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::Node(bccx.tcx.hir.body(body).value.hir_id.local_id),
  47         move_data: MoveData::default(),
  48         move_error_collector: move_error::MoveErrorCollector::new(),
  49     };
  50
  51     let rvalue_promotable_map = bccx.tcx.rvalue_promotable_map(def_id);
  52     euv::ExprUseVisitor::new(&mut glcx,
  53                              bccx.tcx,
  54                              param_env,
  55                              &bccx.region_scope_tree,
  56                              bccx.tables,
  57                              Some(rvalue_promotable_map))
  58         .consume_body(bccx.body);
  59
  60     glcx.report_potential_errors();
  61     let GatherLoanCtxt { all_loans, move_data, .. } = glcx;
  62     (all_loans, move_data)
  63 }
  64
  65 struct GatherLoanCtxt<'a, 'tcx: 'a> {
  66     bccx: &'a BorrowckCtxt<'a, 'tcx>,
  67     move_data: move_data::MoveData<'tcx>,
  68     move_error_collector: move_error::MoveErrorCollector<'tcx>,
  69     all_loans: Vec<Loan<'tcx>>,
  70     /// `item_ub` is used as an upper-bound on the lifetime whenever we
  71     /// ask for the scope of an expression categorized as an upvar.
  72     item_ub: region::Scope,
  73 }
  74
  75 impl<'a, 'tcx> euv::Delegate<'tcx> for GatherLoanCtxt<'a, 'tcx> {
  76     fn consume(&mut self,
  77                consume_id: ast::NodeId,
  78                _consume_span: Span,
  79                cmt: &mc::cmt_<'tcx>,
  80                mode: euv::ConsumeMode) {
  81         debug!("consume(consume_id={}, cmt={:?}, mode={:?})",
  82                consume_id, cmt, mode);
  83
  84         match mode {
  85             euv::Move(move_reason) => {
  86                 gather_moves::gather_move_from_expr(
  87                     self.bccx, &self.move_data, &mut self.move_error_collector,
  88                     self.bccx.tcx.hir.node_to_hir_id(consume_id).local_id, cmt, move_reason);
  89             }
  90             euv::Copy => { }
  91         }
  92     }
  93
  94     fn matched_pat(&mut self,
  95                    matched_pat: &hir::Pat,
  96                    cmt: &mc::cmt_<'tcx>,
  97                    mode: euv::MatchMode) {
  98         debug!("matched_pat(matched_pat={:?}, cmt={:?}, mode={:?})",
  99                matched_pat,
 100                cmt,
 101                mode);
 102     }
 103
 104     fn consume_pat(&mut self,
 105                    consume_pat: &hir::Pat,
 106                    cmt: &mc::cmt_<'tcx>,
 107                    mode: euv::ConsumeMode) {
 108         debug!("consume_pat(consume_pat={:?}, cmt={:?}, mode={:?})",
 109                consume_pat,
 110                cmt,
 111                mode);
 112
 113         match mode {
 114             euv::Copy => { return; }
 115             euv::Move(_) => { }
 116         }
 117
 118         gather_moves::gather_move_from_pat(
 119             self.bccx, &self.move_data, &mut self.move_error_collector,
 120             consume_pat, cmt);
 121     }
 122
 123     fn borrow(&mut self,
 124               borrow_id: ast::NodeId,
 125               borrow_span: Span,
 126               cmt: &mc::cmt_<'tcx>,
 127               loan_region: ty::Region<'tcx>,
 128               bk: ty::BorrowKind,
 129               loan_cause: euv::LoanCause)
 130     {
 131         debug!("borrow(borrow_id={}, cmt={:?}, loan_region={:?}, \
 132                bk={:?}, loan_cause={:?})",
 133                borrow_id, cmt, loan_region,
 134                bk, loan_cause);
 135         let hir_id = self.bccx.tcx.hir.node_to_hir_id(borrow_id);
 136         self.guarantee_valid(hir_id.local_id,
 137                              borrow_span,
 138                              cmt,
 139                              bk,
 140                              loan_region,
 141                              loan_cause);
 142     }
 143
 144     fn mutate(&mut self,
 145               assignment_id: ast::NodeId,
 146               assignment_span: Span,
 147               assignee_cmt: &mc::cmt_<'tcx>,
 148               _: euv::MutateMode)
 149     {
 150         self.guarantee_assignment_valid(assignment_id,
 151                                         assignment_span,
 152                                         assignee_cmt);
 153     }
 154
 155     fn decl_without_init(&mut self, id: ast::NodeId, _span: Span) {
 156         let ty = self.bccx
 157                      .tables
 158                      .node_id_to_type(self.bccx.tcx.hir.node_to_hir_id(id));
 159         gather_moves::gather_decl(self.bccx, &self.move_data, id, ty);
 160     }
 161 }
 162
 163 /// Implements the A-* rules in README.md.
 164 fn check_aliasability<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
 165                                 borrow_span: Span,
 166                                 loan_cause: AliasableViolationKind,
 167                                 cmt: &mc::cmt_<'tcx>,
 168                                 req_kind: ty::BorrowKind)
 169                                 -> Result<(),()> {
 170
 171     let aliasability = cmt.freely_aliasable();
 172     debug!("check_aliasability aliasability={:?} req_kind={:?}",
 173            aliasability, req_kind);
 174
 175     match (aliasability, req_kind) {
 176         (mc::Aliasability::NonAliasable, _) => {
 177             /* Uniquely accessible path -- OK for `&` and `&mut` */
 178             Ok(())
 179         }
 180         (mc::Aliasability::FreelyAliasable(mc::AliasableStatic), ty::ImmBorrow) => {
 181             // Borrow of an immutable static item.
 182             Ok(())
 183         }
 184         (mc::Aliasability::FreelyAliasable(mc::AliasableStaticMut), _) => {
 185             // Even touching a static mut is considered unsafe. We assume the
 186             // user knows what they're doing in these cases.
 187             Ok(())
 188         }
 189         (mc::Aliasability::FreelyAliasable(alias_cause), ty::UniqueImmBorrow) |
 190         (mc::Aliasability::FreelyAliasable(alias_cause), ty::MutBorrow) => {
 191             bccx.report_aliasability_violation(
 192                         borrow_span,
 193                         loan_cause,
 194                         alias_cause,
 195                         cmt);
 196             Err(())
 197         }
 198         (..) => {
 199             Ok(())
 200         }
 201     }
 202 }
 203
 204 /// Implements the M-* rules in README.md.
 205 fn check_mutability<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
 206                               borrow_span: Span,
 207                               cause: AliasableViolationKind,
 208                               cmt: &mc::cmt_<'tcx>,
 209                               req_kind: ty::BorrowKind)
 210                               -> Result<(),()> {
 211     debug!("check_mutability(cause={:?} cmt={:?} req_kind={:?}",
 212            cause, cmt, req_kind);
 213     match req_kind {
 214         ty::UniqueImmBorrow | ty::ImmBorrow => {
 215             match cmt.mutbl {
 216                 // I am intentionally leaving this here to help
 217                 // refactoring if, in the future, we should add new
 218                 // kinds of mutability.
 219                 mc::McImmutable | mc::McDeclared | mc::McInherited => {
 220                     // both imm and mut data can be lent as imm;
 221                     // for mutable data, this is a freeze
 222                     Ok(())
 223                 }
 224             }
 225         }
 226
 227         ty::MutBorrow => {
 228             // Only mutable data can be lent as mutable.
 229             if !cmt.mutbl.is_mutable() {
 230                 Err(bccx.report(BckError { span: borrow_span,
 231                                            cause,
 232                                            cmt,
 233                                            code: err_mutbl }))
 234             } else {
 235                 Ok(())
 236             }
 237         }
 238     }
 239 }
 240
 241 impl<'a, 'tcx> GatherLoanCtxt<'a, 'tcx> {
 242     pub fn tcx(&self) -> TyCtxt<'a, 'tcx, 'tcx> { self.bccx.tcx }
 243
 244     /// Guarantees that `cmt` is assignable, or reports an error.
 245     fn guarantee_assignment_valid(&mut self,
 246                                   assignment_id: ast::NodeId,
 247                                   assignment_span: Span,
 248                                   cmt: &mc::cmt_<'tcx>) {
 249
 250         let opt_lp = opt_loan_path(cmt);
 251         debug!("guarantee_assignment_valid(assignment_id={}, cmt={:?}) opt_lp={:?}",
 252                assignment_id, cmt, opt_lp);
 253
 254         if let Categorization::Local(..) = cmt.cat {
 255             // Only re-assignments to locals require it to be
 256             // mutable - this is checked in check_loans.
 257         } else {
 258             // Check that we don't allow assignments to non-mutable data.
 259             if check_mutability(self.bccx, assignment_span, MutabilityViolation,
 260                                 cmt, ty::MutBorrow).is_err() {
 261                 return; // reported an error, no sense in reporting more.
 262             }
 263         }
 264
 265         // Check that we don't allow assignments to aliasable data
 266         if check_aliasability(self.bccx, assignment_span, MutabilityViolation,
 267                               cmt, ty::MutBorrow).is_err() {
 268             return; // reported an error, no sense in reporting more.
 269         }
 270
 271         match opt_lp {
 272             Some(lp) => {
 273                 if let Categorization::Local(..) = cmt.cat {
 274                     // Only re-assignments to locals require it to be
 275                     // mutable - this is checked in check_loans.
 276                 } else {
 277                     self.mark_loan_path_as_mutated(&lp);
 278                 }
 279                 gather_moves::gather_assignment(self.bccx, &self.move_data,
 280                                                 self.bccx.tcx.hir.node_to_hir_id(assignment_id)
 281                                                     .local_id,
 282                                                 assignment_span,
 283                                                 lp);
 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: hir::ItemLocalId,
 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, loan_region).is_err() {
 320             return; // reported an error, no sense in reporting more.
 321         }
 322
 323         // Check that we don't allow mutable borrows of non-mutable data.
 324         if check_mutability(self.bccx, borrow_span, BorrowViolation(cause),
 325                             cmt, req_kind).is_err() {
 326             return; // reported an error, no sense in reporting more.
 327         }
 328
 329         // Check that we don't allow mutable borrows of aliasable data.
 330         if check_aliasability(self.bccx, borrow_span, BorrowViolation(cause),
 331                               cmt, req_kind).is_err() {
 332             return; // reported an error, no sense in reporting more.
 333         }
 334
 335         // Compute the restrictions that are required to enforce the
 336         // loan is safe.
 337         let restr = restrictions::compute_restrictions(
 338             self.bccx, borrow_span, cause, &cmt, loan_region);
 339
 340         debug!("guarantee_valid(): restrictions={:?}", restr);
 341
 342         // Create the loan record (if needed).
 343         let loan = match restr {
 344             RestrictionResult::Safe => {
 345                 // No restrictions---no loan record necessary
 346                 return;
 347             }
 348
 349             RestrictionResult::SafeIf(loan_path, restricted_paths) => {
 350                 let loan_scope = match *loan_region {
 351                     ty::ReScope(scope) => scope,
 352
 353                     ty::ReEarlyBound(ref br) => {
 354                         self.bccx.region_scope_tree.early_free_scope(self.tcx(), br)
 355                     }
 356
 357                     ty::ReFree(ref fr) => {
 358                         self.bccx.region_scope_tree.free_scope(self.tcx(), fr)
 359                     }
 360
 361                     ty::ReStatic => self.item_ub,
 362
 363                     ty::ReCanonical(_) |
 364                     ty::ReEmpty |
 365                     ty::ReClosureBound(..) |
 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::Scope::Node(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,
 392                     kind: req_kind,
 393                     gen_scope,
 394                     kill_scope,
 395                     span: borrow_span,
 396                     restricted_paths,
 397                     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(https://github.com/rust-lang/rfcs/issues/811) 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             //    Loan {
 423             //        index: self.all_loans.len(),
 424             //        loan_path,
 425             //        cmt,
 426             //        mutbl: ConstMutability,
 427             //        gen_scope: borrow_id,
 428             //        kill_scope,
 429             //        span: borrow_span,
 430             //        restrictions,
 431             //    }
 432         // }
 433     }
 434
 435     pub fn mark_loan_path_as_mutated(&self, loan_path: &LoanPath) {
 436         //! For mutable loans of content whose mutability derives
 437         //! from a local variable, mark the mutability decl as necessary.
 438
 439         let mut wrapped_path = Some(loan_path);
 440         let mut through_borrow = false;
 441
 442         while let Some(current_path) = wrapped_path {
 443             wrapped_path = match current_path.kind {
 444                 LpVar(local_id) => {
 445                     if !through_borrow {
 446                         let hir_id = self.bccx.tcx.hir.node_to_hir_id(local_id);
 447                         self.bccx.used_mut_nodes.borrow_mut().insert(hir_id);
 448                     }
 449                     None
 450                 }
 451                 LpUpvar(ty::UpvarId{ var_id, closure_expr_id: _ }) => {
 452                     self.bccx.used_mut_nodes.borrow_mut().insert(var_id);
 453                     None
 454                 }
 455                 LpExtend(ref base, mc::McInherited, LpDeref(pointer_kind)) |
 456                 LpExtend(ref base, mc::McDeclared, LpDeref(pointer_kind)) => {
 457                     if pointer_kind != mc::Unique {
 458                         through_borrow = true;
 459                     }
 460                     Some(base)
 461                 }
 462                 LpDowncast(ref base, _) |
 463                 LpExtend(ref base, mc::McInherited, _) |
 464                 LpExtend(ref base, mc::McDeclared, _) => {
 465                     Some(base)
 466                 }
 467                 LpExtend(_, mc::McImmutable, _) => {
 468                     // Nothing to do.
 469                     None
 470                 }
 471             }
 472         }
 473
 474     }
 475
 476     pub fn compute_gen_scope(&self,
 477                              borrow_scope: region::Scope,
 478                              loan_scope: region::Scope)
 479                              -> region::Scope {
 480         //! Determine when to introduce the loan. Typically the loan
 481         //! is introduced at the point of the borrow, but in some cases,
 482         //! notably method arguments, the loan may be introduced only
 483         //! later, once it comes into scope.
 484
 485         if self.bccx.region_scope_tree.is_subscope_of(borrow_scope, loan_scope) {
 486             borrow_scope
 487         } else {
 488             loan_scope
 489         }
 490     }
 491
 492     pub fn compute_kill_scope(&self, loan_scope: region::Scope, lp: &LoanPath<'tcx>)
 493                               -> region::Scope {
 494         //! Determine when the loan restrictions go out of scope.
 495         //! This is either when the lifetime expires or when the
 496         //! local variable which roots the loan-path goes out of scope,
 497         //! whichever happens faster.
 498         //!
 499         //! It may seem surprising that we might have a loan region
 500         //! larger than the variable which roots the loan-path; this can
 501         //! come about when variables of `&mut` type are re-borrowed,
 502         //! as in this example:
 503         //!
 504         //!     struct Foo { counter: u32 }
 505         //!
 506         //!     fn counter<'a>(v: &'a mut Foo) -> &'a mut u32 {
 507         //!         &mut v.counter
 508         //!     }
 509         //!
 510         //! In this case, the reference (`'a`) outlives the
 511         //! variable `v` that hosts it. Note that this doesn't come up
 512         //! with immutable `&` pointers, because borrows of such pointers
 513         //! do not require restrictions and hence do not cause a loan.
 514
 515         let lexical_scope = lp.kill_scope(self.bccx);
 516         if self.bccx.region_scope_tree.is_subscope_of(lexical_scope, loan_scope) {
 517             lexical_scope
 518         } else {
 519             assert!(self.bccx.region_scope_tree.is_subscope_of(loan_scope, lexical_scope));
 520             loan_scope
 521         }
 522     }
 523
 524     pub fn report_potential_errors(&self) {
 525         self.move_error_collector.report_potential_errors(self.bccx);
 526     }
 527 }