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 middle::borrowck::*;
  22 use middle::expr_use_visitor as euv;
  23 use middle::mem_categorization as mc;
  24 use middle::ty;
  25 use syntax::ast;
  26 use syntax::codemap::Span;
  27 use util::ppaux::Repr;
  28
  29 use std::rc::Rc;
  30
  31 // FIXME (#16118): These functions are intended to allow the borrow checker to
  32 // be less precise in its handling of Box while still allowing moves out of a
  33 // Box. They should be removed when OwnedPtr is removed from LoanPath.
  34
  35 fn owned_ptr_base_path<'a>(loan_path: &'a LoanPath) -> &'a LoanPath {
  36     //! Returns the base of the leftmost dereference of an OwnedPtr in
  37     //! `loan_path`. If there is no dereference of an OwnedPtr in `loan_path`,
  38     //! then it just returns `loan_path` itself.
  39
  40     return match owned_ptr_base_path_helper(loan_path) {
  41         Some(new_loan_path) => new_loan_path,
  42         None => loan_path.clone()
  43     };
  44
  45     fn owned_ptr_base_path_helper<'a>(loan_path: &'a LoanPath) -> Option<&'a LoanPath> {
  46         match *loan_path {
  47             LpVar(_) | LpUpvar(_) => None,
  48             LpExtend(ref lp_base, _, LpDeref(mc::OwnedPtr)) => {
  49                 match owned_ptr_base_path_helper(&**lp_base) {
  50                     v @ Some(_) => v,
  51                     None => Some(&**lp_base)
  52                 }
  53             }
  54             LpExtend(ref lp_base, _, _) => owned_ptr_base_path_helper(&**lp_base)
  55         }
  56     }
  57 }
  58
  59 fn owned_ptr_base_path_rc(loan_path: &Rc<LoanPath>) -> Rc<LoanPath> {
  60     //! The equivalent of `owned_ptr_base_path` for an &Rc<LoanPath> rather than
  61     //! a &LoanPath.
  62
  63     return match owned_ptr_base_path_helper(loan_path) {
  64         Some(new_loan_path) => new_loan_path,
  65         None => loan_path.clone()
  66     };
  67
  68     fn owned_ptr_base_path_helper(loan_path: &Rc<LoanPath>) -> Option<Rc<LoanPath>> {
  69         match **loan_path {
  70             LpVar(_) | LpUpvar(_) => None,
  71             LpExtend(ref lp_base, _, LpDeref(mc::OwnedPtr)) => {
  72                 match owned_ptr_base_path_helper(lp_base) {
  73                     v @ Some(_) => v,
  74                     None => Some(lp_base.clone())
  75                 }
  76             }
  77             LpExtend(ref lp_base, _, _) => owned_ptr_base_path_helper(lp_base)
  78         }
  79     }
  80 }
  81
  82 struct CheckLoanCtxt<'a, 'tcx: 'a> {
  83     bccx: &'a BorrowckCtxt<'a, 'tcx>,
  84     dfcx_loans: &'a LoanDataFlow<'a, 'tcx>,
  85     move_data: move_data::FlowedMoveData<'a, 'tcx>,
  86     all_loans: &'a [Loan],
  87 }
  88
  89 impl<'a, 'tcx> euv::Delegate for CheckLoanCtxt<'a, 'tcx> {
  90     fn consume(&mut self,
  91                consume_id: ast::NodeId,
  92                consume_span: Span,
  93                cmt: mc::cmt,
  94                mode: euv::ConsumeMode) {
  95         debug!("consume(consume_id={}, cmt={}, mode={})",
  96                consume_id, cmt.repr(self.tcx()), mode);
  97
  98         self.consume_common(consume_id, consume_span, cmt, mode);
  99     }
 100
 101     fn consume_pat(&mut self,
 102                    consume_pat: &ast::Pat,
 103                    cmt: mc::cmt,
 104                    mode: euv::ConsumeMode) {
 105         debug!("consume_pat(consume_pat={}, cmt={}, mode={})",
 106                consume_pat.repr(self.tcx()),
 107                cmt.repr(self.tcx()),
 108                mode);
 109
 110         self.consume_common(consume_pat.id, consume_pat.span, cmt, mode);
 111     }
 112
 113     fn borrow(&mut self,
 114               borrow_id: ast::NodeId,
 115               borrow_span: Span,
 116               cmt: mc::cmt,
 117               loan_region: ty::Region,
 118               bk: ty::BorrowKind,
 119               loan_cause: euv::LoanCause)
 120     {
 121         debug!("borrow(borrow_id={}, cmt={}, loan_region={}, \
 122                bk={}, loan_cause={:?})",
 123                borrow_id, cmt.repr(self.tcx()), loan_region,
 124                bk, loan_cause);
 125
 126         match opt_loan_path(&cmt) {
 127             Some(lp) => {
 128                 let moved_value_use_kind = match loan_cause {
 129                     euv::ClosureCapture(_) => MovedInCapture,
 130                     _ => MovedInUse,
 131                 };
 132                 self.check_if_path_is_moved(borrow_id, borrow_span, moved_value_use_kind, &lp);
 133             }
 134             None => { }
 135         }
 136
 137         self.check_for_conflicting_loans(borrow_id);
 138     }
 139
 140     fn mutate(&mut self,
 141               assignment_id: ast::NodeId,
 142               assignment_span: Span,
 143               assignee_cmt: mc::cmt,
 144               mode: euv::MutateMode)
 145     {
 146         debug!("mutate(assignment_id={}, assignee_cmt={})",
 147                assignment_id, assignee_cmt.repr(self.tcx()));
 148
 149         match opt_loan_path(&assignee_cmt) {
 150             Some(lp) => {
 151                 match mode {
 152                     euv::Init | euv::JustWrite => {
 153                         // In a case like `path = 1`, then path does not
 154                         // have to be *FULLY* initialized, but we still
 155                         // must be careful lest it contains derefs of
 156                         // pointers.
 157                         self.check_if_assigned_path_is_moved(assignee_cmt.id,
 158                                                              assignment_span,
 159                                                              MovedInUse,
 160                                                              &lp);
 161                     }
 162                     euv::WriteAndRead => {
 163                         // In a case like `path += 1`, then path must be
 164                         // fully initialized, since we will read it before
 165                         // we write it.
 166                         self.check_if_path_is_moved(assignee_cmt.id,
 167                                                     assignment_span,
 168                                                     MovedInUse,
 169                                                     &lp);
 170                     }
 171                 }
 172             }
 173             None => { }
 174         }
 175
 176         self.check_assignment(assignment_id, assignment_span, assignee_cmt, mode);
 177     }
 178
 179     fn decl_without_init(&mut self, _id: ast::NodeId, _span: Span) { }
 180 }
 181
 182 pub fn check_loans<'a, 'b, 'c, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
 183                                      dfcx_loans: &LoanDataFlow<'b, 'tcx>,
 184                                      move_data: move_data::FlowedMoveData<'c, 'tcx>,
 185                                      all_loans: &[Loan],
 186                                      decl: &ast::FnDecl,
 187                                      body: &ast::Block) {
 188     debug!("check_loans(body id={:?})", body.id);
 189
 190     let mut clcx = CheckLoanCtxt {
 191         bccx: bccx,
 192         dfcx_loans: dfcx_loans,
 193         move_data: move_data,
 194         all_loans: all_loans,
 195     };
 196
 197     {
 198         let mut euv = euv::ExprUseVisitor::new(&mut clcx, bccx.tcx);
 199         euv.walk_fn(decl, body);
 200     }
 201 }
 202
 203 #[deriving(PartialEq)]
 204 enum UseError {
 205     UseOk,
 206     UseWhileBorrowed(/*loan*/Rc<LoanPath>, /*loan*/Span)
 207 }
 208
 209 fn compatible_borrow_kinds(borrow_kind1: ty::BorrowKind,
 210                            borrow_kind2: ty::BorrowKind)
 211                            -> bool {
 212     borrow_kind1 == ty::ImmBorrow && borrow_kind2 == ty::ImmBorrow
 213 }
 214
 215 impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
 216     pub fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.bccx.tcx }
 217
 218     pub fn each_issued_loan(&self, scope_id: ast::NodeId, op: |&Loan| -> bool)
 219                             -> bool {
 220         //! Iterates over each loan that has been issued
 221         //! on entrance to `scope_id`, regardless of whether it is
 222         //! actually *in scope* at that point.  Sometimes loans
 223         //! are issued for future scopes and thus they may have been
 224         //! *issued* but not yet be in effect.
 225
 226         self.dfcx_loans.each_bit_on_entry(scope_id, |loan_index| {
 227             let loan = &self.all_loans[loan_index];
 228             op(loan)
 229         })
 230     }
 231
 232     pub fn each_in_scope_loan(&self,
 233                               scope_id: ast::NodeId,
 234                               op: |&Loan| -> bool)
 235                               -> bool {
 236         //! Like `each_issued_loan()`, but only considers loans that are
 237         //! currently in scope.
 238
 239         let tcx = self.tcx();
 240         self.each_issued_loan(scope_id, |loan| {
 241             if tcx.region_maps.is_subscope_of(scope_id, loan.kill_scope) {
 242                 op(loan)
 243             } else {
 244                 true
 245             }
 246         })
 247     }
 248
 249     fn each_in_scope_loan_affecting_path(&self,
 250                                          scope_id: ast::NodeId,
 251                                          loan_path: &LoanPath,
 252                                          op: |&Loan| -> bool)
 253                                          -> bool {
 254         //! Iterates through all of the in-scope loans affecting `loan_path`,
 255         //! calling `op`, and ceasing iteration if `false` is returned.
 256
 257         // First, we check for a loan restricting the path P being used. This
 258         // accounts for borrows of P but also borrows of subpaths, like P.a.b.
 259         // Consider the following example:
 260         //
 261         //     let x = &mut a.b.c; // Restricts a, a.b, and a.b.c
 262         //     let y = a;          // Conflicts with restriction
 263
 264         let loan_path = owned_ptr_base_path(loan_path);
 265         let cont = self.each_in_scope_loan(scope_id, |loan| {
 266             let mut ret = true;
 267             for restr_path in loan.restricted_paths.iter() {
 268                 if **restr_path == *loan_path {
 269                     if !op(loan) {
 270                         ret = false;
 271                         break;
 272                     }
 273                 }
 274             }
 275             ret
 276         });
 277
 278         if !cont {
 279             return false;
 280         }
 281
 282         // Next, we must check for *loans* (not restrictions) on the path P or
 283         // any base path. This rejects examples like the following:
 284         //
 285         //     let x = &mut a.b;
 286         //     let y = a.b.c;
 287         //
 288         // Limiting this search to *loans* and not *restrictions* means that
 289         // examples like the following continue to work:
 290         //
 291         //     let x = &mut a.b;
 292         //     let y = a.c;
 293
 294         let mut loan_path = loan_path;
 295         loop {
 296             match *loan_path {
 297                 LpVar(_) | LpUpvar(_) => {
 298                     break;
 299                 }
 300                 LpExtend(ref lp_base, _, _) => {
 301                     loan_path = &**lp_base;
 302                 }
 303             }
 304
 305             let cont = self.each_in_scope_loan(scope_id, |loan| {
 306                 if *loan.loan_path == *loan_path {
 307                     op(loan)
 308                 } else {
 309                     true
 310                 }
 311             });
 312
 313             if !cont {
 314                 return false;
 315             }
 316         }
 317
 318         return true;
 319     }
 320
 321     pub fn loans_generated_by(&self, scope_id: ast::NodeId) -> Vec<uint> {
 322         //! Returns a vector of the loans that are generated as
 323         //! we encounter `scope_id`.
 324
 325         let mut result = Vec::new();
 326         self.dfcx_loans.each_gen_bit(scope_id, |loan_index| {
 327             result.push(loan_index);
 328             true
 329         });
 330         return result;
 331     }
 332
 333     pub fn check_for_conflicting_loans(&self, scope_id: ast::NodeId) {
 334         //! Checks to see whether any of the loans that are issued
 335         //! by `scope_id` conflict with loans that have already been
 336         //! issued when we enter `scope_id` (for example, we do not
 337         //! permit two `&mut` borrows of the same variable).
 338
 339         debug!("check_for_conflicting_loans(scope_id={:?})", scope_id);
 340
 341         let new_loan_indices = self.loans_generated_by(scope_id);
 342         debug!("new_loan_indices = {:?}", new_loan_indices);
 343
 344         self.each_issued_loan(scope_id, |issued_loan| {
 345             for &new_loan_index in new_loan_indices.iter() {
 346                 let new_loan = &self.all_loans[new_loan_index];
 347                 self.report_error_if_loans_conflict(issued_loan, new_loan);
 348             }
 349             true
 350         });
 351
 352         for (i, &x) in new_loan_indices.iter().enumerate() {
 353             let old_loan = &self.all_loans[x];
 354             for &y in new_loan_indices.slice_from(i+1).iter() {
 355                 let new_loan = &self.all_loans[y];
 356                 self.report_error_if_loans_conflict(old_loan, new_loan);
 357             }
 358         }
 359     }
 360
 361     pub fn report_error_if_loans_conflict(&self,
 362                                           old_loan: &Loan,
 363                                           new_loan: &Loan) {
 364         //! Checks whether `old_loan` and `new_loan` can safely be issued
 365         //! simultaneously.
 366
 367         debug!("report_error_if_loans_conflict(old_loan={}, new_loan={})",
 368                old_loan.repr(self.tcx()),
 369                new_loan.repr(self.tcx()));
 370
 371         // Should only be called for loans that are in scope at the same time.
 372         assert!(self.tcx().region_maps.scopes_intersect(old_loan.kill_scope,
 373                                                         new_loan.kill_scope));
 374
 375         self.report_error_if_loan_conflicts_with_restriction(
 376             old_loan, new_loan, old_loan, new_loan) &&
 377         self.report_error_if_loan_conflicts_with_restriction(
 378             new_loan, old_loan, old_loan, new_loan);
 379     }
 380
 381     pub fn report_error_if_loan_conflicts_with_restriction(&self,
 382                                                            loan1: &Loan,
 383                                                            loan2: &Loan,
 384                                                            old_loan: &Loan,
 385                                                            new_loan: &Loan)
 386                                                            -> bool {
 387         //! Checks whether the restrictions introduced by `loan1` would
 388         //! prohibit `loan2`. Returns false if an error is reported.
 389
 390         debug!("report_error_if_loan_conflicts_with_restriction(\
 391                 loan1={}, loan2={})",
 392                loan1.repr(self.tcx()),
 393                loan2.repr(self.tcx()));
 394
 395         if compatible_borrow_kinds(loan1.kind, loan2.kind) {
 396             return true;
 397         }
 398
 399         let loan2_base_path = owned_ptr_base_path_rc(&loan2.loan_path);
 400         for restr_path in loan1.restricted_paths.iter() {
 401             if *restr_path != loan2_base_path { continue; }
 402
 403             let old_pronoun = if new_loan.loan_path == old_loan.loan_path {
 404                 "it".to_string()
 405             } else {
 406                 format!("`{}`",
 407                         self.bccx.loan_path_to_string(&*old_loan.loan_path))
 408             };
 409
 410             match (new_loan.kind, old_loan.kind) {
 411                 (ty::MutBorrow, ty::MutBorrow) => {
 412                     self.bccx.span_err(
 413                         new_loan.span,
 414                         format!("cannot borrow `{}` as mutable \
 415                                 more than once at a time",
 416                                 self.bccx.loan_path_to_string(
 417                                     &*new_loan.loan_path)).as_slice());
 418                 }
 419
 420                 (ty::UniqueImmBorrow, _) => {
 421                     self.bccx.span_err(
 422                         new_loan.span,
 423                         format!("closure requires unique access to `{}` \
 424                                 but {} is already borrowed",
 425                                 self.bccx.loan_path_to_string(&*new_loan.loan_path),
 426                                 old_pronoun).as_slice());
 427                 }
 428
 429                 (_, ty::UniqueImmBorrow) => {
 430                     self.bccx.span_err(
 431                         new_loan.span,
 432                         format!("cannot borrow `{}` as {} because \
 433                                 previous closure requires unique access",
 434                                 self.bccx.loan_path_to_string(&*new_loan.loan_path),
 435                                 new_loan.kind.to_user_str()).as_slice());
 436                 }
 437
 438                 (_, _) => {
 439                     self.bccx.span_err(
 440                         new_loan.span,
 441                         format!("cannot borrow `{}` as {} because \
 442                                 {} is also borrowed as {}",
 443                                 self.bccx.loan_path_to_string(&*new_loan.loan_path),
 444                                 new_loan.kind.to_user_str(),
 445                                 old_pronoun,
 446                                 old_loan.kind.to_user_str()).as_slice());
 447                 }
 448             }
 449
 450             match new_loan.cause {
 451                 euv::ClosureCapture(span) => {
 452                     self.bccx.span_note(
 453                         span,
 454                         format!("borrow occurs due to use of `{}` in closure",
 455                                 self.bccx.loan_path_to_string(
 456                                     &*new_loan.loan_path)).as_slice());
 457                 }
 458                 _ => { }
 459             }
 460
 461             let rule_summary = match old_loan.kind {
 462                 ty::MutBorrow => {
 463                     format!("the mutable borrow prevents subsequent \
 464                             moves, borrows, or modification of `{0}` \
 465                             until the borrow ends",
 466                             self.bccx.loan_path_to_string(
 467                                 &*old_loan.loan_path))
 468                 }
 469
 470                 ty::ImmBorrow => {
 471                     format!("the immutable borrow prevents subsequent \
 472                             moves or mutable borrows of `{0}` \
 473                             until the borrow ends",
 474                             self.bccx.loan_path_to_string(&*old_loan.loan_path))
 475                 }
 476
 477                 ty::UniqueImmBorrow => {
 478                     format!("the unique capture prevents subsequent \
 479                             moves or borrows of `{0}` \
 480                             until the borrow ends",
 481                             self.bccx.loan_path_to_string(&*old_loan.loan_path))
 482                 }
 483             };
 484
 485             let borrow_summary = match old_loan.cause {
 486                 euv::ClosureCapture(_) => {
 487                     format!("previous borrow of `{}` occurs here due to \
 488                             use in closure",
 489                             self.bccx.loan_path_to_string(&*old_loan.loan_path))
 490                 }
 491
 492                 euv::OverloadedOperator(..) |
 493                 euv::AddrOf(..) |
 494                 euv::AutoRef(..) |
 495                 euv::ClosureInvocation(..) |
 496                 euv::ForLoop(..) |
 497                 euv::RefBinding(..) => {
 498                     format!("previous borrow of `{}` occurs here",
 499                             self.bccx.loan_path_to_string(&*old_loan.loan_path))
 500                 }
 501             };
 502
 503             self.bccx.span_note(
 504                 old_loan.span,
 505                 format!("{}; {}", borrow_summary, rule_summary).as_slice());
 506
 507             let old_loan_span = self.tcx().map.span(old_loan.kill_scope);
 508             self.bccx.span_end_note(old_loan_span,
 509                                     "previous borrow ends here");
 510
 511             return false;
 512         }
 513
 514         true
 515     }
 516
 517     pub fn is_local_variable_or_arg(&self, cmt: mc::cmt) -> bool {
 518         match cmt.cat {
 519           mc::cat_local(_) | mc::cat_arg(_) => true,
 520           _ => false
 521         }
 522     }
 523
 524     fn consume_common(&self,
 525                       id: ast::NodeId,
 526                       span: Span,
 527                       cmt: mc::cmt,
 528                       mode: euv::ConsumeMode) {
 529         match opt_loan_path(&cmt) {
 530             Some(lp) => {
 531                 let moved_value_use_kind = match mode {
 532                     euv::Copy => {
 533                         self.check_for_copy_of_frozen_path(id, span, &*lp);
 534                         MovedInUse
 535                     }
 536                     euv::Move(_) => {
 537                         match self.move_data.kind_of_move_of_path(id, &lp) {
 538                             None => {
 539                                 // Sometimes moves don't have a move kind;
 540                                 // this either means that the original move
 541                                 // was from something illegal to move,
 542                                 // or was moved from referent of an unsafe
 543                                 // pointer or something like that.
 544                                 MovedInUse
 545                             }
 546                             Some(move_kind) => {
 547                                 self.check_for_move_of_borrowed_path(id, span,
 548                                                                      &*lp, move_kind);
 549                                 if move_kind == move_data::Captured {
 550                                     MovedInCapture
 551                                 } else {
 552                                     MovedInUse
 553                                 }
 554                             }
 555                         }
 556                     }
 557                 };
 558
 559                 self.check_if_path_is_moved(id, span, moved_value_use_kind, &lp);
 560             }
 561             None => { }
 562         }
 563     }
 564
 565     fn check_for_copy_of_frozen_path(&self,
 566                                      id: ast::NodeId,
 567                                      span: Span,
 568                                      copy_path: &LoanPath) {
 569         match self.analyze_restrictions_on_use(id, copy_path, ty::ImmBorrow) {
 570             UseOk => { }
 571             UseWhileBorrowed(loan_path, loan_span) => {
 572                 self.bccx.span_err(
 573                     span,
 574                     format!("cannot use `{}` because it was mutably borrowed",
 575                             self.bccx.loan_path_to_string(copy_path).as_slice())
 576                     .as_slice());
 577                 self.bccx.span_note(
 578                     loan_span,
 579                     format!("borrow of `{}` occurs here",
 580                             self.bccx.loan_path_to_string(&*loan_path).as_slice())
 581                     .as_slice());
 582             }
 583         }
 584     }
 585
 586     fn check_for_move_of_borrowed_path(&self,
 587                                        id: ast::NodeId,
 588                                        span: Span,
 589                                        move_path: &LoanPath,
 590                                        move_kind: move_data::MoveKind) {
 591         // We want to detect if there are any loans at all, so we search for
 592         // any loans incompatible with MutBorrrow, since all other kinds of
 593         // loans are incompatible with that.
 594         match self.analyze_restrictions_on_use(id, move_path, ty::MutBorrow) {
 595             UseOk => { }
 596             UseWhileBorrowed(loan_path, loan_span) => {
 597                 let err_message = match move_kind {
 598                     move_data::Captured =>
 599                         format!("cannot move `{}` into closure because it is borrowed",
 600                                 self.bccx.loan_path_to_string(move_path).as_slice()),
 601                     move_data::Declared |
 602                     move_data::MoveExpr |
 603                     move_data::MovePat =>
 604                         format!("cannot move out of `{}` because it is borrowed",
 605                                 self.bccx.loan_path_to_string(move_path).as_slice())
 606                 };
 607
 608                 self.bccx.span_err(span, err_message.as_slice());
 609                 self.bccx.span_note(
 610                     loan_span,
 611                     format!("borrow of `{}` occurs here",
 612                             self.bccx.loan_path_to_string(&*loan_path).as_slice())
 613                     .as_slice());
 614             }
 615         }
 616     }
 617
 618     pub fn analyze_restrictions_on_use(&self,
 619                                        expr_id: ast::NodeId,
 620                                        use_path: &LoanPath,
 621                                        borrow_kind: ty::BorrowKind)
 622                                        -> UseError {
 623         debug!("analyze_restrictions_on_use(expr_id={:?}, use_path={})",
 624                self.tcx().map.node_to_string(expr_id),
 625                use_path.repr(self.tcx()));
 626
 627         let mut ret = UseOk;
 628
 629         self.each_in_scope_loan_affecting_path(expr_id, use_path, |loan| {
 630             if !compatible_borrow_kinds(loan.kind, borrow_kind) {
 631                 ret = UseWhileBorrowed(loan.loan_path.clone(), loan.span);
 632                 false
 633             } else {
 634                 true
 635             }
 636         });
 637
 638         return ret;
 639     }
 640
 641     fn check_if_path_is_moved(&self,
 642                               id: ast::NodeId,
 643                               span: Span,
 644                               use_kind: MovedValueUseKind,
 645                               lp: &Rc<LoanPath>) {
 646         /*!
 647          * Reports an error if `expr` (which should be a path)
 648          * is using a moved/uninitialized value
 649          */
 650
 651         debug!("check_if_path_is_moved(id={:?}, use_kind={:?}, lp={})",
 652                id, use_kind, lp.repr(self.bccx.tcx));
 653         let base_lp = owned_ptr_base_path_rc(lp);
 654         self.move_data.each_move_of(id, &base_lp, |move, moved_lp| {
 655             self.bccx.report_use_of_moved_value(
 656                 span,
 657                 use_kind,
 658                 &**lp,
 659                 move,
 660                 moved_lp);
 661             false
 662         });
 663     }
 664
 665     fn check_if_assigned_path_is_moved(&self,
 666                                        id: ast::NodeId,
 667                                        span: Span,
 668                                        use_kind: MovedValueUseKind,
 669                                        lp: &Rc<LoanPath>)
 670     {
 671         /*!
 672          * Reports an error if assigning to `lp` will use a
 673          * moved/uninitialized value. Mainly this is concerned with
 674          * detecting derefs of uninitialized pointers.
 675          *
 676          * For example:
 677          *
 678          *     let a: int;
 679          *     a = 10; // ok, even though a is uninitialized
 680          *
 681          *     struct Point { x: uint, y: uint }
 682          *     let p: Point;
 683          *     p.x = 22; // ok, even though `p` is uninitialized
 684          *
 685          *     let p: ~Point;
 686          *     (*p).x = 22; // not ok, p is uninitialized, can't deref
 687          */
 688
 689         match **lp {
 690             LpVar(_) | LpUpvar(_) => {
 691                 // assigning to `x` does not require that `x` is initialized
 692             }
 693             LpExtend(ref lp_base, _, LpInterior(_)) => {
 694                 // assigning to `P.f` is ok if assigning to `P` is ok
 695                 self.check_if_assigned_path_is_moved(id, span,
 696                                                      use_kind, lp_base);
 697             }
 698             LpExtend(ref lp_base, _, LpDeref(_)) => {
 699                 // assigning to `(*P)` requires that `P` be initialized
 700                 self.check_if_path_is_moved(id, span,
 701                                             use_kind, lp_base);
 702             }
 703         }
 704     }
 705
 706     fn check_assignment(&self,
 707                         assignment_id: ast::NodeId,
 708                         assignment_span: Span,
 709                         assignee_cmt: mc::cmt,
 710                         mode: euv::MutateMode) {
 711         debug!("check_assignment(assignee_cmt={})", assignee_cmt.repr(self.tcx()));
 712
 713         // Mutable values can be assigned, as long as they obey loans
 714         // and aliasing restrictions:
 715         if assignee_cmt.mutbl.is_mutable() {
 716             if check_for_aliasable_mutable_writes(self, assignment_span, assignee_cmt.clone()) {
 717                 if mode != euv::Init {
 718                     check_for_assignment_to_borrowed_path(
 719                         self, assignment_id, assignment_span, assignee_cmt.clone());
 720                     mark_variable_as_used_mut(self, assignee_cmt);
 721                 }
 722             }
 723             return;
 724         }
 725
 726         // Initializations are OK.
 727         if mode == euv::Init {
 728             return
 729         }
 730
 731         // For immutable local variables, assignments are legal
 732         // if they cannot already have been assigned
 733         if self.is_local_variable_or_arg(assignee_cmt.clone()) {
 734             assert!(assignee_cmt.mutbl.is_immutable()); // no "const" locals
 735             let lp = opt_loan_path(&assignee_cmt).unwrap();
 736             self.move_data.each_assignment_of(assignment_id, &lp, |assign| {
 737                 self.bccx.report_reassigned_immutable_variable(
 738                     assignment_span,
 739                     &*lp,
 740                     assign);
 741                 false
 742             });
 743             return;
 744         }
 745
 746         // Otherwise, just a plain error.
 747         match opt_loan_path(&assignee_cmt) {
 748             Some(lp) => {
 749                 self.bccx.span_err(
 750                     assignment_span,
 751                     format!("cannot assign to {} {} `{}`",
 752                             assignee_cmt.mutbl.to_user_str(),
 753                             self.bccx.cmt_to_string(&*assignee_cmt),
 754                             self.bccx.loan_path_to_string(&*lp)).as_slice());
 755             }
 756             None => {
 757                 self.bccx.span_err(
 758                     assignment_span,
 759                     format!("cannot assign to {} {}",
 760                             assignee_cmt.mutbl.to_user_str(),
 761                             self.bccx.cmt_to_string(&*assignee_cmt)).as_slice());
 762             }
 763         }
 764         return;
 765
 766         fn mark_variable_as_used_mut(this: &CheckLoanCtxt,
 767                                      mut cmt: mc::cmt) {
 768             //! If the mutability of the `cmt` being written is inherited
 769             //! from a local variable, liveness will
 770             //! not have been able to detect that this variable's mutability
 771             //! is important, so we must add the variable to the
 772             //! `used_mut_nodes` table here.
 773
 774             loop {
 775                 debug!("mark_variable_as_used_mut(cmt={})", cmt.repr(this.tcx()));
 776                 match cmt.cat.clone() {
 777                     mc::cat_copied_upvar(mc::CopiedUpvar { upvar_id: id, .. }) |
 778                     mc::cat_local(id) | mc::cat_arg(id) => {
 779                         this.tcx().used_mut_nodes.borrow_mut().insert(id);
 780                         return;
 781                     }
 782
 783                     mc::cat_upvar(..) => {
 784                         return;
 785                     }
 786
 787                     mc::cat_deref(_, _, mc::GcPtr) => {
 788                         assert_eq!(cmt.mutbl, mc::McImmutable);
 789                         return;
 790                     }
 791
 792                     mc::cat_rvalue(..) |
 793                     mc::cat_static_item |
 794                     mc::cat_deref(_, _, mc::UnsafePtr(..)) |
 795                     mc::cat_deref(_, _, mc::BorrowedPtr(..)) |
 796                     mc::cat_deref(_, _, mc::Implicit(..)) => {
 797                         assert_eq!(cmt.mutbl, mc::McDeclared);
 798                         return;
 799                     }
 800
 801                     mc::cat_discr(b, _) |
 802                     mc::cat_deref(b, _, mc::OwnedPtr) => {
 803                         assert_eq!(cmt.mutbl, mc::McInherited);
 804                         cmt = b;
 805                     }
 806
 807                     mc::cat_downcast(b) |
 808                     mc::cat_interior(b, _) => {
 809                         assert_eq!(cmt.mutbl, mc::McInherited);
 810                         cmt = b;
 811                     }
 812                 }
 813             }
 814         }
 815
 816         fn check_for_aliasable_mutable_writes(this: &CheckLoanCtxt,
 817                                               span: Span,
 818                                               cmt: mc::cmt) -> bool {
 819             //! Safety checks related to writes to aliasable, mutable locations
 820
 821             let guarantor = cmt.guarantor();
 822             debug!("check_for_aliasable_mutable_writes(cmt={}, guarantor={})",
 823                    cmt.repr(this.tcx()), guarantor.repr(this.tcx()));
 824             match guarantor.cat {
 825                 mc::cat_deref(ref b, _, mc::BorrowedPtr(ty::MutBorrow, _)) => {
 826                     // Statically prohibit writes to `&mut` when aliasable
 827
 828                     check_for_aliasability_violation(this, span, b.clone());
 829                 }
 830
 831                 _ => {}
 832             }
 833
 834             return true; // no errors reported
 835         }
 836
 837         fn check_for_aliasability_violation(this: &CheckLoanCtxt,
 838                                             span: Span,
 839                                             cmt: mc::cmt)
 840                                             -> bool {
 841             match cmt.freely_aliasable(this.tcx()) {
 842                 None => {
 843                     return true;
 844                 }
 845                 Some(mc::AliasableStaticMut(..)) => {
 846                     return true;
 847                 }
 848                 Some(cause) => {
 849                     this.bccx.report_aliasability_violation(
 850                         span,
 851                         MutabilityViolation,
 852                         cause);
 853                     return false;
 854                 }
 855             }
 856         }
 857
 858         fn check_for_assignment_to_borrowed_path(
 859             this: &CheckLoanCtxt,
 860             assignment_id: ast::NodeId,
 861             assignment_span: Span,
 862             assignee_cmt: mc::cmt)
 863         {
 864             //! Check for assignments that violate the terms of an
 865             //! outstanding loan.
 866
 867             let loan_path = match opt_loan_path(&assignee_cmt) {
 868                 Some(lp) => lp,
 869                 None => { return; /* no loan path, can't be any loans */ }
 870             };
 871
 872             this.each_in_scope_loan_affecting_path(assignment_id, &*loan_path, |loan| {
 873                 this.report_illegal_mutation(assignment_span, &*loan_path, loan);
 874                 false
 875             });
 876         }
 877     }
 878
 879     pub fn report_illegal_mutation(&self,
 880                                    span: Span,
 881                                    loan_path: &LoanPath,
 882                                    loan: &Loan) {
 883         self.bccx.span_err(
 884             span,
 885             format!("cannot assign to `{}` because it is borrowed",
 886                     self.bccx.loan_path_to_string(loan_path)).as_slice());
 887         self.bccx.span_note(
 888             loan.span,
 889             format!("borrow of `{}` occurs here",
 890                     self.bccx.loan_path_to_string(loan_path)).as_slice());
 891     }
 892 }
 893