src/librustc_typeck/check/upvar.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 //! ### Inferring borrow kinds for upvars
  12 //!
  13 //! Whenever there is a closure expression, we need to determine how each
  14 //! upvar is used. We do this by initially assigning each upvar an
  15 //! immutable "borrow kind" (see `ty::BorrowKind` for details) and then
  16 //! "escalating" the kind as needed. The borrow kind proceeds according to
  17 //! the following lattice:
  18 //!
  19 //!     ty::ImmBorrow -> ty::UniqueImmBorrow -> ty::MutBorrow
  20 //!
  21 //! So, for example, if we see an assignment `x = 5` to an upvar `x`, we
  22 //! will promote its borrow kind to mutable borrow. If we see an `&mut x`
  23 //! we'll do the same. Naturally, this applies not just to the upvar, but
  24 //! to everything owned by `x`, so the result is the same for something
  25 //! like `x.f = 5` and so on (presuming `x` is not a borrowed pointer to a
  26 //! struct). These adjustments are performed in
  27 //! `adjust_upvar_borrow_kind()` (you can trace backwards through the code
  28 //! from there).
  29 //!
  30 //! The fact that we are inferring borrow kinds as we go results in a
  31 //! semi-hacky interaction with mem-categorization. In particular,
  32 //! mem-categorization will query the current borrow kind as it
  33 //! categorizes, and we'll return the *current* value, but this may get
  34 //! adjusted later. Therefore, in this module, we generally ignore the
  35 //! borrow kind (and derived mutabilities) that are returned from
  36 //! mem-categorization, since they may be inaccurate. (Another option
  37 //! would be to use a unification scheme, where instead of returning a
  38 //! concrete borrow kind like `ty::ImmBorrow`, we return a
  39 //! `ty::InferBorrow(upvar_id)` or something like that, but this would
  40 //! then mean that all later passes would have to check for these figments
  41 //! and report an error, and it just seems like more mess in the end.)
  42
  43 use super::FnCtxt;
  44
  45 use middle::expr_use_visitor as euv;
  46 use middle::mem_categorization as mc;
  47 use middle::ty::{self};
  48 use middle::infer::{InferCtxt, UpvarRegion};
  49 use std::collections::HashSet;
  50 use syntax::ast;
  51 use syntax::ast_util;
  52 use syntax::codemap::Span;
  53 use syntax::visit::{self, Visitor};
  54
  55 ///////////////////////////////////////////////////////////////////////////
  56 // PUBLIC ENTRY POINTS
  57
  58 pub fn closure_analyze_fn(fcx: &FnCtxt,
  59                           _id: ast::NodeId,
  60                           _decl: &ast::FnDecl,
  61                           body: &ast::Block)
  62 {
  63     let mut seed = SeedBorrowKind::new(fcx);
  64     seed.visit_block(body);
  65     let closures_with_inferred_kinds = seed.closures_with_inferred_kinds;
  66
  67     let mut adjust = AdjustBorrowKind::new(fcx, &closures_with_inferred_kinds);
  68     adjust.visit_block(body);
  69
  70     // it's our job to process these.
  71     assert!(fcx.inh.deferred_call_resolutions.borrow().is_empty());
  72 }
  73
  74 ///////////////////////////////////////////////////////////////////////////
  75 // SEED BORROW KIND
  76
  77 struct SeedBorrowKind<'a,'tcx:'a> {
  78     fcx: &'a FnCtxt<'a,'tcx>,
  79     closures_with_inferred_kinds: HashSet<ast::NodeId>,
  80 }
  81
  82 impl<'a, 'tcx, 'v> Visitor<'v> for SeedBorrowKind<'a, 'tcx> {
  83     fn visit_expr(&mut self, expr: &ast::Expr) {
  84         match expr.node {
  85             ast::ExprClosure(cc, _, ref body) => {
  86                 self.check_closure(expr, cc, &**body);
  87             }
  88
  89             _ => { }
  90         }
  91
  92         visit::walk_expr(self, expr);
  93     }
  94
  95     fn visit_fn(&mut self,
  96                 fn_kind: visit::FnKind<'v>,
  97                 decl: &'v ast::FnDecl,
  98                 block: &'v ast::Block,
  99                 span: Span,
 100                 _id: ast::NodeId)
 101     {
 102         match fn_kind {
 103             visit::FkItemFn(..) | visit::FkMethod(..) => {
 104                 // ignore nested fn items
 105             }
 106             visit::FkFnBlock => {
 107                 visit::walk_fn(self, fn_kind, decl, block, span);
 108             }
 109         }
 110     }
 111 }
 112
 113 impl<'a,'tcx> SeedBorrowKind<'a,'tcx> {
 114     fn new(fcx: &'a FnCtxt<'a,'tcx>) -> SeedBorrowKind<'a,'tcx> {
 115         SeedBorrowKind { fcx: fcx, closures_with_inferred_kinds: HashSet::new() }
 116     }
 117
 118     fn tcx(&self) -> &'a ty::ctxt<'tcx> {
 119         self.fcx.tcx()
 120     }
 121
 122     fn infcx(&self) -> &'a InferCtxt<'a,'tcx> {
 123         self.fcx.infcx()
 124     }
 125
 126     fn check_closure(&mut self,
 127                      expr: &ast::Expr,
 128                      capture_clause: ast::CaptureClause,
 129                      _body: &ast::Block)
 130     {
 131         let closure_def_id = ast_util::local_def(expr.id);
 132         if !self.fcx.inh.closure_kinds.borrow().contains_key(&closure_def_id) {
 133             self.closures_with_inferred_kinds.insert(expr.id);
 134             self.fcx.inh.closure_kinds.borrow_mut().insert(closure_def_id, ty::FnClosureKind);
 135             debug!("check_closure: adding closure_id={:?} to closures_with_inferred_kinds",
 136                    closure_def_id);
 137         }
 138
 139         ty::with_freevars(self.tcx(), expr.id, |freevars| {
 140             for freevar in freevars {
 141                 let var_node_id = freevar.def.local_node_id();
 142                 let upvar_id = ty::UpvarId { var_id: var_node_id,
 143                                              closure_expr_id: expr.id };
 144                 debug!("seed upvar_id {:?}", upvar_id);
 145
 146                 let capture_kind = match capture_clause {
 147                     ast::CaptureByValue => {
 148                         ty::UpvarCapture::ByValue
 149                     }
 150                     ast::CaptureByRef => {
 151                         let origin = UpvarRegion(upvar_id, expr.span);
 152                         let freevar_region = self.infcx().next_region_var(origin);
 153                         let upvar_borrow = ty::UpvarBorrow { kind: ty::ImmBorrow,
 154                                                              region: freevar_region };
 155                         ty::UpvarCapture::ByRef(upvar_borrow)
 156                     }
 157                 };
 158
 159                 self.fcx.inh.upvar_capture_map.borrow_mut().insert(upvar_id, capture_kind);
 160             }
 161         });
 162     }
 163 }
 164
 165 ///////////////////////////////////////////////////////////////////////////
 166 // ADJUST BORROW KIND
 167
 168 struct AdjustBorrowKind<'a,'tcx:'a> {
 169     fcx: &'a FnCtxt<'a,'tcx>,
 170     closures_with_inferred_kinds: &'a HashSet<ast::NodeId>,
 171 }
 172
 173 impl<'a,'tcx> AdjustBorrowKind<'a,'tcx> {
 174     fn new(fcx: &'a FnCtxt<'a,'tcx>,
 175            closures_with_inferred_kinds: &'a HashSet<ast::NodeId>)
 176            -> AdjustBorrowKind<'a,'tcx> {
 177         AdjustBorrowKind { fcx: fcx, closures_with_inferred_kinds: closures_with_inferred_kinds }
 178     }
 179
 180     fn analyze_closure(&mut self, id: ast::NodeId, decl: &ast::FnDecl, body: &ast::Block) {
 181         /*!
 182          * Analysis starting point.
 183          */
 184
 185         self.visit_block(body);
 186
 187         debug!("analyzing closure `{}` with fn body id `{}`", id, body.id);
 188
 189         let mut euv = euv::ExprUseVisitor::new(self, self.fcx);
 190         euv.walk_fn(decl, body);
 191
 192         // If we had not yet settled on a closure kind for this closure,
 193         // then we should have by now. Process and remove any deferred resolutions.
 194         //
 195         // Interesting fact: all calls to this closure must come
 196         // *after* its definition.  Initially, I thought that some
 197         // kind of fixed-point iteration would be required, due to the
 198         // possibility of twisted examples like this one:
 199         //
 200         // ```rust
 201         // let mut closure0 = None;
 202         // let vec = vec!(1, 2, 3);
 203         //
 204         // loop {
 205         //     {
 206         //         let closure1 = || {
 207         //             match closure0.take() {
 208         //                 Some(c) => {
 209         //                     return c(); // (*) call to `closure0` before it is defined
 210         //                 }
 211         //                 None => { }
 212         //             }
 213         //         };
 214         //         closure1();
 215         //     }
 216         //
 217         //     closure0 = || vec;
 218         // }
 219         // ```
 220         //
 221         // However, this turns out to be wrong. Examples like this
 222         // fail to compile because the type of the variable `c` above
 223         // is an inference variable.  And in fact since closure types
 224         // cannot be written, there is no way to make this example
 225         // work without a boxed closure. This implies that we can't
 226         // have two closures that recursively call one another without
 227         // some form of boxing (and hence explicit writing of a
 228         // closure kind) involved. Huzzah. -nmatsakis
 229         let closure_def_id = ast_util::local_def(id);
 230         if self.closures_with_inferred_kinds.contains(&id) {
 231             let mut deferred_call_resolutions =
 232                 self.fcx.remove_deferred_call_resolutions(closure_def_id);
 233             for deferred_call_resolution in &mut deferred_call_resolutions {
 234                 deferred_call_resolution.resolve(self.fcx);
 235             }
 236         }
 237     }
 238
 239     fn adjust_upvar_borrow_kind_for_consume(&self,
 240                                             cmt: mc::cmt<'tcx>,
 241                                             mode: euv::ConsumeMode)
 242     {
 243         debug!("adjust_upvar_borrow_kind_for_consume(cmt={:?}, mode={:?})",
 244                cmt, mode);
 245
 246         // we only care about moves
 247         match mode {
 248             euv::Copy => { return; }
 249             euv::Move(_) => { }
 250         }
 251
 252         // watch out for a move of the deref of a borrowed pointer;
 253         // for that to be legal, the upvar would have to be borrowed
 254         // by value instead
 255         let guarantor = cmt.guarantor();
 256         debug!("adjust_upvar_borrow_kind_for_consume: guarantor={:?}",
 257                guarantor);
 258         match guarantor.cat {
 259             mc::cat_deref(_, _, mc::BorrowedPtr(..)) |
 260             mc::cat_deref(_, _, mc::Implicit(..)) => {
 261                 match cmt.note {
 262                     mc::NoteUpvarRef(upvar_id) => {
 263                         debug!("adjust_upvar_borrow_kind_for_consume: \
 264                                 setting upvar_id={:?} to by value",
 265                                upvar_id);
 266
 267                         // to move out of an upvar, this must be a FnOnce closure
 268                         self.adjust_closure_kind(upvar_id.closure_expr_id, ty::FnOnceClosureKind);
 269
 270                         let mut upvar_capture_map = self.fcx.inh.upvar_capture_map.borrow_mut();
 271                         upvar_capture_map.insert(upvar_id, ty::UpvarCapture::ByValue);
 272                     }
 273                     mc::NoteClosureEnv(upvar_id) => {
 274                         // we get just a closureenv ref if this is a
 275                         // `move` closure, or if the upvar has already
 276                         // been inferred to by-value. In any case, we
 277                         // must still adjust the kind of the closure
 278                         // to be a FnOnce closure to permit moves out
 279                         // of the environment.
 280                         self.adjust_closure_kind(upvar_id.closure_expr_id, ty::FnOnceClosureKind);
 281                     }
 282                     mc::NoteNone => {
 283                     }
 284                 }
 285             }
 286             _ => { }
 287         }
 288     }
 289
 290     /// Indicates that `cmt` is being directly mutated (e.g., assigned
 291     /// to). If cmt contains any by-ref upvars, this implies that
 292     /// those upvars must be borrowed using an `&mut` borrow.
 293     fn adjust_upvar_borrow_kind_for_mut(&mut self, cmt: mc::cmt<'tcx>) {
 294         debug!("adjust_upvar_borrow_kind_for_mut(cmt={:?})",
 295                cmt);
 296
 297         match cmt.cat.clone() {
 298             mc::cat_deref(base, _, mc::Unique) |
 299             mc::cat_interior(base, _) |
 300             mc::cat_downcast(base, _) => {
 301                 // Interior or owned data is mutable if base is
 302                 // mutable, so iterate to the base.
 303                 self.adjust_upvar_borrow_kind_for_mut(base);
 304             }
 305
 306             mc::cat_deref(base, _, mc::BorrowedPtr(..)) |
 307             mc::cat_deref(base, _, mc::Implicit(..)) => {
 308                 if !self.try_adjust_upvar_deref(&cmt.note, ty::MutBorrow) {
 309                     // assignment to deref of an `&mut`
 310                     // borrowed pointer implies that the
 311                     // pointer itself must be unique, but not
 312                     // necessarily *mutable*
 313                     self.adjust_upvar_borrow_kind_for_unique(base);
 314                 }
 315             }
 316
 317             mc::cat_deref(_, _, mc::UnsafePtr(..)) |
 318             mc::cat_static_item |
 319             mc::cat_rvalue(_) |
 320             mc::cat_local(_) |
 321             mc::cat_upvar(..) => {
 322                 return;
 323             }
 324         }
 325     }
 326
 327     fn adjust_upvar_borrow_kind_for_unique(&self, cmt: mc::cmt<'tcx>) {
 328         debug!("adjust_upvar_borrow_kind_for_unique(cmt={:?})",
 329                cmt);
 330
 331         match cmt.cat.clone() {
 332             mc::cat_deref(base, _, mc::Unique) |
 333             mc::cat_interior(base, _) |
 334             mc::cat_downcast(base, _) => {
 335                 // Interior or owned data is unique if base is
 336                 // unique.
 337                 self.adjust_upvar_borrow_kind_for_unique(base);
 338             }
 339
 340             mc::cat_deref(base, _, mc::BorrowedPtr(..)) |
 341             mc::cat_deref(base, _, mc::Implicit(..)) => {
 342                 if !self.try_adjust_upvar_deref(&cmt.note, ty::UniqueImmBorrow) {
 343                     // for a borrowed pointer to be unique, its
 344                     // base must be unique
 345                     self.adjust_upvar_borrow_kind_for_unique(base);
 346                 }
 347             }
 348
 349             mc::cat_deref(_, _, mc::UnsafePtr(..)) |
 350             mc::cat_static_item |
 351             mc::cat_rvalue(_) |
 352             mc::cat_local(_) |
 353             mc::cat_upvar(..) => {
 354             }
 355         }
 356     }
 357
 358     fn try_adjust_upvar_deref(&self,
 359                               note: &mc::Note,
 360                               borrow_kind: ty::BorrowKind)
 361                               -> bool
 362     {
 363         assert!(match borrow_kind {
 364             ty::MutBorrow => true,
 365             ty::UniqueImmBorrow => true,
 366
 367             // imm borrows never require adjusting any kinds, so we don't wind up here
 368             ty::ImmBorrow => false,
 369         });
 370
 371         match *note {
 372             mc::NoteUpvarRef(upvar_id) => {
 373                 // if this is an implicit deref of an
 374                 // upvar, then we need to modify the
 375                 // borrow_kind of the upvar to make sure it
 376                 // is inferred to mutable if necessary
 377                 let mut upvar_capture_map = self.fcx.inh.upvar_capture_map.borrow_mut();
 378                 let ub = upvar_capture_map.get_mut(&upvar_id).unwrap();
 379                 self.adjust_upvar_borrow_kind(upvar_id, ub, borrow_kind);
 380
 381                 // also need to be in an FnMut closure since this is not an ImmBorrow
 382                 self.adjust_closure_kind(upvar_id.closure_expr_id, ty::FnMutClosureKind);
 383
 384                 true
 385             }
 386             mc::NoteClosureEnv(upvar_id) => {
 387                 // this kind of deref occurs in a `move` closure, or
 388                 // for a by-value upvar; in either case, to mutate an
 389                 // upvar, we need to be an FnMut closure
 390                 self.adjust_closure_kind(upvar_id.closure_expr_id, ty::FnMutClosureKind);
 391
 392                 true
 393             }
 394             mc::NoteNone => {
 395                 false
 396             }
 397         }
 398     }
 399
 400     /// We infer the borrow_kind with which to borrow upvars in a stack closure. The borrow_kind
 401     /// basically follows a lattice of `imm < unique-imm < mut`, moving from left to right as needed
 402     /// (but never right to left). Here the argument `mutbl` is the borrow_kind that is required by
 403     /// some particular use.
 404     fn adjust_upvar_borrow_kind(&self,
 405                                 upvar_id: ty::UpvarId,
 406                                 upvar_capture: &mut ty::UpvarCapture,
 407                                 kind: ty::BorrowKind) {
 408         debug!("adjust_upvar_borrow_kind(upvar_id={:?}, upvar_capture={:?}, kind={:?})",
 409                upvar_id, upvar_capture, kind);
 410
 411         match *upvar_capture {
 412             ty::UpvarCapture::ByValue => {
 413                 // Upvar is already by-value, the strongest criteria.
 414             }
 415             ty::UpvarCapture::ByRef(ref mut upvar_borrow) => {
 416                 match (upvar_borrow.kind, kind) {
 417                     // Take RHS:
 418                     (ty::ImmBorrow, ty::UniqueImmBorrow) |
 419                     (ty::ImmBorrow, ty::MutBorrow) |
 420                     (ty::UniqueImmBorrow, ty::MutBorrow) => {
 421                         upvar_borrow.kind = kind;
 422                     }
 423                     // Take LHS:
 424                     (ty::ImmBorrow, ty::ImmBorrow) |
 425                     (ty::UniqueImmBorrow, ty::ImmBorrow) |
 426                     (ty::UniqueImmBorrow, ty::UniqueImmBorrow) |
 427                     (ty::MutBorrow, _) => {
 428                     }
 429                 }
 430             }
 431         }
 432     }
 433
 434     fn adjust_closure_kind(&self,
 435                            closure_id: ast::NodeId,
 436                            new_kind: ty::ClosureKind) {
 437         debug!("adjust_closure_kind(closure_id={}, new_kind={:?})",
 438                closure_id, new_kind);
 439
 440         if !self.closures_with_inferred_kinds.contains(&closure_id) {
 441             return;
 442         }
 443
 444         let closure_def_id = ast_util::local_def(closure_id);
 445         let mut closure_kinds = self.fcx.inh.closure_kinds.borrow_mut();
 446         let existing_kind = *closure_kinds.get(&closure_def_id).unwrap();
 447
 448         debug!("adjust_closure_kind: closure_id={}, existing_kind={:?}, new_kind={:?}",
 449                closure_id, existing_kind, new_kind);
 450
 451         match (existing_kind, new_kind) {
 452             (ty::FnClosureKind, ty::FnClosureKind) |
 453             (ty::FnMutClosureKind, ty::FnClosureKind) |
 454             (ty::FnMutClosureKind, ty::FnMutClosureKind) |
 455             (ty::FnOnceClosureKind, _) => {
 456                 // no change needed
 457             }
 458
 459             (ty::FnClosureKind, ty::FnMutClosureKind) |
 460             (ty::FnClosureKind, ty::FnOnceClosureKind) |
 461             (ty::FnMutClosureKind, ty::FnOnceClosureKind) => {
 462                 // new kind is stronger than the old kind
 463                 closure_kinds.insert(closure_def_id, new_kind);
 464             }
 465         }
 466     }
 467 }
 468
 469 impl<'a, 'tcx, 'v> Visitor<'v> for AdjustBorrowKind<'a, 'tcx> {
 470     fn visit_fn(&mut self,
 471                 fn_kind: visit::FnKind<'v>,
 472                 decl: &'v ast::FnDecl,
 473                 body: &'v ast::Block,
 474                 span: Span,
 475                 id: ast::NodeId)
 476     {
 477         match fn_kind {
 478             visit::FkItemFn(..) | visit::FkMethod(..) => {
 479                 // ignore nested fn items
 480             }
 481             visit::FkFnBlock => {
 482                 self.analyze_closure(id, decl, body);
 483                 visit::walk_fn(self, fn_kind, decl, body, span);
 484             }
 485         }
 486     }
 487 }
 488
 489 impl<'a,'tcx> euv::Delegate<'tcx> for AdjustBorrowKind<'a,'tcx> {
 490     fn consume(&mut self,
 491                _consume_id: ast::NodeId,
 492                _consume_span: Span,
 493                cmt: mc::cmt<'tcx>,
 494                mode: euv::ConsumeMode)
 495     {
 496         debug!("consume(cmt={:?},mode={:?})", cmt, mode);
 497         self.adjust_upvar_borrow_kind_for_consume(cmt, mode);
 498     }
 499
 500     fn matched_pat(&mut self,
 501                    _matched_pat: &ast::Pat,
 502                    _cmt: mc::cmt<'tcx>,
 503                    _mode: euv::MatchMode)
 504     {}
 505
 506     fn consume_pat(&mut self,
 507                    _consume_pat: &ast::Pat,
 508                    cmt: mc::cmt<'tcx>,
 509                    mode: euv::ConsumeMode)
 510     {
 511         debug!("consume_pat(cmt={:?},mode={:?})", cmt, mode);
 512         self.adjust_upvar_borrow_kind_for_consume(cmt, mode);
 513     }
 514
 515     fn borrow(&mut self,
 516               borrow_id: ast::NodeId,
 517               _borrow_span: Span,
 518               cmt: mc::cmt<'tcx>,
 519               _loan_region: ty::Region,
 520               bk: ty::BorrowKind,
 521               _loan_cause: euv::LoanCause)
 522     {
 523         debug!("borrow(borrow_id={}, cmt={:?}, bk={:?})",
 524                borrow_id, cmt, bk);
 525
 526         match bk {
 527             ty::ImmBorrow => { }
 528             ty::UniqueImmBorrow => {
 529                 self.adjust_upvar_borrow_kind_for_unique(cmt);
 530             }
 531             ty::MutBorrow => {
 532                 self.adjust_upvar_borrow_kind_for_mut(cmt);
 533             }
 534         }
 535     }
 536
 537     fn decl_without_init(&mut self,
 538                          _id: ast::NodeId,
 539                          _span: Span)
 540     {}
 541
 542     fn mutate(&mut self,
 543               _assignment_id: ast::NodeId,
 544               _assignment_span: Span,
 545               assignee_cmt: mc::cmt<'tcx>,
 546               _mode: euv::MutateMode)
 547     {
 548         debug!("mutate(assignee_cmt={:?})",
 549                assignee_cmt);
 550
 551         self.adjust_upvar_borrow_kind_for_mut(assignee_cmt);
 552     }
 553 }