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 middle::typeck::MethodCall;
  26 use std::vec_ng::Vec;
  27 use syntax::ast;
  28 use syntax::ast_util;
  29 use syntax::codemap::Span;
  30 use syntax::visit::Visitor;
  31 use syntax::visit;
  32 use util::ppaux::Repr;
  33
  34 struct CheckLoanCtxt<'a> {
  35     bccx: &'a BorrowckCtxt,
  36     dfcx_loans: &'a LoanDataFlow,
  37     move_data: move_data::FlowedMoveData,
  38     all_loans: &'a [Loan],
  39 }
  40
  41 impl<'a> Visitor<()> for CheckLoanCtxt<'a> {
  42
  43     fn visit_expr(&mut self, ex: &ast::Expr, _: ()) {
  44         check_loans_in_expr(self, ex);
  45     }
  46     fn visit_local(&mut self, l: &ast::Local, _: ()) {
  47         check_loans_in_local(self, l);
  48     }
  49     fn visit_block(&mut self, b: &ast::Block, _: ()) {
  50         check_loans_in_block(self, b);
  51     }
  52     fn visit_pat(&mut self, p: &ast::Pat, _: ()) {
  53         check_loans_in_pat(self, p);
  54     }
  55     fn visit_fn(&mut self, _fk: &visit::FnKind, _fd: &ast::FnDecl,
  56                 _b: &ast::Block, _s: Span, _n: ast::NodeId, _: ()) {
  57         // Don't process nested items or closures here,
  58         // the outer loop will take care of it.
  59         return;
  60     }
  61
  62     // FIXME(#10894) should continue recursing
  63     fn visit_ty(&mut self, _t: &ast::Ty, _: ()) {}
  64 }
  65
  66 pub fn check_loans(bccx: &BorrowckCtxt,
  67                    dfcx_loans: &LoanDataFlow,
  68                    move_data: move_data::FlowedMoveData,
  69                    all_loans: &[Loan],
  70                    body: &ast::Block) {
  71     debug!("check_loans(body id={:?})", body.id);
  72
  73     let mut clcx = CheckLoanCtxt {
  74         bccx: bccx,
  75         dfcx_loans: dfcx_loans,
  76         move_data: move_data,
  77         all_loans: all_loans,
  78     };
  79
  80     clcx.visit_block(body, ());
  81 }
  82
  83 #[deriving(Eq)]
  84 enum MoveError {
  85     MoveOk,
  86     MoveWhileBorrowed(/*loan*/@LoanPath, /*loan*/Span)
  87 }
  88
  89 impl<'a> CheckLoanCtxt<'a> {
  90     pub fn tcx(&self) -> ty::ctxt { self.bccx.tcx }
  91
  92     pub fn each_issued_loan(&self, scope_id: ast::NodeId, op: |&Loan| -> bool)
  93                             -> bool {
  94         //! Iterates over each loan that has been issued
  95         //! on entrance to `scope_id`, regardless of whether it is
  96         //! actually *in scope* at that point.  Sometimes loans
  97         //! are issued for future scopes and thus they may have been
  98         //! *issued* but not yet be in effect.
  99
 100         self.dfcx_loans.each_bit_on_entry_frozen(scope_id, |loan_index| {
 101             let loan = &self.all_loans[loan_index];
 102             op(loan)
 103         })
 104     }
 105
 106     pub fn each_in_scope_loan(&self,
 107                               scope_id: ast::NodeId,
 108                               op: |&Loan| -> bool)
 109                               -> bool {
 110         //! Like `each_issued_loan()`, but only considers loans that are
 111         //! currently in scope.
 112
 113         let tcx = self.tcx();
 114         self.each_issued_loan(scope_id, |loan| {
 115             if tcx.region_maps.is_subscope_of(scope_id, loan.kill_scope) {
 116                 op(loan)
 117             } else {
 118                 true
 119             }
 120         })
 121     }
 122
 123     pub fn each_in_scope_restriction(&self,
 124                                      scope_id: ast::NodeId,
 125                                      loan_path: @LoanPath,
 126                                      op: |&Loan, &Restriction| -> bool)
 127                                      -> bool {
 128         //! Iterates through all the in-scope restrictions for the
 129         //! given `loan_path`
 130
 131         self.each_in_scope_loan(scope_id, |loan| {
 132             debug!("each_in_scope_restriction found loan: {:?}",
 133                    loan.repr(self.tcx()));
 134
 135             let mut ret = true;
 136             for restr in loan.restrictions.iter() {
 137                 if restr.loan_path == loan_path {
 138                     if !op(loan, restr) {
 139                         ret = false;
 140                         break;
 141                     }
 142                 }
 143             }
 144             ret
 145         })
 146     }
 147
 148     pub fn loans_generated_by(&self, scope_id: ast::NodeId) -> Vec<uint> {
 149         //! Returns a vector of the loans that are generated as
 150         //! we encounter `scope_id`.
 151
 152         let mut result = Vec::new();
 153         self.dfcx_loans.each_gen_bit_frozen(scope_id, |loan_index| {
 154             result.push(loan_index);
 155             true
 156         });
 157         return result;
 158     }
 159
 160     pub fn check_for_conflicting_loans(&self, scope_id: ast::NodeId) {
 161         //! Checks to see whether any of the loans that are issued
 162         //! by `scope_id` conflict with loans that have already been
 163         //! issued when we enter `scope_id` (for example, we do not
 164         //! permit two `&mut` borrows of the same variable).
 165
 166         debug!("check_for_conflicting_loans(scope_id={:?})", scope_id);
 167
 168         let new_loan_indices = self.loans_generated_by(scope_id);
 169         debug!("new_loan_indices = {:?}", new_loan_indices);
 170
 171         self.each_issued_loan(scope_id, |issued_loan| {
 172             for &new_loan_index in new_loan_indices.iter() {
 173                 let new_loan = &self.all_loans[new_loan_index];
 174                 self.report_error_if_loans_conflict(issued_loan, new_loan);
 175             }
 176             true
 177         });
 178
 179         for (i, &x) in new_loan_indices.iter().enumerate() {
 180             let old_loan = &self.all_loans[x];
 181             for &y in new_loan_indices.slice_from(i+1).iter() {
 182                 let new_loan = &self.all_loans[y];
 183                 self.report_error_if_loans_conflict(old_loan, new_loan);
 184             }
 185         }
 186     }
 187
 188     pub fn report_error_if_loans_conflict(&self,
 189                                           old_loan: &Loan,
 190                                           new_loan: &Loan) {
 191         //! Checks whether `old_loan` and `new_loan` can safely be issued
 192         //! simultaneously.
 193
 194         debug!("report_error_if_loans_conflict(old_loan={}, new_loan={})",
 195                old_loan.repr(self.tcx()),
 196                new_loan.repr(self.tcx()));
 197
 198         // Should only be called for loans that are in scope at the same time.
 199         assert!(self.tcx().region_maps.scopes_intersect(old_loan.kill_scope,
 200                                                         new_loan.kill_scope));
 201
 202         self.report_error_if_loan_conflicts_with_restriction(
 203             old_loan, new_loan, old_loan, new_loan) &&
 204         self.report_error_if_loan_conflicts_with_restriction(
 205             new_loan, old_loan, old_loan, new_loan);
 206     }
 207
 208     pub fn report_error_if_loan_conflicts_with_restriction(&self,
 209                                                            loan1: &Loan,
 210                                                            loan2: &Loan,
 211                                                            old_loan: &Loan,
 212                                                            new_loan: &Loan)
 213                                                            -> bool {
 214         //! Checks whether the restrictions introduced by `loan1` would
 215         //! prohibit `loan2`. Returns false if an error is reported.
 216
 217         debug!("report_error_if_loan_conflicts_with_restriction(\
 218                 loan1={}, loan2={})",
 219                loan1.repr(self.tcx()),
 220                loan2.repr(self.tcx()));
 221
 222         // Restrictions that would cause the new loan to be illegal:
 223         let illegal_if = match loan2.kind {
 224             // Look for restrictions against mutation. These are
 225             // generated by all other borrows.
 226             ty::MutBorrow => RESTR_MUTATE,
 227
 228             // Look for restrictions against freezing (immutable borrows).
 229             // These are generated by `&mut` borrows.
 230             ty::ImmBorrow => RESTR_FREEZE,
 231
 232             // No matter how the data is borrowed (as `&`, as `&mut`,
 233             // or as `&unique imm`) it will always generate a
 234             // restriction against mutating the data. So look for those.
 235             ty::UniqueImmBorrow => RESTR_MUTATE,
 236         };
 237         debug!("illegal_if={:?}", illegal_if);
 238
 239         for restr in loan1.restrictions.iter() {
 240             if !restr.set.intersects(illegal_if) { continue; }
 241             if restr.loan_path != loan2.loan_path { continue; }
 242
 243             let old_pronoun = if new_loan.loan_path == old_loan.loan_path {
 244                 ~"it"
 245             } else {
 246                 format!("`{}`",
 247                         self.bccx.loan_path_to_str(old_loan.loan_path))
 248             };
 249
 250             match (new_loan.kind, old_loan.kind) {
 251                 (ty::MutBorrow, ty::MutBorrow) => {
 252                     self.bccx.span_err(
 253                         new_loan.span,
 254                         format!("cannot borrow `{}` as mutable \
 255                                 more than once at a time",
 256                                 self.bccx.loan_path_to_str(new_loan.loan_path)));
 257                 }
 258
 259                 (ty::UniqueImmBorrow, _) => {
 260                     self.bccx.span_err(
 261                         new_loan.span,
 262                         format!("closure requires unique access to `{}` \
 263                                 but {} is already borrowed",
 264                                 self.bccx.loan_path_to_str(new_loan.loan_path),
 265                                 old_pronoun));
 266                 }
 267
 268                 (_, ty::UniqueImmBorrow) => {
 269                     self.bccx.span_err(
 270                         new_loan.span,
 271                         format!("cannot borrow `{}` as {} because \
 272                                 previous closure requires unique access",
 273                                 self.bccx.loan_path_to_str(new_loan.loan_path),
 274                                 new_loan.kind.to_user_str()));
 275                 }
 276
 277                 (_, _) => {
 278                     self.bccx.span_err(
 279                         new_loan.span,
 280                         format!("cannot borrow `{}` as {} because \
 281                                 {} is also borrowed as {}",
 282                                 self.bccx.loan_path_to_str(new_loan.loan_path),
 283                                 new_loan.kind.to_user_str(),
 284                                 old_pronoun,
 285                                 old_loan.kind.to_user_str()));
 286                 }
 287             }
 288
 289             match new_loan.cause {
 290                 ClosureCapture(span) => {
 291                     self.bccx.span_note(
 292                         span,
 293                         format!("borrow occurs due to use of `{}` in closure",
 294                                 self.bccx.loan_path_to_str(new_loan.loan_path)));
 295                 }
 296                 _ => { }
 297             }
 298
 299             let rule_summary = match old_loan.kind {
 300                 ty::MutBorrow => {
 301                     format!("the mutable borrow prevents subsequent \
 302                             moves, borrows, or modification of `{0}` \
 303                             until the borrow ends",
 304                             self.bccx.loan_path_to_str(old_loan.loan_path))
 305                 }
 306
 307                 ty::ImmBorrow => {
 308                     format!("the immutable borrow prevents subsequent \
 309                             moves or mutable borrows of `{0}` \
 310                             until the borrow ends",
 311                             self.bccx.loan_path_to_str(old_loan.loan_path))
 312                 }
 313
 314                 ty::UniqueImmBorrow => {
 315                     format!("the unique capture prevents subsequent \
 316                             moves or borrows of `{0}` \
 317                             until the borrow ends",
 318                             self.bccx.loan_path_to_str(old_loan.loan_path))
 319                 }
 320             };
 321
 322             let borrow_summary = match old_loan.cause {
 323                 ClosureCapture(_) => {
 324                     format!("previous borrow of `{}` occurs here due to \
 325                             use in closure",
 326                             self.bccx.loan_path_to_str(old_loan.loan_path))
 327                 }
 328
 329                 AddrOf | AutoRef | RefBinding => {
 330                     format!("previous borrow of `{}` occurs here",
 331                             self.bccx.loan_path_to_str(old_loan.loan_path))
 332                 }
 333             };
 334
 335             self.bccx.span_note(
 336                 old_loan.span,
 337                 format!("{}; {}", borrow_summary, rule_summary));
 338
 339             let old_loan_span = self.tcx().map.span(old_loan.kill_scope);
 340             self.bccx.span_end_note(old_loan_span,
 341                                     "previous borrow ends here");
 342
 343             return false;
 344         }
 345
 346         true
 347     }
 348
 349     pub fn is_local_variable(&self, cmt: mc::cmt) -> bool {
 350         match cmt.cat {
 351           mc::cat_local(_) => true,
 352           _ => false
 353         }
 354     }
 355
 356     pub fn check_if_path_is_moved(&self,
 357                                   id: ast::NodeId,
 358                                   span: Span,
 359                                   use_kind: MovedValueUseKind,
 360                                   lp: @LoanPath) {
 361         /*!
 362          * Reports an error if `expr` (which should be a path)
 363          * is using a moved/uninitialized value
 364          */
 365
 366         debug!("check_if_path_is_moved(id={:?}, use_kind={:?}, lp={})",
 367                id, use_kind, lp.repr(self.bccx.tcx));
 368         self.move_data.each_move_of(id, lp, |move, moved_lp| {
 369             self.bccx.report_use_of_moved_value(
 370                 span,
 371                 use_kind,
 372                 lp,
 373                 move,
 374                 moved_lp);
 375             false
 376         });
 377     }
 378
 379     pub fn check_assignment(&self, expr: &ast::Expr) {
 380         // We don't use cat_expr() here because we don't want to treat
 381         // auto-ref'd parameters in overloaded operators as rvalues.
 382         let adj = {
 383             let adjustments = self.bccx.tcx.adjustments.borrow();
 384             adjustments.get().find_copy(&expr.id)
 385         };
 386         let cmt = match adj {
 387             None => self.bccx.cat_expr_unadjusted(expr),
 388             Some(adj) => self.bccx.cat_expr_autoderefd(expr, adj)
 389         };
 390
 391         debug!("check_assignment(cmt={})", cmt.repr(self.tcx()));
 392
 393         // Mutable values can be assigned, as long as they obey loans
 394         // and aliasing restrictions:
 395         if cmt.mutbl.is_mutable() {
 396             if check_for_aliasable_mutable_writes(self, expr, cmt) {
 397                 if check_for_assignment_to_restricted_or_frozen_location(
 398                     self, expr, cmt)
 399                 {
 400                     // Safe, but record for lint pass later:
 401                     mark_variable_as_used_mut(self, cmt);
 402                 }
 403             }
 404             return;
 405         }
 406
 407         // For immutable local variables, assignments are legal
 408         // if they cannot already have been assigned
 409         if self.is_local_variable(cmt) {
 410             assert!(cmt.mutbl.is_immutable()); // no "const" locals
 411             let lp = opt_loan_path(cmt).unwrap();
 412             self.move_data.each_assignment_of(expr.id, lp, |assign| {
 413                 self.bccx.report_reassigned_immutable_variable(
 414                     expr.span,
 415                     lp,
 416                     assign);
 417                 false
 418             });
 419             return;
 420         }
 421
 422         // Otherwise, just a plain error.
 423         match opt_loan_path(cmt) {
 424             Some(lp) => {
 425                 self.bccx.span_err(
 426                     expr.span,
 427                     format!("cannot assign to {} {} `{}`",
 428                             cmt.mutbl.to_user_str(),
 429                             self.bccx.cmt_to_str(cmt),
 430                             self.bccx.loan_path_to_str(lp)));
 431             }
 432             None => {
 433                 self.bccx.span_err(
 434                     expr.span,
 435                     format!("cannot assign to {} {}",
 436                             cmt.mutbl.to_user_str(),
 437                             self.bccx.cmt_to_str(cmt)));
 438             }
 439         }
 440         return;
 441
 442         fn mark_variable_as_used_mut(this: &CheckLoanCtxt,
 443                                      cmt: mc::cmt) {
 444             //! If the mutability of the `cmt` being written is inherited
 445             //! from a local variable, liveness will
 446             //! not have been able to detect that this variable's mutability
 447             //! is important, so we must add the variable to the
 448             //! `used_mut_nodes` table here.
 449
 450             let mut cmt = cmt;
 451             loop {
 452                 debug!("mark_writes_through_upvars_as_used_mut(cmt={})",
 453                        cmt.repr(this.tcx()));
 454                 match cmt.cat {
 455                     mc::cat_local(id) | mc::cat_arg(id) => {
 456                         let mut used_mut_nodes = this.tcx()
 457                                                      .used_mut_nodes
 458                                                      .borrow_mut();
 459                         used_mut_nodes.get().insert(id);
 460                         return;
 461                     }
 462
 463                     mc::cat_upvar(..) => {
 464                         return;
 465                     }
 466
 467                     mc::cat_deref(_, _, mc::GcPtr) => {
 468                         assert_eq!(cmt.mutbl, mc::McImmutable);
 469                         return;
 470                     }
 471
 472                     mc::cat_rvalue(..) |
 473                     mc::cat_static_item |
 474                     mc::cat_copied_upvar(..) |
 475                     mc::cat_deref(_, _, mc::UnsafePtr(..)) |
 476                     mc::cat_deref(_, _, mc::BorrowedPtr(..)) => {
 477                         assert_eq!(cmt.mutbl, mc::McDeclared);
 478                         return;
 479                     }
 480
 481                     mc::cat_discr(b, _) |
 482                     mc::cat_deref(b, _, mc::OwnedPtr) => {
 483                         assert_eq!(cmt.mutbl, mc::McInherited);
 484                         cmt = b;
 485                     }
 486
 487                     mc::cat_downcast(b) |
 488                     mc::cat_interior(b, _) => {
 489                         assert_eq!(cmt.mutbl, mc::McInherited);
 490                         cmt = b;
 491                     }
 492                 }
 493             }
 494         }
 495
 496         fn check_for_aliasable_mutable_writes(this: &CheckLoanCtxt,
 497                                               expr: &ast::Expr,
 498                                               cmt: mc::cmt) -> bool {
 499             //! Safety checks related to writes to aliasable, mutable locations
 500
 501             let guarantor = cmt.guarantor();
 502             debug!("check_for_aliasable_mutable_writes(cmt={}, guarantor={})",
 503                    cmt.repr(this.tcx()), guarantor.repr(this.tcx()));
 504             match guarantor.cat {
 505                 mc::cat_deref(b, _, mc::BorrowedPtr(ty::MutBorrow, _)) => {
 506                     // Statically prohibit writes to `&mut` when aliasable
 507
 508                     check_for_aliasability_violation(this, expr, b);
 509                 }
 510
 511                 _ => {}
 512             }
 513
 514             return true; // no errors reported
 515         }
 516
 517         fn check_for_aliasability_violation(this: &CheckLoanCtxt,
 518                                             expr: &ast::Expr,
 519                                             cmt: mc::cmt)
 520                                             -> bool {
 521             match cmt.freely_aliasable() {
 522                 None => {
 523                     return true;
 524                 }
 525                 Some(cause) => {
 526                     this.bccx.report_aliasability_violation(
 527                         expr.span,
 528                         MutabilityViolation,
 529                         cause);
 530                     return false;
 531                 }
 532             }
 533         }
 534
 535         fn check_for_assignment_to_restricted_or_frozen_location(
 536             this: &CheckLoanCtxt,
 537             expr: &ast::Expr,
 538             cmt: mc::cmt) -> bool
 539         {
 540             //! Check for assignments that violate the terms of an
 541             //! outstanding loan.
 542
 543             let loan_path = match opt_loan_path(cmt) {
 544                 Some(lp) => lp,
 545                 None => { return true; /* no loan path, can't be any loans */ }
 546             };
 547
 548             // Start by searching for an assignment to a *restricted*
 549             // location. Here is one example of the kind of error caught
 550             // by this check:
 551             //
 552             //    let mut v = ~[1, 2, 3];
 553             //    let p = &v;
 554             //    v = ~[4];
 555             //
 556             // In this case, creating `p` triggers a RESTR_MUTATE
 557             // restriction on the path `v`.
 558             //
 559             // Here is a second, more subtle example:
 560             //
 561             //    let mut v = ~[1, 2, 3];
 562             //    let p = &const v[0];
 563             //    v[0] = 4;                   // OK
 564             //    v[1] = 5;                   // OK
 565             //    v = ~[4, 5, 3];             // Error
 566             //
 567             // In this case, `p` is pointing to `v[0]`, and it is a
 568             // `const` pointer in any case. So the first two
 569             // assignments are legal (and would be permitted by this
 570             // check). However, the final assignment (which is
 571             // logically equivalent) is forbidden, because it would
 572             // cause the existing `v` array to be freed, thus
 573             // invalidating `p`. In the code, this error results
 574             // because `gather_loans::restrictions` adds a
 575             // `RESTR_MUTATE` restriction whenever the contents of an
 576             // owned pointer are borrowed, and hence while `v[*]` is not
 577             // restricted from being written, `v` is.
 578             let cont = this.each_in_scope_restriction(expr.id,
 579                                                       loan_path,
 580                                                       |loan, restr| {
 581                 if restr.set.intersects(RESTR_MUTATE) {
 582                     this.report_illegal_mutation(expr, loan_path, loan);
 583                     false
 584                 } else {
 585                     true
 586                 }
 587             });
 588
 589             if !cont { return false }
 590
 591             // The previous code handled assignments to paths that
 592             // have been restricted. This covers paths that have been
 593             // directly lent out and their base paths, but does not
 594             // cover random extensions of those paths. For example,
 595             // the following program is not declared illegal by the
 596             // previous check:
 597             //
 598             //    let mut v = ~[1, 2, 3];
 599             //    let p = &v;
 600             //    v[0] = 4; // declared error by loop below, not code above
 601             //
 602             // The reason that this passes the previous check whereas
 603             // an assignment like `v = ~[4]` fails is because the assignment
 604             // here is to `v[*]`, and the existing restrictions were issued
 605             // for `v`, not `v[*]`.
 606             //
 607             // So in this loop, we walk back up the loan path so long
 608             // as the mutability of the path is dependent on a super
 609             // path, and check that the super path was not lent out as
 610             // mutable or immutable (a const loan is ok).
 611             //
 612             // Mutability of a path can be dependent on the super path
 613             // in two ways. First, it might be inherited mutability.
 614             // Second, the pointee of an `&mut` pointer can only be
 615             // mutated if it is found in an unaliased location, so we
 616             // have to check that the owner location is not borrowed.
 617             //
 618             // Note that we are *not* checking for any and all
 619             // restrictions.  We are only interested in the pointers
 620             // that the user created, whereas we add restrictions for
 621             // all kinds of paths that are not directly aliased. If we checked
 622             // for all restrictions, and not just loans, then the following
 623             // valid program would be considered illegal:
 624             //
 625             //    let mut v = ~[1, 2, 3];
 626             //    let p = &const v[0];
 627             //    v[1] = 5; // ok
 628             //
 629             // Here the restriction that `v` not be mutated would be misapplied
 630             // to block the subpath `v[1]`.
 631             let full_loan_path = loan_path;
 632             let mut loan_path = loan_path;
 633             loop {
 634                 match *loan_path {
 635                     // Peel back one layer if, for `loan_path` to be
 636                     // mutable, `lp_base` must be mutable. This occurs
 637                     // with inherited mutability and with `&mut`
 638                     // pointers.
 639                     LpExtend(lp_base, mc::McInherited, _) |
 640                     LpExtend(lp_base, _, LpDeref(mc::BorrowedPtr(ty::MutBorrow, _))) => {
 641                         loan_path = lp_base;
 642                     }
 643
 644                     // Otherwise stop iterating
 645                     LpExtend(_, mc::McDeclared, _) |
 646                     LpExtend(_, mc::McImmutable, _) |
 647                     LpVar(_) => {
 648                         return true;
 649                     }
 650                 }
 651
 652                 // Check for a non-const loan of `loan_path`
 653                 let cont = this.each_in_scope_loan(expr.id, |loan| {
 654                     if loan.loan_path == loan_path {
 655                         this.report_illegal_mutation(expr, full_loan_path, loan);
 656                         false
 657                     } else {
 658                         true
 659                     }
 660                 });
 661
 662                 if !cont { return false }
 663             }
 664         }
 665     }
 666
 667     pub fn report_illegal_mutation(&self,
 668                                    expr: &ast::Expr,
 669                                    loan_path: &LoanPath,
 670                                    loan: &Loan) {
 671         self.bccx.span_err(
 672             expr.span,
 673             format!("cannot assign to `{}` because it is borrowed",
 674                  self.bccx.loan_path_to_str(loan_path)));
 675         self.bccx.span_note(
 676             loan.span,
 677             format!("borrow of `{}` occurs here",
 678                  self.bccx.loan_path_to_str(loan_path)));
 679     }
 680
 681     fn check_move_out_from_expr(&self, expr: &ast::Expr) {
 682         match expr.node {
 683             ast::ExprFnBlock(..) | ast::ExprProc(..) => {
 684                 // Moves due to captures are checked in
 685                 // check_captured_variables() because it allows
 686                 // us to give a more precise error message with
 687                 // a more precise span.
 688             }
 689             _ => {
 690                 self.check_move_out_from_id(expr.id, expr.span)
 691             }
 692         }
 693     }
 694
 695     fn check_move_out_from_id(&self, id: ast::NodeId, span: Span) {
 696         self.move_data.each_path_moved_by(id, |_, move_path| {
 697             match self.analyze_move_out_from(id, move_path) {
 698                 MoveOk => {}
 699                 MoveWhileBorrowed(loan_path, loan_span) => {
 700                     self.bccx.span_err(
 701                         span,
 702                         format!("cannot move out of `{}` \
 703                                 because it is borrowed",
 704                              self.bccx.loan_path_to_str(move_path)));
 705                     self.bccx.span_note(
 706                         loan_span,
 707                         format!("borrow of `{}` occurs here",
 708                                 self.bccx.loan_path_to_str(loan_path)));
 709                 }
 710             }
 711             true
 712         });
 713     }
 714
 715     fn check_captured_variables(&self,
 716                                 closure_id: ast::NodeId,
 717                                 span: Span) {
 718         let capture_map = self.bccx.capture_map.borrow();
 719         let cap_vars = capture_map.get().get(&closure_id);
 720         for cap_var in cap_vars.deref().iter() {
 721             let var_id = ast_util::def_id_of_def(cap_var.def).node;
 722             let var_path = @LpVar(var_id);
 723             self.check_if_path_is_moved(closure_id, span,
 724                                         MovedInCapture, var_path);
 725             match cap_var.mode {
 726                 moves::CapRef | moves::CapCopy => {}
 727                 moves::CapMove => {
 728                     check_by_move_capture(self, closure_id, cap_var, var_path);
 729                 }
 730             }
 731         }
 732         return;
 733
 734         fn check_by_move_capture(this: &CheckLoanCtxt,
 735                                  closure_id: ast::NodeId,
 736                                  cap_var: &moves::CaptureVar,
 737                                  move_path: @LoanPath) {
 738             let move_err = this.analyze_move_out_from(closure_id, move_path);
 739             match move_err {
 740                 MoveOk => {}
 741                 MoveWhileBorrowed(loan_path, loan_span) => {
 742                     this.bccx.span_err(
 743                         cap_var.span,
 744                         format!("cannot move `{}` into closure \
 745                                 because it is borrowed",
 746                                 this.bccx.loan_path_to_str(move_path)));
 747                     this.bccx.span_note(
 748                         loan_span,
 749                         format!("borrow of `{}` occurs here",
 750                                 this.bccx.loan_path_to_str(loan_path)));
 751                 }
 752             }
 753         }
 754     }
 755
 756     pub fn analyze_move_out_from(&self,
 757                                  expr_id: ast::NodeId,
 758                                  mut move_path: @LoanPath)
 759                                  -> MoveError {
 760         debug!("analyze_move_out_from(expr_id={:?}, move_path={})",
 761                self.tcx().map.node_to_str(expr_id),
 762                move_path.repr(self.tcx()));
 763
 764         // We must check every element of a move path. See
 765         // `borrowck-move-subcomponent.rs` for a test case.
 766         loop {
 767             // check for a conflicting loan:
 768             let mut ret = MoveOk;
 769             self.each_in_scope_restriction(expr_id, move_path, |loan, _| {
 770                 // Any restriction prevents moves.
 771                 ret = MoveWhileBorrowed(loan.loan_path, loan.span);
 772                 false
 773             });
 774
 775             if ret != MoveOk {
 776                 return ret
 777             }
 778
 779             match *move_path {
 780                 LpVar(_) => return MoveOk,
 781                 LpExtend(subpath, _, _) => move_path = subpath,
 782             }
 783         }
 784     }
 785
 786     pub fn check_call(&self,
 787                       _expr: &ast::Expr,
 788                       _callee: Option<@ast::Expr>,
 789                       _callee_span: Span,
 790                       _args: &[@ast::Expr]) {
 791         // NB: This call to check for conflicting loans is not truly
 792         // necessary, because the callee_id never issues new loans.
 793         // However, I added it for consistency and lest the system
 794         // should change in the future.
 795         //
 796         // FIXME(#6268) nested method calls
 797         // self.check_for_conflicting_loans(callee_id);
 798     }
 799 }
 800
 801 fn check_loans_in_local<'a>(this: &mut CheckLoanCtxt<'a>,
 802                             local: &ast::Local) {
 803     visit::walk_local(this, local, ());
 804 }
 805
 806 fn check_loans_in_expr<'a>(this: &mut CheckLoanCtxt<'a>,
 807                            expr: &ast::Expr) {
 808     visit::walk_expr(this, expr, ());
 809
 810     debug!("check_loans_in_expr(expr={})",
 811            expr.repr(this.tcx()));
 812
 813     this.check_for_conflicting_loans(expr.id);
 814     this.check_move_out_from_expr(expr);
 815
 816     let method_map = this.bccx.method_map.borrow();
 817     match expr.node {
 818       ast::ExprPath(..) => {
 819           if !this.move_data.is_assignee(expr.id) {
 820               let cmt = this.bccx.cat_expr_unadjusted(expr);
 821               debug!("path cmt={}", cmt.repr(this.tcx()));
 822               let r = opt_loan_path(cmt);
 823               for &lp in r.iter() {
 824                   this.check_if_path_is_moved(expr.id, expr.span, MovedInUse, lp);
 825               }
 826           }
 827       }
 828       ast::ExprFnBlock(..) | ast::ExprProc(..) => {
 829           this.check_captured_variables(expr.id, expr.span)
 830       }
 831       ast::ExprAssign(dest, _) |
 832       ast::ExprAssignOp(_, dest, _) => {
 833         this.check_assignment(dest);
 834       }
 835       ast::ExprCall(f, ref args) => {
 836         this.check_call(expr, Some(f), f.span, args.as_slice());
 837       }
 838       ast::ExprMethodCall(_, _, ref args) => {
 839         this.check_call(expr, None, expr.span, args.as_slice());
 840       }
 841       ast::ExprIndex(_, rval) | ast::ExprBinary(_, _, rval)
 842       if method_map.get().contains_key(&MethodCall::expr(expr.id)) => {
 843         this.check_call(expr, None, expr.span, [rval]);
 844       }
 845       ast::ExprUnary(_, _) | ast::ExprIndex(_, _)
 846       if method_map.get().contains_key(&MethodCall::expr(expr.id)) => {
 847         this.check_call(expr, None, expr.span, []);
 848       }
 849       ast::ExprInlineAsm(ref ia) => {
 850           for &(_, out) in ia.outputs.iter() {
 851               this.check_assignment(out);
 852           }
 853       }
 854       _ => {}
 855     }
 856 }
 857
 858 fn check_loans_in_pat<'a>(this: &mut CheckLoanCtxt<'a>,
 859                           pat: &ast::Pat)
 860 {
 861     this.check_for_conflicting_loans(pat.id);
 862     this.check_move_out_from_id(pat.id, pat.span);
 863     visit::walk_pat(this, pat, ());
 864 }
 865
 866 fn check_loans_in_block<'a>(this: &mut CheckLoanCtxt<'a>,
 867                             blk: &ast::Block)
 868 {
 869     visit::walk_block(this, blk, ());
 870     this.check_for_conflicting_loans(blk.id);
 871 }