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::{IntType, UintType};
  45 use middle::ty::{self, Ty};
  46 use middle::ty::error::TypeError;
  47 use middle::ty::fold::{TypeFolder, TypeFoldable};
  48 use middle::ty::relate::{Relate, RelateResult, TypeRelation};
  49
  50 use syntax::ast;
  51 use syntax::codemap::Span;
  52
  53 #[derive(Clone)]
  54 pub struct CombineFields<'a, 'tcx: 'a> {
  55     pub infcx: &'a InferCtxt<'a, 'tcx>,
  56     pub a_is_expected: bool,
  57     pub trace: TypeTrace<'tcx>,
  58     pub cause: Option<ty::relate::Cause>,
  59 }
  60
  61 pub fn super_combine_tys<'a,'tcx:'a,R>(infcx: &InferCtxt<'a, 'tcx>,
  62                                        relation: &mut R,
  63                                        a: Ty<'tcx>,
  64                                        b: Ty<'tcx>)
  65                                        -> RelateResult<'tcx, Ty<'tcx>>
  66     where R: TypeRelation<'a,'tcx>
  67 {
  68     let a_is_expected = relation.a_is_expected();
  69
  70     match (&a.sty, &b.sty) {
  71         // Relate integral variables to other types
  72         (&ty::TyInfer(ty::IntVar(a_id)), &ty::TyInfer(ty::IntVar(b_id))) => {
  73             try!(infcx.int_unification_table
  74                       .borrow_mut()
  75                       .unify_var_var(a_id, b_id)
  76                       .map_err(|e| int_unification_error(a_is_expected, e)));
  77             Ok(a)
  78         }
  79         (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyInt(v)) => {
  80             unify_integral_variable(infcx, a_is_expected, v_id, IntType(v))
  81         }
  82         (&ty::TyInt(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  83             unify_integral_variable(infcx, !a_is_expected, v_id, IntType(v))
  84         }
  85         (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyUint(v)) => {
  86             unify_integral_variable(infcx, a_is_expected, v_id, UintType(v))
  87         }
  88         (&ty::TyUint(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  89             unify_integral_variable(infcx, !a_is_expected, v_id, UintType(v))
  90         }
  91
  92         // Relate floating-point variables to other types
  93         (&ty::TyInfer(ty::FloatVar(a_id)), &ty::TyInfer(ty::FloatVar(b_id))) => {
  94             try!(infcx.float_unification_table
  95                       .borrow_mut()
  96                       .unify_var_var(a_id, b_id)
  97                       .map_err(|e| float_unification_error(relation.a_is_expected(), e)));
  98             Ok(a)
  99         }
 100         (&ty::TyInfer(ty::FloatVar(v_id)), &ty::TyFloat(v)) => {
 101             unify_float_variable(infcx, a_is_expected, v_id, v)
 102         }
 103         (&ty::TyFloat(v), &ty::TyInfer(ty::FloatVar(v_id))) => {
 104             unify_float_variable(infcx, !a_is_expected, v_id, v)
 105         }
 106
 107         // All other cases of inference are errors
 108         (&ty::TyInfer(_), _) |
 109         (_, &ty::TyInfer(_)) => {
 110             Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
 111         }
 112
 113
 114         _ => {
 115             ty::relate::super_relate_tys(relation, a, b)
 116         }
 117     }
 118 }
 119
 120 fn unify_integral_variable<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
 121                                     vid_is_expected: bool,
 122                                     vid: ty::IntVid,
 123                                     val: ty::IntVarValue)
 124                                     -> RelateResult<'tcx, Ty<'tcx>>
 125 {
 126     try!(infcx
 127          .int_unification_table
 128          .borrow_mut()
 129          .unify_var_value(vid, val)
 130          .map_err(|e| int_unification_error(vid_is_expected, e)));
 131     match val {
 132         IntType(v) => Ok(infcx.tcx.mk_mach_int(v)),
 133         UintType(v) => Ok(infcx.tcx.mk_mach_uint(v)),
 134     }
 135 }
 136
 137 fn unify_float_variable<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
 138                                  vid_is_expected: bool,
 139                                  vid: ty::FloatVid,
 140                                  val: ast::FloatTy)
 141                                  -> RelateResult<'tcx, Ty<'tcx>>
 142 {
 143     try!(infcx
 144          .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 ty::ctxt<'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
 214             debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})",
 215                    a_ty,
 216                    dir,
 217                    b_vid);
 218
 219             // Check whether `vid` has been instantiated yet.  If not,
 220             // make a generalized form of `ty` and instantiate with
 221             // that.
 222             let b_ty = self.infcx.type_variables.borrow().probe(b_vid);
 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 = try!(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             try!(match dir {
 251                 BiTo => self.bivariate().relate(&a_ty, &b_ty),
 252                 EqTo => self.equate().relate(&a_ty, &b_ty),
 253                 SubtypeOf => self.sub().relate(&a_ty, &b_ty),
 254                 SupertypeOf => self.sub().relate_with_variance(ty::Contravariant, &a_ty, &b_ty),
 255             });
 256         }
 257
 258         Ok(())
 259     }
 260
 261     /// Attempts to generalize `ty` for the type variable `for_vid`.  This checks for cycle -- that
 262     /// is, whether the type `ty` references `for_vid`. If `make_region_vars` is true, it will also
 263     /// replace all regions with fresh variables. Returns `TyError` in the case of a cycle, `Ok`
 264     /// otherwise.
 265     fn generalize(&self,
 266                   ty: Ty<'tcx>,
 267                   for_vid: ty::TyVid,
 268                   make_region_vars: bool)
 269                   -> RelateResult<'tcx, Ty<'tcx>>
 270     {
 271         let mut generalize = Generalizer {
 272             infcx: self.infcx,
 273             span: self.trace.origin.span(),
 274             for_vid: for_vid,
 275             make_region_vars: make_region_vars,
 276             cycle_detected: false
 277         };
 278         let u = ty.fold_with(&mut generalize);
 279         if generalize.cycle_detected {
 280             Err(TypeError::CyclicTy)
 281         } else {
 282             Ok(u)
 283         }
 284     }
 285 }
 286
 287 struct Generalizer<'cx, 'tcx:'cx> {
 288     infcx: &'cx InferCtxt<'cx, 'tcx>,
 289     span: Span,
 290     for_vid: ty::TyVid,
 291     make_region_vars: bool,
 292     cycle_detected: bool,
 293 }
 294
 295 impl<'cx, 'tcx> ty::fold::TypeFolder<'tcx> for Generalizer<'cx, 'tcx> {
 296     fn tcx(&self) -> &ty::ctxt<'tcx> {
 297         self.infcx.tcx
 298     }
 299
 300     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 301         // Check to see whether the type we are genealizing references
 302         // `vid`. At the same time, also update any type variables to
 303         // the values that they are bound to. This is needed to truly
 304         // check for cycles, but also just makes things readable.
 305         //
 306         // (In particular, you could have something like `$0 = Box<$1>`
 307         //  where `$1` has already been instantiated with `Box<$0>`)
 308         match t.sty {
 309             ty::TyInfer(ty::TyVar(vid)) => {
 310                 if vid == self.for_vid {
 311                     self.cycle_detected = true;
 312                     self.tcx().types.err
 313                 } else {
 314                     match self.infcx.type_variables.borrow().probe(vid) {
 315                         Some(u) => self.fold_ty(u),
 316                         None => t,
 317                     }
 318                 }
 319             }
 320             _ => {
 321                 t.super_fold_with(self)
 322             }
 323         }
 324     }
 325
 326     fn fold_region(&mut self, r: ty::Region) -> ty::Region {
 327         match r {
 328             // Never make variables for regions bound within the type itself.
 329             ty::ReLateBound(..) => { return r; }
 330
 331             // Early-bound regions should really have been substituted away before
 332             // we get to this point.
 333             ty::ReEarlyBound(..) => {
 334                 self.tcx().sess.span_bug(
 335                     self.span,
 336                     &format!("Encountered early bound region when generalizing: {:?}",
 337                             r));
 338             }
 339
 340             // Always make a fresh region variable for skolemized regions;
 341             // the higher-ranked decision procedures rely on this.
 342             ty::ReSkolemized(..) => { }
 343
 344             // For anything else, we make a region variable, unless we
 345             // are *equating*, in which case it's just wasteful.
 346             ty::ReEmpty |
 347             ty::ReStatic |
 348             ty::ReScope(..) |
 349             ty::ReVar(..) |
 350             ty::ReFree(..) => {
 351                 if !self.make_region_vars {
 352                     return r;
 353                 }
 354             }
 355         }
 356
 357         // FIXME: This is non-ideal because we don't give a
 358         // very descriptive origin for this region variable.
 359         self.infcx.next_region_var(MiscVariable(self.span))
 360     }
 361 }
 362
 363 pub trait RelateResultCompare<'tcx, T> {
 364     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 365         F: FnOnce() -> TypeError<'tcx>;
 366 }
 367
 368 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
 369     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 370         F: FnOnce() -> TypeError<'tcx>,
 371     {
 372         self.clone().and_then(|s| {
 373             if s == t {
 374                 self.clone()
 375             } else {
 376                 Err(f())
 377             }
 378         })
 379     }
 380 }
 381
 382 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
 383                                -> TypeError<'tcx>
 384 {
 385     let (a, b) = v;
 386     TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 387 }
 388
 389 fn float_unification_error<'tcx>(a_is_expected: bool,
 390                                  v: (ast::FloatTy, ast::FloatTy))
 391                                  -> TypeError<'tcx>
 392 {
 393     let (a, b) = v;
 394     TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 395 }