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::middle::mem_categorization as mc;
78 use crate::middle::mem_categorization::Categorization;
79 use crate::middle::region;
80 use rustc::hir::def_id::DefId;
81 use rustc::infer::outlives::env::OutlivesEnvironment;
82 use rustc::infer::{self, RegionObligation, SuppressRegionErrors};
83 use rustc::ty::adjustment;
84 use rustc::ty::subst::{SubstsRef, UnpackedKind};
85 use rustc::ty::{self, Ty};
87 use rustc::hir::intravisit::{self, NestedVisitorMap, Visitor};
88 use rustc::hir::{self, PatKind};
89 use rustc_data_structures::sync::Lrc;
95 // a variation on try that just returns unit
96 macro_rules! ignore_err {
101 debug!("ignoring mem-categorization error!");
108 ///////////////////////////////////////////////////////////////////////////
109 // PUBLIC ENTRY POINTS
111 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
112 pub fn regionck_expr(&self, body: &'gcx hir::Body) {
113 let subject = self.tcx.hir().body_owner_def_id(body.id());
114 let id = body.value.hir_id;
115 let mut rcx = RegionCtxt::new(
123 // There are no add'l implied bounds when checking a
124 // standalone expr (e.g., the `E` in a type like `[u32; E]`).
125 rcx.outlives_environment.save_implied_bounds(id);
127 if self.err_count_since_creation() == 0 {
128 // regionck assumes typeck succeeded
129 rcx.visit_body(body);
130 rcx.visit_region_obligations(id);
132 rcx.resolve_regions_and_report_errors(SuppressRegionErrors::when_nll_is_enabled(self.tcx));
134 assert!(self.tables.borrow().free_region_map.is_empty());
135 self.tables.borrow_mut().free_region_map = rcx.outlives_environment.into_free_region_map();
138 /// Region checking during the WF phase for items. `wf_tys` are the
139 /// types from which we should derive implied bounds, if any.
140 pub fn regionck_item(&self, item_id: hir::HirId, span: Span, wf_tys: &[Ty<'tcx>]) {
141 debug!("regionck_item(item.id={:?}, wf_tys={:?})", item_id, wf_tys);
142 let subject = self.tcx.hir().local_def_id_from_hir_id(item_id);
143 let mut rcx = RegionCtxt::new(
145 RepeatingScope(item_id),
150 rcx.outlives_environment
151 .add_implied_bounds(self, wf_tys, item_id, span);
152 rcx.outlives_environment.save_implied_bounds(item_id);
153 rcx.visit_region_obligations(item_id);
154 rcx.resolve_regions_and_report_errors(SuppressRegionErrors::default());
157 /// Region check a function body. Not invoked on closures, but
158 /// only on the "root" fn item (in which closures may be
159 /// embedded). Walks the function body and adds various add'l
160 /// constraints that are needed for region inference. This is
161 /// separated both to isolate "pure" region constraints from the
162 /// rest of type check and because sometimes we need type
163 /// inference to have completed before we can determine which
164 /// constraints to add.
165 pub fn regionck_fn(&self, fn_id: hir::HirId, body: &'gcx hir::Body) {
166 debug!("regionck_fn(id={})", fn_id);
167 let subject = self.tcx.hir().body_owner_def_id(body.id());
168 let hir_id = body.value.hir_id;
169 let mut rcx = RegionCtxt::new(
171 RepeatingScope(hir_id),
177 if self.err_count_since_creation() == 0 {
178 // regionck assumes typeck succeeded
179 rcx.visit_fn_body(fn_id, body, self.tcx.hir().span_by_hir_id(fn_id));
182 rcx.resolve_regions_and_report_errors(SuppressRegionErrors::when_nll_is_enabled(self.tcx));
184 // In this mode, we also copy the free-region-map into the
185 // tables of the enclosing fcx. In the other regionck modes
186 // (e.g., `regionck_item`), we don't have an enclosing tables.
187 assert!(self.tables.borrow().free_region_map.is_empty());
188 self.tables.borrow_mut().free_region_map = rcx.outlives_environment.into_free_region_map();
192 ///////////////////////////////////////////////////////////////////////////
195 pub struct RegionCtxt<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
196 pub fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
198 pub region_scope_tree: Lrc<region::ScopeTree>,
200 outlives_environment: OutlivesEnvironment<'tcx>,
202 // id of innermost fn body id
205 // call_site scope of innermost fn
206 call_site_scope: Option<region::Scope>,
208 // id of innermost fn or loop
209 repeating_scope: hir::HirId,
211 // id of AST node being analyzed (the subject of the analysis).
212 subject_def_id: DefId,
215 impl<'a, 'gcx, 'tcx> Deref for RegionCtxt<'a, 'gcx, 'tcx> {
216 type Target = FnCtxt<'a, 'gcx, 'tcx>;
217 fn deref(&self) -> &Self::Target {
222 pub struct RepeatingScope(hir::HirId);
223 pub struct Subject(DefId);
225 impl<'a, 'gcx, 'tcx> RegionCtxt<'a, 'gcx, 'tcx> {
227 fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
228 RepeatingScope(initial_repeating_scope): RepeatingScope,
229 initial_body_id: hir::HirId,
230 Subject(subject): Subject,
231 param_env: ty::ParamEnv<'tcx>,
232 ) -> RegionCtxt<'a, 'gcx, 'tcx> {
233 let region_scope_tree = fcx.tcx.region_scope_tree(subject);
234 let outlives_environment = OutlivesEnvironment::new(param_env);
238 repeating_scope: initial_repeating_scope,
239 body_id: initial_body_id,
240 call_site_scope: None,
241 subject_def_id: subject,
242 outlives_environment,
246 fn set_repeating_scope(&mut self, scope: hir::HirId) -> hir::HirId {
247 mem::replace(&mut self.repeating_scope, scope)
250 /// Try to resolve the type for the given node, returning `t_err` if an error results. Note that
251 /// we never care about the details of the error, the same error will be detected and reported
252 /// in the writeback phase.
254 /// Note one important point: we do not attempt to resolve *region variables* here. This is
255 /// because regionck is essentially adding constraints to those region variables and so may yet
256 /// influence how they are resolved.
258 /// Consider this silly example:
261 /// fn borrow(x: &i32) -> &i32 {x}
262 /// fn foo(x: @i32) -> i32 { // block: B
263 /// let b = borrow(x); // region: <R0>
268 /// Here, the region of `b` will be `<R0>`. `<R0>` is constrained to be some subregion of the
269 /// block B and some superregion of the call. If we forced it now, we'd choose the smaller
270 /// region (the call). But that would make the *b illegal. Since we don't resolve, the type
271 /// of b will be `&<R0>.i32` and then `*b` will require that `<R0>` be bigger than the let and
272 /// the `*b` expression, so we will effectively resolve `<R0>` to be the block B.
273 pub fn resolve_type(&self, unresolved_ty: Ty<'tcx>) -> Ty<'tcx> {
274 self.resolve_type_vars_if_possible(&unresolved_ty)
277 /// Try to resolve the type for the given node.
278 fn resolve_node_type(&self, id: hir::HirId) -> Ty<'tcx> {
279 let t = self.node_ty(id);
283 /// Try to resolve the type for the given node.
284 pub fn resolve_expr_type_adjusted(&mut self, expr: &hir::Expr) -> Ty<'tcx> {
285 let ty = self.tables.borrow().expr_ty_adjusted(expr);
286 self.resolve_type(ty)
289 /// This is the "main" function when region-checking a function item or a closure
290 /// within a function item. It begins by updating various fields (e.g., `call_site_scope`
291 /// and `outlives_environment`) to be appropriate to the function and then adds constraints
292 /// derived from the function body.
294 /// Note that it does **not** restore the state of the fields that
295 /// it updates! This is intentional, since -- for the main
296 /// function -- we wish to be able to read the final
297 /// `outlives_environment` and other fields from the caller. For
298 /// closures, however, we save and restore any "scoped state"
299 /// before we invoke this function. (See `visit_fn` in the
300 /// `intravisit::Visitor` impl below.)
303 id: hir::HirId, // the id of the fn itself
304 body: &'gcx hir::Body,
307 // When we enter a function, we can derive
308 debug!("visit_fn_body(id={:?})", id);
310 let body_id = body.id();
311 self.body_id = body_id.hir_id;
313 let call_site = region::Scope {
314 id: body.value.hir_id.local_id,
315 data: region::ScopeData::CallSite,
317 self.call_site_scope = Some(call_site);
320 match self.tables.borrow().liberated_fn_sigs().get(id) {
321 Some(f) => f.clone(),
323 bug!("No fn-sig entry for id={:?}", id);
328 // Collect the types from which we create inferred bounds.
329 // For the return type, if diverging, substitute `bool` just
330 // because it will have no effect.
332 // FIXME(#27579) return types should not be implied bounds
333 let fn_sig_tys: Vec<_> = fn_sig
337 .chain(Some(fn_sig.output()))
340 self.outlives_environment.add_implied_bounds(
346 self.outlives_environment
347 .save_implied_bounds(body_id.hir_id);
350 id: body.value.hir_id.local_id,
351 data: region::ScopeData::Node,
355 self.visit_body(body);
356 self.visit_region_obligations(body_id.hir_id);
358 let call_site_scope = self.call_site_scope.unwrap();
360 "visit_fn_body body.id {:?} call_site_scope: {:?}",
364 let call_site_region = self.tcx.mk_region(ty::ReScope(call_site_scope));
366 self.type_of_node_must_outlive(infer::CallReturn(span), body_id.hir_id, call_site_region);
368 self.constrain_opaque_types(
369 &self.fcx.opaque_types.borrow(),
370 self.outlives_environment.free_region_map(),
374 fn visit_region_obligations(&mut self, hir_id: hir::HirId) {
375 debug!("visit_region_obligations: hir_id={:?}", hir_id);
377 // region checking can introduce new pending obligations
378 // which, when processed, might generate new region
379 // obligations. So make sure we process those.
380 self.select_all_obligations_or_error();
383 fn resolve_regions_and_report_errors(&self, suppress: SuppressRegionErrors) {
384 self.infcx.process_registered_region_obligations(
385 self.outlives_environment.region_bound_pairs_map(),
386 self.implicit_region_bound,
390 self.fcx.resolve_regions_and_report_errors(
392 &self.region_scope_tree,
393 &self.outlives_environment,
398 fn constrain_bindings_in_pat(&mut self, pat: &hir::Pat) {
399 debug!("regionck::visit_pat(pat={:?})", pat);
400 pat.each_binding(|_, hir_id, span, _| {
401 // If we have a variable that contains region'd data, that
402 // data will be accessible from anywhere that the variable is
403 // accessed. We must be wary of loops like this:
405 // // from src/test/compile-fail/borrowck-lend-flow.rs
406 // let mut v = box 3, w = box 4;
407 // let mut x = &mut w;
410 // borrow(v); //~ ERROR cannot borrow
411 // x = &mut v; // (1)
414 // Typically, we try to determine the region of a borrow from
415 // those points where it is dereferenced. In this case, one
416 // might imagine that the lifetime of `x` need only be the
417 // body of the loop. But of course this is incorrect because
418 // the pointer that is created at point (1) is consumed at
419 // point (2), meaning that it must be live across the loop
420 // iteration. The easiest way to guarantee this is to require
421 // that the lifetime of any regions that appear in a
422 // variable's type enclose at least the variable's scope.
423 let var_scope = self.region_scope_tree.var_scope(hir_id.local_id);
424 let var_region = self.tcx.mk_region(ty::ReScope(var_scope));
426 let origin = infer::BindingTypeIsNotValidAtDecl(span);
427 self.type_of_node_must_outlive(origin, hir_id, var_region);
429 let typ = self.resolve_node_type(hir_id);
430 let body_id = self.body_id;
431 let _ = dropck::check_safety_of_destructor_if_necessary(
432 self, typ, span, body_id, var_scope,
438 impl<'a, 'gcx, 'tcx> Visitor<'gcx> for RegionCtxt<'a, 'gcx, 'tcx> {
439 // (..) FIXME(#3238) should use visit_pat, not visit_arm/visit_local,
440 // However, right now we run into an issue whereby some free
441 // regions are not properly related if they appear within the
442 // types of arguments that must be inferred. This could be
443 // addressed by deferring the construction of the region
444 // hierarchy, and in particular the relationships between free
445 // regions, until regionck, as described in #3238.
447 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'gcx> {
448 NestedVisitorMap::None
453 fk: intravisit::FnKind<'gcx>,
454 _: &'gcx hir::FnDecl,
455 body_id: hir::BodyId,
461 intravisit::FnKind::Closure(..) => true,
464 "visit_fn invoked for something other than a closure"
467 // Save state of current function before invoking
468 // `visit_fn_body`. We will restore afterwards.
469 let old_body_id = self.body_id;
470 let old_call_site_scope = self.call_site_scope;
471 let env_snapshot = self.outlives_environment.push_snapshot_pre_closure();
473 let body = self.tcx.hir().body(body_id);
474 self.visit_fn_body(hir_id, body, span);
476 // Restore state from previous function.
477 self.outlives_environment
478 .pop_snapshot_post_closure(env_snapshot);
479 self.call_site_scope = old_call_site_scope;
480 self.body_id = old_body_id;
483 //visit_pat: visit_pat, // (..) see above
485 fn visit_arm(&mut self, arm: &'gcx hir::Arm) {
488 self.constrain_bindings_in_pat(p);
490 intravisit::walk_arm(self, arm);
493 fn visit_local(&mut self, l: &'gcx hir::Local) {
495 self.constrain_bindings_in_pat(&l.pat);
497 intravisit::walk_local(self, l);
500 fn visit_expr(&mut self, expr: &'gcx hir::Expr) {
502 "regionck::visit_expr(e={:?}, repeating_scope={:?})",
503 expr, self.repeating_scope
506 // No matter what, the type of each expression must outlive the
507 // scope of that expression. This also guarantees basic WF.
508 let expr_ty = self.resolve_node_type(expr.hir_id);
509 // the region corresponding to this expression
510 let expr_region = self.tcx.mk_region(ty::ReScope(region::Scope {
511 id: expr.hir_id.local_id,
512 data: region::ScopeData::Node,
514 self.type_must_outlive(
515 infer::ExprTypeIsNotInScope(expr_ty, expr.span),
520 let is_method_call = self.tables.borrow().is_method_call(expr);
522 // If we are calling a method (either explicitly or via an
523 // overloaded operator), check that all of the types provided as
524 // arguments for its type parameters are well-formed, and all the regions
525 // provided as arguments outlive the call.
527 let origin = match expr.node {
528 hir::ExprKind::MethodCall(..) => infer::ParameterOrigin::MethodCall,
529 hir::ExprKind::Unary(op, _) if op == hir::UnDeref => {
530 infer::ParameterOrigin::OverloadedDeref
532 _ => infer::ParameterOrigin::OverloadedOperator,
535 let substs = self.tables.borrow().node_substs(expr.hir_id);
536 self.substs_wf_in_scope(origin, substs, expr.span, expr_region);
537 // Arguments (sub-expressions) are checked via `constrain_call`, below.
540 // Check any autoderefs or autorefs that appear.
541 let cmt_result = self.constrain_adjustments(expr);
543 // If necessary, constrain destructors in this expression. This will be
544 // the adjusted form if there is an adjustment.
547 self.check_safety_of_rvalue_destructor_if_necessary(&head_cmt, expr.span);
550 self.tcx.sess.delay_span_bug(expr.span, "cat_expr Errd");
555 "regionck::visit_expr(e={:?}, repeating_scope={:?}) - visiting subexprs",
556 expr, self.repeating_scope
559 hir::ExprKind::Path(_) => {
560 let substs = self.tables.borrow().node_substs(expr.hir_id);
561 let origin = infer::ParameterOrigin::Path;
562 self.substs_wf_in_scope(origin, substs, expr.span, expr_region);
565 hir::ExprKind::Call(ref callee, ref args) => {
567 self.constrain_call(expr, Some(&callee), args.iter().map(|e| &*e));
569 self.constrain_callee(&callee);
570 self.constrain_call(expr, None, args.iter().map(|e| &*e));
573 intravisit::walk_expr(self, expr);
576 hir::ExprKind::MethodCall(.., ref args) => {
577 self.constrain_call(expr, Some(&args[0]), args[1..].iter().map(|e| &*e));
579 intravisit::walk_expr(self, expr);
582 hir::ExprKind::AssignOp(_, ref lhs, ref rhs) => {
584 self.constrain_call(expr, Some(&lhs), Some(&**rhs).into_iter());
587 intravisit::walk_expr(self, expr);
590 hir::ExprKind::Index(ref lhs, ref rhs) if is_method_call => {
591 self.constrain_call(expr, Some(&lhs), Some(&**rhs).into_iter());
593 intravisit::walk_expr(self, expr);
596 hir::ExprKind::Binary(_, ref lhs, ref rhs) if is_method_call => {
597 // As `ExprKind::MethodCall`, but the call is via an overloaded op.
598 self.constrain_call(expr, Some(&lhs), Some(&**rhs).into_iter());
600 intravisit::walk_expr(self, expr);
603 hir::ExprKind::Binary(_, ref lhs, ref rhs) => {
604 // If you do `x OP y`, then the types of `x` and `y` must
605 // outlive the operation you are performing.
606 let lhs_ty = self.resolve_expr_type_adjusted(&lhs);
607 let rhs_ty = self.resolve_expr_type_adjusted(&rhs);
608 for &ty in &[lhs_ty, rhs_ty] {
609 self.type_must_outlive(infer::Operand(expr.span), ty, expr_region);
611 intravisit::walk_expr(self, expr);
614 hir::ExprKind::Unary(hir::UnDeref, ref base) => {
615 // For *a, the lifetime of a must enclose the deref
617 self.constrain_call(expr, Some(base), None::<hir::Expr>.iter());
619 // For overloaded derefs, base_ty is the input to `Deref::deref`,
620 // but it's a reference type uing the same region as the output.
621 let base_ty = self.resolve_expr_type_adjusted(base);
622 if let ty::Ref(r_ptr, _, _) = base_ty.sty {
623 self.mk_subregion_due_to_dereference(expr.span, expr_region, r_ptr);
626 intravisit::walk_expr(self, expr);
629 hir::ExprKind::Unary(_, ref lhs) if is_method_call => {
631 self.constrain_call(expr, Some(&lhs), None::<hir::Expr>.iter());
633 intravisit::walk_expr(self, expr);
636 hir::ExprKind::Index(ref vec_expr, _) => {
637 // For a[b], the lifetime of a must enclose the deref
638 let vec_type = self.resolve_expr_type_adjusted(&vec_expr);
639 self.constrain_index(expr, vec_type);
641 intravisit::walk_expr(self, expr);
644 hir::ExprKind::Cast(ref source, _) => {
645 // Determine if we are casting `source` to a trait
646 // instance. If so, we have to be sure that the type of
647 // the source obeys the trait's region bound.
648 self.constrain_cast(expr, &source);
649 intravisit::walk_expr(self, expr);
652 hir::ExprKind::AddrOf(m, ref base) => {
653 self.link_addr_of(expr, m, &base);
655 // Require that when you write a `&expr` expression, the
656 // resulting pointer has a lifetime that encompasses the
657 // `&expr` expression itself. Note that we constraining
658 // the type of the node expr.id here *before applying
661 // FIXME(https://github.com/rust-lang/rfcs/issues/811)
662 // nested method calls requires that this rule change
663 let ty0 = self.resolve_node_type(expr.hir_id);
664 self.type_must_outlive(infer::AddrOf(expr.span), ty0, expr_region);
665 intravisit::walk_expr(self, expr);
668 hir::ExprKind::Match(ref discr, ref arms, _) => {
669 self.link_match(&discr, &arms[..]);
671 intravisit::walk_expr(self, expr);
674 hir::ExprKind::Closure(.., body_id, _, _) => {
675 self.check_expr_fn_block(expr, body_id);
678 hir::ExprKind::Loop(ref body, _, _) => {
679 let repeating_scope = self.set_repeating_scope(body.hir_id);
680 intravisit::walk_expr(self, expr);
681 self.set_repeating_scope(repeating_scope);
684 hir::ExprKind::While(ref cond, ref body, _) => {
685 let repeating_scope = self.set_repeating_scope(cond.hir_id);
686 self.visit_expr(&cond);
688 self.set_repeating_scope(body.hir_id);
689 self.visit_block(&body);
691 self.set_repeating_scope(repeating_scope);
694 hir::ExprKind::Ret(Some(ref ret_expr)) => {
695 let call_site_scope = self.call_site_scope;
697 "visit_expr ExprKind::Ret ret_expr.hir_id {} call_site_scope: {:?}",
698 ret_expr.hir_id, call_site_scope
700 let call_site_region = self.tcx.mk_region(ty::ReScope(call_site_scope.unwrap()));
701 self.type_of_node_must_outlive(
702 infer::CallReturn(ret_expr.span),
706 intravisit::walk_expr(self, expr);
710 intravisit::walk_expr(self, expr);
716 impl<'a, 'gcx, 'tcx> RegionCtxt<'a, 'gcx, 'tcx> {
717 fn constrain_cast(&mut self, cast_expr: &hir::Expr, source_expr: &hir::Expr) {
719 "constrain_cast(cast_expr={:?}, source_expr={:?})",
720 cast_expr, source_expr
723 let source_ty = self.resolve_node_type(source_expr.hir_id);
724 let target_ty = self.resolve_node_type(cast_expr.hir_id);
726 self.walk_cast(cast_expr, source_ty, target_ty);
729 fn walk_cast(&mut self, cast_expr: &hir::Expr, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) {
730 debug!("walk_cast(from_ty={:?}, to_ty={:?})", from_ty, to_ty);
731 match (&from_ty.sty, &to_ty.sty) {
733 (&ty::Ref(from_r, from_ty, _), /*To: */ &ty::Ref(to_r, to_ty, _)) => {
734 // Target cannot outlive source, naturally.
735 self.sub_regions(infer::Reborrow(cast_expr.span), to_r, from_r);
736 self.walk_cast(cast_expr, from_ty, to_ty);
740 (_, /*To: */ &ty::Dynamic(.., r)) => {
741 // When T is existentially quantified as a trait
742 // `Foo+'to`, it must outlive the region bound `'to`.
743 self.type_must_outlive(infer::RelateObjectBound(cast_expr.span), from_ty, r);
747 (&ty::Adt(from_def, _), /*To: */ &ty::Adt(to_def, _))
748 if from_def.is_box() && to_def.is_box() =>
750 self.walk_cast(cast_expr, from_ty.boxed_ty(), to_ty.boxed_ty());
757 fn check_expr_fn_block(&mut self, expr: &'gcx hir::Expr, body_id: hir::BodyId) {
758 let repeating_scope = self.set_repeating_scope(body_id.hir_id);
759 intravisit::walk_expr(self, expr);
760 self.set_repeating_scope(repeating_scope);
763 fn constrain_callee(&mut self, callee_expr: &hir::Expr) {
764 let callee_ty = self.resolve_node_type(callee_expr.hir_id);
765 match callee_ty.sty {
766 ty::FnDef(..) | ty::FnPtr(_) => {}
768 // this should not happen, but it does if the program is
773 // "Calling non-function: {}",
779 fn constrain_call<'b, I: Iterator<Item = &'b hir::Expr>>(
781 call_expr: &hir::Expr,
782 receiver: Option<&hir::Expr>,
785 //! Invoked on every call site (i.e., normal calls, method calls,
786 //! and overloaded operators). Constrains the regions which appear
787 //! in the type of the function. Also constrains the regions that
788 //! appear in the arguments appropriately.
791 "constrain_call(call_expr={:?}, receiver={:?})",
795 // `callee_region` is the scope representing the time in which the
798 // FIXME(#6268) to support nested method calls, should be callee_id
799 let callee_scope = region::Scope {
800 id: call_expr.hir_id.local_id,
801 data: region::ScopeData::Node,
803 let callee_region = self.tcx.mk_region(ty::ReScope(callee_scope));
805 debug!("callee_region={:?}", callee_region);
807 for arg_expr in arg_exprs {
808 debug!("Argument: {:?}", arg_expr);
810 // ensure that any regions appearing in the argument type are
811 // valid for at least the lifetime of the function:
812 self.type_of_node_must_outlive(
813 infer::CallArg(arg_expr.span),
819 // as loop above, but for receiver
820 if let Some(r) = receiver {
821 debug!("receiver: {:?}", r);
822 self.type_of_node_must_outlive(infer::CallRcvr(r.span), r.hir_id, callee_region);
826 /// Creates a temporary `MemCategorizationContext` and pass it to the closure.
827 fn with_mc<F, R>(&self, f: F) -> R
829 F: for<'b> FnOnce(mc::MemCategorizationContext<'b, 'gcx, 'tcx>) -> R,
831 f(mc::MemCategorizationContext::with_infer(
833 &self.region_scope_tree,
834 &self.tables.borrow(),
838 /// Invoked on any adjustments that occur. Checks that if this is a region pointer being
839 /// dereferenced, the lifetime of the pointer includes the deref expr.
840 fn constrain_adjustments(&mut self, expr: &hir::Expr) -> mc::McResult<mc::cmt_<'tcx>> {
841 debug!("constrain_adjustments(expr={:?})", expr);
843 let mut cmt = self.with_mc(|mc| mc.cat_expr_unadjusted(expr))?;
845 let tables = self.tables.borrow();
846 let adjustments = tables.expr_adjustments(&expr);
847 if adjustments.is_empty() {
851 debug!("constrain_adjustments: adjustments={:?}", adjustments);
853 // If necessary, constrain destructors in the unadjusted form of this
855 self.check_safety_of_rvalue_destructor_if_necessary(&cmt, expr.span);
857 let expr_region = self.tcx.mk_region(ty::ReScope(region::Scope {
858 id: expr.hir_id.local_id,
859 data: region::ScopeData::Node,
861 for adjustment in adjustments {
863 "constrain_adjustments: adjustment={:?}, cmt={:?}",
867 if let adjustment::Adjust::Deref(Some(deref)) = adjustment.kind {
868 debug!("constrain_adjustments: overloaded deref: {:?}", deref);
870 // Treat overloaded autoderefs as if an AutoBorrow adjustment
871 // was applied on the base type, as that is always the case.
872 let input = self.tcx.mk_ref(
879 let output = self.tcx.mk_ref(
882 ty: adjustment.target,
890 ty::BorrowKind::from_mutbl(deref.mutbl),
894 // Specialized version of constrain_call.
895 self.type_must_outlive(infer::CallRcvr(expr.span), input, expr_region);
896 self.type_must_outlive(infer::CallReturn(expr.span), output, expr_region);
899 if let adjustment::Adjust::Borrow(ref autoref) = adjustment.kind {
900 self.link_autoref(expr, &cmt, autoref);
902 // Require that the resulting region encompasses
905 // FIXME(#6268) remove to support nested method calls
906 self.type_of_node_must_outlive(
907 infer::AutoBorrow(expr.span),
913 cmt = self.with_mc(|mc| mc.cat_expr_adjusted(expr, cmt, &adjustment))?;
915 if let Categorization::Deref(_, mc::BorrowedPtr(_, r_ptr)) = cmt.cat {
916 self.mk_subregion_due_to_dereference(expr.span, expr_region, r_ptr);
923 pub fn mk_subregion_due_to_dereference(
926 minimum_lifetime: ty::Region<'tcx>,
927 maximum_lifetime: ty::Region<'tcx>,
930 infer::DerefPointer(deref_span),
936 fn check_safety_of_rvalue_destructor_if_necessary(&mut self, cmt: &mc::cmt_<'tcx>, span: Span) {
937 if let Categorization::Rvalue(region) = cmt.cat {
939 ty::ReScope(rvalue_scope) => {
940 let typ = self.resolve_type(cmt.ty);
941 let body_id = self.body_id;
942 let _ = dropck::check_safety_of_destructor_if_necessary(
954 "unexpected rvalue region in rvalue \
955 destructor safety checking: `{:?}`",
963 /// Invoked on any index expression that occurs. Checks that if this is a slice
964 /// being indexed, the lifetime of the pointer includes the deref expr.
965 fn constrain_index(&mut self, index_expr: &hir::Expr, indexed_ty: Ty<'tcx>) {
967 "constrain_index(index_expr=?, indexed_ty={}",
968 self.ty_to_string(indexed_ty)
971 let r_index_expr = ty::ReScope(region::Scope {
972 id: index_expr.hir_id.local_id,
973 data: region::ScopeData::Node,
975 if let ty::Ref(r_ptr, r_ty, _) = indexed_ty.sty {
977 ty::Slice(_) | ty::Str => {
979 infer::IndexSlice(index_expr.span),
980 self.tcx.mk_region(r_index_expr),
989 /// Guarantees that any lifetimes which appear in the type of the node `id` (after applying
990 /// adjustments) are valid for at least `minimum_lifetime`
991 fn type_of_node_must_outlive(
993 origin: infer::SubregionOrigin<'tcx>,
995 minimum_lifetime: ty::Region<'tcx>,
997 // Try to resolve the type. If we encounter an error, then typeck
998 // is going to fail anyway, so just stop here and let typeck
999 // report errors later on in the writeback phase.
1000 let ty0 = self.resolve_node_type(hir_id);
1002 let ty = self.tables
1006 .and_then(|adj| adj.last())
1007 .map_or(ty0, |adj| adj.target);
1008 let ty = self.resolve_type(ty);
1010 "constrain_regions_in_type_of_node(\
1011 ty={}, ty0={}, id={:?}, minimum_lifetime={:?})",
1012 ty, ty0, hir_id, minimum_lifetime
1014 self.type_must_outlive(origin, ty, minimum_lifetime);
1017 /// Adds constraints to inference such that `T: 'a` holds (or
1018 /// reports an error if it cannot).
1022 /// - `origin`, the reason we need this constraint
1023 /// - `ty`, the type `T`
1024 /// - `region`, the region `'a`
1025 pub fn type_must_outlive(
1027 origin: infer::SubregionOrigin<'tcx>,
1029 region: ty::Region<'tcx>,
1031 self.infcx.register_region_obligation(
1041 /// Computes the guarantor for an expression `&base` and then ensures that the lifetime of the
1042 /// resulting pointer is linked to the lifetime of its guarantor (if any).
1043 fn link_addr_of(&mut self, expr: &hir::Expr, mutability: hir::Mutability, base: &hir::Expr) {
1044 debug!("link_addr_of(expr={:?}, base={:?})", expr, base);
1046 let cmt = ignore_err!(self.with_mc(|mc| mc.cat_expr(base)));
1048 debug!("link_addr_of: cmt={:?}", cmt);
1050 self.link_region_from_node_type(expr.span, expr.hir_id, mutability, &cmt);
1053 /// Computes the guarantors for any ref bindings in a `let` and
1054 /// then ensures that the lifetime of the resulting pointer is
1055 /// linked to the lifetime of the initialization expression.
1056 fn link_local(&self, local: &hir::Local) {
1057 debug!("regionck::for_local()");
1058 let init_expr = match local.init {
1062 Some(ref expr) => &**expr,
1064 let discr_cmt = Rc::new(ignore_err!(self.with_mc(|mc| mc.cat_expr(init_expr))));
1065 self.link_pattern(discr_cmt, &local.pat);
1068 /// Computes the guarantors for any ref bindings in a match and
1069 /// then ensures that the lifetime of the resulting pointer is
1070 /// linked to the lifetime of its guarantor (if any).
1071 fn link_match(&self, discr: &hir::Expr, arms: &[hir::Arm]) {
1072 debug!("regionck::for_match()");
1073 let discr_cmt = Rc::new(ignore_err!(self.with_mc(|mc| mc.cat_expr(discr))));
1074 debug!("discr_cmt={:?}", discr_cmt);
1076 for root_pat in &arm.pats {
1077 self.link_pattern(discr_cmt.clone(), &root_pat);
1082 /// Computes the guarantors for any ref bindings in a match and
1083 /// then ensures that the lifetime of the resulting pointer is
1084 /// linked to the lifetime of its guarantor (if any).
1085 fn link_fn_args(&self, body_scope: region::Scope, args: &[hir::Arg]) {
1086 debug!("regionck::link_fn_args(body_scope={:?})", body_scope);
1088 let arg_ty = self.node_ty(arg.hir_id);
1089 let re_scope = self.tcx.mk_region(ty::ReScope(body_scope));
1090 let arg_cmt = self.with_mc(|mc| {
1091 Rc::new(mc.cat_rvalue(arg.hir_id, arg.pat.span, re_scope, arg_ty))
1093 debug!("arg_ty={:?} arg_cmt={:?} arg={:?}", arg_ty, arg_cmt, arg);
1094 self.link_pattern(arg_cmt, &arg.pat);
1098 /// Link lifetimes of any ref bindings in `root_pat` to the pointers found
1099 /// in the discriminant, if needed.
1100 fn link_pattern(&self, discr_cmt: mc::cmt<'tcx>, root_pat: &hir::Pat) {
1102 "link_pattern(discr_cmt={:?}, root_pat={:?})",
1105 ignore_err!(self.with_mc(|mc| {
1106 mc.cat_pattern(discr_cmt, root_pat, |sub_cmt, sub_pat| {
1108 if let PatKind::Binding(..) = sub_pat.node {
1109 if let Some(&bm) = mc.tables.pat_binding_modes().get(sub_pat.hir_id) {
1110 if let ty::BindByReference(mutbl) = bm {
1111 self.link_region_from_node_type(
1121 .delay_span_bug(sub_pat.span, "missing binding mode");
1128 /// Link lifetime of borrowed pointer resulting from autoref to lifetimes in the value being
1133 expr_cmt: &mc::cmt_<'tcx>,
1134 autoref: &adjustment::AutoBorrow<'tcx>,
1137 "link_autoref(autoref={:?}, expr_cmt={:?})",
1142 adjustment::AutoBorrow::Ref(r, m) => {
1143 self.link_region(expr.span, r, ty::BorrowKind::from_mutbl(m.into()), expr_cmt);
1146 adjustment::AutoBorrow::RawPtr(m) => {
1147 let r = self.tcx.mk_region(ty::ReScope(region::Scope {
1148 id: expr.hir_id.local_id,
1149 data: region::ScopeData::Node,
1151 self.link_region(expr.span, r, ty::BorrowKind::from_mutbl(m), expr_cmt);
1156 /// Like `link_region()`, except that the region is extracted from the type of `id`,
1157 /// which must be some reference (`&T`, `&str`, etc).
1158 fn link_region_from_node_type(
1162 mutbl: hir::Mutability,
1163 cmt_borrowed: &mc::cmt_<'tcx>,
1166 "link_region_from_node_type(id={:?}, mutbl={:?}, cmt_borrowed={:?})",
1167 id, mutbl, cmt_borrowed
1170 let rptr_ty = self.resolve_node_type(id);
1171 if let ty::Ref(r, _, _) = rptr_ty.sty {
1172 debug!("rptr_ty={}", rptr_ty);
1173 self.link_region(span, r, ty::BorrowKind::from_mutbl(mutbl), cmt_borrowed);
1177 /// Informs the inference engine that `borrow_cmt` is being borrowed with
1178 /// kind `borrow_kind` and lifetime `borrow_region`.
1179 /// In order to ensure borrowck is satisfied, this may create constraints
1180 /// between regions, as explained in `link_reborrowed_region()`.
1184 borrow_region: ty::Region<'tcx>,
1185 borrow_kind: ty::BorrowKind,
1186 borrow_cmt: &mc::cmt_<'tcx>,
1188 let origin = infer::DataBorrowed(borrow_cmt.ty, span);
1189 self.type_must_outlive(origin, borrow_cmt.ty, borrow_region);
1191 let mut borrow_kind = borrow_kind;
1192 let mut borrow_cmt_cat = borrow_cmt.cat.clone();
1196 "link_region(borrow_region={:?}, borrow_kind={:?}, borrow_cmt={:?})",
1197 borrow_region, borrow_kind, borrow_cmt
1199 match borrow_cmt_cat {
1200 Categorization::Deref(ref_cmt, mc::BorrowedPtr(ref_kind, ref_region)) => {
1201 match self.link_reborrowed_region(
1211 borrow_cmt_cat = c.cat.clone();
1220 Categorization::Downcast(cmt_base, _)
1221 | Categorization::Deref(cmt_base, mc::Unique)
1222 | Categorization::Interior(cmt_base, _) => {
1223 // Borrowing interior or owned data requires the base
1224 // to be valid and borrowable in the same fashion.
1225 borrow_cmt_cat = cmt_base.cat.clone();
1226 borrow_kind = borrow_kind;
1229 Categorization::Deref(_, mc::UnsafePtr(..))
1230 | Categorization::StaticItem
1231 | Categorization::Upvar(..)
1232 | Categorization::Local(..)
1233 | Categorization::ThreadLocal(..)
1234 | Categorization::Rvalue(..) => {
1235 // These are all "base cases" with independent lifetimes
1236 // that are not subject to inference
1243 /// This is the most complicated case: the path being borrowed is
1244 /// itself the referent of a borrowed pointer. Let me give an
1245 /// example fragment of code to make clear(er) the situation:
1247 /// let r: &'a mut T = ...; // the original reference "r" has lifetime 'a
1249 /// &'z *r // the reborrow has lifetime 'z
1251 /// Now, in this case, our primary job is to add the inference
1252 /// constraint that `'z <= 'a`. Given this setup, let's clarify the
1253 /// parameters in (roughly) terms of the example:
1255 /// ```plain,ignore (pseudo-Rust)
1256 /// A borrow of: `& 'z bk * r` where `r` has type `& 'a bk T`
1257 /// borrow_region ^~ ref_region ^~
1258 /// borrow_kind ^~ ref_kind ^~
1262 /// Here `bk` stands for some borrow-kind (e.g., `mut`, `uniq`, etc).
1264 /// Unfortunately, there are some complications beyond the simple
1265 /// scenario I just painted:
1267 /// 1. The reference `r` might in fact be a "by-ref" upvar. In that
1268 /// case, we have two jobs. First, we are inferring whether this reference
1269 /// should be an `&T`, `&mut T`, or `&uniq T` reference, and we must
1270 /// adjust that based on this borrow (e.g., if this is an `&mut` borrow,
1271 /// then `r` must be an `&mut` reference). Second, whenever we link
1272 /// two regions (here, `'z <= 'a`), we supply a *cause*, and in this
1273 /// case we adjust the cause to indicate that the reference being
1274 /// "reborrowed" is itself an upvar. This provides a nicer error message
1275 /// should something go wrong.
1277 /// 2. There may in fact be more levels of reborrowing. In the
1278 /// example, I said the borrow was like `&'z *r`, but it might
1279 /// in fact be a borrow like `&'z **q` where `q` has type `&'a
1280 /// &'b mut T`. In that case, we want to ensure that `'z <= 'a`
1281 /// and `'z <= 'b`. This is explained more below.
1283 /// The return value of this function indicates whether we need to
1284 /// recurse and process `ref_cmt` (see case 2 above).
1285 fn link_reborrowed_region(
1288 borrow_region: ty::Region<'tcx>,
1289 borrow_kind: ty::BorrowKind,
1290 ref_cmt: mc::cmt<'tcx>,
1291 ref_region: ty::Region<'tcx>,
1292 mut ref_kind: ty::BorrowKind,
1294 ) -> Option<(mc::cmt<'tcx>, ty::BorrowKind)> {
1295 // Possible upvar ID we may need later to create an entry in the
1298 // Detect by-ref upvar `x`:
1299 let cause = match note {
1300 mc::NoteUpvarRef(ref upvar_id) => {
1301 match self.tables.borrow().upvar_capture_map.get(upvar_id) {
1302 Some(&ty::UpvarCapture::ByRef(ref upvar_borrow)) => {
1303 // The mutability of the upvar may have been modified
1304 // by the above adjustment, so update our local variable.
1305 ref_kind = upvar_borrow.kind;
1307 infer::ReborrowUpvar(span, *upvar_id)
1310 span_bug!(span, "Illegal upvar id: {:?}", upvar_id);
1314 mc::NoteClosureEnv(ref upvar_id) => {
1315 // We don't have any mutability changes to propagate, but
1316 // we do want to note that an upvar reborrow caused this
1318 infer::ReborrowUpvar(span, *upvar_id)
1320 _ => infer::Reborrow(span),
1324 "link_reborrowed_region: {:?} <= {:?}",
1325 borrow_region, ref_region
1327 self.sub_regions(cause, borrow_region, ref_region);
1329 // If we end up needing to recurse and establish a region link
1330 // with `ref_cmt`, calculate what borrow kind we will end up
1331 // needing. This will be used below.
1333 // One interesting twist is that we can weaken the borrow kind
1334 // when we recurse: to reborrow an `&mut` referent as mutable,
1335 // borrowck requires a unique path to the `&mut` reference but not
1336 // necessarily a *mutable* path.
1337 let new_borrow_kind = match borrow_kind {
1338 ty::ImmBorrow => ty::ImmBorrow,
1339 ty::MutBorrow | ty::UniqueImmBorrow => ty::UniqueImmBorrow,
1342 // Decide whether we need to recurse and link any regions within
1343 // the `ref_cmt`. This is concerned for the case where the value
1344 // being reborrowed is in fact a borrowed pointer found within
1345 // another borrowed pointer. For example:
1347 // let p: &'b &'a mut T = ...;
1351 // What makes this case particularly tricky is that, if the data
1352 // being borrowed is a `&mut` or `&uniq` borrow, borrowck requires
1353 // not only that `'z <= 'a`, (as before) but also `'z <= 'b`
1354 // (otherwise the user might mutate through the `&mut T` reference
1355 // after `'b` expires and invalidate the borrow we are looking at
1358 // So let's re-examine our parameters in light of this more
1359 // complicated (possible) scenario:
1361 // A borrow of: `& 'z bk * * p` where `p` has type `&'b bk & 'a bk T`
1362 // borrow_region ^~ ref_region ^~
1363 // borrow_kind ^~ ref_kind ^~
1366 // (Note that since we have not examined `ref_cmt.cat`, we don't
1367 // know whether this scenario has occurred; but I wanted to show
1368 // how all the types get adjusted.)
1371 // The reference being reborrowed is a shareable ref of
1372 // type `&'a T`. In this case, it doesn't matter where we
1373 // *found* the `&T` pointer, the memory it references will
1374 // be valid and immutable for `'a`. So we can stop here.
1376 // (Note that the `borrow_kind` must also be ImmBorrow or
1377 // else the user is borrowed imm memory as mut memory,
1378 // which means they'll get an error downstream in borrowck
1383 ty::MutBorrow | ty::UniqueImmBorrow => {
1384 // The reference being reborrowed is either an `&mut T` or
1385 // `&uniq T`. This is the case where recursion is needed.
1386 return Some((ref_cmt, new_borrow_kind));
1391 /// Checks that the values provided for type/region arguments in a given
1392 /// expression are well-formed and in-scope.
1393 fn substs_wf_in_scope(
1395 origin: infer::ParameterOrigin,
1396 substs: SubstsRef<'tcx>,
1398 expr_region: ty::Region<'tcx>,
1401 "substs_wf_in_scope(substs={:?}, \
1405 substs, expr_region, origin, expr_span
1408 let origin = infer::ParameterInScope(origin, expr_span);
1410 for kind in substs {
1411 match kind.unpack() {
1412 UnpackedKind::Lifetime(lt) => {
1413 self.sub_regions(origin.clone(), expr_region, lt);
1415 UnpackedKind::Type(ty) => {
1416 let ty = self.resolve_type(ty);
1417 self.type_must_outlive(origin.clone(), ty, expr_region);
1419 UnpackedKind::Const(_) => {
1420 // Const parameters don't impose constraints.