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//! Check whether a type has (potentially) non-trivial drop glue.
use rustc_data_structures::fx::FxHashSet;
use rustc_hir::def_id::DefId;
use rustc_middle::bug;
use rustc_middle::query::Providers;
use rustc_middle::ty::util::{needs_drop_components, AlwaysRequiresDrop};
use rustc_middle::ty::{self, EarlyBinder, GenericArgsRef, Ty, TyCtxt};
use rustc_session::Limit;
use rustc_span::sym;
use tracing::debug;
use crate::errors::NeedsDropOverflow;
type NeedsDropResult<T> = Result<T, AlwaysRequiresDrop>;
fn needs_drop_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
// If we don't know a type doesn't need drop, for example if it's a type
// parameter without a `Copy` bound, then we conservatively return that it
// needs drop.
let adt_has_dtor =
|adt_def: ty::AdtDef<'tcx>| adt_def.destructor(tcx).map(|_| DtorType::Significant);
let res = drop_tys_helper(tcx, query.value, query.param_env, adt_has_dtor, false)
.filter(filter_array_elements(tcx, query.param_env))
.next()
.is_some();
debug!("needs_drop_raw({:?}) = {:?}", query, res);
res
}
fn needs_async_drop_raw<'tcx>(tcx: TyCtxt<'tcx>, query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
// If we don't know a type doesn't need async drop, for example if it's a
// type parameter without a `Copy` bound, then we conservatively return that
// it needs async drop.
let adt_has_async_dtor =
|adt_def: ty::AdtDef<'tcx>| adt_def.async_destructor(tcx).map(|_| DtorType::Significant);
let res = drop_tys_helper(tcx, query.value, query.param_env, adt_has_async_dtor, false)
.filter(filter_array_elements(tcx, query.param_env))
.next()
.is_some();
debug!("needs_drop_raw({:?}) = {:?}", query, res);
res
}
/// HACK: in order to not mistakenly assume that `[PhantomData<T>; N]` requires drop glue
/// we check the element type for drop glue. The correct fix would be looking at the
/// entirety of the code around `needs_drop_components` and this file and come up with
/// logic that is easier to follow while not repeating any checks that may thus diverge.
fn filter_array_elements<'tcx>(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
) -> impl Fn(&Result<Ty<'tcx>, AlwaysRequiresDrop>) -> bool {
move |ty| match ty {
Ok(ty) => match *ty.kind() {
ty::Array(elem, _) => tcx.needs_drop_raw(param_env.and(elem)),
_ => true,
},
Err(AlwaysRequiresDrop) => true,
}
}
fn has_significant_drop_raw<'tcx>(
tcx: TyCtxt<'tcx>,
query: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
) -> bool {
let res = drop_tys_helper(
tcx,
query.value,
query.param_env,
adt_consider_insignificant_dtor(tcx),
true,
)
.filter(filter_array_elements(tcx, query.param_env))
.next()
.is_some();
debug!("has_significant_drop_raw({:?}) = {:?}", query, res);
res
}
struct NeedsDropTypes<'tcx, F> {
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
// Whether to reveal coroutine witnesses, this is set
// to `false` unless we compute `needs_drop` for a coroutine witness.
reveal_coroutine_witnesses: bool,
query_ty: Ty<'tcx>,
seen_tys: FxHashSet<Ty<'tcx>>,
/// A stack of types left to process, and the recursion depth when we
/// pushed that type. Each round, we pop something from the stack and check
/// if it needs drop. If the result depends on whether some other types
/// need drop we push them onto the stack.
unchecked_tys: Vec<(Ty<'tcx>, usize)>,
recursion_limit: Limit,
adt_components: F,
}
impl<'tcx, F> NeedsDropTypes<'tcx, F> {
fn new(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
ty: Ty<'tcx>,
adt_components: F,
) -> Self {
let mut seen_tys = FxHashSet::default();
seen_tys.insert(ty);
Self {
tcx,
param_env,
reveal_coroutine_witnesses: false,
seen_tys,
query_ty: ty,
unchecked_tys: vec![(ty, 0)],
recursion_limit: tcx.recursion_limit(),
adt_components,
}
}
}
impl<'tcx, F, I> Iterator for NeedsDropTypes<'tcx, F>
where
F: Fn(ty::AdtDef<'tcx>, GenericArgsRef<'tcx>) -> NeedsDropResult<I>,
I: Iterator<Item = Ty<'tcx>>,
{
type Item = NeedsDropResult<Ty<'tcx>>;
fn next(&mut self) -> Option<NeedsDropResult<Ty<'tcx>>> {
let tcx = self.tcx;
while let Some((ty, level)) = self.unchecked_tys.pop() {
if !self.recursion_limit.value_within_limit(level) {
// Not having a `Span` isn't great. But there's hopefully some other
// recursion limit error as well.
tcx.dcx().emit_err(NeedsDropOverflow { query_ty: self.query_ty });
return Some(Err(AlwaysRequiresDrop));
}
let components = match needs_drop_components(tcx, ty) {
Err(e) => return Some(Err(e)),
Ok(components) => components,
};
debug!("needs_drop_components({:?}) = {:?}", ty, components);
let queue_type = move |this: &mut Self, component: Ty<'tcx>| {
if this.seen_tys.insert(component) {
this.unchecked_tys.push((component, level + 1));
}
};
for component in components {
match *component.kind() {
// The information required to determine whether a coroutine has drop is
// computed on MIR, while this very method is used to build MIR.
// To avoid cycles, we consider that coroutines always require drop.
//
// HACK: Because we erase regions contained in the coroutine witness, we
// have to conservatively assume that every region captured by the
// coroutine has to be live when dropped. This results in a lot of
// undesirable borrowck errors. During borrowck, we call `needs_drop`
// for the coroutine witness and check whether any of the contained types
// need to be dropped, and only require the captured types to be live
// if they do.
ty::Coroutine(_, args) => {
if self.reveal_coroutine_witnesses {
queue_type(self, args.as_coroutine().witness());
} else {
return Some(Err(AlwaysRequiresDrop));
}
}
ty::CoroutineWitness(def_id, args) => {
if let Some(witness) = tcx.mir_coroutine_witnesses(def_id) {
self.reveal_coroutine_witnesses = true;
for field_ty in &witness.field_tys {
queue_type(
self,
EarlyBinder::bind(field_ty.ty).instantiate(tcx, args),
);
}
}
}
_ if component.is_copy_modulo_regions(tcx, self.param_env) => (),
ty::Closure(_, args) => {
for upvar in args.as_closure().upvar_tys() {
queue_type(self, upvar);
}
}
ty::CoroutineClosure(_, args) => {
for upvar in args.as_coroutine_closure().upvar_tys() {
queue_type(self, upvar);
}
}
// Check for a `Drop` impl and whether this is a union or
// `ManuallyDrop`. If it's a struct or enum without a `Drop`
// impl then check whether the field types need `Drop`.
ty::Adt(adt_def, args) => {
let tys = match (self.adt_components)(adt_def, args) {
Err(e) => return Some(Err(e)),
Ok(tys) => tys,
};
for required_ty in tys {
let required = tcx
.try_normalize_erasing_regions(self.param_env, required_ty)
.unwrap_or(required_ty);
queue_type(self, required);
}
}
ty::Alias(..) | ty::Array(..) | ty::Placeholder(_) | ty::Param(_) => {
if ty == component {
// Return the type to the caller: they may be able
// to normalize further than we can.
return Some(Ok(component));
} else {
// Store the type for later. We can't return here
// because we would then lose any other components
// of the type.
queue_type(self, component);
}
}
ty::Foreign(_) | ty::Dynamic(..) => {
return Some(Err(AlwaysRequiresDrop));
}
ty::Bool
| ty::Char
| ty::Int(_)
| ty::Uint(_)
| ty::Float(_)
| ty::Str
| ty::Slice(_)
| ty::Ref(..)
| ty::RawPtr(..)
| ty::FnDef(..)
| ty::Pat(..)
| ty::FnPtr(..)
| ty::Tuple(_)
| ty::Bound(..)
| ty::Never
| ty::Infer(_)
| ty::Error(_) => {
bug!("unexpected type returned by `needs_drop_components`: {component}")
}
}
}
}
None
}
}
enum DtorType {
/// Type has a `Drop` but it is considered insignificant.
/// Check the query `adt_significant_drop_tys` for understanding
/// "significant" / "insignificant".
Insignificant,
/// Type has a `Drop` implantation.
Significant,
}
// This is a helper function for `adt_drop_tys` and `adt_significant_drop_tys`.
// Depending on the implantation of `adt_has_dtor`, it is used to check if the
// ADT has a destructor or if the ADT only has a significant destructor. For
// understanding significant destructor look at `adt_significant_drop_tys`.
fn drop_tys_helper<'tcx>(
tcx: TyCtxt<'tcx>,
ty: Ty<'tcx>,
param_env: rustc_middle::ty::ParamEnv<'tcx>,
adt_has_dtor: impl Fn(ty::AdtDef<'tcx>) -> Option<DtorType>,
only_significant: bool,
) -> impl Iterator<Item = NeedsDropResult<Ty<'tcx>>> {
fn with_query_cache<'tcx>(
tcx: TyCtxt<'tcx>,
iter: impl IntoIterator<Item = Ty<'tcx>>,
) -> NeedsDropResult<Vec<Ty<'tcx>>> {
iter.into_iter().try_fold(Vec::new(), |mut vec, subty| {
match subty.kind() {
ty::Adt(adt_id, args) => {
for subty in tcx.adt_drop_tys(adt_id.did())? {
vec.push(EarlyBinder::bind(subty).instantiate(tcx, args));
}
}
_ => vec.push(subty),
};
Ok(vec)
})
}
let adt_components = move |adt_def: ty::AdtDef<'tcx>, args: GenericArgsRef<'tcx>| {
if adt_def.is_manually_drop() {
debug!("drop_tys_helper: `{:?}` is manually drop", adt_def);
Ok(Vec::new())
} else if let Some(dtor_info) = adt_has_dtor(adt_def) {
match dtor_info {
DtorType::Significant => {
debug!("drop_tys_helper: `{:?}` implements `Drop`", adt_def);
Err(AlwaysRequiresDrop)
}
DtorType::Insignificant => {
debug!("drop_tys_helper: `{:?}` drop is insignificant", adt_def);
// Since the destructor is insignificant, we just want to make sure all of
// the passed in type parameters are also insignificant.
// Eg: Vec<T> dtor is insignificant when T=i32 but significant when T=Mutex.
Ok(args.types().collect())
}
}
} else if adt_def.is_union() {
debug!("drop_tys_helper: `{:?}` is a union", adt_def);
Ok(Vec::new())
} else {
let field_tys = adt_def.all_fields().map(|field| {
let r = tcx.type_of(field.did).instantiate(tcx, args);
debug!(
"drop_tys_helper: Instantiate into {:?} with {:?} getting {:?}",
field, args, r
);
r
});
if only_significant {
// We can't recurse through the query system here because we might induce a cycle
Ok(field_tys.collect())
} else {
// We can use the query system if we consider all drops significant. In that case,
// ADTs are `needs_drop` exactly if they `impl Drop` or if any of their "transitive"
// fields do. There can be no cycles here, because ADTs cannot contain themselves as
// fields.
with_query_cache(tcx, field_tys)
}
}
.map(|v| v.into_iter())
};
NeedsDropTypes::new(tcx, param_env, ty, adt_components)
}
fn adt_consider_insignificant_dtor<'tcx>(
tcx: TyCtxt<'tcx>,
) -> impl Fn(ty::AdtDef<'tcx>) -> Option<DtorType> + 'tcx {
move |adt_def: ty::AdtDef<'tcx>| {
let is_marked_insig = tcx.has_attr(adt_def.did(), sym::rustc_insignificant_dtor);
if is_marked_insig {
// In some cases like `std::collections::HashMap` where the struct is a wrapper around
// a type that is a Drop type, and the wrapped type (eg: `hashbrown::HashMap`) lies
// outside stdlib, we might choose to still annotate the wrapper (std HashMap) with
// `rustc_insignificant_dtor`, even if the type itself doesn't have a `Drop` impl.
Some(DtorType::Insignificant)
} else if adt_def.destructor(tcx).is_some() {
// There is a Drop impl and the type isn't marked insignificant, therefore Drop must be
// significant.
Some(DtorType::Significant)
} else {
// No destructor found nor the type is annotated with `rustc_insignificant_dtor`, we
// treat this as the simple case of Drop impl for type.
None
}
}
}
fn adt_drop_tys<'tcx>(
tcx: TyCtxt<'tcx>,
def_id: DefId,
) -> Result<&'tcx ty::List<Ty<'tcx>>, AlwaysRequiresDrop> {
// This is for the "adt_drop_tys" query, that considers all `Drop` impls, therefore all dtors are
// significant.
let adt_has_dtor =
|adt_def: ty::AdtDef<'tcx>| adt_def.destructor(tcx).map(|_| DtorType::Significant);
// `tcx.type_of(def_id)` identical to `tcx.make_adt(def, identity_args)`
drop_tys_helper(
tcx,
tcx.type_of(def_id).instantiate_identity(),
tcx.param_env(def_id),
adt_has_dtor,
false,
)
.collect::<Result<Vec<_>, _>>()
.map(|components| tcx.mk_type_list(&components))
}
// If `def_id` refers to a generic ADT, the queries above and below act as if they had been handed
// a `tcx.make_ty(def, identity_args)` and as such it is legal to instantiate the generic parameters
// of the ADT into the outputted `ty`s.
fn adt_significant_drop_tys(
tcx: TyCtxt<'_>,
def_id: DefId,
) -> Result<&ty::List<Ty<'_>>, AlwaysRequiresDrop> {
drop_tys_helper(
tcx,
tcx.type_of(def_id).instantiate_identity(), // identical to `tcx.make_adt(def, identity_args)`
tcx.param_env(def_id),
adt_consider_insignificant_dtor(tcx),
true,
)
.collect::<Result<Vec<_>, _>>()
.map(|components| tcx.mk_type_list(&components))
}
pub(crate) fn provide(providers: &mut Providers) {
*providers = Providers {
needs_drop_raw,
needs_async_drop_raw,
has_significant_drop_raw,
adt_drop_tys,
adt_significant_drop_tys,
..*providers
};
}