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};
20 use util::nodemap::FnvHashSet;
23 use syntax::codemap::{self, Span};
24 use syntax::parse::token::special_idents;
26 /// check_drop_impl confirms that the Drop implementation identfied by
27 /// `drop_impl_did` is not any more specialized than the type it is
28 /// attached to (Issue #8142).
32 /// 1. The self type must be nominal (this is already checked during
35 /// 2. The generic region/type parameters of the impl's self-type must
36 /// all be parameters of the Drop impl itself (i.e. no
37 /// specialization like `impl Drop for Foo<i32>`), and,
39 /// 3. Any bounds on the generic parameters must be reflected in the
40 /// struct/enum definition for the nominal type itself (i.e.
41 /// cannot do `struct S<T>; impl<T:Clone> Drop for S<T> { ... }`).
43 pub fn check_drop_impl(tcx: &ty::ctxt, drop_impl_did: DefId) -> Result<(), ()> {
44 let ty::TypeScheme { generics: ref dtor_generics,
45 ty: dtor_self_type } = tcx.lookup_item_type(drop_impl_did);
46 let dtor_predicates = tcx.lookup_predicates(drop_impl_did);
47 match dtor_self_type.sty {
48 ty::TyEnum(adt_def, self_to_impl_substs) |
49 ty::TyStruct(adt_def, self_to_impl_substs) => {
50 try!(ensure_drop_params_and_item_params_correspond(tcx,
56 ensure_drop_predicates_are_implied_by_item_defn(tcx,
63 // Destructors only work on nominal types. This was
64 // already checked by coherence, so we can panic here.
65 let span = tcx.map.def_id_span(drop_impl_did, codemap::DUMMY_SP);
67 span, &format!("should have been rejected by coherence check: {}",
73 fn ensure_drop_params_and_item_params_correspond<'tcx>(
76 drop_impl_generics: &ty::Generics<'tcx>,
77 drop_impl_ty: &ty::Ty<'tcx>,
78 self_type_did: DefId) -> Result<(), ()>
80 let drop_impl_node_id = tcx.map.as_local_node_id(drop_impl_did).unwrap();
81 let self_type_node_id = tcx.map.as_local_node_id(self_type_did).unwrap();
83 // check that the impl type can be made to match the trait type.
85 let impl_param_env = ty::ParameterEnvironment::for_item(tcx, self_type_node_id);
86 let infcx = infer::new_infer_ctxt(tcx, &tcx.tables, Some(impl_param_env));
87 let mut fulfillment_cx = traits::FulfillmentContext::new();
89 let named_type = tcx.lookup_item_type(self_type_did).ty;
90 let named_type = named_type.subst(tcx, &infcx.parameter_environment.free_substs);
92 let drop_impl_span = tcx.map.def_id_span(drop_impl_did, codemap::DUMMY_SP);
93 let fresh_impl_substs =
94 infcx.fresh_substs_for_generics(drop_impl_span, drop_impl_generics);
95 let fresh_impl_self_ty = drop_impl_ty.subst(tcx, &fresh_impl_substs);
97 if let Err(_) = infer::mk_eqty(&infcx, true, infer::TypeOrigin::Misc(drop_impl_span),
98 named_type, fresh_impl_self_ty) {
99 let item_span = tcx.map.span(self_type_node_id);
100 struct_span_err!(tcx.sess, drop_impl_span, E0366,
101 "Implementations of Drop cannot be specialized")
102 .span_note(item_span,
103 "Use same sequence of generic type and region \
104 parameters that is on the struct/enum definition")
109 if let Err(ref errors) = fulfillment_cx.select_all_or_error(&infcx) {
110 // this could be reached when we get lazy normalization
111 traits::report_fulfillment_errors(&infcx, errors);
115 let free_regions = FreeRegionMap::new();
116 infcx.resolve_regions_and_report_errors(&free_regions, drop_impl_node_id);
120 /// Confirms that every predicate imposed by dtor_predicates is
121 /// implied by assuming the predicates attached to self_type_did.
122 fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
123 tcx: &ty::ctxt<'tcx>,
124 drop_impl_did: DefId,
125 dtor_predicates: &ty::GenericPredicates<'tcx>,
126 self_type_did: DefId,
127 self_to_impl_substs: &subst::Substs<'tcx>) -> Result<(), ()> {
129 // Here is an example, analogous to that from
130 // `compare_impl_method`.
132 // Consider a struct type:
134 // struct Type<'c, 'b:'c, 'a> {
135 // x: &'a Contents // (contents are irrelevant;
136 // y: &'c Cell<&'b Contents>, // only the bounds matter for our purposes.)
141 // impl<'z, 'y:'z, 'x:'y> Drop for P<'z, 'y, 'x> {
142 // fn drop(&mut self) { self.y.set(self.x); } // (only legal if 'x: 'y)
145 // We start out with self_to_impl_substs, that maps the generic
146 // parameters of Type to that of the Drop impl.
148 // self_to_impl_substs = {'c => 'z, 'b => 'y, 'a => 'x}
150 // Applying this to the predicates (i.e. assumptions) provided by the item
151 // definition yields the instantiated assumptions:
155 // We then check all of the predicates of the Drop impl:
159 // and ensure each is in the list of instantiated
160 // assumptions. Here, `'y:'z` is present, but `'x:'y` is
161 // absent. So we report an error that the Drop impl injected a
162 // predicate that is not present on the struct definition.
164 let self_type_node_id = tcx.map.as_local_node_id(self_type_did).unwrap();
166 let drop_impl_span = tcx.map.def_id_span(drop_impl_did, codemap::DUMMY_SP);
168 // We can assume the predicates attached to struct/enum definition
170 let generic_assumptions = tcx.lookup_predicates(self_type_did);
172 let assumptions_in_impl_context = generic_assumptions.instantiate(tcx, &self_to_impl_substs);
173 assert!(assumptions_in_impl_context.predicates.is_empty_in(subst::SelfSpace));
174 assert!(assumptions_in_impl_context.predicates.is_empty_in(subst::FnSpace));
175 let assumptions_in_impl_context =
176 assumptions_in_impl_context.predicates.get_slice(subst::TypeSpace);
178 // An earlier version of this code attempted to do this checking
179 // via the traits::fulfill machinery. However, it ran into trouble
180 // since the fulfill machinery merely turns outlives-predicates
181 // 'a:'b and T:'b into region inference constraints. It is simpler
182 // just to look for all the predicates directly.
184 assert!(dtor_predicates.predicates.is_empty_in(subst::SelfSpace));
185 assert!(dtor_predicates.predicates.is_empty_in(subst::FnSpace));
186 let predicates = dtor_predicates.predicates.get_slice(subst::TypeSpace);
187 for predicate in predicates {
188 // (We do not need to worry about deep analysis of type
189 // expressions etc because the Drop impls are already forced
190 // to take on a structure that is roughly an alpha-renaming of
191 // the generic parameters of the item definition.)
193 // This path now just checks *all* predicates via the direct
194 // lookup, rather than using fulfill machinery.
196 // However, it may be more efficient in the future to batch
197 // the analysis together via the fulfill , rather than the
198 // repeated `contains` calls.
200 if !assumptions_in_impl_context.contains(&predicate) {
201 let item_span = tcx.map.span(self_type_node_id);
202 struct_span_err!(tcx.sess, drop_impl_span, E0367,
203 "The requirement `{}` is added only by the Drop impl.", predicate)
204 .span_note(item_span,
205 "The same requirement must be part of \
206 the struct/enum definition")
211 if tcx.sess.has_errors() {
217 /// check_safety_of_destructor_if_necessary confirms that the type
218 /// expression `typ` conforms to the "Drop Check Rule" from the Sound
219 /// Generic Drop (RFC 769).
223 /// The simplified (*) Drop Check Rule is the following:
225 /// Let `v` be some value (either temporary or named) and 'a be some
226 /// lifetime (scope). If the type of `v` owns data of type `D`, where
228 /// * (1.) `D` has a lifetime- or type-parametric Drop implementation,
229 /// (where that `Drop` implementation does not opt-out of
230 /// this check via the `unsafe_destructor_blind_to_params`
232 /// * (2.) the structure of `D` can reach a reference of type `&'a _`,
234 /// then 'a must strictly outlive the scope of v.
238 /// This function is meant to by applied to the type for every
239 /// expression in the program.
243 /// (*) The qualifier "simplified" is attached to the above
244 /// definition of the Drop Check Rule, because it is a simplification
245 /// of the original Drop Check rule, which attempted to prove that
246 /// some `Drop` implementations could not possibly access data even if
247 /// it was technically reachable, due to parametricity.
249 /// However, (1.) parametricity on its own turned out to be a
250 /// necessary but insufficient condition, and (2.) future changes to
251 /// the language are expected to make it impossible to ensure that a
252 /// `Drop` implementation is actually parametric with respect to any
253 /// particular type parameter. (In particular, impl specialization is
254 /// expected to break the needed parametricity property beyond
257 /// Therefore we have scaled back Drop-Check to a more conservative
258 /// rule that does not attempt to deduce whether a `Drop`
259 /// implementation could not possible access data of a given lifetime;
260 /// instead Drop-Check now simply assumes that if a destructor has
261 /// access (direct or indirect) to a lifetime parameter, then that
262 /// lifetime must be forced to outlive that destructor's dynamic
263 /// extent. We then provide the `unsafe_destructor_blind_to_params`
264 /// attribute as a way for destructor implementations to opt-out of
265 /// this conservative assumption (and thus assume the obligation of
266 /// ensuring that they do not access data nor invoke methods of
267 /// values that have been previously dropped).
269 pub fn check_safety_of_destructor_if_necessary<'a, 'tcx>(rcx: &mut Rcx<'a, 'tcx>,
272 scope: region::CodeExtent) {
273 debug!("check_safety_of_destructor_if_necessary typ: {:?} scope: {:?}",
276 let parent_scope = rcx.tcx().region_maps.opt_encl_scope(scope).unwrap_or_else(|| {
277 rcx.tcx().sess.span_bug(
278 span, &format!("no enclosing scope found for scope: {:?}", scope))
281 let result = iterate_over_potentially_unsafe_regions_in_type(
285 parent_scope: parent_scope,
286 breadcrumbs: FnvHashSet()
293 Err(Error::Overflow(ref ctxt, ref detected_on_typ)) => {
295 let mut err = struct_span_err!(tcx.sess, span, E0320,
296 "overflow while adding drop-check rules for {}", typ);
298 TypeContext::Root => {
299 // no need for an additional note if the overflow
300 // was somehow on the root.
302 TypeContext::ADT { def_id, variant, field, field_index } => {
303 let adt = tcx.lookup_adt_def(def_id);
304 let variant_name = match adt.adt_kind() {
305 ty::AdtKind::Enum => format!("enum {} variant {}",
306 tcx.item_path_str(def_id),
308 ty::AdtKind::Struct => format!("struct {}",
309 tcx.item_path_str(def_id))
311 let field_name = if field == special_idents::unnamed_field.name {
312 format!("#{}", field_index)
314 format!("`{}`", field)
319 "overflowed on {} field {} type: {}",
331 Overflow(TypeContext, ty::Ty<'tcx>),
334 #[derive(Copy, Clone)]
345 struct DropckContext<'a, 'b: 'a, 'tcx: 'b> {
346 rcx: &'a mut Rcx<'b, 'tcx>,
347 /// types that have already been traversed
348 breadcrumbs: FnvHashSet<Ty<'tcx>>,
349 /// span for error reporting
351 /// the scope reachable dtorck types must outlive
352 parent_scope: region::CodeExtent
355 // `context` is used for reporting overflow errors
356 fn iterate_over_potentially_unsafe_regions_in_type<'a, 'b, 'tcx>(
357 cx: &mut DropckContext<'a, 'b, 'tcx>,
358 context: TypeContext,
360 depth: usize) -> Result<(), Error<'tcx>>
362 let tcx = cx.rcx.tcx();
363 // Issue #22443: Watch out for overflow. While we are careful to
364 // handle regular types properly, non-regular ones cause problems.
365 let recursion_limit = tcx.sess.recursion_limit.get();
366 if depth / 4 >= recursion_limit {
367 // This can get into rather deep recursion, especially in the
368 // presence of things like Vec<T> -> Unique<T> -> PhantomData<T> -> T.
369 // use a higher recursion limit to avoid errors.
370 return Err(Error::Overflow(context, ty))
373 // canoncialize the regions in `ty` before inserting - infinitely many
374 // region variables can refer to the same region.
375 let ty = cx.rcx.infcx().resolve_type_and_region_vars_if_possible(&ty);
377 if !cx.breadcrumbs.insert(ty) {
378 debug!("iterate_over_potentially_unsafe_regions_in_type \
379 {}ty: {} scope: {:?} - cached",
380 (0..depth).map(|_| ' ').collect::<String>(),
381 ty, cx.parent_scope);
382 return Ok(()); // we already visited this type
384 debug!("iterate_over_potentially_unsafe_regions_in_type \
385 {}ty: {} scope: {:?}",
386 (0..depth).map(|_| ' ').collect::<String>(),
387 ty, cx.parent_scope);
389 // If `typ` has a destructor, then we must ensure that all
390 // borrowed data reachable via `typ` must outlive the parent
391 // of `scope`. This is handled below.
393 // However, there is an important special case: for any Drop
394 // impl that is tagged as "blind" to their parameters,
395 // we assume that data borrowed via such type parameters
396 // remains unreachable via that Drop impl.
398 // For example, consider:
401 // #[unsafe_destructor_blind_to_params]
402 // impl<T> Drop for Vec<T> { ... }
405 // which does have to be able to drop instances of `T`, but
406 // otherwise cannot read data from `T`.
408 // Of course, for the type expression passed in for any such
409 // unbounded type parameter `T`, we must resume the recursive
410 // analysis on `T` (since it would be ignored by
411 // type_must_outlive).
412 if has_dtor_of_interest(tcx, ty) {
413 debug!("iterate_over_potentially_unsafe_regions_in_type \
414 {}ty: {} - is a dtorck type!",
415 (0..depth).map(|_| ' ').collect::<String>(),
418 regionck::type_must_outlive(cx.rcx,
419 infer::SubregionOrigin::SafeDestructor(cx.span),
421 ty::ReScope(cx.parent_scope));
426 debug!("iterate_over_potentially_unsafe_regions_in_type \
427 {}ty: {} scope: {:?} - checking interior",
428 (0..depth).map(|_| ' ').collect::<String>(),
429 ty, cx.parent_scope);
431 // We still need to ensure all referenced data is safe.
433 ty::TyBool | ty::TyChar | ty::TyInt(_) | ty::TyUint(_) |
434 ty::TyFloat(_) | ty::TyStr => {
435 // primitive - definitely safe
439 ty::TyBox(ity) | ty::TyArray(ity, _) | ty::TySlice(ity) => {
440 // single-element containers, behave like their element
441 iterate_over_potentially_unsafe_regions_in_type(
442 cx, context, ity, depth+1)
445 ty::TyStruct(def, substs) if def.is_phantom_data() => {
446 // PhantomData<T> - behaves identically to T
447 let ity = *substs.types.get(subst::TypeSpace, 0);
448 iterate_over_potentially_unsafe_regions_in_type(
449 cx, context, ity, depth+1)
452 ty::TyStruct(def, substs) | ty::TyEnum(def, substs) => {
454 for variant in &def.variants {
455 for (i, field) in variant.fields.iter().enumerate() {
456 let fty = field.ty(tcx, substs);
457 let fty = cx.rcx.fcx.resolve_type_vars_if_possible(
458 cx.rcx.fcx.normalize_associated_types_in(cx.span, &fty));
459 try!(iterate_over_potentially_unsafe_regions_in_type(
464 variant: variant.name,
474 ty::TyTuple(ref tys) |
475 ty::TyClosure(_, box ty::ClosureSubsts { upvar_tys: ref tys, .. }) => {
477 try!(iterate_over_potentially_unsafe_regions_in_type(
478 cx, context, ty, depth+1))
483 ty::TyRawPtr(..) | ty::TyRef(..) | ty::TyParam(..) => {
484 // these always come with a witness of liveness (references
485 // explicitly, pointers implicitly, parameters by the
490 ty::TyBareFn(..) => {
491 // FIXME(#26656): this type is always destruction-safe, but
492 // it implicitly witnesses Self: Fn, which can be false.
496 ty::TyInfer(..) | ty::TyError => {
497 tcx.sess.delay_span_bug(cx.span, "unresolved type in regionck");
501 // these are always dtorck
502 ty::TyTrait(..) | ty::TyProjection(_) => unreachable!(),
506 fn has_dtor_of_interest<'tcx>(tcx: &ty::ctxt<'tcx>,
507 ty: ty::Ty<'tcx>) -> bool {
509 ty::TyEnum(def, _) | ty::TyStruct(def, _) => {
512 ty::TyTrait(..) | ty::TyProjection(..) => {
513 debug!("ty: {:?} isn't known, and therefore is a dropck type", ty);