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//! Helper routines for higher-ranked things. See the `doc` module at
//! the end of the file for details.
use super::combine::CombineFields;
use super::{HigherRankedType, InferCtxt};
use crate::infer::CombinedSnapshot;
use rustc_middle::ty::fold::FnMutDelegate;
use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
use rustc_middle::ty::{self, Binder, TypeFoldable};
impl<'a, 'tcx> CombineFields<'a, 'tcx> {
/// Checks whether `for<..> sub <: for<..> sup` holds.
///
/// For this to hold, **all** instantiations of the super type
/// have to be a super type of **at least one** instantiation of
/// the subtype.
///
/// This is implemented by first entering a new universe.
/// We then replace all bound variables in `sup` with placeholders,
/// and all bound variables in `sub` with inference vars.
/// We can then just relate the two resulting types as normal.
///
/// Note: this is a subtle algorithm. For a full explanation, please see
/// the [rustc dev guide][rd]
///
/// [rd]: https://rustc-dev-guide.rust-lang.org/borrow_check/region_inference/placeholders_and_universes.html
#[instrument(skip(self), level = "debug")]
pub fn higher_ranked_sub<T>(
&mut self,
sub: Binder<'tcx, T>,
sup: Binder<'tcx, T>,
sub_is_expected: bool,
) -> RelateResult<'tcx, ()>
where
T: Relate<'tcx>,
{
let span = self.trace.cause.span;
// First, we instantiate each bound region in the supertype with a
// fresh placeholder region. Note that this automatically creates
// a new universe if needed.
let sup_prime = self.infcx.replace_bound_vars_with_placeholders(sup);
// Next, we instantiate each bound region in the subtype
// with a fresh region variable. These region variables --
// but no other pre-existing region variables -- can name
// the placeholders.
let sub_prime = self.infcx.replace_bound_vars_with_fresh_vars(span, HigherRankedType, sub);
debug!("a_prime={:?}", sub_prime);
debug!("b_prime={:?}", sup_prime);
// Compare types now that bound regions have been replaced.
let result = self.sub(sub_is_expected).relate(sub_prime, sup_prime)?;
debug!("OK result={result:?}");
// NOTE: returning the result here would be dangerous as it contains
// placeholders which **must not** be named afterwards.
Ok(())
}
}
impl<'tcx> InferCtxt<'tcx> {
/// Replaces all bound variables (lifetimes, types, and constants) bound by
/// `binder` with placeholder variables in a new universe. This means that the
/// new placeholders can only be named by inference variables created after
/// this method has been called.
///
/// This is the first step of checking subtyping when higher-ranked things are involved.
/// For more details visit the relevant sections of the [rustc dev guide].
///
/// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
#[instrument(level = "debug", skip(self), ret)]
pub fn replace_bound_vars_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T
where
T: TypeFoldable<'tcx> + Copy,
{
if let Some(inner) = binder.no_bound_vars() {
return inner;
}
let next_universe = self.create_next_universe();
let delegate = FnMutDelegate {
regions: &mut |br: ty::BoundRegion| {
self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion {
universe: next_universe,
name: br.kind,
}))
},
types: &mut |bound_ty: ty::BoundTy| {
self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType {
universe: next_universe,
name: bound_ty.var,
}))
},
consts: &mut |bound_var: ty::BoundVar, ty| {
self.tcx
.mk_const(ty::PlaceholderConst { universe: next_universe, name: bound_var }, ty)
},
};
debug!(?next_universe);
self.tcx.replace_bound_vars_uncached(binder, delegate)
}
/// See [RegionConstraintCollector::leak_check][1].
///
/// [1]: crate::infer::region_constraints::RegionConstraintCollector::leak_check
pub fn leak_check(
&self,
overly_polymorphic: bool,
snapshot: &CombinedSnapshot<'tcx>,
) -> RelateResult<'tcx, ()> {
// If the user gave `-Zno-leak-check`, or we have been
// configured to skip the leak check, then skip the leak check
// completely. The leak check is deprecated. Any legitimate
// subtyping errors that it would have caught will now be
// caught later on, during region checking. However, we
// continue to use it for a transition period.
if self.tcx.sess.opts.unstable_opts.no_leak_check || self.skip_leak_check.get() {
return Ok(());
}
self.inner.borrow_mut().unwrap_region_constraints().leak_check(
self.tcx,
overly_polymorphic,
self.universe(),
snapshot,
)
}
}
|
