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, EarlyBinder, 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<'tcx>| adt_def.destructor(tcx).map(|_| DtorType::Significant);
21 drop_tys_helper(tcx, query.value, query.param_env, adt_has_dtor, false).next().is_some();
23 debug!("needs_drop_raw({:?}) = {:?}", query, res);
27 fn has_significant_drop_raw<'tcx>(
29 query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
31 let res = drop_tys_helper(
35 adt_consider_insignificant_dtor(tcx),
40 debug!("has_significant_drop_raw({:?}) = {:?}", query, res);
44 struct NeedsDropTypes<'tcx, F> {
46 param_env: ty::ParamEnv<'tcx>,
48 seen_tys: FxHashSet<Ty<'tcx>>,
49 /// A stack of types left to process, and the recursion depth when we
50 /// pushed that type. Each round, we pop something from the stack and check
51 /// if it needs drop. If the result depends on whether some other types
52 /// need drop we push them onto the stack.
53 unchecked_tys: Vec<(Ty<'tcx>, usize)>,
54 recursion_limit: Limit,
58 impl<'tcx, F> NeedsDropTypes<'tcx, F> {
61 param_env: ty::ParamEnv<'tcx>,
65 let mut seen_tys = FxHashSet::default();
72 unchecked_tys: vec![(ty, 0)],
73 recursion_limit: tcx.recursion_limit(),
79 impl<'tcx, F, I> Iterator for NeedsDropTypes<'tcx, F>
81 F: Fn(ty::AdtDef<'tcx>, SubstsRef<'tcx>) -> NeedsDropResult<I>,
82 I: Iterator<Item = Ty<'tcx>>,
84 type Item = NeedsDropResult<Ty<'tcx>>;
86 fn next(&mut self) -> Option<NeedsDropResult<Ty<'tcx>>> {
89 while let Some((ty, level)) = self.unchecked_tys.pop() {
90 if !self.recursion_limit.value_within_limit(level) {
91 // Not having a `Span` isn't great. But there's hopefully some other
92 // recursion limit error as well.
95 &format!("overflow while checking whether `{}` requires drop", self.query_ty),
97 return Some(Err(AlwaysRequiresDrop));
100 let components = match needs_drop_components(ty, &tcx.data_layout) {
101 Err(e) => return Some(Err(e)),
102 Ok(components) => components,
104 debug!("needs_drop_components({:?}) = {:?}", ty, components);
106 let queue_type = move |this: &mut Self, component: Ty<'tcx>| {
107 if this.seen_tys.insert(component) {
108 this.unchecked_tys.push((component, level + 1));
112 for component in components {
113 match *component.kind() {
114 _ if component.is_copy_modulo_regions(tcx.at(DUMMY_SP), self.param_env) => (),
116 ty::Closure(_, substs) => {
117 queue_type(self, substs.as_closure().tupled_upvars_ty());
120 ty::Generator(def_id, substs, _) => {
121 let substs = substs.as_generator();
122 queue_type(self, substs.tupled_upvars_ty());
124 let witness = substs.witness();
125 let interior_tys = match witness.kind() {
126 &ty::GeneratorWitness(tys) => tcx.erase_late_bound_regions(tys),
128 tcx.sess.delay_span_bug(
129 tcx.hir().span_if_local(def_id).unwrap_or(DUMMY_SP),
130 &format!("unexpected generator witness type {:?}", witness),
132 return Some(Err(AlwaysRequiresDrop));
136 for interior_ty in interior_tys {
137 queue_type(self, interior_ty);
141 // Check for a `Drop` impl and whether this is a union or
142 // `ManuallyDrop`. If it's a struct or enum without a `Drop`
143 // impl then check whether the field types need `Drop`.
144 ty::Adt(adt_def, substs) => {
145 let tys = match (self.adt_components)(adt_def, substs) {
146 Err(e) => return Some(Err(e)),
149 for required_ty in tys {
151 .try_normalize_erasing_regions(self.param_env, required_ty)
152 .unwrap_or(required_ty);
154 queue_type(self, required);
157 ty::Array(..) | ty::Opaque(..) | ty::Projection(..) | ty::Param(_) => {
159 // Return the type to the caller: they may be able
160 // to normalize further than we can.
161 return Some(Ok(component));
163 // Store the type for later. We can't return here
164 // because we would then lose any other components
166 queue_type(self, component);
169 _ => return Some(Err(AlwaysRequiresDrop)),
179 /// Type has a `Drop` but it is considered insignificant.
180 /// Check the query `adt_significant_drop_tys` for understanding
181 /// "significant" / "insignificant".
184 /// Type has a `Drop` implantation.
188 // This is a helper function for `adt_drop_tys` and `adt_significant_drop_tys`.
189 // Depending on the implantation of `adt_has_dtor`, it is used to check if the
190 // ADT has a destructor or if the ADT only has a significant destructor. For
191 // understanding significant destructor look at `adt_significant_drop_tys`.
192 fn drop_tys_helper<'tcx>(
195 param_env: rustc_middle::ty::ParamEnv<'tcx>,
196 adt_has_dtor: impl Fn(ty::AdtDef<'tcx>) -> Option<DtorType>,
197 only_significant: bool,
198 ) -> impl Iterator<Item = NeedsDropResult<Ty<'tcx>>> {
199 fn with_query_cache<'tcx>(
201 iter: impl IntoIterator<Item = Ty<'tcx>>,
202 ) -> NeedsDropResult<Vec<Ty<'tcx>>> {
203 iter.into_iter().try_fold(Vec::new(), |mut vec, subty| {
205 ty::Adt(adt_id, subst) => {
206 for subty in tcx.adt_drop_tys(adt_id.did())? {
207 vec.push(EarlyBinder(subty).subst(tcx, subst));
210 _ => vec.push(subty),
216 let adt_components = move |adt_def: ty::AdtDef<'tcx>, substs: SubstsRef<'tcx>| {
217 if adt_def.is_manually_drop() {
218 debug!("drop_tys_helper: `{:?}` is manually drop", adt_def);
220 } else if let Some(dtor_info) = adt_has_dtor(adt_def) {
222 DtorType::Significant => {
223 debug!("drop_tys_helper: `{:?}` implements `Drop`", adt_def);
224 Err(AlwaysRequiresDrop)
226 DtorType::Insignificant => {
227 debug!("drop_tys_helper: `{:?}` drop is insignificant", adt_def);
229 // Since the destructor is insignificant, we just want to make sure all of
230 // the passed in type parameters are also insignificant.
231 // Eg: Vec<T> dtor is insignificant when T=i32 but significant when T=Mutex.
232 Ok(substs.types().collect())
235 } else if adt_def.is_union() {
236 debug!("drop_tys_helper: `{:?}` is a union", adt_def);
239 let field_tys = adt_def.all_fields().map(|field| {
240 let r = tcx.bound_type_of(field.did).subst(tcx, substs);
241 debug!("drop_tys_helper: Subst into {:?} with {:?} gettng {:?}", field, substs, r);
244 if only_significant {
245 // We can't recurse through the query system here because we might induce a cycle
246 Ok(field_tys.collect())
248 // We can use the query system if we consider all drops significant. In that case,
249 // ADTs are `needs_drop` exactly if they `impl Drop` or if any of their "transitive"
250 // fields do. There can be no cycles here, because ADTs cannot contain themselves as
252 with_query_cache(tcx, field_tys)
255 .map(|v| v.into_iter())
258 NeedsDropTypes::new(tcx, param_env, ty, adt_components)
261 fn adt_consider_insignificant_dtor<'tcx>(
263 ) -> impl Fn(ty::AdtDef<'tcx>) -> Option<DtorType> + 'tcx {
264 move |adt_def: ty::AdtDef<'tcx>| {
265 let is_marked_insig = tcx.has_attr(adt_def.did(), sym::rustc_insignificant_dtor);
267 // In some cases like `std::collections::HashMap` where the struct is a wrapper around
268 // a type that is a Drop type, and the wrapped type (eg: `hashbrown::HashMap`) lies
269 // outside stdlib, we might choose to still annotate the the wrapper (std HashMap) with
270 // `rustc_insignificant_dtor`, even if the type itself doesn't have a `Drop` impl.
271 Some(DtorType::Insignificant)
272 } else if adt_def.destructor(tcx).is_some() {
273 // There is a Drop impl and the type isn't marked insignificant, therefore Drop must be
275 Some(DtorType::Significant)
277 // No destructor found nor the type is annotated with `rustc_insignificant_dtor`, we
278 // treat this as the simple case of Drop impl for type.
284 fn adt_drop_tys<'tcx>(
287 ) -> Result<&ty::List<Ty<'tcx>>, AlwaysRequiresDrop> {
288 // This is for the "adt_drop_tys" query, that considers all `Drop` impls, therefore all dtors are
291 |adt_def: ty::AdtDef<'tcx>| adt_def.destructor(tcx).map(|_| DtorType::Significant);
292 // `tcx.type_of(def_id)` identical to `tcx.make_adt(def, identity_substs)`
293 drop_tys_helper(tcx, tcx.type_of(def_id), tcx.param_env(def_id), adt_has_dtor, false)
294 .collect::<Result<Vec<_>, _>>()
295 .map(|components| tcx.intern_type_list(&components))
297 // If `def_id` refers to a generic ADT, the queries above and below act as if they had been handed
298 // a `tcx.make_ty(def, identity_substs)` and as such it is legal to substitute the generic parameters
299 // of the ADT into the outputted `ty`s.
300 fn adt_significant_drop_tys(
303 ) -> Result<&ty::List<Ty<'_>>, AlwaysRequiresDrop> {
306 tcx.type_of(def_id), // identical to `tcx.make_adt(def, identity_substs)`
307 tcx.param_env(def_id),
308 adt_consider_insignificant_dtor(tcx),
311 .collect::<Result<Vec<_>, _>>()
312 .map(|components| tcx.intern_type_list(&components))
315 pub(crate) fn provide(providers: &mut ty::query::Providers) {
316 *providers = ty::query::Providers {
318 has_significant_drop_raw,
320 adt_significant_drop_tys,