1 // Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 use check::regionck::{self, Rcx};
13 use middle::def_id::DefId;
14 use middle::free_region::FreeRegionMap;
17 use middle::subst::{self, Subst};
19 use middle::ty::{self, Ty};
22 use syntax::codemap::{self, Span};
23 use syntax::parse::token::special_idents;
25 /// check_drop_impl confirms that the Drop implementation identfied by
26 /// `drop_impl_did` is not any more specialized than the type it is
27 /// attached to (Issue #8142).
31 /// 1. The self type must be nominal (this is already checked during
34 /// 2. The generic region/type parameters of the impl's self-type must
35 /// all be parameters of the Drop impl itself (i.e. no
36 /// specialization like `impl Drop for Foo<i32>`), and,
38 /// 3. Any bounds on the generic parameters must be reflected in the
39 /// struct/enum definition for the nominal type itself (i.e.
40 /// cannot do `struct S<T>; impl<T:Clone> Drop for S<T> { ... }`).
42 pub fn check_drop_impl(tcx: &ty::ctxt, drop_impl_did: DefId) -> Result<(), ()> {
43 let ty::TypeScheme { generics: ref dtor_generics,
44 ty: dtor_self_type } = tcx.lookup_item_type(drop_impl_did);
45 let dtor_predicates = tcx.lookup_predicates(drop_impl_did);
46 match dtor_self_type.sty {
47 ty::TyEnum(adt_def, self_to_impl_substs) |
48 ty::TyStruct(adt_def, self_to_impl_substs) => {
49 try!(ensure_drop_params_and_item_params_correspond(tcx,
55 ensure_drop_predicates_are_implied_by_item_defn(tcx,
62 // Destructors only work on nominal types. This was
63 // already checked by coherence, so we can panic here.
64 let span = tcx.map.def_id_span(drop_impl_did, codemap::DUMMY_SP);
66 span, &format!("should have been rejected by coherence check: {}",
72 fn ensure_drop_params_and_item_params_correspond<'tcx>(
75 drop_impl_generics: &ty::Generics<'tcx>,
76 drop_impl_ty: &ty::Ty<'tcx>,
77 self_type_did: DefId) -> Result<(), ()>
79 let drop_impl_node_id = tcx.map.as_local_node_id(drop_impl_did).unwrap();
80 let self_type_node_id = tcx.map.as_local_node_id(self_type_did).unwrap();
82 // check that the impl type can be made to match the trait type.
84 let impl_param_env = ty::ParameterEnvironment::for_item(tcx, self_type_node_id);
85 let infcx = infer::new_infer_ctxt(tcx, &tcx.tables, Some(impl_param_env), true);
87 let named_type = tcx.lookup_item_type(self_type_did).ty;
88 let named_type = named_type.subst(tcx, &infcx.parameter_environment.free_substs);
90 let drop_impl_span = tcx.map.def_id_span(drop_impl_did, codemap::DUMMY_SP);
91 let fresh_impl_substs =
92 infcx.fresh_substs_for_generics(drop_impl_span, drop_impl_generics);
93 let fresh_impl_self_ty = drop_impl_ty.subst(tcx, &fresh_impl_substs);
95 if let Err(_) = infer::mk_eqty(&infcx, true, infer::TypeOrigin::Misc(drop_impl_span),
96 named_type, fresh_impl_self_ty) {
97 span_err!(tcx.sess, drop_impl_span, E0366,
98 "Implementations of Drop cannot be specialized");
99 let item_span = tcx.map.span(self_type_node_id);
100 tcx.sess.span_note(item_span,
101 "Use same sequence of generic type and region \
102 parameters that is on the struct/enum definition");
106 if let Err(ref errors) = infcx.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
107 // this could be reached when we get lazy normalization
108 traits::report_fulfillment_errors(&infcx, errors);
112 let free_regions = FreeRegionMap::new();
113 infcx.resolve_regions_and_report_errors(&free_regions, drop_impl_node_id);
117 /// Confirms that every predicate imposed by dtor_predicates is
118 /// implied by assuming the predicates attached to self_type_did.
119 fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
120 tcx: &ty::ctxt<'tcx>,
121 drop_impl_did: DefId,
122 dtor_predicates: &ty::GenericPredicates<'tcx>,
123 self_type_did: DefId,
124 self_to_impl_substs: &subst::Substs<'tcx>) -> Result<(), ()> {
126 // Here is an example, analogous to that from
127 // `compare_impl_method`.
129 // Consider a struct type:
131 // struct Type<'c, 'b:'c, 'a> {
132 // x: &'a Contents // (contents are irrelevant;
133 // y: &'c Cell<&'b Contents>, // only the bounds matter for our purposes.)
138 // impl<'z, 'y:'z, 'x:'y> Drop for P<'z, 'y, 'x> {
139 // fn drop(&mut self) { self.y.set(self.x); } // (only legal if 'x: 'y)
142 // We start out with self_to_impl_substs, that maps the generic
143 // parameters of Type to that of the Drop impl.
145 // self_to_impl_substs = {'c => 'z, 'b => 'y, 'a => 'x}
147 // Applying this to the predicates (i.e. assumptions) provided by the item
148 // definition yields the instantiated assumptions:
152 // We then check all of the predicates of the Drop impl:
156 // and ensure each is in the list of instantiated
157 // assumptions. Here, `'y:'z` is present, but `'x:'y` is
158 // absent. So we report an error that the Drop impl injected a
159 // predicate that is not present on the struct definition.
161 let self_type_node_id = tcx.map.as_local_node_id(self_type_did).unwrap();
163 let drop_impl_span = tcx.map.def_id_span(drop_impl_did, codemap::DUMMY_SP);
165 // We can assume the predicates attached to struct/enum definition
167 let generic_assumptions = tcx.lookup_predicates(self_type_did);
169 let assumptions_in_impl_context = generic_assumptions.instantiate(tcx, &self_to_impl_substs);
170 assert!(assumptions_in_impl_context.predicates.is_empty_in(subst::SelfSpace));
171 assert!(assumptions_in_impl_context.predicates.is_empty_in(subst::FnSpace));
172 let assumptions_in_impl_context =
173 assumptions_in_impl_context.predicates.get_slice(subst::TypeSpace);
175 // An earlier version of this code attempted to do this checking
176 // via the traits::fulfill machinery. However, it ran into trouble
177 // since the fulfill machinery merely turns outlives-predicates
178 // 'a:'b and T:'b into region inference constraints. It is simpler
179 // just to look for all the predicates directly.
181 assert!(dtor_predicates.predicates.is_empty_in(subst::SelfSpace));
182 assert!(dtor_predicates.predicates.is_empty_in(subst::FnSpace));
183 let predicates = dtor_predicates.predicates.get_slice(subst::TypeSpace);
184 for predicate in predicates {
185 // (We do not need to worry about deep analysis of type
186 // expressions etc because the Drop impls are already forced
187 // to take on a structure that is roughly an alpha-renaming of
188 // the generic parameters of the item definition.)
190 // This path now just checks *all* predicates via the direct
191 // lookup, rather than using fulfill machinery.
193 // However, it may be more efficient in the future to batch
194 // the analysis together via the fulfill , rather than the
195 // repeated `contains` calls.
197 if !assumptions_in_impl_context.contains(&predicate) {
198 let item_span = tcx.map.span(self_type_node_id);
199 span_err!(tcx.sess, drop_impl_span, E0367,
200 "The requirement `{}` is added only by the Drop impl.", predicate);
201 tcx.sess.span_note(item_span,
202 "The same requirement must be part of \
203 the struct/enum definition");
207 if tcx.sess.has_errors() {
213 /// check_safety_of_destructor_if_necessary confirms that the type
214 /// expression `typ` conforms to the "Drop Check Rule" from the Sound
215 /// Generic Drop (RFC 769).
219 /// The simplified (*) Drop Check Rule is the following:
221 /// Let `v` be some value (either temporary or named) and 'a be some
222 /// lifetime (scope). If the type of `v` owns data of type `D`, where
224 /// * (1.) `D` has a lifetime- or type-parametric Drop implementation,
225 /// (where that `Drop` implementation does not opt-out of
226 /// this check via the `unsafe_destructor_blind_to_params`
228 /// * (2.) the structure of `D` can reach a reference of type `&'a _`,
230 /// then 'a must strictly outlive the scope of v.
234 /// This function is meant to by applied to the type for every
235 /// expression in the program.
239 /// (*) The qualifier "simplified" is attached to the above
240 /// definition of the Drop Check Rule, because it is a simplification
241 /// of the original Drop Check rule, which attempted to prove that
242 /// some `Drop` implementations could not possibly access data even if
243 /// it was technically reachable, due to parametricity.
245 /// However, (1.) parametricity on its own turned out to be a
246 /// necessary but insufficient condition, and (2.) future changes to
247 /// the language are expected to make it impossible to ensure that a
248 /// `Drop` implementation is actually parametric with respect to any
249 /// particular type parameter. (In particular, impl specialization is
250 /// expected to break the needed parametricity property beyond
253 /// Therefore we have scaled back Drop-Check to a more conservative
254 /// rule that does not attempt to deduce whether a `Drop`
255 /// implementation could not possible access data of a given lifetime;
256 /// instead Drop-Check now simply assumes that if a destructor has
257 /// access (direct or indirect) to a lifetime parameter, then that
258 /// lifetime must be forced to outlive that destructor's dynamic
259 /// extent. We then provide the `unsafe_destructor_blind_to_params`
260 /// attribute as a way for destructor implementations to opt-out of
261 /// this conservative assumption (and thus assume the obligation of
262 /// ensuring that they do not access data nor invoke methods of
263 /// values that have been previously dropped).
265 pub fn check_safety_of_destructor_if_necessary<'a, 'tcx>(rcx: &mut Rcx<'a, 'tcx>,
268 scope: region::CodeExtent) {
269 debug!("check_safety_of_destructor_if_necessary typ: {:?} scope: {:?}",
272 let parent_scope = rcx.tcx().region_maps.opt_encl_scope(scope).unwrap_or_else(|| {
273 rcx.tcx().sess.span_bug(
274 span, &format!("no enclosing scope found for scope: {:?}", scope))
277 let result = iterate_over_potentially_unsafe_regions_in_type(
281 parent_scope: parent_scope,
282 breadcrumbs: Vec::new(),
289 Err(Error::Overflow(ref ctxt, ref detected_on_typ)) => {
291 span_err!(tcx.sess, span, E0320,
292 "overflow while adding drop-check rules for {}", typ);
294 TypeContext::Root => {
295 // no need for an additional note if the overflow
296 // was somehow on the root.
298 TypeContext::ADT { def_id, variant, field, field_index } => {
299 let adt = tcx.lookup_adt_def(def_id);
300 let variant_name = match adt.adt_kind() {
301 ty::AdtKind::Enum => format!("enum {} variant {}",
302 tcx.item_path_str(def_id),
304 ty::AdtKind::Struct => format!("struct {}",
305 tcx.item_path_str(def_id))
307 let field_name = if field == special_idents::unnamed_field.name {
308 format!("#{}", field_index)
310 format!("`{}`", field)
315 "overflowed on {} field {} type: {}",
326 Overflow(TypeContext, ty::Ty<'tcx>),
329 #[derive(Copy, Clone)]
340 struct DropckContext<'a, 'b: 'a, 'tcx: 'b> {
341 rcx: &'a mut Rcx<'b, 'tcx>,
342 /// types that have already been traversed
343 breadcrumbs: Vec<Ty<'tcx>>,
344 /// span for error reporting
346 /// the scope reachable dtorck types must outlive
347 parent_scope: region::CodeExtent
350 // `context` is used for reporting overflow errors
351 fn iterate_over_potentially_unsafe_regions_in_type<'a, 'b, 'tcx>(
352 cx: &mut DropckContext<'a, 'b, 'tcx>,
353 context: TypeContext,
355 depth: usize) -> Result<(), Error<'tcx>>
357 let tcx = cx.rcx.tcx();
358 let ty = cx.rcx.infcx().resolve_type_and_region_vars_if_possible(&ty);
360 // Issue #22443: Watch out for overflow. While we are careful to
361 // handle regular types properly, non-regular ones cause problems.
362 let recursion_limit = tcx.sess.recursion_limit.get();
363 if depth / 4 >= recursion_limit {
364 // This can get into rather deep recursion, especially in the
365 // presence of things like Vec<T> -> Unique<T> -> PhantomData<T> -> T.
366 // use a higher recursion limit to avoid errors.
367 return Err(Error::Overflow(context, ty))
370 for breadcrumb in &mut cx.breadcrumbs {
372 cx.rcx.infcx().resolve_type_and_region_vars_if_possible(breadcrumb);
373 if *breadcrumb == ty {
374 debug!("iterate_over_potentially_unsafe_regions_in_type \
375 {}ty: {} scope: {:?} - cached",
376 (0..depth).map(|_| ' ').collect::<String>(),
377 ty, cx.parent_scope);
378 return Ok(()); // we already visited this type
381 cx.breadcrumbs.push(ty);
383 debug!("iterate_over_potentially_unsafe_regions_in_type \
384 {}ty: {} scope: {:?}",
385 (0..depth).map(|_| ' ').collect::<String>(),
386 ty, cx.parent_scope);
388 // If `typ` has a destructor, then we must ensure that all
389 // borrowed data reachable via `typ` must outlive the parent
390 // of `scope`. This is handled below.
392 // However, there is an important special case: for any Drop
393 // impl that is tagged as "blind" to their parameters,
394 // we assume that data borrowed via such type parameters
395 // remains unreachable via that Drop impl.
397 // For example, consider:
400 // #[unsafe_destructor_blind_to_params]
401 // impl<T> Drop for Vec<T> { ... }
404 // which does have to be able to drop instances of `T`, but
405 // otherwise cannot read data from `T`.
407 // Of course, for the type expression passed in for any such
408 // unbounded type parameter `T`, we must resume the recursive
409 // analysis on `T` (since it would be ignored by
410 // type_must_outlive).
411 if has_dtor_of_interest(tcx, ty) {
412 debug!("iterate_over_potentially_unsafe_regions_in_type \
413 {}ty: {} - is a dtorck type!",
414 (0..depth).map(|_| ' ').collect::<String>(),
417 regionck::type_must_outlive(cx.rcx,
418 infer::SubregionOrigin::SafeDestructor(cx.span),
420 ty::ReScope(cx.parent_scope));
425 debug!("iterate_over_potentially_unsafe_regions_in_type \
426 {}ty: {} scope: {:?} - checking interior",
427 (0..depth).map(|_| ' ').collect::<String>(),
428 ty, cx.parent_scope);
430 // We still need to ensure all referenced data is safe.
432 ty::TyBool | ty::TyChar | ty::TyInt(_) | ty::TyUint(_) |
433 ty::TyFloat(_) | ty::TyStr => {
434 // primitive - definitely safe
438 ty::TyBox(ity) | ty::TyArray(ity, _) | ty::TySlice(ity) => {
439 // single-element containers, behave like their element
440 iterate_over_potentially_unsafe_regions_in_type(
441 cx, context, ity, depth+1)
444 ty::TyStruct(def, substs) if def.is_phantom_data() => {
445 // PhantomData<T> - behaves identically to T
446 let ity = *substs.types.get(subst::TypeSpace, 0);
447 iterate_over_potentially_unsafe_regions_in_type(
448 cx, context, ity, depth+1)
451 ty::TyStruct(def, substs) | ty::TyEnum(def, substs) => {
453 for variant in &def.variants {
454 for (i, field) in variant.fields.iter().enumerate() {
455 let fty = field.ty(tcx, substs);
456 let fty = cx.rcx.fcx.resolve_type_vars_if_possible(
457 cx.rcx.fcx.normalize_associated_types_in(cx.span, &fty));
458 try!(iterate_over_potentially_unsafe_regions_in_type(
463 variant: variant.name,
473 ty::TyTuple(ref tys) |
474 ty::TyClosure(_, box ty::ClosureSubsts { upvar_tys: ref tys, .. }) => {
476 try!(iterate_over_potentially_unsafe_regions_in_type(
477 cx, context, ty, depth+1))
482 ty::TyRawPtr(..) | ty::TyRef(..) | ty::TyParam(..) => {
483 // these always come with a witness of liveness (references
484 // explicitly, pointers implicitly, parameters by the
489 ty::TyBareFn(..) => {
490 // FIXME(#26656): this type is always destruction-safe, but
491 // it implicitly witnesses Self: Fn, which can be false.
495 ty::TyInfer(..) | ty::TyError => {
496 tcx.sess.delay_span_bug(cx.span, "unresolved type in regionck");
500 // these are always dtorck
501 ty::TyTrait(..) | ty::TyProjection(_) => unreachable!(),
505 fn has_dtor_of_interest<'tcx>(tcx: &ty::ctxt<'tcx>,
506 ty: ty::Ty<'tcx>) -> bool {
508 ty::TyEnum(def, _) | ty::TyStruct(def, _) => {
511 ty::TyTrait(..) | ty::TyProjection(..) => {
512 debug!("ty: {:?} isn't known, and therefore is a dropck type", ty);