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1 | //! Helper routines for higher-ranked things. See the `doc` module at | |
2 | //! the end of the file for details. | |
3 | ||
4 | use super::combine::CombineFields; | |
5 | use super::{HigherRankedType, InferCtxt, PlaceholderMap}; | |
6 | ||
7 | use crate::infer::CombinedSnapshot; | |
8 | use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation}; | |
9 | use rustc_middle::ty::{self, Binder, TypeFoldable}; | |
10 | ||
11 | impl<'a, 'tcx> CombineFields<'a, 'tcx> { | |
12 | pub fn higher_ranked_sub<T>( | |
13 | &mut self, | |
14 | a: Binder<T>, | |
15 | b: Binder<T>, | |
16 | a_is_expected: bool, | |
17 | ) -> RelateResult<'tcx, Binder<T>> | |
18 | where | |
19 | T: Relate<'tcx>, | |
20 | { | |
21 | debug!("higher_ranked_sub(a={:?}, b={:?})", a, b); | |
22 | ||
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 | |
26 | // | |
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 | |
29 | // `doc`. | |
30 | ||
31 | let span = self.trace.cause.span; | |
32 | ||
33 | self.infcx.commit_if_ok(|_| { | |
34 | // First, we instantiate each bound region in the supertype with a | |
35 | // fresh placeholder region. | |
36 | let (b_prime, _) = self.infcx.replace_bound_vars_with_placeholders(&b); | |
37 | ||
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 | |
41 | // the placeholders. | |
42 | let (a_prime, _) = | |
43 | self.infcx.replace_bound_vars_with_fresh_vars(span, HigherRankedType, &a); | |
44 | ||
45 | debug!("a_prime={:?}", a_prime); | |
46 | debug!("b_prime={:?}", b_prime); | |
47 | ||
48 | // Compare types now that bound regions have been replaced. | |
49 | let result = self.sub(a_is_expected).relate(a_prime, b_prime)?; | |
50 | ||
51 | debug!("higher_ranked_sub: OK result={:?}", result); | |
52 | ||
53 | Ok(ty::Binder::bind(result)) | |
54 | }) | |
55 | } | |
56 | } | |
57 | ||
58 | impl<'a, 'tcx> InferCtxt<'a, 'tcx> { | |
59 | /// Replaces all regions (resp. types) bound by `binder` with placeholder | |
60 | /// regions (resp. types) and return a map indicating which bound-region | |
61 | /// placeholder region. This is the first step of checking subtyping | |
62 | /// when higher-ranked things are involved. | |
63 | /// | |
64 | /// **Important:** You have to be careful to not leak these placeholders, | |
65 | /// for more information about how placeholders and HRTBs work, see | |
66 | /// the [rustc dev guide]. | |
67 | /// | |
68 | /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html | |
69 | pub fn replace_bound_vars_with_placeholders<T>( | |
70 | &self, | |
71 | binder: &ty::Binder<T>, | |
72 | ) -> (T, PlaceholderMap<'tcx>) | |
73 | where | |
74 | T: TypeFoldable<'tcx>, | |
75 | { | |
76 | // Figure out what the next universe will be, but don't actually create | |
77 | // it until after we've done the substitution (in particular there may | |
78 | // be no bound variables). This is a performance optimization, since the | |
79 | // leak check for example can be skipped if no new universes are created | |
80 | // (i.e., if there are no placeholders). | |
81 | let next_universe = self.universe().next_universe(); | |
82 | ||
83 | let fld_r = |br| { | |
84 | self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion { | |
85 | universe: next_universe, | |
86 | name: br, | |
87 | })) | |
88 | }; | |
89 | ||
90 | let fld_t = |bound_ty: ty::BoundTy| { | |
91 | self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType { | |
92 | universe: next_universe, | |
93 | name: bound_ty.var, | |
94 | })) | |
95 | }; | |
96 | ||
97 | let fld_c = |bound_var: ty::BoundVar, ty| { | |
98 | self.tcx.mk_const(ty::Const { | |
99 | val: ty::ConstKind::Placeholder(ty::PlaceholderConst { | |
100 | universe: next_universe, | |
101 | name: bound_var, | |
102 | }), | |
103 | ty, | |
104 | }) | |
105 | }; | |
106 | ||
107 | let (result, map) = self.tcx.replace_bound_vars(binder, fld_r, fld_t, fld_c); | |
108 | ||
109 | // If there were higher-ranked regions to replace, then actually create | |
110 | // the next universe (this avoids needlessly creating universes). | |
111 | if !map.is_empty() { | |
112 | let n_u = self.create_next_universe(); | |
113 | assert_eq!(n_u, next_universe); | |
114 | } | |
115 | ||
116 | debug!( | |
117 | "replace_bound_vars_with_placeholders(\ | |
118 | next_universe={:?}, \ | |
119 | binder={:?}, \ | |
120 | result={:?}, \ | |
121 | map={:?})", | |
122 | next_universe, binder, result, map, | |
123 | ); | |
124 | ||
125 | (result, map) | |
126 | } | |
127 | ||
128 | /// See `infer::region_constraints::RegionConstraintCollector::leak_check`. | |
129 | pub fn leak_check( | |
130 | &self, | |
131 | overly_polymorphic: bool, | |
132 | snapshot: &CombinedSnapshot<'_, 'tcx>, | |
133 | ) -> RelateResult<'tcx, ()> { | |
134 | // If the user gave `-Zno-leak-check`, or we have been | |
135 | // configured to skip the leak check, then skip the leak check | |
136 | // completely. The leak check is deprecated. Any legitimate | |
137 | // subtyping errors that it would have caught will now be | |
138 | // caught later on, during region checking. However, we | |
139 | // continue to use it for a transition period. | |
140 | if self.tcx.sess.opts.debugging_opts.no_leak_check || self.skip_leak_check.get() { | |
141 | return Ok(()); | |
142 | } | |
143 | ||
144 | self.inner.borrow_mut().unwrap_region_constraints().leak_check( | |
145 | self.tcx, | |
146 | overly_polymorphic, | |
147 | self.universe(), | |
148 | snapshot, | |
149 | ) | |
150 | } | |
151 | } |