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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}; | |
6 | use crate::infer::CombinedSnapshot; | |
7 | use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation}; | |
8 | use rustc_middle::ty::{self, Binder, TypeFoldable}; | |
9 | ||
10 | impl<'a, 'tcx> CombineFields<'a, 'tcx> { | |
11 | /// Checks whether `for<..> sub <: for<..> sup` holds. | |
12 | /// | |
13 | /// For this to hold, **all** instantiations of the super type | |
14 | /// have to be a super type of **at least one** instantiation of | |
15 | /// the subtype. | |
16 | /// | |
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. | |
21 | /// | |
22 | /// Note: this is a subtle algorithm. For a full explanation, please see | |
23 | /// the [rustc dev guide][rd] | |
24 | /// | |
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>( | |
28 | &mut self, | |
29 | sub: Binder<'tcx, T>, | |
30 | sup: Binder<'tcx, T>, | |
31 | sub_is_expected: bool, | |
32 | ) -> RelateResult<'tcx, ()> | |
33 | where | |
34 | T: Relate<'tcx>, | |
35 | { | |
36 | let span = self.trace.cause.span; | |
37 | ||
38 | self.infcx.commit_if_ok(|_| { | |
39 | // First, we instantiate each bound region in the supertype with a | |
40 | // fresh placeholder region. Note that this automatically creates | |
41 | // a new universe if needed. | |
42 | let sup_prime = self.infcx.replace_bound_vars_with_placeholders(sup); | |
43 | ||
44 | // Next, we instantiate each bound region in the subtype | |
45 | // with a fresh region variable. These region variables -- | |
46 | // but no other pre-existing region variables -- can name | |
47 | // the placeholders. | |
48 | let sub_prime = | |
49 | self.infcx.replace_bound_vars_with_fresh_vars(span, HigherRankedType, sub); | |
50 | ||
51 | debug!("a_prime={:?}", sub_prime); | |
52 | debug!("b_prime={:?}", sup_prime); | |
53 | ||
54 | // Compare types now that bound regions have been replaced. | |
55 | let result = self.sub(sub_is_expected).relate(sub_prime, sup_prime)?; | |
56 | ||
57 | debug!("higher_ranked_sub: OK result={result:?}"); | |
58 | // NOTE: returning the result here would be dangerous as it contains | |
59 | // placeholders which **must not** be named afterwards. | |
60 | Ok(()) | |
61 | }) | |
62 | } | |
63 | } | |
64 | ||
65 | impl<'a, 'tcx> InferCtxt<'a, 'tcx> { | |
66 | /// Replaces all bound variables (lifetimes, types, and constants) bound by | |
67 | /// `binder` with placeholder variables in a new universe. This means that the | |
68 | /// new placeholders can only be named by inference variables created after | |
69 | /// this method has been called. | |
70 | /// | |
71 | /// This is the first step of checking subtyping when higher-ranked things are involved. | |
72 | /// For more details visit the relevant sections of the [rustc dev guide]. | |
73 | /// | |
74 | /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html | |
75 | #[instrument(level = "debug", skip(self))] | |
76 | pub fn replace_bound_vars_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T | |
77 | where | |
78 | T: TypeFoldable<'tcx> + Copy, | |
79 | { | |
80 | if let Some(inner) = binder.no_bound_vars() { | |
81 | return inner; | |
82 | } | |
83 | ||
84 | let next_universe = self.create_next_universe(); | |
85 | ||
86 | let fld_r = |br: ty::BoundRegion| { | |
87 | self.tcx.mk_region(ty::RePlaceholder(ty::PlaceholderRegion { | |
88 | universe: next_universe, | |
89 | name: br.kind, | |
90 | })) | |
91 | }; | |
92 | ||
93 | let fld_t = |bound_ty: ty::BoundTy| { | |
94 | self.tcx.mk_ty(ty::Placeholder(ty::PlaceholderType { | |
95 | universe: next_universe, | |
96 | name: bound_ty.var, | |
97 | })) | |
98 | }; | |
99 | ||
100 | let fld_c = |bound_var: ty::BoundVar, ty| { | |
101 | self.tcx.mk_const(ty::ConstS { | |
102 | kind: ty::ConstKind::Placeholder(ty::PlaceholderConst { | |
103 | universe: next_universe, | |
104 | name: ty::BoundConst { var: bound_var, ty }, | |
105 | }), | |
106 | ty, | |
107 | }) | |
108 | }; | |
109 | ||
110 | let result = self.tcx.replace_bound_vars_uncached(binder, fld_r, fld_t, fld_c); | |
111 | debug!(?next_universe, ?result); | |
112 | result | |
113 | } | |
114 | ||
115 | /// See [RegionConstraintCollector::leak_check][1]. | |
116 | /// | |
117 | /// [1]: crate::infer::region_constraints::RegionConstraintCollector::leak_check | |
118 | pub fn leak_check( | |
119 | &self, | |
120 | overly_polymorphic: bool, | |
121 | snapshot: &CombinedSnapshot<'_, 'tcx>, | |
122 | ) -> RelateResult<'tcx, ()> { | |
123 | // If the user gave `-Zno-leak-check`, or we have been | |
124 | // configured to skip the leak check, then skip the leak check | |
125 | // completely. The leak check is deprecated. Any legitimate | |
126 | // subtyping errors that it would have caught will now be | |
127 | // caught later on, during region checking. However, we | |
128 | // continue to use it for a transition period. | |
129 | if self.tcx.sess.opts.debugging_opts.no_leak_check || self.skip_leak_check.get() { | |
130 | return Ok(()); | |
131 | } | |
132 | ||
133 | self.inner.borrow_mut().unwrap_region_constraints().leak_check( | |
134 | self.tcx, | |
135 | overly_polymorphic, | |
136 | self.universe(), | |
137 | snapshot, | |
138 | ) | |
139 | } | |
140 | } |