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), true);
88 let named_type = tcx.lookup_item_type(self_type_did).ty;
89 let named_type = named_type.subst(tcx, &infcx.parameter_environment.free_substs);
91 let drop_impl_span = tcx.map.def_id_span(drop_impl_did, codemap::DUMMY_SP);
92 let fresh_impl_substs =
93 infcx.fresh_substs_for_generics(drop_impl_span, drop_impl_generics);
94 let fresh_impl_self_ty = drop_impl_ty.subst(tcx, &fresh_impl_substs);
96 if let Err(_) = infer::mk_eqty(&infcx, true, infer::TypeOrigin::Misc(drop_impl_span),
97 named_type, fresh_impl_self_ty) {
98 span_err!(tcx.sess, drop_impl_span, E0366,
99 "Implementations of Drop cannot be specialized");
100 let item_span = tcx.map.span(self_type_node_id);
101 tcx.sess.span_note(item_span,
102 "Use same sequence of generic type and region \
103 parameters that is on the struct/enum definition");
107 if let Err(ref errors) = infcx.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
108 // this could be reached when we get lazy normalization
109 traits::report_fulfillment_errors(&infcx, errors);
113 let free_regions = FreeRegionMap::new();
114 infcx.resolve_regions_and_report_errors(&free_regions, drop_impl_node_id);
118 /// Confirms that every predicate imposed by dtor_predicates is
119 /// implied by assuming the predicates attached to self_type_did.
120 fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
121 tcx: &ty::ctxt<'tcx>,
122 drop_impl_did: DefId,
123 dtor_predicates: &ty::GenericPredicates<'tcx>,
124 self_type_did: DefId,
125 self_to_impl_substs: &subst::Substs<'tcx>) -> Result<(), ()> {
127 // Here is an example, analogous to that from
128 // `compare_impl_method`.
130 // Consider a struct type:
132 // struct Type<'c, 'b:'c, 'a> {
133 // x: &'a Contents // (contents are irrelevant;
134 // y: &'c Cell<&'b Contents>, // only the bounds matter for our purposes.)
139 // impl<'z, 'y:'z, 'x:'y> Drop for P<'z, 'y, 'x> {
140 // fn drop(&mut self) { self.y.set(self.x); } // (only legal if 'x: 'y)
143 // We start out with self_to_impl_substs, that maps the generic
144 // parameters of Type to that of the Drop impl.
146 // self_to_impl_substs = {'c => 'z, 'b => 'y, 'a => 'x}
148 // Applying this to the predicates (i.e. assumptions) provided by the item
149 // definition yields the instantiated assumptions:
153 // We then check all of the predicates of the Drop impl:
157 // and ensure each is in the list of instantiated
158 // assumptions. Here, `'y:'z` is present, but `'x:'y` is
159 // absent. So we report an error that the Drop impl injected a
160 // predicate that is not present on the struct definition.
162 let self_type_node_id = tcx.map.as_local_node_id(self_type_did).unwrap();
164 let drop_impl_span = tcx.map.def_id_span(drop_impl_did, codemap::DUMMY_SP);
166 // We can assume the predicates attached to struct/enum definition
168 let generic_assumptions = tcx.lookup_predicates(self_type_did);
170 let assumptions_in_impl_context = generic_assumptions.instantiate(tcx, &self_to_impl_substs);
171 assert!(assumptions_in_impl_context.predicates.is_empty_in(subst::SelfSpace));
172 assert!(assumptions_in_impl_context.predicates.is_empty_in(subst::FnSpace));
173 let assumptions_in_impl_context =
174 assumptions_in_impl_context.predicates.get_slice(subst::TypeSpace);
176 // An earlier version of this code attempted to do this checking
177 // via the traits::fulfill machinery. However, it ran into trouble
178 // since the fulfill machinery merely turns outlives-predicates
179 // 'a:'b and T:'b into region inference constraints. It is simpler
180 // just to look for all the predicates directly.
182 assert!(dtor_predicates.predicates.is_empty_in(subst::SelfSpace));
183 assert!(dtor_predicates.predicates.is_empty_in(subst::FnSpace));
184 let predicates = dtor_predicates.predicates.get_slice(subst::TypeSpace);
185 for predicate in predicates {
186 // (We do not need to worry about deep analysis of type
187 // expressions etc because the Drop impls are already forced
188 // to take on a structure that is roughly an alpha-renaming of
189 // the generic parameters of the item definition.)
191 // This path now just checks *all* predicates via the direct
192 // lookup, rather than using fulfill machinery.
194 // However, it may be more efficient in the future to batch
195 // the analysis together via the fulfill , rather than the
196 // repeated `contains` calls.
198 if !assumptions_in_impl_context.contains(&predicate) {
199 let item_span = tcx.map.span(self_type_node_id);
200 span_err!(tcx.sess, drop_impl_span, E0367,
201 "The requirement `{}` is added only by the Drop impl.", predicate);
202 tcx.sess.span_note(item_span,
203 "The same requirement must be part of \
204 the struct/enum definition");
208 if tcx.sess.has_errors() {
214 /// check_safety_of_destructor_if_necessary confirms that the type
215 /// expression `typ` conforms to the "Drop Check Rule" from the Sound
216 /// Generic Drop (RFC 769).
220 /// The simplified (*) Drop Check Rule is the following:
222 /// Let `v` be some value (either temporary or named) and 'a be some
223 /// lifetime (scope). If the type of `v` owns data of type `D`, where
225 /// * (1.) `D` has a lifetime- or type-parametric Drop implementation,
226 /// (where that `Drop` implementation does not opt-out of
227 /// this check via the `unsafe_destructor_blind_to_params`
229 /// * (2.) the structure of `D` can reach a reference of type `&'a _`,
231 /// then 'a must strictly outlive the scope of v.
235 /// This function is meant to by applied to the type for every
236 /// expression in the program.
240 /// (*) The qualifier "simplified" is attached to the above
241 /// definition of the Drop Check Rule, because it is a simplification
242 /// of the original Drop Check rule, which attempted to prove that
243 /// some `Drop` implementations could not possibly access data even if
244 /// it was technically reachable, due to parametricity.
246 /// However, (1.) parametricity on its own turned out to be a
247 /// necessary but insufficient condition, and (2.) future changes to
248 /// the language are expected to make it impossible to ensure that a
249 /// `Drop` implementation is actually parametric with respect to any
250 /// particular type parameter. (In particular, impl specialization is
251 /// expected to break the needed parametricity property beyond
254 /// Therefore we have scaled back Drop-Check to a more conservative
255 /// rule that does not attempt to deduce whether a `Drop`
256 /// implementation could not possible access data of a given lifetime;
257 /// instead Drop-Check now simply assumes that if a destructor has
258 /// access (direct or indirect) to a lifetime parameter, then that
259 /// lifetime must be forced to outlive that destructor's dynamic
260 /// extent. We then provide the `unsafe_destructor_blind_to_params`
261 /// attribute as a way for destructor implementations to opt-out of
262 /// this conservative assumption (and thus assume the obligation of
263 /// ensuring that they do not access data nor invoke methods of
264 /// values that have been previously dropped).
266 pub fn check_safety_of_destructor_if_necessary<'a, 'tcx>(rcx: &mut Rcx<'a, 'tcx>,
269 scope: region::CodeExtent) {
270 debug!("check_safety_of_destructor_if_necessary typ: {:?} scope: {:?}",
273 let parent_scope = rcx.tcx().region_maps.opt_encl_scope(scope).unwrap_or_else(|| {
274 rcx.tcx().sess.span_bug(
275 span, &format!("no enclosing scope found for scope: {:?}", scope))
278 let result = iterate_over_potentially_unsafe_regions_in_type(
282 parent_scope: parent_scope,
283 breadcrumbs: FnvHashSet()
290 Err(Error::Overflow(ref ctxt, ref detected_on_typ)) => {
292 span_err!(tcx.sess, span, E0320,
293 "overflow while adding drop-check rules for {}", typ);
295 TypeContext::Root => {
296 // no need for an additional note if the overflow
297 // was somehow on the root.
299 TypeContext::ADT { def_id, variant, field, field_index } => {
300 let adt = tcx.lookup_adt_def(def_id);
301 let variant_name = match adt.adt_kind() {
302 ty::AdtKind::Enum => format!("enum {} variant {}",
303 tcx.item_path_str(def_id),
305 ty::AdtKind::Struct => format!("struct {}",
306 tcx.item_path_str(def_id))
308 let field_name = if field == special_idents::unnamed_field.name {
309 format!("#{}", field_index)
311 format!("`{}`", field)
316 "overflowed on {} field {} type: {}",
327 Overflow(TypeContext, ty::Ty<'tcx>),
330 #[derive(Copy, Clone)]
341 struct DropckContext<'a, 'b: 'a, 'tcx: 'b> {
342 rcx: &'a mut Rcx<'b, 'tcx>,
343 /// types that have already been traversed
344 breadcrumbs: FnvHashSet<Ty<'tcx>>,
345 /// span for error reporting
347 /// the scope reachable dtorck types must outlive
348 parent_scope: region::CodeExtent
351 // `context` is used for reporting overflow errors
352 fn iterate_over_potentially_unsafe_regions_in_type<'a, 'b, 'tcx>(
353 cx: &mut DropckContext<'a, 'b, 'tcx>,
354 context: TypeContext,
356 depth: usize) -> Result<(), Error<'tcx>>
358 let tcx = cx.rcx.tcx();
359 // Issue #22443: Watch out for overflow. While we are careful to
360 // handle regular types properly, non-regular ones cause problems.
361 let recursion_limit = tcx.sess.recursion_limit.get();
362 if depth / 4 >= recursion_limit {
363 // This can get into rather deep recursion, especially in the
364 // presence of things like Vec<T> -> Unique<T> -> PhantomData<T> -> T.
365 // use a higher recursion limit to avoid errors.
366 return Err(Error::Overflow(context, ty))
369 if !cx.breadcrumbs.insert(ty) {
370 debug!("iterate_over_potentially_unsafe_regions_in_type \
371 {}ty: {} scope: {:?} - cached",
372 (0..depth).map(|_| ' ').collect::<String>(),
373 ty, cx.parent_scope);
374 return Ok(()); // we already visited this type
376 debug!("iterate_over_potentially_unsafe_regions_in_type \
377 {}ty: {} scope: {:?}",
378 (0..depth).map(|_| ' ').collect::<String>(),
379 ty, cx.parent_scope);
381 // If `typ` has a destructor, then we must ensure that all
382 // borrowed data reachable via `typ` must outlive the parent
383 // of `scope`. This is handled below.
385 // However, there is an important special case: for any Drop
386 // impl that is tagged as "blind" to their parameters,
387 // we assume that data borrowed via such type parameters
388 // remains unreachable via that Drop impl.
390 // For example, consider:
393 // #[unsafe_destructor_blind_to_params]
394 // impl<T> Drop for Vec<T> { ... }
397 // which does have to be able to drop instances of `T`, but
398 // otherwise cannot read data from `T`.
400 // Of course, for the type expression passed in for any such
401 // unbounded type parameter `T`, we must resume the recursive
402 // analysis on `T` (since it would be ignored by
403 // type_must_outlive).
404 if has_dtor_of_interest(tcx, ty) {
405 debug!("iterate_over_potentially_unsafe_regions_in_type \
406 {}ty: {} - is a dtorck type!",
407 (0..depth).map(|_| ' ').collect::<String>(),
410 regionck::type_must_outlive(cx.rcx,
411 infer::SubregionOrigin::SafeDestructor(cx.span),
413 ty::ReScope(cx.parent_scope));
418 debug!("iterate_over_potentially_unsafe_regions_in_type \
419 {}ty: {} scope: {:?} - checking interior",
420 (0..depth).map(|_| ' ').collect::<String>(),
421 ty, cx.parent_scope);
423 // We still need to ensure all referenced data is safe.
425 ty::TyBool | ty::TyChar | ty::TyInt(_) | ty::TyUint(_) |
426 ty::TyFloat(_) | ty::TyStr => {
427 // primitive - definitely safe
431 ty::TyBox(ity) | ty::TyArray(ity, _) | ty::TySlice(ity) => {
432 // single-element containers, behave like their element
433 iterate_over_potentially_unsafe_regions_in_type(
434 cx, context, ity, depth+1)
437 ty::TyStruct(def, substs) if def.is_phantom_data() => {
438 // PhantomData<T> - behaves identically to T
439 let ity = *substs.types.get(subst::TypeSpace, 0);
440 iterate_over_potentially_unsafe_regions_in_type(
441 cx, context, ity, depth+1)
444 ty::TyStruct(def, substs) | ty::TyEnum(def, substs) => {
446 for variant in &def.variants {
447 for (i, field) in variant.fields.iter().enumerate() {
448 let fty = field.ty(tcx, substs);
449 let fty = cx.rcx.fcx.resolve_type_vars_if_possible(
450 cx.rcx.fcx.normalize_associated_types_in(cx.span, &fty));
451 try!(iterate_over_potentially_unsafe_regions_in_type(
456 variant: variant.name,
466 ty::TyTuple(ref tys) |
467 ty::TyClosure(_, box ty::ClosureSubsts { upvar_tys: ref tys, .. }) => {
469 try!(iterate_over_potentially_unsafe_regions_in_type(
470 cx, context, ty, depth+1))
475 ty::TyRawPtr(..) | ty::TyRef(..) | ty::TyParam(..) => {
476 // these always come with a witness of liveness (references
477 // explicitly, pointers implicitly, parameters by the
482 ty::TyBareFn(..) => {
483 // FIXME(#26656): this type is always destruction-safe, but
484 // it implicitly witnesses Self: Fn, which can be false.
488 ty::TyInfer(..) | ty::TyError => {
489 tcx.sess.delay_span_bug(cx.span, "unresolved type in regionck");
493 // these are always dtorck
494 ty::TyTrait(..) | ty::TyProjection(_) => unreachable!(),
498 fn has_dtor_of_interest<'tcx>(tcx: &ty::ctxt<'tcx>,
499 ty: ty::Ty<'tcx>) -> bool {
501 ty::TyEnum(def, _) | ty::TyStruct(def, _) => {
504 ty::TyTrait(..) | ty::TyProjection(..) => {
505 debug!("ty: {:?} isn't known, and therefore is a dropck type", ty);