rustc_middle/ty/opaque_types.rs
1use rustc_data_structures::fx::FxHashMap;
2use rustc_span::Span;
3use rustc_span::def_id::DefId;
4use tracing::{debug, instrument, trace};
5
6use crate::error::ConstNotUsedTraitAlias;
7use crate::ty::{
8 self, GenericArg, GenericArgKind, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable,
9};
10
11pub type OpaqueTypeKey<'tcx> = rustc_type_ir::OpaqueTypeKey<TyCtxt<'tcx>>;
12
13/// Converts generic params of a TypeFoldable from one
14/// item's generics to another. Usually from a function's generics
15/// list to the opaque type's own generics.
16pub(super) struct ReverseMapper<'tcx> {
17 tcx: TyCtxt<'tcx>,
18 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
19 /// see call sites to fold_kind_no_missing_regions_error
20 /// for an explanation of this field.
21 do_not_error: bool,
22
23 /// Span of function being checked.
24 span: Span,
25}
26
27impl<'tcx> ReverseMapper<'tcx> {
28 pub(super) fn new(
29 tcx: TyCtxt<'tcx>,
30 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
31 span: Span,
32 ) -> Self {
33 Self { tcx, map, do_not_error: false, span }
34 }
35
36 fn fold_kind_no_missing_regions_error(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
37 assert!(!self.do_not_error);
38 self.do_not_error = true;
39 let kind = kind.fold_with(self);
40 self.do_not_error = false;
41 kind
42 }
43
44 fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
45 assert!(!self.do_not_error);
46 kind.fold_with(self)
47 }
48
49 fn fold_closure_args(
50 &mut self,
51 def_id: DefId,
52 args: ty::GenericArgsRef<'tcx>,
53 ) -> ty::GenericArgsRef<'tcx> {
54 // I am a horrible monster and I pray for death. When
55 // we encounter a closure here, it is always a closure
56 // from within the function that we are currently
57 // type-checking -- one that is now being encapsulated
58 // in an opaque type. Ideally, we would
59 // go through the types/lifetimes that it references
60 // and treat them just like we would any other type,
61 // which means we would error out if we find any
62 // reference to a type/region that is not in the
63 // "reverse map".
64 //
65 // **However,** in the case of closures, there is a
66 // somewhat subtle (read: hacky) consideration. The
67 // problem is that our closure types currently include
68 // all the lifetime parameters declared on the
69 // enclosing function, even if they are unused by the
70 // closure itself. We can't readily filter them out,
71 // so here we replace those values with `'empty`. This
72 // can't really make a difference to the rest of the
73 // compiler; those regions are ignored for the
74 // outlives relation, and hence don't affect trait
75 // selection or auto traits, and they are erased
76 // during codegen.
77
78 let generics = self.tcx.generics_of(def_id);
79 self.tcx.mk_args_from_iter(args.iter().enumerate().map(|(index, kind)| {
80 if index < generics.parent_count {
81 // Accommodate missing regions in the parent kinds...
82 self.fold_kind_no_missing_regions_error(kind)
83 } else {
84 // ...but not elsewhere.
85 self.fold_kind_normally(kind)
86 }
87 }))
88 }
89}
90
91impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ReverseMapper<'tcx> {
92 fn cx(&self) -> TyCtxt<'tcx> {
93 self.tcx
94 }
95
96 #[instrument(skip(self), level = "debug")]
97 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
98 match r.kind() {
99 // Ignore bound regions and `'static` regions that appear in the
100 // type, we only need to remap regions that reference lifetimes
101 // from the function declaration.
102 //
103 // E.g. We ignore `'r` in a type like `for<'r> fn(&'r u32)`.
104 ty::ReBound(..) | ty::ReStatic => return r,
105
106 // If regions have been erased (by writeback), don't try to unerase
107 // them.
108 ty::ReErased => return r,
109
110 ty::ReError(_) => return r,
111
112 // The regions that we expect from borrow checking.
113 ty::ReEarlyParam(_) | ty::ReLateParam(_) => {}
114
115 ty::RePlaceholder(_) | ty::ReVar(_) => {
116 // All of the regions in the type should either have been
117 // erased by writeback, or mapped back to named regions by
118 // borrow checking.
119 bug!("unexpected region kind in opaque type: {:?}", r);
120 }
121 }
122
123 match self.map.get(&r.into()).map(|k| k.unpack()) {
124 Some(GenericArgKind::Lifetime(r1)) => r1,
125 Some(u) => panic!("region mapped to unexpected kind: {u:?}"),
126 None if self.do_not_error => self.tcx.lifetimes.re_static,
127 None => {
128 let e = self
129 .tcx
130 .dcx()
131 .struct_span_err(self.span, "non-defining opaque type use in defining scope")
132 .with_span_label(
133 self.span,
134 format!(
135 "lifetime `{r}` is part of concrete type but not used in \
136 parameter list of the `impl Trait` type alias"
137 ),
138 )
139 .emit();
140
141 ty::Region::new_error(self.cx(), e)
142 }
143 }
144 }
145
146 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
147 match *ty.kind() {
148 ty::Closure(def_id, args) => {
149 let args = self.fold_closure_args(def_id, args);
150 Ty::new_closure(self.tcx, def_id, args)
151 }
152
153 ty::Coroutine(def_id, args) => {
154 let args = self.fold_closure_args(def_id, args);
155 Ty::new_coroutine(self.tcx, def_id, args)
156 }
157
158 ty::CoroutineWitness(def_id, args) => {
159 let args = self.fold_closure_args(def_id, args);
160 Ty::new_coroutine_witness(self.tcx, def_id, args)
161 }
162
163 ty::Param(param) => {
164 // Look it up in the generic parameters list.
165 match self.map.get(&ty.into()).map(|k| k.unpack()) {
166 // Found it in the generic parameters list; replace with the parameter from the
167 // opaque type.
168 Some(GenericArgKind::Type(t1)) => t1,
169 Some(u) => panic!("type mapped to unexpected kind: {u:?}"),
170 None => {
171 debug!(?param, ?self.map);
172 let guar = self
173 .tcx
174 .dcx()
175 .struct_span_err(
176 self.span,
177 format!(
178 "type parameter `{ty}` is part of concrete type but not \
179 used in parameter list for the `impl Trait` type alias"
180 ),
181 )
182 .emit();
183 Ty::new_error(self.tcx, guar)
184 }
185 }
186 }
187
188 _ => ty.super_fold_with(self),
189 }
190 }
191
192 fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
193 trace!("checking const {:?}", ct);
194 // Find a const parameter
195 match ct.kind() {
196 ty::ConstKind::Param(..) => {
197 // Look it up in the generic parameters list.
198 match self.map.get(&ct.into()).map(|k| k.unpack()) {
199 // Found it in the generic parameters list, replace with the parameter from the
200 // opaque type.
201 Some(GenericArgKind::Const(c1)) => c1,
202 Some(u) => panic!("const mapped to unexpected kind: {u:?}"),
203 None => {
204 let guar = self
205 .tcx
206 .dcx()
207 .create_err(ConstNotUsedTraitAlias {
208 ct: ct.to_string(),
209 span: self.span,
210 })
211 .emit();
212 ty::Const::new_error(self.tcx, guar)
213 }
214 }
215 }
216
217 _ => ct,
218 }
219 }
220}