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//! Helper routines for higher-ranked things. See the `doc` module at
//! the end of the file for details.
use rustc_middle::ty::{self, FnMutDelegate, Ty, TyCtxt, TypeFoldable, TypeVisitableExt};
use tracing::{debug, instrument};
use super::RelateResult;
use crate::infer::InferCtxt;
use crate::infer::snapshot::CombinedSnapshot;
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].
///
/// `fn enter_forall` should be preferred over this method.
///
/// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
#[instrument(level = "debug", skip(self), ret)]
pub fn enter_forall_and_leak_universe<T>(&self, binder: ty::Binder<'tcx, T>) -> T
where
T: TypeFoldable<TyCtxt<'tcx>>,
{
// Inlined `no_bound_vars`.
if !binder.as_ref().skip_binder().has_escaping_bound_vars() {
return binder.skip_binder();
}
let next_universe = self.create_next_universe();
let delegate = FnMutDelegate {
regions: &mut |br: ty::BoundRegion| {
ty::Region::new_placeholder(
self.tcx,
ty::PlaceholderRegion { universe: next_universe, bound: br },
)
},
types: &mut |bound_ty: ty::BoundTy| {
Ty::new_placeholder(
self.tcx,
ty::PlaceholderType { universe: next_universe, bound: bound_ty },
)
},
consts: &mut |bound_const: ty::BoundConst| {
ty::Const::new_placeholder(
self.tcx,
ty::PlaceholderConst { universe: next_universe, bound: bound_const },
)
},
};
debug!(?next_universe);
self.tcx.replace_bound_vars_uncached(binder, delegate)
}
/// Replaces all bound variables (lifetimes, types, and constants) bound by
/// `binder` with placeholder variables in a new universe and then calls the
/// closure `f` with the instantiated value. The new placeholders can only be
/// named by inference variables created inside of the closure `f` or afterwards.
///
/// 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].
///
/// This method should be preferred over `fn enter_forall_and_leak_universe`.
///
/// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
#[instrument(level = "debug", skip(self, f))]
pub fn enter_forall<T, U>(&self, forall: ty::Binder<'tcx, T>, f: impl FnOnce(T) -> U) -> U
where
T: TypeFoldable<TyCtxt<'tcx>>,
{
// FIXME: currently we do nothing to prevent placeholders with the new universe being
// used after exiting `f`. For example region subtyping can result in outlives constraints
// that name placeholders created in this function. Nested goals from type relations can
// also contain placeholders created by this function.
let value = self.enter_forall_and_leak_universe(forall);
debug!(?value);
f(value)
}
/// See [RegionConstraintCollector::leak_check][1]. We only check placeholder
/// leaking into `outer_universe`, i.e. placeholders which cannot be named by that
/// universe.
///
/// [1]: crate::infer::region_constraints::RegionConstraintCollector::leak_check
pub fn leak_check(
&self,
outer_universe: ty::UniverseIndex,
only_consider_snapshot: Option<&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 {
return Ok(());
}
self.inner.borrow_mut().unwrap_region_constraints().leak_check(
self.tcx,
outer_universe,
self.universe(),
only_consider_snapshot,
)
}
}
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