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, PlaceholderMap};
7 use crate::infer::CombinedSnapshot;
8 use rustc::ty::relate::{Relate, RelateResult, TypeRelation};
9 use rustc::ty::{self, Binder, TypeFoldable};
11 impl<'a, 'tcx> CombineFields<'a, 'tcx> {
12 pub fn higher_ranked_sub<T>(
17 ) -> RelateResult<'tcx, Binder<T>>
21 debug!("higher_ranked_sub(a={:?}, b={:?})", a, b);
23 // Rather than checking the subtype relationship between `a` and `b`
24 // as-is, we need to do some extra work here in order to make sure
25 // that function subtyping works correctly with respect to regions
27 // Note: this is a subtle algorithm. For a full explanation,
28 // please see the large comment at the end of the file in the (inlined) module
31 let span = self.trace.cause.span;
33 return self.infcx.commit_if_ok(|snapshot| {
34 // First, we instantiate each bound region in the supertype with a
35 // fresh placeholder region.
36 let (b_prime, placeholder_map) = self.infcx.replace_bound_vars_with_placeholders(b);
38 // Next, we instantiate each bound region in the subtype
39 // with a fresh region variable. These region variables --
40 // but no other pre-existing region variables -- can name
43 self.infcx.replace_bound_vars_with_fresh_vars(span, HigherRankedType, a);
45 debug!("a_prime={:?}", a_prime);
46 debug!("b_prime={:?}", b_prime);
48 // Compare types now that bound regions have been replaced.
49 let result = self.sub(a_is_expected).relate(&a_prime, &b_prime)?;
51 self.infcx.leak_check(!a_is_expected, &placeholder_map, snapshot)?;
53 debug!("higher_ranked_sub: OK result={:?}", result);
55 Ok(ty::Binder::bind(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 must call this function from within a snapshot.
67 /// Moreover, before committing the snapshot, you must eventually call
68 /// either `plug_leaks` or `pop_placeholders` to remove the placeholder
69 /// regions. If you rollback the snapshot (or are using a probe), then
70 /// the pop occurs as part of the rollback, so an explicit call is not
71 /// needed (but is also permitted).
73 /// For more information about how placeholders and HRTBs work, see
74 /// the [rustc dev guide].
76 /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
77 pub fn replace_bound_vars_with_placeholders<T>(
79 binder: &ty::Binder<T>,
80 ) -> (T, PlaceholderMap<'tcx>)
82 T: TypeFoldable<'tcx>,
84 let next_universe = self.create_next_universe();
87 self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion {
88 universe: next_universe,
93 let fld_t = |bound_ty: ty::BoundTy| {
94 self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType {
95 universe: next_universe,
100 let fld_c = |bound_var: ty::BoundVar, ty| {
101 self.tcx.mk_const(ty::Const {
102 val: ty::ConstKind::Placeholder(ty::PlaceholderConst {
103 universe: next_universe,
110 let (result, map) = self.tcx.replace_bound_vars(binder, fld_r, fld_t, fld_c);
113 "replace_bound_vars_with_placeholders(\
114 next_universe={:?}, \
118 next_universe, binder, result, map,
124 /// See `infer::region_constraints::RegionConstraintCollector::leak_check`.
127 overly_polymorphic: bool,
128 placeholder_map: &PlaceholderMap<'tcx>,
129 snapshot: &CombinedSnapshot<'_, 'tcx>,
130 ) -> RelateResult<'tcx, ()> {
131 // If the user gave `-Zno-leak-check`, or we have been
132 // configured to skip the leak check, then skip the leak check
133 // completely. The leak check is deprecated. Any legitimate
134 // subtyping errors that it would have caught will now be
135 // caught later on, during region checking. However, we
136 // continue to use it for a transition period.
137 if self.tcx.sess.opts.debugging_opts.no_leak_check || self.skip_leak_check.get() {
141 self.inner.borrow_mut().unwrap_region_constraints().leak_check(