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