1 // Copyright 2012 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.
13 The region check is a final pass that runs over the AST after we have
14 inferred the type constraints but before we have actually finalized
15 the types. Its purpose is to embed a variety of region constraints.
16 Inserting these constraints as a separate pass is good because (1) it
17 localizes the code that has to do with region inference and (2) often
18 we cannot know what constraints are needed until the basic types have
21 ### Interaction with the borrow checker
23 In general, the job of the borrowck module (which runs later) is to
24 check that all soundness criteria are met, given a particular set of
25 regions. The job of *this* module is to anticipate the needs of the
26 borrow checker and infer regions that will satisfy its requirements.
27 It is generally true that the inference doesn't need to be sound,
28 meaning that if there is a bug and we inferred bad regions, the borrow
29 checker should catch it. This is not entirely true though; for
30 example, the borrow checker doesn't check subtyping, and it doesn't
31 check that region pointers are always live when they are used. It
32 might be worthwhile to fix this so that borrowck serves as a kind of
33 verification step -- that would add confidence in the overall
34 correctness of the compiler, at the cost of duplicating some type
37 ### Inferring the duration of borrows, automatic and otherwise
39 Whenever we introduce a borrowed pointer, for example as the result of
40 a borrow expression `let x = &data`, the lifetime of the pointer `x`
41 is always specified as a region inference variable. `regionck` has the
42 job of adding constraints such that this inference variable is as
43 narrow as possible while still accommodating all uses (that is, every
44 dereference of the resulting pointer must be within the lifetime).
48 Generally speaking, `regionck` does NOT try to ensure that the data
49 `data` will outlive the pointer `x`. That is the job of borrowck. The
50 one exception is when "re-borrowing" the contents of another borrowed
51 pointer. For example, imagine you have a borrowed pointer `b` with
52 lifetime L1 and you have an expression `&*b`. The result of this
53 expression will be another borrowed pointer with lifetime L2 (which is
54 an inference variable). The borrow checker is going to enforce the
55 constraint that L2 < L1, because otherwise you are re-borrowing data
56 for a lifetime larger than the original loan. However, without the
57 routines in this module, the region inferencer would not know of this
58 dependency and thus it might infer the lifetime of L2 to be greater
59 than L1 (issue #3148).
61 There are a number of troublesome scenarios in the tests
62 `region-dependent-*.rs`, but here is one example:
65 struct Bar { foo: Foo }
66 fn get_i(x: &'a Bar) -> &'a int {
67 let foo = &x.foo; // Lifetime L1
71 Note that this comes up either with `&` expressions, `ref`
72 bindings, and `autorefs`, which are the three ways to introduce
75 The key point here is that when you are borrowing a value that
76 is "guaranteed" by a borrowed pointer, you must link the
77 lifetime of that borrowed pointer (L1, here) to the lifetime of
78 the borrow itself (L2). What do I mean by "guaranteed" by a
79 borrowed pointer? I mean any data that is reached by first
80 dereferencing a borrowed pointer and then either traversing
81 interior offsets or owned pointers. We say that the guarantor
82 of such data it the region of the borrowed pointer that was
83 traversed. This is essentially the same as the ownership
84 relation, except that a borrowed pointer never owns its
87 ### Inferring borrow kinds for upvars
89 Whenever there is a closure expression, we need to determine how each
90 upvar is used. We do this by initially assigning each upvar an
91 immutable "borrow kind" (see `ty::BorrowKind` for details) and then
92 "escalating" the kind as needed. The borrow kind proceeds according to
93 the following lattice:
95 ty::ImmBorrow -> ty::UniqueImmBorrow -> ty::MutBorrow
97 So, for example, if we see an assignment `x = 5` to an upvar `x`, we
98 will promote its borrow kind to mutable borrow. If we see an `&mut x`
99 we'll do the same. Naturally, this applies not just to the upvar, but
100 to everything owned by `x`, so the result is the same for something
101 like `x.f = 5` and so on (presuming `x` is not a borrowed pointer to a
102 struct). These adjustments are performed in
103 `adjust_upvar_borrow_kind()` (you can trace backwards through the code
106 The fact that we are inferring borrow kinds as we go results in a
107 semi-hacky interaction with mem-categorization. In particular,
108 mem-categorization will query the current borrow kind as it
109 categorizes, and we'll return the *current* value, but this may get
110 adjusted later. Therefore, in this module, we generally ignore the
111 borrow kind (and derived mutabilities) that are returned from
112 mem-categorization, since they may be inaccurate. (Another option
113 would be to use a unification scheme, where instead of returning a
114 concrete borrow kind like `ty::ImmBorrow`, we return a
115 `ty::InferBorrow(upvar_id)` or something like that, but this would
116 then mean that all later passes would have to check for these figments
117 and report an error, and it just seems like more mess in the end.)
122 use middle::freevars;
123 use mc = middle::mem_categorization;
124 use middle::ty::{ReScope};
126 use middle::typeck::astconv::AstConv;
127 use middle::typeck::check::FnCtxt;
128 use middle::typeck::check::regionmanip::relate_nested_regions;
129 use middle::typeck::infer::resolve_and_force_all_but_regions;
130 use middle::typeck::infer::resolve_type;
131 use middle::typeck::infer;
132 use middle::typeck::MethodCall;
133 use middle::pat_util;
134 use util::nodemap::NodeMap;
135 use util::ppaux::{ty_to_str, region_to_str, Repr};
137 use syntax::ast::{DefArg, DefBinding, DefLocal, DefUpvar};
139 use syntax::ast_util;
140 use syntax::codemap::Span;
142 use syntax::visit::Visitor;
144 use std::cell::RefCell;
146 // If mem categorization results in an error, it's because the type
147 // check failed (or will fail, when the error is uncovered and
148 // reported during writeback). In this case, we just ignore this part
149 // of the code and don't try to add any more region constraints.
150 macro_rules! ignore_err(
162 // id of innermost fn or loop
163 repeating_scope: ast::NodeId,
166 fn region_of_def(fcx: &FnCtxt, def: ast::Def) -> ty::Region {
168 * Returns the validity region of `def` -- that is, how long
174 DefLocal(node_id, _) | DefArg(node_id, _) |
175 DefBinding(node_id, _) => {
176 tcx.region_maps.var_region(node_id)
178 DefUpvar(_, subdef, closure_id, body_id) => {
179 match ty::ty_closure_store(fcx.node_ty(closure_id)) {
180 ty::RegionTraitStore(..) => region_of_def(fcx, *subdef),
181 ty::UniqTraitStore => ReScope(body_id)
185 tcx.sess.bug(format!("unexpected def in region_of_def: {:?}",
192 pub fn tcx(&self) -> &'a ty::ctxt {
196 pub fn set_repeating_scope(&mut self, scope: ast::NodeId) -> ast::NodeId {
197 let old_scope = self.repeating_scope;
198 self.repeating_scope = scope;
202 pub fn resolve_type(&self, unresolved_ty: ty::t) -> ty::t {
204 * Try to resolve the type for the given node, returning
205 * t_err if an error results. Note that we never care
206 * about the details of the error, the same error will be
207 * detected and reported in the writeback phase.
209 * Note one important point: we do not attempt to resolve
210 * *region variables* here. This is because regionck is
211 * essentially adding constraints to those region variables
212 * and so may yet influence how they are resolved.
214 * Consider this silly example:
216 * fn borrow(x: &int) -> &int {x}
217 * fn foo(x: @int) -> int { // block: B
218 * let b = borrow(x); // region: <R0>
222 * Here, the region of `b` will be `<R0>`. `<R0>` is
223 * constrainted to be some subregion of the block B and some
224 * superregion of the call. If we forced it now, we'd choose
225 * the smaller region (the call). But that would make the *b
226 * illegal. Since we don't resolve, the type of b will be
227 * `&<R0>.int` and then `*b` will require that `<R0>` be
228 * bigger than the let and the `*b` expression, so we will
229 * effectively resolve `<R0>` to be the block B.
231 match resolve_type(self.fcx.infcx(), unresolved_ty,
232 resolve_and_force_all_but_regions) {
234 Err(_) => ty::mk_err()
238 /// Try to resolve the type for the given node.
239 fn resolve_node_type(&self, id: ast::NodeId) -> ty::t {
240 let t = self.fcx.node_ty(id);
244 fn resolve_method_type(&self, method_call: MethodCall) -> Option<ty::t> {
245 let method_ty = self.fcx.inh.method_map.borrow()
246 .find(&method_call).map(|method| method.ty);
247 method_ty.map(|method_ty| self.resolve_type(method_ty))
250 /// Try to resolve the type for the given node.
251 pub fn resolve_expr_type_adjusted(&mut self, expr: &ast::Expr) -> ty::t {
252 let ty_unadjusted = self.resolve_node_type(expr.id);
253 if ty::type_is_error(ty_unadjusted) || ty::type_is_bot(ty_unadjusted) {
256 let tcx = self.fcx.tcx();
257 ty::adjust_ty(tcx, expr.span, expr.id, ty_unadjusted,
258 self.fcx.inh.adjustments.borrow().find(&expr.id),
259 |method_call| self.resolve_method_type(method_call))
264 impl<'fcx> mc::Typer for Rcx<'fcx> {
265 fn tcx<'a>(&'a self) -> &'a ty::ctxt {
269 fn node_ty(&self, id: ast::NodeId) -> mc::McResult<ty::t> {
270 let t = self.resolve_node_type(id);
271 if ty::type_is_error(t) {Err(())} else {Ok(t)}
274 fn node_method_ty(&self, method_call: MethodCall) -> Option<ty::t> {
275 self.resolve_method_type(method_call)
278 fn adjustments<'a>(&'a self) -> &'a RefCell<NodeMap<ty::AutoAdjustment>> {
279 &self.fcx.inh.adjustments
282 fn is_method_call(&self, id: ast::NodeId) -> bool {
283 self.fcx.inh.method_map.borrow().contains_key(&MethodCall::expr(id))
286 fn temporary_scope(&self, id: ast::NodeId) -> Option<ast::NodeId> {
287 self.tcx().region_maps.temporary_scope(id)
290 fn upvar_borrow(&self, id: ty::UpvarId) -> ty::UpvarBorrow {
291 self.fcx.inh.upvar_borrow_map.borrow().get_copy(&id)
295 pub fn regionck_expr(fcx: &FnCtxt, e: &ast::Expr) {
296 let mut rcx = Rcx { fcx: fcx, repeating_scope: e.id };
298 if fcx.err_count_since_creation() == 0 {
299 // regionck assumes typeck succeeded
300 rcx.visit_expr(e, ());
302 fcx.infcx().resolve_regions();
305 pub fn regionck_fn(fcx: &FnCtxt, blk: &ast::Block) {
306 let mut rcx = Rcx { fcx: fcx, repeating_scope: blk.id };
308 if fcx.err_count_since_creation() == 0 {
309 // regionck assumes typeck succeeded
310 rcx.visit_block(blk, ());
312 fcx.infcx().resolve_regions();
315 impl<'a> Visitor<()> for Rcx<'a> {
316 // (..) FIXME(#3238) should use visit_pat, not visit_arm/visit_local,
317 // However, right now we run into an issue whereby some free
318 // regions are not properly related if they appear within the
319 // types of arguments that must be inferred. This could be
320 // addressed by deferring the construction of the region
321 // hierarchy, and in particular the relationships between free
322 // regions, until regionck, as described in #3238.
324 fn visit_item(&mut self, i: &ast::Item, _: ()) { visit_item(self, i); }
326 fn visit_expr(&mut self, ex: &ast::Expr, _: ()) { visit_expr(self, ex); }
328 //visit_pat: visit_pat, // (..) see above
330 fn visit_arm(&mut self, a: &ast::Arm, _: ()) { visit_arm(self, a); }
332 fn visit_local(&mut self, l: &ast::Local, _: ()) { visit_local(self, l); }
334 fn visit_block(&mut self, b: &ast::Block, _: ()) { visit_block(self, b); }
337 fn visit_item(_rcx: &mut Rcx, _item: &ast::Item) {
341 fn visit_block(rcx: &mut Rcx, b: &ast::Block) {
342 visit::walk_block(rcx, b, ());
345 fn visit_arm(rcx: &mut Rcx, arm: &ast::Arm) {
347 for &p in arm.pats.iter() {
348 constrain_bindings_in_pat(p, rcx);
351 visit::walk_arm(rcx, arm, ());
354 fn visit_local(rcx: &mut Rcx, l: &ast::Local) {
356 constrain_bindings_in_pat(l.pat, rcx);
358 visit::walk_local(rcx, l, ());
361 fn constrain_bindings_in_pat(pat: &ast::Pat, rcx: &mut Rcx) {
362 let tcx = rcx.fcx.tcx();
363 debug!("regionck::visit_pat(pat={})", pat.repr(tcx));
364 pat_util::pat_bindings(&tcx.def_map, pat, |_, id, span, _| {
365 // If we have a variable that contains region'd data, that
366 // data will be accessible from anywhere that the variable is
367 // accessed. We must be wary of loops like this:
369 // // from src/test/compile-fail/borrowck-lend-flow.rs
370 // let mut v = ~3, w = ~4;
371 // let mut x = &mut w;
374 // borrow(v); //~ ERROR cannot borrow
375 // x = &mut v; // (1)
378 // Typically, we try to determine the region of a borrow from
379 // those points where it is dereferenced. In this case, one
380 // might imagine that the lifetime of `x` need only be the
381 // body of the loop. But of course this is incorrect because
382 // the pointer that is created at point (1) is consumed at
383 // point (2), meaning that it must be live across the loop
384 // iteration. The easiest way to guarantee this is to require
385 // that the lifetime of any regions that appear in a
386 // variable's type enclose at least the variable's scope.
388 let var_region = tcx.region_maps.var_region(id);
389 constrain_regions_in_type_of_node(
391 infer::BindingTypeIsNotValidAtDecl(span));
395 fn visit_expr(rcx: &mut Rcx, expr: &ast::Expr) {
396 debug!("regionck::visit_expr(e={}, repeating_scope={:?})",
397 expr.repr(rcx.fcx.tcx()), rcx.repeating_scope);
399 let method_call = MethodCall::expr(expr.id);
400 let has_method_map = rcx.fcx.inh.method_map.borrow().contains_key(&method_call);
402 // Check any autoderefs or autorefs that appear.
403 for &adjustment in rcx.fcx.inh.adjustments.borrow().find(&expr.id).iter() {
404 debug!("adjustment={:?}", adjustment);
406 ty::AutoDerefRef(ty::AutoDerefRef {autoderefs, autoref: opt_autoref}) => {
407 let expr_ty = rcx.resolve_node_type(expr.id);
408 constrain_autoderefs(rcx, expr, autoderefs, expr_ty);
409 for autoref in opt_autoref.iter() {
410 link_autoref(rcx, expr, autoderefs, autoref);
412 // Require that the resulting region encompasses
415 // FIXME(#6268) remove to support nested method calls
416 constrain_regions_in_type_of_node(
417 rcx, expr.id, ty::ReScope(expr.id),
418 infer::AutoBorrow(expr.span));
421 ty::AutoObject(ty::RegionTraitStore(trait_region, _), _, _, _) => {
422 // Determine if we are casting `expr` to a trait
423 // instance. If so, we have to be sure that the type of
424 // the source obeys the trait's region bound.
426 // Note: there is a subtle point here concerning type
427 // parameters. It is possible that the type of `source`
428 // contains type parameters, which in turn may contain
429 // regions that are not visible to us (only the caller
430 // knows about them). The kind checker is ultimately
431 // responsible for guaranteeing region safety in that
432 // particular case. There is an extensive comment on the
433 // function check_cast_for_escaping_regions() in kind.rs
434 // explaining how it goes about doing that.
436 let source_ty = rcx.fcx.expr_ty(expr);
437 constrain_regions_in_type(rcx, trait_region,
438 infer::RelateObjectBound(expr.span), source_ty);
445 ast::ExprCall(callee, ref args) => {
446 constrain_callee(rcx, callee.id, expr, callee);
454 visit::walk_expr(rcx, expr, ());
457 ast::ExprMethodCall(_, _, ref args) => {
458 constrain_call(rcx, None, expr, Some(*args.get(0)),
459 args.slice_from(1), false);
461 visit::walk_expr(rcx, expr, ());
464 ast::ExprAssign(lhs, _) => {
465 adjust_borrow_kind_for_assignment_lhs(rcx, lhs);
466 visit::walk_expr(rcx, expr, ());
469 ast::ExprAssignOp(_, lhs, rhs) => {
471 constrain_call(rcx, None, expr, Some(lhs), [rhs], true);
474 adjust_borrow_kind_for_assignment_lhs(rcx, lhs);
476 visit::walk_expr(rcx, expr, ());
479 ast::ExprIndex(lhs, rhs) |
480 ast::ExprBinary(_, lhs, rhs) if has_method_map => {
481 // As `expr_method_call`, but the call is via an
482 // overloaded op. Note that we (sadly) currently use an
483 // implicit "by ref" sort of passing style here. This
484 // should be converted to an adjustment!
485 constrain_call(rcx, None, expr, Some(lhs), [rhs], true);
487 visit::walk_expr(rcx, expr, ());
490 ast::ExprUnary(_, lhs) if has_method_map => {
492 constrain_call(rcx, None, expr, Some(lhs), [], true);
494 visit::walk_expr(rcx, expr, ());
497 ast::ExprUnary(ast::UnDeref, base) => {
498 // For *a, the lifetime of a must enclose the deref
499 let method_call = MethodCall::expr(expr.id);
500 let base_ty = match rcx.fcx.inh.method_map.borrow().find(&method_call) {
502 constrain_call(rcx, None, expr, Some(base), [], true);
503 ty::ty_fn_ret(method.ty)
505 None => rcx.resolve_node_type(base.id)
507 match ty::get(base_ty).sty {
508 ty::ty_rptr(r_ptr, _) => {
509 mk_subregion_due_to_dereference(rcx, expr.span,
510 ty::ReScope(expr.id), r_ptr);
515 visit::walk_expr(rcx, expr, ());
518 ast::ExprIndex(vec_expr, _) => {
519 // For a[b], the lifetime of a must enclose the deref
520 let vec_type = rcx.resolve_expr_type_adjusted(vec_expr);
521 constrain_index(rcx, expr, vec_type);
523 visit::walk_expr(rcx, expr, ());
526 ast::ExprCast(source, _) => {
527 // Determine if we are casting `source` to a trait
528 // instance. If so, we have to be sure that the type of
529 // the source obeys the trait's region bound.
531 // Note: there is a subtle point here concerning type
532 // parameters. It is possible that the type of `source`
533 // contains type parameters, which in turn may contain
534 // regions that are not visible to us (only the caller
535 // knows about them). The kind checker is ultimately
536 // responsible for guaranteeing region safety in that
537 // particular case. There is an extensive comment on the
538 // function check_cast_for_escaping_regions() in kind.rs
539 // explaining how it goes about doing that.
540 let target_ty = rcx.resolve_node_type(expr.id);
541 match ty::get(target_ty).sty {
542 ty::ty_trait(~ty::TyTrait {
543 store: ty::RegionTraitStore(trait_region, _), ..
545 let source_ty = rcx.resolve_expr_type_adjusted(source);
546 constrain_regions_in_type(
549 infer::RelateObjectBound(expr.span),
555 visit::walk_expr(rcx, expr, ());
558 ast::ExprAddrOf(m, base) => {
559 link_addr_of(rcx, expr, m, base);
561 // Require that when you write a `&expr` expression, the
562 // resulting pointer has a lifetime that encompasses the
563 // `&expr` expression itself. Note that we constraining
564 // the type of the node expr.id here *before applying
567 // FIXME(#6268) nested method calls requires that this rule change
568 let ty0 = rcx.resolve_node_type(expr.id);
569 constrain_regions_in_type(rcx, ty::ReScope(expr.id),
570 infer::AddrOf(expr.span), ty0);
571 visit::walk_expr(rcx, expr, ());
574 ast::ExprMatch(discr, ref arms) => {
575 link_match(rcx, discr, arms.as_slice());
577 visit::walk_expr(rcx, expr, ());
580 ast::ExprFnBlock(_, ref body) | ast::ExprProc(_, ref body) => {
581 check_expr_fn_block(rcx, expr, &**body);
584 ast::ExprLoop(body, _) => {
585 let repeating_scope = rcx.set_repeating_scope(body.id);
586 visit::walk_expr(rcx, expr, ());
587 rcx.set_repeating_scope(repeating_scope);
590 ast::ExprWhile(cond, body) => {
591 let repeating_scope = rcx.set_repeating_scope(cond.id);
592 rcx.visit_expr(cond, ());
594 rcx.set_repeating_scope(body.id);
595 rcx.visit_block(body, ());
597 rcx.set_repeating_scope(repeating_scope);
601 visit::walk_expr(rcx, expr, ());
606 fn check_expr_fn_block(rcx: &mut Rcx,
609 let tcx = rcx.fcx.tcx();
610 let function_type = rcx.resolve_node_type(expr.id);
611 match ty::get(function_type).sty {
612 ty::ty_closure(~ty::ClosureTy {
613 store: ty::RegionTraitStore(region, _), ..}) => {
614 freevars::with_freevars(tcx, expr.id, |freevars| {
615 if freevars.is_empty() {
616 // No free variables means that the environment
617 // will be NULL at runtime and hence the closure
618 // has static lifetime.
620 // Closure must not outlive the variables it closes over.
621 constrain_free_variables(rcx, region, expr, freevars);
623 // Closure cannot outlive the appropriate temporary scope.
624 let s = rcx.repeating_scope;
625 rcx.fcx.mk_subr(true, infer::InfStackClosure(expr.span),
626 region, ty::ReScope(s));
633 let repeating_scope = rcx.set_repeating_scope(body.id);
634 visit::walk_expr(rcx, expr, ());
635 rcx.set_repeating_scope(repeating_scope);
637 match ty::get(function_type).sty {
638 ty::ty_closure(~ty::ClosureTy {store: ty::RegionTraitStore(..), ..}) => {
639 freevars::with_freevars(tcx, expr.id, |freevars| {
640 propagate_upupvar_borrow_kind(rcx, expr, freevars);
646 fn constrain_free_variables(rcx: &mut Rcx,
649 freevars: &[freevars::freevar_entry]) {
651 * Make sure that all free variables referenced inside the closure
652 * outlive the closure itself. Also, create an entry in the
653 * upvar_borrows map with a region.
656 let tcx = rcx.fcx.ccx.tcx;
657 let infcx = rcx.fcx.infcx();
658 debug!("constrain_free_variables({}, {})",
659 region.repr(tcx), expr.repr(tcx));
660 for freevar in freevars.iter() {
661 debug!("freevar def is {:?}", freevar.def);
663 // Identify the variable being closed over and its node-id.
664 let def = freevar.def;
665 let def_id = ast_util::def_id_of_def(def);
666 assert!(def_id.krate == ast::LOCAL_CRATE);
667 let upvar_id = ty::UpvarId { var_id: def_id.node,
668 closure_expr_id: expr.id };
670 // Create a region variable to represent this borrow. This borrow
671 // must outlive the region on the closure.
672 let origin = infer::UpvarRegion(upvar_id, expr.span);
673 let freevar_region = infcx.next_region_var(origin);
674 rcx.fcx.mk_subr(true, infer::FreeVariable(freevar.span, def_id.node),
675 region, freevar_region);
677 // Create a UpvarBorrow entry. Note that we begin with a
678 // const borrow_kind, but change it to either mut or
679 // immutable as dictated by the uses.
680 let upvar_borrow = ty::UpvarBorrow { kind: ty::ImmBorrow,
681 region: freevar_region };
682 rcx.fcx.inh.upvar_borrow_map.borrow_mut().insert(upvar_id,
685 // Guarantee that the closure does not outlive the variable itself.
686 let en_region = region_of_def(rcx.fcx, def);
687 debug!("en_region = {}", en_region.repr(tcx));
688 rcx.fcx.mk_subr(true, infer::FreeVariable(freevar.span, def_id.node),
693 fn propagate_upupvar_borrow_kind(rcx: &mut Rcx,
695 freevars: &[freevars::freevar_entry]) {
696 let tcx = rcx.fcx.ccx.tcx;
697 debug!("propagate_upupvar_borrow_kind({})", expr.repr(tcx));
698 for freevar in freevars.iter() {
699 // Because of the semi-hokey way that we are doing
700 // borrow_kind inference, we need to check for
701 // indirect dependencies, like so:
710 // Here, the `inner_call` is basically "reborrowing" the
711 // outer pointer. With no other changes, `inner_call`
712 // would infer that it requires a mutable borrow, but
713 // `outer_call` would infer that a const borrow is
714 // sufficient. This is because we haven't linked the
715 // borrow_kind of the borrow that occurs in the inner
716 // closure to the borrow_kind of the borrow in the outer
717 // closure. Note that regions *are* naturally linked
718 // because we have a proper inference scheme there.
720 // Anyway, for borrow_kind, we basically go back over now
721 // after checking the inner closure (and hence
722 // determining the final borrow_kind) and propagate that as
723 // a constraint on the outer closure.
725 ast::DefUpvar(var_id, _, outer_closure_id, _) => {
726 // thing being captured is itself an upvar:
727 let outer_upvar_id = ty::UpvarId {
729 closure_expr_id: outer_closure_id };
730 let inner_upvar_id = ty::UpvarId {
732 closure_expr_id: expr.id };
733 link_upvar_borrow_kind_for_nested_closures(rcx,
743 fn constrain_callee(rcx: &mut Rcx,
744 callee_id: ast::NodeId,
745 call_expr: &ast::Expr,
746 callee_expr: &ast::Expr) {
747 let call_region = ty::ReScope(call_expr.id);
749 let callee_ty = rcx.resolve_node_type(callee_id);
750 match ty::get(callee_ty).sty {
751 ty::ty_bare_fn(..) => { }
752 ty::ty_closure(ref closure_ty) => {
753 let region = match closure_ty.store {
754 ty::RegionTraitStore(r, _) => {
755 // While we're here, link the closure's region with a unique
756 // immutable borrow (gathered later in borrowck)
757 let mc = mc::MemCategorizationContext::new(rcx);
758 let expr_cmt = ignore_err!(mc.cat_expr(callee_expr));
759 link_region(rcx, callee_expr.span, call_region,
760 ty::UniqueImmBorrow, expr_cmt);
763 ty::UniqTraitStore => ty::ReStatic
765 rcx.fcx.mk_subr(true, infer::InvokeClosure(callee_expr.span),
766 call_region, region);
769 // this should not happen, but it does if the program is
772 // tcx.sess.span_bug(
774 // format!("Calling non-function: {}", callee_ty.repr(tcx)));
779 fn constrain_call(rcx: &mut Rcx,
780 // might be expr_call, expr_method_call, or an overloaded
782 fn_expr_id: Option<ast::NodeId>,
783 call_expr: &ast::Expr,
784 receiver: Option<@ast::Expr>,
785 arg_exprs: &[@ast::Expr],
786 implicitly_ref_args: bool) {
787 //! Invoked on every call site (i.e., normal calls, method calls,
788 //! and overloaded operators). Constrains the regions which appear
789 //! in the type of the function. Also constrains the regions that
790 //! appear in the arguments appropriately.
792 let tcx = rcx.fcx.tcx();
793 debug!("constrain_call(call_expr={}, \
796 implicitly_ref_args={:?})",
800 implicitly_ref_args);
801 let callee_ty = match fn_expr_id {
802 Some(id) => rcx.resolve_node_type(id),
803 None => rcx.resolve_method_type(MethodCall::expr(call_expr.id))
804 .expect("call should have been to a method")
806 if ty::type_is_error(callee_ty) {
807 // Bail, as function type is unknown
810 let fn_sig = ty::ty_fn_sig(callee_ty);
812 // `callee_region` is the scope representing the time in which the
815 // FIXME(#6268) to support nested method calls, should be callee_id
816 let callee_scope = call_expr.id;
817 let callee_region = ty::ReScope(callee_scope);
819 for &arg_expr in arg_exprs.iter() {
822 // ensure that any regions appearing in the argument type are
823 // valid for at least the lifetime of the function:
824 constrain_regions_in_type_of_node(
825 rcx, arg_expr.id, callee_region,
826 infer::CallArg(arg_expr.span));
828 // unfortunately, there are two means of taking implicit
829 // references, and we need to propagate constraints as a
830 // result. modes are going away and the "DerefArgs" code
831 // should be ported to use adjustments
832 if implicitly_ref_args {
833 link_by_ref(rcx, arg_expr, callee_scope);
837 // as loop above, but for receiver
838 for &r in receiver.iter() {
840 constrain_regions_in_type_of_node(
841 rcx, r.id, callee_region, infer::CallRcvr(r.span));
842 if implicitly_ref_args {
843 link_by_ref(rcx, r, callee_scope);
847 // constrain regions that may appear in the return type to be
848 // valid for the function call:
849 constrain_regions_in_type(
850 rcx, callee_region, infer::CallReturn(call_expr.span),
854 fn constrain_autoderefs(rcx: &mut Rcx,
855 deref_expr: &ast::Expr,
857 mut derefd_ty: ty::t) {
859 * Invoked on any auto-dereference that occurs. Checks that if
860 * this is a region pointer being dereferenced, the lifetime of
861 * the pointer includes the deref expr.
863 let r_deref_expr = ty::ReScope(deref_expr.id);
864 for i in range(0u, derefs) {
865 debug!("constrain_autoderefs(deref_expr=?, derefd_ty={}, derefs={:?}/{:?}",
866 rcx.fcx.infcx().ty_to_str(derefd_ty),
869 let method_call = MethodCall::autoderef(deref_expr.id, i as u32);
870 derefd_ty = match rcx.fcx.inh.method_map.borrow().find(&method_call) {
872 // Treat overloaded autoderefs as if an AutoRef adjustment
873 // was applied on the base type, as that is always the case.
874 let fn_sig = ty::ty_fn_sig(method.ty);
875 let self_ty = *fn_sig.inputs.get(0);
876 let (m, r) = match ty::get(self_ty).sty {
877 ty::ty_rptr(r, ref m) => (m.mutbl, r),
878 _ => rcx.tcx().sess.span_bug(deref_expr.span,
879 format!("bad overloaded deref type {}",
880 method.ty.repr(rcx.tcx())))
883 let mc = mc::MemCategorizationContext::new(rcx);
884 let self_cmt = ignore_err!(mc.cat_expr_autoderefd(deref_expr, i));
885 link_region(rcx, deref_expr.span, r,
886 ty::BorrowKind::from_mutbl(m), self_cmt);
889 // Specialized version of constrain_call.
890 constrain_regions_in_type(rcx, r_deref_expr,
891 infer::CallRcvr(deref_expr.span),
893 constrain_regions_in_type(rcx, r_deref_expr,
894 infer::CallReturn(deref_expr.span),
901 match ty::get(derefd_ty).sty {
902 ty::ty_rptr(r_ptr, _) => {
903 mk_subregion_due_to_dereference(rcx, deref_expr.span,
904 r_deref_expr, r_ptr);
909 match ty::deref(derefd_ty, true) {
910 Some(mt) => derefd_ty = mt.ty,
911 /* if this type can't be dereferenced, then there's already an error
912 in the session saying so. Just bail out for now */
918 pub fn mk_subregion_due_to_dereference(rcx: &mut Rcx,
920 minimum_lifetime: ty::Region,
921 maximum_lifetime: ty::Region) {
922 rcx.fcx.mk_subr(true, infer::DerefPointer(deref_span),
923 minimum_lifetime, maximum_lifetime)
927 fn constrain_index(rcx: &mut Rcx,
928 index_expr: &ast::Expr,
932 * Invoked on any index expression that occurs. Checks that if
933 * this is a slice being indexed, the lifetime of the pointer
934 * includes the deref expr.
937 debug!("constrain_index(index_expr=?, indexed_ty={}",
938 rcx.fcx.infcx().ty_to_str(indexed_ty));
940 let r_index_expr = ty::ReScope(index_expr.id);
941 match ty::get(indexed_ty).sty {
942 ty::ty_rptr(r_ptr, mt) => match ty::get(mt.ty).sty {
943 ty::ty_vec(_, None) | ty::ty_str => {
944 rcx.fcx.mk_subr(true, infer::IndexSlice(index_expr.span),
945 r_index_expr, r_ptr);
954 fn constrain_regions_in_type_of_node(
957 minimum_lifetime: ty::Region,
958 origin: infer::SubregionOrigin) {
959 //! Guarantees that any lifetimes which appear in the type of
960 //! the node `id` (after applying adjustments) are valid for at
961 //! least `minimum_lifetime`
963 let tcx = rcx.fcx.tcx();
965 // Try to resolve the type. If we encounter an error, then typeck
966 // is going to fail anyway, so just stop here and let typeck
967 // report errors later on in the writeback phase.
968 let ty0 = rcx.resolve_node_type(id);
969 let ty = ty::adjust_ty(tcx, origin.span(), id, ty0,
970 rcx.fcx.inh.adjustments.borrow().find(&id),
971 |method_call| rcx.resolve_method_type(method_call));
972 debug!("constrain_regions_in_type_of_node(\
973 ty={}, ty0={}, id={}, minimum_lifetime={:?})",
974 ty_to_str(tcx, ty), ty_to_str(tcx, ty0),
975 id, minimum_lifetime);
976 constrain_regions_in_type(rcx, minimum_lifetime, origin, ty);
979 fn constrain_regions_in_type(
981 minimum_lifetime: ty::Region,
982 origin: infer::SubregionOrigin,
985 * Requires that any regions which appear in `ty` must be
986 * superregions of `minimum_lifetime`. Also enforces the constraint
987 * that given a pointer type `&'r T`, T must not contain regions
988 * that outlive 'r, as well as analogous constraints for other
991 * This check prevents regions from being used outside of the block in
992 * which they are valid. Recall that regions represent blocks of
993 * code or expressions: this requirement basically says "any place
994 * that uses or may use a region R must be within the block of
995 * code that R corresponds to."
998 let tcx = rcx.fcx.ccx.tcx;
1000 debug!("constrain_regions_in_type(minimum_lifetime={}, ty={})",
1001 region_to_str(tcx, "", false, minimum_lifetime),
1002 ty_to_str(tcx, ty));
1004 relate_nested_regions(tcx, Some(minimum_lifetime), ty, |r_sub, r_sup| {
1005 debug!("relate_nested_regions(r_sub={}, r_sup={})",
1009 if r_sup.is_bound() || r_sub.is_bound() {
1010 // a bound region is one which appears inside an fn type.
1011 // (e.g., the `&` in `fn(&T)`). Such regions need not be
1012 // constrained by `minimum_lifetime` as they are placeholders
1013 // for regions that are as-yet-unknown.
1014 } else if r_sub == minimum_lifetime {
1016 true, origin.clone(),
1020 true, infer::ReferenceOutlivesReferent(ty, origin.span()),
1026 fn link_addr_of(rcx: &mut Rcx, expr: &ast::Expr,
1027 mutability: ast::Mutability, base: &ast::Expr) {
1029 * Computes the guarantor for an expression `&base` and then
1030 * ensures that the lifetime of the resulting pointer is linked
1031 * to the lifetime of its guarantor (if any).
1034 debug!("link_addr_of(base=?)");
1037 let mc = mc::MemCategorizationContext::new(rcx);
1038 ignore_err!(mc.cat_expr(base))
1040 link_region_from_node_type(rcx, expr.span, expr.id, mutability, cmt);
1043 fn link_local(rcx: &Rcx, local: &ast::Local) {
1045 * Computes the guarantors for any ref bindings in a `let` and
1046 * then ensures that the lifetime of the resulting pointer is
1047 * linked to the lifetime of the initialization expression.
1050 debug!("regionck::for_local()");
1051 let init_expr = match local.init {
1055 let mc = mc::MemCategorizationContext::new(rcx);
1056 let discr_cmt = ignore_err!(mc.cat_expr(init_expr));
1057 link_pattern(rcx, mc, discr_cmt, local.pat);
1060 fn link_match(rcx: &Rcx, discr: &ast::Expr, arms: &[ast::Arm]) {
1062 * Computes the guarantors for any ref bindings in a match and
1063 * then ensures that the lifetime of the resulting pointer is
1064 * linked to the lifetime of its guarantor (if any).
1067 debug!("regionck::for_match()");
1068 let mc = mc::MemCategorizationContext::new(rcx);
1069 let discr_cmt = ignore_err!(mc.cat_expr(discr));
1070 debug!("discr_cmt={}", discr_cmt.repr(rcx.tcx()));
1071 for arm in arms.iter() {
1072 for &root_pat in arm.pats.iter() {
1073 link_pattern(rcx, mc, discr_cmt.clone(), root_pat);
1078 fn link_pattern(rcx: &Rcx,
1079 mc: mc::MemCategorizationContext<Rcx>,
1081 root_pat: &ast::Pat) {
1083 * Link lifetimes of any ref bindings in `root_pat` to
1084 * the pointers found in the discriminant, if needed.
1087 let _ = mc.cat_pattern(discr_cmt, root_pat, |mc, sub_cmt, sub_pat| {
1088 match sub_pat.node {
1090 ast::PatIdent(ast::BindByRef(mutbl), _, _) => {
1091 link_region_from_node_type(
1092 rcx, sub_pat.span, sub_pat.id,
1096 // `[_, ..slice, _]` pattern
1097 ast::PatVec(_, Some(slice_pat), _) => {
1098 match mc.cat_slice_pattern(sub_cmt, slice_pat) {
1099 Ok((slice_cmt, slice_mutbl, slice_r)) => {
1100 link_region(rcx, sub_pat.span, slice_r,
1101 ty::BorrowKind::from_mutbl(slice_mutbl),
1112 fn link_autoref(rcx: &Rcx,
1115 autoref: &ty::AutoRef) {
1117 * Link lifetime of borrowed pointer resulting from autoref
1118 * to lifetimes in the value being autoref'd.
1121 debug!("link_autoref(autoref={:?})", autoref);
1122 let mc = mc::MemCategorizationContext::new(rcx);
1123 let expr_cmt = ignore_err!(mc.cat_expr_autoderefd(expr, autoderefs));
1124 debug!("expr_cmt={}", expr_cmt.repr(rcx.tcx()));
1127 ty::AutoPtr(r, m) => {
1128 link_region(rcx, expr.span, r,
1129 ty::BorrowKind::from_mutbl(m), expr_cmt);
1132 ty::AutoBorrowVec(r, m) | ty::AutoBorrowVecRef(r, m) => {
1133 let cmt_index = mc.cat_index(expr, expr_cmt, autoderefs+1);
1134 link_region(rcx, expr.span, r,
1135 ty::BorrowKind::from_mutbl(m), cmt_index);
1138 ty::AutoBorrowObj(r, m) => {
1139 let cmt_deref = mc.cat_deref_obj(expr, expr_cmt);
1140 link_region(rcx, expr.span, r,
1141 ty::BorrowKind::from_mutbl(m), cmt_deref);
1144 ty::AutoUnsafe(_) => {}
1148 fn link_by_ref(rcx: &Rcx,
1150 callee_scope: ast::NodeId) {
1152 * Computes the guarantor for cases where the `expr` is
1153 * being passed by implicit reference and must outlive
1157 let tcx = rcx.tcx();
1158 debug!("link_by_ref(expr={}, callee_scope={})",
1159 expr.repr(tcx), callee_scope);
1160 let mc = mc::MemCategorizationContext::new(rcx);
1161 let expr_cmt = ignore_err!(mc.cat_expr(expr));
1162 let region_min = ty::ReScope(callee_scope);
1163 link_region(rcx, expr.span, region_min, ty::ImmBorrow, expr_cmt);
1166 fn link_region_from_node_type(rcx: &Rcx,
1169 mutbl: ast::Mutability,
1170 cmt_borrowed: mc::cmt) {
1172 * Like `link_region()`, except that the region is
1173 * extracted from the type of `id`, which must be some
1174 * reference (`&T`, `&str`, etc).
1177 let rptr_ty = rcx.resolve_node_type(id);
1178 if !ty::type_is_bot(rptr_ty) && !ty::type_is_error(rptr_ty) {
1179 let tcx = rcx.fcx.ccx.tcx;
1180 debug!("rptr_ty={}", ty_to_str(tcx, rptr_ty));
1181 let r = ty::ty_region(tcx, span, rptr_ty);
1182 link_region(rcx, span, r, ty::BorrowKind::from_mutbl(mutbl),
1187 fn link_region(rcx: &Rcx,
1189 region_min: ty::Region,
1190 kind: ty::BorrowKind,
1191 cmt_borrowed: mc::cmt) {
1193 * Informs the inference engine that a borrow of `cmt`
1194 * must have the borrow kind `kind` and lifetime `region_min`.
1195 * If `cmt` is a deref of a region pointer with
1196 * lifetime `r_borrowed`, this will add the constraint that
1197 * `region_min <= r_borrowed`.
1200 // Iterate through all the things that must be live at least
1201 // for the lifetime `region_min` for the borrow to be valid:
1202 let mut cmt_borrowed = cmt_borrowed;
1204 debug!("link_region(region_min={}, kind={}, cmt_borrowed={})",
1205 region_min.repr(rcx.tcx()),
1206 kind.repr(rcx.tcx()),
1207 cmt_borrowed.repr(rcx.tcx()));
1208 match cmt_borrowed.cat.clone() {
1209 mc::cat_deref(base, _, mc::BorrowedPtr(_, r_borrowed)) => {
1210 // References to an upvar `x` are translated to
1211 // `*x`, since that is what happens in the
1212 // underlying machine. We detect such references
1213 // and treat them slightly differently, both to
1214 // offer better error messages and because we need
1215 // to infer the kind of borrow (mut, const, etc)
1216 // to use for each upvar.
1217 let cause = match base.cat {
1218 mc::cat_upvar(ref upvar_id, _) => {
1219 match rcx.fcx.inh.upvar_borrow_map.borrow_mut()
1220 .find_mut(upvar_id) {
1221 Some(upvar_borrow) => {
1222 debug!("link_region: {} <= {}",
1223 region_min.repr(rcx.tcx()),
1224 upvar_borrow.region.repr(rcx.tcx()));
1225 adjust_upvar_borrow_kind_for_loan(
1229 infer::ReborrowUpvar(span, *upvar_id)
1232 rcx.tcx().sess.span_bug(
1234 format!("Illegal upvar id: {}",
1235 upvar_id.repr(rcx.tcx())));
1241 infer::Reborrow(span)
1245 debug!("link_region: {} <= {}",
1246 region_min.repr(rcx.tcx()),
1247 r_borrowed.repr(rcx.tcx()));
1248 rcx.fcx.mk_subr(true, cause, region_min, r_borrowed);
1250 if kind != ty::ImmBorrow {
1251 // If this is a mutable borrow, then the thing
1252 // being borrowed will have to be unique.
1253 // In user code, this means it must be an `&mut`
1254 // borrow, but for an upvar, we might opt
1255 // for an immutable-unique borrow.
1256 adjust_upvar_borrow_kind_for_unique(rcx, base);
1259 // Borrowing an `&mut` pointee for `region_min` is
1260 // only valid if the pointer resides in a unique
1261 // location which is itself valid for
1262 // `region_min`. We don't care about the unique
1263 // part, but we may need to influence the
1264 // inference to ensure that the location remains
1267 // FIXME(#8624) fixing borrowck will require this
1268 // if m == ast::m_mutbl {
1269 // cmt_borrowed = cmt_base;
1275 mc::cat_discr(cmt_base, _) |
1276 mc::cat_downcast(cmt_base) |
1277 mc::cat_deref(cmt_base, _, mc::GcPtr(..)) |
1278 mc::cat_deref(cmt_base, _, mc::OwnedPtr) |
1279 mc::cat_interior(cmt_base, _) => {
1280 // Interior or owned data requires its base to be valid
1281 cmt_borrowed = cmt_base;
1283 mc::cat_deref(_, _, mc::UnsafePtr(..)) |
1284 mc::cat_static_item |
1285 mc::cat_copied_upvar(..) |
1289 mc::cat_rvalue(..) => {
1290 // These are all "base cases" with independent lifetimes
1291 // that are not subject to inference
1298 fn adjust_borrow_kind_for_assignment_lhs(rcx: &Rcx,
1301 * Adjusts the inferred borrow_kind as needed to account
1302 * for upvars that are assigned to in an assignment
1306 let mc = mc::MemCategorizationContext::new(rcx);
1307 let cmt = ignore_err!(mc.cat_expr(lhs));
1308 adjust_upvar_borrow_kind_for_mut(rcx, cmt);
1311 fn adjust_upvar_borrow_kind_for_mut(rcx: &Rcx,
1315 debug!("adjust_upvar_borrow_kind_for_mut(cmt={})",
1316 cmt.repr(rcx.tcx()));
1318 match cmt.cat.clone() {
1319 mc::cat_deref(base, _, mc::OwnedPtr) |
1320 mc::cat_interior(base, _) |
1321 mc::cat_downcast(base) |
1322 mc::cat_discr(base, _) => {
1323 // Interior or owned data is mutable if base is
1324 // mutable, so iterate to the base.
1329 mc::cat_deref(base, _, mc::BorrowedPtr(..)) => {
1331 mc::cat_upvar(ref upvar_id, _) => {
1332 // if this is an implicit deref of an
1333 // upvar, then we need to modify the
1334 // borrow_kind of the upvar to make sure it
1335 // is inferred to mutable if necessary
1336 let mut upvar_borrow_map =
1337 rcx.fcx.inh.upvar_borrow_map.borrow_mut();
1338 let ub = upvar_borrow_map.get_mut(upvar_id);
1339 return adjust_upvar_borrow_kind(*upvar_id, ub, ty::MutBorrow);
1345 // assignment to deref of an `&mut`
1346 // borrowed pointer implies that the
1347 // pointer itself must be unique, but not
1348 // necessarily *mutable*
1349 return adjust_upvar_borrow_kind_for_unique(rcx, base);
1352 mc::cat_deref(_, _, mc::UnsafePtr(..)) |
1353 mc::cat_deref(_, _, mc::GcPtr) |
1354 mc::cat_static_item |
1356 mc::cat_copied_upvar(_) |
1359 mc::cat_upvar(..) => {
1366 fn adjust_upvar_borrow_kind_for_unique(rcx: &Rcx, cmt: mc::cmt) {
1369 debug!("adjust_upvar_borrow_kind_for_unique(cmt={})",
1370 cmt.repr(rcx.tcx()));
1372 match cmt.cat.clone() {
1373 mc::cat_deref(base, _, mc::OwnedPtr) |
1374 mc::cat_interior(base, _) |
1375 mc::cat_downcast(base) |
1376 mc::cat_discr(base, _) => {
1377 // Interior or owned data is unique if base is
1383 mc::cat_deref(base, _, mc::BorrowedPtr(..)) => {
1385 mc::cat_upvar(ref upvar_id, _) => {
1386 // if this is an implicit deref of an
1387 // upvar, then we need to modify the
1388 // borrow_kind of the upvar to make sure it
1389 // is inferred to unique if necessary
1390 let mut ub = rcx.fcx.inh.upvar_borrow_map.borrow_mut();
1391 let ub = ub.get_mut(upvar_id);
1392 return adjust_upvar_borrow_kind(*upvar_id, ub, ty::UniqueImmBorrow);
1398 // for a borrowed pointer to be unique, its
1399 // base must be unique
1400 return adjust_upvar_borrow_kind_for_unique(rcx, base);
1403 mc::cat_deref(_, _, mc::UnsafePtr(..)) |
1404 mc::cat_deref(_, _, mc::GcPtr) |
1405 mc::cat_static_item |
1407 mc::cat_copied_upvar(_) |
1410 mc::cat_upvar(..) => {
1417 fn link_upvar_borrow_kind_for_nested_closures(rcx: &mut Rcx,
1418 inner_upvar_id: ty::UpvarId,
1419 outer_upvar_id: ty::UpvarId) {
1421 * Indicates that the borrow_kind of `outer_upvar_id` must
1422 * permit a reborrowing with the borrow_kind of `inner_upvar_id`.
1423 * This occurs in nested closures, see comment above at the call to
1427 debug!("link_upvar_borrow_kind: inner_upvar_id={:?} outer_upvar_id={:?}",
1428 inner_upvar_id, outer_upvar_id);
1430 let mut upvar_borrow_map = rcx.fcx.inh.upvar_borrow_map.borrow_mut();
1431 let inner_borrow = upvar_borrow_map.get_copy(&inner_upvar_id);
1432 match upvar_borrow_map.find_mut(&outer_upvar_id) {
1433 Some(outer_borrow) => {
1434 adjust_upvar_borrow_kind(outer_upvar_id, outer_borrow, inner_borrow.kind);
1436 None => { /* outer closure is not a stack closure */ }
1440 fn adjust_upvar_borrow_kind_for_loan(upvar_id: ty::UpvarId,
1441 upvar_borrow: &mut ty::UpvarBorrow,
1442 kind: ty::BorrowKind) {
1443 debug!("adjust_upvar_borrow_kind_for_loan: upvar_id={:?} kind={:?} -> {:?}",
1444 upvar_id, upvar_borrow.kind, kind);
1446 adjust_upvar_borrow_kind(upvar_id, upvar_borrow, kind)
1449 fn adjust_upvar_borrow_kind(upvar_id: ty::UpvarId,
1450 upvar_borrow: &mut ty::UpvarBorrow,
1451 kind: ty::BorrowKind) {
1453 * We infer the borrow_kind with which to borrow upvars in a stack
1454 * closure. The borrow_kind basically follows a lattice of
1455 * `imm < unique-imm < mut`, moving from left to right as needed (but never
1456 * right to left). Here the argument `mutbl` is the borrow_kind that
1457 * is required by some particular use.
1460 debug!("adjust_upvar_borrow_kind: id={:?} kind=({:?} -> {:?})",
1461 upvar_id, upvar_borrow.kind, kind);
1463 match (upvar_borrow.kind, kind) {
1465 (ty::ImmBorrow, ty::UniqueImmBorrow) |
1466 (ty::ImmBorrow, ty::MutBorrow) |
1467 (ty::UniqueImmBorrow, ty::MutBorrow) => {
1468 upvar_borrow.kind = kind;
1471 (ty::ImmBorrow, ty::ImmBorrow) |
1472 (ty::UniqueImmBorrow, ty::ImmBorrow) |
1473 (ty::UniqueImmBorrow, ty::UniqueImmBorrow) |
1474 (ty::MutBorrow, _) => {