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,
 146                                      &self.infcx,
 147                                      self.param_env,
 148                                      region_maps,
 149                                      &self.tables.borrow())
 150                 .consume_body(body);
 151
 152             // Write the adjusted values back into the main tables.
 153             if infer_kind {
 154                 if let Some(kind) = delegate.adjust_closure_kinds.remove(&id) {
 155                     self.tables.borrow_mut().closure_kinds.insert(id, kind);
 156                 }
 157             }
 158             self.tables.borrow_mut().upvar_capture_map.extend(
 159                 delegate.adjust_upvar_captures);
 160         }
 161
 162         // Now that we've analyzed the closure, we know how each
 163         // variable is borrowed, and we know what traits the closure
 164         // implements (Fn vs FnMut etc). We now have some updates to do
 165         // with that information.
 166         //
 167         // Note that no closure type C may have an upvar of type C
 168         // (though it may reference itself via a trait object). This
 169         // results from the desugaring of closures to a struct like
 170         // `Foo<..., UV0...UVn>`. If one of those upvars referenced
 171         // C, then the type would have infinite size (and the
 172         // inference algorithm will reject it).
 173
 174         // Extract the type variables UV0...UVn.
 175         let (def_id, closure_substs) = match self.node_ty(id).sty {
 176             ty::TyClosure(def_id, substs) => (def_id, substs),
 177             ref t => {
 178                 span_bug!(
 179                     span,
 180                     "type of closure expr {:?} is not a closure {:?}",
 181                     id, t);
 182             }
 183         };
 184
 185         // Equate the type variables with the actual types.
 186         let final_upvar_tys = self.final_upvar_tys(id);
 187         debug!("analyze_closure: id={:?} closure_substs={:?} final_upvar_tys={:?}",
 188                id, closure_substs, final_upvar_tys);
 189         for (upvar_ty, final_upvar_ty) in
 190             closure_substs.upvar_tys(def_id, self.tcx).zip(final_upvar_tys)
 191         {
 192             self.demand_eqtype(span, final_upvar_ty, upvar_ty);
 193         }
 194
 195         // If we are also inferred the closure kind here,
 196         // process any deferred resolutions.
 197         if infer_kind {
 198             let closure_def_id = self.tcx.hir.local_def_id(id);
 199             let deferred_call_resolutions =
 200                 self.remove_deferred_call_resolutions(closure_def_id);
 201             for deferred_call_resolution in deferred_call_resolutions {
 202                 deferred_call_resolution.resolve(self);
 203             }
 204         }
 205     }
 206
 207     // Returns a list of `ClosureUpvar`s for each upvar.
 208     fn final_upvar_tys(&self, closure_id: ast::NodeId) -> Vec<Ty<'tcx>> {
 209         // Presently an unboxed closure type cannot "escape" out of a
 210         // function, so we will only encounter ones that originated in the
 211         // local crate or were inlined into it along with some function.
 212         // This may change if abstract return types of some sort are
 213         // implemented.
 214         let tcx = self.tcx;
 215         tcx.with_freevars(closure_id, |freevars| {
 216             freevars.iter().map(|freevar| {
 217                 let def_id = freevar.def.def_id();
 218                 let var_id = tcx.hir.as_local_node_id(def_id).unwrap();
 219                 let freevar_ty = self.node_ty(var_id);
 220                 let upvar_id = ty::UpvarId {
 221                     var_id: var_id,
 222                     closure_expr_id: closure_id
 223                 };
 224                 let capture = self.tables.borrow().upvar_capture(upvar_id);
 225
 226                 debug!("var_id={:?} freevar_ty={:?} capture={:?}",
 227                        var_id, freevar_ty, capture);
 228
 229                 match capture {
 230                     ty::UpvarCapture::ByValue => freevar_ty,
 231                     ty::UpvarCapture::ByRef(borrow) =>
 232                         tcx.mk_ref(borrow.region,
 233                                     ty::TypeAndMut {
 234                                         ty: freevar_ty,
 235                                         mutbl: borrow.kind.to_mutbl_lossy(),
 236                                     }),
 237                 }
 238             }).collect()
 239         })
 240     }
 241 }
 242
 243 struct InferBorrowKind<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 244     fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
 245     adjust_closure_kinds: NodeMap<(ty::ClosureKind, Option<(Span, ast::Name)>)>,
 246     adjust_upvar_captures: ty::UpvarCaptureMap<'tcx>,
 247 }
 248
 249 impl<'a, 'gcx, 'tcx> InferBorrowKind<'a, 'gcx, 'tcx> {
 250     fn adjust_upvar_borrow_kind_for_consume(&mut self,
 251                                             cmt: mc::cmt<'tcx>,
 252                                             mode: euv::ConsumeMode)
 253     {
 254         debug!("adjust_upvar_borrow_kind_for_consume(cmt={:?}, mode={:?})",
 255                cmt, mode);
 256
 257         // we only care about moves
 258         match mode {
 259             euv::Copy => { return; }
 260             euv::Move(_) => { }
 261         }
 262
 263         let tcx = self.fcx.tcx;
 264
 265         // watch out for a move of the deref of a borrowed pointer;
 266         // for that to be legal, the upvar would have to be borrowed
 267         // by value instead
 268         let guarantor = cmt.guarantor();
 269         debug!("adjust_upvar_borrow_kind_for_consume: guarantor={:?}",
 270                guarantor);
 271         match guarantor.cat {
 272             Categorization::Deref(_, mc::BorrowedPtr(..)) |
 273             Categorization::Deref(_, mc::Implicit(..)) => {
 274                 match cmt.note {
 275                     mc::NoteUpvarRef(upvar_id) => {
 276                         debug!("adjust_upvar_borrow_kind_for_consume: \
 277                                 setting upvar_id={:?} to by value",
 278                                upvar_id);
 279
 280                         // to move out of an upvar, this must be a FnOnce closure
 281                         self.adjust_closure_kind(upvar_id.closure_expr_id,
 282                                                  ty::ClosureKind::FnOnce,
 283                                                  guarantor.span,
 284                                                  tcx.hir.name(upvar_id.var_id));
 285
 286                         self.adjust_upvar_captures.insert(upvar_id, ty::UpvarCapture::ByValue);
 287                     }
 288                     mc::NoteClosureEnv(upvar_id) => {
 289                         // we get just a closureenv ref if this is a
 290                         // `move` closure, or if the upvar has already
 291                         // been inferred to by-value. In any case, we
 292                         // must still adjust the kind of the closure
 293                         // to be a FnOnce closure to permit moves out
 294                         // of the environment.
 295                         self.adjust_closure_kind(upvar_id.closure_expr_id,
 296                                                  ty::ClosureKind::FnOnce,
 297                                                  guarantor.span,
 298                                                  tcx.hir.name(upvar_id.var_id));
 299                     }
 300                     mc::NoteNone => {
 301                     }
 302                 }
 303             }
 304             _ => { }
 305         }
 306     }
 307
 308     /// Indicates that `cmt` is being directly mutated (e.g., assigned
 309     /// to). If cmt contains any by-ref upvars, this implies that
 310     /// those upvars must be borrowed using an `&mut` borrow.
 311     fn adjust_upvar_borrow_kind_for_mut(&mut self, cmt: mc::cmt<'tcx>) {
 312         debug!("adjust_upvar_borrow_kind_for_mut(cmt={:?})",
 313                cmt);
 314
 315         match cmt.cat.clone() {
 316             Categorization::Deref(base, mc::Unique) |
 317             Categorization::Interior(base, _) |
 318             Categorization::Downcast(base, _) => {
 319                 // Interior or owned data is mutable if base is
 320                 // mutable, so iterate to the base.
 321                 self.adjust_upvar_borrow_kind_for_mut(base);
 322             }
 323
 324             Categorization::Deref(base, mc::BorrowedPtr(..)) |
 325             Categorization::Deref(base, mc::Implicit(..)) => {
 326                 if !self.try_adjust_upvar_deref(cmt, ty::MutBorrow) {
 327                     // assignment to deref of an `&mut`
 328                     // borrowed pointer implies that the
 329                     // pointer itself must be unique, but not
 330                     // necessarily *mutable*
 331                     self.adjust_upvar_borrow_kind_for_unique(base);
 332                 }
 333             }
 334
 335             Categorization::Deref(_, mc::UnsafePtr(..)) |
 336             Categorization::StaticItem |
 337             Categorization::Rvalue(..) |
 338             Categorization::Local(_) |
 339             Categorization::Upvar(..) => {
 340                 return;
 341             }
 342         }
 343     }
 344
 345     fn adjust_upvar_borrow_kind_for_unique(&mut self, cmt: mc::cmt<'tcx>) {
 346         debug!("adjust_upvar_borrow_kind_for_unique(cmt={:?})",
 347                cmt);
 348
 349         match cmt.cat.clone() {
 350             Categorization::Deref(base, mc::Unique) |
 351             Categorization::Interior(base, _) |
 352             Categorization::Downcast(base, _) => {
 353                 // Interior or owned data is unique if base is
 354                 // unique.
 355                 self.adjust_upvar_borrow_kind_for_unique(base);
 356             }
 357
 358             Categorization::Deref(base, mc::BorrowedPtr(..)) |
 359             Categorization::Deref(base, mc::Implicit(..)) => {
 360                 if !self.try_adjust_upvar_deref(cmt, ty::UniqueImmBorrow) {
 361                     // for a borrowed pointer to be unique, its
 362                     // base must be unique
 363                     self.adjust_upvar_borrow_kind_for_unique(base);
 364                 }
 365             }
 366
 367             Categorization::Deref(_, mc::UnsafePtr(..)) |
 368             Categorization::StaticItem |
 369             Categorization::Rvalue(..) |
 370             Categorization::Local(_) |
 371             Categorization::Upvar(..) => {
 372             }
 373         }
 374     }
 375
 376     fn try_adjust_upvar_deref(&mut self,
 377                               cmt: mc::cmt<'tcx>,
 378                               borrow_kind: ty::BorrowKind)
 379                               -> bool
 380     {
 381         assert!(match borrow_kind {
 382             ty::MutBorrow => true,
 383             ty::UniqueImmBorrow => true,
 384
 385             // imm borrows never require adjusting any kinds, so we don't wind up here
 386             ty::ImmBorrow => false,
 387         });
 388
 389         let tcx = self.fcx.tcx;
 390
 391         match cmt.note {
 392             mc::NoteUpvarRef(upvar_id) => {
 393                 // if this is an implicit deref of an
 394                 // upvar, then we need to modify the
 395                 // borrow_kind of the upvar to make sure it
 396                 // is inferred to mutable if necessary
 397                 self.adjust_upvar_borrow_kind(upvar_id, borrow_kind);
 398
 399                 // also need to be in an FnMut closure since this is not an ImmBorrow
 400                 self.adjust_closure_kind(upvar_id.closure_expr_id,
 401                                          ty::ClosureKind::FnMut,
 402                                          cmt.span,
 403                                          tcx.hir.name(upvar_id.var_id));
 404
 405                 true
 406             }
 407             mc::NoteClosureEnv(upvar_id) => {
 408                 // this kind of deref occurs in a `move` closure, or
 409                 // for a by-value upvar; in either case, to mutate an
 410                 // upvar, we need to be an FnMut closure
 411                 self.adjust_closure_kind(upvar_id.closure_expr_id,
 412                                          ty::ClosureKind::FnMut,
 413                                          cmt.span,
 414                                          tcx.hir.name(upvar_id.var_id));
 415
 416                 true
 417             }
 418             mc::NoteNone => {
 419                 false
 420             }
 421         }
 422     }
 423
 424     /// We infer the borrow_kind with which to borrow upvars in a stack closure.
 425     /// The borrow_kind basically follows a lattice of `imm < unique-imm < mut`,
 426     /// moving from left to right as needed (but never right to left).
 427     /// Here the argument `mutbl` is the borrow_kind that is required by
 428     /// some particular use.
 429     fn adjust_upvar_borrow_kind(&mut self,
 430                                 upvar_id: ty::UpvarId,
 431                                 kind: ty::BorrowKind) {
 432         let upvar_capture = self.adjust_upvar_captures.get(&upvar_id).cloned()
 433             .unwrap_or_else(|| self.fcx.tables.borrow().upvar_capture(upvar_id));
 434         debug!("adjust_upvar_borrow_kind(upvar_id={:?}, upvar_capture={:?}, kind={:?})",
 435                upvar_id, upvar_capture, kind);
 436
 437         match upvar_capture {
 438             ty::UpvarCapture::ByValue => {
 439                 // Upvar is already by-value, the strongest criteria.
 440             }
 441             ty::UpvarCapture::ByRef(mut upvar_borrow) => {
 442                 match (upvar_borrow.kind, kind) {
 443                     // Take RHS:
 444                     (ty::ImmBorrow, ty::UniqueImmBorrow) |
 445                     (ty::ImmBorrow, ty::MutBorrow) |
 446                     (ty::UniqueImmBorrow, ty::MutBorrow) => {
 447                         upvar_borrow.kind = kind;
 448                         self.adjust_upvar_captures.insert(upvar_id,
 449                             ty::UpvarCapture::ByRef(upvar_borrow));
 450                     }
 451                     // Take LHS:
 452                     (ty::ImmBorrow, ty::ImmBorrow) |
 453                     (ty::UniqueImmBorrow, ty::ImmBorrow) |
 454                     (ty::UniqueImmBorrow, ty::UniqueImmBorrow) |
 455                     (ty::MutBorrow, _) => {
 456                     }
 457                 }
 458             }
 459         }
 460     }
 461
 462     fn adjust_closure_kind(&mut self,
 463                            closure_id: ast::NodeId,
 464                            new_kind: ty::ClosureKind,
 465                            upvar_span: Span,
 466                            var_name: ast::Name) {
 467         debug!("adjust_closure_kind(closure_id={}, new_kind={:?}, upvar_span={:?}, var_name={})",
 468                closure_id, new_kind, upvar_span, var_name);
 469
 470         let closure_kind = self.adjust_closure_kinds.get(&closure_id).cloned()
 471             .or_else(|| self.fcx.tables.borrow().closure_kinds.get(&closure_id).cloned());
 472         if let Some((existing_kind, _)) = closure_kind {
 473             debug!("adjust_closure_kind: closure_id={}, existing_kind={:?}, new_kind={:?}",
 474                    closure_id, existing_kind, new_kind);
 475
 476             match (existing_kind, new_kind) {
 477                 (ty::ClosureKind::Fn, ty::ClosureKind::Fn) |
 478                 (ty::ClosureKind::FnMut, ty::ClosureKind::Fn) |
 479                 (ty::ClosureKind::FnMut, ty::ClosureKind::FnMut) |
 480                 (ty::ClosureKind::FnOnce, _) => {
 481                     // no change needed
 482                 }
 483
 484                 (ty::ClosureKind::Fn, ty::ClosureKind::FnMut) |
 485                 (ty::ClosureKind::Fn, ty::ClosureKind::FnOnce) |
 486                 (ty::ClosureKind::FnMut, ty::ClosureKind::FnOnce) => {
 487                     // new kind is stronger than the old kind
 488                     self.adjust_closure_kinds.insert(
 489                         closure_id,
 490                         (new_kind, Some((upvar_span, var_name)))
 491                     );
 492                 }
 493             }
 494         }
 495     }
 496 }
 497
 498 impl<'a, 'gcx, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'gcx, 'tcx> {
 499     fn consume(&mut self,
 500                _consume_id: ast::NodeId,
 501                _consume_span: Span,
 502                cmt: mc::cmt<'tcx>,
 503                mode: euv::ConsumeMode)
 504     {
 505         debug!("consume(cmt={:?},mode={:?})", cmt, mode);
 506         self.adjust_upvar_borrow_kind_for_consume(cmt, mode);
 507     }
 508
 509     fn matched_pat(&mut self,
 510                    _matched_pat: &hir::Pat,
 511                    _cmt: mc::cmt<'tcx>,
 512                    _mode: euv::MatchMode)
 513     {}
 514
 515     fn consume_pat(&mut self,
 516                    _consume_pat: &hir::Pat,
 517                    cmt: mc::cmt<'tcx>,
 518                    mode: euv::ConsumeMode)
 519     {
 520         debug!("consume_pat(cmt={:?},mode={:?})", cmt, mode);
 521         self.adjust_upvar_borrow_kind_for_consume(cmt, mode);
 522     }
 523
 524     fn borrow(&mut self,
 525               borrow_id: ast::NodeId,
 526               _borrow_span: Span,
 527               cmt: mc::cmt<'tcx>,
 528               _loan_region: ty::Region<'tcx>,
 529               bk: ty::BorrowKind,
 530               _loan_cause: euv::LoanCause)
 531     {
 532         debug!("borrow(borrow_id={}, cmt={:?}, bk={:?})",
 533                borrow_id, cmt, bk);
 534
 535         match bk {
 536             ty::ImmBorrow => { }
 537             ty::UniqueImmBorrow => {
 538                 self.adjust_upvar_borrow_kind_for_unique(cmt);
 539             }
 540             ty::MutBorrow => {
 541                 self.adjust_upvar_borrow_kind_for_mut(cmt);
 542             }
 543         }
 544     }
 545
 546     fn decl_without_init(&mut self,
 547                          _id: ast::NodeId,
 548                          _span: Span)
 549     {}
 550
 551     fn mutate(&mut self,
 552               _assignment_id: ast::NodeId,
 553               _assignment_span: Span,
 554               assignee_cmt: mc::cmt<'tcx>,
 555               _mode: euv::MutateMode)
 556     {
 557         debug!("mutate(assignee_cmt={:?})",
 558                assignee_cmt);
 559
 560         self.adjust_upvar_borrow_kind_for_mut(assignee_cmt);
 561     }
 562 }