src/librustc/middle/borrowck/check_loans.rs

   1 // Copyright 2012-2013 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.
   4 //
   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.
  10
  11 // ----------------------------------------------------------------------
  12 // Checking loans
  13 //
  14 // Phase 2 of check: we walk down the tree and check that:
  15 // 1. assignments are always made to mutable locations;
  16 // 2. loans made in overlapping scopes do not conflict
  17 // 3. assignments do not affect things loaned out as immutable
  18 // 4. moves do not affect things loaned out in any way
  19
  20
  21 use mc = middle::mem_categorization;
  22 use middle::borrowck::*;
  23 use middle::moves;
  24 use middle::ty;
  25 use std::vec_ng::Vec;
  26 use syntax::ast;
  27 use syntax::ast_util;
  28 use syntax::codemap::Span;
  29 use syntax::visit::Visitor;
  30 use syntax::visit;
  31 use util::ppaux::Repr;
  32
  33 struct CheckLoanCtxt<'a> {
  34     bccx: &'a BorrowckCtxt,
  35     dfcx_loans: &'a LoanDataFlow,
  36     move_data: move_data::FlowedMoveData,
  37     all_loans: &'a [Loan],
  38 }
  39
  40 impl<'a> Visitor<()> for CheckLoanCtxt<'a> {
  41
  42     fn visit_expr(&mut self, ex: &ast::Expr, _: ()) {
  43         check_loans_in_expr(self, ex);
  44     }
  45     fn visit_local(&mut self, l: &ast::Local, _: ()) {
  46         check_loans_in_local(self, l);
  47     }
  48     fn visit_block(&mut self, b: &ast::Block, _: ()) {
  49         check_loans_in_block(self, b);
  50     }
  51     fn visit_pat(&mut self, p: &ast::Pat, _: ()) {
  52         check_loans_in_pat(self, p);
  53     }
  54     fn visit_fn(&mut self, _fk: &visit::FnKind, _fd: &ast::FnDecl,
  55                 _b: &ast::Block, _s: Span, _n: ast::NodeId, _: ()) {
  56         // Don't process nested items or closures here,
  57         // the outer loop will take care of it.
  58         return;
  59     }
  60
  61     // FIXME(#10894) should continue recursing
  62     fn visit_ty(&mut self, _t: &ast::Ty, _: ()) {}
  63 }
  64
  65 pub fn check_loans(bccx: &BorrowckCtxt,
  66                    dfcx_loans: &LoanDataFlow,
  67                    move_data: move_data::FlowedMoveData,
  68                    all_loans: &[Loan],
  69                    body: &ast::Block) {
  70     debug!("check_loans(body id={:?})", body.id);
  71
  72     let mut clcx = CheckLoanCtxt {
  73         bccx: bccx,
  74         dfcx_loans: dfcx_loans,
  75         move_data: move_data,
  76         all_loans: all_loans,
  77     };
  78
  79     clcx.visit_block(body, ());
  80 }
  81
  82 #[deriving(Eq)]
  83 enum MoveError {
  84     MoveOk,
  85     MoveWhileBorrowed(/*loan*/@LoanPath, /*loan*/Span)
  86 }
  87
  88 impl<'a> CheckLoanCtxt<'a> {
  89     pub fn tcx(&self) -> ty::ctxt { self.bccx.tcx }
  90
  91     pub fn each_issued_loan(&self, scope_id: ast::NodeId, op: |&Loan| -> bool)
  92                             -> bool {
  93         //! Iterates over each loan that has been issued
  94         //! on entrance to `scope_id`, regardless of whether it is
  95         //! actually *in scope* at that point.  Sometimes loans
  96         //! are issued for future scopes and thus they may have been
  97         //! *issued* but not yet be in effect.
  98
  99         self.dfcx_loans.each_bit_on_entry_frozen(scope_id, |loan_index| {
 100             let loan = &self.all_loans[loan_index];
 101             op(loan)
 102         })
 103     }
 104
 105     pub fn each_in_scope_loan(&self,
 106                               scope_id: ast::NodeId,
 107                               op: |&Loan| -> bool)
 108                               -> bool {
 109         //! Like `each_issued_loan()`, but only considers loans that are
 110         //! currently in scope.
 111
 112         let tcx = self.tcx();
 113         self.each_issued_loan(scope_id, |loan| {
 114             if tcx.region_maps.is_subscope_of(scope_id, loan.kill_scope) {
 115                 op(loan)
 116             } else {
 117                 true
 118             }
 119         })
 120     }
 121
 122     pub fn each_in_scope_restriction(&self,
 123                                      scope_id: ast::NodeId,
 124                                      loan_path: @LoanPath,
 125                                      op: |&Loan, &Restriction| -> bool)
 126                                      -> bool {
 127         //! Iterates through all the in-scope restrictions for the
 128         //! given `loan_path`
 129
 130         self.each_in_scope_loan(scope_id, |loan| {
 131             debug!("each_in_scope_restriction found loan: {:?}",
 132                    loan.repr(self.tcx()));
 133
 134             let mut ret = true;
 135             for restr in loan.restrictions.iter() {
 136                 if restr.loan_path == loan_path {
 137                     if !op(loan, restr) {
 138                         ret = false;
 139                         break;
 140                     }
 141                 }
 142             }
 143             ret
 144         })
 145     }
 146
 147     pub fn loans_generated_by(&self, scope_id: ast::NodeId) -> Vec<uint> {
 148         //! Returns a vector of the loans that are generated as
 149         //! we encounter `scope_id`.
 150
 151         let mut result = Vec::new();
 152         self.dfcx_loans.each_gen_bit_frozen(scope_id, |loan_index| {
 153             result.push(loan_index);
 154             true
 155         });
 156         return result;
 157     }
 158
 159     pub fn check_for_conflicting_loans(&self, scope_id: ast::NodeId) {
 160         //! Checks to see whether any of the loans that are issued
 161         //! by `scope_id` conflict with loans that have already been
 162         //! issued when we enter `scope_id` (for example, we do not
 163         //! permit two `&mut` borrows of the same variable).
 164
 165         debug!("check_for_conflicting_loans(scope_id={:?})", scope_id);
 166
 167         let new_loan_indices = self.loans_generated_by(scope_id);
 168         debug!("new_loan_indices = {:?}", new_loan_indices);
 169
 170         self.each_issued_loan(scope_id, |issued_loan| {
 171             for &new_loan_index in new_loan_indices.iter() {
 172                 let new_loan = &self.all_loans[new_loan_index];
 173                 self.report_error_if_loans_conflict(issued_loan, new_loan);
 174             }
 175             true
 176         });
 177
 178         for (i, &x) in new_loan_indices.iter().enumerate() {
 179             let old_loan = &self.all_loans[x];
 180             for &y in new_loan_indices.slice_from(i+1).iter() {
 181                 let new_loan = &self.all_loans[y];
 182                 self.report_error_if_loans_conflict(old_loan, new_loan);
 183             }
 184         }
 185     }
 186
 187     pub fn report_error_if_loans_conflict(&self,
 188                                           old_loan: &Loan,
 189                                           new_loan: &Loan) {
 190         //! Checks whether `old_loan` and `new_loan` can safely be issued
 191         //! simultaneously.
 192
 193         debug!("report_error_if_loans_conflict(old_loan={}, new_loan={})",
 194                old_loan.repr(self.tcx()),
 195                new_loan.repr(self.tcx()));
 196
 197         // Should only be called for loans that are in scope at the same time.
 198         assert!(self.tcx().region_maps.scopes_intersect(old_loan.kill_scope,
 199                                                         new_loan.kill_scope));
 200
 201         self.report_error_if_loan_conflicts_with_restriction(
 202             old_loan, new_loan, old_loan, new_loan) &&
 203         self.report_error_if_loan_conflicts_with_restriction(
 204             new_loan, old_loan, old_loan, new_loan);
 205     }
 206
 207     pub fn report_error_if_loan_conflicts_with_restriction(&self,
 208                                                            loan1: &Loan,
 209                                                            loan2: &Loan,
 210                                                            old_loan: &Loan,
 211                                                            new_loan: &Loan)
 212                                                            -> bool {
 213         //! Checks whether the restrictions introduced by `loan1` would
 214         //! prohibit `loan2`. Returns false if an error is reported.
 215
 216         debug!("report_error_if_loan_conflicts_with_restriction(\
 217                 loan1={}, loan2={})",
 218                loan1.repr(self.tcx()),
 219                loan2.repr(self.tcx()));
 220
 221         // Restrictions that would cause the new loan to be illegal:
 222         let illegal_if = match loan2.kind {
 223             // Look for restrictions against mutation. These are
 224             // generated by all other borrows.
 225             ty::MutBorrow => RESTR_MUTATE,
 226
 227             // Look for restrictions against freezing (immutable borrows).
 228             // These are generated by `&mut` borrows.
 229             ty::ImmBorrow => RESTR_FREEZE,
 230
 231             // No matter how the data is borrowed (as `&`, as `&mut`,
 232             // or as `&unique imm`) it will always generate a
 233             // restriction against mutating the data. So look for those.
 234             ty::UniqueImmBorrow => RESTR_MUTATE,
 235         };
 236         debug!("illegal_if={:?}", illegal_if);
 237
 238         for restr in loan1.restrictions.iter() {
 239             if !restr.set.intersects(illegal_if) { continue; }
 240             if restr.loan_path != loan2.loan_path { continue; }
 241
 242             let old_pronoun = if new_loan.loan_path == old_loan.loan_path {
 243                 ~"it"
 244             } else {
 245                 format!("`{}`",
 246                         self.bccx.loan_path_to_str(old_loan.loan_path))
 247             };
 248
 249             match (new_loan.kind, old_loan.kind) {
 250                 (ty::MutBorrow, ty::MutBorrow) => {
 251                     self.bccx.span_err(
 252                         new_loan.span,
 253                         format!("cannot borrow `{}` as mutable \
 254                                 more than once at a time",
 255                                 self.bccx.loan_path_to_str(new_loan.loan_path)));
 256                 }
 257
 258                 (ty::UniqueImmBorrow, _) => {
 259                     self.bccx.span_err(
 260                         new_loan.span,
 261                         format!("closure requires unique access to `{}` \
 262                                 but {} is already borrowed",
 263                                 self.bccx.loan_path_to_str(new_loan.loan_path),
 264                                 old_pronoun));
 265                 }
 266
 267                 (_, ty::UniqueImmBorrow) => {
 268                     self.bccx.span_err(
 269                         new_loan.span,
 270                         format!("cannot borrow `{}` as {} because \
 271                                 previous closure requires unique access",
 272                                 self.bccx.loan_path_to_str(new_loan.loan_path),
 273                                 new_loan.kind.to_user_str()));
 274                 }
 275
 276                 (_, _) => {
 277                     self.bccx.span_err(
 278                         new_loan.span,
 279                         format!("cannot borrow `{}` as {} because \
 280                                 {} is also borrowed as {}",
 281                                 self.bccx.loan_path_to_str(new_loan.loan_path),
 282                                 new_loan.kind.to_user_str(),
 283                                 old_pronoun,
 284                                 old_loan.kind.to_user_str()));
 285                 }
 286             }
 287
 288             match new_loan.cause {
 289                 ClosureCapture(span) => {
 290                     self.bccx.span_note(
 291                         span,
 292                         format!("borrow occurs due to use of `{}` in closure",
 293                                 self.bccx.loan_path_to_str(new_loan.loan_path)));
 294                 }
 295                 _ => { }
 296             }
 297
 298             let rule_summary = match old_loan.kind {
 299                 ty::MutBorrow => {
 300                     format!("the mutable borrow prevents subsequent \
 301                             moves, borrows, or modification of `{0}` \
 302                             until the borrow ends",
 303                             self.bccx.loan_path_to_str(old_loan.loan_path))
 304                 }
 305
 306                 ty::ImmBorrow => {
 307                     format!("the immutable borrow prevents subsequent \
 308                             moves or mutable borrows of `{0}` \
 309                             until the borrow ends",
 310                             self.bccx.loan_path_to_str(old_loan.loan_path))
 311                 }
 312
 313                 ty::UniqueImmBorrow => {
 314                     format!("the unique capture prevents subsequent \
 315                             moves or borrows of `{0}` \
 316                             until the borrow ends",
 317                             self.bccx.loan_path_to_str(old_loan.loan_path))
 318                 }
 319             };
 320
 321             let borrow_summary = match old_loan.cause {
 322                 ClosureCapture(_) => {
 323                     format!("previous borrow of `{}` occurs here due to \
 324                             use in closure",
 325                             self.bccx.loan_path_to_str(old_loan.loan_path))
 326                 }
 327
 328                 AddrOf | AutoRef | RefBinding => {
 329                     format!("previous borrow of `{}` occurs here",
 330                             self.bccx.loan_path_to_str(old_loan.loan_path))
 331                 }
 332             };
 333
 334             self.bccx.span_note(
 335                 old_loan.span,
 336                 format!("{}; {}", borrow_summary, rule_summary));
 337
 338             let old_loan_span = self.tcx().map.span(old_loan.kill_scope);
 339             self.bccx.span_end_note(old_loan_span,
 340                                     "previous borrow ends here");
 341
 342             return false;
 343         }
 344
 345         true
 346     }
 347
 348     pub fn is_local_variable(&self, cmt: mc::cmt) -> bool {
 349         match cmt.cat {
 350           mc::cat_local(_) => true,
 351           _ => false
 352         }
 353     }
 354
 355     pub fn check_if_path_is_moved(&self,
 356                                   id: ast::NodeId,
 357                                   span: Span,
 358                                   use_kind: MovedValueUseKind,
 359                                   lp: @LoanPath) {
 360         /*!
 361          * Reports an error if `expr` (which should be a path)
 362          * is using a moved/uninitialized value
 363          */
 364
 365         debug!("check_if_path_is_moved(id={:?}, use_kind={:?}, lp={})",
 366                id, use_kind, lp.repr(self.bccx.tcx));
 367         self.move_data.each_move_of(id, lp, |move, moved_lp| {
 368             self.bccx.report_use_of_moved_value(
 369                 span,
 370                 use_kind,
 371                 lp,
 372                 move,
 373                 moved_lp);
 374             false
 375         });
 376     }
 377
 378     pub fn check_assignment(&self, expr: &ast::Expr) {
 379         // We don't use cat_expr() here because we don't want to treat
 380         // auto-ref'd parameters in overloaded operators as rvalues.
 381         let adj = {
 382             let adjustments = self.bccx.tcx.adjustments.borrow();
 383             adjustments.get().find_copy(&expr.id)
 384         };
 385         let cmt = match adj {
 386             None => self.bccx.cat_expr_unadjusted(expr),
 387             Some(adj) => self.bccx.cat_expr_autoderefd(expr, adj)
 388         };
 389
 390         debug!("check_assignment(cmt={})", cmt.repr(self.tcx()));
 391
 392         // Mutable values can be assigned, as long as they obey loans
 393         // and aliasing restrictions:
 394         if cmt.mutbl.is_mutable() {
 395             if check_for_aliasable_mutable_writes(self, expr, cmt) {
 396                 if check_for_assignment_to_restricted_or_frozen_location(
 397                     self, expr, cmt)
 398                 {
 399                     // Safe, but record for lint pass later:
 400                     mark_variable_as_used_mut(self, cmt);
 401                 }
 402             }
 403             return;
 404         }
 405
 406         // For immutable local variables, assignments are legal
 407         // if they cannot already have been assigned
 408         if self.is_local_variable(cmt) {
 409             assert!(cmt.mutbl.is_immutable()); // no "const" locals
 410             let lp = opt_loan_path(cmt).unwrap();
 411             self.move_data.each_assignment_of(expr.id, lp, |assign| {
 412                 self.bccx.report_reassigned_immutable_variable(
 413                     expr.span,
 414                     lp,
 415                     assign);
 416                 false
 417             });
 418             return;
 419         }
 420
 421         // Otherwise, just a plain error.
 422         match opt_loan_path(cmt) {
 423             Some(lp) => {
 424                 self.bccx.span_err(
 425                     expr.span,
 426                     format!("cannot assign to {} {} `{}`",
 427                             cmt.mutbl.to_user_str(),
 428                             self.bccx.cmt_to_str(cmt),
 429                             self.bccx.loan_path_to_str(lp)));
 430             }
 431             None => {
 432                 self.bccx.span_err(
 433                     expr.span,
 434                     format!("cannot assign to {} {}",
 435                             cmt.mutbl.to_user_str(),
 436                             self.bccx.cmt_to_str(cmt)));
 437             }
 438         }
 439         return;
 440
 441         fn mark_variable_as_used_mut(this: &CheckLoanCtxt,
 442                                      cmt: mc::cmt) {
 443             //! If the mutability of the `cmt` being written is inherited
 444             //! from a local variable, liveness will
 445             //! not have been able to detect that this variable's mutability
 446             //! is important, so we must add the variable to the
 447             //! `used_mut_nodes` table here.
 448
 449             let mut cmt = cmt;
 450             loop {
 451                 debug!("mark_writes_through_upvars_as_used_mut(cmt={})",
 452                        cmt.repr(this.tcx()));
 453                 match cmt.cat {
 454                     mc::cat_local(id) | mc::cat_arg(id) => {
 455                         let mut used_mut_nodes = this.tcx()
 456                                                      .used_mut_nodes
 457                                                      .borrow_mut();
 458                         used_mut_nodes.get().insert(id);
 459                         return;
 460                     }
 461
 462                     mc::cat_upvar(..) => {
 463                         return;
 464                     }
 465
 466                     mc::cat_deref(_, _, mc::GcPtr) => {
 467                         assert_eq!(cmt.mutbl, mc::McImmutable);
 468                         return;
 469                     }
 470
 471                     mc::cat_rvalue(..) |
 472                     mc::cat_static_item |
 473                     mc::cat_copied_upvar(..) |
 474                     mc::cat_deref(_, _, mc::UnsafePtr(..)) |
 475                     mc::cat_deref(_, _, mc::BorrowedPtr(..)) => {
 476                         assert_eq!(cmt.mutbl, mc::McDeclared);
 477                         return;
 478                     }
 479
 480                     mc::cat_discr(b, _) |
 481                     mc::cat_deref(b, _, mc::OwnedPtr) => {
 482                         assert_eq!(cmt.mutbl, mc::McInherited);
 483                         cmt = b;
 484                     }
 485
 486                     mc::cat_downcast(b) |
 487                     mc::cat_interior(b, _) => {
 488                         assert_eq!(cmt.mutbl, mc::McInherited);
 489                         cmt = b;
 490                     }
 491                 }
 492             }
 493         }
 494
 495         fn check_for_aliasable_mutable_writes(this: &CheckLoanCtxt,
 496                                               expr: &ast::Expr,
 497                                               cmt: mc::cmt) -> bool {
 498             //! Safety checks related to writes to aliasable, mutable locations
 499
 500             let guarantor = cmt.guarantor();
 501             debug!("check_for_aliasable_mutable_writes(cmt={}, guarantor={})",
 502                    cmt.repr(this.tcx()), guarantor.repr(this.tcx()));
 503             match guarantor.cat {
 504                 mc::cat_deref(b, _, mc::BorrowedPtr(ty::MutBorrow, _)) => {
 505                     // Statically prohibit writes to `&mut` when aliasable
 506
 507                     check_for_aliasability_violation(this, expr, b);
 508                 }
 509
 510                 _ => {}
 511             }
 512
 513             return true; // no errors reported
 514         }
 515
 516         fn check_for_aliasability_violation(this: &CheckLoanCtxt,
 517                                             expr: &ast::Expr,
 518                                             cmt: mc::cmt)
 519                                             -> bool {
 520             match cmt.freely_aliasable() {
 521                 None => {
 522                     return true;
 523                 }
 524                 Some(cause) => {
 525                     this.bccx.report_aliasability_violation(
 526                         expr.span,
 527                         MutabilityViolation,
 528                         cause);
 529                     return false;
 530                 }
 531             }
 532         }
 533
 534         fn check_for_assignment_to_restricted_or_frozen_location(
 535             this: &CheckLoanCtxt,
 536             expr: &ast::Expr,
 537             cmt: mc::cmt) -> bool
 538         {
 539             //! Check for assignments that violate the terms of an
 540             //! outstanding loan.
 541
 542             let loan_path = match opt_loan_path(cmt) {
 543                 Some(lp) => lp,
 544                 None => { return true; /* no loan path, can't be any loans */ }
 545             };
 546
 547             // Start by searching for an assignment to a *restricted*
 548             // location. Here is one example of the kind of error caught
 549             // by this check:
 550             //
 551             //    let mut v = ~[1, 2, 3];
 552             //    let p = &v;
 553             //    v = ~[4];
 554             //
 555             // In this case, creating `p` triggers a RESTR_MUTATE
 556             // restriction on the path `v`.
 557             //
 558             // Here is a second, more subtle example:
 559             //
 560             //    let mut v = ~[1, 2, 3];
 561             //    let p = &const v[0];
 562             //    v[0] = 4;                   // OK
 563             //    v[1] = 5;                   // OK
 564             //    v = ~[4, 5, 3];             // Error
 565             //
 566             // In this case, `p` is pointing to `v[0]`, and it is a
 567             // `const` pointer in any case. So the first two
 568             // assignments are legal (and would be permitted by this
 569             // check). However, the final assignment (which is
 570             // logically equivalent) is forbidden, because it would
 571             // cause the existing `v` array to be freed, thus
 572             // invalidating `p`. In the code, this error results
 573             // because `gather_loans::restrictions` adds a
 574             // `RESTR_MUTATE` restriction whenever the contents of an
 575             // owned pointer are borrowed, and hence while `v[*]` is not
 576             // restricted from being written, `v` is.
 577             let cont = this.each_in_scope_restriction(expr.id,
 578                                                       loan_path,
 579                                                       |loan, restr| {
 580                 if restr.set.intersects(RESTR_MUTATE) {
 581                     this.report_illegal_mutation(expr, loan_path, loan);
 582                     false
 583                 } else {
 584                     true
 585                 }
 586             });
 587
 588             if !cont { return false }
 589
 590             // The previous code handled assignments to paths that
 591             // have been restricted. This covers paths that have been
 592             // directly lent out and their base paths, but does not
 593             // cover random extensions of those paths. For example,
 594             // the following program is not declared illegal by the
 595             // previous check:
 596             //
 597             //    let mut v = ~[1, 2, 3];
 598             //    let p = &v;
 599             //    v[0] = 4; // declared error by loop below, not code above
 600             //
 601             // The reason that this passes the previous check whereas
 602             // an assignment like `v = ~[4]` fails is because the assignment
 603             // here is to `v[*]`, and the existing restrictions were issued
 604             // for `v`, not `v[*]`.
 605             //
 606             // So in this loop, we walk back up the loan path so long
 607             // as the mutability of the path is dependent on a super
 608             // path, and check that the super path was not lent out as
 609             // mutable or immutable (a const loan is ok).
 610             //
 611             // Mutability of a path can be dependent on the super path
 612             // in two ways. First, it might be inherited mutability.
 613             // Second, the pointee of an `&mut` pointer can only be
 614             // mutated if it is found in an unaliased location, so we
 615             // have to check that the owner location is not borrowed.
 616             //
 617             // Note that we are *not* checking for any and all
 618             // restrictions.  We are only interested in the pointers
 619             // that the user created, whereas we add restrictions for
 620             // all kinds of paths that are not directly aliased. If we checked
 621             // for all restrictions, and not just loans, then the following
 622             // valid program would be considered illegal:
 623             //
 624             //    let mut v = ~[1, 2, 3];
 625             //    let p = &const v[0];
 626             //    v[1] = 5; // ok
 627             //
 628             // Here the restriction that `v` not be mutated would be misapplied
 629             // to block the subpath `v[1]`.
 630             let full_loan_path = loan_path;
 631             let mut loan_path = loan_path;
 632             loop {
 633                 match *loan_path {
 634                     // Peel back one layer if, for `loan_path` to be
 635                     // mutable, `lp_base` must be mutable. This occurs
 636                     // with inherited mutability and with `&mut`
 637                     // pointers.
 638                     LpExtend(lp_base, mc::McInherited, _) |
 639                     LpExtend(lp_base, _, LpDeref(mc::BorrowedPtr(ty::MutBorrow, _))) => {
 640                         loan_path = lp_base;
 641                     }
 642
 643                     // Otherwise stop iterating
 644                     LpExtend(_, mc::McDeclared, _) |
 645                     LpExtend(_, mc::McImmutable, _) |
 646                     LpVar(_) => {
 647                         return true;
 648                     }
 649                 }
 650
 651                 // Check for a non-const loan of `loan_path`
 652                 let cont = this.each_in_scope_loan(expr.id, |loan| {
 653                     if loan.loan_path == loan_path {
 654                         this.report_illegal_mutation(expr, full_loan_path, loan);
 655                         false
 656                     } else {
 657                         true
 658                     }
 659                 });
 660
 661                 if !cont { return false }
 662             }
 663         }
 664     }
 665
 666     pub fn report_illegal_mutation(&self,
 667                                    expr: &ast::Expr,
 668                                    loan_path: &LoanPath,
 669                                    loan: &Loan) {
 670         self.bccx.span_err(
 671             expr.span,
 672             format!("cannot assign to `{}` because it is borrowed",
 673                  self.bccx.loan_path_to_str(loan_path)));
 674         self.bccx.span_note(
 675             loan.span,
 676             format!("borrow of `{}` occurs here",
 677                  self.bccx.loan_path_to_str(loan_path)));
 678     }
 679
 680     fn check_move_out_from_expr(&self, expr: &ast::Expr) {
 681         match expr.node {
 682             ast::ExprFnBlock(..) | ast::ExprProc(..) => {
 683                 // Moves due to captures are checked in
 684                 // check_captured_variables() because it allows
 685                 // us to give a more precise error message with
 686                 // a more precise span.
 687             }
 688             _ => {
 689                 self.check_move_out_from_id(expr.id, expr.span)
 690             }
 691         }
 692     }
 693
 694     fn check_move_out_from_id(&self, id: ast::NodeId, span: Span) {
 695         self.move_data.each_path_moved_by(id, |_, move_path| {
 696             match self.analyze_move_out_from(id, move_path) {
 697                 MoveOk => {}
 698                 MoveWhileBorrowed(loan_path, loan_span) => {
 699                     self.bccx.span_err(
 700                         span,
 701                         format!("cannot move out of `{}` \
 702                                 because it is borrowed",
 703                              self.bccx.loan_path_to_str(move_path)));
 704                     self.bccx.span_note(
 705                         loan_span,
 706                         format!("borrow of `{}` occurs here",
 707                                 self.bccx.loan_path_to_str(loan_path)));
 708                 }
 709             }
 710             true
 711         });
 712     }
 713
 714     fn check_captured_variables(&self,
 715                                 closure_id: ast::NodeId,
 716                                 span: Span) {
 717         let capture_map = self.bccx.capture_map.borrow();
 718         let cap_vars = capture_map.get().get(&closure_id);
 719         for cap_var in cap_vars.deref().iter() {
 720             let var_id = ast_util::def_id_of_def(cap_var.def).node;
 721             let var_path = @LpVar(var_id);
 722             self.check_if_path_is_moved(closure_id, span,
 723                                         MovedInCapture, var_path);
 724             match cap_var.mode {
 725                 moves::CapRef | moves::CapCopy => {}
 726                 moves::CapMove => {
 727                     check_by_move_capture(self, closure_id, cap_var, var_path);
 728                 }
 729             }
 730         }
 731         return;
 732
 733         fn check_by_move_capture(this: &CheckLoanCtxt,
 734                                  closure_id: ast::NodeId,
 735                                  cap_var: &moves::CaptureVar,
 736                                  move_path: @LoanPath) {
 737             let move_err = this.analyze_move_out_from(closure_id, move_path);
 738             match move_err {
 739                 MoveOk => {}
 740                 MoveWhileBorrowed(loan_path, loan_span) => {
 741                     this.bccx.span_err(
 742                         cap_var.span,
 743                         format!("cannot move `{}` into closure \
 744                                 because it is borrowed",
 745                                 this.bccx.loan_path_to_str(move_path)));
 746                     this.bccx.span_note(
 747                         loan_span,
 748                         format!("borrow of `{}` occurs here",
 749                                 this.bccx.loan_path_to_str(loan_path)));
 750                 }
 751             }
 752         }
 753     }
 754
 755     pub fn analyze_move_out_from(&self,
 756                                  expr_id: ast::NodeId,
 757                                  mut move_path: @LoanPath)
 758                                  -> MoveError {
 759         debug!("analyze_move_out_from(expr_id={:?}, move_path={})",
 760                self.tcx().map.node_to_str(expr_id),
 761                move_path.repr(self.tcx()));
 762
 763         // We must check every element of a move path. See
 764         // `borrowck-move-subcomponent.rs` for a test case.
 765         loop {
 766             // check for a conflicting loan:
 767             let mut ret = MoveOk;
 768             self.each_in_scope_restriction(expr_id, move_path, |loan, _| {
 769                 // Any restriction prevents moves.
 770                 ret = MoveWhileBorrowed(loan.loan_path, loan.span);
 771                 false
 772             });
 773
 774             if ret != MoveOk {
 775                 return ret
 776             }
 777
 778             match *move_path {
 779                 LpVar(_) => return MoveOk,
 780                 LpExtend(subpath, _, _) => move_path = subpath,
 781             }
 782         }
 783     }
 784
 785     pub fn check_call(&self,
 786                       _expr: &ast::Expr,
 787                       _callee: Option<@ast::Expr>,
 788                       _callee_span: Span,
 789                       _args: &[@ast::Expr]) {
 790         // NB: This call to check for conflicting loans is not truly
 791         // necessary, because the callee_id never issues new loans.
 792         // However, I added it for consistency and lest the system
 793         // should change in the future.
 794         //
 795         // FIXME(#6268) nested method calls
 796         // self.check_for_conflicting_loans(callee_id);
 797     }
 798 }
 799
 800 fn check_loans_in_local<'a>(this: &mut CheckLoanCtxt<'a>,
 801                             local: &ast::Local) {
 802     visit::walk_local(this, local, ());
 803 }
 804
 805 fn check_loans_in_expr<'a>(this: &mut CheckLoanCtxt<'a>,
 806                            expr: &ast::Expr) {
 807     visit::walk_expr(this, expr, ());
 808
 809     debug!("check_loans_in_expr(expr={})",
 810            expr.repr(this.tcx()));
 811
 812     this.check_for_conflicting_loans(expr.id);
 813     this.check_move_out_from_expr(expr);
 814
 815     let method_map = this.bccx.method_map.borrow();
 816     match expr.node {
 817       ast::ExprPath(..) => {
 818           if !this.move_data.is_assignee(expr.id) {
 819               let cmt = this.bccx.cat_expr_unadjusted(expr);
 820               debug!("path cmt={}", cmt.repr(this.tcx()));
 821               let r = opt_loan_path(cmt);
 822               for &lp in r.iter() {
 823                   this.check_if_path_is_moved(expr.id, expr.span, MovedInUse, lp);
 824               }
 825           }
 826       }
 827       ast::ExprFnBlock(..) | ast::ExprProc(..) => {
 828           this.check_captured_variables(expr.id, expr.span)
 829       }
 830       ast::ExprAssign(dest, _) |
 831       ast::ExprAssignOp(_, dest, _) => {
 832         this.check_assignment(dest);
 833       }
 834       ast::ExprCall(f, ref args) => {
 835         this.check_call(expr, Some(f), f.span, args.as_slice());
 836       }
 837       ast::ExprMethodCall(_, _, ref args) => {
 838         this.check_call(expr, None, expr.span, args.as_slice());
 839       }
 840       ast::ExprIndex(_, rval) | ast::ExprBinary(_, _, rval)
 841       if method_map.get().contains_key(&expr.id) => {
 842         this.check_call(expr, None, expr.span, [rval]);
 843       }
 844       ast::ExprUnary(_, _) | ast::ExprIndex(_, _)
 845       if method_map.get().contains_key(&expr.id) => {
 846         this.check_call(expr, None, expr.span, []);
 847       }
 848       ast::ExprInlineAsm(ref ia) => {
 849           for &(_, out) in ia.outputs.iter() {
 850               this.check_assignment(out);
 851           }
 852       }
 853       _ => {}
 854     }
 855 }
 856
 857 fn check_loans_in_pat<'a>(this: &mut CheckLoanCtxt<'a>,
 858                           pat: &ast::Pat)
 859 {
 860     this.check_for_conflicting_loans(pat.id);
 861     this.check_move_out_from_id(pat.id, pat.span);
 862     visit::walk_pat(this, pat, ());
 863 }
 864
 865 fn check_loans_in_block<'a>(this: &mut CheckLoanCtxt<'a>,
 866                             blk: &ast::Block)
 867 {
 868     visit::walk_block(this, blk, ());
 869     this.check_for_conflicting_loans(blk.id);
 870 }