118 lines
4.2 KiB
Rust
118 lines
4.2 KiB
Rust
// Copyright 2014 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.
|
|
|
|
//! Applies the "bivariance relationship" to two types and/or regions.
|
|
//! If (A,B) are bivariant then either A <: B or B <: A. It occurs
|
|
//! when type/lifetime parameters are unconstrained. Usually this is
|
|
//! an error, but we permit it in the specific case where a type
|
|
//! parameter is constrained in a where-clause via an associated type.
|
|
//!
|
|
//! There are several ways one could implement bivariance. You could
|
|
//! just do nothing at all, for example, or you could fully verify
|
|
//! that one of the two subtyping relationships hold. We choose to
|
|
//! thread a middle line: we relate types up to regions, but ignore
|
|
//! all region relationships.
|
|
//!
|
|
//! At one point, handling bivariance in this fashion was necessary
|
|
//! for inference, but I'm actually not sure if that is true anymore.
|
|
//! In particular, it might be enough to say (A,B) are bivariant for
|
|
//! all (A,B).
|
|
|
|
use super::combine::{self, CombineFields};
|
|
use super::type_variable::{BiTo};
|
|
|
|
use middle::ty::{self, Ty};
|
|
use middle::ty::TyVar;
|
|
use middle::ty_relate::{Relate, RelateResult, TypeRelation};
|
|
|
|
pub struct Bivariate<'a, 'tcx: 'a> {
|
|
fields: CombineFields<'a, 'tcx>
|
|
}
|
|
|
|
impl<'a, 'tcx> Bivariate<'a, 'tcx> {
|
|
pub fn new(fields: CombineFields<'a, 'tcx>) -> Bivariate<'a, 'tcx> {
|
|
Bivariate { fields: fields }
|
|
}
|
|
}
|
|
|
|
impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Bivariate<'a, 'tcx> {
|
|
fn tag(&self) -> &'static str { "Bivariate" }
|
|
|
|
fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.fields.tcx() }
|
|
|
|
fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
|
|
|
|
fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
|
|
variance: ty::Variance,
|
|
a: &T,
|
|
b: &T)
|
|
-> RelateResult<'tcx, T>
|
|
{
|
|
match variance {
|
|
// If we have Foo<A> and Foo is invariant w/r/t A,
|
|
// and we want to assert that
|
|
//
|
|
// Foo<A> <: Foo<B> ||
|
|
// Foo<B> <: Foo<A>
|
|
//
|
|
// then still A must equal B.
|
|
ty::Invariant => self.relate(a, b),
|
|
|
|
ty::Covariant => self.relate(a, b),
|
|
ty::Bivariant => self.relate(a, b),
|
|
ty::Contravariant => self.relate(a, b),
|
|
}
|
|
}
|
|
|
|
fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
|
|
debug!("{}.tys({:?}, {:?})", self.tag(),
|
|
a, b);
|
|
if a == b { return Ok(a); }
|
|
|
|
let infcx = self.fields.infcx;
|
|
let a = infcx.type_variables.borrow().replace_if_possible(a);
|
|
let b = infcx.type_variables.borrow().replace_if_possible(b);
|
|
match (&a.sty, &b.sty) {
|
|
(&ty::TyInfer(TyVar(a_id)), &ty::TyInfer(TyVar(b_id))) => {
|
|
infcx.type_variables.borrow_mut().relate_vars(a_id, BiTo, b_id);
|
|
Ok(a)
|
|
}
|
|
|
|
(&ty::TyInfer(TyVar(a_id)), _) => {
|
|
try!(self.fields.instantiate(b, BiTo, a_id));
|
|
Ok(a)
|
|
}
|
|
|
|
(_, &ty::TyInfer(TyVar(b_id))) => {
|
|
try!(self.fields.instantiate(a, BiTo, b_id));
|
|
Ok(a)
|
|
}
|
|
|
|
_ => {
|
|
combine::super_combine_tys(self.fields.infcx, self, a, b)
|
|
}
|
|
}
|
|
}
|
|
|
|
fn regions(&mut self, a: ty::Region, _: ty::Region) -> RelateResult<'tcx, ty::Region> {
|
|
Ok(a)
|
|
}
|
|
|
|
fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
|
|
-> RelateResult<'tcx, ty::Binder<T>>
|
|
where T: Relate<'a,'tcx>
|
|
{
|
|
let a1 = self.tcx().erase_late_bound_regions(a);
|
|
let b1 = self.tcx().erase_late_bound_regions(b);
|
|
let c = try!(self.relate(&a1, &b1));
|
|
Ok(ty::Binder(c))
|
|
}
|
|
}
|