rustc_infer/infer/relate/
higher_ranked.rs

1//! Helper routines for higher-ranked things. See the `doc` module at
2//! the end of the file for details.
3
4use rustc_middle::ty::{self, FnMutDelegate, Ty, TyCtxt, TypeFoldable, TypeVisitableExt};
5use tracing::{debug, instrument};
6
7use super::RelateResult;
8use crate::infer::InferCtxt;
9use crate::infer::snapshot::CombinedSnapshot;
10
11impl<'tcx> InferCtxt<'tcx> {
12    /// Replaces all bound variables (lifetimes, types, and constants) bound by
13    /// `binder` with placeholder variables in a new universe. This means that the
14    /// new placeholders can only be named by inference variables created after
15    /// this method has been called.
16    ///
17    /// This is the first step of checking subtyping when higher-ranked things are involved.
18    /// For more details visit the relevant sections of the [rustc dev guide].
19    ///
20    /// `fn enter_forall` should be preferred over this method.
21    ///
22    /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
23    #[instrument(level = "debug", skip(self), ret)]
24    pub fn enter_forall_and_leak_universe<T>(&self, binder: ty::Binder<'tcx, T>) -> T
25    where
26        T: TypeFoldable<TyCtxt<'tcx>>,
27    {
28        // Inlined `no_bound_vars`.
29        if !binder.as_ref().skip_binder().has_escaping_bound_vars() {
30            return binder.skip_binder();
31        }
32
33        let next_universe = self.create_next_universe();
34
35        let delegate = FnMutDelegate {
36            regions: &mut |br: ty::BoundRegion| {
37                ty::Region::new_placeholder(self.tcx, ty::PlaceholderRegion::new(next_universe, br))
38            },
39            types: &mut |bound_ty: ty::BoundTy| {
40                Ty::new_placeholder(self.tcx, ty::PlaceholderType::new(next_universe, bound_ty))
41            },
42            consts: &mut |bound_const: ty::BoundConst| {
43                ty::Const::new_placeholder(
44                    self.tcx,
45                    ty::PlaceholderConst::new(next_universe, bound_const),
46                )
47            },
48        };
49
50        debug!(?next_universe);
51        self.tcx.replace_bound_vars_uncached(binder, delegate)
52    }
53
54    /// Replaces all bound variables (lifetimes, types, and constants) bound by
55    /// `binder` with placeholder variables in a new universe and then calls the
56    /// closure `f` with the instantiated value. The new placeholders can only be
57    /// named by inference variables created inside of the closure `f` or afterwards.
58    ///
59    /// This is the first step of checking subtyping when higher-ranked things are involved.
60    /// For more details visit the relevant sections of the [rustc dev guide].
61    ///
62    /// This method should be preferred over `fn enter_forall_and_leak_universe`.
63    ///
64    /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
65    #[instrument(level = "debug", skip(self, f))]
66    pub fn enter_forall<T, U>(&self, forall: ty::Binder<'tcx, T>, f: impl FnOnce(T) -> U) -> U
67    where
68        T: TypeFoldable<TyCtxt<'tcx>>,
69    {
70        // FIXME: currently we do nothing to prevent placeholders with the new universe being
71        // used after exiting `f`. For example region subtyping can result in outlives constraints
72        // that name placeholders created in this function. Nested goals from type relations can
73        // also contain placeholders created by this function.
74        let value = self.enter_forall_and_leak_universe(forall);
75        debug!(?value);
76        f(value)
77    }
78
79    /// See [RegionConstraintCollector::leak_check][1]. We only check placeholder
80    /// leaking into `outer_universe`, i.e. placeholders which cannot be named by that
81    /// universe.
82    ///
83    /// [1]: crate::infer::region_constraints::RegionConstraintCollector::leak_check
84    pub fn leak_check(
85        &self,
86        outer_universe: ty::UniverseIndex,
87        only_consider_snapshot: Option<&CombinedSnapshot<'tcx>>,
88    ) -> RelateResult<'tcx, ()> {
89        // If the user gave `-Zno-leak-check`, or we have been
90        // configured to skip the leak check, then skip the leak check
91        // completely. The leak check is deprecated. Any legitimate
92        // subtyping errors that it would have caught will now be
93        // caught later on, during region checking. However, we
94        // continue to use it for a transition period.
95        if self.tcx.sess.opts.unstable_opts.no_leak_check || self.skip_leak_check {
96            return Ok(());
97        }
98
99        self.inner.borrow_mut().unwrap_region_constraints().leak_check(
100            self.tcx,
101            outer_universe,
102            self.universe(),
103            only_consider_snapshot,
104        )
105    }
106}
rustc_infer/infer/relate/higher_ranked.rs

rustc_infer/infer/relate/
higher_ranked.rs
