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_pos::Span;
  51
  52 #[derive(Clone)]
  53 pub struct CombineFields<'infcx, 'gcx: 'infcx+'tcx, 'tcx: 'infcx> {
  54     pub infcx: &'infcx InferCtxt<'infcx, 'gcx, 'tcx>,
  55     pub trace: TypeTrace<'tcx>,
  56     pub cause: Option<ty::relate::Cause>,
  57     pub obligations: PredicateObligations<'tcx>,
  58 }
  59
  60 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
  61 pub enum RelationDir {
  62     SubtypeOf, SupertypeOf, EqTo
  63 }
  64
  65 impl<'infcx, 'gcx, 'tcx> InferCtxt<'infcx, 'gcx, 'tcx> {
  66     pub fn super_combine_tys<R>(&self,
  67                                 relation: &mut R,
  68                                 a: Ty<'tcx>,
  69                                 b: Ty<'tcx>)
  70                                 -> RelateResult<'tcx, Ty<'tcx>>
  71         where R: TypeRelation<'infcx, 'gcx, 'tcx>
  72     {
  73         let a_is_expected = relation.a_is_expected();
  74
  75         match (&a.sty, &b.sty) {
  76             // Relate integral variables to other types
  77             (&ty::TyInfer(ty::IntVar(a_id)), &ty::TyInfer(ty::IntVar(b_id))) => {
  78                 self.int_unification_table
  79                     .borrow_mut()
  80                     .unify_var_var(a_id, b_id)
  81                     .map_err(|e| int_unification_error(a_is_expected, e))?;
  82                 Ok(a)
  83             }
  84             (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyInt(v)) => {
  85                 self.unify_integral_variable(a_is_expected, v_id, IntType(v))
  86             }
  87             (&ty::TyInt(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  88                 self.unify_integral_variable(!a_is_expected, v_id, IntType(v))
  89             }
  90             (&ty::TyInfer(ty::IntVar(v_id)), &ty::TyUint(v)) => {
  91                 self.unify_integral_variable(a_is_expected, v_id, UintType(v))
  92             }
  93             (&ty::TyUint(v), &ty::TyInfer(ty::IntVar(v_id))) => {
  94                 self.unify_integral_variable(!a_is_expected, v_id, UintType(v))
  95             }
  96
  97             // Relate floating-point variables to other types
  98             (&ty::TyInfer(ty::FloatVar(a_id)), &ty::TyInfer(ty::FloatVar(b_id))) => {
  99                 self.float_unification_table
 100                     .borrow_mut()
 101                     .unify_var_var(a_id, b_id)
 102                     .map_err(|e| float_unification_error(relation.a_is_expected(), e))?;
 103                 Ok(a)
 104             }
 105             (&ty::TyInfer(ty::FloatVar(v_id)), &ty::TyFloat(v)) => {
 106                 self.unify_float_variable(a_is_expected, v_id, v)
 107             }
 108             (&ty::TyFloat(v), &ty::TyInfer(ty::FloatVar(v_id))) => {
 109                 self.unify_float_variable(!a_is_expected, v_id, v)
 110             }
 111
 112             // All other cases of inference are errors
 113             (&ty::TyInfer(_), _) |
 114             (_, &ty::TyInfer(_)) => {
 115                 Err(TypeError::Sorts(ty::relate::expected_found(relation, &a, &b)))
 116             }
 117
 118
 119             _ => {
 120                 ty::relate::super_relate_tys(relation, a, b)
 121             }
 122         }
 123     }
 124
 125     fn unify_integral_variable(&self,
 126                                vid_is_expected: bool,
 127                                vid: ty::IntVid,
 128                                val: ty::IntVarValue)
 129                                -> RelateResult<'tcx, Ty<'tcx>>
 130     {
 131         self.int_unification_table
 132             .borrow_mut()
 133             .unify_var_value(vid, val)
 134             .map_err(|e| int_unification_error(vid_is_expected, e))?;
 135         match val {
 136             IntType(v) => Ok(self.tcx.mk_mach_int(v)),
 137             UintType(v) => Ok(self.tcx.mk_mach_uint(v)),
 138         }
 139     }
 140
 141     fn unify_float_variable(&self,
 142                             vid_is_expected: bool,
 143                             vid: ty::FloatVid,
 144                             val: ast::FloatTy)
 145                             -> RelateResult<'tcx, Ty<'tcx>>
 146     {
 147         self.float_unification_table
 148             .borrow_mut()
 149             .unify_var_value(vid, val)
 150             .map_err(|e| float_unification_error(vid_is_expected, e))?;
 151         Ok(self.tcx.mk_mach_float(val))
 152     }
 153 }
 154
 155 impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
 156     pub fn tcx(&self) -> TyCtxt<'infcx, 'gcx, 'tcx> {
 157         self.infcx.tcx
 158     }
 159
 160     pub fn equate<'a>(&'a mut self, a_is_expected: bool) -> Equate<'a, 'infcx, 'gcx, 'tcx> {
 161         Equate::new(self, a_is_expected)
 162     }
 163
 164     pub fn sub<'a>(&'a mut self, a_is_expected: bool) -> Sub<'a, 'infcx, 'gcx, 'tcx> {
 165         Sub::new(self, a_is_expected)
 166     }
 167
 168     pub fn lub<'a>(&'a mut self, a_is_expected: bool) -> Lub<'a, 'infcx, 'gcx, 'tcx> {
 169         Lub::new(self, a_is_expected)
 170     }
 171
 172     pub fn glb<'a>(&'a mut self, a_is_expected: bool) -> Glb<'a, 'infcx, 'gcx, 'tcx> {
 173         Glb::new(self, a_is_expected)
 174     }
 175
 176     /// Here dir is either EqTo, SubtypeOf, or SupertypeOf. The
 177     /// idea is that we should ensure that the type `a_ty` is equal
 178     /// to, a subtype of, or a supertype of (respectively) the type
 179     /// to which `b_vid` is bound.
 180     ///
 181     /// Since `b_vid` has not yet been instantiated with a type, we
 182     /// will first instantiate `b_vid` with a *generalized* version
 183     /// of `a_ty`. Generalization introduces other inference
 184     /// variables wherever subtyping could occur.
 185     pub fn instantiate(&mut self,
 186                        a_ty: Ty<'tcx>,
 187                        dir: RelationDir,
 188                        b_vid: ty::TyVid,
 189                        a_is_expected: bool)
 190                        -> RelateResult<'tcx, ()>
 191     {
 192         use self::RelationDir::*;
 193
 194         // Get the actual variable that b_vid has been inferred to
 195         debug_assert!(self.infcx.type_variables.borrow_mut().probe(b_vid).is_none());
 196
 197         debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})", a_ty, dir, b_vid);
 198
 199         // Generalize type of `a_ty` appropriately depending on the
 200         // direction.  As an example, assume:
 201         //
 202         // - `a_ty == &'x ?1`, where `'x` is some free region and `?1` is an
 203         //   inference variable,
 204         // - and `dir` == `SubtypeOf`.
 205         //
 206         // Then the generalized form `b_ty` would be `&'?2 ?3`, where
 207         // `'?2` and `?3` are fresh region/type inference
 208         // variables. (Down below, we will relate `a_ty <: b_ty`,
 209         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
 210         let b_ty = self.generalize(a_ty, b_vid, dir == EqTo)?;
 211         debug!("instantiate(a_ty={:?}, dir={:?}, b_vid={:?}, generalized b_ty={:?})",
 212                a_ty, dir, b_vid, b_ty);
 213         self.infcx.type_variables.borrow_mut().instantiate(b_vid, b_ty);
 214
 215         // Finally, relate `b_ty` to `a_ty`, as described in previous comment.
 216         //
 217         // FIXME(#16847): This code is non-ideal because all these subtype
 218         // relations wind up attributed to the same spans. We need
 219         // to associate causes/spans with each of the relations in
 220         // the stack to get this right.
 221         match dir {
 222             EqTo => self.equate(a_is_expected).relate(&a_ty, &b_ty),
 223             SubtypeOf => self.sub(a_is_expected).relate(&a_ty, &b_ty),
 224             SupertypeOf => self.sub(a_is_expected).relate_with_variance(
 225                 ty::Contravariant, &a_ty, &b_ty),
 226         }?;
 227
 228         Ok(())
 229     }
 230
 231     /// Attempts to generalize `ty` for the type variable `for_vid`.
 232     /// This checks for cycle -- that is, whether the type `ty`
 233     /// references `for_vid`. If `is_eq_relation` is false, it will
 234     /// also replace all regions/unbound-type-variables with fresh
 235     /// variables. Returns `TyError` in the case of a cycle, `Ok`
 236     /// otherwise.
 237     ///
 238     /// Preconditions:
 239     ///
 240     /// - `for_vid` is a "root vid"
 241     fn generalize(&self,
 242                   ty: Ty<'tcx>,
 243                   for_vid: ty::TyVid,
 244                   is_eq_relation: bool)
 245                   -> RelateResult<'tcx, Ty<'tcx>>
 246     {
 247         let mut generalize = Generalizer {
 248             infcx: self.infcx,
 249             span: self.trace.cause.span,
 250             for_vid_sub_root: self.infcx.type_variables.borrow_mut().sub_root_var(for_vid),
 251             is_eq_relation: is_eq_relation,
 252             cycle_detected: false
 253         };
 254         let u = ty.fold_with(&mut generalize);
 255         if generalize.cycle_detected {
 256             Err(TypeError::CyclicTy)
 257         } else {
 258             Ok(u)
 259         }
 260     }
 261 }
 262
 263 struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
 264     infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
 265     span: Span,
 266     for_vid_sub_root: ty::TyVid,
 267     is_eq_relation: bool,
 268     cycle_detected: bool,
 269 }
 270
 271 impl<'cx, 'gcx, 'tcx> ty::fold::TypeFolder<'gcx, 'tcx> for Generalizer<'cx, 'gcx, 'tcx> {
 272     fn tcx<'a>(&'a self) -> TyCtxt<'a, 'gcx, 'tcx> {
 273         self.infcx.tcx
 274     }
 275
 276     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 277         // Check to see whether the type we are genealizing references
 278         // any other type variable related to `vid` via
 279         // subtyping. This is basically our "occurs check", preventing
 280         // us from creating infinitely sized types.
 281         match t.sty {
 282             ty::TyInfer(ty::TyVar(vid)) => {
 283                 let mut variables = self.infcx.type_variables.borrow_mut();
 284                 let vid = variables.root_var(vid);
 285                 let sub_vid = variables.sub_root_var(vid);
 286                 if sub_vid == self.for_vid_sub_root {
 287                     // If sub-roots are equal, then `for_vid` and
 288                     // `vid` are related via subtyping.
 289                     self.cycle_detected = true;
 290                     self.tcx().types.err
 291                 } else {
 292                     match variables.probe_root(vid) {
 293                         Some(u) => {
 294                             drop(variables);
 295                             self.fold_ty(u)
 296                         }
 297                         None => {
 298                             if !self.is_eq_relation {
 299                                 let origin = variables.origin(vid);
 300                                 let new_var_id = variables.new_var(false, origin, None);
 301                                 let u = self.tcx().mk_var(new_var_id);
 302                                 debug!("generalize: replacing original vid={:?} with new={:?}",
 303                                        vid, u);
 304                                 u
 305                             } else {
 306                                 t
 307                             }
 308                         }
 309                     }
 310                 }
 311             }
 312             _ => {
 313                 t.super_fold_with(self)
 314             }
 315         }
 316     }
 317
 318     fn fold_region(&mut self, r: &'tcx ty::Region) -> &'tcx ty::Region {
 319         match *r {
 320             // Never make variables for regions bound within the type itself,
 321             // nor for erased regions.
 322             ty::ReLateBound(..) |
 323             ty::ReErased => { return r; }
 324
 325             // Early-bound regions should really have been substituted away before
 326             // we get to this point.
 327             ty::ReEarlyBound(..) => {
 328                 span_bug!(
 329                     self.span,
 330                     "Encountered early bound region when generalizing: {:?}",
 331                     r);
 332             }
 333
 334             // Always make a fresh region variable for skolemized regions;
 335             // the higher-ranked decision procedures rely on this.
 336             ty::ReSkolemized(..) => { }
 337
 338             // For anything else, we make a region variable, unless we
 339             // are *equating*, in which case it's just wasteful.
 340             ty::ReEmpty |
 341             ty::ReStatic |
 342             ty::ReScope(..) |
 343             ty::ReVar(..) |
 344             ty::ReFree(..) => {
 345                 if self.is_eq_relation {
 346                     return r;
 347                 }
 348             }
 349         }
 350
 351         // FIXME: This is non-ideal because we don't give a
 352         // very descriptive origin for this region variable.
 353         self.infcx.next_region_var(MiscVariable(self.span))
 354     }
 355 }
 356
 357 pub trait RelateResultCompare<'tcx, T> {
 358     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 359         F: FnOnce() -> TypeError<'tcx>;
 360 }
 361
 362 impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
 363     fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
 364         F: FnOnce() -> TypeError<'tcx>,
 365     {
 366         self.clone().and_then(|s| {
 367             if s == t {
 368                 self.clone()
 369             } else {
 370                 Err(f())
 371             }
 372         })
 373     }
 374 }
 375
 376 fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
 377                                -> TypeError<'tcx>
 378 {
 379     let (a, b) = v;
 380     TypeError::IntMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 381 }
 382
 383 fn float_unification_error<'tcx>(a_is_expected: bool,
 384                                  v: (ast::FloatTy, ast::FloatTy))
 385                                  -> TypeError<'tcx>
 386 {
 387     let (a, b) = v;
 388     TypeError::FloatMismatch(ty::relate::expected_found_bool(a_is_expected, &a, &b))
 389 }