src/librustc_borrowck/borrowck/gather_loans/mod.rs

   1 // Copyright 2012-2014 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 // Gathering loans
  13 //
  14 // The borrow check proceeds in two phases. In phase one, we gather the full
  15 // set of loans that are required at any point.  These are sorted according to
  16 // their associated scopes.  In phase two, checking loans, we will then make
  17 // sure that all of these loans are honored.
  18
  19 use borrowck::*;
  20 use borrowck::move_data::MoveData;
  21 use rustc::middle::expr_use_visitor as euv;
  22 use rustc::middle::mem_categorization as mc;
  23 use rustc::middle::mem_categorization::Categorization;
  24 use rustc::middle::region;
  25 use rustc::ty::{self, TyCtxt};
  26
  27 use syntax::ast;
  28 use syntax_pos::Span;
  29 use rustc::hir;
  30
  31 use self::restrictions::RestrictionResult;
  32
  33 mod lifetime;
  34 mod restrictions;
  35 mod gather_moves;
  36 mod move_error;
  37
  38 pub fn gather_loans_in_fn<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
  39                                     body: hir::BodyId)
  40                                     -> (Vec<Loan<'tcx>>, move_data::MoveData<'tcx>) {
  41     let def_id = bccx.tcx.hir().body_owner_def_id(body);
  42     let param_env = bccx.tcx.param_env(def_id);
  43     let mut glcx = GatherLoanCtxt {
  44         bccx,
  45         all_loans: Vec::new(),
  46         item_ub: region::Scope {
  47             id: bccx.tcx.hir().body(body).value.hir_id.local_id,
  48             data: region::ScopeData::Node
  49         },
  50         move_data: MoveData::default(),
  51         move_error_collector: move_error::MoveErrorCollector::new(),
  52     };
  53
  54     let rvalue_promotable_map = bccx.tcx.rvalue_promotable_map(def_id);
  55     euv::ExprUseVisitor::new(&mut glcx,
  56                              bccx.tcx,
  57                              param_env,
  58                              &bccx.region_scope_tree,
  59                              bccx.tables,
  60                              Some(rvalue_promotable_map))
  61         .consume_body(bccx.body);
  62
  63     glcx.report_potential_errors();
  64     let GatherLoanCtxt { all_loans, move_data, .. } = glcx;
  65     (all_loans, move_data)
  66 }
  67
  68 struct GatherLoanCtxt<'a, 'tcx: 'a> {
  69     bccx: &'a BorrowckCtxt<'a, 'tcx>,
  70     move_data: move_data::MoveData<'tcx>,
  71     move_error_collector: move_error::MoveErrorCollector<'tcx>,
  72     all_loans: Vec<Loan<'tcx>>,
  73     /// `item_ub` is used as an upper-bound on the lifetime whenever we
  74     /// ask for the scope of an expression categorized as an upvar.
  75     item_ub: region::Scope,
  76 }
  77
  78 impl<'a, 'tcx> euv::Delegate<'tcx> for GatherLoanCtxt<'a, 'tcx> {
  79     fn consume(&mut self,
  80                consume_id: ast::NodeId,
  81                _consume_span: Span,
  82                cmt: &mc::cmt_<'tcx>,
  83                mode: euv::ConsumeMode) {
  84         debug!("consume(consume_id={}, cmt={:?}, mode={:?})",
  85                consume_id, cmt, mode);
  86
  87         match mode {
  88             euv::Move(move_reason) => {
  89                 gather_moves::gather_move_from_expr(
  90                     self.bccx, &self.move_data, &mut self.move_error_collector,
  91                     self.bccx.tcx.hir().node_to_hir_id(consume_id).local_id, cmt, move_reason);
  92             }
  93             euv::Copy => { }
  94         }
  95     }
  96
  97     fn matched_pat(&mut self,
  98                    matched_pat: &hir::Pat,
  99                    cmt: &mc::cmt_<'tcx>,
 100                    mode: euv::MatchMode) {
 101         debug!("matched_pat(matched_pat={:?}, cmt={:?}, mode={:?})",
 102                matched_pat,
 103                cmt,
 104                mode);
 105     }
 106
 107     fn consume_pat(&mut self,
 108                    consume_pat: &hir::Pat,
 109                    cmt: &mc::cmt_<'tcx>,
 110                    mode: euv::ConsumeMode) {
 111         debug!("consume_pat(consume_pat={:?}, cmt={:?}, mode={:?})",
 112                consume_pat,
 113                cmt,
 114                mode);
 115
 116         match mode {
 117             euv::Copy => { return; }
 118             euv::Move(_) => { }
 119         }
 120
 121         gather_moves::gather_move_from_pat(
 122             self.bccx, &self.move_data, &mut self.move_error_collector,
 123             consume_pat, cmt);
 124     }
 125
 126     fn borrow(&mut self,
 127               borrow_id: ast::NodeId,
 128               borrow_span: Span,
 129               cmt: &mc::cmt_<'tcx>,
 130               loan_region: ty::Region<'tcx>,
 131               bk: ty::BorrowKind,
 132               loan_cause: euv::LoanCause)
 133     {
 134         debug!("borrow(borrow_id={}, cmt={:?}, loan_region={:?}, \
 135                bk={:?}, loan_cause={:?})",
 136                borrow_id, cmt, loan_region,
 137                bk, loan_cause);
 138         let hir_id = self.bccx.tcx.hir().node_to_hir_id(borrow_id);
 139         self.guarantee_valid(hir_id.local_id,
 140                              borrow_span,
 141                              cmt,
 142                              bk,
 143                              loan_region,
 144                              loan_cause);
 145     }
 146
 147     fn mutate(&mut self,
 148               assignment_id: ast::NodeId,
 149               assignment_span: Span,
 150               assignee_cmt: &mc::cmt_<'tcx>,
 151               _: euv::MutateMode)
 152     {
 153         self.guarantee_assignment_valid(assignment_id,
 154                                         assignment_span,
 155                                         assignee_cmt);
 156     }
 157
 158     fn decl_without_init(&mut self, id: ast::NodeId, _span: Span) {
 159         let ty = self.bccx
 160                      .tables
 161                      .node_id_to_type(self.bccx.tcx.hir().node_to_hir_id(id));
 162         gather_moves::gather_decl(self.bccx, &self.move_data, id, ty);
 163     }
 164 }
 165
 166 /// Implements the A-* rules in README.md.
 167 fn check_aliasability<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
 168                                 borrow_span: Span,
 169                                 loan_cause: AliasableViolationKind,
 170                                 cmt: &mc::cmt_<'tcx>,
 171                                 req_kind: ty::BorrowKind)
 172                                 -> Result<(),()> {
 173
 174     let aliasability = cmt.freely_aliasable();
 175     debug!("check_aliasability aliasability={:?} req_kind={:?}",
 176            aliasability, req_kind);
 177
 178     match (aliasability, req_kind) {
 179         (mc::Aliasability::NonAliasable, _) => {
 180             /* Uniquely accessible path -- OK for `&` and `&mut` */
 181             Ok(())
 182         }
 183         (mc::Aliasability::FreelyAliasable(mc::AliasableStatic), ty::ImmBorrow) => {
 184             // Borrow of an immutable static item.
 185             Ok(())
 186         }
 187         (mc::Aliasability::FreelyAliasable(mc::AliasableStaticMut), _) => {
 188             // Even touching a static mut is considered unsafe. We assume the
 189             // user knows what they're doing in these cases.
 190             Ok(())
 191         }
 192         (mc::Aliasability::FreelyAliasable(alias_cause), ty::UniqueImmBorrow) |
 193         (mc::Aliasability::FreelyAliasable(alias_cause), ty::MutBorrow) => {
 194             bccx.report_aliasability_violation(
 195                         borrow_span,
 196                         loan_cause,
 197                         alias_cause,
 198                         cmt);
 199             Err(())
 200         }
 201         (..) => {
 202             Ok(())
 203         }
 204     }
 205 }
 206
 207 /// Implements the M-* rules in README.md.
 208 fn check_mutability<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
 209                               borrow_span: Span,
 210                               cause: AliasableViolationKind,
 211                               cmt: &mc::cmt_<'tcx>,
 212                               req_kind: ty::BorrowKind)
 213                               -> Result<(),()> {
 214     debug!("check_mutability(cause={:?} cmt={:?} req_kind={:?}",
 215            cause, cmt, req_kind);
 216     match req_kind {
 217         ty::UniqueImmBorrow | ty::ImmBorrow => {
 218             match cmt.mutbl {
 219                 // I am intentionally leaving this here to help
 220                 // refactoring if, in the future, we should add new
 221                 // kinds of mutability.
 222                 mc::McImmutable | mc::McDeclared | mc::McInherited => {
 223                     // both imm and mut data can be lent as imm;
 224                     // for mutable data, this is a freeze
 225                     Ok(())
 226                 }
 227             }
 228         }
 229
 230         ty::MutBorrow => {
 231             // Only mutable data can be lent as mutable.
 232             if !cmt.mutbl.is_mutable() {
 233                 Err(bccx.report(BckError { span: borrow_span,
 234                                            cause,
 235                                            cmt,
 236                                            code: err_mutbl }))
 237             } else {
 238                 Ok(())
 239             }
 240         }
 241     }
 242 }
 243
 244 impl<'a, 'tcx> GatherLoanCtxt<'a, 'tcx> {
 245     pub fn tcx(&self) -> TyCtxt<'a, 'tcx, 'tcx> { self.bccx.tcx }
 246
 247     /// Guarantees that `cmt` is assignable, or reports an error.
 248     fn guarantee_assignment_valid(&mut self,
 249                                   assignment_id: ast::NodeId,
 250                                   assignment_span: Span,
 251                                   cmt: &mc::cmt_<'tcx>) {
 252
 253         let opt_lp = opt_loan_path(cmt);
 254         debug!("guarantee_assignment_valid(assignment_id={}, cmt={:?}) opt_lp={:?}",
 255                assignment_id, cmt, opt_lp);
 256
 257         if let Categorization::Local(..) = cmt.cat {
 258             // Only re-assignments to locals require it to be
 259             // mutable - this is checked in check_loans.
 260         } else {
 261             // Check that we don't allow assignments to non-mutable data.
 262             if check_mutability(self.bccx, assignment_span, MutabilityViolation,
 263                                 cmt, ty::MutBorrow).is_err() {
 264                 return; // reported an error, no sense in reporting more.
 265             }
 266         }
 267
 268         // Check that we don't allow assignments to aliasable data
 269         if check_aliasability(self.bccx, assignment_span, MutabilityViolation,
 270                               cmt, ty::MutBorrow).is_err() {
 271             return; // reported an error, no sense in reporting more.
 272         }
 273
 274         match opt_lp {
 275             Some(lp) => {
 276                 if let Categorization::Local(..) = cmt.cat {
 277                     // Only re-assignments to locals require it to be
 278                     // mutable - this is checked in check_loans.
 279                 } else {
 280                     self.mark_loan_path_as_mutated(&lp);
 281                 }
 282                 gather_moves::gather_assignment(self.bccx, &self.move_data,
 283                                                 self.bccx.tcx.hir().node_to_hir_id(assignment_id)
 284                                                     .local_id,
 285                                                 assignment_span,
 286                                                 lp);
 287             }
 288             None => {
 289                 // This can occur with e.g. `*foo() = 5`.  In such
 290                 // cases, there is no need to check for conflicts
 291                 // with moves etc, just ignore.
 292             }
 293         }
 294     }
 295
 296     /// Guarantees that `addr_of(cmt)` will be valid for the duration of `static_scope_r`, or
 297     /// reports an error.  This may entail taking out loans, which will be added to the
 298     /// `req_loan_map`.
 299     fn guarantee_valid(&mut self,
 300                        borrow_id: hir::ItemLocalId,
 301                        borrow_span: Span,
 302                        cmt: &mc::cmt_<'tcx>,
 303                        req_kind: ty::BorrowKind,
 304                        loan_region: ty::Region<'tcx>,
 305                        cause: euv::LoanCause) {
 306         debug!("guarantee_valid(borrow_id={:?}, cmt={:?}, \
 307                 req_mutbl={:?}, loan_region={:?})",
 308                borrow_id,
 309                cmt,
 310                req_kind,
 311                loan_region);
 312
 313         // a loan for the empty region can never be dereferenced, so
 314         // it is always safe
 315         if *loan_region == ty::ReEmpty {
 316             return;
 317         }
 318
 319         // Check that the lifetime of the borrow does not exceed
 320         // the lifetime of the data being borrowed.
 321         if lifetime::guarantee_lifetime(self.bccx, self.item_ub,
 322                                         borrow_span, cause, cmt, loan_region).is_err() {
 323             return; // reported an error, no sense in reporting more.
 324         }
 325
 326         // Check that we don't allow mutable borrows of non-mutable data.
 327         if check_mutability(self.bccx, borrow_span, BorrowViolation(cause),
 328                             cmt, req_kind).is_err() {
 329             return; // reported an error, no sense in reporting more.
 330         }
 331
 332         // Check that we don't allow mutable borrows of aliasable data.
 333         if check_aliasability(self.bccx, borrow_span, BorrowViolation(cause),
 334                               cmt, req_kind).is_err() {
 335             return; // reported an error, no sense in reporting more.
 336         }
 337
 338         // Compute the restrictions that are required to enforce the
 339         // loan is safe.
 340         let restr = restrictions::compute_restrictions(
 341             self.bccx, borrow_span, cause, &cmt, loan_region);
 342
 343         debug!("guarantee_valid(): restrictions={:?}", restr);
 344
 345         // Create the loan record (if needed).
 346         let loan = match restr {
 347             RestrictionResult::Safe => {
 348                 // No restrictions---no loan record necessary
 349                 return;
 350             }
 351
 352             RestrictionResult::SafeIf(loan_path, restricted_paths) => {
 353                 let loan_scope = match *loan_region {
 354                     ty::ReScope(scope) => scope,
 355
 356                     ty::ReEarlyBound(ref br) => {
 357                         self.bccx.region_scope_tree.early_free_scope(self.tcx(), br)
 358                     }
 359
 360                     ty::ReFree(ref fr) => {
 361                         self.bccx.region_scope_tree.free_scope(self.tcx(), fr)
 362                     }
 363
 364                     ty::ReStatic => self.item_ub,
 365
 366                     ty::ReEmpty |
 367                     ty::ReClosureBound(..) |
 368                     ty::ReLateBound(..) |
 369                     ty::ReVar(..) |
 370                     ty::RePlaceholder(..) |
 371                     ty::ReErased => {
 372                         span_bug!(
 373                             cmt.span,
 374                             "invalid borrow lifetime: {:?}",
 375                             loan_region);
 376                     }
 377                 };
 378                 debug!("loan_scope = {:?}", loan_scope);
 379
 380                 let borrow_scope = region::Scope {
 381                     id: borrow_id,
 382                     data: region::ScopeData::Node
 383                 };
 384                 let gen_scope = self.compute_gen_scope(borrow_scope, loan_scope);
 385                 debug!("gen_scope = {:?}", gen_scope);
 386
 387                 let kill_scope = self.compute_kill_scope(loan_scope, &loan_path);
 388                 debug!("kill_scope = {:?}", kill_scope);
 389
 390                 if req_kind == ty::MutBorrow {
 391                     self.mark_loan_path_as_mutated(&loan_path);
 392                 }
 393
 394                 Loan {
 395                     index: self.all_loans.len(),
 396                     loan_path,
 397                     kind: req_kind,
 398                     gen_scope,
 399                     kill_scope,
 400                     span: borrow_span,
 401                     restricted_paths,
 402                     cause,
 403                 }
 404             }
 405         };
 406
 407         debug!("guarantee_valid(borrow_id={:?}), loan={:?}",
 408                borrow_id, loan);
 409
 410         // let loan_path = loan.loan_path;
 411         // let loan_gen_scope = loan.gen_scope;
 412         // let loan_kill_scope = loan.kill_scope;
 413         self.all_loans.push(loan);
 414
 415         // if loan_gen_scope != borrow_id {
 416             // FIXME(https://github.com/rust-lang/rfcs/issues/811) Nested method calls
 417             //
 418             // Typically, the scope of the loan includes the point at
 419             // which the loan is originated. This
 420             // This is a subtle case. See the test case
 421             // <compile-fail/borrowck-bad-nested-calls-free.rs>
 422             // to see what we are guarding against.
 423
 424             //let restr = restrictions::compute_restrictions(
 425             //    self.bccx, borrow_span, cmt, RESTR_EMPTY);
 426             //let loan = {
 427             //    Loan {
 428             //        index: self.all_loans.len(),
 429             //        loan_path,
 430             //        cmt,
 431             //        mutbl: ConstMutability,
 432             //        gen_scope: borrow_id,
 433             //        kill_scope,
 434             //        span: borrow_span,
 435             //        restrictions,
 436             //    }
 437         // }
 438     }
 439
 440     pub fn mark_loan_path_as_mutated(&self, loan_path: &LoanPath) {
 441         //! For mutable loans of content whose mutability derives
 442         //! from a local variable, mark the mutability decl as necessary.
 443
 444         let mut wrapped_path = Some(loan_path);
 445         let mut through_borrow = false;
 446
 447         while let Some(current_path) = wrapped_path {
 448             wrapped_path = match current_path.kind {
 449                 LpVar(local_id) => {
 450                     if !through_borrow {
 451                         let hir_id = self.bccx.tcx.hir().node_to_hir_id(local_id);
 452                         self.bccx.used_mut_nodes.borrow_mut().insert(hir_id);
 453                     }
 454                     None
 455                 }
 456                 LpUpvar(ty::UpvarId{ var_path: ty::UpvarPath { hir_id }, closure_expr_id: _ }) => {
 457                     self.bccx.used_mut_nodes.borrow_mut().insert(hir_id);
 458                     None
 459                 }
 460                 LpExtend(ref base, mc::McInherited, LpDeref(pointer_kind)) |
 461                 LpExtend(ref base, mc::McDeclared, LpDeref(pointer_kind)) => {
 462                     if pointer_kind != mc::Unique {
 463                         through_borrow = true;
 464                     }
 465                     Some(base)
 466                 }
 467                 LpDowncast(ref base, _) |
 468                 LpExtend(ref base, mc::McInherited, _) |
 469                 LpExtend(ref base, mc::McDeclared, _) => {
 470                     Some(base)
 471                 }
 472                 LpExtend(_, mc::McImmutable, _) => {
 473                     // Nothing to do.
 474                     None
 475                 }
 476             }
 477         }
 478
 479     }
 480
 481     pub fn compute_gen_scope(&self,
 482                              borrow_scope: region::Scope,
 483                              loan_scope: region::Scope)
 484                              -> region::Scope {
 485         //! Determine when to introduce the loan. Typically the loan
 486         //! is introduced at the point of the borrow, but in some cases,
 487         //! notably method arguments, the loan may be introduced only
 488         //! later, once it comes into scope.
 489
 490         if self.bccx.region_scope_tree.is_subscope_of(borrow_scope, loan_scope) {
 491             borrow_scope
 492         } else {
 493             loan_scope
 494         }
 495     }
 496
 497     pub fn compute_kill_scope(&self, loan_scope: region::Scope, lp: &LoanPath<'tcx>)
 498                               -> region::Scope {
 499         //! Determine when the loan restrictions go out of scope.
 500         //! This is either when the lifetime expires or when the
 501         //! local variable which roots the loan-path goes out of scope,
 502         //! whichever happens faster.
 503         //!
 504         //! It may seem surprising that we might have a loan region
 505         //! larger than the variable which roots the loan-path; this can
 506         //! come about when variables of `&mut` type are re-borrowed,
 507         //! as in this example:
 508         //!
 509         //!     struct Foo { counter: u32 }
 510         //!
 511         //!     fn counter<'a>(v: &'a mut Foo) -> &'a mut u32 {
 512         //!         &mut v.counter
 513         //!     }
 514         //!
 515         //! In this case, the reference (`'a`) outlives the
 516         //! variable `v` that hosts it. Note that this doesn't come up
 517         //! with immutable `&` pointers, because borrows of such pointers
 518         //! do not require restrictions and hence do not cause a loan.
 519
 520         let lexical_scope = lp.kill_scope(self.bccx);
 521         if self.bccx.region_scope_tree.is_subscope_of(lexical_scope, loan_scope) {
 522             lexical_scope
 523         } else {
 524             assert!(self.bccx.region_scope_tree.is_subscope_of(loan_scope, lexical_scope));
 525             loan_scope
 526         }
 527     }
 528
 529     pub fn report_potential_errors(&self) {
 530         self.move_error_collector.report_potential_errors(self.bccx);
 531     }
 532 }