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//! Greatest lower bound. See [`lattice`].
use super::combine::{CombineFields, ObligationEmittingRelation};
use super::lattice::{self, LatticeDir};
use super::Subtype;
use super::{DefineOpaqueTypes, InferCtxt};
use crate::traits::{ObligationCause, PredicateObligations};
use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
/// "Greatest lower bound" (common subtype)
pub struct Glb<'combine, 'infcx, 'tcx> {
fields: &'combine mut CombineFields<'infcx, 'tcx>,
a_is_expected: bool,
}
impl<'combine, 'infcx, 'tcx> Glb<'combine, 'infcx, 'tcx> {
pub fn new(
fields: &'combine mut CombineFields<'infcx, 'tcx>,
a_is_expected: bool,
) -> Glb<'combine, 'infcx, 'tcx> {
Glb { fields, a_is_expected }
}
}
impl<'tcx> TypeRelation<'tcx> for Glb<'_, '_, 'tcx> {
fn tag(&self) -> &'static str {
"Glb"
}
fn tcx(&self) -> TyCtxt<'tcx> {
self.fields.tcx()
}
fn param_env(&self) -> ty::ParamEnv<'tcx> {
self.fields.param_env
}
fn a_is_expected(&self) -> bool {
self.a_is_expected
}
fn relate_with_variance<T: Relate<'tcx>>(
&mut self,
variance: ty::Variance,
_info: ty::VarianceDiagInfo<'tcx>,
a: T,
b: T,
) -> RelateResult<'tcx, T> {
match variance {
ty::Invariant => self.fields.equate(self.a_is_expected).relate(a, b),
ty::Covariant => self.relate(a, b),
// FIXME(#41044) -- not correct, need test
ty::Bivariant => Ok(a),
ty::Contravariant => self.fields.lub(self.a_is_expected).relate(a, b),
}
}
fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
lattice::super_lattice_tys(self, a, b)
}
fn regions(
&mut self,
a: ty::Region<'tcx>,
b: ty::Region<'tcx>,
) -> RelateResult<'tcx, ty::Region<'tcx>> {
debug!("{}.regions({:?}, {:?})", self.tag(), a, b);
let origin = Subtype(Box::new(self.fields.trace.clone()));
// GLB(&'static u8, &'a u8) == &RegionLUB('static, 'a) u8 == &'static u8
Ok(self.fields.infcx.inner.borrow_mut().unwrap_region_constraints().lub_regions(
self.tcx(),
origin,
a,
b,
))
}
fn consts(
&mut self,
a: ty::Const<'tcx>,
b: ty::Const<'tcx>,
) -> RelateResult<'tcx, ty::Const<'tcx>> {
self.fields.infcx.super_combine_consts(self, a, b)
}
fn binders<T>(
&mut self,
a: ty::Binder<'tcx, T>,
b: ty::Binder<'tcx, T>,
) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
where
T: Relate<'tcx>,
{
// GLB of a binder and itself is just itself
if a == b {
return Ok(a);
}
debug!("binders(a={:?}, b={:?})", a, b);
if a.skip_binder().has_escaping_bound_vars() || b.skip_binder().has_escaping_bound_vars() {
// When higher-ranked types are involved, computing the GLB is
// very challenging, switch to invariance. This is obviously
// overly conservative but works ok in practice.
self.relate_with_variance(
ty::Variance::Invariant,
ty::VarianceDiagInfo::default(),
a,
b,
)?;
Ok(a)
} else {
Ok(ty::Binder::dummy(self.relate(a.skip_binder(), b.skip_binder())?))
}
}
}
impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Glb<'combine, 'infcx, 'tcx> {
fn infcx(&self) -> &'infcx InferCtxt<'tcx> {
self.fields.infcx
}
fn cause(&self) -> &ObligationCause<'tcx> {
&self.fields.trace.cause
}
fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
let mut sub = self.fields.sub(self.a_is_expected);
sub.relate(v, a)?;
sub.relate(v, b)?;
Ok(())
}
fn define_opaque_types(&self) -> DefineOpaqueTypes {
self.fields.define_opaque_types
}
}
impl<'tcx> ObligationEmittingRelation<'tcx> for Glb<'_, '_, 'tcx> {
fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ty::ToPredicate<'tcx>>) {
self.fields.register_predicates(obligations);
}
fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
self.fields.register_obligations(obligations);
}
fn alias_relate_direction(&self) -> ty::AliasRelationDirection {
// FIXME(deferred_projection_equality): This isn't right, I think?
ty::AliasRelationDirection::Equate
}
}
|
