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     euv::ExprUseVisitor::new(&mut glcx, bccx.tcx, param_env, &bccx.region_maps, bccx.tables)
  53         .consume_body(body);
  54
  55     glcx.report_potential_errors();
  56     let GatherLoanCtxt { all_loans, move_data, .. } = glcx;
  57     (all_loans, move_data)
  58 }
  59
  60 struct GatherLoanCtxt<'a, 'tcx: 'a> {
  61     bccx: &'a BorrowckCtxt<'a, 'tcx>,
  62     move_data: move_data::MoveData<'tcx>,
  63     move_error_collector: move_error::MoveErrorCollector<'tcx>,
  64     all_loans: Vec<Loan<'tcx>>,
  65     /// `item_ub` is used as an upper-bound on the lifetime whenever we
  66     /// ask for the scope of an expression categorized as an upvar.
  67     item_ub: region::CodeExtent,
  68 }
  69
  70 impl<'a, 'tcx> euv::Delegate<'tcx> for GatherLoanCtxt<'a, 'tcx> {
  71     fn consume(&mut self,
  72                consume_id: ast::NodeId,
  73                _consume_span: Span,
  74                cmt: mc::cmt<'tcx>,
  75                mode: euv::ConsumeMode) {
  76         debug!("consume(consume_id={}, cmt={:?}, mode={:?})",
  77                consume_id, cmt, mode);
  78
  79         match mode {
  80             euv::Move(move_reason) => {
  81                 gather_moves::gather_move_from_expr(
  82                     self.bccx, &self.move_data, &mut self.move_error_collector,
  83                     consume_id, cmt, move_reason);
  84             }
  85             euv::Copy => { }
  86         }
  87     }
  88
  89     fn matched_pat(&mut self,
  90                    matched_pat: &hir::Pat,
  91                    cmt: mc::cmt<'tcx>,
  92                    mode: euv::MatchMode) {
  93         debug!("matched_pat(matched_pat={:?}, cmt={:?}, mode={:?})",
  94                matched_pat,
  95                cmt,
  96                mode);
  97
  98         if let Categorization::Downcast(..) = cmt.cat {
  99             gather_moves::gather_match_variant(
 100                 self.bccx, &self.move_data, &mut self.move_error_collector,
 101                 matched_pat, cmt, mode);
 102         }
 103     }
 104
 105     fn consume_pat(&mut self,
 106                    consume_pat: &hir::Pat,
 107                    cmt: mc::cmt<'tcx>,
 108                    mode: euv::ConsumeMode) {
 109         debug!("consume_pat(consume_pat={:?}, cmt={:?}, mode={:?})",
 110                consume_pat,
 111                cmt,
 112                mode);
 113
 114         match mode {
 115             euv::Copy => { return; }
 116             euv::Move(_) => { }
 117         }
 118
 119         gather_moves::gather_move_from_pat(
 120             self.bccx, &self.move_data, &mut self.move_error_collector,
 121             consume_pat, cmt);
 122     }
 123
 124     fn borrow(&mut self,
 125               borrow_id: ast::NodeId,
 126               borrow_span: Span,
 127               cmt: mc::cmt<'tcx>,
 128               loan_region: ty::Region<'tcx>,
 129               bk: ty::BorrowKind,
 130               loan_cause: euv::LoanCause)
 131     {
 132         debug!("borrow(borrow_id={}, cmt={:?}, loan_region={:?}, \
 133                bk={:?}, loan_cause={:?})",
 134                borrow_id, cmt, loan_region,
 135                bk, loan_cause);
 136
 137         self.guarantee_valid(borrow_id,
 138                              borrow_span,
 139                              cmt,
 140                              bk,
 141                              loan_region,
 142                              loan_cause);
 143     }
 144
 145     fn mutate(&mut self,
 146               assignment_id: ast::NodeId,
 147               assignment_span: Span,
 148               assignee_cmt: mc::cmt<'tcx>,
 149               mode: euv::MutateMode)
 150     {
 151         self.guarantee_assignment_valid(assignment_id,
 152                                         assignment_span,
 153                                         assignee_cmt,
 154                                         mode);
 155     }
 156
 157     fn decl_without_init(&mut self, id: ast::NodeId, _span: Span) {
 158         let ty = self.bccx.tables.node_id_to_type(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: cause,
 232                                            cmt: 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                                   mode: euv::MutateMode) {
 250
 251         let opt_lp = opt_loan_path(&cmt);
 252         debug!("guarantee_assignment_valid(assignment_id={}, cmt={:?}) opt_lp={:?}",
 253                assignment_id, cmt, opt_lp);
 254
 255         if let Categorization::Local(..) = cmt.cat {
 256             // Only re-assignments to locals require it to be
 257             // mutable - this is checked in check_loans.
 258         } else {
 259             // Check that we don't allow assignments to non-mutable data.
 260             if check_mutability(self.bccx, assignment_span, MutabilityViolation,
 261                                 cmt.clone(), ty::MutBorrow).is_err() {
 262                 return; // reported an error, no sense in reporting more.
 263             }
 264         }
 265
 266         // Check that we don't allow assignments to aliasable data
 267         if check_aliasability(self.bccx, assignment_span, MutabilityViolation,
 268                               cmt.clone(), ty::MutBorrow).is_err() {
 269             return; // reported an error, no sense in reporting more.
 270         }
 271
 272         match opt_lp {
 273             Some(lp) => {
 274                 if let Categorization::Local(..) = cmt.cat {
 275                     // Only re-assignments to locals require it to be
 276                     // mutable - this is checked in check_loans.
 277                 } else {
 278                     self.mark_loan_path_as_mutated(&lp);
 279                 }
 280                 gather_moves::gather_assignment(self.bccx, &self.move_data,
 281                                                 assignment_id, assignment_span,
 282                                                 lp, cmt.id, mode);
 283             }
 284             None => {
 285                 // This can occur with e.g. `*foo() = 5`.  In such
 286                 // cases, there is no need to check for conflicts
 287                 // with moves etc, just ignore.
 288             }
 289         }
 290     }
 291
 292     /// Guarantees that `addr_of(cmt)` will be valid for the duration of `static_scope_r`, or
 293     /// reports an error.  This may entail taking out loans, which will be added to the
 294     /// `req_loan_map`.
 295     fn guarantee_valid(&mut self,
 296                        borrow_id: ast::NodeId,
 297                        borrow_span: Span,
 298                        cmt: mc::cmt<'tcx>,
 299                        req_kind: ty::BorrowKind,
 300                        loan_region: ty::Region<'tcx>,
 301                        cause: euv::LoanCause) {
 302         debug!("guarantee_valid(borrow_id={}, cmt={:?}, \
 303                 req_mutbl={:?}, loan_region={:?})",
 304                borrow_id,
 305                cmt,
 306                req_kind,
 307                loan_region);
 308
 309         // a loan for the empty region can never be dereferenced, so
 310         // it is always safe
 311         if *loan_region == ty::ReEmpty {
 312             return;
 313         }
 314
 315         // Check that the lifetime of the borrow does not exceed
 316         // the lifetime of the data being borrowed.
 317         if lifetime::guarantee_lifetime(self.bccx, self.item_ub,
 318                                         borrow_span, cause, cmt.clone(), loan_region,
 319                                         req_kind).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.clone(), 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.clone(), 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,
 339             cmt.clone(), loan_region);
 340
 341         debug!("guarantee_valid(): restrictions={:?}", restr);
 342
 343         // Create the loan record (if needed).
 344         let loan = match restr {
 345             RestrictionResult::Safe => {
 346                 // No restrictions---no loan record necessary
 347                 return;
 348             }
 349
 350             RestrictionResult::SafeIf(loan_path, restricted_paths) => {
 351                 let loan_scope = match *loan_region {
 352                     ty::ReScope(scope) => scope,
 353
 354                     ty::ReEarlyBound(ref br) => {
 355                         self.bccx.region_maps.early_free_extent(self.tcx(), br)
 356                     }
 357
 358                     ty::ReFree(ref fr) => {
 359                         self.bccx.region_maps.free_extent(self.tcx(), fr)
 360                     }
 361
 362                     ty::ReStatic => self.item_ub,
 363
 364                     ty::ReEmpty |
 365                     ty::ReLateBound(..) |
 366                     ty::ReVar(..) |
 367                     ty::ReSkolemized(..) |
 368                     ty::ReErased => {
 369                         span_bug!(
 370                             cmt.span,
 371                             "invalid borrow lifetime: {:?}",
 372                             loan_region);
 373                     }
 374                 };
 375                 debug!("loan_scope = {:?}", loan_scope);
 376
 377                 let borrow_scope = region::CodeExtent::Misc(borrow_id);
 378                 let gen_scope = self.compute_gen_scope(borrow_scope, loan_scope);
 379                 debug!("gen_scope = {:?}", gen_scope);
 380
 381                 let kill_scope = self.compute_kill_scope(loan_scope, &loan_path);
 382                 debug!("kill_scope = {:?}", kill_scope);
 383
 384                 if req_kind == ty::MutBorrow {
 385                     self.mark_loan_path_as_mutated(&loan_path);
 386                 }
 387
 388                 Loan {
 389                     index: self.all_loans.len(),
 390                     loan_path: loan_path,
 391                     kind: req_kind,
 392                     gen_scope: gen_scope,
 393                     kill_scope: kill_scope,
 394                     span: borrow_span,
 395                     restricted_paths: restricted_paths,
 396                     cause: cause,
 397                 }
 398             }
 399         };
 400
 401         debug!("guarantee_valid(borrow_id={}), loan={:?}",
 402                borrow_id, loan);
 403
 404         // let loan_path = loan.loan_path;
 405         // let loan_gen_scope = loan.gen_scope;
 406         // let loan_kill_scope = loan.kill_scope;
 407         self.all_loans.push(loan);
 408
 409         // if loan_gen_scope != borrow_id {
 410             // FIXME(#6268) Nested method calls
 411             //
 412             // Typically, the scope of the loan includes the point at
 413             // which the loan is originated. This
 414             // This is a subtle case. See the test case
 415             // <compile-fail/borrowck-bad-nested-calls-free.rs>
 416             // to see what we are guarding against.
 417
 418             //let restr = restrictions::compute_restrictions(
 419             //    self.bccx, borrow_span, cmt, RESTR_EMPTY);
 420             //let loan = {
 421             //    let all_loans = &mut *self.all_loans; // FIXME(#5074)
 422             //    Loan {
 423             //        index: all_loans.len(),
 424             //        loan_path: loan_path,
 425             //        cmt: cmt,
 426             //        mutbl: ConstMutability,
 427             //        gen_scope: borrow_id,
 428             //        kill_scope: kill_scope,
 429             //        span: borrow_span,
 430             //        restrictions: 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         match loan_path.kind {
 440             LpVar(local_id) |
 441             LpUpvar(ty::UpvarId{ var_id: local_id, closure_expr_id: _ }) => {
 442                 self.tcx().used_mut_nodes.borrow_mut().insert(local_id);
 443             }
 444             LpDowncast(ref base, _) |
 445             LpExtend(ref base, mc::McInherited, _) |
 446             LpExtend(ref base, mc::McDeclared, _) => {
 447                 self.mark_loan_path_as_mutated(&base);
 448             }
 449             LpExtend(_, mc::McImmutable, _) => {
 450                 // Nothing to do.
 451             }
 452         }
 453     }
 454
 455     pub fn compute_gen_scope(&self,
 456                              borrow_scope: region::CodeExtent,
 457                              loan_scope: region::CodeExtent)
 458                              -> region::CodeExtent {
 459         //! Determine when to introduce the loan. Typically the loan
 460         //! is introduced at the point of the borrow, but in some cases,
 461         //! notably method arguments, the loan may be introduced only
 462         //! later, once it comes into scope.
 463
 464         if self.bccx.region_maps.is_subscope_of(borrow_scope, loan_scope) {
 465             borrow_scope
 466         } else {
 467             loan_scope
 468         }
 469     }
 470
 471     pub fn compute_kill_scope(&self, loan_scope: region::CodeExtent, lp: &LoanPath<'tcx>)
 472                               -> region::CodeExtent {
 473         //! Determine when the loan restrictions go out of scope.
 474         //! This is either when the lifetime expires or when the
 475         //! local variable which roots the loan-path goes out of scope,
 476         //! whichever happens faster.
 477         //!
 478         //! It may seem surprising that we might have a loan region
 479         //! larger than the variable which roots the loan-path; this can
 480         //! come about when variables of `&mut` type are re-borrowed,
 481         //! as in this example:
 482         //!
 483         //!     struct Foo { counter: u32 }
 484         //!
 485         //!     fn counter<'a>(v: &'a mut Foo) -> &'a mut u32 {
 486         //!         &mut v.counter
 487         //!     }
 488         //!
 489         //! In this case, the reference (`'a`) outlives the
 490         //! variable `v` that hosts it. Note that this doesn't come up
 491         //! with immutable `&` pointers, because borrows of such pointers
 492         //! do not require restrictions and hence do not cause a loan.
 493
 494         let lexical_scope = lp.kill_scope(self.bccx);
 495         if self.bccx.region_maps.is_subscope_of(lexical_scope, loan_scope) {
 496             lexical_scope
 497         } else {
 498             assert!(self.bccx.region_maps.is_subscope_of(loan_scope, lexical_scope));
 499             loan_scope
 500         }
 501     }
 502
 503     pub fn report_potential_errors(&self) {
 504         self.move_error_collector.report_potential_errors(self.bccx);
 505     }
 506 }