src/librustc_typeck/variance/xform.rs

// 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 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,
    }
}