1 // Copyright 2012-2014 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 //! The region check is a final pass that runs over the AST after we have
12 //! inferred the type constraints but before we have actually finalized
13 //! the types. Its purpose is to embed a variety of region constraints.
14 //! Inserting these constraints as a separate pass is good because (1) it
15 //! localizes the code that has to do with region inference and (2) often
16 //! we cannot know what constraints are needed until the basic types have
19 //! ### Interaction with the borrow checker
21 //! In general, the job of the borrowck module (which runs later) is to
22 //! check that all soundness criteria are met, given a particular set of
23 //! regions. The job of *this* module is to anticipate the needs of the
24 //! borrow checker and infer regions that will satisfy its requirements.
25 //! It is generally true that the inference doesn't need to be sound,
26 //! meaning that if there is a bug and we inferred bad regions, the borrow
27 //! checker should catch it. This is not entirely true though; for
28 //! example, the borrow checker doesn't check subtyping, and it doesn't
29 //! check that region pointers are always live when they are used. It
30 //! might be worthwhile to fix this so that borrowck serves as a kind of
31 //! verification step -- that would add confidence in the overall
32 //! correctness of the compiler, at the cost of duplicating some type
33 //! checks and effort.
35 //! ### Inferring the duration of borrows, automatic and otherwise
37 //! Whenever we introduce a borrowed pointer, for example as the result of
38 //! a borrow expression `let x = &data`, the lifetime of the pointer `x`
39 //! is always specified as a region inference variable. `regionck` has the
40 //! job of adding constraints such that this inference variable is as
41 //! narrow as possible while still accommodating all uses (that is, every
42 //! dereference of the resulting pointer must be within the lifetime).
46 //! Generally speaking, `regionck` does NOT try to ensure that the data
47 //! `data` will outlive the pointer `x`. That is the job of borrowck. The
48 //! one exception is when "re-borrowing" the contents of another borrowed
49 //! pointer. For example, imagine you have a borrowed pointer `b` with
50 //! lifetime L1 and you have an expression `&*b`. The result of this
51 //! expression will be another borrowed pointer with lifetime L2 (which is
52 //! an inference variable). The borrow checker is going to enforce the
53 //! constraint that L2 < L1, because otherwise you are re-borrowing data
54 //! for a lifetime larger than the original loan. However, without the
55 //! routines in this module, the region inferencer would not know of this
56 //! dependency and thus it might infer the lifetime of L2 to be greater
57 //! than L1 (issue #3148).
59 //! There are a number of troublesome scenarios in the tests
60 //! `region-dependent-*.rs`, but here is one example:
62 //! struct Foo { i: i32 }
63 //! struct Bar { foo: Foo }
64 //! fn get_i<'a>(x: &'a Bar) -> &'a i32 {
65 //! let foo = &x.foo; // Lifetime L1
66 //! &foo.i // Lifetime L2
69 //! Note that this comes up either with `&` expressions, `ref`
70 //! bindings, and `autorefs`, which are the three ways to introduce
73 //! The key point here is that when you are borrowing a value that
74 //! is "guaranteed" by a borrowed pointer, you must link the
75 //! lifetime of that borrowed pointer (L1, here) to the lifetime of
76 //! the borrow itself (L2). What do I mean by "guaranteed" by a
77 //! borrowed pointer? I mean any data that is reached by first
78 //! dereferencing a borrowed pointer and then either traversing
79 //! interior offsets or boxes. We say that the guarantor
80 //! of such data is the region of the borrowed pointer that was
81 //! traversed. This is essentially the same as the ownership
82 //! relation, except that a borrowed pointer never owns its
87 use middle::free_region::FreeRegionMap;
88 use middle::mem_categorization as mc;
89 use middle::mem_categorization::Categorization;
90 use middle::region::{CodeExtent, RegionMaps};
91 use rustc::hir::def_id::DefId;
92 use rustc::ty::subst::Substs;
94 use rustc::ty::{self, Ty, TypeFoldable};
95 use rustc::infer::{self, GenericKind, SubregionOrigin, VerifyBound};
96 use rustc::ty::adjustment;
97 use rustc::ty::wf::ImpliedBound;
103 use syntax_pos::Span;
104 use rustc::hir::intravisit::{self, Visitor, NestedVisitorMap};
105 use rustc::hir::{self, PatKind};
107 // a variation on try that just returns unit
108 macro_rules! ignore_err {
109 ($e:expr) => (match $e { Ok(e) => e, Err(_) => return () })
112 ///////////////////////////////////////////////////////////////////////////
113 // PUBLIC ENTRY POINTS
115 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
116 pub fn regionck_expr(&self, body: &'gcx hir::Body) {
117 let subject = self.tcx.hir.body_owner_def_id(body.id());
118 let id = body.value.id;
119 let mut rcx = RegionCtxt::new(self, RepeatingScope(id), id, Subject(subject));
120 if self.err_count_since_creation() == 0 {
121 // regionck assumes typeck succeeded
122 rcx.visit_body(body);
123 rcx.visit_region_obligations(id);
125 rcx.resolve_regions_and_report_errors();
127 assert!(self.tables.borrow().free_region_map.is_empty());
128 self.tables.borrow_mut().free_region_map = rcx.free_region_map;
131 /// Region checking during the WF phase for items. `wf_tys` are the
132 /// types from which we should derive implied bounds, if any.
133 pub fn regionck_item(&self,
134 item_id: ast::NodeId,
136 wf_tys: &[Ty<'tcx>]) {
137 debug!("regionck_item(item.id={:?}, wf_tys={:?}", item_id, wf_tys);
138 let subject = self.tcx.hir.local_def_id(item_id);
139 let mut rcx = RegionCtxt::new(self, RepeatingScope(item_id), item_id, Subject(subject));
140 rcx.free_region_map.relate_free_regions_from_predicates(
141 &self.param_env.caller_bounds);
142 rcx.relate_free_regions(wf_tys, item_id, span);
143 rcx.visit_region_obligations(item_id);
144 rcx.resolve_regions_and_report_errors();
147 pub fn regionck_fn(&self,
149 body: &'gcx hir::Body) {
150 debug!("regionck_fn(id={})", fn_id);
151 let subject = self.tcx.hir.body_owner_def_id(body.id());
152 let node_id = body.value.id;
153 let mut rcx = RegionCtxt::new(self, RepeatingScope(node_id), node_id, Subject(subject));
155 if self.err_count_since_creation() == 0 {
156 // regionck assumes typeck succeeded
157 rcx.visit_fn_body(fn_id, body, self.tcx.hir.span(fn_id));
160 rcx.free_region_map.relate_free_regions_from_predicates(
161 &self.param_env.caller_bounds);
163 rcx.resolve_regions_and_report_errors();
165 // In this mode, we also copy the free-region-map into the
166 // tables of the enclosing fcx. In the other regionck modes
167 // (e.g., `regionck_item`), we don't have an enclosing tables.
168 assert!(self.tables.borrow().free_region_map.is_empty());
169 self.tables.borrow_mut().free_region_map = rcx.free_region_map;
173 ///////////////////////////////////////////////////////////////////////////
176 pub struct RegionCtxt<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
177 pub fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
179 region_bound_pairs: Vec<(ty::Region<'tcx>, GenericKind<'tcx>)>,
181 pub region_maps: Rc<RegionMaps>,
183 free_region_map: FreeRegionMap<'tcx>,
185 // id of innermost fn body id
186 body_id: ast::NodeId,
188 // call_site scope of innermost fn
189 call_site_scope: Option<CodeExtent>,
191 // id of innermost fn or loop
192 repeating_scope: ast::NodeId,
194 // id of AST node being analyzed (the subject of the analysis).
195 subject_def_id: DefId,
199 impl<'a, 'gcx, 'tcx> Deref for RegionCtxt<'a, 'gcx, 'tcx> {
200 type Target = FnCtxt<'a, 'gcx, 'tcx>;
201 fn deref(&self) -> &Self::Target {
206 pub struct RepeatingScope(ast::NodeId);
207 pub struct Subject(DefId);
209 impl<'a, 'gcx, 'tcx> RegionCtxt<'a, 'gcx, 'tcx> {
210 pub fn new(fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
211 RepeatingScope(initial_repeating_scope): RepeatingScope,
212 initial_body_id: ast::NodeId,
213 Subject(subject): Subject) -> RegionCtxt<'a, 'gcx, 'tcx> {
214 let region_maps = fcx.tcx.region_maps(subject);
217 region_maps: region_maps,
218 repeating_scope: initial_repeating_scope,
219 body_id: initial_body_id,
220 call_site_scope: None,
221 subject_def_id: subject,
222 region_bound_pairs: Vec::new(),
223 free_region_map: FreeRegionMap::new(),
227 fn set_call_site_scope(&mut self, call_site_scope: Option<CodeExtent>)
228 -> Option<CodeExtent> {
229 mem::replace(&mut self.call_site_scope, call_site_scope)
232 fn set_body_id(&mut self, body_id: ast::NodeId) -> ast::NodeId {
233 mem::replace(&mut self.body_id, body_id)
236 fn set_repeating_scope(&mut self, scope: ast::NodeId) -> ast::NodeId {
237 mem::replace(&mut self.repeating_scope, scope)
240 /// Try to resolve the type for the given node, returning t_err if an error results. Note that
241 /// we never care about the details of the error, the same error will be detected and reported
242 /// in the writeback phase.
244 /// Note one important point: we do not attempt to resolve *region variables* here. This is
245 /// because regionck is essentially adding constraints to those region variables and so may yet
246 /// influence how they are resolved.
248 /// Consider this silly example:
251 /// fn borrow(x: &i32) -> &i32 {x}
252 /// fn foo(x: @i32) -> i32 { // block: B
253 /// let b = borrow(x); // region: <R0>
258 /// Here, the region of `b` will be `<R0>`. `<R0>` is constrained to be some subregion of the
259 /// block B and some superregion of the call. If we forced it now, we'd choose the smaller
260 /// region (the call). But that would make the *b illegal. Since we don't resolve, the type
261 /// of b will be `&<R0>.i32` and then `*b` will require that `<R0>` be bigger than the let and
262 /// the `*b` expression, so we will effectively resolve `<R0>` to be the block B.
263 pub fn resolve_type(&self, unresolved_ty: Ty<'tcx>) -> Ty<'tcx> {
264 self.resolve_type_vars_if_possible(&unresolved_ty)
267 /// Try to resolve the type for the given node.
268 fn resolve_node_type(&self, id: ast::NodeId) -> Ty<'tcx> {
269 let t = self.node_ty(id);
273 /// Try to resolve the type for the given node.
274 pub fn resolve_expr_type_adjusted(&mut self, expr: &hir::Expr) -> Ty<'tcx> {
275 let ty = self.tables.borrow().expr_ty_adjusted(expr);
276 self.resolve_type(ty)
279 fn visit_fn_body(&mut self,
280 id: ast::NodeId, // the id of the fn itself
281 body: &'gcx hir::Body,
284 // When we enter a function, we can derive
285 debug!("visit_fn_body(id={})", id);
287 let body_id = body.id();
289 let call_site = CodeExtent::CallSiteScope(body_id);
290 let old_call_site_scope = self.set_call_site_scope(Some(call_site));
293 let fn_sig_map = &self.tables.borrow().liberated_fn_sigs;
294 match fn_sig_map.get(&id) {
295 Some(f) => f.clone(),
297 bug!("No fn-sig entry for id={}", id);
302 let old_region_bounds_pairs_len = self.region_bound_pairs.len();
304 // Collect the types from which we create inferred bounds.
305 // For the return type, if diverging, substitute `bool` just
306 // because it will have no effect.
308 // FIXME(#27579) return types should not be implied bounds
309 let fn_sig_tys: Vec<_> =
310 fn_sig.inputs().iter().cloned().chain(Some(fn_sig.output())).collect();
312 let old_body_id = self.set_body_id(body_id.node_id);
313 self.relate_free_regions(&fn_sig_tys[..], body_id.node_id, span);
314 self.link_fn_args(CodeExtent::Misc(body_id.node_id), &body.arguments);
315 self.visit_body(body);
316 self.visit_region_obligations(body_id.node_id);
318 let call_site_scope = self.call_site_scope.unwrap();
319 debug!("visit_fn_body body.id {:?} call_site_scope: {:?}",
320 body.id(), call_site_scope);
321 let call_site_region = self.tcx.mk_region(ty::ReScope(call_site_scope));
322 self.type_of_node_must_outlive(infer::CallReturn(span),
326 self.region_bound_pairs.truncate(old_region_bounds_pairs_len);
328 self.set_body_id(old_body_id);
329 self.set_call_site_scope(old_call_site_scope);
332 fn visit_region_obligations(&mut self, node_id: ast::NodeId)
334 debug!("visit_region_obligations: node_id={}", node_id);
336 // region checking can introduce new pending obligations
337 // which, when processed, might generate new region
338 // obligations. So make sure we process those.
339 self.select_all_obligations_or_error();
341 // Make a copy of the region obligations vec because we'll need
342 // to be able to borrow the fulfillment-cx below when projecting.
343 let region_obligations =
346 .region_obligations(node_id)
349 for r_o in ®ion_obligations {
350 debug!("visit_region_obligations: r_o={:?} cause={:?}",
352 let sup_type = self.resolve_type(r_o.sup_type);
353 let origin = self.code_to_origin(&r_o.cause, sup_type);
354 self.type_must_outlive(origin, sup_type, r_o.sub_region);
357 // Processing the region obligations should not cause the list to grow further:
358 assert_eq!(region_obligations.len(),
359 self.fulfillment_cx.borrow().region_obligations(node_id).len());
362 fn code_to_origin(&self,
363 cause: &traits::ObligationCause<'tcx>,
365 -> SubregionOrigin<'tcx> {
366 SubregionOrigin::from_obligation_cause(cause,
367 || infer::RelateParamBound(cause.span, sup_type))
370 /// This method populates the region map's `free_region_map`. It walks over the transformed
371 /// argument and return types for each function just before we check the body of that function,
372 /// looking for types where you have a borrowed pointer to other borrowed data (e.g., `&'a &'b
373 /// [usize]`. We do not allow references to outlive the things they point at, so we can assume
374 /// that `'a <= 'b`. This holds for both the argument and return types, basically because, on
375 /// the caller side, the caller is responsible for checking that the type of every expression
376 /// (including the actual values for the arguments, as well as the return type of the fn call)
379 /// Tests: `src/test/compile-fail/regions-free-region-ordering-*.rs`
380 fn relate_free_regions(&mut self,
381 fn_sig_tys: &[Ty<'tcx>],
382 body_id: ast::NodeId,
384 debug!("relate_free_regions >>");
386 for &ty in fn_sig_tys {
387 let ty = self.resolve_type(ty);
388 debug!("relate_free_regions(t={:?})", ty);
389 let implied_bounds = ty::wf::implied_bounds(self, body_id, ty, span);
391 // Record any relations between free regions that we observe into the free-region-map.
392 self.free_region_map.relate_free_regions_from_implied_bounds(&implied_bounds);
394 // But also record other relationships, such as `T:'x`,
395 // that don't go into the free-region-map but which we use
397 for implication in implied_bounds {
398 debug!("implication: {:?}", implication);
400 ImpliedBound::RegionSubRegion(r_a @ &ty::ReEarlyBound(_),
402 ImpliedBound::RegionSubRegion(r_a @ &ty::ReFree(_),
403 &ty::ReVar(vid_b)) => {
404 self.add_given(r_a, vid_b);
406 ImpliedBound::RegionSubParam(r_a, param_b) => {
407 self.region_bound_pairs.push((r_a, GenericKind::Param(param_b)));
409 ImpliedBound::RegionSubProjection(r_a, projection_b) => {
410 self.region_bound_pairs.push((r_a, GenericKind::Projection(projection_b)));
412 ImpliedBound::RegionSubRegion(..) => {
413 // In principle, we could record (and take
414 // advantage of) every relationship here, but
415 // we are also free not to -- it simply means
416 // strictly less that we can successfully type
417 // check. (It may also be that we should
418 // revise our inference system to be more
419 // general and to make use of *every*
420 // relationship that arises here, but
421 // presently we do not.)
427 debug!("<< relate_free_regions");
430 fn resolve_regions_and_report_errors(&self) {
431 self.fcx.resolve_regions_and_report_errors(self.subject_def_id,
433 &self.free_region_map);
436 fn constrain_bindings_in_pat(&mut self, pat: &hir::Pat) {
437 debug!("regionck::visit_pat(pat={:?})", pat);
438 pat.each_binding(|_, id, span, _| {
439 // If we have a variable that contains region'd data, that
440 // data will be accessible from anywhere that the variable is
441 // accessed. We must be wary of loops like this:
443 // // from src/test/compile-fail/borrowck-lend-flow.rs
444 // let mut v = box 3, w = box 4;
445 // let mut x = &mut w;
448 // borrow(v); //~ ERROR cannot borrow
449 // x = &mut v; // (1)
452 // Typically, we try to determine the region of a borrow from
453 // those points where it is dereferenced. In this case, one
454 // might imagine that the lifetime of `x` need only be the
455 // body of the loop. But of course this is incorrect because
456 // the pointer that is created at point (1) is consumed at
457 // point (2), meaning that it must be live across the loop
458 // iteration. The easiest way to guarantee this is to require
459 // that the lifetime of any regions that appear in a
460 // variable's type enclose at least the variable's scope.
462 let var_scope = self.region_maps.var_scope(id);
463 let var_region = self.tcx.mk_region(ty::ReScope(var_scope));
465 let origin = infer::BindingTypeIsNotValidAtDecl(span);
466 self.type_of_node_must_outlive(origin, id, var_region);
468 let typ = self.resolve_node_type(id);
469 let _ = dropck::check_safety_of_destructor_if_necessary(
470 self, typ, span, var_scope);
475 impl<'a, 'gcx, 'tcx> Visitor<'gcx> for RegionCtxt<'a, 'gcx, 'tcx> {
476 // (..) FIXME(#3238) should use visit_pat, not visit_arm/visit_local,
477 // However, right now we run into an issue whereby some free
478 // regions are not properly related if they appear within the
479 // types of arguments that must be inferred. This could be
480 // addressed by deferring the construction of the region
481 // hierarchy, and in particular the relationships between free
482 // regions, until regionck, as described in #3238.
484 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'gcx> {
485 NestedVisitorMap::None
488 fn visit_fn(&mut self, _fk: intravisit::FnKind<'gcx>, _: &'gcx hir::FnDecl,
489 b: hir::BodyId, span: Span, id: ast::NodeId) {
490 let body = self.tcx.hir.body(b);
491 self.visit_fn_body(id, body, span)
494 //visit_pat: visit_pat, // (..) see above
496 fn visit_arm(&mut self, arm: &'gcx hir::Arm) {
499 self.constrain_bindings_in_pat(p);
501 intravisit::walk_arm(self, arm);
504 fn visit_local(&mut self, l: &'gcx hir::Local) {
506 self.constrain_bindings_in_pat(&l.pat);
508 intravisit::walk_local(self, l);
511 fn visit_expr(&mut self, expr: &'gcx hir::Expr) {
512 debug!("regionck::visit_expr(e={:?}, repeating_scope={})",
513 expr, self.repeating_scope);
515 // No matter what, the type of each expression must outlive the
516 // scope of that expression. This also guarantees basic WF.
517 let expr_ty = self.resolve_node_type(expr.id);
518 // the region corresponding to this expression
519 let expr_region = self.tcx.node_scope_region(expr.id);
520 self.type_must_outlive(infer::ExprTypeIsNotInScope(expr_ty, expr.span),
521 expr_ty, expr_region);
523 let is_method_call = self.tables.borrow().is_method_call(expr);
525 // If we are calling a method (either explicitly or via an
526 // overloaded operator), check that all of the types provided as
527 // arguments for its type parameters are well-formed, and all the regions
528 // provided as arguments outlive the call.
530 let origin = match expr.node {
531 hir::ExprMethodCall(..) =>
532 infer::ParameterOrigin::MethodCall,
533 hir::ExprUnary(op, _) if op == hir::UnDeref =>
534 infer::ParameterOrigin::OverloadedDeref,
536 infer::ParameterOrigin::OverloadedOperator
539 let substs = self.tables.borrow().node_substs(expr.id);
540 self.substs_wf_in_scope(origin, substs, expr.span, expr_region);
541 // Arguments (sub-expressions) are checked via `constrain_call`, below.
544 // Check any autoderefs or autorefs that appear.
545 let adjustment = self.tables.borrow().adjustments.get(&expr.id).map(|a| a.clone());
546 if let Some(adjustment) = adjustment {
547 debug!("adjustment={:?}", adjustment);
548 match adjustment.kind {
549 adjustment::Adjust::Deref(ref autoderefs) => {
550 let cmt = ignore_err!(self.constrain_autoderefs(expr, autoderefs));
551 if let Some(ref autoref) = adjustment.autoref {
552 self.link_autoref(expr, cmt, autoref);
554 // Require that the resulting region encompasses
557 // FIXME(#6268) remove to support nested method calls
558 self.type_of_node_must_outlive(infer::AutoBorrow(expr.span),
559 expr.id, expr_region);
563 adjustment::AutoObject(_, ref bounds, ..) => {
564 // Determine if we are casting `expr` to a trait
565 // instance. If so, we have to be sure that the type
566 // of the source obeys the new region bound.
567 let source_ty = self.resolve_node_type(expr.id);
568 self.type_must_outlive(infer::RelateObjectBound(expr.span),
569 source_ty, bounds.region_bound);
573 assert!(adjustment.autoref.is_none());
577 // If necessary, constrain destructors in the unadjusted form of this
580 let mc = mc::MemCategorizationContext::new(self, &self.region_maps);
581 mc.cat_expr_unadjusted(expr)
585 self.check_safety_of_rvalue_destructor_if_necessary(head_cmt,
589 self.tcx.sess.delay_span_bug(expr.span, "cat_expr_unadjusted Errd");
594 // If necessary, constrain destructors in this expression. This will be
595 // the adjusted form if there is an adjustment.
597 let mc = mc::MemCategorizationContext::new(self, &self.region_maps);
602 self.check_safety_of_rvalue_destructor_if_necessary(head_cmt, expr.span);
605 self.tcx.sess.delay_span_bug(expr.span, "cat_expr Errd");
609 debug!("regionck::visit_expr(e={:?}, repeating_scope={}) - visiting subexprs",
610 expr, self.repeating_scope);
612 hir::ExprPath(_) => {
613 let substs = self.tables.borrow().node_substs(expr.id);
614 let origin = infer::ParameterOrigin::Path;
615 self.substs_wf_in_scope(origin, substs, expr.span, expr_region);
618 hir::ExprCall(ref callee, ref args) => {
620 self.constrain_call(expr, Some(&callee),
621 args.iter().map(|e| &*e), false);
623 self.constrain_callee(callee.id, expr, &callee);
624 self.constrain_call(expr, None,
625 args.iter().map(|e| &*e), false);
628 intravisit::walk_expr(self, expr);
631 hir::ExprMethodCall(.., ref args) => {
632 self.constrain_call(expr, Some(&args[0]),
633 args[1..].iter().map(|e| &*e), false);
635 intravisit::walk_expr(self, expr);
638 hir::ExprAssignOp(_, ref lhs, ref rhs) => {
640 self.constrain_call(expr, Some(&lhs),
641 Some(&**rhs).into_iter(), false);
644 intravisit::walk_expr(self, expr);
647 hir::ExprIndex(ref lhs, ref rhs) if is_method_call => {
648 self.constrain_call(expr, Some(&lhs),
649 Some(&**rhs).into_iter(), true);
651 intravisit::walk_expr(self, expr);
654 hir::ExprBinary(op, ref lhs, ref rhs) if is_method_call => {
655 let implicitly_ref_args = !op.node.is_by_value();
657 // As `expr_method_call`, but the call is via an
658 // overloaded op. Note that we (sadly) currently use an
659 // implicit "by ref" sort of passing style here. This
660 // should be converted to an adjustment!
661 self.constrain_call(expr, Some(&lhs),
662 Some(&**rhs).into_iter(), implicitly_ref_args);
664 intravisit::walk_expr(self, expr);
667 hir::ExprBinary(_, ref lhs, ref rhs) => {
668 // If you do `x OP y`, then the types of `x` and `y` must
669 // outlive the operation you are performing.
670 let lhs_ty = self.resolve_expr_type_adjusted(&lhs);
671 let rhs_ty = self.resolve_expr_type_adjusted(&rhs);
672 for &ty in &[lhs_ty, rhs_ty] {
673 self.type_must_outlive(infer::Operand(expr.span),
676 intravisit::walk_expr(self, expr);
679 hir::ExprUnary(op, ref lhs) if is_method_call => {
680 let implicitly_ref_args = !op.is_by_value();
683 self.constrain_call(expr, Some(&lhs),
684 None::<hir::Expr>.iter(), implicitly_ref_args);
686 intravisit::walk_expr(self, expr);
689 hir::ExprUnary(hir::UnDeref, ref base) => {
690 // For *a, the lifetime of a must enclose the deref
691 if self.tables.borrow().is_method_call(expr) {
692 self.constrain_call(expr, Some(base),
693 None::<hir::Expr>.iter(), true);
695 // For overloaded derefs, base_ty is the input to `Deref::deref`,
696 // but it's a reference type uing the same region as the output.
697 let base_ty = self.resolve_expr_type_adjusted(base);
698 if let ty::TyRef(r_ptr, _) = base_ty.sty {
699 self.mk_subregion_due_to_dereference(expr.span, expr_region, r_ptr);
702 intravisit::walk_expr(self, expr);
705 hir::ExprIndex(ref vec_expr, _) => {
706 // For a[b], the lifetime of a must enclose the deref
707 let vec_type = self.resolve_expr_type_adjusted(&vec_expr);
708 self.constrain_index(expr, vec_type);
710 intravisit::walk_expr(self, expr);
713 hir::ExprCast(ref source, _) => {
714 // Determine if we are casting `source` to a trait
715 // instance. If so, we have to be sure that the type of
716 // the source obeys the trait's region bound.
717 self.constrain_cast(expr, &source);
718 intravisit::walk_expr(self, expr);
721 hir::ExprAddrOf(m, ref base) => {
722 self.link_addr_of(expr, m, &base);
724 // Require that when you write a `&expr` expression, the
725 // resulting pointer has a lifetime that encompasses the
726 // `&expr` expression itself. Note that we constraining
727 // the type of the node expr.id here *before applying
730 // FIXME(#6268) nested method calls requires that this rule change
731 let ty0 = self.resolve_node_type(expr.id);
732 self.type_must_outlive(infer::AddrOf(expr.span), ty0, expr_region);
733 intravisit::walk_expr(self, expr);
736 hir::ExprMatch(ref discr, ref arms, _) => {
737 self.link_match(&discr, &arms[..]);
739 intravisit::walk_expr(self, expr);
742 hir::ExprClosure(.., body_id, _) => {
743 self.check_expr_fn_block(expr, body_id);
746 hir::ExprLoop(ref body, _, _) => {
747 let repeating_scope = self.set_repeating_scope(body.id);
748 intravisit::walk_expr(self, expr);
749 self.set_repeating_scope(repeating_scope);
752 hir::ExprWhile(ref cond, ref body, _) => {
753 let repeating_scope = self.set_repeating_scope(cond.id);
754 self.visit_expr(&cond);
756 self.set_repeating_scope(body.id);
757 self.visit_block(&body);
759 self.set_repeating_scope(repeating_scope);
762 hir::ExprRet(Some(ref ret_expr)) => {
763 let call_site_scope = self.call_site_scope;
764 debug!("visit_expr ExprRet ret_expr.id {} call_site_scope: {:?}",
765 ret_expr.id, call_site_scope);
766 let call_site_region = self.tcx.mk_region(ty::ReScope(call_site_scope.unwrap()));
767 self.type_of_node_must_outlive(infer::CallReturn(ret_expr.span),
770 intravisit::walk_expr(self, expr);
774 intravisit::walk_expr(self, expr);
780 impl<'a, 'gcx, 'tcx> RegionCtxt<'a, 'gcx, 'tcx> {
781 fn constrain_cast(&mut self,
782 cast_expr: &hir::Expr,
783 source_expr: &hir::Expr)
785 debug!("constrain_cast(cast_expr={:?}, source_expr={:?})",
789 let source_ty = self.resolve_node_type(source_expr.id);
790 let target_ty = self.resolve_node_type(cast_expr.id);
792 self.walk_cast(cast_expr, source_ty, target_ty);
795 fn walk_cast(&mut self,
796 cast_expr: &hir::Expr,
799 debug!("walk_cast(from_ty={:?}, to_ty={:?})",
802 match (&from_ty.sty, &to_ty.sty) {
803 /*From:*/ (&ty::TyRef(from_r, ref from_mt),
804 /*To: */ &ty::TyRef(to_r, ref to_mt)) => {
805 // Target cannot outlive source, naturally.
806 self.sub_regions(infer::Reborrow(cast_expr.span), to_r, from_r);
807 self.walk_cast(cast_expr, from_mt.ty, to_mt.ty);
811 /*To: */ &ty::TyDynamic(.., r)) => {
812 // When T is existentially quantified as a trait
813 // `Foo+'to`, it must outlive the region bound `'to`.
814 self.type_must_outlive(infer::RelateObjectBound(cast_expr.span), from_ty, r);
817 /*From:*/ (&ty::TyAdt(from_def, _),
818 /*To: */ &ty::TyAdt(to_def, _)) if from_def.is_box() && to_def.is_box() => {
819 self.walk_cast(cast_expr, from_ty.boxed_ty(), to_ty.boxed_ty());
826 fn check_expr_fn_block(&mut self,
827 expr: &'gcx hir::Expr,
828 body_id: hir::BodyId) {
829 let repeating_scope = self.set_repeating_scope(body_id.node_id);
830 intravisit::walk_expr(self, expr);
831 self.set_repeating_scope(repeating_scope);
834 fn constrain_callee(&mut self,
835 callee_id: ast::NodeId,
836 _call_expr: &hir::Expr,
837 _callee_expr: &hir::Expr) {
838 let callee_ty = self.resolve_node_type(callee_id);
839 match callee_ty.sty {
840 ty::TyFnDef(..) | ty::TyFnPtr(_) => { }
842 // this should not happen, but it does if the program is
847 // "Calling non-function: {}",
853 fn constrain_call<'b, I: Iterator<Item=&'b hir::Expr>>(&mut self,
854 call_expr: &hir::Expr,
855 receiver: Option<&hir::Expr>,
857 implicitly_ref_args: bool) {
858 //! Invoked on every call site (i.e., normal calls, method calls,
859 //! and overloaded operators). Constrains the regions which appear
860 //! in the type of the function. Also constrains the regions that
861 //! appear in the arguments appropriately.
863 debug!("constrain_call(call_expr={:?}, \
865 implicitly_ref_args={})",
868 implicitly_ref_args);
870 // `callee_region` is the scope representing the time in which the
873 // FIXME(#6268) to support nested method calls, should be callee_id
874 let callee_scope = CodeExtent::Misc(call_expr.id);
875 let callee_region = self.tcx.mk_region(ty::ReScope(callee_scope));
877 debug!("callee_region={:?}", callee_region);
879 for arg_expr in arg_exprs {
880 debug!("Argument: {:?}", arg_expr);
882 // ensure that any regions appearing in the argument type are
883 // valid for at least the lifetime of the function:
884 self.type_of_node_must_outlive(infer::CallArg(arg_expr.span),
885 arg_expr.id, callee_region);
887 // unfortunately, there are two means of taking implicit
888 // references, and we need to propagate constraints as a
889 // result. modes are going away and the "DerefArgs" code
890 // should be ported to use adjustments
891 if implicitly_ref_args {
892 self.link_by_ref(arg_expr, callee_scope);
896 // as loop above, but for receiver
897 if let Some(r) = receiver {
898 debug!("receiver: {:?}", r);
899 self.type_of_node_must_outlive(infer::CallRcvr(r.span),
900 r.id, callee_region);
901 if implicitly_ref_args {
902 self.link_by_ref(&r, callee_scope);
907 /// Invoked on any auto-dereference that occurs. Checks that if this is a region pointer being
908 /// dereferenced, the lifetime of the pointer includes the deref expr.
909 fn constrain_autoderefs(&mut self,
910 deref_expr: &hir::Expr,
911 autoderefs: &[Option<adjustment::OverloadedDeref<'tcx>>])
912 -> mc::McResult<mc::cmt<'tcx>>
914 debug!("constrain_autoderefs(deref_expr={:?}, autoderefs={:?})",
919 let mc = mc::MemCategorizationContext::new(self, &self.region_maps);
920 mc.cat_expr_unadjusted(deref_expr)?
923 let r_deref_expr = self.tcx.node_scope_region(deref_expr.id);
924 for &overloaded in autoderefs {
925 if let Some(deref) = overloaded {
926 debug!("constrain_autoderefs: overloaded, {:?}", deref);
928 // Treat overloaded autoderefs as if an AutoBorrow adjustment
929 // was applied on the base type, as that is always the case.
930 let input = self.tcx.mk_ref(deref.region, ty::TypeAndMut {
934 let output = self.tcx.mk_ref(deref.region, ty::TypeAndMut {
939 debug!("constrain_autoderefs: self_cmt={:?}", cmt);
940 self.link_region(deref_expr.span, deref.region,
941 ty::BorrowKind::from_mutbl(deref.mutbl), cmt.clone());
943 // Specialized version of constrain_call.
944 self.type_must_outlive(infer::CallRcvr(deref_expr.span),
945 input, r_deref_expr);
946 self.type_must_outlive(infer::CallReturn(deref_expr.span),
947 output, r_deref_expr);
951 let mc = mc::MemCategorizationContext::new(self, &self.region_maps);
952 if let Some(deref) = overloaded {
953 cmt = mc.cat_overloaded_autoderef(deref_expr, deref)?;
955 cmt = mc.cat_deref(deref_expr, cmt, false)?;
959 if let Categorization::Deref(_, mc::BorrowedPtr(_, r_ptr)) = cmt.cat {
960 self.mk_subregion_due_to_dereference(deref_expr.span,
961 r_deref_expr, r_ptr);
968 pub fn mk_subregion_due_to_dereference(&mut self,
970 minimum_lifetime: ty::Region<'tcx>,
971 maximum_lifetime: ty::Region<'tcx>) {
972 self.sub_regions(infer::DerefPointer(deref_span),
973 minimum_lifetime, maximum_lifetime)
976 fn check_safety_of_rvalue_destructor_if_necessary(&mut self,
980 Categorization::Rvalue(region, _) => {
982 ty::ReScope(rvalue_scope) => {
983 let typ = self.resolve_type(cmt.ty);
984 let _ = dropck::check_safety_of_destructor_if_necessary(
985 self, typ, span, rvalue_scope);
990 "unexpected rvalue region in rvalue \
991 destructor safety checking: `{:?}`",
1000 /// Invoked on any index expression that occurs. Checks that if this is a slice
1001 /// being indexed, the lifetime of the pointer includes the deref expr.
1002 fn constrain_index(&mut self,
1003 index_expr: &hir::Expr,
1004 indexed_ty: Ty<'tcx>)
1006 debug!("constrain_index(index_expr=?, indexed_ty={}",
1007 self.ty_to_string(indexed_ty));
1009 let r_index_expr = ty::ReScope(CodeExtent::Misc(index_expr.id));
1010 if let ty::TyRef(r_ptr, mt) = indexed_ty.sty {
1012 ty::TySlice(_) | ty::TyStr => {
1013 self.sub_regions(infer::IndexSlice(index_expr.span),
1014 self.tcx.mk_region(r_index_expr), r_ptr);
1021 /// Guarantees that any lifetimes which appear in the type of the node `id` (after applying
1022 /// adjustments) are valid for at least `minimum_lifetime`
1023 fn type_of_node_must_outlive(&mut self,
1024 origin: infer::SubregionOrigin<'tcx>,
1026 minimum_lifetime: ty::Region<'tcx>)
1028 // Try to resolve the type. If we encounter an error, then typeck
1029 // is going to fail anyway, so just stop here and let typeck
1030 // report errors later on in the writeback phase.
1031 let ty0 = self.resolve_node_type(id);
1032 let ty = self.tables.borrow().adjustments.get(&id).map_or(ty0, |adj| adj.target);
1033 let ty = self.resolve_type(ty);
1034 debug!("constrain_regions_in_type_of_node(\
1035 ty={}, ty0={}, id={}, minimum_lifetime={:?})",
1037 id, minimum_lifetime);
1038 self.type_must_outlive(origin, ty, minimum_lifetime);
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,
1044 mutability: hir::Mutability, base: &hir::Expr) {
1045 debug!("link_addr_of(expr={:?}, base={:?})", expr, base);
1048 let mc = mc::MemCategorizationContext::new(self, &self.region_maps);
1049 ignore_err!(mc.cat_expr(base))
1052 debug!("link_addr_of: cmt={:?}", cmt);
1054 self.link_region_from_node_type(expr.span, expr.id, mutability, cmt);
1057 /// Computes the guarantors for any ref bindings in a `let` and
1058 /// then ensures that the lifetime of the resulting pointer is
1059 /// linked to the lifetime of the initialization expression.
1060 fn link_local(&self, local: &hir::Local) {
1061 debug!("regionck::for_local()");
1062 let init_expr = match local.init {
1064 Some(ref expr) => &**expr,
1066 let mc = &mc::MemCategorizationContext::new(self, &self.region_maps);
1067 let discr_cmt = ignore_err!(mc.cat_expr(init_expr));
1068 self.link_pattern(mc, discr_cmt, &local.pat);
1071 /// Computes the guarantors for any ref bindings in a match and
1072 /// then ensures that the lifetime of the resulting pointer is
1073 /// linked to the lifetime of its guarantor (if any).
1074 fn link_match(&self, discr: &hir::Expr, arms: &[hir::Arm]) {
1075 debug!("regionck::for_match()");
1076 let mc = &mc::MemCategorizationContext::new(self, &self.region_maps);
1077 let discr_cmt = ignore_err!(mc.cat_expr(discr));
1078 debug!("discr_cmt={:?}", discr_cmt);
1080 for root_pat in &arm.pats {
1081 self.link_pattern(mc, discr_cmt.clone(), &root_pat);
1086 /// Computes the guarantors for any ref bindings in a match and
1087 /// then ensures that the lifetime of the resulting pointer is
1088 /// linked to the lifetime of its guarantor (if any).
1089 fn link_fn_args(&self, body_scope: CodeExtent, args: &[hir::Arg]) {
1090 debug!("regionck::link_fn_args(body_scope={:?})", body_scope);
1091 let mc = &mc::MemCategorizationContext::new(self, &self.region_maps);
1093 let arg_ty = self.node_ty(arg.id);
1094 let re_scope = self.tcx.mk_region(ty::ReScope(body_scope));
1095 let arg_cmt = mc.cat_rvalue(
1096 arg.id, arg.pat.span, re_scope, re_scope, arg_ty);
1097 debug!("arg_ty={:?} arg_cmt={:?} arg={:?}",
1101 self.link_pattern(mc, arg_cmt, &arg.pat);
1105 /// Link lifetimes of any ref bindings in `root_pat` to the pointers found
1106 /// in the discriminant, if needed.
1107 fn link_pattern<'t>(&self,
1108 mc: &mc::MemCategorizationContext<'a, 'gcx, 'tcx>,
1109 discr_cmt: mc::cmt<'tcx>,
1110 root_pat: &hir::Pat) {
1111 debug!("link_pattern(discr_cmt={:?}, root_pat={:?})",
1114 let _ = mc.cat_pattern(discr_cmt, root_pat, |_, sub_cmt, sub_pat| {
1115 match sub_pat.node {
1117 PatKind::Binding(hir::BindByRef(mutbl), ..) => {
1118 self.link_region_from_node_type(sub_pat.span, sub_pat.id,
1126 /// Link lifetime of borrowed pointer resulting from autoref to lifetimes in the value being
1128 fn link_autoref(&self,
1130 expr_cmt: mc::cmt<'tcx>,
1131 autoref: &adjustment::AutoBorrow<'tcx>)
1133 debug!("link_autoref(autoref={:?}, expr_cmt={:?})", autoref, expr_cmt);
1136 adjustment::AutoBorrow::Ref(r, m) => {
1137 self.link_region(expr.span, r,
1138 ty::BorrowKind::from_mutbl(m), expr_cmt);
1141 adjustment::AutoBorrow::RawPtr(m) => {
1142 let r = self.tcx.node_scope_region(expr.id);
1143 self.link_region(expr.span, r, ty::BorrowKind::from_mutbl(m), expr_cmt);
1148 /// Computes the guarantor for cases where the `expr` is being passed by implicit reference and
1149 /// must outlive `callee_scope`.
1150 fn link_by_ref(&self,
1152 callee_scope: CodeExtent) {
1153 debug!("link_by_ref(expr={:?}, callee_scope={:?})",
1154 expr, callee_scope);
1155 let mc = mc::MemCategorizationContext::new(self, &self.region_maps);
1156 let expr_cmt = ignore_err!(mc.cat_expr(expr));
1157 let borrow_region = self.tcx.mk_region(ty::ReScope(callee_scope));
1158 self.link_region(expr.span, borrow_region, ty::ImmBorrow, expr_cmt);
1161 /// Like `link_region()`, except that the region is extracted from the type of `id`,
1162 /// which must be some reference (`&T`, `&str`, etc).
1163 fn link_region_from_node_type(&self,
1166 mutbl: hir::Mutability,
1167 cmt_borrowed: mc::cmt<'tcx>) {
1168 debug!("link_region_from_node_type(id={:?}, mutbl={:?}, cmt_borrowed={:?})",
1169 id, mutbl, cmt_borrowed);
1171 let rptr_ty = self.resolve_node_type(id);
1172 if let ty::TyRef(r, _) = rptr_ty.sty {
1173 debug!("rptr_ty={}", rptr_ty);
1174 self.link_region(span, r, ty::BorrowKind::from_mutbl(mutbl),
1179 /// Informs the inference engine that `borrow_cmt` is being borrowed with
1180 /// kind `borrow_kind` and lifetime `borrow_region`.
1181 /// In order to ensure borrowck is satisfied, this may create constraints
1182 /// between regions, as explained in `link_reborrowed_region()`.
1183 fn link_region(&self,
1185 borrow_region: ty::Region<'tcx>,
1186 borrow_kind: ty::BorrowKind,
1187 borrow_cmt: mc::cmt<'tcx>) {
1188 let mut borrow_cmt = borrow_cmt;
1189 let mut borrow_kind = borrow_kind;
1191 let origin = infer::DataBorrowed(borrow_cmt.ty, span);
1192 self.type_must_outlive(origin, borrow_cmt.ty, borrow_region);
1195 debug!("link_region(borrow_region={:?}, borrow_kind={:?}, borrow_cmt={:?})",
1199 match borrow_cmt.cat.clone() {
1200 Categorization::Deref(ref_cmt, mc::Implicit(ref_kind, ref_region)) |
1201 Categorization::Deref(ref_cmt, mc::BorrowedPtr(ref_kind, ref_region)) => {
1202 match self.link_reborrowed_region(span,
1203 borrow_region, borrow_kind,
1204 ref_cmt, ref_region, ref_kind,
1216 Categorization::Downcast(cmt_base, _) |
1217 Categorization::Deref(cmt_base, mc::Unique) |
1218 Categorization::Interior(cmt_base, _) => {
1219 // Borrowing interior or owned data requires the base
1220 // to be valid and borrowable in the same fashion.
1221 borrow_cmt = cmt_base;
1222 borrow_kind = borrow_kind;
1225 Categorization::Deref(_, mc::UnsafePtr(..)) |
1226 Categorization::StaticItem |
1227 Categorization::Upvar(..) |
1228 Categorization::Local(..) |
1229 Categorization::Rvalue(..) => {
1230 // These are all "base cases" with independent lifetimes
1231 // that are not subject to inference
1238 /// This is the most complicated case: the path being borrowed is
1239 /// itself the referent of a borrowed pointer. Let me give an
1240 /// example fragment of code to make clear(er) the situation:
1242 /// let r: &'a mut T = ...; // the original reference "r" has lifetime 'a
1244 /// &'z *r // the reborrow has lifetime 'z
1246 /// Now, in this case, our primary job is to add the inference
1247 /// constraint that `'z <= 'a`. Given this setup, let's clarify the
1248 /// parameters in (roughly) terms of the example:
1250 /// A borrow of: `& 'z bk * r` where `r` has type `& 'a bk T`
1251 /// borrow_region ^~ ref_region ^~
1252 /// borrow_kind ^~ ref_kind ^~
1255 /// Here `bk` stands for some borrow-kind (e.g., `mut`, `uniq`, etc).
1257 /// Unfortunately, there are some complications beyond the simple
1258 /// scenario I just painted:
1260 /// 1. The reference `r` might in fact be a "by-ref" upvar. In that
1261 /// case, we have two jobs. First, we are inferring whether this reference
1262 /// should be an `&T`, `&mut T`, or `&uniq T` reference, and we must
1263 /// adjust that based on this borrow (e.g., if this is an `&mut` borrow,
1264 /// then `r` must be an `&mut` reference). Second, whenever we link
1265 /// two regions (here, `'z <= 'a`), we supply a *cause*, and in this
1266 /// case we adjust the cause to indicate that the reference being
1267 /// "reborrowed" is itself an upvar. This provides a nicer error message
1268 /// should something go wrong.
1270 /// 2. There may in fact be more levels of reborrowing. In the
1271 /// example, I said the borrow was like `&'z *r`, but it might
1272 /// in fact be a borrow like `&'z **q` where `q` has type `&'a
1273 /// &'b mut T`. In that case, we want to ensure that `'z <= 'a`
1274 /// and `'z <= 'b`. This is explained more below.
1276 /// The return value of this function indicates whether we need to
1277 /// recurse and process `ref_cmt` (see case 2 above).
1278 fn link_reborrowed_region(&self,
1280 borrow_region: ty::Region<'tcx>,
1281 borrow_kind: ty::BorrowKind,
1282 ref_cmt: mc::cmt<'tcx>,
1283 ref_region: ty::Region<'tcx>,
1284 mut ref_kind: ty::BorrowKind,
1286 -> Option<(mc::cmt<'tcx>, ty::BorrowKind)>
1288 // Possible upvar ID we may need later to create an entry in the
1291 // Detect by-ref upvar `x`:
1292 let cause = match note {
1293 mc::NoteUpvarRef(ref upvar_id) => {
1294 let upvar_capture_map = &self.tables.borrow_mut().upvar_capture_map;
1295 match upvar_capture_map.get(upvar_id) {
1296 Some(&ty::UpvarCapture::ByRef(ref upvar_borrow)) => {
1297 // The mutability of the upvar may have been modified
1298 // by the above adjustment, so update our local variable.
1299 ref_kind = upvar_borrow.kind;
1301 infer::ReborrowUpvar(span, *upvar_id)
1304 span_bug!( span, "Illegal upvar id: {:?}", upvar_id);
1308 mc::NoteClosureEnv(ref upvar_id) => {
1309 // We don't have any mutability changes to propagate, but
1310 // we do want to note that an upvar reborrow caused this
1312 infer::ReborrowUpvar(span, *upvar_id)
1315 infer::Reborrow(span)
1319 debug!("link_reborrowed_region: {:?} <= {:?}",
1322 self.sub_regions(cause, borrow_region, ref_region);
1324 // If we end up needing to recurse and establish a region link
1325 // with `ref_cmt`, calculate what borrow kind we will end up
1326 // needing. This will be used below.
1328 // One interesting twist is that we can weaken the borrow kind
1329 // when we recurse: to reborrow an `&mut` referent as mutable,
1330 // borrowck requires a unique path to the `&mut` reference but not
1331 // necessarily a *mutable* path.
1332 let new_borrow_kind = match borrow_kind {
1335 ty::MutBorrow | ty::UniqueImmBorrow =>
1339 // Decide whether we need to recurse and link any regions within
1340 // the `ref_cmt`. This is concerned for the case where the value
1341 // being reborrowed is in fact a borrowed pointer found within
1342 // another borrowed pointer. For example:
1344 // let p: &'b &'a mut T = ...;
1348 // What makes this case particularly tricky is that, if the data
1349 // being borrowed is a `&mut` or `&uniq` borrow, borrowck requires
1350 // not only that `'z <= 'a`, (as before) but also `'z <= 'b`
1351 // (otherwise the user might mutate through the `&mut T` reference
1352 // after `'b` expires and invalidate the borrow we are looking at
1355 // So let's re-examine our parameters in light of this more
1356 // complicated (possible) scenario:
1358 // A borrow of: `& 'z bk * * p` where `p` has type `&'b bk & 'a bk T`
1359 // borrow_region ^~ ref_region ^~
1360 // borrow_kind ^~ ref_kind ^~
1363 // (Note that since we have not examined `ref_cmt.cat`, we don't
1364 // know whether this scenario has occurred; but I wanted to show
1365 // how all the types get adjusted.)
1368 // The reference being reborrowed is a sharable ref of
1369 // type `&'a T`. In this case, it doesn't matter where we
1370 // *found* the `&T` pointer, the memory it references will
1371 // be valid and immutable for `'a`. So we can stop here.
1373 // (Note that the `borrow_kind` must also be ImmBorrow or
1374 // else the user is borrowed imm memory as mut memory,
1375 // which means they'll get an error downstream in borrowck
1380 ty::MutBorrow | ty::UniqueImmBorrow => {
1381 // The reference being reborrowed is either an `&mut T` or
1382 // `&uniq T`. This is the case where recursion is needed.
1383 return Some((ref_cmt, new_borrow_kind));
1388 /// Checks that the values provided for type/region arguments in a given
1389 /// expression are well-formed and in-scope.
1390 fn substs_wf_in_scope(&mut self,
1391 origin: infer::ParameterOrigin,
1392 substs: &Substs<'tcx>,
1394 expr_region: ty::Region<'tcx>) {
1395 debug!("substs_wf_in_scope(substs={:?}, \
1399 substs, expr_region, origin, expr_span);
1401 let origin = infer::ParameterInScope(origin, expr_span);
1403 for region in substs.regions() {
1404 self.sub_regions(origin.clone(), expr_region, region);
1407 for ty in substs.types() {
1408 let ty = self.resolve_type(ty);
1409 self.type_must_outlive(origin.clone(), ty, expr_region);
1413 /// Ensures that type is well-formed in `region`, which implies (among
1414 /// other things) that all borrowed data reachable via `ty` outlives
1416 pub fn type_must_outlive(&self,
1417 origin: infer::SubregionOrigin<'tcx>,
1419 region: ty::Region<'tcx>)
1421 let ty = self.resolve_type(ty);
1423 debug!("type_must_outlive(ty={:?}, region={:?}, origin={:?})",
1428 assert!(!ty.has_escaping_regions());
1430 let components = self.tcx.outlives_components(ty);
1431 self.components_must_outlive(origin, components, region);
1434 fn components_must_outlive(&self,
1435 origin: infer::SubregionOrigin<'tcx>,
1436 components: Vec<ty::outlives::Component<'tcx>>,
1437 region: ty::Region<'tcx>)
1439 for component in components {
1440 let origin = origin.clone();
1442 ty::outlives::Component::Region(region1) => {
1443 self.sub_regions(origin, region, region1);
1445 ty::outlives::Component::Param(param_ty) => {
1446 self.param_ty_must_outlive(origin, region, param_ty);
1448 ty::outlives::Component::Projection(projection_ty) => {
1449 self.projection_must_outlive(origin, region, projection_ty);
1451 ty::outlives::Component::EscapingProjection(subcomponents) => {
1452 self.components_must_outlive(origin, subcomponents, region);
1454 ty::outlives::Component::UnresolvedInferenceVariable(v) => {
1455 // ignore this, we presume it will yield an error
1456 // later, since if a type variable is not resolved by
1457 // this point it never will be
1458 self.tcx.sess.delay_span_bug(
1460 &format!("unresolved inference variable in outlives: {:?}", v));
1466 fn param_ty_must_outlive(&self,
1467 origin: infer::SubregionOrigin<'tcx>,
1468 region: ty::Region<'tcx>,
1469 param_ty: ty::ParamTy) {
1470 debug!("param_ty_must_outlive(region={:?}, param_ty={:?}, origin={:?})",
1471 region, param_ty, origin);
1473 let verify_bound = self.param_bound(param_ty);
1474 let generic = GenericKind::Param(param_ty);
1475 self.verify_generic_bound(origin, generic, region, verify_bound);
1478 fn projection_must_outlive(&self,
1479 origin: infer::SubregionOrigin<'tcx>,
1480 region: ty::Region<'tcx>,
1481 projection_ty: ty::ProjectionTy<'tcx>)
1483 debug!("projection_must_outlive(region={:?}, projection_ty={:?}, origin={:?})",
1484 region, projection_ty, origin);
1486 // This case is thorny for inference. The fundamental problem is
1487 // that there are many cases where we have choice, and inference
1488 // doesn't like choice (the current region inference in
1489 // particular). :) First off, we have to choose between using the
1490 // OutlivesProjectionEnv, OutlivesProjectionTraitDef, and
1491 // OutlivesProjectionComponent rules, any one of which is
1492 // sufficient. If there are no inference variables involved, it's
1493 // not hard to pick the right rule, but if there are, we're in a
1494 // bit of a catch 22: if we picked which rule we were going to
1495 // use, we could add constraints to the region inference graph
1496 // that make it apply, but if we don't add those constraints, the
1497 // rule might not apply (but another rule might). For now, we err
1498 // on the side of adding too few edges into the graph.
1500 // Compute the bounds we can derive from the environment or trait
1501 // definition. We know that the projection outlives all the
1502 // regions in this list.
1503 let env_bounds = self.projection_declared_bounds(origin.span(), projection_ty);
1505 debug!("projection_must_outlive: env_bounds={:?}",
1508 // If we know that the projection outlives 'static, then we're
1510 if env_bounds.contains(&&ty::ReStatic) {
1511 debug!("projection_must_outlive: 'static as declared bound");
1515 // If declared bounds list is empty, the only applicable rule is
1516 // OutlivesProjectionComponent. If there are inference variables,
1517 // then, we can break down the outlives into more primitive
1518 // components without adding unnecessary edges.
1520 // If there are *no* inference variables, however, we COULD do
1521 // this, but we choose not to, because the error messages are less
1522 // good. For example, a requirement like `T::Item: 'r` would be
1523 // translated to a requirement that `T: 'r`; when this is reported
1524 // to the user, it will thus say "T: 'r must hold so that T::Item:
1525 // 'r holds". But that makes it sound like the only way to fix
1526 // the problem is to add `T: 'r`, which isn't true. So, if there are no
1527 // inference variables, we use a verify constraint instead of adding
1528 // edges, which winds up enforcing the same condition.
1529 let needs_infer = projection_ty.trait_ref.needs_infer();
1530 if env_bounds.is_empty() && needs_infer {
1531 debug!("projection_must_outlive: no declared bounds");
1533 for component_ty in projection_ty.trait_ref.substs.types() {
1534 self.type_must_outlive(origin.clone(), component_ty, region);
1537 for r in projection_ty.trait_ref.substs.regions() {
1538 self.sub_regions(origin.clone(), region, r);
1544 // If we find that there is a unique declared bound `'b`, and this bound
1545 // appears in the trait reference, then the best action is to require that `'b:'r`,
1546 // so do that. This is best no matter what rule we use:
1548 // - OutlivesProjectionEnv or OutlivesProjectionTraitDef: these would translate to
1549 // the requirement that `'b:'r`
1550 // - OutlivesProjectionComponent: this would require `'b:'r` in addition to
1552 if !env_bounds.is_empty() && env_bounds[1..].iter().all(|b| *b == env_bounds[0]) {
1553 let unique_bound = env_bounds[0];
1554 debug!("projection_must_outlive: unique declared bound = {:?}", unique_bound);
1555 if projection_ty.trait_ref.substs.regions().any(|r| env_bounds.contains(&r)) {
1556 debug!("projection_must_outlive: unique declared bound appears in trait ref");
1557 self.sub_regions(origin.clone(), region, unique_bound);
1562 // Fallback to verifying after the fact that there exists a
1563 // declared bound, or that all the components appearing in the
1564 // projection outlive; in some cases, this may add insufficient
1565 // edges into the inference graph, leading to inference failures
1566 // even though a satisfactory solution exists.
1567 let verify_bound = self.projection_bound(origin.span(), env_bounds, projection_ty);
1568 let generic = GenericKind::Projection(projection_ty);
1569 self.verify_generic_bound(origin, generic.clone(), region, verify_bound);
1572 fn type_bound(&self, span: Span, ty: Ty<'tcx>) -> VerifyBound<'tcx> {
1577 ty::TyProjection(data) => {
1578 let declared_bounds = self.projection_declared_bounds(span, data);
1579 self.projection_bound(span, declared_bounds, data)
1582 self.recursive_type_bound(span, ty)
1587 fn param_bound(&self, param_ty: ty::ParamTy) -> VerifyBound<'tcx> {
1588 debug!("param_bound(param_ty={:?})",
1591 let mut param_bounds = self.declared_generic_bounds_from_env(GenericKind::Param(param_ty));
1593 // Add in the default bound of fn body that applies to all in
1594 // scope type parameters:
1595 param_bounds.extend(self.implicit_region_bound);
1597 VerifyBound::AnyRegion(param_bounds)
1600 fn projection_declared_bounds(&self,
1602 projection_ty: ty::ProjectionTy<'tcx>)
1603 -> Vec<ty::Region<'tcx>>
1605 // First assemble bounds from where clauses and traits.
1607 let mut declared_bounds =
1608 self.declared_generic_bounds_from_env(GenericKind::Projection(projection_ty));
1610 declared_bounds.extend_from_slice(
1611 &self.declared_projection_bounds_from_trait(span, projection_ty));
1616 fn projection_bound(&self,
1618 declared_bounds: Vec<ty::Region<'tcx>>,
1619 projection_ty: ty::ProjectionTy<'tcx>)
1620 -> VerifyBound<'tcx> {
1621 debug!("projection_bound(declared_bounds={:?}, projection_ty={:?})",
1622 declared_bounds, projection_ty);
1624 // see the extensive comment in projection_must_outlive
1626 let ty = self.tcx.mk_projection(projection_ty.trait_ref, projection_ty.item_name);
1627 let recursive_bound = self.recursive_type_bound(span, ty);
1629 VerifyBound::AnyRegion(declared_bounds).or(recursive_bound)
1632 fn recursive_type_bound(&self, span: Span, ty: Ty<'tcx>) -> VerifyBound<'tcx> {
1633 let mut bounds = vec![];
1635 for subty in ty.walk_shallow() {
1636 bounds.push(self.type_bound(span, subty));
1639 let mut regions = ty.regions();
1640 regions.retain(|r| !r.is_late_bound()); // ignore late-bound regions
1641 bounds.push(VerifyBound::AllRegions(regions));
1643 // remove bounds that must hold, since they are not interesting
1644 bounds.retain(|b| !b.must_hold());
1646 if bounds.len() == 1 {
1647 bounds.pop().unwrap()
1649 VerifyBound::AllBounds(bounds)
1653 fn declared_generic_bounds_from_env(&self, generic: GenericKind<'tcx>)
1654 -> Vec<ty::Region<'tcx>>
1656 let param_env = &self.param_env;
1658 // To start, collect bounds from user:
1659 let mut param_bounds = self.tcx.required_region_bounds(generic.to_ty(self.tcx),
1660 param_env.caller_bounds.to_vec());
1662 // Next, collect regions we scraped from the well-formedness
1663 // constraints in the fn signature. To do that, we walk the list
1664 // of known relations from the fn ctxt.
1666 // This is crucial because otherwise code like this fails:
1668 // fn foo<'a, A>(x: &'a A) { x.bar() }
1670 // The problem is that the type of `x` is `&'a A`. To be
1671 // well-formed, then, A must be lower-generic by `'a`, but we
1672 // don't know that this holds from first principles.
1673 for &(r, p) in &self.region_bound_pairs {
1674 debug!("generic={:?} p={:?}",
1678 param_bounds.push(r);
1685 fn declared_projection_bounds_from_trait(&self,
1687 projection_ty: ty::ProjectionTy<'tcx>)
1688 -> Vec<ty::Region<'tcx>>
1690 debug!("projection_bounds(projection_ty={:?})",
1693 let ty = self.tcx.mk_projection(projection_ty.trait_ref.clone(),
1694 projection_ty.item_name);
1696 // Say we have a projection `<T as SomeTrait<'a>>::SomeType`. We are interested
1697 // in looking for a trait definition like:
1700 // trait SomeTrait<'a> {
1701 // type SomeType : 'a;
1705 // we can thus deduce that `<T as SomeTrait<'a>>::SomeType : 'a`.
1706 let trait_predicates = self.tcx.predicates_of(projection_ty.trait_ref.def_id);
1707 assert_eq!(trait_predicates.parent, None);
1708 let predicates = trait_predicates.predicates.as_slice().to_vec();
1709 traits::elaborate_predicates(self.tcx, predicates)
1710 .filter_map(|predicate| {
1711 // we're only interesting in `T : 'a` style predicates:
1712 let outlives = match predicate {
1713 ty::Predicate::TypeOutlives(data) => data,
1714 _ => { return None; }
1717 debug!("projection_bounds: outlives={:?} (1)",
1720 // apply the substitutions (and normalize any projected types)
1721 let outlives = self.instantiate_type_scheme(span,
1722 projection_ty.trait_ref.substs,
1725 debug!("projection_bounds: outlives={:?} (2)",
1728 let region_result = self.commit_if_ok(|_| {
1730 self.replace_late_bound_regions_with_fresh_var(
1732 infer::AssocTypeProjection(projection_ty.item_name),
1735 debug!("projection_bounds: outlives={:?} (3)",
1738 // check whether this predicate applies to our current projection
1739 let cause = self.fcx.misc(span);
1740 match self.eq_types(false, &cause, ty, outlives.0) {
1742 self.register_infer_ok_obligations(ok);
1745 Err(_) => { Err(()) }
1749 debug!("projection_bounds: region_result={:?}",