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.
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.
11 //! # Lattice Variables
13 //! This file contains generic code for operating on inference variables
14 //! that are characterized by an upper- and lower-bound. The logic and
15 //! reasoning is explained in detail in the large comment in `infer.rs`.
17 //! The code in here is defined quite generically so that it can be
18 //! applied both to type variables, which represent types being inferred,
19 //! and fn variables, which represent function types being inferred.
20 //! It may eventually be applied to their types as well, who knows.
21 //! In some cases, the functions are also generic with respect to the
22 //! operation on the lattice (GLB vs LUB).
24 //! Although all the functions are generic, we generally write the
25 //! comments in a way that is specific to type variables and the LUB
26 //! operation. It's just easier that way.
28 //! In general all of the functions are defined parametrically
29 //! over a `LatticeValue`, which is a value defined with respect to
33 use super::type_variable::TypeVariableOrigin;
35 use traits::ObligationCause;
38 use ty::relate::{RelateResult, TypeRelation};
40 pub trait LatticeDir<'f, 'gcx: 'f+'tcx, 'tcx: 'f> : TypeRelation<'f, 'gcx, 'tcx> {
41 fn infcx(&self) -> &'f InferCtxt<'f, 'gcx, 'tcx>;
43 fn cause(&self) -> &ObligationCause<'tcx>;
45 // Relates the type `v` to `a` and `b` such that `v` represents
46 // the LUB/GLB of `a` and `b` as appropriate.
48 // Subtle hack: ordering *may* be significant here. This method
49 // relates `v` to `a` first, which may help us to avoid unnecessary
50 // type variable obligations. See caller for details.
51 fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()>;
54 pub fn super_lattice_tys<'a, 'gcx, 'tcx, L>(this: &mut L,
57 -> RelateResult<'tcx, Ty<'tcx>>
58 where L: LatticeDir<'a, 'gcx, 'tcx>, 'gcx: 'a+'tcx, 'tcx: 'a
60 debug!("{}.lattice_tys({:?}, {:?})",
69 let infcx = this.infcx();
70 let a = infcx.type_variables.borrow_mut().replace_if_possible(a);
71 let b = infcx.type_variables.borrow_mut().replace_if_possible(b);
72 match (&a.sty, &b.sty) {
73 // If one side is known to be a variable and one is not,
74 // create a variable (`v`) to represent the LUB. Make sure to
75 // relate `v` to the non-type-variable first (by passing it
76 // first to `relate_bound`). Otherwise, we would produce a
77 // subtype obligation that must then be processed.
79 // Example: if the LHS is a type variable, and RHS is
80 // `Box<i32>`, then we current compare `v` to the RHS first,
81 // which will instantiate `v` with `Box<i32>`. Then when `v`
82 // is compared to the LHS, we instantiate LHS with `Box<i32>`.
83 // But if we did in reverse order, we would create a `v <:
84 // LHS` (or vice versa) constraint and then instantiate
85 // `v`. This would require further processing to achieve same
86 // end-result; in partiular, this screws up some of the logic
87 // in coercion, which expects LUB to figure out that the LHS
88 // is (e.g.) `Box<i32>`. A more obvious solution might be to
89 // iterate on the subtype obligations that are returned, but I
90 // think this suffices. -nmatsakis
91 (&ty::TyInfer(TyVar(..)), _) => {
92 let v = infcx.next_ty_var(TypeVariableOrigin::LatticeVariable(this.cause().span));
93 this.relate_bound(v, b, a)?;
96 (_, &ty::TyInfer(TyVar(..))) => {
97 let v = infcx.next_ty_var(TypeVariableOrigin::LatticeVariable(this.cause().span));
98 this.relate_bound(v, a, b)?;
103 infcx.super_combine_tys(this, a, b)