1 //! Check whether a type has (potentially) non-trivial drop glue.
3 use rustc_data_structures::fx::FxHashSet;
4 use rustc_hir::def_id::DefId;
5 use rustc_middle::ty::subst::Subst;
6 use rustc_middle::ty::subst::SubstsRef;
7 use rustc_middle::ty::util::{needs_drop_components, AlwaysRequiresDrop};
8 use rustc_middle::ty::{self, Ty, TyCtxt};
9 use rustc_session::Limit;
10 use rustc_span::{sym, DUMMY_SP};
12 type NeedsDropResult<T> = Result<T, AlwaysRequiresDrop>;
14 fn needs_drop_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
15 // If we don't know a type doesn't need drop, for example if it's a type
16 // parameter without a `Copy` bound, then we conservatively return that it
19 |adt_def: &ty::AdtDef| adt_def.destructor(tcx).map(|_| DtorType::Significant);
20 let res = drop_tys_helper(tcx, query.value, query.param_env, adt_has_dtor).next().is_some();
22 debug!("needs_drop_raw({:?}) = {:?}", query, res);
26 fn has_significant_drop_raw<'tcx>(
28 query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
31 drop_tys_helper(tcx, query.value, query.param_env, adt_consider_insignificant_dtor(tcx))
34 debug!("has_significant_drop_raw({:?}) = {:?}", query, res);
38 struct NeedsDropTypes<'tcx, F> {
40 param_env: ty::ParamEnv<'tcx>,
42 seen_tys: FxHashSet<Ty<'tcx>>,
43 /// A stack of types left to process, and the recursion depth when we
44 /// pushed that type. Each round, we pop something from the stack and check
45 /// if it needs drop. If the result depends on whether some other types
46 /// need drop we push them onto the stack.
47 unchecked_tys: Vec<(Ty<'tcx>, usize)>,
48 recursion_limit: Limit,
52 impl<'tcx, F> NeedsDropTypes<'tcx, F> {
55 param_env: ty::ParamEnv<'tcx>,
59 let mut seen_tys = FxHashSet::default();
66 unchecked_tys: vec![(ty, 0)],
67 recursion_limit: tcx.recursion_limit(),
73 impl<'tcx, F, I> Iterator for NeedsDropTypes<'tcx, F>
75 F: Fn(&ty::AdtDef, SubstsRef<'tcx>) -> NeedsDropResult<I>,
76 I: Iterator<Item = Ty<'tcx>>,
78 type Item = NeedsDropResult<Ty<'tcx>>;
80 fn next(&mut self) -> Option<NeedsDropResult<Ty<'tcx>>> {
83 while let Some((ty, level)) = self.unchecked_tys.pop() {
84 if !self.recursion_limit.value_within_limit(level) {
85 // Not having a `Span` isn't great. But there's hopefully some other
86 // recursion limit error as well.
89 &format!("overflow while checking whether `{}` requires drop", self.query_ty),
91 return Some(Err(AlwaysRequiresDrop));
94 let components = match needs_drop_components(ty, &tcx.data_layout) {
95 Err(e) => return Some(Err(e)),
96 Ok(components) => components,
98 debug!("needs_drop_components({:?}) = {:?}", ty, components);
100 let queue_type = move |this: &mut Self, component: Ty<'tcx>| {
101 if this.seen_tys.insert(component) {
102 this.unchecked_tys.push((component, level + 1));
106 for component in components {
107 match *component.kind() {
108 _ if component.is_copy_modulo_regions(tcx.at(DUMMY_SP), self.param_env) => (),
110 ty::Closure(_, substs) => {
111 queue_type(self, substs.as_closure().tupled_upvars_ty());
114 ty::Generator(def_id, substs, _) => {
115 let substs = substs.as_generator();
116 queue_type(self, substs.tupled_upvars_ty());
118 let witness = substs.witness();
119 let interior_tys = match witness.kind() {
120 &ty::GeneratorWitness(tys) => tcx.erase_late_bound_regions(tys),
122 tcx.sess.delay_span_bug(
123 tcx.hir().span_if_local(def_id).unwrap_or(DUMMY_SP),
124 &format!("unexpected generator witness type {:?}", witness),
126 return Some(Err(AlwaysRequiresDrop));
130 for interior_ty in interior_tys {
131 queue_type(self, interior_ty);
135 // Check for a `Drop` impl and whether this is a union or
136 // `ManuallyDrop`. If it's a struct or enum without a `Drop`
137 // impl then check whether the field types need `Drop`.
138 ty::Adt(adt_def, substs) => {
139 let tys = match (self.adt_components)(adt_def, substs) {
140 Err(e) => return Some(Err(e)),
143 for required_ty in tys {
145 tcx.normalize_erasing_regions(self.param_env, required_ty);
146 queue_type(self, subst_ty);
149 ty::Array(..) | ty::Opaque(..) | ty::Projection(..) | ty::Param(_) => {
151 // Return the type to the caller: they may be able
152 // to normalize further than we can.
153 return Some(Ok(component));
155 // Store the type for later. We can't return here
156 // because we would then lose any other components
158 queue_type(self, component);
161 _ => return Some(Err(AlwaysRequiresDrop)),
171 /// Type has a `Drop` but it is considered insignificant.
172 /// Check the query `adt_significant_drop_tys` for understanding
173 /// "significant" / "insignificant".
176 /// Type has a `Drop` implentation.
180 // This is a helper function for `adt_drop_tys` and `adt_significant_drop_tys`.
181 // Depending on the implentation of `adt_has_dtor`, it is used to check if the
182 // ADT has a destructor or if the ADT only has a significant destructor. For
183 // understanding significant destructor look at `adt_significant_drop_tys`.
184 fn drop_tys_helper<'tcx>(
187 param_env: rustc_middle::ty::ParamEnv<'tcx>,
188 adt_has_dtor: impl Fn(&ty::AdtDef) -> Option<DtorType>,
189 ) -> impl Iterator<Item = NeedsDropResult<Ty<'tcx>>> {
190 let adt_components = move |adt_def: &ty::AdtDef, substs: SubstsRef<'tcx>| {
191 if adt_def.is_manually_drop() {
192 debug!("drop_tys_helper: `{:?}` is manually drop", adt_def);
193 return Ok(Vec::new().into_iter());
194 } else if let Some(dtor_info) = adt_has_dtor(adt_def) {
196 DtorType::Significant => {
197 debug!("drop_tys_helper: `{:?}` implements `Drop`", adt_def);
198 return Err(AlwaysRequiresDrop);
200 DtorType::Insignificant => {
201 debug!("drop_tys_helper: `{:?}` drop is insignificant", adt_def);
203 // Since the destructor is insignificant, we just want to make sure all of
204 // the passed in type parameters are also insignificant.
205 // Eg: Vec<T> dtor is insignificant when T=i32 but significant when T=Mutex.
206 return Ok(substs.types().collect::<Vec<Ty<'_>>>().into_iter());
209 } else if adt_def.is_union() {
210 debug!("drop_tys_helper: `{:?}` is a union", adt_def);
211 return Ok(Vec::new().into_iter());
216 let r = tcx.type_of(field.did).subst(tcx, substs);
217 debug!("drop_tys_helper: Subst into {:?} with {:?} gettng {:?}", field, substs, r);
224 NeedsDropTypes::new(tcx, param_env, ty, adt_components)
227 fn adt_consider_insignificant_dtor<'tcx>(
229 ) -> impl Fn(&ty::AdtDef) -> Option<DtorType> + 'tcx {
230 move |adt_def: &ty::AdtDef| {
231 let is_marked_insig = tcx.has_attr(adt_def.did, sym::rustc_insignificant_dtor);
233 // In some cases like `std::collections::HashMap` where the struct is a wrapper around
234 // a type that is a Drop type, and the wrapped type (eg: `hashbrown::HashMap`) lies
235 // outside stdlib, we might choose to still annotate the the wrapper (std HashMap) with
236 // `rustc_insignificant_dtor`, even if the type itself doesn't have a `Drop` impl.
237 Some(DtorType::Insignificant)
238 } else if adt_def.destructor(tcx).is_some() {
239 // There is a Drop impl and the type isn't marked insignificant, therefore Drop must be
241 Some(DtorType::Significant)
243 // No destructor found nor the type is annotated with `rustc_insignificant_dtor`, we
244 // treat this as the simple case of Drop impl for type.
250 fn adt_drop_tys(tcx: TyCtxt<'_>, def_id: DefId) -> Result<&ty::List<Ty<'_>>, AlwaysRequiresDrop> {
251 // This is for the "adt_drop_tys" query, that considers all `Drop` impls, therefore all dtors are
254 |adt_def: &ty::AdtDef| adt_def.destructor(tcx).map(|_| DtorType::Significant);
255 drop_tys_helper(tcx, tcx.type_of(def_id), tcx.param_env(def_id), adt_has_dtor)
256 .collect::<Result<Vec<_>, _>>()
257 .map(|components| tcx.intern_type_list(&components))
260 fn adt_significant_drop_tys(
263 ) -> Result<&ty::List<Ty<'_>>, AlwaysRequiresDrop> {
267 tcx.param_env(def_id),
268 adt_consider_insignificant_dtor(tcx),
270 .collect::<Result<Vec<_>, _>>()
271 .map(|components| tcx.intern_type_list(&components))
274 pub(crate) fn provide(providers: &mut ty::query::Providers) {
275 *providers = ty::query::Providers {
277 has_significant_drop_raw,
279 adt_significant_drop_tys,