| // Copyright 2013 The Rust Project Developers. See the COPYRIGHT |
| // file at the top-level directory of this distribution and at |
| // http://rust-lang.org/COPYRIGHT. |
| // |
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or |
| // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license |
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your |
| // option. This file may not be copied, modified, or distributed |
| // except according to those terms. |
| |
| use rustc::ty; |
| |
| pub trait Xform { |
| fn xform(self, v: Self) -> Self; |
| } |
| |
| impl Xform for ty::Variance { |
| fn xform(self, v: ty::Variance) -> ty::Variance { |
| // "Variance transformation", Figure 1 of The Paper |
| match (self, v) { |
| // Figure 1, column 1. |
| (ty::Covariant, ty::Covariant) => ty::Covariant, |
| (ty::Covariant, ty::Contravariant) => ty::Contravariant, |
| (ty::Covariant, ty::Invariant) => ty::Invariant, |
| (ty::Covariant, ty::Bivariant) => ty::Bivariant, |
| |
| // Figure 1, column 2. |
| (ty::Contravariant, ty::Covariant) => ty::Contravariant, |
| (ty::Contravariant, ty::Contravariant) => ty::Covariant, |
| (ty::Contravariant, ty::Invariant) => ty::Invariant, |
| (ty::Contravariant, ty::Bivariant) => ty::Bivariant, |
| |
| // Figure 1, column 3. |
| (ty::Invariant, _) => ty::Invariant, |
| |
| // Figure 1, column 4. |
| (ty::Bivariant, _) => ty::Bivariant, |
| } |
| } |
| } |
| |
| pub fn glb(v1: ty::Variance, v2: ty::Variance) -> ty::Variance { |
| // Greatest lower bound of the variance lattice as |
| // defined in The Paper: |
| // |
| // * |
| // - + |
| // o |
| match (v1, v2) { |
| (ty::Invariant, _) | (_, ty::Invariant) => ty::Invariant, |
| |
| (ty::Covariant, ty::Contravariant) => ty::Invariant, |
| (ty::Contravariant, ty::Covariant) => ty::Invariant, |
| |
| (ty::Covariant, ty::Covariant) => ty::Covariant, |
| |
| (ty::Contravariant, ty::Contravariant) => ty::Contravariant, |
| |
| (x, ty::Bivariant) | (ty::Bivariant, x) => x, |
| } |
| } |