1 //! Helper routines for higher-ranked things. See the `doc` module at
2 //! the end of the file for details.
4 use super::combine::CombineFields;
5 use super::{HigherRankedType, InferCtxt};
6 use crate::infer::CombinedSnapshot;
7 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
8 use rustc_middle::ty::{self, Binder, TypeFoldable};
10 impl<'a, 'tcx> CombineFields<'a, 'tcx> {
11 /// Checks whether `for<..> sub <: for<..> sup` holds.
13 /// For this to hold, **all** instantiations of the super type
14 /// have to be a super type of **at least one** instantiation of
17 /// This is implemented by first entering a new universe.
18 /// We then replace all bound variables in `sup` with placeholders,
19 /// and all bound variables in `sup` with inference vars.
20 /// We can then just relate the two resulting types as normal.
22 /// Note: this is a subtle algorithm. For a full explanation, please see
23 /// the [rustc dev guide][rd]
25 /// [rd]: https://rustc-dev-guide.rust-lang.org/borrow_check/region_inference/placeholders_and_universes.html
26 #[instrument(skip(self), level = "debug")]
27 pub fn higher_ranked_sub<T>(
31 sub_is_expected: bool,
32 ) -> RelateResult<'tcx, ()>
36 let span = self.trace.cause.span;
37 // First, we instantiate each bound region in the supertype with a
38 // fresh placeholder region. Note that this automatically creates
39 // a new universe if needed.
40 let sup_prime = self.infcx.replace_bound_vars_with_placeholders(sup);
42 // Next, we instantiate each bound region in the subtype
43 // with a fresh region variable. These region variables --
44 // but no other pre-existing region variables -- can name
46 let sub_prime = self.infcx.replace_bound_vars_with_fresh_vars(span, HigherRankedType, sub);
48 debug!("a_prime={:?}", sub_prime);
49 debug!("b_prime={:?}", sup_prime);
51 // Compare types now that bound regions have been replaced.
52 let result = self.sub(sub_is_expected).relate(sub_prime, sup_prime)?;
54 debug!("OK result={result:?}");
55 // NOTE: returning the result here would be dangerous as it contains
56 // placeholders which **must not** be named afterwards.
61 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
62 /// Replaces all bound variables (lifetimes, types, and constants) bound by
63 /// `binder` with placeholder variables in a new universe. This means that the
64 /// new placeholders can only be named by inference variables created after
65 /// this method has been called.
67 /// This is the first step of checking subtyping when higher-ranked things are involved.
68 /// For more details visit the relevant sections of the [rustc dev guide].
70 /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
71 #[instrument(level = "debug", skip(self))]
72 pub fn replace_bound_vars_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T
74 T: TypeFoldable<'tcx> + Copy,
76 if let Some(inner) = binder.no_bound_vars() {
80 let next_universe = self.create_next_universe();
82 let fld_r = |br: ty::BoundRegion| {
83 self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion {
84 universe: next_universe,
89 let fld_t = |bound_ty: ty::BoundTy| {
90 self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType {
91 universe: next_universe,
96 let fld_c = |bound_var: ty::BoundVar, ty| {
97 self.tcx.mk_const(ty::ConstS {
98 kind: ty::ConstKind::Placeholder(ty::PlaceholderConst {
99 universe: next_universe,
100 name: ty::BoundConst { var: bound_var, ty },
106 let result = self.tcx.replace_bound_vars_uncached(binder, fld_r, fld_t, fld_c);
107 debug!(?next_universe, ?result);
111 /// See [RegionConstraintCollector::leak_check][1].
113 /// [1]: crate::infer::region_constraints::RegionConstraintCollector::leak_check
116 overly_polymorphic: bool,
117 snapshot: &CombinedSnapshot<'_, 'tcx>,
118 ) -> RelateResult<'tcx, ()> {
119 // If the user gave `-Zno-leak-check`, or we have been
120 // configured to skip the leak check, then skip the leak check
121 // completely. The leak check is deprecated. Any legitimate
122 // subtyping errors that it would have caught will now be
123 // caught later on, during region checking. However, we
124 // continue to use it for a transition period.
125 if self.tcx.sess.opts.debugging_opts.no_leak_check || self.skip_leak_check.get() {
129 self.inner.borrow_mut().unwrap_region_constraints().leak_check(