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74b04a01 XL |
1 | use rustc_data_structures::fx::FxHashMap; |
2 | use rustc_hir::def_id::DefId; | |
3 | use rustc_infer::infer::InferCtxt; | |
ba9703b0 | 4 | use rustc_middle::ty::{self, TyCtxt, TypeFoldable}; |
74b04a01 | 5 | use rustc_span::Span; |
ba9703b0 | 6 | use rustc_trait_selection::opaque_types::InferCtxtExt; |
74b04a01 XL |
7 | |
8 | use super::RegionInferenceContext; | |
9 | ||
10 | impl<'tcx> RegionInferenceContext<'tcx> { | |
11 | /// Resolve any opaque types that were encountered while borrow checking | |
12 | /// this item. This is then used to get the type in the `type_of` query. | |
13 | /// | |
14 | /// For example consider `fn f<'a>(x: &'a i32) -> impl Sized + 'a { x }`. | |
15 | /// This is lowered to give HIR something like | |
16 | /// | |
17 | /// type f<'a>::_Return<'_a> = impl Sized + '_a; | |
18 | /// fn f<'a>(x: &'a i32) -> f<'static>::_Return<'a> { x } | |
19 | /// | |
20 | /// When checking the return type record the type from the return and the | |
21 | /// type used in the return value. In this case they might be `_Return<'1>` | |
22 | /// and `&'2 i32` respectively. | |
23 | /// | |
24 | /// Once we to this method, we have completed region inference and want to | |
25 | /// call `infer_opaque_definition_from_instantiation` to get the inferred | |
26 | /// type of `_Return<'_a>`. `infer_opaque_definition_from_instantiation` | |
27 | /// compares lifetimes directly, so we need to map the inference variables | |
28 | /// back to concrete lifetimes: `'static`, `ReEarlyBound` or `ReFree`. | |
29 | /// | |
30 | /// First we map all the lifetimes in the concrete type to an equal | |
31 | /// universal region that occurs in the concrete type's substs, in this case | |
32 | /// this would result in `&'1 i32`. We only consider regions in the substs | |
33 | /// in case there is an equal region that does not. For example, this should | |
34 | /// be allowed: | |
35 | /// `fn f<'a: 'b, 'b: 'a>(x: *mut &'b i32) -> impl Sized + 'a { x }` | |
36 | /// | |
37 | /// Then we map the regions in both the type and the subst to their | |
38 | /// `external_name` giving `concrete_type = &'a i32`, | |
39 | /// `substs = ['static, 'a]`. This will then allow | |
40 | /// `infer_opaque_definition_from_instantiation` to determine that | |
41 | /// `_Return<'_a> = &'_a i32`. | |
42 | /// | |
43 | /// There's a slight complication around closures. Given | |
44 | /// `fn f<'a: 'a>() { || {} }` the closure's type is something like | |
45 | /// `f::<'a>::{{closure}}`. The region parameter from f is essentially | |
46 | /// ignored by type checking so ends up being inferred to an empty region. | |
47 | /// Calling `universal_upper_bound` for such a region gives `fr_fn_body`, | |
48 | /// which has no `external_name` in which case we use `'empty` as the | |
49 | /// region to pass to `infer_opaque_definition_from_instantiation`. | |
50 | pub(in crate::borrow_check) fn infer_opaque_types( | |
51 | &self, | |
52 | infcx: &InferCtxt<'_, 'tcx>, | |
53 | opaque_ty_decls: FxHashMap<DefId, ty::ResolvedOpaqueTy<'tcx>>, | |
54 | span: Span, | |
55 | ) -> FxHashMap<DefId, ty::ResolvedOpaqueTy<'tcx>> { | |
56 | opaque_ty_decls | |
57 | .into_iter() | |
58 | .map(|(opaque_def_id, ty::ResolvedOpaqueTy { concrete_type, substs })| { | |
59 | debug!( | |
60 | "infer_opaque_types(concrete_type = {:?}, substs = {:?})", | |
61 | concrete_type, substs | |
62 | ); | |
63 | ||
64 | let mut subst_regions = vec![self.universal_regions.fr_static]; | |
65 | let universal_substs = | |
66 | infcx.tcx.fold_regions(&substs, &mut false, |region, _| match *region { | |
67 | ty::ReVar(vid) => { | |
68 | subst_regions.push(vid); | |
69 | self.definitions[vid].external_name.unwrap_or_else(|| { | |
70 | infcx.tcx.sess.delay_span_bug( | |
71 | span, | |
72 | "opaque type with non-universal region substs", | |
73 | ); | |
74 | infcx.tcx.lifetimes.re_static | |
75 | }) | |
76 | } | |
77 | // We don't fold regions in the predicates of opaque | |
78 | // types to `ReVar`s. This means that in a case like | |
79 | // | |
80 | // fn f<'a: 'a>() -> impl Iterator<Item = impl Sized> | |
81 | // | |
82 | // The inner opaque type has `'static` in its substs. | |
83 | ty::ReStatic => region, | |
84 | _ => { | |
85 | infcx.tcx.sess.delay_span_bug( | |
86 | span, | |
87 | &format!("unexpected concrete region in borrowck: {:?}", region), | |
88 | ); | |
89 | region | |
90 | } | |
91 | }); | |
92 | ||
93 | subst_regions.sort(); | |
94 | subst_regions.dedup(); | |
95 | ||
96 | let universal_concrete_type = | |
97 | infcx.tcx.fold_regions(&concrete_type, &mut false, |region, _| match *region { | |
98 | ty::ReVar(vid) => subst_regions | |
99 | .iter() | |
100 | .find(|ur_vid| self.eval_equal(vid, **ur_vid)) | |
101 | .and_then(|ur_vid| self.definitions[*ur_vid].external_name) | |
102 | .unwrap_or(infcx.tcx.lifetimes.re_root_empty), | |
103 | ty::ReLateBound(..) => region, | |
104 | _ => { | |
105 | infcx.tcx.sess.delay_span_bug( | |
106 | span, | |
107 | &format!("unexpected concrete region in borrowck: {:?}", region), | |
108 | ); | |
109 | region | |
110 | } | |
111 | }); | |
112 | ||
113 | debug!( | |
114 | "infer_opaque_types(universal_concrete_type = {:?}, universal_substs = {:?})", | |
115 | universal_concrete_type, universal_substs | |
116 | ); | |
117 | ||
118 | let remapped_type = infcx.infer_opaque_definition_from_instantiation( | |
119 | opaque_def_id, | |
120 | universal_substs, | |
121 | universal_concrete_type, | |
122 | span, | |
123 | ); | |
124 | ( | |
125 | opaque_def_id, | |
126 | ty::ResolvedOpaqueTy { concrete_type: remapped_type, substs: universal_substs }, | |
127 | ) | |
128 | }) | |
129 | .collect() | |
130 | } | |
131 | ||
132 | /// Map the regions in the type to named regions. This is similar to what | |
133 | /// `infer_opaque_types` does, but can infer any universal region, not only | |
134 | /// ones from the substs for the opaque type. It also doesn't double check | |
135 | /// that the regions produced are in fact equal to the named region they are | |
136 | /// replaced with. This is fine because this function is only to improve the | |
137 | /// region names in error messages. | |
138 | pub(in crate::borrow_check) fn name_regions<T>(&self, tcx: TyCtxt<'tcx>, ty: T) -> T | |
139 | where | |
140 | T: TypeFoldable<'tcx>, | |
141 | { | |
142 | tcx.fold_regions(&ty, &mut false, |region, _| match *region { | |
143 | ty::ReVar(vid) => { | |
144 | let upper_bound = self.universal_upper_bound(vid); | |
145 | self.definitions[upper_bound].external_name.unwrap_or(region) | |
146 | } | |
147 | _ => region, | |
148 | }) | |
149 | } | |
150 | } |