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};
7 use crate::infer::CombinedSnapshot;
8 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
9 use rustc_middle::ty::{self, Binder, TypeFoldable};
11 impl<'a, 'tcx> CombineFields<'a, 'tcx> {
12 #[instrument(skip(self), level = "debug")]
13 pub fn higher_ranked_sub<T>(
18 ) -> RelateResult<'tcx, Binder<'tcx, T>>
22 // Rather than checking the subtype relationship between `a` and `b`
23 // as-is, we need to do some extra work here in order to make sure
24 // that function subtyping works correctly with respect to regions
26 // Note: this is a subtle algorithm. For a full explanation, please see
27 // the rustc dev guide:
28 // <https://rustc-dev-guide.rust-lang.org/borrow_check/region_inference/placeholders_and_universes.html>
30 let span = self.trace.cause.span;
32 self.infcx.commit_if_ok(|_| {
33 // First, we instantiate each bound region in the supertype with a
34 // fresh placeholder region.
35 let b_prime = self.infcx.replace_bound_vars_with_placeholders(b);
37 // Next, we instantiate each bound region in the subtype
38 // with a fresh region variable. These region variables --
39 // but no other pre-existing region variables -- can name
42 self.infcx.replace_bound_vars_with_fresh_vars(span, HigherRankedType, a);
44 debug!("a_prime={:?}", a_prime);
45 debug!("b_prime={:?}", b_prime);
47 // Compare types now that bound regions have been replaced.
48 let result = self.sub(a_is_expected).relate(a_prime, b_prime)?;
50 debug!("higher_ranked_sub: OK result={:?}", result);
52 // We related `a_prime` and `b_prime`, which just had any bound vars
53 // replaced with placeholders or infer vars, respectively. Relating
54 // them should not introduce new bound vars.
55 Ok(ty::Binder::dummy(result))
60 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
61 /// Replaces all regions (resp. types) bound by `binder` with placeholder
62 /// regions (resp. types) and return a map indicating which bound-region
63 /// placeholder region. This is the first step of checking subtyping
64 /// when higher-ranked things are involved.
66 /// **Important:** You have to be careful to not leak these placeholders,
67 /// for more information about how placeholders and HRTBs work, see
68 /// the [rustc dev guide].
70 /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
71 pub fn replace_bound_vars_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T
73 T: TypeFoldable<'tcx>,
75 // Figure out what the next universe will be, but don't actually create
76 // it until after we've done the substitution (in particular there may
77 // be no bound variables). This is a performance optimization, since the
78 // leak check for example can be skipped if no new universes are created
79 // (i.e., if there are no placeholders).
80 let next_universe = self.universe().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::Const {
98 val: ty::ConstKind::Placeholder(ty::PlaceholderConst {
99 universe: next_universe,
100 name: ty::BoundConst { var: bound_var, ty },
106 let (result, map) = self.tcx.replace_bound_vars(binder, fld_r, fld_t, fld_c);
108 // If there were higher-ranked regions to replace, then actually create
109 // the next universe (this avoids needlessly creating universes).
111 let n_u = self.create_next_universe();
112 assert_eq!(n_u, next_universe);
116 "replace_bound_vars_with_placeholders(\
117 next_universe={:?}, \
120 next_universe, result, map,
126 /// See `infer::region_constraints::RegionConstraintCollector::leak_check`.
129 overly_polymorphic: bool,
130 snapshot: &CombinedSnapshot<'_, 'tcx>,
131 ) -> RelateResult<'tcx, ()> {
132 // If the user gave `-Zno-leak-check`, or we have been
133 // configured to skip the leak check, then skip the leak check
134 // completely. The leak check is deprecated. Any legitimate
135 // subtyping errors that it would have caught will now be
136 // caught later on, during region checking. However, we
137 // continue to use it for a transition period.
138 if self.tcx.sess.opts.debugging_opts.no_leak_check || self.skip_leak_check.get() {
142 self.inner.borrow_mut().unwrap_region_constraints().leak_check(