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: bccx,
  45         all_loans: Vec::new(),
  46         item_ub: region::CodeExtent::Misc(body.node_id),
  47         move_data: MoveData::new(),
  48         move_error_collector: move_error::MoveErrorCollector::new(),
  49     };
  50
  51     let body = glcx.bccx.tcx.hir.body(body);
  52     let infcx = bccx.tcx.borrowck_fake_infer_ctxt();
  53     euv::ExprUseVisitor::new(&mut glcx, &infcx, param_env, &bccx.region_maps, bccx.tables)
  54         .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     move_data: move_data::MoveData<'tcx>,
  64     move_error_collector: move_error::MoveErrorCollector<'tcx>,
  65     all_loans: Vec<Loan<'tcx>>,
  66     /// `item_ub` is used as an upper-bound on the lifetime whenever we
  67     /// ask for the scope of an expression categorized as an upvar.
  68     item_ub: region::CodeExtent,
  69 }
  70
  71 impl<'a, 'tcx> euv::Delegate<'tcx> for GatherLoanCtxt<'a, 'tcx> {
  72     fn consume(&mut self,
  73                consume_id: ast::NodeId,
  74                _consume_span: Span,
  75                cmt: mc::cmt<'tcx>,
  76                mode: euv::ConsumeMode) {
  77         debug!("consume(consume_id={}, cmt={:?}, mode={:?})",
  78                consume_id, cmt, mode);
  79
  80         match mode {
  81             euv::Move(move_reason) => {
  82                 gather_moves::gather_move_from_expr(
  83                     self.bccx, &self.move_data, &mut self.move_error_collector,
  84                     consume_id, cmt, move_reason);
  85             }
  86             euv::Copy => { }
  87         }
  88     }
  89
  90     fn matched_pat(&mut self,
  91                    matched_pat: &hir::Pat,
  92                    cmt: mc::cmt<'tcx>,
  93                    mode: euv::MatchMode) {
  94         debug!("matched_pat(matched_pat={:?}, cmt={:?}, mode={:?})",
  95                matched_pat,
  96                cmt,
  97                mode);
  98
  99         if let Categorization::Downcast(..) = cmt.cat {
 100             gather_moves::gather_match_variant(
 101                 self.bccx, &self.move_data, &mut self.move_error_collector,
 102                 matched_pat, cmt, mode);
 103         }
 104     }
 105
 106     fn consume_pat(&mut self,
 107                    consume_pat: &hir::Pat,
 108                    cmt: mc::cmt<'tcx>,
 109                    mode: euv::ConsumeMode) {
 110         debug!("consume_pat(consume_pat={:?}, cmt={:?}, mode={:?})",
 111                consume_pat,
 112                cmt,
 113                mode);
 114
 115         match mode {
 116             euv::Copy => { return; }
 117             euv::Move(_) => { }
 118         }
 119
 120         gather_moves::gather_move_from_pat(
 121             self.bccx, &self.move_data, &mut self.move_error_collector,
 122             consume_pat, cmt);
 123     }
 124
 125     fn borrow(&mut self,
 126               borrow_id: ast::NodeId,
 127               borrow_span: Span,
 128               cmt: mc::cmt<'tcx>,
 129               loan_region: ty::Region<'tcx>,
 130               bk: ty::BorrowKind,
 131               loan_cause: euv::LoanCause)
 132     {
 133         debug!("borrow(borrow_id={}, cmt={:?}, loan_region={:?}, \
 134                bk={:?}, loan_cause={:?})",
 135                borrow_id, cmt, loan_region,
 136                bk, loan_cause);
 137
 138         self.guarantee_valid(borrow_id,
 139                              borrow_span,
 140                              cmt,
 141                              bk,
 142                              loan_region,
 143                              loan_cause);
 144     }
 145
 146     fn mutate(&mut self,
 147               assignment_id: ast::NodeId,
 148               assignment_span: Span,
 149               assignee_cmt: mc::cmt<'tcx>,
 150               mode: euv::MutateMode)
 151     {
 152         self.guarantee_assignment_valid(assignment_id,
 153                                         assignment_span,
 154                                         assignee_cmt,
 155                                         mode);
 156     }
 157
 158     fn decl_without_init(&mut self, id: ast::NodeId, _span: Span) {
 159         let ty = self.bccx.tables.node_id_to_type(id);
 160         gather_moves::gather_decl(self.bccx, &self.move_data, id, ty);
 161     }
 162 }
 163
 164 /// Implements the A-* rules in README.md.
 165 fn check_aliasability<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
 166                                 borrow_span: Span,
 167                                 loan_cause: AliasableViolationKind,
 168                                 cmt: mc::cmt<'tcx>,
 169                                 req_kind: ty::BorrowKind)
 170                                 -> Result<(),()> {
 171
 172     let aliasability = cmt.freely_aliasable();
 173     debug!("check_aliasability aliasability={:?} req_kind={:?}",
 174            aliasability, req_kind);
 175
 176     match (aliasability, req_kind) {
 177         (mc::Aliasability::NonAliasable, _) => {
 178             /* Uniquely accessible path -- OK for `&` and `&mut` */
 179             Ok(())
 180         }
 181         (mc::Aliasability::FreelyAliasable(mc::AliasableStatic), ty::ImmBorrow) => {
 182             // Borrow of an immutable static item.
 183             Ok(())
 184         }
 185         (mc::Aliasability::FreelyAliasable(mc::AliasableStaticMut), _) => {
 186             // Even touching a static mut is considered unsafe. We assume the
 187             // user knows what they're doing in these cases.
 188             Ok(())
 189         }
 190         (mc::Aliasability::FreelyAliasable(alias_cause), ty::UniqueImmBorrow) |
 191         (mc::Aliasability::FreelyAliasable(alias_cause), ty::MutBorrow) => {
 192             bccx.report_aliasability_violation(
 193                         borrow_span,
 194                         loan_cause,
 195                         alias_cause,
 196                         cmt);
 197             Err(())
 198         }
 199         (..) => {
 200             Ok(())
 201         }
 202     }
 203 }
 204
 205 /// Implements the M-* rules in README.md.
 206 fn check_mutability<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
 207                               borrow_span: Span,
 208                               cause: AliasableViolationKind,
 209                               cmt: mc::cmt<'tcx>,
 210                               req_kind: ty::BorrowKind)
 211                               -> Result<(),()> {
 212     debug!("check_mutability(cause={:?} cmt={:?} req_kind={:?}",
 213            cause, cmt, req_kind);
 214     match req_kind {
 215         ty::UniqueImmBorrow | ty::ImmBorrow => {
 216             match cmt.mutbl {
 217                 // I am intentionally leaving this here to help
 218                 // refactoring if, in the future, we should add new
 219                 // kinds of mutability.
 220                 mc::McImmutable | mc::McDeclared | mc::McInherited => {
 221                     // both imm and mut data can be lent as imm;
 222                     // for mutable data, this is a freeze
 223                     Ok(())
 224                 }
 225             }
 226         }
 227
 228         ty::MutBorrow => {
 229             // Only mutable data can be lent as mutable.
 230             if !cmt.mutbl.is_mutable() {
 231                 Err(bccx.report(BckError { span: borrow_span,
 232                                            cause: cause,
 233                                            cmt: cmt,
 234                                            code: err_mutbl }))
 235             } else {
 236                 Ok(())
 237             }
 238         }
 239     }
 240 }
 241
 242 impl<'a, 'tcx> GatherLoanCtxt<'a, 'tcx> {
 243     pub fn tcx(&self) -> TyCtxt<'a, 'tcx, 'tcx> { self.bccx.tcx }
 244
 245     /// Guarantees that `cmt` is assignable, or reports an error.
 246     fn guarantee_assignment_valid(&mut self,
 247                                   assignment_id: ast::NodeId,
 248                                   assignment_span: Span,
 249                                   cmt: mc::cmt<'tcx>,
 250                                   mode: euv::MutateMode) {
 251
 252         let opt_lp = opt_loan_path(&cmt);
 253         debug!("guarantee_assignment_valid(assignment_id={}, cmt={:?}) opt_lp={:?}",
 254                assignment_id, cmt, opt_lp);
 255
 256         if let Categorization::Local(..) = cmt.cat {
 257             // Only re-assignments to locals require it to be
 258             // mutable - this is checked in check_loans.
 259         } else {
 260             // Check that we don't allow assignments to non-mutable data.
 261             if check_mutability(self.bccx, assignment_span, MutabilityViolation,
 262                                 cmt.clone(), ty::MutBorrow).is_err() {
 263                 return; // reported an error, no sense in reporting more.
 264             }
 265         }
 266
 267         // Check that we don't allow assignments to aliasable data
 268         if check_aliasability(self.bccx, assignment_span, MutabilityViolation,
 269                               cmt.clone(), ty::MutBorrow).is_err() {
 270             return; // reported an error, no sense in reporting more.
 271         }
 272
 273         match opt_lp {
 274             Some(lp) => {
 275                 if let Categorization::Local(..) = cmt.cat {
 276                     // Only re-assignments to locals require it to be
 277                     // mutable - this is checked in check_loans.
 278                 } else {
 279                     self.mark_loan_path_as_mutated(&lp);
 280                 }
 281                 gather_moves::gather_assignment(self.bccx, &self.move_data,
 282                                                 assignment_id, assignment_span,
 283                                                 lp, cmt.id, mode);
 284             }
 285             None => {
 286                 // This can occur with e.g. `*foo() = 5`.  In such
 287                 // cases, there is no need to check for conflicts
 288                 // with moves etc, just ignore.
 289             }
 290         }
 291     }
 292
 293     /// Guarantees that `addr_of(cmt)` will be valid for the duration of `static_scope_r`, or
 294     /// reports an error.  This may entail taking out loans, which will be added to the
 295     /// `req_loan_map`.
 296     fn guarantee_valid(&mut self,
 297                        borrow_id: ast::NodeId,
 298                        borrow_span: Span,
 299                        cmt: mc::cmt<'tcx>,
 300                        req_kind: ty::BorrowKind,
 301                        loan_region: ty::Region<'tcx>,
 302                        cause: euv::LoanCause) {
 303         debug!("guarantee_valid(borrow_id={}, cmt={:?}, \
 304                 req_mutbl={:?}, loan_region={:?})",
 305                borrow_id,
 306                cmt,
 307                req_kind,
 308                loan_region);
 309
 310         // a loan for the empty region can never be dereferenced, so
 311         // it is always safe
 312         if *loan_region == ty::ReEmpty {
 313             return;
 314         }
 315
 316         // Check that the lifetime of the borrow does not exceed
 317         // the lifetime of the data being borrowed.
 318         if lifetime::guarantee_lifetime(self.bccx, self.item_ub,
 319                                         borrow_span, cause, cmt.clone(), loan_region,
 320                                         req_kind).is_err() {
 321             return; // reported an error, no sense in reporting more.
 322         }
 323
 324         // Check that we don't allow mutable borrows of non-mutable data.
 325         if check_mutability(self.bccx, borrow_span, BorrowViolation(cause),
 326                             cmt.clone(), req_kind).is_err() {
 327             return; // reported an error, no sense in reporting more.
 328         }
 329
 330         // Check that we don't allow mutable borrows of aliasable data.
 331         if check_aliasability(self.bccx, borrow_span, BorrowViolation(cause),
 332                               cmt.clone(), req_kind).is_err() {
 333             return; // reported an error, no sense in reporting more.
 334         }
 335
 336         // Compute the restrictions that are required to enforce the
 337         // loan is safe.
 338         let restr = restrictions::compute_restrictions(
 339             self.bccx, borrow_span, cause,
 340             cmt.clone(), loan_region);
 341
 342         debug!("guarantee_valid(): restrictions={:?}", restr);
 343
 344         // Create the loan record (if needed).
 345         let loan = match restr {
 346             RestrictionResult::Safe => {
 347                 // No restrictions---no loan record necessary
 348                 return;
 349             }
 350
 351             RestrictionResult::SafeIf(loan_path, restricted_paths) => {
 352                 let loan_scope = match *loan_region {
 353                     ty::ReScope(scope) => scope,
 354
 355                     ty::ReEarlyBound(ref br) => {
 356                         self.bccx.region_maps.early_free_extent(self.tcx(), br)
 357                     }
 358
 359                     ty::ReFree(ref fr) => {
 360                         self.bccx.region_maps.free_extent(self.tcx(), fr)
 361                     }
 362
 363                     ty::ReStatic => self.item_ub,
 364
 365                     ty::ReEmpty |
 366                     ty::ReLateBound(..) |
 367                     ty::ReVar(..) |
 368                     ty::ReSkolemized(..) |
 369                     ty::ReErased => {
 370                         span_bug!(
 371                             cmt.span,
 372                             "invalid borrow lifetime: {:?}",
 373                             loan_region);
 374                     }
 375                 };
 376                 debug!("loan_scope = {:?}", loan_scope);
 377
 378                 let borrow_scope = region::CodeExtent::Misc(borrow_id);
 379                 let gen_scope = self.compute_gen_scope(borrow_scope, loan_scope);
 380                 debug!("gen_scope = {:?}", gen_scope);
 381
 382                 let kill_scope = self.compute_kill_scope(loan_scope, &loan_path);
 383                 debug!("kill_scope = {:?}", kill_scope);
 384
 385                 if req_kind == ty::MutBorrow {
 386                     self.mark_loan_path_as_mutated(&loan_path);
 387                 }
 388
 389                 Loan {
 390                     index: self.all_loans.len(),
 391                     loan_path: loan_path,
 392                     kind: req_kind,
 393                     gen_scope: gen_scope,
 394                     kill_scope: kill_scope,
 395                     span: borrow_span,
 396                     restricted_paths: restricted_paths,
 397                     cause: cause,
 398                 }
 399             }
 400         };
 401
 402         debug!("guarantee_valid(borrow_id={}), loan={:?}",
 403                borrow_id, loan);
 404
 405         // let loan_path = loan.loan_path;
 406         // let loan_gen_scope = loan.gen_scope;
 407         // let loan_kill_scope = loan.kill_scope;
 408         self.all_loans.push(loan);
 409
 410         // if loan_gen_scope != borrow_id {
 411             // FIXME(#6268) Nested method calls
 412             //
 413             // Typically, the scope of the loan includes the point at
 414             // which the loan is originated. This
 415             // This is a subtle case. See the test case
 416             // <compile-fail/borrowck-bad-nested-calls-free.rs>
 417             // to see what we are guarding against.
 418
 419             //let restr = restrictions::compute_restrictions(
 420             //    self.bccx, borrow_span, cmt, RESTR_EMPTY);
 421             //let loan = {
 422             //    let all_loans = &mut *self.all_loans; // FIXME(#5074)
 423             //    Loan {
 424             //        index: all_loans.len(),
 425             //        loan_path: loan_path,
 426             //        cmt: cmt,
 427             //        mutbl: ConstMutability,
 428             //        gen_scope: borrow_id,
 429             //        kill_scope: kill_scope,
 430             //        span: borrow_span,
 431             //        restrictions: restrictions
 432             //    }
 433         // }
 434     }
 435
 436     pub fn mark_loan_path_as_mutated(&self, loan_path: &LoanPath) {
 437         //! For mutable loans of content whose mutability derives
 438         //! from a local variable, mark the mutability decl as necessary.
 439
 440         match loan_path.kind {
 441             LpVar(local_id) |
 442             LpUpvar(ty::UpvarId{ var_id: local_id, closure_expr_id: _ }) => {
 443                 self.tcx().used_mut_nodes.borrow_mut().insert(local_id);
 444             }
 445             LpDowncast(ref base, _) |
 446             LpExtend(ref base, mc::McInherited, _) |
 447             LpExtend(ref base, mc::McDeclared, _) => {
 448                 self.mark_loan_path_as_mutated(&base);
 449             }
 450             LpExtend(_, mc::McImmutable, _) => {
 451                 // Nothing to do.
 452             }
 453         }
 454     }
 455
 456     pub fn compute_gen_scope(&self,
 457                              borrow_scope: region::CodeExtent,
 458                              loan_scope: region::CodeExtent)
 459                              -> region::CodeExtent {
 460         //! Determine when to introduce the loan. Typically the loan
 461         //! is introduced at the point of the borrow, but in some cases,
 462         //! notably method arguments, the loan may be introduced only
 463         //! later, once it comes into scope.
 464
 465         if self.bccx.region_maps.is_subscope_of(borrow_scope, loan_scope) {
 466             borrow_scope
 467         } else {
 468             loan_scope
 469         }
 470     }
 471
 472     pub fn compute_kill_scope(&self, loan_scope: region::CodeExtent, lp: &LoanPath<'tcx>)
 473                               -> region::CodeExtent {
 474         //! Determine when the loan restrictions go out of scope.
 475         //! This is either when the lifetime expires or when the
 476         //! local variable which roots the loan-path goes out of scope,
 477         //! whichever happens faster.
 478         //!
 479         //! It may seem surprising that we might have a loan region
 480         //! larger than the variable which roots the loan-path; this can
 481         //! come about when variables of `&mut` type are re-borrowed,
 482         //! as in this example:
 483         //!
 484         //!     struct Foo { counter: u32 }
 485         //!
 486         //!     fn counter<'a>(v: &'a mut Foo) -> &'a mut u32 {
 487         //!         &mut v.counter
 488         //!     }
 489         //!
 490         //! In this case, the reference (`'a`) outlives the
 491         //! variable `v` that hosts it. Note that this doesn't come up
 492         //! with immutable `&` pointers, because borrows of such pointers
 493         //! do not require restrictions and hence do not cause a loan.
 494
 495         let lexical_scope = lp.kill_scope(self.bccx);
 496         if self.bccx.region_maps.is_subscope_of(lexical_scope, loan_scope) {
 497             lexical_scope
 498         } else {
 499             assert!(self.bccx.region_maps.is_subscope_of(loan_scope, lexical_scope));
 500             loan_scope
 501         }
 502     }
 503
 504     pub fn report_potential_errors(&self) {
 505         self.move_error_collector.report_potential_errors(self.bccx);
 506     }
 507 }