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::SubstsRef;
6 use rustc_middle::ty::util::{needs_drop_components, AlwaysRequiresDrop};
7 use rustc_middle::ty::{self, EarlyBinder, Ty, TyCtxt};
8 use rustc_session::Limit;
9 use rustc_span::{sym, DUMMY_SP};
11 use crate::errors::NeedsDropOverflow;
13 type NeedsDropResult<T> = Result<T, AlwaysRequiresDrop>;
15 fn needs_drop_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
16 // If we don't know a type doesn't need drop, for example if it's a type
17 // parameter without a `Copy` bound, then we conservatively return that it
20 |adt_def: ty::AdtDef<'tcx>| adt_def.destructor(tcx).map(|_| DtorType::Significant);
22 drop_tys_helper(tcx, query.value, query.param_env, adt_has_dtor, false).next().is_some();
24 debug!("needs_drop_raw({:?}) = {:?}", query, res);
28 fn has_significant_drop_raw<'tcx>(
30 query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
32 let res = drop_tys_helper(
36 adt_consider_insignificant_dtor(tcx),
41 debug!("has_significant_drop_raw({:?}) = {:?}", query, res);
45 struct NeedsDropTypes<'tcx, F> {
47 param_env: ty::ParamEnv<'tcx>,
49 seen_tys: FxHashSet<Ty<'tcx>>,
50 /// A stack of types left to process, and the recursion depth when we
51 /// pushed that type. Each round, we pop something from the stack and check
52 /// if it needs drop. If the result depends on whether some other types
53 /// need drop we push them onto the stack.
54 unchecked_tys: Vec<(Ty<'tcx>, usize)>,
55 recursion_limit: Limit,
59 impl<'tcx, F> NeedsDropTypes<'tcx, F> {
62 param_env: ty::ParamEnv<'tcx>,
66 let mut seen_tys = FxHashSet::default();
73 unchecked_tys: vec![(ty, 0)],
74 recursion_limit: tcx.recursion_limit(),
80 impl<'tcx, F, I> Iterator for NeedsDropTypes<'tcx, F>
82 F: Fn(ty::AdtDef<'tcx>, SubstsRef<'tcx>) -> NeedsDropResult<I>,
83 I: Iterator<Item = Ty<'tcx>>,
85 type Item = NeedsDropResult<Ty<'tcx>>;
87 fn next(&mut self) -> Option<NeedsDropResult<Ty<'tcx>>> {
90 while let Some((ty, level)) = self.unchecked_tys.pop() {
91 if !self.recursion_limit.value_within_limit(level) {
92 // Not having a `Span` isn't great. But there's hopefully some other
93 // recursion limit error as well.
94 tcx.sess.emit_err(NeedsDropOverflow { query_ty: self.query_ty });
95 return Some(Err(AlwaysRequiresDrop));
98 let components = match needs_drop_components(ty, &tcx.data_layout) {
99 Err(e) => return Some(Err(e)),
100 Ok(components) => components,
102 debug!("needs_drop_components({:?}) = {:?}", ty, components);
104 let queue_type = move |this: &mut Self, component: Ty<'tcx>| {
105 if this.seen_tys.insert(component) {
106 this.unchecked_tys.push((component, level + 1));
110 for component in components {
111 match *component.kind() {
112 // The information required to determine whether a generator has drop is
113 // computed on MIR, while this very method is used to build MIR. To avoid
114 // cycles, we consider that generators always require drop.
115 ty::Generator(..) if tcx.sess.opts.unstable_opts.drop_tracking_mir => {
116 return Some(Err(AlwaysRequiresDrop));
119 _ if component.is_copy_modulo_regions(tcx, self.param_env) => (),
121 ty::Closure(_, substs) => {
122 queue_type(self, substs.as_closure().tupled_upvars_ty());
125 ty::Generator(def_id, substs, _) => {
126 let substs = substs.as_generator();
127 queue_type(self, substs.tupled_upvars_ty());
129 let witness = substs.witness();
130 let interior_tys = match witness.kind() {
131 &ty::GeneratorWitness(tys) => tcx.erase_late_bound_regions(tys),
133 tcx.sess.delay_span_bug(
134 tcx.hir().span_if_local(def_id).unwrap_or(DUMMY_SP),
135 &format!("unexpected generator witness type {:?}", witness),
137 return Some(Err(AlwaysRequiresDrop));
141 for interior_ty in interior_tys {
142 queue_type(self, interior_ty);
146 // Check for a `Drop` impl and whether this is a union or
147 // `ManuallyDrop`. If it's a struct or enum without a `Drop`
148 // impl then check whether the field types need `Drop`.
149 ty::Adt(adt_def, substs) => {
150 let tys = match (self.adt_components)(adt_def, substs) {
151 Err(e) => return Some(Err(e)),
154 for required_ty in tys {
156 .try_normalize_erasing_regions(self.param_env, required_ty)
157 .unwrap_or(required_ty);
159 queue_type(self, required);
162 ty::Array(..) | ty::Alias(..) | ty::Param(_) => {
164 // Return the type to the caller: they may be able
165 // to normalize further than we can.
166 return Some(Ok(component));
168 // Store the type for later. We can't return here
169 // because we would then lose any other components
171 queue_type(self, component);
174 _ => return Some(Err(AlwaysRequiresDrop)),
184 /// Type has a `Drop` but it is considered insignificant.
185 /// Check the query `adt_significant_drop_tys` for understanding
186 /// "significant" / "insignificant".
189 /// Type has a `Drop` implantation.
193 // This is a helper function for `adt_drop_tys` and `adt_significant_drop_tys`.
194 // Depending on the implantation of `adt_has_dtor`, it is used to check if the
195 // ADT has a destructor or if the ADT only has a significant destructor. For
196 // understanding significant destructor look at `adt_significant_drop_tys`.
197 fn drop_tys_helper<'tcx>(
200 param_env: rustc_middle::ty::ParamEnv<'tcx>,
201 adt_has_dtor: impl Fn(ty::AdtDef<'tcx>) -> Option<DtorType>,
202 only_significant: bool,
203 ) -> impl Iterator<Item = NeedsDropResult<Ty<'tcx>>> {
204 fn with_query_cache<'tcx>(
206 iter: impl IntoIterator<Item = Ty<'tcx>>,
207 ) -> NeedsDropResult<Vec<Ty<'tcx>>> {
208 iter.into_iter().try_fold(Vec::new(), |mut vec, subty| {
210 ty::Adt(adt_id, subst) => {
211 for subty in tcx.adt_drop_tys(adt_id.did())? {
212 vec.push(EarlyBinder(subty).subst(tcx, subst));
215 _ => vec.push(subty),
221 let adt_components = move |adt_def: ty::AdtDef<'tcx>, substs: SubstsRef<'tcx>| {
222 if adt_def.is_manually_drop() {
223 debug!("drop_tys_helper: `{:?}` is manually drop", adt_def);
225 } else if let Some(dtor_info) = adt_has_dtor(adt_def) {
227 DtorType::Significant => {
228 debug!("drop_tys_helper: `{:?}` implements `Drop`", adt_def);
229 Err(AlwaysRequiresDrop)
231 DtorType::Insignificant => {
232 debug!("drop_tys_helper: `{:?}` drop is insignificant", adt_def);
234 // Since the destructor is insignificant, we just want to make sure all of
235 // the passed in type parameters are also insignificant.
236 // Eg: Vec<T> dtor is insignificant when T=i32 but significant when T=Mutex.
237 Ok(substs.types().collect())
240 } else if adt_def.is_union() {
241 debug!("drop_tys_helper: `{:?}` is a union", adt_def);
244 let field_tys = adt_def.all_fields().map(|field| {
245 let r = tcx.bound_type_of(field.did).subst(tcx, substs);
246 debug!("drop_tys_helper: Subst into {:?} with {:?} gettng {:?}", field, substs, r);
249 if only_significant {
250 // We can't recurse through the query system here because we might induce a cycle
251 Ok(field_tys.collect())
253 // We can use the query system if we consider all drops significant. In that case,
254 // ADTs are `needs_drop` exactly if they `impl Drop` or if any of their "transitive"
255 // fields do. There can be no cycles here, because ADTs cannot contain themselves as
257 with_query_cache(tcx, field_tys)
260 .map(|v| v.into_iter())
263 NeedsDropTypes::new(tcx, param_env, ty, adt_components)
266 fn adt_consider_insignificant_dtor<'tcx>(
268 ) -> impl Fn(ty::AdtDef<'tcx>) -> Option<DtorType> + 'tcx {
269 move |adt_def: ty::AdtDef<'tcx>| {
270 let is_marked_insig = tcx.has_attr(adt_def.did(), sym::rustc_insignificant_dtor);
272 // In some cases like `std::collections::HashMap` where the struct is a wrapper around
273 // a type that is a Drop type, and the wrapped type (eg: `hashbrown::HashMap`) lies
274 // outside stdlib, we might choose to still annotate the wrapper (std HashMap) with
275 // `rustc_insignificant_dtor`, even if the type itself doesn't have a `Drop` impl.
276 Some(DtorType::Insignificant)
277 } else if adt_def.destructor(tcx).is_some() {
278 // There is a Drop impl and the type isn't marked insignificant, therefore Drop must be
280 Some(DtorType::Significant)
282 // No destructor found nor the type is annotated with `rustc_insignificant_dtor`, we
283 // treat this as the simple case of Drop impl for type.
289 fn adt_drop_tys<'tcx>(
292 ) -> Result<&ty::List<Ty<'tcx>>, AlwaysRequiresDrop> {
293 // This is for the "adt_drop_tys" query, that considers all `Drop` impls, therefore all dtors are
296 |adt_def: ty::AdtDef<'tcx>| adt_def.destructor(tcx).map(|_| DtorType::Significant);
297 // `tcx.type_of(def_id)` identical to `tcx.make_adt(def, identity_substs)`
298 drop_tys_helper(tcx, tcx.type_of(def_id), tcx.param_env(def_id), adt_has_dtor, false)
299 .collect::<Result<Vec<_>, _>>()
300 .map(|components| tcx.intern_type_list(&components))
302 // If `def_id` refers to a generic ADT, the queries above and below act as if they had been handed
303 // a `tcx.make_ty(def, identity_substs)` and as such it is legal to substitute the generic parameters
304 // of the ADT into the outputted `ty`s.
305 fn adt_significant_drop_tys(
308 ) -> Result<&ty::List<Ty<'_>>, AlwaysRequiresDrop> {
311 tcx.type_of(def_id), // identical to `tcx.make_adt(def, identity_substs)`
312 tcx.param_env(def_id),
313 adt_consider_insignificant_dtor(tcx),
316 .collect::<Result<Vec<_>, _>>()
317 .map(|components| tcx.intern_type_list(&components))
320 pub(crate) fn provide(providers: &mut ty::query::Providers) {
321 *providers = ty::query::Providers {
323 has_significant_drop_raw,
325 adt_significant_drop_tys,