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::infer::InferCtxt;
  22 use rustc::middle::expr_use_visitor as euv;
  23 use rustc::middle::mem_categorization as mc;
  24 use rustc::middle::mem_categorization::Categorization;
  25 use rustc::middle::region;
  26 use rustc::ty::{self, TyCtxt};
  27
  28 use syntax::ast;
  29 use syntax_pos::Span;
  30 use rustc::hir;
  31
  32 use self::restrictions::RestrictionResult;
  33
  34 mod lifetime;
  35 mod restrictions;
  36 mod gather_moves;
  37 mod move_error;
  38
  39 pub fn gather_loans_in_fn<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
  40                                     body: hir::BodyId)
  41                                     -> (Vec<Loan<'tcx>>,
  42                                         move_data::MoveData<'tcx>) {
  43     let infcx = bccx.tcx.borrowck_fake_infer_ctxt(body);
  44     let mut glcx = GatherLoanCtxt {
  45         bccx: bccx,
  46         infcx: &infcx,
  47         all_loans: Vec::new(),
  48         item_ub: bccx.tcx.node_extent(body.node_id),
  49         move_data: MoveData::new(),
  50         move_error_collector: move_error::MoveErrorCollector::new(),
  51     };
  52
  53     let body = glcx.bccx.tcx.hir.body(body);
  54     euv::ExprUseVisitor::new(&mut glcx, &bccx.region_maps, &infcx).consume_body(body);
  55
  56     glcx.report_potential_errors();
  57     let GatherLoanCtxt { all_loans, move_data, .. } = glcx;
  58     (all_loans, move_data)
  59 }
  60
  61 struct GatherLoanCtxt<'a, 'tcx: 'a> {
  62     bccx: &'a BorrowckCtxt<'a, 'tcx>,
  63     infcx: &'a InferCtxt<'a, 'tcx, 'tcx>,
  64     move_data: move_data::MoveData<'tcx>,
  65     move_error_collector: move_error::MoveErrorCollector<'tcx>,
  66     all_loans: Vec<Loan<'tcx>>,
  67     /// `item_ub` is used as an upper-bound on the lifetime whenever we
  68     /// ask for the scope of an expression categorized as an upvar.
  69     item_ub: region::CodeExtent<'tcx>,
  70 }
  71
  72 impl<'a, 'tcx> euv::Delegate<'tcx> for GatherLoanCtxt<'a, 'tcx> {
  73     fn consume(&mut self,
  74                consume_id: ast::NodeId,
  75                _consume_span: Span,
  76                cmt: mc::cmt<'tcx>,
  77                mode: euv::ConsumeMode) {
  78         debug!("consume(consume_id={}, cmt={:?}, mode={:?})",
  79                consume_id, cmt, mode);
  80
  81         match mode {
  82             euv::Move(move_reason) => {
  83                 gather_moves::gather_move_from_expr(
  84                     self.bccx, &self.move_data, &mut self.move_error_collector,
  85                     consume_id, cmt, move_reason);
  86             }
  87             euv::Copy => { }
  88         }
  89     }
  90
  91     fn matched_pat(&mut self,
  92                    matched_pat: &hir::Pat,
  93                    cmt: mc::cmt<'tcx>,
  94                    mode: euv::MatchMode) {
  95         debug!("matched_pat(matched_pat={:?}, cmt={:?}, mode={:?})",
  96                matched_pat,
  97                cmt,
  98                mode);
  99
 100         if let Categorization::Downcast(..) = cmt.cat {
 101             gather_moves::gather_match_variant(
 102                 self.bccx, &self.move_data, &mut self.move_error_collector,
 103                 matched_pat, cmt, mode);
 104         }
 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
 139         self.guarantee_valid(borrow_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               mode: euv::MutateMode)
 152     {
 153         self.guarantee_assignment_valid(assignment_id,
 154                                         assignment_span,
 155                                         assignee_cmt,
 156                                         mode);
 157     }
 158
 159     fn decl_without_init(&mut self, id: ast::NodeId, _span: Span) {
 160         let ty = self.infcx.tables.borrow().node_id_to_type(id);
 161         gather_moves::gather_decl(self.bccx, &self.move_data, id, ty);
 162     }
 163 }
 164
 165 /// Implements the A-* rules in README.md.
 166 fn check_aliasability<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
 167                                 borrow_span: Span,
 168                                 loan_cause: AliasableViolationKind,
 169                                 cmt: mc::cmt<'tcx>,
 170                                 req_kind: ty::BorrowKind)
 171                                 -> Result<(),()> {
 172
 173     let aliasability = cmt.freely_aliasable();
 174     debug!("check_aliasability aliasability={:?} req_kind={:?}",
 175            aliasability, req_kind);
 176
 177     match (aliasability, req_kind) {
 178         (mc::Aliasability::NonAliasable, _) => {
 179             /* Uniquely accessible path -- OK for `&` and `&mut` */
 180             Ok(())
 181         }
 182         (mc::Aliasability::FreelyAliasable(mc::AliasableStatic), ty::ImmBorrow) => {
 183             // Borrow of an immutable static item.
 184             Ok(())
 185         }
 186         (mc::Aliasability::FreelyAliasable(mc::AliasableStaticMut), _) => {
 187             // Even touching a static mut is considered unsafe. We assume the
 188             // user knows what they're doing in these cases.
 189             Ok(())
 190         }
 191         (mc::Aliasability::FreelyAliasable(alias_cause), ty::UniqueImmBorrow) |
 192         (mc::Aliasability::FreelyAliasable(alias_cause), ty::MutBorrow) => {
 193             bccx.report_aliasability_violation(
 194                         borrow_span,
 195                         loan_cause,
 196                         alias_cause,
 197                         cmt);
 198             Err(())
 199         }
 200         (..) => {
 201             Ok(())
 202         }
 203     }
 204 }
 205
 206 /// Implements the M-* rules in README.md.
 207 fn check_mutability<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
 208                               borrow_span: Span,
 209                               cause: AliasableViolationKind,
 210                               cmt: mc::cmt<'tcx>,
 211                               req_kind: ty::BorrowKind)
 212                               -> Result<(),()> {
 213     debug!("check_mutability(cause={:?} cmt={:?} req_kind={:?}",
 214            cause, cmt, req_kind);
 215     match req_kind {
 216         ty::UniqueImmBorrow | ty::ImmBorrow => {
 217             match cmt.mutbl {
 218                 // I am intentionally leaving this here to help
 219                 // refactoring if, in the future, we should add new
 220                 // kinds of mutability.
 221                 mc::McImmutable | mc::McDeclared | mc::McInherited => {
 222                     // both imm and mut data can be lent as imm;
 223                     // for mutable data, this is a freeze
 224                     Ok(())
 225                 }
 226             }
 227         }
 228
 229         ty::MutBorrow => {
 230             // Only mutable data can be lent as mutable.
 231             if !cmt.mutbl.is_mutable() {
 232                 Err(bccx.report(BckError { span: borrow_span,
 233                                            cause: cause,
 234                                            cmt: cmt,
 235                                            code: err_mutbl }))
 236             } else {
 237                 Ok(())
 238             }
 239         }
 240     }
 241 }
 242
 243 impl<'a, 'tcx> GatherLoanCtxt<'a, 'tcx> {
 244     pub fn tcx(&self) -> TyCtxt<'a, 'tcx, 'tcx> { self.bccx.tcx }
 245
 246     /// Guarantees that `cmt` is assignable, or reports an error.
 247     fn guarantee_assignment_valid(&mut self,
 248                                   assignment_id: ast::NodeId,
 249                                   assignment_span: Span,
 250                                   cmt: mc::cmt<'tcx>,
 251                                   mode: euv::MutateMode) {
 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.clone(), 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.clone(), 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                                                 assignment_id, assignment_span,
 284                                                 lp, cmt.id, mode);
 285             }
 286             None => {
 287                 // This can occur with e.g. `*foo() = 5`.  In such
 288                 // cases, there is no need to check for conflicts
 289                 // with moves etc, just ignore.
 290             }
 291         }
 292     }
 293
 294     /// Guarantees that `addr_of(cmt)` will be valid for the duration of `static_scope_r`, or
 295     /// reports an error.  This may entail taking out loans, which will be added to the
 296     /// `req_loan_map`.
 297     fn guarantee_valid(&mut self,
 298                        borrow_id: ast::NodeId,
 299                        borrow_span: Span,
 300                        cmt: mc::cmt<'tcx>,
 301                        req_kind: ty::BorrowKind,
 302                        loan_region: ty::Region<'tcx>,
 303                        cause: euv::LoanCause) {
 304         debug!("guarantee_valid(borrow_id={}, cmt={:?}, \
 305                 req_mutbl={:?}, loan_region={:?})",
 306                borrow_id,
 307                cmt,
 308                req_kind,
 309                loan_region);
 310
 311         // a loan for the empty region can never be dereferenced, so
 312         // it is always safe
 313         if *loan_region == ty::ReEmpty {
 314             return;
 315         }
 316
 317         // Check that the lifetime of the borrow does not exceed
 318         // the lifetime of the data being borrowed.
 319         if lifetime::guarantee_lifetime(self.bccx, self.item_ub,
 320                                         borrow_span, cause, cmt.clone(), loan_region,
 321                                         req_kind).is_err() {
 322             return; // reported an error, no sense in reporting more.
 323         }
 324
 325         // Check that we don't allow mutable borrows of non-mutable data.
 326         if check_mutability(self.bccx, borrow_span, BorrowViolation(cause),
 327                             cmt.clone(), req_kind).is_err() {
 328             return; // reported an error, no sense in reporting more.
 329         }
 330
 331         // Check that we don't allow mutable borrows of aliasable data.
 332         if check_aliasability(self.bccx, borrow_span, BorrowViolation(cause),
 333                               cmt.clone(), req_kind).is_err() {
 334             return; // reported an error, no sense in reporting more.
 335         }
 336
 337         // Compute the restrictions that are required to enforce the
 338         // loan is safe.
 339         let restr = restrictions::compute_restrictions(
 340             self.bccx, borrow_span, cause,
 341             cmt.clone(), 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::ReFree(ref fr) => {
 357                         self.bccx.region_maps.free_extent(self.tcx(), fr)
 358                     }
 359
 360                     ty::ReStatic => self.item_ub,
 361
 362                     ty::ReEmpty |
 363                     ty::ReLateBound(..) |
 364                     ty::ReEarlyBound(..) |
 365                     ty::ReVar(..) |
 366                     ty::ReSkolemized(..) |
 367                     ty::ReErased => {
 368                         span_bug!(
 369                             cmt.span,
 370                             "invalid borrow lifetime: {:?}",
 371                             loan_region);
 372                     }
 373                 };
 374                 debug!("loan_scope = {:?}", loan_scope);
 375
 376                 let borrow_scope = self.tcx().node_extent(borrow_id);
 377                 let gen_scope = self.compute_gen_scope(borrow_scope, loan_scope);
 378                 debug!("gen_scope = {:?}", gen_scope);
 379
 380                 let kill_scope = self.compute_kill_scope(loan_scope, &loan_path);
 381                 debug!("kill_scope = {:?}", kill_scope);
 382
 383                 if req_kind == ty::MutBorrow {
 384                     self.mark_loan_path_as_mutated(&loan_path);
 385                 }
 386
 387                 Loan {
 388                     index: self.all_loans.len(),
 389                     loan_path: loan_path,
 390                     kind: req_kind,
 391                     gen_scope: gen_scope,
 392                     kill_scope: kill_scope,
 393                     span: borrow_span,
 394                     restricted_paths: restricted_paths,
 395                     cause: cause,
 396                 }
 397             }
 398         };
 399
 400         debug!("guarantee_valid(borrow_id={}), loan={:?}",
 401                borrow_id, loan);
 402
 403         // let loan_path = loan.loan_path;
 404         // let loan_gen_scope = loan.gen_scope;
 405         // let loan_kill_scope = loan.kill_scope;
 406         self.all_loans.push(loan);
 407
 408         // if loan_gen_scope != borrow_id {
 409             // FIXME(#6268) Nested method calls
 410             //
 411             // Typically, the scope of the loan includes the point at
 412             // which the loan is originated. This
 413             // This is a subtle case. See the test case
 414             // <compile-fail/borrowck-bad-nested-calls-free.rs>
 415             // to see what we are guarding against.
 416
 417             //let restr = restrictions::compute_restrictions(
 418             //    self.bccx, borrow_span, cmt, RESTR_EMPTY);
 419             //let loan = {
 420             //    let all_loans = &mut *self.all_loans; // FIXME(#5074)
 421             //    Loan {
 422             //        index: all_loans.len(),
 423             //        loan_path: loan_path,
 424             //        cmt: cmt,
 425             //        mutbl: ConstMutability,
 426             //        gen_scope: borrow_id,
 427             //        kill_scope: kill_scope,
 428             //        span: borrow_span,
 429             //        restrictions: restrictions
 430             //    }
 431         // }
 432     }
 433
 434     pub fn mark_loan_path_as_mutated(&self, loan_path: &LoanPath) {
 435         //! For mutable loans of content whose mutability derives
 436         //! from a local variable, mark the mutability decl as necessary.
 437
 438         match loan_path.kind {
 439             LpVar(local_id) |
 440             LpUpvar(ty::UpvarId{ var_id: local_id, closure_expr_id: _ }) => {
 441                 self.tcx().used_mut_nodes.borrow_mut().insert(local_id);
 442             }
 443             LpDowncast(ref base, _) |
 444             LpExtend(ref base, mc::McInherited, _) |
 445             LpExtend(ref base, mc::McDeclared, _) => {
 446                 self.mark_loan_path_as_mutated(&base);
 447             }
 448             LpExtend(_, mc::McImmutable, _) => {
 449                 // Nothing to do.
 450             }
 451         }
 452     }
 453
 454     pub fn compute_gen_scope(&self,
 455                              borrow_scope: region::CodeExtent<'tcx>,
 456                              loan_scope: region::CodeExtent<'tcx>)
 457                              -> region::CodeExtent<'tcx> {
 458         //! Determine when to introduce the loan. Typically the loan
 459         //! is introduced at the point of the borrow, but in some cases,
 460         //! notably method arguments, the loan may be introduced only
 461         //! later, once it comes into scope.
 462
 463         if self.bccx.region_maps.is_subscope_of(borrow_scope, loan_scope) {
 464             borrow_scope
 465         } else {
 466             loan_scope
 467         }
 468     }
 469
 470     pub fn compute_kill_scope(&self, loan_scope: region::CodeExtent<'tcx>, lp: &LoanPath<'tcx>)
 471                               -> region::CodeExtent<'tcx> {
 472         //! Determine when the loan restrictions go out of scope.
 473         //! This is either when the lifetime expires or when the
 474         //! local variable which roots the loan-path goes out of scope,
 475         //! whichever happens faster.
 476         //!
 477         //! It may seem surprising that we might have a loan region
 478         //! larger than the variable which roots the loan-path; this can
 479         //! come about when variables of `&mut` type are re-borrowed,
 480         //! as in this example:
 481         //!
 482         //!     struct Foo { counter: u32 }
 483         //!
 484         //!     fn counter<'a>(v: &'a mut Foo) -> &'a mut u32 {
 485         //!         &mut v.counter
 486         //!     }
 487         //!
 488         //! In this case, the reference (`'a`) outlives the
 489         //! variable `v` that hosts it. Note that this doesn't come up
 490         //! with immutable `&` pointers, because borrows of such pointers
 491         //! do not require restrictions and hence do not cause a loan.
 492
 493         let lexical_scope = lp.kill_scope(self.bccx);
 494         if self.bccx.region_maps.is_subscope_of(lexical_scope, loan_scope) {
 495             lexical_scope
 496         } else {
 497             assert!(self.bccx.region_maps.is_subscope_of(loan_scope, lexical_scope));
 498             loan_scope
 499         }
 500     }
 501
 502     pub fn report_potential_errors(&self) {
 503         self.move_error_collector.report_potential_errors(self.bccx);
 504     }
 505 }