src/librustc_typeck/variance/xform.rs

   1 // Copyright 2013 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 rustc::ty;
  12
  13 pub trait Xform {
  14     fn xform(self, v: Self) -> Self;
  15 }
  16
  17 impl Xform for ty::Variance {
  18     fn xform(self, v: ty::Variance) -> ty::Variance {
  19         // "Variance transformation", Figure 1 of The Paper
  20         match (self, v) {
  21             // Figure 1, column 1.
  22             (ty::Covariant, ty::Covariant) => ty::Covariant,
  23             (ty::Covariant, ty::Contravariant) => ty::Contravariant,
  24             (ty::Covariant, ty::Invariant) => ty::Invariant,
  25             (ty::Covariant, ty::Bivariant) => ty::Bivariant,
  26
  27             // Figure 1, column 2.
  28             (ty::Contravariant, ty::Covariant) => ty::Contravariant,
  29             (ty::Contravariant, ty::Contravariant) => ty::Covariant,
  30             (ty::Contravariant, ty::Invariant) => ty::Invariant,
  31             (ty::Contravariant, ty::Bivariant) => ty::Bivariant,
  32
  33             // Figure 1, column 3.
  34             (ty::Invariant, _) => ty::Invariant,
  35
  36             // Figure 1, column 4.
  37             (ty::Bivariant, _) => ty::Bivariant,
  38         }
  39     }
  40 }
  41
  42 pub fn glb(v1: ty::Variance, v2: ty::Variance) -> ty::Variance {
  43     // Greatest lower bound of the variance lattice as
  44     // defined in The Paper:
  45     //
  46     //       *
  47     //    -     +
  48     //       o
  49     match (v1, v2) {
  50         (ty::Invariant, _) | (_, ty::Invariant) => ty::Invariant,
  51
  52         (ty::Covariant, ty::Contravariant) => ty::Invariant,
  53         (ty::Contravariant, ty::Covariant) => ty::Invariant,
  54
  55         (ty::Covariant, ty::Covariant) => ty::Covariant,
  56
  57         (ty::Contravariant, ty::Contravariant) => ty::Contravariant,
  58
  59         (x, ty::Bivariant) | (ty::Bivariant, x) => x,
  60     }
  61 }