }
  97
  98             // Relate floating-point variables to other types
  99             (&ty::TyInfer(ty::FloatVar(a_id)), &ty::TyInfer(ty::FloatVar(b_id))) => {
 100                 self.float_unification_table
 101                     .borrow_mut()
 102                     .unify_var_var(a_id, b_id)
 103                     .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
 104                 Ok(a)
 105             }
 106             (&ty::TyInfer(ty::FloatVar(v_id)), &ty::TyFloat(v)) => {
 107                 self.unify_float_variable(a_is_expected, v_id, v)
 108             }
 109             (&ty::TyFloat(v), &ty::TyInfer(ty::FloatVar(v_id))) => {
 110                 self.unify_float_variable(!a_is_expected, v_id, v)
 111             }
 112
 113             // All other cases of inference are errors
 114             (&ty::TyInfer(_), _) |
 115             (_, &ty::TyInfer(_)) => {
 116                 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
 117             }
 118
 119
 120             _ => {
 121                 ty::relate::super_relate_tys(relation, a, b)
 122             }
 123         }
 124     }
 125
 126     fn unify_integral_variable(&self,
 127                                vid_is_expected: bool,
 128                                vid: ty::IntVid,
 129                                val: ty::IntVarValue)
 130                                -> RelateResult<'tcx, Ty<'tcx>>
 131     {
 132         self.int_unification_table
 133             .borrow_mut()
 134             .unify_var_value(vid, val)
 135             .map_err(|e| int_unification_error(vid_is_expected, e))?;
 136         match val {
 137             IntType(v) => Ok(self.tcx.mk_mach_int(v)),
 138             UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
 139         }
 140     }
 141
 142     fn unify_float_variable(&self,
 143                             vid_is_expected: bool,
 144                             vid: ty::FloatVid,
 145                             val: ast::FloatTy)
 146                             -> RelateResult<'tcx, Ty<'tcx>>
 147     {
 148         self.float_unification_table
 149             .borrow_mut()
 150             .unify_var_value(vid, val)
 151             .map_err(|e| float_unification_error(vid_is_expected, e))?;
 152         Ok(self.tcx.mk_mach_float(val))
 153     }
 154 }
 155
 156 impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
 157     pub fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> {
 158         self.infcx.tcx
 159     }
 160
 161     pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'gcx, 'tcx> {
 162         Equate::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         use self::RelationDir::*;
 185
 186         // We use SmallVector here instead of Vec because this code is hot and
 187         // it's rare that the stack length exceeds 1.
 188         let mut stack = SmallVector::new();
 189         stack.push((a_ty, dir, b_vid));
 190         loop {
 191             // For each turn of the loop, we extract a tuple
 192             //
 193             //     (a_ty, dir, b_vid)
 194             //
 195             // to relate. Here dir is either SubtypeOf or
 196             // SupertypeOf. The idea is that we should ensure that
 197             // the type `a_ty` is a subtype or supertype (respectively) of the
 198             // type to which `b_vid` is bound.
 199             //
 200             // If `b_vid` has not yet been instantiated with a type
 201             // (which is always true on the first iteration, but not
 202             // necessarily true on later iterations), we will first
 203             // instantiate `b_vid` with a *generalized* version of
 204             // `a_ty`. Generalization introduces other inference
 205             // variables wherever subtyping could occur (at time of
 206             // this writing, this means replacing free regions with
 207             // region variables).
 208             let (a_ty, dir, b_vid) = match stack.pop() {
 209                 None => break,
 210                 Some(e) => e,
 211             };
 212             // Get the actual variable that b_vid has been inferred to
 213             let (b_vid, b_ty) = {
 214                 let mut variables = self.infcx.type_variables.borrow_mut();
 215                 let b_vid = variables.root_var(b_vid);
 216                 (b_vid, variables.probe_root(b_vid))
 217             };
 218
 219             debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})",
 220                    a_ty,
 221                    dir,
 222                    b_vid);
 223
 224             // Check whether `vid` has been instantiated yet.  If not,
 225             // make a generalized form of `ty` and instantiate with
 226             // that.
 227             //
 228             // FIXME(#18653) -- we need to generalize nested type
 229             // variables too.
 230             let b_ty = match b_ty {
 231                 Some(t) => t, // ...already instantiated.
 232                 None => {     // ...not yet instantiated:
 233                     // Generalize type if necessary.
 234                     let generalized_ty = match dir {
 235                         EqTo => self.generalize(a_ty, b_vid, false),
 236                         SupertypeOf | SubtypeOf => self.generalize(a_ty, b_vid, true),
 237                     }?;
 238                     debug!("instantiate(a_ty={:?}, dir={:?}, \
 239                                         b_vid={:?}, generalized_ty={:?})",
 240                            a_ty, dir, b_vid,
 241                            generalized_ty);
 242                     self.infcx.type_variables
 243                         .borrow_mut()
 244                         .instantiate(b_vid, generalized_ty);
 245                     generalized_ty
 246                 }
 247             };
 248
 249             // The original triple was `(a_ty, dir, b_vid)` -- now we have
 250             // resolved `b_vid` to `b_ty`, so apply `(a_ty, dir, b_ty)`:
 251             //
 252             // FIXME(#16847): This code is non-ideal because all these subtype
 253             // relations wind up attributed to the same spans. We need
 254             // to associate causes/spans with each of the relations in
 255             // the stack to get this right.
 256             match dir {
 257                 EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
 258                 SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
 259                 SupertypeOf => self.sub(a_is_expected).relate_with_variance(
 260                     ty::Contravariant, &a_ty, &b_ty),
 261             }?;
 262         }
 263
 264         Ok(())
 265     }
 266
 267     /// Attempts to generalize `ty` for the type variable `for_vid`.  This checks for cycle -- that
 268     /// is, whether the type `ty` references `for_vid`. If `make_region_vars` is true, it will also
 269     /// replace all regions with fresh variables. Returns `TyError` in the case of a cycle, `Ok`
 270     /// otherwise.
 271     fn generalize(&self,
 272                   ty: Ty<'tcx>,
 273                   for_vid: ty::TyVid,
 274                   make_region_vars: bool)
 275                   -> RelateResult<'tcx, Ty<'tcx>>
 276     {
 277         let mut generalize = Generalizer {
 278             infcx: self.infcx,
 279             span: self.trace.cause.span,
 280             for_vid: for_vid,
 281             make_region_vars: make_region_vars,
 282             cycle_detected: false
 283         };
 284         let u = ty.fold_with(&mut generalize);
 285         if generalize.cycle_detected {
 286             Err(TypeError::CyclicTy)
 287         } else {
 288             Ok(u)
 289         }
 290     }
 291 }
 292
 293 struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
 294     infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
 295     span: Span,
 296     for_vid: ty::TyVid,
 297     make_region_vars: bool,
 298     cycle_detected: bool,
 299 }
 300
 301 impl<'cx, 'gcx, 'tcx> ty::fold::TypeFolder<'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> {
 302     fn tcx<'a>(&'a self) -> TyCtxt<'a, 'gcx, 'tcx> {
 303         self.infcx.tcx
 304     }
 305
 306     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 307         // Check to see whether the type we are genealizing references
 308         // `vid`. At the same time, also update any type variables to
 309         // the values that they are bound to. This is needed to truly
 310         // check for cycles, but also just makes things readable.
 311         //
 312         // (In particular, you could have something like `$0 = Box<$1>`
 313         //  where `$1` has already been instantiated with `Box<$0>`)
 314         match t.sty {
 315             ty::TyInfer(ty::TyVar(vid)) => {
 316                 let mut variables = self.infcx.type_variables.borrow_mut();
 317                 let vid = variables.root_var(vid);
 318                 if vid == self.for_vid {
 319                     self.cycle_detected = true;
 320                     self.tcx().types.err
 321                 } else {
 322                     match variables.probe_root(vid) {
 323                         Some(u) => {
 324                             drop(variables);
 325                             self.fold_ty(u)
 326                         }
 327                         None => t,
 328                     }
 329                 }
 330             }
 331             _ => {
 332                 t.super_fold_with(self)
 333             }
 334         }
 335     }
 336
 337     fn fold_region(&mut self, r: &'tcx ty::Region) -> &'tcx ty::Region {
 338         match *r {
 339             // Never make variables for regions bound within the type itself,
 340             // nor for erased regions.
 341             ty::ReLateBound(..) |
 342             ty::ReErased => { return r; }
 343
 344             // Early-bound regions should really have been substituted away before
 345             // we get to this point.
 346             ty::ReEarlyBound(..) => {
 347                 span_bug!(
 348                     self.span,
 349                     "Encountered early bound region when generalizing: {:?}",
 350                     r);
 351             }
 352
 353             // Always make a fresh region variable for skolemized regions;
 354             // the higher-ranked decision procedures rely on this.
 355             ty::ReSkolemized(..) => { }
 356
 357             // For anything else, we make a region variable, unless we
 358             // are *equating*, in which case it's just wasteful.
 359             ty::ReEmpty |
 360             ty::ReStatic |
 361             ty::ReScope(..) |
 362             ty::ReVar(..) |
 363             ty::ReFree(..) => {
 364                 if !self.make_region_vars {
 365                     return r;
 366                 }
 367             }
 368         }
 369
 370         // FIXME: This is non-ideal because we don't give a
 371         // very descriptive origin for this region variable.
 372         self.infcx.next_region_var(MiscVariable(self.span))
 373     }
 374 }
 375
 376 pub trait RelateResultCompare<'tcx, T> {
 377     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 378         F: FnOnce() -> TypeError<'tcx>;
 379 }
 380
 381 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
 382     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 383         F: FnOnce() -> TypeError<'tcx>,
 384     {
 385         self.clone().and_then(|s| {
 386             if s == t {
 387                 self.clone()
 388             } else {
 389                 Err(f())
 390             }
 391         })
 392     }
 393 }
 394
 395 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
 396                                -> TypeError<'tcx>
 397 {
 398     let (a, b) = v;
 399     TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 400 }
 401
 402 fn float_unification_error<'tcx>(a_is_expected: bool,
 403                                  v: (ast::FloatTy, ast::FloatTy))
 404                                  -> TypeError<'tcx>
 405 {
 406     let (a, b) = v;
 407     TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 408 }