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 use crate::errors::NeedsDropOverflow;
14 type NeedsDropResult<T> = Result<T, AlwaysRequiresDrop>;
16 fn needs_drop_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
17 // If we don't know a type doesn't need drop, for example if it's a type
18 // parameter without a `Copy` bound, then we conservatively return that it
21 |adt_def: ty::AdtDef<'tcx>| adt_def.destructor(tcx).map(|_| DtorType::Significant);
23 drop_tys_helper(tcx, query.value, query.param_env, adt_has_dtor, false).next().is_some();
25 debug!("needs_drop_raw({:?}) = {:?}", query, res);
29 fn has_significant_drop_raw<'tcx>(
31 query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
33 let res = drop_tys_helper(
37 adt_consider_insignificant_dtor(tcx),
42 debug!("has_significant_drop_raw({:?}) = {:?}", query, res);
46 struct NeedsDropTypes<'tcx, F> {
48 param_env: ty::ParamEnv<'tcx>,
50 seen_tys: FxHashSet<Ty<'tcx>>,
51 /// A stack of types left to process, and the recursion depth when we
52 /// pushed that type. Each round, we pop something from the stack and check
53 /// if it needs drop. If the result depends on whether some other types
54 /// need drop we push them onto the stack.
55 unchecked_tys: Vec<(Ty<'tcx>, usize)>,
56 recursion_limit: Limit,
60 impl<'tcx, F> NeedsDropTypes<'tcx, F> {
63 param_env: ty::ParamEnv<'tcx>,
67 let mut seen_tys = FxHashSet::default();
74 unchecked_tys: vec![(ty, 0)],
75 recursion_limit: tcx.recursion_limit(),
81 impl<'tcx, F, I> Iterator for NeedsDropTypes<'tcx, F>
83 F: Fn(ty::AdtDef<'tcx>, SubstsRef<'tcx>) -> NeedsDropResult<I>,
84 I: Iterator<Item = Ty<'tcx>>,
86 type Item = NeedsDropResult<Ty<'tcx>>;
88 fn next(&mut self) -> Option<NeedsDropResult<Ty<'tcx>>> {
91 while let Some((ty, level)) = self.unchecked_tys.pop() {
92 if !self.recursion_limit.value_within_limit(level) {
93 // Not having a `Span` isn't great. But there's hopefully some other
94 // recursion limit error as well.
95 tcx.sess.emit_err(NeedsDropOverflow { query_ty: self.query_ty });
96 return Some(Err(AlwaysRequiresDrop));
99 let components = match needs_drop_components(ty, &tcx.data_layout) {
100 Err(e) => return Some(Err(e)),
101 Ok(components) => components,
103 debug!("needs_drop_components({:?}) = {:?}", ty, components);
105 let queue_type = move |this: &mut Self, component: Ty<'tcx>| {
106 if this.seen_tys.insert(component) {
107 this.unchecked_tys.push((component, level + 1));
111 for component in components {
112 match *component.kind() {
113 _ if component.is_copy_modulo_regions(tcx.at(DUMMY_SP), self.param_env) => (),
115 ty::Closure(_, substs) => {
116 queue_type(self, substs.as_closure().tupled_upvars_ty());
119 ty::Generator(def_id, substs, _) => {
120 let substs = substs.as_generator();
121 queue_type(self, substs.tupled_upvars_ty());
123 let witness = substs.witness();
124 let interior_tys = match witness.kind() {
125 &ty::GeneratorWitness(tys) => tcx.erase_late_bound_regions(tys),
127 tcx.sess.delay_span_bug(
128 tcx.hir().span_if_local(def_id).unwrap_or(DUMMY_SP),
129 &format!("unexpected generator witness type {:?}", witness),
131 return Some(Err(AlwaysRequiresDrop));
135 for interior_ty in interior_tys {
136 queue_type(self, interior_ty);
140 // Check for a `Drop` impl and whether this is a union or
141 // `ManuallyDrop`. If it's a struct or enum without a `Drop`
142 // impl then check whether the field types need `Drop`.
143 ty::Adt(adt_def, substs) => {
144 let tys = match (self.adt_components)(adt_def, substs) {
145 Err(e) => return Some(Err(e)),
148 for required_ty in tys {
150 .try_normalize_erasing_regions(self.param_env, required_ty)
151 .unwrap_or(required_ty);
153 queue_type(self, required);
156 ty::Array(..) | ty::Opaque(..) | ty::Projection(..) | ty::Param(_) => {
158 // Return the type to the caller: they may be able
159 // to normalize further than we can.
160 return Some(Ok(component));
162 // Store the type for later. We can't return here
163 // because we would then lose any other components
165 queue_type(self, component);
168 _ => return Some(Err(AlwaysRequiresDrop)),
178 /// Type has a `Drop` but it is considered insignificant.
179 /// Check the query `adt_significant_drop_tys` for understanding
180 /// "significant" / "insignificant".
183 /// Type has a `Drop` implantation.
187 // This is a helper function for `adt_drop_tys` and `adt_significant_drop_tys`.
188 // Depending on the implantation of `adt_has_dtor`, it is used to check if the
189 // ADT has a destructor or if the ADT only has a significant destructor. For
190 // understanding significant destructor look at `adt_significant_drop_tys`.
191 fn drop_tys_helper<'tcx>(
194 param_env: rustc_middle::ty::ParamEnv<'tcx>,
195 adt_has_dtor: impl Fn(ty::AdtDef<'tcx>) -> Option<DtorType>,
196 only_significant: bool,
197 ) -> impl Iterator<Item = NeedsDropResult<Ty<'tcx>>> {
198 fn with_query_cache<'tcx>(
200 iter: impl IntoIterator<Item = Ty<'tcx>>,
201 ) -> NeedsDropResult<Vec<Ty<'tcx>>> {
202 iter.into_iter().try_fold(Vec::new(), |mut vec, subty| {
204 ty::Adt(adt_id, subst) => {
205 for subty in tcx.adt_drop_tys(adt_id.did())? {
206 vec.push(EarlyBinder(subty).subst(tcx, subst));
209 _ => vec.push(subty),
215 let adt_components = move |adt_def: ty::AdtDef<'tcx>, substs: SubstsRef<'tcx>| {
216 if adt_def.is_manually_drop() {
217 debug!("drop_tys_helper: `{:?}` is manually drop", adt_def);
219 } else if let Some(dtor_info) = adt_has_dtor(adt_def) {
221 DtorType::Significant => {
222 debug!("drop_tys_helper: `{:?}` implements `Drop`", adt_def);
223 Err(AlwaysRequiresDrop)
225 DtorType::Insignificant => {
226 debug!("drop_tys_helper: `{:?}` drop is insignificant", adt_def);
228 // Since the destructor is insignificant, we just want to make sure all of
229 // the passed in type parameters are also insignificant.
230 // Eg: Vec<T> dtor is insignificant when T=i32 but significant when T=Mutex.
231 Ok(substs.types().collect())
234 } else if adt_def.is_union() {
235 debug!("drop_tys_helper: `{:?}` is a union", adt_def);
238 let field_tys = adt_def.all_fields().map(|field| {
239 let r = tcx.bound_type_of(field.did).subst(tcx, substs);
240 debug!("drop_tys_helper: Subst into {:?} with {:?} gettng {:?}", field, substs, r);
243 if only_significant {
244 // We can't recurse through the query system here because we might induce a cycle
245 Ok(field_tys.collect())
247 // We can use the query system if we consider all drops significant. In that case,
248 // ADTs are `needs_drop` exactly if they `impl Drop` or if any of their "transitive"
249 // fields do. There can be no cycles here, because ADTs cannot contain themselves as
251 with_query_cache(tcx, field_tys)
254 .map(|v| v.into_iter())
257 NeedsDropTypes::new(tcx, param_env, ty, adt_components)
260 fn adt_consider_insignificant_dtor<'tcx>(
262 ) -> impl Fn(ty::AdtDef<'tcx>) -> Option<DtorType> + 'tcx {
263 move |adt_def: ty::AdtDef<'tcx>| {
264 let is_marked_insig = tcx.has_attr(adt_def.did(), sym::rustc_insignificant_dtor);
266 // In some cases like `std::collections::HashMap` where the struct is a wrapper around
267 // a type that is a Drop type, and the wrapped type (eg: `hashbrown::HashMap`) lies
268 // outside stdlib, we might choose to still annotate the the wrapper (std HashMap) with
269 // `rustc_insignificant_dtor`, even if the type itself doesn't have a `Drop` impl.
270 Some(DtorType::Insignificant)
271 } else if adt_def.destructor(tcx).is_some() {
272 // There is a Drop impl and the type isn't marked insignificant, therefore Drop must be
274 Some(DtorType::Significant)
276 // No destructor found nor the type is annotated with `rustc_insignificant_dtor`, we
277 // treat this as the simple case of Drop impl for type.
283 fn adt_drop_tys<'tcx>(
286 ) -> Result<&ty::List<Ty<'tcx>>, AlwaysRequiresDrop> {
287 // This is for the "adt_drop_tys" query, that considers all `Drop` impls, therefore all dtors are
290 |adt_def: ty::AdtDef<'tcx>| adt_def.destructor(tcx).map(|_| DtorType::Significant);
291 // `tcx.type_of(def_id)` identical to `tcx.make_adt(def, identity_substs)`
292 drop_tys_helper(tcx, tcx.type_of(def_id), tcx.param_env(def_id), adt_has_dtor, false)
293 .collect::<Result<Vec<_>, _>>()
294 .map(|components| tcx.intern_type_list(&components))
296 // If `def_id` refers to a generic ADT, the queries above and below act as if they had been handed
297 // a `tcx.make_ty(def, identity_substs)` and as such it is legal to substitute the generic parameters
298 // of the ADT into the outputted `ty`s.
299 fn adt_significant_drop_tys(
302 ) -> Result<&ty::List<Ty<'_>>, AlwaysRequiresDrop> {
305 tcx.type_of(def_id), // identical to `tcx.make_adt(def, identity_substs)`
306 tcx.param_env(def_id),
307 adt_consider_insignificant_dtor(tcx),
310 .collect::<Result<Vec<_>, _>>()
311 .map(|components| tcx.intern_type_list(&components))
314 pub(crate) fn provide(providers: &mut ty::query::Providers) {
315 *providers = ty::query::Providers {
317 has_significant_drop_raw,
319 adt_significant_drop_tys,