1 //! The region check is a final pass that runs over the AST after we have
2 //! inferred the type constraints but before we have actually finalized
3 //! the types. Its purpose is to embed a variety of region constraints.
4 //! Inserting these constraints as a separate pass is good because (1) it
5 //! localizes the code that has to do with region inference and (2) often
6 //! we cannot know what constraints are needed until the basic types have
9 //! ### Interaction with the borrow checker
11 //! In general, the job of the borrowck module (which runs later) is to
12 //! check that all soundness criteria are met, given a particular set of
13 //! regions. The job of *this* module is to anticipate the needs of the
14 //! borrow checker and infer regions that will satisfy its requirements.
15 //! It is generally true that the inference doesn't need to be sound,
16 //! meaning that if there is a bug and we inferred bad regions, the borrow
17 //! checker should catch it. This is not entirely true though; for
18 //! example, the borrow checker doesn't check subtyping, and it doesn't
19 //! check that region pointers are always live when they are used. It
20 //! might be worthwhile to fix this so that borrowck serves as a kind of
21 //! verification step -- that would add confidence in the overall
22 //! correctness of the compiler, at the cost of duplicating some type
23 //! checks and effort.
25 //! ### Inferring the duration of borrows, automatic and otherwise
27 //! Whenever we introduce a borrowed pointer, for example as the result of
28 //! a borrow expression `let x = &data`, the lifetime of the pointer `x`
29 //! is always specified as a region inference variable. `regionck` has the
30 //! job of adding constraints such that this inference variable is as
31 //! narrow as possible while still accommodating all uses (that is, every
32 //! dereference of the resulting pointer must be within the lifetime).
36 //! Generally speaking, `regionck` does NOT try to ensure that the data
37 //! `data` will outlive the pointer `x`. That is the job of borrowck. The
38 //! one exception is when "re-borrowing" the contents of another borrowed
39 //! pointer. For example, imagine you have a borrowed pointer `b` with
40 //! lifetime `L1` and you have an expression `&*b`. The result of this
41 //! expression will be another borrowed pointer with lifetime `L2` (which is
42 //! an inference variable). The borrow checker is going to enforce the
43 //! constraint that `L2 < L1`, because otherwise you are re-borrowing data
44 //! for a lifetime larger than the original loan. However, without the
45 //! routines in this module, the region inferencer would not know of this
46 //! dependency and thus it might infer the lifetime of `L2` to be greater
47 //! than `L1` (issue #3148).
49 //! There are a number of troublesome scenarios in the tests
50 //! `region-dependent-*.rs`, but here is one example:
52 //! struct Foo { i: i32 }
53 //! struct Bar { foo: Foo }
54 //! fn get_i<'a>(x: &'a Bar) -> &'a i32 {
55 //! let foo = &x.foo; // Lifetime L1
56 //! &foo.i // Lifetime L2
59 //! Note that this comes up either with `&` expressions, `ref`
60 //! bindings, and `autorefs`, which are the three ways to introduce
63 //! The key point here is that when you are borrowing a value that
64 //! is "guaranteed" by a borrowed pointer, you must link the
65 //! lifetime of that borrowed pointer (`L1`, here) to the lifetime of
66 //! the borrow itself (`L2`). What do I mean by "guaranteed" by a
67 //! borrowed pointer? I mean any data that is reached by first
68 //! dereferencing a borrowed pointer and then either traversing
69 //! interior offsets or boxes. We say that the guarantor
70 //! of such data is the region of the borrowed pointer that was
71 //! traversed. This is essentially the same as the ownership
72 //! relation, except that a borrowed pointer never owns its
75 use crate::check::dropck;
76 use crate::check::FnCtxt;
77 use crate::mem_categorization as mc;
78 use crate::middle::region;
79 use rustc_data_structures::stable_set::FxHashSet;
81 use rustc_hir::def_id::LocalDefId;
82 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
83 use rustc_hir::PatKind;
84 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
85 use rustc_infer::infer::{self, RegionObligation, RegionckMode};
86 use rustc_middle::hir::place::{PlaceBase, PlaceWithHirId};
87 use rustc_middle::ty::adjustment;
88 use rustc_middle::ty::{self, Ty};
90 use rustc_trait_selection::infer::OutlivesEnvironmentExt;
91 use rustc_trait_selection::opaque_types::InferCtxtExt;
94 // a variation on try that just returns unit
95 macro_rules! ignore_err {
100 debug!("ignoring mem-categorization error!");
107 ///////////////////////////////////////////////////////////////////////////
108 // PUBLIC ENTRY POINTS
110 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
111 pub fn regionck_expr(&self, body: &'tcx hir::Body<'tcx>) {
112 let subject = self.tcx.hir().body_owner_def_id(body.id());
113 let id = body.value.hir_id;
114 let mut rcx = RegionCtxt::new(self, id, Subject(subject), self.param_env);
116 // There are no add'l implied bounds when checking a
117 // standalone expr (e.g., the `E` in a type like `[u32; E]`).
118 rcx.outlives_environment.save_implied_bounds(id);
120 if !self.errors_reported_since_creation() {
121 // regionck assumes typeck succeeded
122 rcx.visit_body(body);
123 rcx.visit_region_obligations(id);
125 rcx.resolve_regions_and_report_errors(RegionckMode::for_item_body(self.tcx));
128 /// Region checking during the WF phase for items. `wf_tys` are the
129 /// types from which we should derive implied bounds, if any.
130 pub fn regionck_item(&self, item_id: hir::HirId, span: Span, wf_tys: FxHashSet<Ty<'tcx>>) {
131 debug!("regionck_item(item.id={:?}, wf_tys={:?})", item_id, wf_tys);
132 let subject = self.tcx.hir().local_def_id(item_id);
133 let mut rcx = RegionCtxt::new(self, item_id, Subject(subject), self.param_env);
134 rcx.outlives_environment.add_implied_bounds(self, wf_tys, item_id, span);
135 rcx.outlives_environment.save_implied_bounds(item_id);
136 rcx.visit_region_obligations(item_id);
137 rcx.resolve_regions_and_report_errors(RegionckMode::default());
140 /// Region check a function body. Not invoked on closures, but
141 /// only on the "root" fn item (in which closures may be
142 /// embedded). Walks the function body and adds various add'l
143 /// constraints that are needed for region inference. This is
144 /// separated both to isolate "pure" region constraints from the
145 /// rest of type check and because sometimes we need type
146 /// inference to have completed before we can determine which
147 /// constraints to add.
148 pub(crate) fn regionck_fn(
151 body: &'tcx hir::Body<'tcx>,
153 wf_tys: FxHashSet<Ty<'tcx>>,
155 debug!("regionck_fn(id={})", fn_id);
156 let subject = self.tcx.hir().body_owner_def_id(body.id());
157 let hir_id = body.value.hir_id;
158 let mut rcx = RegionCtxt::new(self, hir_id, Subject(subject), self.param_env);
159 // We need to add the implied bounds from the function signature
160 rcx.outlives_environment.add_implied_bounds(self, wf_tys, fn_id, span);
161 rcx.outlives_environment.save_implied_bounds(fn_id);
163 if !self.errors_reported_since_creation() {
164 // regionck assumes typeck succeeded
165 rcx.visit_fn_body(fn_id, body, self.tcx.hir().span(fn_id));
168 rcx.resolve_regions_and_report_errors(RegionckMode::for_item_body(self.tcx));
172 ///////////////////////////////////////////////////////////////////////////
175 pub struct RegionCtxt<'a, 'tcx> {
176 pub fcx: &'a FnCtxt<'a, 'tcx>,
178 pub region_scope_tree: &'tcx region::ScopeTree,
180 outlives_environment: OutlivesEnvironment<'tcx>,
182 // id of innermost fn body id
184 body_owner: LocalDefId,
186 // id of AST node being analyzed (the subject of the analysis).
187 subject_def_id: LocalDefId,
190 impl<'a, 'tcx> Deref for RegionCtxt<'a, 'tcx> {
191 type Target = FnCtxt<'a, 'tcx>;
192 fn deref(&self) -> &Self::Target {
197 pub struct Subject(LocalDefId);
199 impl<'a, 'tcx> RegionCtxt<'a, 'tcx> {
201 fcx: &'a FnCtxt<'a, 'tcx>,
202 initial_body_id: hir::HirId,
203 Subject(subject): Subject,
204 param_env: ty::ParamEnv<'tcx>,
205 ) -> RegionCtxt<'a, 'tcx> {
206 let region_scope_tree = fcx.tcx.region_scope_tree(subject);
207 let outlives_environment = OutlivesEnvironment::new(param_env);
211 body_id: initial_body_id,
213 subject_def_id: subject,
214 outlives_environment,
218 /// Try to resolve the type for the given node, returning `t_err` if an error results. Note that
219 /// we never care about the details of the error, the same error will be detected and reported
220 /// in the writeback phase.
222 /// Note one important point: we do not attempt to resolve *region variables* here. This is
223 /// because regionck is essentially adding constraints to those region variables and so may yet
224 /// influence how they are resolved.
226 /// Consider this silly example:
229 /// fn borrow(x: &i32) -> &i32 {x}
230 /// fn foo(x: @i32) -> i32 { // block: B
231 /// let b = borrow(x); // region: <R0>
236 /// Here, the region of `b` will be `<R0>`. `<R0>` is constrained to be some subregion of the
237 /// block B and some superregion of the call. If we forced it now, we'd choose the smaller
238 /// region (the call). But that would make the *b illegal. Since we don't resolve, the type
239 /// of b will be `&<R0>.i32` and then `*b` will require that `<R0>` be bigger than the let and
240 /// the `*b` expression, so we will effectively resolve `<R0>` to be the block B.
241 pub fn resolve_type(&self, unresolved_ty: Ty<'tcx>) -> Ty<'tcx> {
242 self.resolve_vars_if_possible(unresolved_ty)
245 /// Try to resolve the type for the given node.
246 fn resolve_node_type(&self, id: hir::HirId) -> Ty<'tcx> {
247 let t = self.node_ty(id);
251 /// This is the "main" function when region-checking a function item or a
252 /// closure within a function item. It begins by updating various fields
253 /// (e.g., `outlives_environment`) to be appropriate to the function and
254 /// then adds constraints derived from the function body.
256 /// Note that it does **not** restore the state of the fields that
257 /// it updates! This is intentional, since -- for the main
258 /// function -- we wish to be able to read the final
259 /// `outlives_environment` and other fields from the caller. For
260 /// closures, however, we save and restore any "scoped state"
261 /// before we invoke this function. (See `visit_fn` in the
262 /// `intravisit::Visitor` impl below.)
265 id: hir::HirId, // the id of the fn itself
266 body: &'tcx hir::Body<'tcx>,
269 // When we enter a function, we can derive
270 debug!("visit_fn_body(id={:?})", id);
272 let body_id = body.id();
273 self.body_id = body_id.hir_id;
274 self.body_owner = self.tcx.hir().body_owner_def_id(body_id);
277 match self.typeck_results.borrow().liberated_fn_sigs().get(id) {
280 bug!("No fn-sig entry for id={:?}", id);
285 // Collect the types from which we create inferred bounds.
286 // For the return type, if diverging, substitute `bool` just
287 // because it will have no effect.
289 // FIXME(#27579) return types should not be implied bounds
290 let fn_sig_tys: FxHashSet<_> =
291 fn_sig.inputs().iter().cloned().chain(Some(fn_sig.output())).collect();
293 self.outlives_environment.add_implied_bounds(self.fcx, fn_sig_tys, body_id.hir_id, span);
294 self.outlives_environment.save_implied_bounds(body_id.hir_id);
295 self.link_fn_params(&body.params);
296 self.visit_body(body);
297 self.visit_region_obligations(body_id.hir_id);
299 self.constrain_opaque_types(self.outlives_environment.free_region_map());
302 fn visit_region_obligations(&mut self, hir_id: hir::HirId) {
303 debug!("visit_region_obligations: hir_id={:?}", hir_id);
305 // region checking can introduce new pending obligations
306 // which, when processed, might generate new region
307 // obligations. So make sure we process those.
308 self.select_all_obligations_or_error();
311 fn resolve_regions_and_report_errors(&self, mode: RegionckMode) {
312 self.infcx.process_registered_region_obligations(
313 self.outlives_environment.region_bound_pairs_map(),
314 Some(self.tcx.lifetimes.re_root_empty),
318 self.fcx.resolve_regions_and_report_errors(
319 self.subject_def_id.to_def_id(),
320 &self.outlives_environment,
325 fn constrain_bindings_in_pat(&mut self, pat: &hir::Pat<'_>) {
326 debug!("regionck::visit_pat(pat={:?})", pat);
327 pat.each_binding(|_, hir_id, span, _| {
328 let typ = self.resolve_node_type(hir_id);
329 let body_id = self.body_id;
330 dropck::check_drop_obligations(self, typ, span, body_id);
335 impl<'a, 'tcx> Visitor<'tcx> for RegionCtxt<'a, 'tcx> {
336 // (..) FIXME(#3238) should use visit_pat, not visit_arm/visit_local,
337 // However, right now we run into an issue whereby some free
338 // regions are not properly related if they appear within the
339 // types of arguments that must be inferred. This could be
340 // addressed by deferring the construction of the region
341 // hierarchy, and in particular the relationships between free
342 // regions, until regionck, as described in #3238.
344 type Map = intravisit::ErasedMap<'tcx>;
346 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
347 NestedVisitorMap::None
352 fk: intravisit::FnKind<'tcx>,
353 _: &'tcx hir::FnDecl<'tcx>,
354 body_id: hir::BodyId,
359 matches!(fk, intravisit::FnKind::Closure),
360 "visit_fn invoked for something other than a closure"
363 // Save state of current function before invoking
364 // `visit_fn_body`. We will restore afterwards.
365 let old_body_id = self.body_id;
366 let old_body_owner = self.body_owner;
367 let env_snapshot = self.outlives_environment.push_snapshot_pre_closure();
369 let body = self.tcx.hir().body(body_id);
370 self.visit_fn_body(hir_id, body, span);
372 // Restore state from previous function.
373 self.outlives_environment.pop_snapshot_post_closure(env_snapshot);
374 self.body_id = old_body_id;
375 self.body_owner = old_body_owner;
378 //visit_pat: visit_pat, // (..) see above
380 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
382 self.constrain_bindings_in_pat(&arm.pat);
383 intravisit::walk_arm(self, arm);
386 fn visit_local(&mut self, l: &'tcx hir::Local<'tcx>) {
388 self.constrain_bindings_in_pat(&l.pat);
390 intravisit::walk_local(self, l);
393 fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
394 // Check any autoderefs or autorefs that appear.
395 let cmt_result = self.constrain_adjustments(expr);
397 // If necessary, constrain destructors in this expression. This will be
398 // the adjusted form if there is an adjustment.
401 self.check_safety_of_rvalue_destructor_if_necessary(&head_cmt, expr.span);
404 self.tcx.sess.delay_span_bug(expr.span, "cat_expr Errd");
409 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, m, ref base) => {
410 self.link_addr_of(expr, m, &base);
412 intravisit::walk_expr(self, expr);
415 hir::ExprKind::Match(ref discr, ref arms, _) => {
416 self.link_match(&discr, &arms[..]);
418 intravisit::walk_expr(self, expr);
421 _ => intravisit::walk_expr(self, expr),
426 impl<'a, 'tcx> RegionCtxt<'a, 'tcx> {
427 /// Creates a temporary `MemCategorizationContext` and pass it to the closure.
428 fn with_mc<F, R>(&self, f: F) -> R
430 F: for<'b> FnOnce(mc::MemCategorizationContext<'b, 'tcx>) -> R,
432 f(mc::MemCategorizationContext::new(
434 self.outlives_environment.param_env,
436 &self.typeck_results.borrow(),
440 /// Invoked on any adjustments that occur. Checks that if this is a region pointer being
441 /// dereferenced, the lifetime of the pointer includes the deref expr.
442 fn constrain_adjustments(
444 expr: &hir::Expr<'_>,
445 ) -> mc::McResult<PlaceWithHirId<'tcx>> {
446 debug!("constrain_adjustments(expr={:?})", expr);
448 let mut place = self.with_mc(|mc| mc.cat_expr_unadjusted(expr))?;
450 let typeck_results = self.typeck_results.borrow();
451 let adjustments = typeck_results.expr_adjustments(&expr);
452 if adjustments.is_empty() {
456 debug!("constrain_adjustments: adjustments={:?}", adjustments);
458 // If necessary, constrain destructors in the unadjusted form of this
460 self.check_safety_of_rvalue_destructor_if_necessary(&place, expr.span);
462 for adjustment in adjustments {
463 debug!("constrain_adjustments: adjustment={:?}, place={:?}", adjustment, place);
465 if let adjustment::Adjust::Deref(Some(deref)) = adjustment.kind {
469 ty::BorrowKind::from_mutbl(deref.mutbl),
474 if let adjustment::Adjust::Borrow(ref autoref) = adjustment.kind {
475 self.link_autoref(expr, &place, autoref);
478 place = self.with_mc(|mc| mc.cat_expr_adjusted(expr, place, &adjustment))?;
484 fn check_safety_of_rvalue_destructor_if_necessary(
486 place_with_id: &PlaceWithHirId<'tcx>,
489 if let PlaceBase::Rvalue = place_with_id.place.base {
490 if place_with_id.place.projections.is_empty() {
491 let typ = self.resolve_type(place_with_id.place.ty());
492 let body_id = self.body_id;
493 dropck::check_drop_obligations(self, typ, span, body_id);
497 /// Adds constraints to inference such that `T: 'a` holds (or
498 /// reports an error if it cannot).
502 /// - `origin`, the reason we need this constraint
503 /// - `ty`, the type `T`
504 /// - `region`, the region `'a`
505 pub fn type_must_outlive(
507 origin: infer::SubregionOrigin<'tcx>,
509 region: ty::Region<'tcx>,
511 self.infcx.register_region_obligation(
513 RegionObligation { sub_region: region, sup_type: ty, origin },
517 /// Computes the guarantor for an expression `&base` and then ensures that the lifetime of the
518 /// resulting pointer is linked to the lifetime of its guarantor (if any).
521 expr: &hir::Expr<'_>,
522 mutability: hir::Mutability,
523 base: &hir::Expr<'_>,
525 debug!("link_addr_of(expr={:?}, base={:?})", expr, base);
527 let cmt = ignore_err!(self.with_mc(|mc| mc.cat_expr(base)));
529 debug!("link_addr_of: cmt={:?}", cmt);
531 self.link_region_from_node_type(expr.span, expr.hir_id, mutability, &cmt);
534 /// Computes the guarantors for any ref bindings in a `let` and
535 /// then ensures that the lifetime of the resulting pointer is
536 /// linked to the lifetime of the initialization expression.
537 fn link_local(&self, local: &hir::Local<'_>) {
538 debug!("regionck::for_local()");
539 let init_expr = match local.init {
543 Some(ref expr) => &**expr,
545 let discr_cmt = ignore_err!(self.with_mc(|mc| mc.cat_expr(init_expr)));
546 self.link_pattern(discr_cmt, &local.pat);
549 /// Computes the guarantors for any ref bindings in a match and
550 /// then ensures that the lifetime of the resulting pointer is
551 /// linked to the lifetime of its guarantor (if any).
552 fn link_match(&self, discr: &hir::Expr<'_>, arms: &[hir::Arm<'_>]) {
553 debug!("regionck::for_match()");
554 let discr_cmt = ignore_err!(self.with_mc(|mc| mc.cat_expr(discr)));
555 debug!("discr_cmt={:?}", discr_cmt);
557 self.link_pattern(discr_cmt.clone(), &arm.pat);
561 /// Computes the guarantors for any ref bindings in a match and
562 /// then ensures that the lifetime of the resulting pointer is
563 /// linked to the lifetime of its guarantor (if any).
564 fn link_fn_params(&self, params: &[hir::Param<'_>]) {
565 for param in params {
566 let param_ty = self.node_ty(param.hir_id);
568 self.with_mc(|mc| mc.cat_rvalue(param.hir_id, param.pat.span, param_ty));
569 debug!("param_ty={:?} param_cmt={:?} param={:?}", param_ty, param_cmt, param);
570 self.link_pattern(param_cmt, ¶m.pat);
574 /// Link lifetimes of any ref bindings in `root_pat` to the pointers found
575 /// in the discriminant, if needed.
576 fn link_pattern(&self, discr_cmt: PlaceWithHirId<'tcx>, root_pat: &hir::Pat<'_>) {
577 debug!("link_pattern(discr_cmt={:?}, root_pat={:?})", discr_cmt, root_pat);
578 ignore_err!(self.with_mc(|mc| {
579 mc.cat_pattern(discr_cmt, root_pat, |sub_cmt, hir::Pat { kind, span, hir_id, .. }| {
581 if let PatKind::Binding(..) = kind {
582 if let Some(ty::BindByReference(mutbl)) =
583 mc.typeck_results.extract_binding_mode(self.tcx.sess, *hir_id, *span)
585 self.link_region_from_node_type(*span, *hir_id, mutbl, &sub_cmt);
592 /// Link lifetime of borrowed pointer resulting from autoref to lifetimes in the value being
596 expr: &hir::Expr<'_>,
597 expr_cmt: &PlaceWithHirId<'tcx>,
598 autoref: &adjustment::AutoBorrow<'tcx>,
600 debug!("link_autoref(autoref={:?}, expr_cmt={:?})", autoref, expr_cmt);
603 adjustment::AutoBorrow::Ref(r, m) => {
604 self.link_region(expr.span, r, ty::BorrowKind::from_mutbl(m.into()), expr_cmt);
607 adjustment::AutoBorrow::RawPtr(_) => {}
611 /// Like `link_region()`, except that the region is extracted from the type of `id`,
612 /// which must be some reference (`&T`, `&str`, etc).
613 fn link_region_from_node_type(
617 mutbl: hir::Mutability,
618 cmt_borrowed: &PlaceWithHirId<'tcx>,
621 "link_region_from_node_type(id={:?}, mutbl={:?}, cmt_borrowed={:?})",
622 id, mutbl, cmt_borrowed
625 let rptr_ty = self.resolve_node_type(id);
626 if let ty::Ref(r, _, _) = rptr_ty.kind() {
627 debug!("rptr_ty={}", rptr_ty);
628 self.link_region(span, r, ty::BorrowKind::from_mutbl(mutbl), cmt_borrowed);
632 /// Informs the inference engine that `borrow_cmt` is being borrowed with
633 /// kind `borrow_kind` and lifetime `borrow_region`.
634 /// In order to ensure borrowck is satisfied, this may create constraints
635 /// between regions, as explained in `link_reborrowed_region()`.
639 borrow_region: ty::Region<'tcx>,
640 borrow_kind: ty::BorrowKind,
641 borrow_place: &PlaceWithHirId<'tcx>,
643 let origin = infer::DataBorrowed(borrow_place.place.ty(), span);
644 self.type_must_outlive(origin, borrow_place.place.ty(), borrow_region);
646 for pointer_ty in borrow_place.place.deref_tys() {
648 "link_region(borrow_region={:?}, borrow_kind={:?}, pointer_ty={:?})",
649 borrow_region, borrow_kind, borrow_place
651 match *pointer_ty.kind() {
652 ty::RawPtr(_) => return,
653 ty::Ref(ref_region, _, ref_mutability) => {
654 if self.link_reborrowed_region(span, borrow_region, ref_region, ref_mutability)
659 _ => assert!(pointer_ty.is_box(), "unexpected built-in deref type {}", pointer_ty),
662 if let PlaceBase::Upvar(upvar_id) = borrow_place.place.base {
663 self.link_upvar_region(span, borrow_region, upvar_id);
667 /// This is the most complicated case: the path being borrowed is
668 /// itself the referent of a borrowed pointer. Let me give an
669 /// example fragment of code to make clear(er) the situation:
671 /// ```ignore (incomplete Rust code)
672 /// let r: &'a mut T = ...; // the original reference "r" has lifetime 'a
674 /// &'z *r // the reborrow has lifetime 'z
677 /// Now, in this case, our primary job is to add the inference
678 /// constraint that `'z <= 'a`. Given this setup, let's clarify the
679 /// parameters in (roughly) terms of the example:
681 /// ```plain,ignore (pseudo-Rust)
682 /// A borrow of: `& 'z bk * r` where `r` has type `& 'a bk T`
683 /// borrow_region ^~ ref_region ^~
684 /// borrow_kind ^~ ref_kind ^~
688 /// Here `bk` stands for some borrow-kind (e.g., `mut`, `uniq`, etc).
690 /// There is a complication beyond the simple scenario I just painted: there
691 /// may in fact be more levels of reborrowing. In the example, I said the
692 /// borrow was like `&'z *r`, but it might in fact be a borrow like
693 /// `&'z **q` where `q` has type `&'a &'b mut T`. In that case, we want to
694 /// ensure that `'z <= 'a` and `'z <= 'b`.
696 /// The return value of this function indicates whether we *don't* need to
697 /// the recurse to the next reference up.
699 /// This is explained more below.
700 fn link_reborrowed_region(
703 borrow_region: ty::Region<'tcx>,
704 ref_region: ty::Region<'tcx>,
705 ref_mutability: hir::Mutability,
707 debug!("link_reborrowed_region: {:?} <= {:?}", borrow_region, ref_region);
708 self.sub_regions(infer::Reborrow(span), borrow_region, ref_region);
710 // Decide whether we need to recurse and link any regions within
711 // the `ref_cmt`. This is concerned for the case where the value
712 // being reborrowed is in fact a borrowed pointer found within
713 // another borrowed pointer. For example:
715 // let p: &'b &'a mut T = ...;
719 // What makes this case particularly tricky is that, if the data
720 // being borrowed is a `&mut` or `&uniq` borrow, borrowck requires
721 // not only that `'z <= 'a`, (as before) but also `'z <= 'b`
722 // (otherwise the user might mutate through the `&mut T` reference
723 // after `'b` expires and invalidate the borrow we are looking at
726 // So let's re-examine our parameters in light of this more
727 // complicated (possible) scenario:
729 // A borrow of: `& 'z bk * * p` where `p` has type `&'b bk & 'a bk T`
730 // borrow_region ^~ ref_region ^~
731 // borrow_kind ^~ ref_kind ^~
734 // (Note that since we have not examined `ref_cmt.cat`, we don't
735 // know whether this scenario has occurred; but I wanted to show
736 // how all the types get adjusted.)
737 match ref_mutability {
738 hir::Mutability::Not => {
739 // The reference being reborrowed is a shareable ref of
740 // type `&'a T`. In this case, it doesn't matter where we
741 // *found* the `&T` pointer, the memory it references will
742 // be valid and immutable for `'a`. So we can stop here.
746 hir::Mutability::Mut => {
747 // The reference being reborrowed is either an `&mut T`. This is
748 // the case where recursion is needed.
754 /// An upvar may be behind up to 2 references:
756 /// * One can come from the reference to a "by-reference" upvar.
757 /// * Another one can come from the reference to the closure itself if it's
758 /// a `FnMut` or `Fn` closure.
760 /// This function links the lifetimes of those references to the lifetime
761 /// of the borrow that's provided. See [RegionCtxt::link_reborrowed_region] for some
762 /// more explanation of this in the general case.
764 /// We also supply a *cause*, and in this case we set the cause to
765 /// indicate that the reference being "reborrowed" is itself an upvar. This
766 /// provides a nicer error message should something go wrong.
767 fn link_upvar_region(
770 borrow_region: ty::Region<'tcx>,
771 upvar_id: ty::UpvarId,
773 debug!("link_upvar_region(borrorw_region={:?}, upvar_id={:?}", borrow_region, upvar_id);
774 // A by-reference upvar can't be borrowed for longer than the
775 // upvar is borrowed from the environment.
776 let closure_local_def_id = upvar_id.closure_expr_id;
777 let mut all_captures_are_imm_borrow = true;
778 for captured_place in self
781 .closure_min_captures
782 .get(&closure_local_def_id.to_def_id())
783 .and_then(|root_var_min_cap| root_var_min_cap.get(&upvar_id.var_path.hir_id))
787 match captured_place.info.capture_kind {
788 ty::UpvarCapture::ByRef(upvar_borrow) => {
790 infer::ReborrowUpvar(span, upvar_id),
794 if let ty::ImmBorrow = upvar_borrow.kind {
795 debug!("link_upvar_region: capture by shared ref");
797 all_captures_are_imm_borrow = false;
800 ty::UpvarCapture::ByValue(_) => {
801 all_captures_are_imm_borrow = false;
805 if all_captures_are_imm_borrow {
808 let fn_hir_id = self.tcx.hir().local_def_id_to_hir_id(closure_local_def_id);
809 let ty = self.resolve_node_type(fn_hir_id);
810 debug!("link_upvar_region: ty={:?}", ty);
812 // A closure capture can't be borrowed for longer than the
813 // reference to the closure.
814 if let ty::Closure(_, substs) = ty.kind() {
815 match self.infcx.closure_kind(substs) {
816 Some(ty::ClosureKind::Fn | ty::ClosureKind::FnMut) => {
817 // Region of environment pointer
818 let env_region = self.tcx.mk_region(ty::ReFree(ty::FreeRegion {
819 scope: upvar_id.closure_expr_id.to_def_id(),
820 bound_region: ty::BrEnv,
823 infer::ReborrowUpvar(span, upvar_id),
828 Some(ty::ClosureKind::FnOnce) => {}
830 span_bug!(span, "Have not inferred closure kind before regionck");