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}
;
15 use middle
::subst
::{self, Subst}
;
16 use middle
::ty
::{self, Ty}
;
17 use util
::nodemap
::FnvHashSet
;
20 use syntax
::codemap
::{self, Span}
;
22 /// check_drop_impl confirms that the Drop implementation identfied by
23 /// `drop_impl_did` is not any more specialized than the type it is
24 /// attached to (Issue #8142).
28 /// 1. The self type must be nominal (this is already checked during
31 /// 2. The generic region/type parameters of the impl's self-type must
32 /// all be parameters of the Drop impl itself (i.e. no
33 /// specialization like `impl Drop for Foo<i32>`), and,
35 /// 3. Any bounds on the generic parameters must be reflected in the
36 /// struct/enum definition for the nominal type itself (i.e.
37 /// cannot do `struct S<T>; impl<T:Clone> Drop for S<T> { ... }`).
39 pub fn check_drop_impl(tcx
: &ty
::ctxt
, drop_impl_did
: ast
::DefId
) -> Result
<(), ()> {
40 let ty
::TypeScheme
{ generics
: ref dtor_generics
,
41 ty
: dtor_self_type
} = tcx
.lookup_item_type(drop_impl_did
);
42 let dtor_predicates
= tcx
.lookup_predicates(drop_impl_did
);
43 match dtor_self_type
.sty
{
44 ty
::TyEnum(self_type_did
, self_to_impl_substs
) |
45 ty
::TyStruct(self_type_did
, self_to_impl_substs
) => {
46 try
!(ensure_drop_params_and_item_params_correspond(tcx
,
52 ensure_drop_predicates_are_implied_by_item_defn(tcx
,
59 // Destructors only work on nominal types. This was
60 // already checked by coherence, so we can panic here.
61 let span
= tcx
.map
.def_id_span(drop_impl_did
, codemap
::DUMMY_SP
);
63 span
, &format
!("should have been rejected by coherence check: {}",
69 fn ensure_drop_params_and_item_params_correspond
<'tcx
>(
71 drop_impl_did
: ast
::DefId
,
72 drop_impl_generics
: &ty
::Generics
<'tcx
>,
73 drop_impl_ty
: &ty
::Ty
<'tcx
>,
74 self_type_did
: ast
::DefId
) -> Result
<(), ()>
76 // New strategy based on review suggestion from nikomatsakis.
78 // (In the text and code below, "named" denotes "struct/enum", and
79 // "generic params" denotes "type and region params")
81 // 1. Create fresh skolemized type/region "constants" for each of
82 // the named type's generic params. Instantiate the named type
83 // with the fresh constants, yielding `named_skolem`.
85 // 2. Create unification variables for each of the Drop impl's
86 // generic params. Instantiate the impl's Self's type with the
87 // unification-vars, yielding `drop_unifier`.
89 // 3. Attempt to unify Self_unif with Type_skolem. If unification
90 // succeeds, continue (i.e. with the predicate checks).
92 let ty
::TypeScheme
{ generics
: ref named_type_generics
,
94 tcx
.lookup_item_type(self_type_did
);
96 let infcx
= infer
::new_infer_ctxt(tcx
, &tcx
.tables
, None
, false);
98 infcx
.commit_if_ok(|snapshot
| {
99 let (named_type_to_skolem
, skol_map
) =
100 infcx
.construct_skolemized_subst(named_type_generics
, snapshot
);
101 let named_type_skolem
= named_type
.subst(tcx
, &named_type_to_skolem
);
103 let drop_impl_span
= tcx
.map
.def_id_span(drop_impl_did
, codemap
::DUMMY_SP
);
104 let drop_to_unifier
=
105 infcx
.fresh_substs_for_generics(drop_impl_span
, drop_impl_generics
);
106 let drop_unifier
= drop_impl_ty
.subst(tcx
, &drop_to_unifier
);
108 if let Ok(()) = infer
::mk_eqty(&infcx
, true, infer
::TypeOrigin
::Misc(drop_impl_span
),
109 named_type_skolem
, drop_unifier
) {
110 // Even if we did manage to equate the types, the process
111 // may have just gathered unsolvable region constraints
112 // like `R == 'static` (represented as a pair of subregion
113 // constraints) for some skolemization constant R.
115 // However, the leak_check method allows us to confirm
116 // that no skolemized regions escaped (i.e. were related
117 // to other regions in the constraint graph).
118 if let Ok(()) = infcx
.leak_check(&skol_map
, snapshot
) {
123 span_err
!(tcx
.sess
, drop_impl_span
, E0366
,
124 "Implementations of Drop cannot be specialized");
125 let item_span
= tcx
.map
.span(self_type_did
.node
);
126 tcx
.sess
.span_note(item_span
,
127 "Use same sequence of generic type and region \
128 parameters that is on the struct/enum definition");
133 /// Confirms that every predicate imposed by dtor_predicates is
134 /// implied by assuming the predicates attached to self_type_did.
135 fn ensure_drop_predicates_are_implied_by_item_defn
<'tcx
>(
136 tcx
: &ty
::ctxt
<'tcx
>,
137 drop_impl_did
: ast
::DefId
,
138 dtor_predicates
: &ty
::GenericPredicates
<'tcx
>,
139 self_type_did
: ast
::DefId
,
140 self_to_impl_substs
: &subst
::Substs
<'tcx
>) -> Result
<(), ()> {
142 // Here is an example, analogous to that from
143 // `compare_impl_method`.
145 // Consider a struct type:
147 // struct Type<'c, 'b:'c, 'a> {
148 // x: &'a Contents // (contents are irrelevant;
149 // y: &'c Cell<&'b Contents>, // only the bounds matter for our purposes.)
154 // impl<'z, 'y:'z, 'x:'y> Drop for P<'z, 'y, 'x> {
155 // fn drop(&mut self) { self.y.set(self.x); } // (only legal if 'x: 'y)
158 // We start out with self_to_impl_substs, that maps the generic
159 // parameters of Type to that of the Drop impl.
161 // self_to_impl_substs = {'c => 'z, 'b => 'y, 'a => 'x}
163 // Applying this to the predicates (i.e. assumptions) provided by the item
164 // definition yields the instantiated assumptions:
168 // We then check all of the predicates of the Drop impl:
172 // and ensure each is in the list of instantiated
173 // assumptions. Here, `'y:'z` is present, but `'x:'y` is
174 // absent. So we report an error that the Drop impl injected a
175 // predicate that is not present on the struct definition.
177 assert_eq
!(self_type_did
.krate
, ast
::LOCAL_CRATE
);
179 let drop_impl_span
= tcx
.map
.def_id_span(drop_impl_did
, codemap
::DUMMY_SP
);
181 // We can assume the predicates attached to struct/enum definition
183 let generic_assumptions
= tcx
.lookup_predicates(self_type_did
);
185 let assumptions_in_impl_context
= generic_assumptions
.instantiate(tcx
, &self_to_impl_substs
);
186 assert
!(assumptions_in_impl_context
.predicates
.is_empty_in(subst
::SelfSpace
));
187 assert
!(assumptions_in_impl_context
.predicates
.is_empty_in(subst
::FnSpace
));
188 let assumptions_in_impl_context
=
189 assumptions_in_impl_context
.predicates
.get_slice(subst
::TypeSpace
);
191 // An earlier version of this code attempted to do this checking
192 // via the traits::fulfill machinery. However, it ran into trouble
193 // since the fulfill machinery merely turns outlives-predicates
194 // 'a:'b and T:'b into region inference constraints. It is simpler
195 // just to look for all the predicates directly.
197 assert
!(dtor_predicates
.predicates
.is_empty_in(subst
::SelfSpace
));
198 assert
!(dtor_predicates
.predicates
.is_empty_in(subst
::FnSpace
));
199 let predicates
= dtor_predicates
.predicates
.get_slice(subst
::TypeSpace
);
200 for predicate
in predicates
{
201 // (We do not need to worry about deep analysis of type
202 // expressions etc because the Drop impls are already forced
203 // to take on a structure that is roughly a alpha-renaming of
204 // the generic parameters of the item definition.)
206 // This path now just checks *all* predicates via the direct
207 // lookup, rather than using fulfill machinery.
209 // However, it may be more efficient in the future to batch
210 // the analysis together via the fulfill , rather than the
211 // repeated `contains` calls.
213 if !assumptions_in_impl_context
.contains(&predicate
) {
214 let item_span
= tcx
.map
.span(self_type_did
.node
);
215 span_err
!(tcx
.sess
, drop_impl_span
, E0367
,
216 "The requirement `{}` is added only by the Drop impl.", predicate
);
217 tcx
.sess
.span_note(item_span
,
218 "The same requirement must be part of \
219 the struct/enum definition");
223 if tcx
.sess
.has_errors() {
229 /// check_safety_of_destructor_if_necessary confirms that the type
230 /// expression `typ` conforms to the "Drop Check Rule" from the Sound
231 /// Generic Drop (RFC 769).
235 /// The Drop Check Rule is the following:
237 /// Let `v` be some value (either temporary or named) and 'a be some
238 /// lifetime (scope). If the type of `v` owns data of type `D`, where
240 /// * (1.) `D` has a lifetime- or type-parametric Drop implementation, and
241 /// * (2.) the structure of `D` can reach a reference of type `&'a _`, and
243 /// * (A.) the Drop impl for `D` instantiates `D` at 'a directly,
244 /// i.e. `D<'a>`, or,
245 /// * (B.) the Drop impl for `D` has some type parameter with a
246 /// trait bound `T` where `T` is a trait that has at least
249 /// then 'a must strictly outlive the scope of v.
253 /// This function is meant to by applied to the type for every
254 /// expression in the program.
255 pub fn check_safety_of_destructor_if_necessary
<'a
, 'tcx
>(rcx
: &mut Rcx
<'a
, 'tcx
>,
258 scope
: region
::CodeExtent
) {
259 debug
!("check_safety_of_destructor_if_necessary typ: {:?} scope: {:?}",
262 let parent_scope
= rcx
.tcx().region_maps
.opt_encl_scope(scope
).unwrap_or_else(|| {
263 rcx
.tcx().sess
.span_bug(
264 span
, &format
!("no enclosing scope found for scope: {:?}", scope
))
267 let result
= iterate_over_potentially_unsafe_regions_in_type(
271 parent_scope
: parent_scope
,
272 breadcrumbs
: FnvHashSet()
279 Err(Error
::Overflow(ref ctxt
, ref detected_on_typ
)) => {
281 span_err
!(tcx
.sess
, span
, E0320
,
282 "overflow while adding drop-check rules for {}", typ
);
284 TypeContext
::Root
=> {
285 // no need for an additional note if the overflow
286 // was somehow on the root.
288 TypeContext
::EnumVariant { def_id, variant, arg_index }
=> {
289 // FIXME (pnkfelix): eventually lookup arg_name
290 // for the given index on struct variants.
294 "overflowed on enum {} variant {} argument {} type: {}",
295 tcx
.item_path_str(def_id
),
300 TypeContext
::Struct { def_id, field }
=> {
304 "overflowed on struct {} field {} type: {}",
305 tcx
.item_path_str(def_id
),
315 Overflow(TypeContext
, ty
::Ty
<'tcx
>),
318 #[derive(Copy, Clone)]
332 struct DropckContext
<'a
, 'b
: 'a
, 'tcx
: 'b
> {
333 rcx
: &'a
mut Rcx
<'b
, 'tcx
>,
334 /// types that have already been traversed
335 breadcrumbs
: FnvHashSet
<Ty
<'tcx
>>,
336 /// span for error reporting
338 /// the scope reachable dtorck types must outlive
339 parent_scope
: region
::CodeExtent
342 // `context` is used for reporting overflow errors
343 fn iterate_over_potentially_unsafe_regions_in_type
<'a
, 'b
, 'tcx
>(
344 cx
: &mut DropckContext
<'a
, 'b
, 'tcx
>,
345 context
: TypeContext
,
347 depth
: usize) -> Result
<(), Error
<'tcx
>>
349 let tcx
= cx
.rcx
.tcx();
350 // Issue #22443: Watch out for overflow. While we are careful to
351 // handle regular types properly, non-regular ones cause problems.
352 let recursion_limit
= tcx
.sess
.recursion_limit
.get();
353 if depth
/ 4 >= recursion_limit
{
354 // This can get into rather deep recursion, especially in the
355 // presence of things like Vec<T> -> Unique<T> -> PhantomData<T> -> T.
356 // use a higher recursion limit to avoid errors.
357 return Err(Error
::Overflow(context
, ty
))
360 let opt_phantom_data_def_id
= tcx
.lang_items
.phantom_data();
362 if !cx
.breadcrumbs
.insert(ty
) {
363 debug
!("iterate_over_potentially_unsafe_regions_in_type \
364 {}ty: {} scope: {:?} - cached",
365 (0..depth
).map(|_
| ' '
).collect
::<String
>(),
366 ty
, cx
.parent_scope
);
367 return Ok(()); // we already visited this type
369 debug
!("iterate_over_potentially_unsafe_regions_in_type \
370 {}ty: {} scope: {:?}",
371 (0..depth
).map(|_
| ' '
).collect
::<String
>(),
372 ty
, cx
.parent_scope
);
374 // If `typ` has a destructor, then we must ensure that all
375 // borrowed data reachable via `typ` must outlive the parent
376 // of `scope`. This is handled below.
378 // However, there is an important special case: by
379 // parametricity, any generic type parameters have *no* trait
380 // bounds in the Drop impl can not be used in any way (apart
381 // from being dropped), and thus we can treat data borrowed
382 // via such type parameters remains unreachable.
384 // For example, consider `impl<T> Drop for Vec<T> { ... }`,
385 // which does have to be able to drop instances of `T`, but
386 // otherwise cannot read data from `T`.
388 // Of course, for the type expression passed in for any such
389 // unbounded type parameter `T`, we must resume the recursive
390 // analysis on `T` (since it would be ignored by
391 // type_must_outlive).
393 // FIXME (pnkfelix): Long term, we could be smart and actually
394 // feed which generic parameters can be ignored *into* `fn
395 // type_must_outlive` (or some generalization thereof). But
396 // for the short term, it probably covers most cases of
397 // interest to just special case Drop impls where: (1.) there
398 // are no generic lifetime parameters and (2.) *all* generic
399 // type parameters are unbounded. If both conditions hold, we
400 // simply skip the `type_must_outlive` call entirely (but
401 // resume the recursive checking of the type-substructure).
402 if has_dtor_of_interest(tcx
, ty
, cx
.span
) {
403 debug
!("iterate_over_potentially_unsafe_regions_in_type \
404 {}ty: {} - is a dtorck type!",
405 (0..depth
).map(|_
| ' '
).collect
::<String
>(),
408 regionck
::type_must_outlive(cx
.rcx
,
409 infer
::SubregionOrigin
::SafeDestructor(cx
.span
),
411 ty
::ReScope(cx
.parent_scope
));
416 debug
!("iterate_over_potentially_unsafe_regions_in_type \
417 {}ty: {} scope: {:?} - checking interior",
418 (0..depth
).map(|_
| ' '
).collect
::<String
>(),
419 ty
, cx
.parent_scope
);
421 // We still need to ensure all referenced data is safe.
423 ty
::TyBool
| ty
::TyChar
| ty
::TyInt(_
) | ty
::TyUint(_
) |
424 ty
::TyFloat(_
) | ty
::TyStr
=> {
425 // primitive - definitely safe
429 ty
::TyBox(ity
) | ty
::TyArray(ity
, _
) | ty
::TySlice(ity
) => {
430 // single-element containers, behave like their element
431 iterate_over_potentially_unsafe_regions_in_type(
432 cx
, context
, ity
, depth
+1)
435 ty
::TyStruct(did
, substs
) if Some(did
) == opt_phantom_data_def_id
=> {
436 // PhantomData<T> - behaves identically to T
437 let ity
= *substs
.types
.get(subst
::TypeSpace
, 0);
438 iterate_over_potentially_unsafe_regions_in_type(
439 cx
, context
, ity
, depth
+1)
442 ty
::TyStruct(did
, substs
) => {
443 let fields
= tcx
.lookup_struct_fields(did
);
444 for field
in &fields
{
445 let fty
= tcx
.lookup_field_type(did
, field
.id
, substs
);
446 let fty
= cx
.rcx
.fcx
.resolve_type_vars_if_possible(
447 cx
.rcx
.fcx
.normalize_associated_types_in(cx
.span
, &fty
));
448 try
!(iterate_over_potentially_unsafe_regions_in_type(
450 TypeContext
::Struct
{
460 ty
::TyEnum(did
, substs
) => {
461 let all_variant_info
= tcx
.substd_enum_variants(did
, substs
);
462 for variant_info
in &all_variant_info
{
463 for (i
, fty
) in variant_info
.args
.iter().enumerate() {
464 let fty
= cx
.rcx
.fcx
.resolve_type_vars_if_possible(
465 cx
.rcx
.fcx
.normalize_associated_types_in(cx
.span
, &fty
));
466 try
!(iterate_over_potentially_unsafe_regions_in_type(
468 TypeContext
::EnumVariant
{
470 variant
: variant_info
.name
,
480 ty
::TyTuple(ref tys
) |
481 ty
::TyClosure(_
, box ty
::ClosureSubsts { upvar_tys: ref tys, .. }
) => {
483 try
!(iterate_over_potentially_unsafe_regions_in_type(
484 cx
, context
, ty
, depth
+1))
489 ty
::TyRawPtr(..) | ty
::TyRef(..) | ty
::TyParam(..) => {
490 // these always come with a witness of liveness (references
491 // explicitly, pointers implicitly, parameters by the
496 ty
::TyBareFn(..) => {
497 // FIXME(#26656): this type is always destruction-safe, but
498 // it implicitly witnesses Self: Fn, which can be false.
502 ty
::TyInfer(..) | ty
::TyError
=> {
503 tcx
.sess
.delay_span_bug(cx
.span
, "unresolved type in regionck");
507 // these are always dtorck
508 ty
::TyTrait(..) | ty
::TyProjection(_
) => unreachable
!(),
512 fn has_dtor_of_interest
<'tcx
>(tcx
: &ty
::ctxt
<'tcx
>,
514 span
: Span
) -> bool
{
516 ty
::TyEnum(def_id
, _
) | ty
::TyStruct(def_id
, _
) => {
517 let dtor_method_did
= match tcx
.destructor_for_type
.borrow().get(&def_id
) {
518 Some(def_id
) => *def_id
,
520 debug
!("ty: {:?} has no dtor, and thus isn't a dropck type", ty
);
524 let impl_did
= tcx
.impl_of_method(dtor_method_did
)
527 span
, "no Drop impl found for drop method")
530 let dtor_typescheme
= tcx
.lookup_item_type(impl_did
);
531 let dtor_generics
= dtor_typescheme
.generics
;
533 let mut has_pred_of_interest
= false;
535 let mut seen_items
= Vec
::new();
536 let mut items_to_inspect
= vec
![impl_did
];
537 'items
: while let Some(item_def_id
) = items_to_inspect
.pop() {
538 if seen_items
.contains(&item_def_id
) {
542 for pred
in tcx
.lookup_predicates(item_def_id
).predicates
{
543 let result
= match pred
{
544 ty
::Predicate
::Equate(..) |
545 ty
::Predicate
::RegionOutlives(..) |
546 ty
::Predicate
::TypeOutlives(..) |
547 ty
::Predicate
::Projection(..) => {
548 // For now, assume all these where-clauses
549 // may give drop implementation capabilty
550 // to access borrowed data.
554 ty
::Predicate
::Trait(ty
::Binder(ref t_pred
)) => {
555 let def_id
= t_pred
.trait_ref
.def_id
;
556 if tcx
.trait_items(def_id
).len() != 0 {
557 // If trait has items, assume it adds
558 // capability to access borrowed data.
561 // Trait without items is itself
562 // uninteresting from POV of dropck.
564 // However, may have parent w/ items;
565 // so schedule checking of predicates,
566 items_to_inspect
.push(def_id
);
567 // and say "no capability found" for now.
574 has_pred_of_interest
= true;
575 debug
!("ty: {:?} has interesting dtor due to generic preds, e.g. {:?}",
581 seen_items
.push(item_def_id
);
584 // In `impl<'a> Drop ...`, we automatically assume
585 // `'a` is meaningful and thus represents a bound
586 // through which we could reach borrowed data.
588 // FIXME (pnkfelix): In the future it would be good to
589 // extend the language to allow the user to express,
590 // in the impl signature, that a lifetime is not
591 // actually used (something like `where 'a: ?Live`).
592 let has_region_param_of_interest
=
593 dtor_generics
.has_region_params(subst
::TypeSpace
);
595 let has_dtor_of_interest
=
596 has_region_param_of_interest
||
597 has_pred_of_interest
;
599 if has_dtor_of_interest
{
600 debug
!("ty: {:?} has interesting dtor, due to \
601 region params: {} or pred: {}",
603 has_region_param_of_interest
,
604 has_pred_of_interest
);
606 debug
!("ty: {:?} has dtor, but it is uninteresting", ty
);
610 ty
::TyTrait(..) | ty
::TyProjection(..) => {
611 debug
!("ty: {:?} isn't known, and therefore is a dropck type", ty
);