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Commit | Line | Data |
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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}; |
9fa01778 | 6 | use crate::infer::CombinedSnapshot; |
064997fb | 7 | use rustc_middle::ty::fold::FnMutDelegate; |
ba9703b0 | 8 | use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation}; |
fe692bf9 | 9 | use rustc_middle::ty::{self, Binder, Ty, TyCtxt, TypeFoldable}; |
3157f602 | 10 | |
dc9dc135 | 11 | impl<'a, 'tcx> CombineFields<'a, 'tcx> { |
923072b8 FG |
12 | /// Checks whether `for<..> sub <: for<..> sup` holds. |
13 | /// | |
14 | /// For this to hold, **all** instantiations of the super type | |
15 | /// have to be a super type of **at least one** instantiation of | |
16 | /// the subtype. | |
17 | /// | |
18 | /// This is implemented by first entering a new universe. | |
19 | /// We then replace all bound variables in `sup` with placeholders, | |
064997fb | 20 | /// and all bound variables in `sub` with inference vars. |
923072b8 FG |
21 | /// We can then just relate the two resulting types as normal. |
22 | /// | |
23 | /// Note: this is a subtle algorithm. For a full explanation, please see | |
24 | /// the [rustc dev guide][rd] | |
25 | /// | |
26 | /// [rd]: https://rustc-dev-guide.rust-lang.org/borrow_check/region_inference/placeholders_and_universes.html | |
c295e0f8 | 27 | #[instrument(skip(self), level = "debug")] |
0731742a XL |
28 | pub fn higher_ranked_sub<T>( |
29 | &mut self, | |
923072b8 FG |
30 | sub: Binder<'tcx, T>, |
31 | sup: Binder<'tcx, T>, | |
32 | sub_is_expected: bool, | |
33 | ) -> RelateResult<'tcx, ()> | |
0731742a XL |
34 | where |
35 | T: Relate<'tcx>, | |
1a4d82fc | 36 | { |
0731742a | 37 | let span = self.trace.cause.span; |
064997fb FG |
38 | // First, we instantiate each bound region in the supertype with a |
39 | // fresh placeholder region. Note that this automatically creates | |
40 | // a new universe if needed. | |
9ffffee4 | 41 | let sup_prime = self.infcx.instantiate_binder_with_placeholders(sup); |
3157f602 | 42 | |
064997fb FG |
43 | // Next, we instantiate each bound region in the subtype |
44 | // with a fresh region variable. These region variables -- | |
49aad941 | 45 | // but no other preexisting region variables -- can name |
064997fb | 46 | // the placeholders. |
9ffffee4 | 47 | let sub_prime = self.infcx.instantiate_binder_with_fresh_vars(span, HigherRankedType, sub); |
0bf4aa26 | 48 | |
064997fb FG |
49 | debug!("a_prime={:?}", sub_prime); |
50 | debug!("b_prime={:?}", sup_prime); | |
1a4d82fc | 51 | |
064997fb FG |
52 | // Compare types now that bound regions have been replaced. |
53 | let result = self.sub(sub_is_expected).relate(sub_prime, sup_prime)?; | |
1a4d82fc | 54 | |
064997fb FG |
55 | debug!("OK result={result:?}"); |
56 | // NOTE: returning the result here would be dangerous as it contains | |
57 | // placeholders which **must not** be named afterwards. | |
58 | Ok(()) | |
1a4d82fc | 59 | } |
1a4d82fc JJ |
60 | } |
61 | ||
2b03887a | 62 | impl<'tcx> InferCtxt<'tcx> { |
5e7ed085 | 63 | /// Replaces all bound variables (lifetimes, types, and constants) bound by |
923072b8 FG |
64 | /// `binder` with placeholder variables in a new universe. This means that the |
65 | /// new placeholders can only be named by inference variables created after | |
66 | /// this method has been called. | |
3157f602 | 67 | /// |
5e7ed085 FG |
68 | /// This is the first step of checking subtyping when higher-ranked things are involved. |
69 | /// For more details visit the relevant sections of the [rustc dev guide]. | |
0531ce1d | 70 | /// |
ba9703b0 | 71 | /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html |
f2b60f7d | 72 | #[instrument(level = "debug", skip(self), ret)] |
9ffffee4 | 73 | pub fn instantiate_binder_with_placeholders<T>(&self, binder: ty::Binder<'tcx, T>) -> T |
0bf4aa26 | 74 | where |
9ffffee4 | 75 | T: TypeFoldable<TyCtxt<'tcx>> + Copy, |
a7813a04 | 76 | { |
923072b8 FG |
77 | if let Some(inner) = binder.no_bound_vars() { |
78 | return inner; | |
79 | } | |
80 | ||
81 | let next_universe = self.create_next_universe(); | |
0bf4aa26 | 82 | |
064997fb | 83 | let delegate = FnMutDelegate { |
f2b60f7d | 84 | regions: &mut |br: ty::BoundRegion| { |
fe692bf9 FG |
85 | ty::Region::new_placeholder( |
86 | self.tcx, | |
87 | ty::PlaceholderRegion { universe: next_universe, bound: br }, | |
88 | ) | |
064997fb | 89 | }, |
f2b60f7d | 90 | types: &mut |bound_ty: ty::BoundTy| { |
fe692bf9 FG |
91 | Ty::new_placeholder( |
92 | self.tcx, | |
93 | ty::PlaceholderType { universe: next_universe, bound: bound_ty }, | |
94 | ) | |
064997fb | 95 | }, |
f2b60f7d | 96 | consts: &mut |bound_var: ty::BoundVar, ty| { |
fe692bf9 FG |
97 | ty::Const::new_placeholder( |
98 | self.tcx, | |
353b0b11 FG |
99 | ty::PlaceholderConst { universe: next_universe, bound: bound_var }, |
100 | ty, | |
101 | ) | |
064997fb | 102 | }, |
48663c56 XL |
103 | }; |
104 | ||
f2b60f7d FG |
105 | debug!(?next_universe); |
106 | self.tcx.replace_bound_vars_uncached(binder, delegate) | |
1a4d82fc | 107 | } |
1a4d82fc | 108 | |
fe692bf9 FG |
109 | /// See [RegionConstraintCollector::leak_check][1]. We only check placeholder |
110 | /// leaking into `outer_universe`, i.e. placeholders which cannot be named by that | |
111 | /// universe. | |
5e7ed085 FG |
112 | /// |
113 | /// [1]: crate::infer::region_constraints::RegionConstraintCollector::leak_check | |
0731742a | 114 | pub fn leak_check( |
0bf4aa26 | 115 | &self, |
fe692bf9 FG |
116 | outer_universe: ty::UniverseIndex, |
117 | only_consider_snapshot: Option<&CombinedSnapshot<'tcx>>, | |
0731742a | 118 | ) -> RelateResult<'tcx, ()> { |
74b04a01 XL |
119 | // If the user gave `-Zno-leak-check`, or we have been |
120 | // configured to skip the leak check, then skip the leak check | |
121 | // completely. The leak check is deprecated. Any legitimate | |
122 | // subtyping errors that it would have caught will now be | |
123 | // caught later on, during region checking. However, we | |
124 | // continue to use it for a transition period. | |
fe692bf9 | 125 | if self.tcx.sess.opts.unstable_opts.no_leak_check || self.skip_leak_check { |
74b04a01 XL |
126 | return Ok(()); |
127 | } | |
128 | ||
129 | self.inner.borrow_mut().unwrap_region_constraints().leak_check( | |
dfeec247 | 130 | self.tcx, |
fe692bf9 | 131 | outer_universe, |
f035d41b | 132 | self.universe(), |
fe692bf9 | 133 | only_consider_snapshot, |
dfeec247 | 134 | ) |
3157f602 | 135 | } |
1a4d82fc | 136 | } |