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 /// We do not want to emit any errors in typeck because
24 /// the spans in typeck are subpar at the moment.
25 /// Borrowck will do the same work again (this time with
26 /// lifetime information) and thus report better errors.
27 ignore_errors: bool,
28
29 /// Span of function being checked.
30 span: Span,
31}
32
33impl<'tcx> ReverseMapper<'tcx> {
34 pub(super) fn new(
35 tcx: TyCtxt<'tcx>,
36 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
37 span: Span,
38 ignore_errors: bool,
39 ) -> Self {
40 Self { tcx, map, do_not_error: false, ignore_errors, span }
41 }
42
43 fn fold_kind_no_missing_regions_error(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
44 assert!(!self.do_not_error);
45 self.do_not_error = true;
46 let kind = kind.fold_with(self);
47 self.do_not_error = false;
48 kind
49 }
50
51 fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
52 assert!(!self.do_not_error);
53 kind.fold_with(self)
54 }
55
56 fn fold_closure_args(
57 &mut self,
58 def_id: DefId,
59 args: ty::GenericArgsRef<'tcx>,
60 ) -> ty::GenericArgsRef<'tcx> {
61 // I am a horrible monster and I pray for death. When
62 // we encounter a closure here, it is always a closure
63 // from within the function that we are currently
64 // type-checking -- one that is now being encapsulated
65 // in an opaque type. Ideally, we would
66 // go through the types/lifetimes that it references
67 // and treat them just like we would any other type,
68 // which means we would error out if we find any
69 // reference to a type/region that is not in the
70 // "reverse map".
71 //
72 // **However,** in the case of closures, there is a
73 // somewhat subtle (read: hacky) consideration. The
74 // problem is that our closure types currently include
75 // all the lifetime parameters declared on the
76 // enclosing function, even if they are unused by the
77 // closure itself. We can't readily filter them out,
78 // so here we replace those values with `'empty`. This
79 // can't really make a difference to the rest of the
80 // compiler; those regions are ignored for the
81 // outlives relation, and hence don't affect trait
82 // selection or auto traits, and they are erased
83 // during codegen.
84
85 let generics = self.tcx.generics_of(def_id);
86 self.tcx.mk_args_from_iter(args.iter().enumerate().map(|(index, kind)| {
87 if index < generics.parent_count {
88 // Accommodate missing regions in the parent kinds...
89 self.fold_kind_no_missing_regions_error(kind)
90 } else {
91 // ...but not elsewhere.
92 self.fold_kind_normally(kind)
93 }
94 }))
95 }
96}
97
98impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ReverseMapper<'tcx> {
99 fn cx(&self) -> TyCtxt<'tcx> {
100 self.tcx
101 }
102
103 #[instrument(skip(self), level = "debug")]
104 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
105 match *r {
106 // Ignore bound regions and `'static` regions that appear in the
107 // type, we only need to remap regions that reference lifetimes
108 // from the function declaration.
109 //
110 // E.g. We ignore `'r` in a type like `for<'r> fn(&'r u32)`.
111 ty::ReBound(..) | ty::ReStatic => return r,
112
113 // If regions have been erased (by writeback), don't try to unerase
114 // them.
115 ty::ReErased => return r,
116
117 ty::ReError(_) => return r,
118
119 // The regions that we expect from borrow checking.
120 ty::ReEarlyParam(_) | ty::ReLateParam(_) => {}
121
122 ty::RePlaceholder(_) | ty::ReVar(_) => {
123 // All of the regions in the type should either have been
124 // erased by writeback, or mapped back to named regions by
125 // borrow checking.
126 bug!("unexpected region kind in opaque type: {:?}", r);
127 }
128 }
129
130 match self.map.get(&r.into()).map(|k| k.unpack()) {
131 Some(GenericArgKind::Lifetime(r1)) => r1,
132 Some(u) => panic!("region mapped to unexpected kind: {u:?}"),
133 None if self.do_not_error => self.tcx.lifetimes.re_static,
134 None => {
135 let e = self
136 .tcx
137 .dcx()
138 .struct_span_err(self.span, "non-defining opaque type use in defining scope")
139 .with_span_label(
140 self.span,
141 format!(
142 "lifetime `{r}` is part of concrete type but not used in \
143 parameter list of the `impl Trait` type alias"
144 ),
145 )
146 .emit();
147
148 ty::Region::new_error(self.cx(), e)
149 }
150 }
151 }
152
153 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
154 match *ty.kind() {
155 ty::Closure(def_id, args) => {
156 let args = self.fold_closure_args(def_id, args);
157 Ty::new_closure(self.tcx, def_id, args)
158 }
159
160 ty::Coroutine(def_id, args) => {
161 let args = self.fold_closure_args(def_id, args);
162 Ty::new_coroutine(self.tcx, def_id, args)
163 }
164
165 ty::CoroutineWitness(def_id, args) => {
166 let args = self.fold_closure_args(def_id, args);
167 Ty::new_coroutine_witness(self.tcx, def_id, args)
168 }
169
170 ty::Param(param) => {
171 // Look it up in the generic parameters list.
172 match self.map.get(&ty.into()).map(|k| k.unpack()) {
173 // Found it in the generic parameters list; replace with the parameter from the
174 // opaque type.
175 Some(GenericArgKind::Type(t1)) => t1,
176 Some(u) => panic!("type mapped to unexpected kind: {u:?}"),
177 None => {
178 debug!(?param, ?self.map);
179 if !self.ignore_errors {
180 self.tcx
181 .dcx()
182 .struct_span_err(
183 self.span,
184 format!(
185 "type parameter `{ty}` is part of concrete type but not \
186 used in parameter list for the `impl Trait` type alias"
187 ),
188 )
189 .emit();
190 }
191
192 Ty::new_misc_error(self.tcx)
193 }
194 }
195 }
196
197 _ => ty.super_fold_with(self),
198 }
199 }
200
201 fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
202 trace!("checking const {:?}", ct);
203 // Find a const parameter
204 match ct.kind() {
205 ty::ConstKind::Param(..) => {
206 // Look it up in the generic parameters list.
207 match self.map.get(&ct.into()).map(|k| k.unpack()) {
208 // Found it in the generic parameters list, replace with the parameter from the
209 // opaque type.
210 Some(GenericArgKind::Const(c1)) => c1,
211 Some(u) => panic!("const mapped to unexpected kind: {u:?}"),
212 None => {
213 let guar = self
214 .tcx
215 .dcx()
216 .create_err(ConstNotUsedTraitAlias {
217 ct: ct.to_string(),
218 span: self.span,
219 })
220 .emit_unless(self.ignore_errors);
221
222 ty::Const::new_error(self.tcx, guar)
223 }
224 }
225 }
226
227 _ => ct,
228 }
229 }
230}