]>
Commit | Line | Data |
---|---|---|
1a4d82fc JJ |
1 | //! Helper routines for higher-ranked things. See the `doc` module at |
2 | //! the end of the file for details. | |
3 | ||
c34b1796 | 4 | use super::combine::CombineFields; |
29967ef6 | 5 | use super::{HigherRankedType, InferCtxt}; |
1a4d82fc | 6 | |
9fa01778 | 7 | use crate::infer::CombinedSnapshot; |
ba9703b0 XL |
8 | use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation}; |
9 | use rustc_middle::ty::{self, Binder, TypeFoldable}; | |
3157f602 | 10 | |
dc9dc135 | 11 | impl<'a, 'tcx> CombineFields<'a, 'tcx> { |
c295e0f8 | 12 | #[instrument(skip(self), level = "debug")] |
0731742a XL |
13 | pub fn higher_ranked_sub<T>( |
14 | &mut self, | |
cdc7bbd5 XL |
15 | a: Binder<'tcx, T>, |
16 | b: Binder<'tcx, T>, | |
0731742a | 17 | a_is_expected: bool, |
cdc7bbd5 | 18 | ) -> RelateResult<'tcx, Binder<'tcx, T>> |
0731742a XL |
19 | where |
20 | T: Relate<'tcx>, | |
1a4d82fc | 21 | { |
1a4d82fc JJ |
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 | |
25 | // | |
fc512014 XL |
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> | |
1a4d82fc | 29 | |
0731742a | 30 | let span = self.trace.cause.span; |
3157f602 | 31 | |
f035d41b | 32 | self.infcx.commit_if_ok(|_| { |
0bf4aa26 XL |
33 | // First, we instantiate each bound region in the supertype with a |
34 | // fresh placeholder region. | |
fc512014 | 35 | let b_prime = self.infcx.replace_bound_vars_with_placeholders(b); |
0bf4aa26 XL |
36 | |
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 | |
40 | // the placeholders. | |
0731742a | 41 | let (a_prime, _) = |
fc512014 | 42 | self.infcx.replace_bound_vars_with_fresh_vars(span, HigherRankedType, a); |
1a4d82fc | 43 | |
62682a34 SL |
44 | debug!("a_prime={:?}", a_prime); |
45 | debug!("b_prime={:?}", b_prime); | |
1a4d82fc JJ |
46 | |
47 | // Compare types now that bound regions have been replaced. | |
f035d41b | 48 | let result = self.sub(a_is_expected).relate(a_prime, b_prime)?; |
1a4d82fc | 49 | |
a7813a04 | 50 | debug!("higher_ranked_sub: OK result={:?}", result); |
1a4d82fc | 51 | |
cdc7bbd5 XL |
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)) | |
ba9703b0 | 56 | }) |
1a4d82fc | 57 | } |
1a4d82fc JJ |
58 | } |
59 | ||
dc9dc135 | 60 | impl<'a, 'tcx> InferCtxt<'a, 'tcx> { |
9fa01778 | 61 | /// Replaces all regions (resp. types) bound by `binder` with placeholder |
a1dfa0c6 | 62 | /// regions (resp. types) and return a map indicating which bound-region |
0731742a XL |
63 | /// placeholder region. This is the first step of checking subtyping |
64 | /// when higher-ranked things are involved. | |
3157f602 | 65 | /// |
f035d41b XL |
66 | /// **Important:** You have to be careful to not leak these placeholders, |
67 | /// for more information about how placeholders and HRTBs work, see | |
ba9703b0 | 68 | /// the [rustc dev guide]. |
0531ce1d | 69 | /// |
ba9703b0 | 70 | /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html |
cdc7bbd5 | 71 | pub fn replace_bound_vars_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T |
0bf4aa26 | 72 | where |
0731742a | 73 | T: TypeFoldable<'tcx>, |
a7813a04 | 74 | { |
f035d41b XL |
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(); | |
0bf4aa26 | 81 | |
fc512014 | 82 | let fld_r = |br: ty::BoundRegion| { |
a1dfa0c6 | 83 | self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion { |
0bf4aa26 | 84 | universe: next_universe, |
fc512014 | 85 | name: br.kind, |
0bf4aa26 | 86 | })) |
a1dfa0c6 XL |
87 | }; |
88 | ||
89 | let fld_t = |bound_ty: ty::BoundTy| { | |
90 | self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType { | |
91 | universe: next_universe, | |
92 | name: bound_ty.var, | |
93 | })) | |
94 | }; | |
1a4d82fc | 95 | |
48663c56 | 96 | let fld_c = |bound_var: ty::BoundVar, ty| { |
dfeec247 XL |
97 | self.tcx.mk_const(ty::Const { |
98 | val: ty::ConstKind::Placeholder(ty::PlaceholderConst { | |
99 | universe: next_universe, | |
fc512014 | 100 | name: ty::BoundConst { var: bound_var, ty }, |
dfeec247 XL |
101 | }), |
102 | ty, | |
103 | }) | |
48663c56 XL |
104 | }; |
105 | ||
106 | let (result, map) = self.tcx.replace_bound_vars(binder, fld_r, fld_t, fld_c); | |
a1dfa0c6 | 107 | |
f035d41b XL |
108 | // If there were higher-ranked regions to replace, then actually create |
109 | // the next universe (this avoids needlessly creating universes). | |
110 | if !map.is_empty() { | |
111 | let n_u = self.create_next_universe(); | |
112 | assert_eq!(n_u, next_universe); | |
113 | } | |
114 | ||
a1dfa0c6 | 115 | debug!( |
0731742a XL |
116 | "replace_bound_vars_with_placeholders(\ |
117 | next_universe={:?}, \ | |
0731742a XL |
118 | result={:?}, \ |
119 | map={:?})", | |
fc512014 | 120 | next_universe, result, map, |
a1dfa0c6 | 121 | ); |
1a4d82fc | 122 | |
29967ef6 | 123 | result |
1a4d82fc | 124 | } |
1a4d82fc | 125 | |
0731742a XL |
126 | /// See `infer::region_constraints::RegionConstraintCollector::leak_check`. |
127 | pub fn leak_check( | |
0bf4aa26 | 128 | &self, |
0731742a | 129 | overly_polymorphic: bool, |
0731742a XL |
130 | snapshot: &CombinedSnapshot<'_, 'tcx>, |
131 | ) -> RelateResult<'tcx, ()> { | |
74b04a01 XL |
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() { | |
139 | return Ok(()); | |
140 | } | |
141 | ||
142 | self.inner.borrow_mut().unwrap_region_constraints().leak_check( | |
dfeec247 XL |
143 | self.tcx, |
144 | overly_polymorphic, | |
f035d41b | 145 | self.universe(), |
dfeec247 XL |
146 | snapshot, |
147 | ) | |
3157f602 | 148 | } |
1a4d82fc | 149 | } |