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::mem_categorization::Categorization;
  48 use rustc::ty::{self, Ty};
  49 use rustc::infer::UpvarRegion;
  50 use syntax::ast;
  51 use syntax_pos::Span;
  52 use rustc::hir;
  53 use rustc::hir::intravisit::{self, Visitor, NestedVisitorMap};
  54 use rustc::util::nodemap::NodeMap;
  55
  56 use std::collections::hash_map::Entry;
  57
  58 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
  59     pub fn closure_analyze(&self, body: &'gcx hir::Body) {
  60         InferBorrowKindVisitor { fcx: self }.visit_body(body);
  61
  62         // it's our job to process these.
  63         assert!(self.deferred_call_resolutions.borrow().is_empty());
  64     }
  65 }
  66
  67 struct InferBorrowKindVisitor<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
  68     fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
  69 }
  70
  71 impl<'a, 'gcx, 'tcx> Visitor<'gcx> for InferBorrowKindVisitor<'a, 'gcx, 'tcx> {
  72     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'gcx> {
  73         NestedVisitorMap::None
  74     }
  75
  76     fn visit_expr(&mut self, expr: &'gcx hir::Expr) {
  77         match expr.node {
  78             hir::ExprClosure(cc, _, body_id, _) => {
  79                 let body = self.fcx.tcx.hir.body(body_id);
  80                 self.visit_body(body);
  81                 self.fcx.analyze_closure(expr.id, expr.span, body, cc);
  82             }
  83
  84             _ => { }
  85         }
  86
  87         intravisit::walk_expr(self, expr);
  88     }
  89 }
  90
  91 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
  92     fn analyze_closure(&self,
  93                        id: ast::NodeId,
  94                        span: Span,
  95                        body: &hir::Body,
  96                        capture_clause: hir::CaptureClause) {
  97         /*!
  98          * Analysis starting point.
  99          */
 100
 101         debug!("analyze_closure(id={:?}, body.id={:?})", id, body.id());
 102
 103         let infer_kind = match self.tables.borrow_mut().closure_kinds.entry(id) {
 104             Entry::Occupied(_) => false,
 105             Entry::Vacant(entry) => {
 106                 debug!("check_closure: adding closure {:?} as Fn", id);
 107                 entry.insert((ty::ClosureKind::Fn, None));
 108                 true
 109             }
 110         };
 111
 112         self.tcx.with_freevars(id, |freevars| {
 113             for freevar in freevars {
 114                 let def_id = freevar.def.def_id();
 115                 let var_node_id = self.tcx.hir.as_local_node_id(def_id).unwrap();
 116                 let upvar_id = ty::UpvarId { var_id: var_node_id,
 117                                              closure_expr_id: id };
 118                 debug!("seed upvar_id {:?}", upvar_id);
 119
 120                 let capture_kind = match capture_clause {
 121                     hir::CaptureByValue => {
 122                         ty::UpvarCapture::ByValue
 123                     }
 124                     hir::CaptureByRef => {
 125                         let origin = UpvarRegion(upvar_id, span);
 126                         let freevar_region = self.next_region_var(origin);
 127                         let upvar_borrow = ty::UpvarBorrow { kind: ty::ImmBorrow,
 128                                                              region: freevar_region };
 129                         ty::UpvarCapture::ByRef(upvar_borrow)
 130                     }
 131                 };
 132
 133                 self.tables.borrow_mut().upvar_capture_map.insert(upvar_id, capture_kind);
 134             }
 135         });
 136
 137         {
 138             let body_owner_def_id = self.tcx.hir.body_owner_def_id(body.id());
 139             let region_maps = &self.tcx.region_maps(body_owner_def_id);
 140             let mut delegate = InferBorrowKind {
 141                 fcx: self,
 142                 adjust_closure_kinds: NodeMap(),
 143                 adjust_upvar_captures: ty::UpvarCaptureMap::default(),
 144             };
 145             euv::ExprUseVisitor::new(&mut delegate, region_maps, self, self.param_env)
 146                 .consume_body(body);
 147
 148             // Write the adjusted values back into the main tables.
 149             if infer_kind {
 150                 if let Some(kind) = delegate.adjust_closure_kinds.remove(&id) {
 151                     self.tables.borrow_mut().closure_kinds.insert(id, kind);
 152                 }
 153             }
 154             self.tables.borrow_mut().upvar_capture_map.extend(
 155                 delegate.adjust_upvar_captures);
 156         }
 157
 158         // Now that we've analyzed the closure, we know how each
 159         // variable is borrowed, and we know what traits the closure
 160         // implements (Fn vs FnMut etc). We now have some updates to do
 161         // with that information.
 162         //
 163         // Note that no closure type C may have an upvar of type C
 164         // (though it may reference itself via a trait object). This
 165         // results from the desugaring of closures to a struct like
 166         // `Foo<..., UV0...UVn>`. If one of those upvars referenced
 167         // C, then the type would have infinite size (and the
 168         // inference algorithm will reject it).
 169
 170         // Extract the type variables UV0...UVn.
 171         let (def_id, closure_substs) = match self.node_ty(id).sty {
 172             ty::TyClosure(def_id, substs) => (def_id, substs),
 173             ref t => {
 174                 span_bug!(
 175                     span,
 176                     "type of closure expr {:?} is not a closure {:?}",
 177                     id, t);
 178             }
 179         };
 180
 181         // Equate the type variables with the actual types.
 182         let final_upvar_tys = self.final_upvar_tys(id);
 183         debug!("analyze_closure: id={:?} closure_substs={:?} final_upvar_tys={:?}",
 184                id, closure_substs, final_upvar_tys);
 185         for (upvar_ty, final_upvar_ty) in
 186             closure_substs.upvar_tys(def_id, self.tcx).zip(final_upvar_tys)
 187         {
 188             self.demand_eqtype(span, final_upvar_ty, upvar_ty);
 189         }
 190
 191         // If we are also inferred the closure kind here,
 192         // process any deferred resolutions.
 193         if infer_kind {
 194             let closure_def_id = self.tcx.hir.local_def_id(id);
 195             let deferred_call_resolutions =
 196                 self.remove_deferred_call_resolutions(closure_def_id);
 197             for deferred_call_resolution in deferred_call_resolutions {
 198                 deferred_call_resolution.resolve(self);
 199             }
 200         }
 201     }
 202
 203     // Returns a list of `ClosureUpvar`s for each upvar.
 204     fn final_upvar_tys(&self, closure_id: ast::NodeId) -> Vec<Ty<'tcx>> {
 205         // Presently an unboxed closure type cannot "escape" out of a
 206         // function, so we will only encounter ones that originated in the
 207         // local crate or were inlined into it along with some function.
 208         // This may change if abstract return types of some sort are
 209         // implemented.
 210         let tcx = self.tcx;
 211         tcx.with_freevars(closure_id, |freevars| {
 212             freevars.iter().map(|freevar| {
 213                 let def_id = freevar.def.def_id();
 214                 let var_id = tcx.hir.as_local_node_id(def_id).unwrap();
 215                 let freevar_ty = self.node_ty(var_id);
 216                 let upvar_id = ty::UpvarId {
 217                     var_id: var_id,
 218                     closure_expr_id: closure_id
 219                 };
 220                 let capture = self.tables.borrow().upvar_capture(upvar_id);
 221
 222                 debug!("var_id={:?} freevar_ty={:?} capture={:?}",
 223                        var_id, freevar_ty, capture);
 224
 225                 match capture {
 226                     ty::UpvarCapture::ByValue => freevar_ty,
 227                     ty::UpvarCapture::ByRef(borrow) =>
 228                         tcx.mk_ref(borrow.region,
 229                                     ty::TypeAndMut {
 230                                         ty: freevar_ty,
 231                                         mutbl: borrow.kind.to_mutbl_lossy(),
 232                                     }),
 233                 }
 234             }).collect()
 235         })
 236     }
 237 }
 238
 239 struct InferBorrowKind<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 240     fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
 241     adjust_closure_kinds: NodeMap<(ty::ClosureKind, Option<(Span, ast::Name)>)>,
 242     adjust_upvar_captures: ty::UpvarCaptureMap<'tcx>,
 243 }
 244
 245 impl<'a, 'gcx, 'tcx> InferBorrowKind<'a, 'gcx, 'tcx> {
 246     fn adjust_upvar_borrow_kind_for_consume(&mut self,
 247                                             cmt: mc::cmt<'tcx>,
 248                                             mode: euv::ConsumeMode)
 249     {
 250         debug!("adjust_upvar_borrow_kind_for_consume(cmt={:?}, mode={:?})",
 251                cmt, mode);
 252
 253         // we only care about moves
 254         match mode {
 255             euv::Copy => { return; }
 256             euv::Move(_) => { }
 257         }
 258
 259         let tcx = self.fcx.tcx;
 260
 261         // watch out for a move of the deref of a borrowed pointer;
 262         // for that to be legal, the upvar would have to be borrowed
 263         // by value instead
 264         let guarantor = cmt.guarantor();
 265         debug!("adjust_upvar_borrow_kind_for_consume: guarantor={:?}",
 266                guarantor);
 267         match guarantor.cat {
 268             Categorization::Deref(_, mc::BorrowedPtr(..)) |
 269             Categorization::Deref(_, mc::Implicit(..)) => {
 270                 match cmt.note {
 271                     mc::NoteUpvarRef(upvar_id) => {
 272                         debug!("adjust_upvar_borrow_kind_for_consume: \
 273                                 setting upvar_id={:?} to by value",
 274                                upvar_id);
 275
 276                         // to move out of an upvar, this must be a FnOnce closure
 277                         self.adjust_closure_kind(upvar_id.closure_expr_id,
 278                                                  ty::ClosureKind::FnOnce,
 279                                                  guarantor.span,
 280                                                  tcx.hir.name(upvar_id.var_id));
 281
 282                         self.adjust_upvar_captures.insert(upvar_id, ty::UpvarCapture::ByValue);
 283                     }
 284                     mc::NoteClosureEnv(upvar_id) => {
 285                         // we get just a closureenv ref if this is a
 286                         // `move` closure, or if the upvar has already
 287                         // been inferred to by-value. In any case, we
 288                         // must still adjust the kind of the closure
 289                         // to be a FnOnce closure to permit moves out
 290                         // of the environment.
 291                         self.adjust_closure_kind(upvar_id.closure_expr_id,
 292                                                  ty::ClosureKind::FnOnce,
 293                                                  guarantor.span,
 294                                                  tcx.hir.name(upvar_id.var_id));
 295                     }
 296                     mc::NoteNone => {
 297                     }
 298                 }
 299             }
 300             _ => { }
 301         }
 302     }
 303
 304     /// Indicates that `cmt` is being directly mutated (e.g., assigned
 305     /// to). If cmt contains any by-ref upvars, this implies that
 306     /// those upvars must be borrowed using an `&mut` borrow.
 307     fn adjust_upvar_borrow_kind_for_mut(&mut self, cmt: mc::cmt<'tcx>) {
 308         debug!("adjust_upvar_borrow_kind_for_mut(cmt={:?})",
 309                cmt);
 310
 311         match cmt.cat.clone() {
 312             Categorization::Deref(base, mc::Unique) |
 313             Categorization::Interior(base, _) |
 314             Categorization::Downcast(base, _) => {
 315                 // Interior or owned data is mutable if base is
 316                 // mutable, so iterate to the base.
 317                 self.adjust_upvar_borrow_kind_for_mut(base);
 318             }
 319
 320             Categorization::Deref(base, mc::BorrowedPtr(..)) |
 321             Categorization::Deref(base, mc::Implicit(..)) => {
 322                 if !self.try_adjust_upvar_deref(cmt, ty::MutBorrow) {
 323                     // assignment to deref of an `&mut`
 324                     // borrowed pointer implies that the
 325                     // pointer itself must be unique, but not
 326                     // necessarily *mutable*
 327                     self.adjust_upvar_borrow_kind_for_unique(base);
 328                 }
 329             }
 330
 331             Categorization::Deref(_, mc::UnsafePtr(..)) |
 332             Categorization::StaticItem |
 333             Categorization::Rvalue(..) |
 334             Categorization::Local(_) |
 335             Categorization::Upvar(..) => {
 336                 return;
 337             }
 338         }
 339     }
 340
 341     fn adjust_upvar_borrow_kind_for_unique(&mut self, cmt: mc::cmt<'tcx>) {
 342         debug!("adjust_upvar_borrow_kind_for_unique(cmt={:?})",
 343                cmt);
 344
 345         match cmt.cat.clone() {
 346             Categorization::Deref(base, mc::Unique) |
 347             Categorization::Interior(base, _) |
 348             Categorization::Downcast(base, _) => {
 349                 // Interior or owned data is unique if base is
 350                 // unique.
 351                 self.adjust_upvar_borrow_kind_for_unique(base);
 352             }
 353
 354             Categorization::Deref(base, mc::BorrowedPtr(..)) |
 355             Categorization::Deref(base, mc::Implicit(..)) => {
 356                 if !self.try_adjust_upvar_deref(cmt, ty::UniqueImmBorrow) {
 357                     // for a borrowed pointer to be unique, its
 358                     // base must be unique
 359                     self.adjust_upvar_borrow_kind_for_unique(base);
 360                 }
 361             }
 362
 363             Categorization::Deref(_, mc::UnsafePtr(..)) |
 364             Categorization::StaticItem |
 365             Categorization::Rvalue(..) |
 366             Categorization::Local(_) |
 367             Categorization::Upvar(..) => {
 368             }
 369         }
 370     }
 371
 372     fn try_adjust_upvar_deref(&mut self,
 373                               cmt: mc::cmt<'tcx>,
 374                               borrow_kind: ty::BorrowKind)
 375                               -> bool
 376     {
 377         assert!(match borrow_kind {
 378             ty::MutBorrow => true,
 379             ty::UniqueImmBorrow => true,
 380
 381             // imm borrows never require adjusting any kinds, so we don't wind up here
 382             ty::ImmBorrow => false,
 383         });
 384
 385         let tcx = self.fcx.tcx;
 386
 387         match cmt.note {
 388             mc::NoteUpvarRef(upvar_id) => {
 389                 // if this is an implicit deref of an
 390                 // upvar, then we need to modify the
 391                 // borrow_kind of the upvar to make sure it
 392                 // is inferred to mutable if necessary
 393                 self.adjust_upvar_borrow_kind(upvar_id, borrow_kind);
 394
 395                 // also need to be in an FnMut closure since this is not an ImmBorrow
 396                 self.adjust_closure_kind(upvar_id.closure_expr_id,
 397                                          ty::ClosureKind::FnMut,
 398                                          cmt.span,
 399                                          tcx.hir.name(upvar_id.var_id));
 400
 401                 true
 402             }
 403             mc::NoteClosureEnv(upvar_id) => {
 404                 // this kind of deref occurs in a `move` closure, or
 405                 // for a by-value upvar; in either case, to mutate an
 406                 // upvar, we need to be an FnMut closure
 407                 self.adjust_closure_kind(upvar_id.closure_expr_id,
 408                                          ty::ClosureKind::FnMut,
 409                                          cmt.span,
 410                                          tcx.hir.name(upvar_id.var_id));
 411
 412                 true
 413             }
 414             mc::NoteNone => {
 415                 false
 416             }
 417         }
 418     }
 419
 420     /// We infer the borrow_kind with which to borrow upvars in a stack closure.
 421     /// The borrow_kind basically follows a lattice of `imm < unique-imm < mut`,
 422     /// moving from left to right as needed (but never right to left).
 423     /// Here the argument `mutbl` is the borrow_kind that is required by
 424     /// some particular use.
 425     fn adjust_upvar_borrow_kind(&mut self,
 426                                 upvar_id: ty::UpvarId,
 427                                 kind: ty::BorrowKind) {
 428         let upvar_capture = self.adjust_upvar_captures.get(&upvar_id).cloned()
 429             .unwrap_or_else(|| self.fcx.tables.borrow().upvar_capture(upvar_id));
 430         debug!("adjust_upvar_borrow_kind(upvar_id={:?}, upvar_capture={:?}, kind={:?})",
 431                upvar_id, upvar_capture, kind);
 432
 433         match upvar_capture {
 434             ty::UpvarCapture::ByValue => {
 435                 // Upvar is already by-value, the strongest criteria.
 436             }
 437             ty::UpvarCapture::ByRef(mut upvar_borrow) => {
 438                 match (upvar_borrow.kind, kind) {
 439                     // Take RHS:
 440                     (ty::ImmBorrow, ty::UniqueImmBorrow) |
 441                     (ty::ImmBorrow, ty::MutBorrow) |
 442                     (ty::UniqueImmBorrow, ty::MutBorrow) => {
 443                         upvar_borrow.kind = kind;
 444                         self.adjust_upvar_captures.insert(upvar_id,
 445                             ty::UpvarCapture::ByRef(upvar_borrow));
 446                     }
 447                     // Take LHS:
 448                     (ty::ImmBorrow, ty::ImmBorrow) |
 449                     (ty::UniqueImmBorrow, ty::ImmBorrow) |
 450                     (ty::UniqueImmBorrow, ty::UniqueImmBorrow) |
 451                     (ty::MutBorrow, _) => {
 452                     }
 453                 }
 454             }
 455         }
 456     }
 457
 458     fn adjust_closure_kind(&mut self,
 459                            closure_id: ast::NodeId,
 460                            new_kind: ty::ClosureKind,
 461                            upvar_span: Span,
 462                            var_name: ast::Name) {
 463         debug!("adjust_closure_kind(closure_id={}, new_kind={:?}, upvar_span={:?}, var_name={})",
 464                closure_id, new_kind, upvar_span, var_name);
 465
 466         let closure_kind = self.adjust_closure_kinds.get(&closure_id).cloned()
 467             .or_else(|| self.fcx.tables.borrow().closure_kinds.get(&closure_id).cloned());
 468         if let Some((existing_kind, _)) = closure_kind {
 469             debug!("adjust_closure_kind: closure_id={}, existing_kind={:?}, new_kind={:?}",
 470                    closure_id, existing_kind, new_kind);
 471
 472             match (existing_kind, new_kind) {
 473                 (ty::ClosureKind::Fn, ty::ClosureKind::Fn) |
 474                 (ty::ClosureKind::FnMut, ty::ClosureKind::Fn) |
 475                 (ty::ClosureKind::FnMut, ty::ClosureKind::FnMut) |
 476                 (ty::ClosureKind::FnOnce, _) => {
 477                     // no change needed
 478                 }
 479
 480                 (ty::ClosureKind::Fn, ty::ClosureKind::FnMut) |
 481                 (ty::ClosureKind::Fn, ty::ClosureKind::FnOnce) |
 482                 (ty::ClosureKind::FnMut, ty::ClosureKind::FnOnce) => {
 483                     // new kind is stronger than the old kind
 484                     self.adjust_closure_kinds.insert(
 485                         closure_id,
 486                         (new_kind, Some((upvar_span, var_name)))
 487                     );
 488                 }
 489             }
 490         }
 491     }
 492 }
 493
 494 impl<'a, 'gcx, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'gcx, 'tcx> {
 495     fn consume(&mut self,
 496                _consume_id: ast::NodeId,
 497                _consume_span: Span,
 498                cmt: mc::cmt<'tcx>,
 499                mode: euv::ConsumeMode)
 500     {
 501         debug!("consume(cmt={:?},mode={:?})", cmt, mode);
 502         self.adjust_upvar_borrow_kind_for_consume(cmt, mode);
 503     }
 504
 505     fn matched_pat(&mut self,
 506                    _matched_pat: &hir::Pat,
 507                    _cmt: mc::cmt<'tcx>,
 508                    _mode: euv::MatchMode)
 509     {}
 510
 511     fn consume_pat(&mut self,
 512                    _consume_pat: &hir::Pat,
 513                    cmt: mc::cmt<'tcx>,
 514                    mode: euv::ConsumeMode)
 515     {
 516         debug!("consume_pat(cmt={:?},mode={:?})", cmt, mode);
 517         self.adjust_upvar_borrow_kind_for_consume(cmt, mode);
 518     }
 519
 520     fn borrow(&mut self,
 521               borrow_id: ast::NodeId,
 522               _borrow_span: Span,
 523               cmt: mc::cmt<'tcx>,
 524               _loan_region: ty::Region<'tcx>,
 525               bk: ty::BorrowKind,
 526               _loan_cause: euv::LoanCause)
 527     {
 528         debug!("borrow(borrow_id={}, cmt={:?}, bk={:?})",
 529                borrow_id, cmt, bk);
 530
 531         match bk {
 532             ty::ImmBorrow => { }
 533             ty::UniqueImmBorrow => {
 534                 self.adjust_upvar_borrow_kind_for_unique(cmt);
 535             }
 536             ty::MutBorrow => {
 537                 self.adjust_upvar_borrow_kind_for_mut(cmt);
 538             }
 539         }
 540     }
 541
 542     fn decl_without_init(&mut self,
 543                          _id: ast::NodeId,
 544                          _span: Span)
 545     {}
 546
 547     fn mutate(&mut self,
 548               _assignment_id: ast::NodeId,
 549               _assignment_span: Span,
 550               assignee_cmt: mc::cmt<'tcx>,
 551               _mode: euv::MutateMode)
 552     {
 553         debug!("mutate(assignee_cmt={:?})",
 554                assignee_cmt);
 555
 556         self.adjust_upvar_borrow_kind_for_mut(assignee_cmt);
 557     }
 558 }