src/librustc/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 ty::{self, Ty, TyCtxt};
  12 use ty::error::TypeError;
  13 use ty::relate::{self, Relate, TypeRelation, RelateResult};
  14
  15 /// A type "A" *matches* "B" if the fresh types in B could be
  16 /// substituted with values so as to make it equal to A. Matching is
  17 /// intended to be used only on freshened types, and it basically
  18 /// indicates if the non-freshened versions of A and B could have been
  19 /// unified.
  20 ///
  21 /// It is only an approximation. If it yields false, unification would
  22 /// definitely fail, but a true result doesn't mean unification would
  23 /// succeed. This is because we don't track the "side-constraints" on
  24 /// type variables, nor do we track if the same freshened type appears
  25 /// more than once. To some extent these approximations could be
  26 /// fixed, given effort.
  27 ///
  28 /// Like subtyping, matching is really a binary relation, so the only
  29 /// important thing about the result is Ok/Err. Also, matching never
  30 /// affects any type variables or unification state.
  31 pub struct Match<'a, 'tcx: 'a> {
  32     tcx: &'a TyCtxt<'tcx>
  33 }
  34
  35 impl<'a, 'tcx> Match<'a, 'tcx> {
  36     pub fn new(tcx: &'a TyCtxt<'tcx>) -> Match<'a, 'tcx> {
  37         Match { tcx: tcx }
  38     }
  39 }
  40
  41 impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Match<'a, 'tcx> {
  42     fn tag(&self) -> &'static str { "Match" }
  43     fn tcx(&self) -> &'a TyCtxt<'tcx> { self.tcx }
  44     fn a_is_expected(&self) -> bool { true } // irrelevant
  45
  46     fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
  47                                                _: ty::Variance,
  48                                                a: &T,
  49                                                b: &T)
  50                                                -> RelateResult<'tcx, T>
  51     {
  52         self.relate(a, b)
  53     }
  54
  55     fn regions(&mut self, a: ty::Region, b: ty::Region) -> RelateResult<'tcx, ty::Region> {
  56         debug!("{}.regions({:?}, {:?})",
  57                self.tag(),
  58                a,
  59                b);
  60         Ok(a)
  61     }
  62
  63     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
  64         debug!("{}.tys({:?}, {:?})", self.tag(),
  65                a, b);
  66         if a == b { return Ok(a); }
  67
  68         match (&a.sty, &b.sty) {
  69             (_, &ty::TyInfer(ty::FreshTy(_))) |
  70             (_, &ty::TyInfer(ty::FreshIntTy(_))) |
  71             (_, &ty::TyInfer(ty::FreshFloatTy(_))) => {
  72                 Ok(a)
  73             }
  74
  75             (&ty::TyInfer(_), _) |
  76             (_, &ty::TyInfer(_)) => {
  77                 Err(TypeError::Sorts(relate::expected_found(self, &a, &b)))
  78             }
  79
  80             (&ty::TyError, _) | (_, &ty::TyError) => {
  81                 Ok(self.tcx().types.err)
  82             }
  83
  84             _ => {
  85                 relate::super_relate_tys(self, a, b)
  86             }
  87         }
  88     }
  89
  90     fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
  91                   -> RelateResult<'tcx, ty::Binder<T>>
  92         where T: Relate<'a,'tcx>
  93     {
  94         Ok(ty::Binder(self.relate(a.skip_binder(), b.skip_binder())?))
  95     }
  96 }