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::infer;
  23 use rustc::middle::mem_categorization as mc;
  24 use rustc::middle::mem_categorization::Categorization;
  25 use rustc::middle::region;
  26 use rustc::middle::ty;
  27
  28 use syntax::ast;
  29 use syntax::codemap::Span;
  30 use syntax::ast::NodeId;
  31 use rustc_front::hir;
  32 use rustc_front::hir::{Expr, FnDecl, Block, Pat};
  33 use rustc_front::intravisit;
  34 use rustc_front::intravisit::Visitor;
  35
  36 mod lifetime;
  37 mod restrictions;
  38 mod gather_moves;
  39 mod move_error;
  40
  41 pub fn gather_loans_in_fn<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
  42                                     fn_id: NodeId,
  43                                     decl: &hir::FnDecl,
  44                                     body: &hir::Block)
  45                                     -> (Vec<Loan<'tcx>>,
  46                                         move_data::MoveData<'tcx>) {
  47     let mut glcx = GatherLoanCtxt {
  48         bccx: bccx,
  49         all_loans: Vec::new(),
  50         item_ub: bccx.tcx.region_maps.node_extent(body.id),
  51         move_data: MoveData::new(),
  52         move_error_collector: move_error::MoveErrorCollector::new(),
  53     };
  54
  55     let param_env = ty::ParameterEnvironment::for_item(bccx.tcx, fn_id);
  56     let infcx = infer::new_infer_ctxt(bccx.tcx, &bccx.tcx.tables, Some(param_env), false);
  57     {
  58         let mut euv = euv::ExprUseVisitor::new(&mut glcx, &infcx);
  59         euv.walk_fn(decl, body);
  60     }
  61
  62     glcx.report_potential_errors();
  63     let GatherLoanCtxt { all_loans, move_data, .. } = glcx;
  64     (all_loans, move_data)
  65 }
  66
  67 struct GatherLoanCtxt<'a, 'tcx: 'a> {
  68     bccx: &'a BorrowckCtxt<'a, 'tcx>,
  69     move_data: move_data::MoveData<'tcx>,
  70     move_error_collector: move_error::MoveErrorCollector<'tcx>,
  71     all_loans: Vec<Loan<'tcx>>,
  72     /// `item_ub` is used as an upper-bound on the lifetime whenever we
  73     /// ask for the scope of an expression categorized as an upvar.
  74     item_ub: region::CodeExtent,
  75 }
  76
  77 impl<'a, 'tcx> euv::Delegate<'tcx> for GatherLoanCtxt<'a, 'tcx> {
  78     fn consume(&mut self,
  79                consume_id: ast::NodeId,
  80                _consume_span: Span,
  81                cmt: mc::cmt<'tcx>,
  82                mode: euv::ConsumeMode) {
  83         debug!("consume(consume_id={}, cmt={:?}, mode={:?})",
  84                consume_id, cmt, mode);
  85
  86         match mode {
  87             euv::Move(move_reason) => {
  88                 gather_moves::gather_move_from_expr(
  89                     self.bccx, &self.move_data, &mut self.move_error_collector,
  90                     consume_id, cmt, move_reason);
  91             }
  92             euv::Copy => { }
  93         }
  94     }
  95
  96     fn matched_pat(&mut self,
  97                    matched_pat: &hir::Pat,
  98                    cmt: mc::cmt<'tcx>,
  99                    mode: euv::MatchMode) {
 100         debug!("matched_pat(matched_pat={:?}, cmt={:?}, mode={:?})",
 101                matched_pat,
 102                cmt,
 103                mode);
 104
 105         if let Categorization::Downcast(..) = cmt.cat {
 106             gather_moves::gather_match_variant(
 107                 self.bccx, &self.move_data, &mut self.move_error_collector,
 108                 matched_pat, cmt, mode);
 109         }
 110     }
 111
 112     fn consume_pat(&mut self,
 113                    consume_pat: &hir::Pat,
 114                    cmt: mc::cmt<'tcx>,
 115                    mode: euv::ConsumeMode) {
 116         debug!("consume_pat(consume_pat={:?}, cmt={:?}, mode={:?})",
 117                consume_pat,
 118                cmt,
 119                mode);
 120
 121         match mode {
 122             euv::Copy => { return; }
 123             euv::Move(_) => { }
 124         }
 125
 126         gather_moves::gather_move_from_pat(
 127             self.bccx, &self.move_data, &mut self.move_error_collector,
 128             consume_pat, cmt);
 129     }
 130
 131     fn borrow(&mut self,
 132               borrow_id: ast::NodeId,
 133               borrow_span: Span,
 134               cmt: mc::cmt<'tcx>,
 135               loan_region: ty::Region,
 136               bk: ty::BorrowKind,
 137               loan_cause: euv::LoanCause)
 138     {
 139         debug!("borrow(borrow_id={}, cmt={:?}, loan_region={:?}, \
 140                bk={:?}, loan_cause={:?})",
 141                borrow_id, cmt, loan_region,
 142                bk, loan_cause);
 143
 144         self.guarantee_valid(borrow_id,
 145                              borrow_span,
 146                              cmt,
 147                              bk,
 148                              loan_region,
 149                              loan_cause);
 150     }
 151
 152     fn mutate(&mut self,
 153               assignment_id: ast::NodeId,
 154               assignment_span: Span,
 155               assignee_cmt: mc::cmt<'tcx>,
 156               mode: euv::MutateMode)
 157     {
 158         self.guarantee_assignment_valid(assignment_id,
 159                                         assignment_span,
 160                                         assignee_cmt,
 161                                         mode);
 162     }
 163
 164     fn decl_without_init(&mut self, id: ast::NodeId, span: Span) {
 165         gather_moves::gather_decl(self.bccx, &self.move_data, id, span, id);
 166     }
 167 }
 168
 169 /// Implements the A-* rules in README.md.
 170 fn check_aliasability<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
 171                                 borrow_span: Span,
 172                                 loan_cause: AliasableViolationKind,
 173                                 cmt: mc::cmt<'tcx>,
 174                                 req_kind: ty::BorrowKind)
 175                                 -> Result<(),()> {
 176
 177     let aliasability = cmt.freely_aliasable(bccx.tcx);
 178     debug!("check_aliasability aliasability={:?} req_kind={:?}",
 179            aliasability, req_kind);
 180
 181     match (aliasability, req_kind) {
 182         (mc::Aliasability::NonAliasable, _) => {
 183             /* Uniquely accessible path -- OK for `&` and `&mut` */
 184             Ok(())
 185         }
 186         (mc::Aliasability::FreelyAliasable(mc::AliasableStatic), ty::ImmBorrow) => {
 187             // Borrow of an immutable static item.
 188             Ok(())
 189         }
 190         (mc::Aliasability::FreelyAliasable(mc::AliasableStaticMut), _) => {
 191             // Even touching a static mut is considered unsafe. We assume the
 192             // user knows what they're doing in these cases.
 193             Ok(())
 194         }
 195         (mc::Aliasability::ImmutableUnique(_), ty::MutBorrow) => {
 196             bccx.report_aliasability_violation(
 197                         borrow_span,
 198                         loan_cause,
 199                         mc::AliasableReason::UnaliasableImmutable);
 200             Err(())
 201         }
 202         (mc::Aliasability::FreelyAliasable(alias_cause), ty::UniqueImmBorrow) |
 203         (mc::Aliasability::FreelyAliasable(alias_cause), ty::MutBorrow) => {
 204             bccx.report_aliasability_violation(
 205                         borrow_span,
 206                         loan_cause,
 207                         alias_cause);
 208             Err(())
 209         }
 210         (_, _) => {
 211             Ok(())
 212         }
 213     }
 214 }
 215
 216 /// Implements the M-* rules in README.md.
 217 fn check_mutability<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
 218                               borrow_span: Span,
 219                               cause: AliasableViolationKind,
 220                               cmt: mc::cmt<'tcx>,
 221                               req_kind: ty::BorrowKind)
 222                               -> Result<(),()> {
 223     debug!("check_mutability(cause={:?} cmt={:?} req_kind={:?}",
 224            cause, cmt, req_kind);
 225     match req_kind {
 226         ty::UniqueImmBorrow | ty::ImmBorrow => {
 227             match cmt.mutbl {
 228                 // I am intentionally leaving this here to help
 229                 // refactoring if, in the future, we should add new
 230                 // kinds of mutability.
 231                 mc::McImmutable | mc::McDeclared | mc::McInherited => {
 232                     // both imm and mut data can be lent as imm;
 233                     // for mutable data, this is a freeze
 234                     Ok(())
 235                 }
 236             }
 237         }
 238
 239         ty::MutBorrow => {
 240             // Only mutable data can be lent as mutable.
 241             if !cmt.mutbl.is_mutable() {
 242                 Err(bccx.report(BckError { span: borrow_span,
 243                                            cause: cause,
 244                                            cmt: cmt,
 245                                            code: err_mutbl }))
 246             } else {
 247                 Ok(())
 248             }
 249         }
 250     }
 251 }
 252
 253 impl<'a, 'tcx> GatherLoanCtxt<'a, 'tcx> {
 254     pub fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.bccx.tcx }
 255
 256     /// Guarantees that `cmt` is assignable, or reports an error.
 257     fn guarantee_assignment_valid(&mut self,
 258                                   assignment_id: ast::NodeId,
 259                                   assignment_span: Span,
 260                                   cmt: mc::cmt<'tcx>,
 261                                   mode: euv::MutateMode) {
 262
 263         let opt_lp = opt_loan_path(&cmt);
 264         debug!("guarantee_assignment_valid(assignment_id={}, cmt={:?}) opt_lp={:?}",
 265                assignment_id, cmt, opt_lp);
 266
 267         if let Categorization::Local(..) = cmt.cat {
 268             // Only re-assignments to locals require it to be
 269             // mutable - this is checked in check_loans.
 270         } else {
 271             // Check that we don't allow assignments to non-mutable data.
 272             if check_mutability(self.bccx, assignment_span, MutabilityViolation,
 273                                 cmt.clone(), ty::MutBorrow).is_err() {
 274                 return; // reported an error, no sense in reporting more.
 275             }
 276         }
 277
 278         // Check that we don't allow assignments to aliasable data
 279         if check_aliasability(self.bccx, assignment_span, MutabilityViolation,
 280                               cmt.clone(), ty::MutBorrow).is_err() {
 281             return; // reported an error, no sense in reporting more.
 282         }
 283
 284         match opt_lp {
 285             Some(lp) => {
 286                 if let Categorization::Local(..) = cmt.cat {
 287                     // Only re-assignments to locals require it to be
 288                     // mutable - this is checked in check_loans.
 289                 } else {
 290                     self.mark_loan_path_as_mutated(&lp);
 291                 }
 292                 gather_moves::gather_assignment(self.bccx, &self.move_data,
 293                                                 assignment_id, assignment_span,
 294                                                 lp, cmt.id, mode);
 295             }
 296             None => {
 297                 // This can occur with e.g. `*foo() = 5`.  In such
 298                 // cases, there is no need to check for conflicts
 299                 // with moves etc, just ignore.
 300             }
 301         }
 302     }
 303
 304     /// Guarantees that `addr_of(cmt)` will be valid for the duration of `static_scope_r`, or
 305     /// reports an error.  This may entail taking out loans, which will be added to the
 306     /// `req_loan_map`.
 307     fn guarantee_valid(&mut self,
 308                        borrow_id: ast::NodeId,
 309                        borrow_span: Span,
 310                        cmt: mc::cmt<'tcx>,
 311                        req_kind: ty::BorrowKind,
 312                        loan_region: ty::Region,
 313                        cause: euv::LoanCause) {
 314         debug!("guarantee_valid(borrow_id={}, cmt={:?}, \
 315                 req_mutbl={:?}, loan_region={:?})",
 316                borrow_id,
 317                cmt,
 318                req_kind,
 319                loan_region);
 320
 321         // a loan for the empty region can never be dereferenced, so
 322         // it is always safe
 323         if loan_region == ty::ReEmpty {
 324             return;
 325         }
 326
 327         // Check that the lifetime of the borrow does not exceed
 328         // the lifetime of the data being borrowed.
 329         if lifetime::guarantee_lifetime(self.bccx, self.item_ub,
 330                                         borrow_span, cause, cmt.clone(), loan_region,
 331                                         req_kind).is_err() {
 332             return; // reported an error, no sense in reporting more.
 333         }
 334
 335         // Check that we don't allow mutable borrows of non-mutable data.
 336         if check_mutability(self.bccx, borrow_span, BorrowViolation(cause),
 337                             cmt.clone(), req_kind).is_err() {
 338             return; // reported an error, no sense in reporting more.
 339         }
 340
 341         // Check that we don't allow mutable borrows of aliasable data.
 342         if check_aliasability(self.bccx, borrow_span, BorrowViolation(cause),
 343                               cmt.clone(), req_kind).is_err() {
 344             return; // reported an error, no sense in reporting more.
 345         }
 346
 347         // Compute the restrictions that are required to enforce the
 348         // loan is safe.
 349         let restr = restrictions::compute_restrictions(
 350             self.bccx, borrow_span, cause,
 351             cmt.clone(), loan_region);
 352
 353         debug!("guarantee_valid(): restrictions={:?}", restr);
 354
 355         // Create the loan record (if needed).
 356         let loan = match restr {
 357             restrictions::Safe => {
 358                 // No restrictions---no loan record necessary
 359                 return;
 360             }
 361
 362             restrictions::SafeIf(loan_path, restricted_paths) => {
 363                 let loan_scope = match loan_region {
 364                     ty::ReScope(scope) => scope,
 365
 366                     ty::ReFree(ref fr) => fr.scope,
 367
 368                     ty::ReStatic => self.item_ub,
 369
 370                     ty::ReEmpty |
 371                     ty::ReLateBound(..) |
 372                     ty::ReEarlyBound(..) |
 373                     ty::ReVar(..) |
 374                     ty::ReSkolemized(..) => {
 375                         self.tcx().sess.span_bug(
 376                             cmt.span,
 377                             &format!("invalid borrow lifetime: {:?}",
 378                                     loan_region));
 379                     }
 380                 };
 381                 debug!("loan_scope = {:?}", loan_scope);
 382
 383                 let borrow_scope = self.tcx().region_maps.node_extent(borrow_id);
 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: loan_path,
 397                     kind: req_kind,
 398                     gen_scope: gen_scope,
 399                     kill_scope: kill_scope,
 400                     span: borrow_span,
 401                     restricted_paths: restricted_paths,
 402                     cause: 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(#6268) 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             //    let all_loans = &mut *self.all_loans; // FIXME(#5074)
 428             //    Loan {
 429             //        index: all_loans.len(),
 430             //        loan_path: loan_path,
 431             //        cmt: cmt,
 432             //        mutbl: ConstMutability,
 433             //        gen_scope: borrow_id,
 434             //        kill_scope: kill_scope,
 435             //        span: borrow_span,
 436             //        restrictions: restrictions
 437             //    }
 438         // }
 439     }
 440
 441     pub fn mark_loan_path_as_mutated(&self, loan_path: &LoanPath) {
 442         //! For mutable loans of content whose mutability derives
 443         //! from a local variable, mark the mutability decl as necessary.
 444
 445         match loan_path.kind {
 446             LpVar(local_id) |
 447             LpUpvar(ty::UpvarId{ var_id: local_id, closure_expr_id: _ }) => {
 448                 self.tcx().used_mut_nodes.borrow_mut().insert(local_id);
 449             }
 450             LpDowncast(ref base, _) |
 451             LpExtend(ref base, mc::McInherited, _) |
 452             LpExtend(ref base, mc::McDeclared, _) => {
 453                 self.mark_loan_path_as_mutated(&**base);
 454             }
 455             LpExtend(_, mc::McImmutable, _) => {
 456                 // Nothing to do.
 457             }
 458         }
 459     }
 460
 461     pub fn compute_gen_scope(&self,
 462                              borrow_scope: region::CodeExtent,
 463                              loan_scope: region::CodeExtent)
 464                              -> region::CodeExtent {
 465         //! Determine when to introduce the loan. Typically the loan
 466         //! is introduced at the point of the borrow, but in some cases,
 467         //! notably method arguments, the loan may be introduced only
 468         //! later, once it comes into scope.
 469
 470         if self.bccx.tcx.region_maps.is_subscope_of(borrow_scope, loan_scope) {
 471             borrow_scope
 472         } else {
 473             loan_scope
 474         }
 475     }
 476
 477     pub fn compute_kill_scope(&self, loan_scope: region::CodeExtent, lp: &LoanPath<'tcx>)
 478                               -> region::CodeExtent {
 479         //! Determine when the loan restrictions go out of scope.
 480         //! This is either when the lifetime expires or when the
 481         //! local variable which roots the loan-path goes out of scope,
 482         //! whichever happens faster.
 483         //!
 484         //! It may seem surprising that we might have a loan region
 485         //! larger than the variable which roots the loan-path; this can
 486         //! come about when variables of `&mut` type are re-borrowed,
 487         //! as in this example:
 488         //!
 489         //!     struct Foo { counter: u32 }
 490         //!
 491         //!     fn counter<'a>(v: &'a mut Foo) -> &'a mut u32 {
 492         //!         &mut v.counter
 493         //!     }
 494         //!
 495         //! In this case, the reference (`'a`) outlives the
 496         //! variable `v` that hosts it. Note that this doesn't come up
 497         //! with immutable `&` pointers, because borrows of such pointers
 498         //! do not require restrictions and hence do not cause a loan.
 499
 500         let lexical_scope = lp.kill_scope(self.bccx.tcx);
 501         let rm = &self.bccx.tcx.region_maps;
 502         if rm.is_subscope_of(lexical_scope, loan_scope) {
 503             lexical_scope
 504         } else {
 505             assert!(self.bccx.tcx.region_maps.is_subscope_of(loan_scope, lexical_scope));
 506             loan_scope
 507         }
 508     }
 509
 510     pub fn report_potential_errors(&self) {
 511         self.move_error_collector.report_potential_errors(self.bccx);
 512     }
 513 }
 514
 515 /// Context used while gathering loans on static initializers
 516 ///
 517 /// This visitor walks static initializer's expressions and makes
 518 /// sure the loans being taken are sound.
 519 struct StaticInitializerCtxt<'a, 'tcx: 'a> {
 520     bccx: &'a BorrowckCtxt<'a, 'tcx>,
 521 }
 522
 523 impl<'a, 'tcx, 'v> Visitor<'v> for StaticInitializerCtxt<'a, 'tcx> {
 524     fn visit_expr(&mut self, ex: &Expr) {
 525         if let hir::ExprAddrOf(mutbl, ref base) = ex.node {
 526             let infcx = infer::new_infer_ctxt(self.bccx.tcx, &self.bccx.tcx.tables, None, false);
 527             let mc = mc::MemCategorizationContext::new(&infcx);
 528             let base_cmt = mc.cat_expr(&**base).unwrap();
 529             let borrow_kind = ty::BorrowKind::from_mutbl(mutbl);
 530             // Check that we don't allow borrows of unsafe static items.
 531             if check_aliasability(self.bccx, ex.span,
 532                                   BorrowViolation(euv::AddrOf),
 533                                   base_cmt, borrow_kind).is_err() {
 534                 return; // reported an error, no sense in reporting more.
 535             }
 536         }
 537
 538         intravisit::walk_expr(self, ex);
 539     }
 540 }
 541
 542 pub fn gather_loans_in_static_initializer(bccx: &mut BorrowckCtxt, expr: &hir::Expr) {
 543
 544     debug!("gather_loans_in_static_initializer(expr={:?})", expr);
 545
 546     let mut sicx = StaticInitializerCtxt {
 547         bccx: bccx
 548     };
 549
 550     sicx.visit_expr(expr);
 551 }