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<'a, 'b> mc::Typer for &'a Rcx<'b> {
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 an 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 an 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, _) => r,
755 ty::UniqTraitStore => ty::ReStatic
757 rcx.fcx.mk_subr(true, infer::InvokeClosure(callee_expr.span),
758 call_region, region);
761 // this should not happen, but it does if the program is
764 // tcx.sess.span_bug(
766 // format!("Calling non-function: {}", callee_ty.repr(tcx)));
771 fn constrain_call(rcx: &mut Rcx,
772 // might be expr_call, expr_method_call, or an overloaded
774 fn_expr_id: Option<ast::NodeId>,
775 call_expr: &ast::Expr,
776 receiver: Option<@ast::Expr>,
777 arg_exprs: &[@ast::Expr],
778 implicitly_ref_args: bool) {
779 //! Invoked on every call site (i.e., normal calls, method calls,
780 //! and overloaded operators). Constrains the regions which appear
781 //! in the type of the function. Also constrains the regions that
782 //! appear in the arguments appropriately.
784 let tcx = rcx.fcx.tcx();
785 debug!("constrain_call(call_expr={}, \
788 implicitly_ref_args={:?})",
792 implicitly_ref_args);
793 let callee_ty = match fn_expr_id {
794 Some(id) => rcx.resolve_node_type(id),
795 None => rcx.resolve_method_type(MethodCall::expr(call_expr.id))
796 .expect("call should have been to a method")
798 if ty::type_is_error(callee_ty) {
799 // Bail, as function type is unknown
802 let fn_sig = ty::ty_fn_sig(callee_ty);
804 // `callee_region` is the scope representing the time in which the
807 // FIXME(#6268) to support nested method calls, should be callee_id
808 let callee_scope = call_expr.id;
809 let callee_region = ty::ReScope(callee_scope);
811 for &arg_expr in arg_exprs.iter() {
814 // ensure that any regions appearing in the argument type are
815 // valid for at least the lifetime of the function:
816 constrain_regions_in_type_of_node(
817 rcx, arg_expr.id, callee_region,
818 infer::CallArg(arg_expr.span));
820 // unfortunately, there are two means of taking implicit
821 // references, and we need to propagate constraints as a
822 // result. modes are going away and the "DerefArgs" code
823 // should be ported to use adjustments
824 if implicitly_ref_args {
825 link_by_ref(rcx, arg_expr, callee_scope);
829 // as loop above, but for receiver
830 for &r in receiver.iter() {
832 constrain_regions_in_type_of_node(
833 rcx, r.id, callee_region, infer::CallRcvr(r.span));
834 if implicitly_ref_args {
835 link_by_ref(rcx, r, callee_scope);
839 // constrain regions that may appear in the return type to be
840 // valid for the function call:
841 constrain_regions_in_type(
842 rcx, callee_region, infer::CallReturn(call_expr.span),
846 fn constrain_autoderefs(rcx: &mut Rcx,
847 deref_expr: &ast::Expr,
849 mut derefd_ty: ty::t) {
851 * Invoked on any auto-dereference that occurs. Checks that if
852 * this is a region pointer being dereferenced, the lifetime of
853 * the pointer includes the deref expr.
855 let r_deref_expr = ty::ReScope(deref_expr.id);
856 for i in range(0u, derefs) {
857 debug!("constrain_autoderefs(deref_expr=?, derefd_ty={}, derefs={:?}/{:?}",
858 rcx.fcx.infcx().ty_to_str(derefd_ty),
861 let method_call = MethodCall::autoderef(deref_expr.id, i as u32);
862 derefd_ty = match rcx.fcx.inh.method_map.borrow().find(&method_call) {
864 // Treat overloaded autoderefs as if an AutoRef adjustment
865 // was applied on the base type, as that is always the case.
866 let fn_sig = ty::ty_fn_sig(method.ty);
867 let self_ty = *fn_sig.inputs.get(0);
868 let (m, r) = match ty::get(self_ty).sty {
869 ty::ty_rptr(r, ref m) => (m.mutbl, r),
870 _ => rcx.tcx().sess.span_bug(deref_expr.span,
871 format!("bad overloaded deref type {}",
872 method.ty.repr(rcx.tcx())))
875 let mc = mc::MemCategorizationContext { typer: &*rcx };
876 let self_cmt = ignore_err!(mc.cat_expr_autoderefd(deref_expr, i));
877 link_region(mc.typer, deref_expr.span, r, m, self_cmt);
880 // Specialized version of constrain_call.
881 constrain_regions_in_type(rcx, r_deref_expr,
882 infer::CallRcvr(deref_expr.span),
884 constrain_regions_in_type(rcx, r_deref_expr,
885 infer::CallReturn(deref_expr.span),
892 match ty::get(derefd_ty).sty {
893 ty::ty_rptr(r_ptr, _) => {
894 mk_subregion_due_to_dereference(rcx, deref_expr.span,
895 r_deref_expr, r_ptr);
900 match ty::deref(derefd_ty, true) {
901 Some(mt) => derefd_ty = mt.ty,
902 /* if this type can't be dereferenced, then there's already an error
903 in the session saying so. Just bail out for now */
909 pub fn mk_subregion_due_to_dereference(rcx: &mut Rcx,
911 minimum_lifetime: ty::Region,
912 maximum_lifetime: ty::Region) {
913 rcx.fcx.mk_subr(true, infer::DerefPointer(deref_span),
914 minimum_lifetime, maximum_lifetime)
918 fn constrain_index(rcx: &mut Rcx,
919 index_expr: &ast::Expr,
923 * Invoked on any index expression that occurs. Checks that if
924 * this is a slice being indexed, the lifetime of the pointer
925 * includes the deref expr.
928 debug!("constrain_index(index_expr=?, indexed_ty={}",
929 rcx.fcx.infcx().ty_to_str(indexed_ty));
931 let r_index_expr = ty::ReScope(index_expr.id);
932 match ty::get(indexed_ty).sty {
933 ty::ty_str(ty::VstoreSlice(r_ptr)) => {
934 rcx.fcx.mk_subr(true, infer::IndexSlice(index_expr.span),
935 r_index_expr, r_ptr);
937 ty::ty_rptr(r_ptr, mt) => match ty::get(mt.ty).sty {
938 ty::ty_vec(_, None) => {
939 rcx.fcx.mk_subr(true, infer::IndexSlice(index_expr.span),
940 r_index_expr, r_ptr);
949 fn constrain_regions_in_type_of_node(
952 minimum_lifetime: ty::Region,
953 origin: infer::SubregionOrigin) {
954 //! Guarantees that any lifetimes which appear in the type of
955 //! the node `id` (after applying adjustments) are valid for at
956 //! least `minimum_lifetime`
958 let tcx = rcx.fcx.tcx();
960 // Try to resolve the type. If we encounter an error, then typeck
961 // is going to fail anyway, so just stop here and let typeck
962 // report errors later on in the writeback phase.
963 let ty0 = rcx.resolve_node_type(id);
964 let ty = ty::adjust_ty(tcx, origin.span(), id, ty0,
965 rcx.fcx.inh.adjustments.borrow().find(&id),
966 |method_call| rcx.resolve_method_type(method_call));
967 debug!("constrain_regions_in_type_of_node(\
968 ty={}, ty0={}, id={}, minimum_lifetime={:?})",
969 ty_to_str(tcx, ty), ty_to_str(tcx, ty0),
970 id, minimum_lifetime);
971 constrain_regions_in_type(rcx, minimum_lifetime, origin, ty);
974 fn constrain_regions_in_type(
976 minimum_lifetime: ty::Region,
977 origin: infer::SubregionOrigin,
980 * Requires that any regions which appear in `ty` must be
981 * superregions of `minimum_lifetime`. Also enforces the constraint
982 * that given a pointer type `&'r T`, T must not contain regions
983 * that outlive 'r, as well as analogous constraints for other
986 * This check prevents regions from being used outside of the block in
987 * which they are valid. Recall that regions represent blocks of
988 * code or expressions: this requirement basically says "any place
989 * that uses or may use a region R must be within the block of
990 * code that R corresponds to."
993 let tcx = rcx.fcx.ccx.tcx;
995 debug!("constrain_regions_in_type(minimum_lifetime={}, ty={})",
996 region_to_str(tcx, "", false, minimum_lifetime),
999 relate_nested_regions(tcx, Some(minimum_lifetime), ty, |r_sub, r_sup| {
1000 debug!("relate_nested_regions(r_sub={}, r_sup={})",
1004 if r_sup.is_bound() || r_sub.is_bound() {
1005 // a bound region is one which appears inside an fn type.
1006 // (e.g., the `&` in `fn(&T)`). Such regions need not be
1007 // constrained by `minimum_lifetime` as they are placeholders
1008 // for regions that are as-yet-unknown.
1009 } else if r_sub == minimum_lifetime {
1011 true, origin.clone(),
1015 true, infer::ReferenceOutlivesReferent(ty, origin.span()),
1021 fn link_addr_of(rcx: &mut Rcx, expr: &ast::Expr,
1022 mutability: ast::Mutability, base: &ast::Expr) {
1024 * Computes the guarantor for an expression `&base` and then
1025 * ensures that the lifetime of the resulting pointer is linked
1026 * to the lifetime of its guarantor (if any).
1029 debug!("link_addr_of(base=?)");
1032 let mc = mc::MemCategorizationContext { typer: &*rcx };
1033 ignore_err!(mc.cat_expr(base))
1035 link_region_from_node_type(rcx, expr.span, expr.id, mutability, cmt);
1038 fn link_local(rcx: &Rcx, local: &ast::Local) {
1040 * Computes the guarantors for any ref bindings in a `let` and
1041 * then ensures that the lifetime of the resulting pointer is
1042 * linked to the lifetime of the initialization expression.
1045 debug!("regionck::for_local()");
1046 let init_expr = match local.init {
1050 let mc = mc::MemCategorizationContext { typer: rcx };
1051 let discr_cmt = ignore_err!(mc.cat_expr(init_expr));
1052 link_pattern(mc, discr_cmt, local.pat);
1055 fn link_match(rcx: &Rcx, discr: &ast::Expr, arms: &[ast::Arm]) {
1057 * Computes the guarantors for any ref bindings in a match and
1058 * then ensures that the lifetime of the resulting pointer is
1059 * linked to the lifetime of its guarantor (if any).
1062 debug!("regionck::for_match()");
1063 let mc = mc::MemCategorizationContext { typer: rcx };
1064 let discr_cmt = ignore_err!(mc.cat_expr(discr));
1065 debug!("discr_cmt={}", discr_cmt.repr(mc.typer.tcx()));
1066 for arm in arms.iter() {
1067 for &root_pat in arm.pats.iter() {
1068 link_pattern(mc, discr_cmt.clone(), root_pat);
1073 fn link_pattern(mc: mc::MemCategorizationContext<&Rcx>,
1075 root_pat: &ast::Pat) {
1077 * Link lifetimes of any ref bindings in `root_pat` to
1078 * the pointers found in the discriminant, if needed.
1081 let _ = mc.cat_pattern(discr_cmt, root_pat, |mc, sub_cmt, sub_pat| {
1082 match sub_pat.node {
1084 ast::PatIdent(ast::BindByRef(mutbl), _, _) => {
1085 link_region_from_node_type(
1086 mc.typer, sub_pat.span, sub_pat.id,
1090 // `[_, ..slice, _]` pattern
1091 ast::PatVec(_, Some(slice_pat), _) => {
1092 match mc.cat_slice_pattern(sub_cmt, slice_pat) {
1093 Ok((slice_cmt, slice_mutbl, slice_r)) => {
1094 link_region(mc.typer, sub_pat.span, slice_r,
1095 slice_mutbl, slice_cmt);
1105 fn link_autoref(rcx: &Rcx,
1108 autoref: &ty::AutoRef) {
1110 * Link lifetime of borrowed pointer resulting from autoref
1111 * to lifetimes in the value being autoref'd.
1114 debug!("link_autoref(autoref={:?})", autoref);
1115 let mc = mc::MemCategorizationContext { typer: rcx };
1116 let expr_cmt = ignore_err!(mc.cat_expr_autoderefd(expr, autoderefs));
1117 debug!("expr_cmt={}", expr_cmt.repr(mc.typer.tcx()));
1120 ty::AutoPtr(r, m) => {
1121 link_region(mc.typer, expr.span, r, m, expr_cmt);
1124 ty::AutoBorrowVec(r, m) | ty::AutoBorrowVecRef(r, m) => {
1125 let cmt_index = mc.cat_index(expr, expr_cmt, autoderefs+1);
1126 link_region(mc.typer, expr.span, r, m, cmt_index);
1129 ty::AutoBorrowObj(r, m) => {
1130 let cmt_deref = mc.cat_deref_obj(expr, expr_cmt);
1131 link_region(mc.typer, expr.span, r, m, cmt_deref);
1134 ty::AutoUnsafe(_) => {}
1138 fn link_by_ref(rcx: &Rcx,
1140 callee_scope: ast::NodeId) {
1142 * Computes the guarantor for cases where the `expr` is
1143 * being passed by implicit reference and must outlive
1147 let tcx = rcx.tcx();
1148 debug!("link_by_ref(expr={}, callee_scope={})",
1149 expr.repr(tcx), callee_scope);
1150 let mc = mc::MemCategorizationContext { typer: rcx };
1151 let expr_cmt = ignore_err!(mc.cat_expr(expr));
1152 let region_min = ty::ReScope(callee_scope);
1153 link_region(mc.typer, expr.span, region_min, ast::MutImmutable, expr_cmt);
1156 fn link_region_from_node_type(rcx: &Rcx,
1159 mutbl: ast::Mutability,
1160 cmt_borrowed: mc::cmt) {
1162 * Like `link_region()`, except that the region is
1163 * extracted from the type of `id`, which must be some
1164 * reference (`&T`, `&str`, etc).
1167 let rptr_ty = rcx.resolve_node_type(id);
1168 if !ty::type_is_bot(rptr_ty) && !ty::type_is_error(rptr_ty) {
1169 let tcx = rcx.fcx.ccx.tcx;
1170 debug!("rptr_ty={}", ty_to_str(tcx, rptr_ty));
1171 let r = ty::ty_region(tcx, span, rptr_ty);
1172 link_region(rcx, span, r, mutbl, cmt_borrowed);
1176 fn link_region(rcx: &Rcx,
1178 region_min: ty::Region,
1179 mutbl: ast::Mutability,
1180 cmt_borrowed: mc::cmt) {
1182 * Informs the inference engine that a borrow of `cmt`
1183 * must have mutability `mutbl` and lifetime `region_min`.
1184 * If `cmt` is a deref of a region pointer with
1185 * lifetime `r_borrowed`, this will add the constraint that
1186 * `region_min <= r_borrowed`.
1189 // Iterate through all the things that must be live at least
1190 // for the lifetime `region_min` for the borrow to be valid:
1191 let mut cmt_borrowed = cmt_borrowed;
1193 debug!("link_region(region_min={}, mutbl={}, cmt_borrowed={})",
1194 region_min.repr(rcx.tcx()),
1195 mutbl.repr(rcx.tcx()),
1196 cmt_borrowed.repr(rcx.tcx()));
1197 match cmt_borrowed.cat.clone() {
1198 mc::cat_deref(base, _, mc::BorrowedPtr(_, r_borrowed)) => {
1199 // References to an upvar `x` are translated to
1200 // `*x`, since that is what happens in the
1201 // underlying machine. We detect such references
1202 // and treat them slightly differently, both to
1203 // offer better error messages and because we need
1204 // to infer the kind of borrow (mut, const, etc)
1205 // to use for each upvar.
1206 let cause = match base.cat {
1207 mc::cat_upvar(ref upvar_id, _) => {
1208 match rcx.fcx.inh.upvar_borrow_map.borrow_mut()
1209 .find_mut(upvar_id) {
1210 Some(upvar_borrow) => {
1211 debug!("link_region: {} <= {}",
1212 region_min.repr(rcx.tcx()),
1213 upvar_borrow.region.repr(rcx.tcx()));
1214 adjust_upvar_borrow_kind_for_loan(
1218 infer::ReborrowUpvar(span, *upvar_id)
1221 rcx.tcx().sess.span_bug(
1223 format!("Illegal upvar id: {}",
1224 upvar_id.repr(rcx.tcx())));
1230 infer::Reborrow(span)
1234 debug!("link_region: {} <= {}",
1235 region_min.repr(rcx.tcx()),
1236 r_borrowed.repr(rcx.tcx()));
1237 rcx.fcx.mk_subr(true, cause, region_min, r_borrowed);
1239 if mutbl == ast::MutMutable {
1240 // If this is a mutable borrow, then the thing
1241 // being borrowed will have to be unique.
1242 // In user code, this means it must be an `&mut`
1243 // borrow, but for an upvar, we might opt
1244 // for an immutable-unique borrow.
1245 adjust_upvar_borrow_kind_for_unique(rcx, base);
1248 // Borrowing an `&mut` pointee for `region_min` is
1249 // only valid if the pointer resides in a unique
1250 // location which is itself valid for
1251 // `region_min`. We don't care about the unique
1252 // part, but we may need to influence the
1253 // inference to ensure that the location remains
1256 // FIXME(#8624) fixing borrowck will require this
1257 // if m == ast::m_mutbl {
1258 // cmt_borrowed = cmt_base;
1264 mc::cat_discr(cmt_base, _) |
1265 mc::cat_downcast(cmt_base) |
1266 mc::cat_deref(cmt_base, _, mc::OwnedPtr) |
1267 mc::cat_interior(cmt_base, _) => {
1268 // Interior or owned data requires its base to be valid
1269 cmt_borrowed = cmt_base;
1271 mc::cat_deref(_, _, mc::GcPtr(..)) |
1272 mc::cat_deref(_, _, mc::UnsafePtr(..)) |
1273 mc::cat_static_item |
1274 mc::cat_copied_upvar(..) |
1278 mc::cat_rvalue(..) => {
1279 // These are all "base cases" with independent lifetimes
1280 // that are not subject to inference
1287 fn adjust_borrow_kind_for_assignment_lhs(rcx: &Rcx,
1290 * Adjusts the inferred borrow_kind as needed to account
1291 * for upvars that are assigned to in an assignment
1295 let mc = mc::MemCategorizationContext { typer: rcx };
1296 let cmt = ignore_err!(mc.cat_expr(lhs));
1297 adjust_upvar_borrow_kind_for_mut(mc.typer, cmt);
1300 fn adjust_upvar_borrow_kind_for_mut(rcx: &Rcx,
1304 debug!("adjust_upvar_borrow_kind_for_mut(cmt={})",
1305 cmt.repr(rcx.tcx()));
1307 match cmt.cat.clone() {
1308 mc::cat_deref(base, _, mc::OwnedPtr) |
1309 mc::cat_interior(base, _) |
1310 mc::cat_downcast(base) |
1311 mc::cat_discr(base, _) => {
1312 // Interior or owned data is mutable if base is
1313 // mutable, so iterate to the base.
1318 mc::cat_deref(base, _, mc::BorrowedPtr(..)) => {
1320 mc::cat_upvar(ref upvar_id, _) => {
1321 // if this is an implicit deref of an
1322 // upvar, then we need to modify the
1323 // borrow_kind of the upvar to make sure it
1324 // is inferred to mutable if necessary
1325 let mut upvar_borrow_map =
1326 rcx.fcx.inh.upvar_borrow_map.borrow_mut();
1327 let ub = upvar_borrow_map.get_mut(upvar_id);
1328 return adjust_upvar_borrow_kind(*upvar_id, ub, ty::MutBorrow);
1334 // assignment to deref of an `&mut`
1335 // borrowed pointer implies that the
1336 // pointer itself must be unique, but not
1337 // necessarily *mutable*
1338 return adjust_upvar_borrow_kind_for_unique(rcx, base);
1341 mc::cat_deref(_, _, mc::UnsafePtr(..)) |
1342 mc::cat_deref(_, _, mc::GcPtr) |
1343 mc::cat_static_item |
1345 mc::cat_copied_upvar(_) |
1348 mc::cat_upvar(..) => {
1355 fn adjust_upvar_borrow_kind_for_unique(rcx: &Rcx, cmt: mc::cmt) {
1358 debug!("adjust_upvar_borrow_kind_for_unique(cmt={})",
1359 cmt.repr(rcx.tcx()));
1361 match cmt.cat.clone() {
1362 mc::cat_deref(base, _, mc::OwnedPtr) |
1363 mc::cat_interior(base, _) |
1364 mc::cat_downcast(base) |
1365 mc::cat_discr(base, _) => {
1366 // Interior or owned data is unique if base is
1372 mc::cat_deref(base, _, mc::BorrowedPtr(..)) => {
1374 mc::cat_upvar(ref upvar_id, _) => {
1375 // if this is an implicit deref of an
1376 // upvar, then we need to modify the
1377 // borrow_kind of the upvar to make sure it
1378 // is inferred to unique if necessary
1379 let mut ub = rcx.fcx.inh.upvar_borrow_map.borrow_mut();
1380 let ub = ub.get_mut(upvar_id);
1381 return adjust_upvar_borrow_kind(*upvar_id, ub, ty::UniqueImmBorrow);
1387 // for a borrowed pointer to be unique, its
1388 // base must be unique
1389 return adjust_upvar_borrow_kind_for_unique(rcx, base);
1392 mc::cat_deref(_, _, mc::UnsafePtr(..)) |
1393 mc::cat_deref(_, _, mc::GcPtr) |
1394 mc::cat_static_item |
1396 mc::cat_copied_upvar(_) |
1399 mc::cat_upvar(..) => {
1406 fn link_upvar_borrow_kind_for_nested_closures(rcx: &mut Rcx,
1407 inner_upvar_id: ty::UpvarId,
1408 outer_upvar_id: ty::UpvarId) {
1410 * Indicates that the borrow_kind of `outer_upvar_id` must
1411 * permit a reborrowing with the borrow_kind of `inner_upvar_id`.
1412 * This occurs in nested closures, see comment above at the call to
1416 debug!("link_upvar_borrow_kind: inner_upvar_id={:?} outer_upvar_id={:?}",
1417 inner_upvar_id, outer_upvar_id);
1419 let mut upvar_borrow_map = rcx.fcx.inh.upvar_borrow_map.borrow_mut();
1420 let inner_borrow = upvar_borrow_map.get_copy(&inner_upvar_id);
1421 match upvar_borrow_map.find_mut(&outer_upvar_id) {
1422 Some(outer_borrow) => {
1423 adjust_upvar_borrow_kind(outer_upvar_id, outer_borrow, inner_borrow.kind);
1425 None => { /* outer closure is not a stack closure */ }
1429 fn adjust_upvar_borrow_kind_for_loan(upvar_id: ty::UpvarId,
1430 upvar_borrow: &mut ty::UpvarBorrow,
1431 mutbl: ast::Mutability) {
1432 debug!("adjust_upvar_borrow_kind_for_loan: upvar_id={:?} kind={:?} -> {:?}",
1433 upvar_id, upvar_borrow.kind, mutbl);
1435 adjust_upvar_borrow_kind(upvar_id, upvar_borrow,
1436 ty::BorrowKind::from_mutbl(mutbl))
1439 fn adjust_upvar_borrow_kind(upvar_id: ty::UpvarId,
1440 upvar_borrow: &mut ty::UpvarBorrow,
1441 kind: ty::BorrowKind) {
1443 * We infer the borrow_kind with which to borrow upvars in a stack
1444 * closure. The borrow_kind basically follows a lattice of
1445 * `imm < unique-imm < mut`, moving from left to right as needed (but never
1446 * right to left). Here the argument `mutbl` is the borrow_kind that
1447 * is required by some particular use.
1450 debug!("adjust_upvar_borrow_kind: id={:?} kind=({:?} -> {:?})",
1451 upvar_id, upvar_borrow.kind, kind);
1453 match (upvar_borrow.kind, kind) {
1455 (ty::ImmBorrow, ty::UniqueImmBorrow) |
1456 (ty::ImmBorrow, ty::MutBorrow) |
1457 (ty::UniqueImmBorrow, ty::MutBorrow) => {
1458 upvar_borrow.kind = kind;
1461 (ty::ImmBorrow, ty::ImmBorrow) |
1462 (ty::UniqueImmBorrow, ty::ImmBorrow) |
1463 (ty::UniqueImmBorrow, ty::UniqueImmBorrow) |
1464 (ty::MutBorrow, _) => {