src/librustc/middle/ty_match.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 use middle::ty::{self, Ty};
  12 use middle::ty_relate::{self, Relate, TypeRelation, RelateResult};
  13
  14 /// A type "A" *matches* "B" if the fresh types in B could be
  15 /// substituted with values so as to make it equal to A. Matching is
  16 /// intended to be used only on freshened types, and it basically
  17 /// indicates if the non-freshened versions of A and B could have been
  18 /// unified.
  19 ///
  20 /// It is only an approximation. If it yields false, unification would
  21 /// definitely fail, but a true result doesn't mean unification would
  22 /// succeed. This is because we don't track the "side-constraints" on
  23 /// type variables, nor do we track if the same freshened type appears
  24 /// more than once. To some extent these approximations could be
  25 /// fixed, given effort.
  26 ///
  27 /// Like subtyping, matching is really a binary relation, so the only
  28 /// important thing about the result is Ok/Err. Also, matching never
  29 /// affects any type variables or unification state.
  30 pub struct Match<'a, 'tcx: 'a> {
  31     tcx: &'a ty::ctxt<'tcx>
  32 }
  33
  34 impl<'a, 'tcx> Match<'a, 'tcx> {
  35     pub fn new(tcx: &'a ty::ctxt<'tcx>) -> Match<'a, 'tcx> {
  36         Match { tcx: tcx }
  37     }
  38 }
  39
  40 impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Match<'a, 'tcx> {
  41     fn tag(&self) -> &'static str { "Match" }
  42     fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.tcx }
  43     fn a_is_expected(&self) -> bool { true } // irrelevant
  44
  45     fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
  46                                                _: ty::Variance,
  47                                                a: &T,
  48                                                b: &T)
  49                                                -> RelateResult<'tcx, T>
  50     {
  51         self.relate(a, b)
  52     }
  53
  54     fn regions(&mut self, a: ty::Region, b: ty::Region) -> RelateResult<'tcx, ty::Region> {
  55         debug!("{}.regions({:?}, {:?})",
  56                self.tag(),
  57                a,
  58                b);
  59         Ok(a)
  60     }
  61
  62     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  63         debug!("{}.tys({:?}, {:?})", self.tag(),
  64                a, b);
  65         if a == b { return Ok(a); }
  66
  67         match (&a.sty, &b.sty) {
  68             (_, &ty::TyInfer(ty::FreshTy(_))) |
  69             (_, &ty::TyInfer(ty::FreshIntTy(_))) |
  70             (_, &ty::TyInfer(ty::FreshFloatTy(_))) => {
  71                 Ok(a)
  72             }
  73
  74             (&ty::TyInfer(_), _) |
  75             (_, &ty::TyInfer(_)) => {
  76                 Err(ty::TypeError::Sorts(ty_relate::expected_found(self, &a, &b)))
  77             }
  78
  79             (&ty::TyError, _) | (_, &ty::TyError) => {
  80                 Ok(self.tcx().types.err)
  81             }
  82
  83             _ => {
  84                 ty_relate::super_relate_tys(self, a, b)
  85             }
  86         }
  87     }
  88
  89     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
  90                   -> RelateResult<'tcx, ty::Binder<T>>
  91         where T: Relate<'a,'tcx>
  92     {
  93         Ok(ty::Binder(try!(self.relate(a.skip_binder(), b.skip_binder()))))
  94     }
  95 }