rust/src/librustc/middle/infer/bivariate.rs

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// 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.
2015-02-18 17:30:02 -05:00
//! 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 middle::ty::BuiltinBounds;
use middle::ty::{self, Ty};
use middle::ty::TyVar;
use middle::infer::combine::*;
use middle::infer::CombineResult;
use middle::infer::type_variable::BiTo;
use util::ppaux::Repr;
pub struct Bivariate<'f, 'tcx: 'f> {
fields: CombineFields<'f, 'tcx>
}
#[allow(non_snake_case)]
pub fn Bivariate<'f, 'tcx>(cf: CombineFields<'f, 'tcx>) -> Bivariate<'f, 'tcx> {
Bivariate { fields: cf }
}
impl<'f, 'tcx> Combine<'tcx> for Bivariate<'f, 'tcx> {
fn tag(&self) -> String { "Bivariate".to_string() }
fn fields<'a>(&'a self) -> &'a CombineFields<'a, 'tcx> { &self.fields }
fn tys_with_variance(&self, v: ty::Variance, a: Ty<'tcx>, b: Ty<'tcx>)
-> CombineResult<'tcx, Ty<'tcx>>
{
match v {
ty::Invariant => self.equate().tys(a, b),
ty::Covariant => self.tys(a, b),
ty::Contravariant => self.tys(a, b),
ty::Bivariant => self.tys(a, b),
}
}
fn regions_with_variance(&self, v: ty::Variance, a: ty::Region, b: ty::Region)
-> CombineResult<'tcx, ty::Region>
{
match v {
ty::Invariant => self.equate().regions(a, b),
ty::Covariant => self.regions(a, b),
ty::Contravariant => self.regions(a, b),
ty::Bivariant => self.regions(a, b),
}
}
fn regions(&self, a: ty::Region, _: ty::Region) -> CombineResult<'tcx, ty::Region> {
Ok(a)
}
fn builtin_bounds(&self,
a: BuiltinBounds,
b: BuiltinBounds)
-> CombineResult<'tcx, BuiltinBounds>
{
if a != b {
Err(ty::terr_builtin_bounds(expected_found(self, a, b)))
} else {
Ok(a)
}
}
fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CombineResult<'tcx, Ty<'tcx>> {
debug!("{}.tys({}, {})", self.tag(),
a.repr(self.fields.infcx.tcx), b.repr(self.fields.infcx.tcx));
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::ty_infer(TyVar(a_id)), &ty::ty_infer(TyVar(b_id))) => {
infcx.type_variables.borrow_mut().relate_vars(a_id, BiTo, b_id);
Ok(a)
}
(&ty::ty_infer(TyVar(a_id)), _) => {
try!(self.fields.instantiate(b, BiTo, a_id));
Ok(a)
}
(_, &ty::ty_infer(TyVar(b_id))) => {
try!(self.fields.instantiate(a, BiTo, b_id));
Ok(a)
}
_ => {
super_tys(self, a, b)
}
}
}
fn binders<T>(&self, a: &ty::Binder<T>, b: &ty::Binder<T>) -> CombineResult<'tcx, ty::Binder<T>>
where T : Combineable<'tcx>
{
let a1 = ty::erase_late_bound_regions(self.tcx(), a);
let b1 = ty::erase_late_bound_regions(self.tcx(), b);
let c = try!(Combineable::combine(self, &a1, &b1));
Ok(ty::Binder(c))
}
}