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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 | ||
54a0048b | 11 | use rustc::ty; |
7453a54e SL |
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 | } |