compiler/rustc_typeck/src/check/upvar.rs

   1 //! ### Inferring borrow kinds for upvars
   2 //!
   3 //! Whenever there is a closure expression, we need to determine how each
   4 //! upvar is used. We do this by initially assigning each upvar an
   5 //! immutable "borrow kind" (see `ty::BorrowKind` for details) and then
   6 //! "escalating" the kind as needed. The borrow kind proceeds according to
   7 //! the following lattice:
   8 //!
   9 //!     ty::ImmBorrow -> ty::UniqueImmBorrow -> ty::MutBorrow
  10 //!
  11 //! So, for example, if we see an assignment `x = 5` to an upvar `x`, we
  12 //! will promote its borrow kind to mutable borrow. If we see an `&mut x`
  13 //! we'll do the same. Naturally, this applies not just to the upvar, but
  14 //! to everything owned by `x`, so the result is the same for something
  15 //! like `x.f = 5` and so on (presuming `x` is not a borrowed pointer to a
  16 //! struct). These adjustments are performed in
  17 //! `adjust_upvar_borrow_kind()` (you can trace backwards through the code
  18 //! from there).
  19 //!
  20 //! The fact that we are inferring borrow kinds as we go results in a
  21 //! semi-hacky interaction with mem-categorization. In particular,
  22 //! mem-categorization will query the current borrow kind as it
  23 //! categorizes, and we'll return the *current* value, but this may get
  24 //! adjusted later. Therefore, in this module, we generally ignore the
  25 //! borrow kind (and derived mutabilities) that are returned from
  26 //! mem-categorization, since they may be inaccurate. (Another option
  27 //! would be to use a unification scheme, where instead of returning a
  28 //! concrete borrow kind like `ty::ImmBorrow`, we return a
  29 //! `ty::InferBorrow(upvar_id)` or something like that, but this would
  30 //! then mean that all later passes would have to check for these figments
  31 //! and report an error, and it just seems like more mess in the end.)
  32
  33 use super::FnCtxt;
  34
  35 use crate::expr_use_visitor as euv;
  36 use rustc_data_structures::fx::FxIndexMap;
  37 use rustc_hir as hir;
  38 use rustc_hir::def_id::DefId;
  39 use rustc_hir::def_id::LocalDefId;
  40 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
  41 use rustc_infer::infer::UpvarRegion;
  42 use rustc_middle::hir::place::{PlaceBase, PlaceWithHirId};
  43 use rustc_middle::ty::{self, Ty, TyCtxt, UpvarSubsts};
  44 use rustc_span::{Span, Symbol};
  45 use std::collections::hash_map::Entry;
  46
  47 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  48     pub fn closure_analyze(&self, body: &'tcx hir::Body<'tcx>) {
  49         InferBorrowKindVisitor { fcx: self }.visit_body(body);
  50
  51         // it's our job to process these.
  52         assert!(self.deferred_call_resolutions.borrow().is_empty());
  53     }
  54 }
  55
  56 struct InferBorrowKindVisitor<'a, 'tcx> {
  57     fcx: &'a FnCtxt<'a, 'tcx>,
  58 }
  59
  60 impl<'a, 'tcx> Visitor<'tcx> for InferBorrowKindVisitor<'a, 'tcx> {
  61     type Map = intravisit::ErasedMap<'tcx>;
  62
  63     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
  64         NestedVisitorMap::None
  65     }
  66
  67     fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
  68         if let hir::ExprKind::Closure(cc, _, body_id, _, _) = expr.kind {
  69             let body = self.fcx.tcx.hir().body(body_id);
  70             self.visit_body(body);
  71             self.fcx.analyze_closure(expr.hir_id, expr.span, body, cc);
  72         }
  73
  74         intravisit::walk_expr(self, expr);
  75     }
  76 }
  77
  78 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  79     /// Analysis starting point.
  80     fn analyze_closure(
  81         &self,
  82         closure_hir_id: hir::HirId,
  83         span: Span,
  84         body: &hir::Body<'_>,
  85         capture_clause: hir::CaptureBy,
  86     ) {
  87         debug!("analyze_closure(id={:?}, body.id={:?})", closure_hir_id, body.id());
  88
  89         // Extract the type of the closure.
  90         let ty = self.node_ty(closure_hir_id);
  91         let (closure_def_id, substs) = match *ty.kind() {
  92             ty::Closure(def_id, substs) => (def_id, UpvarSubsts::Closure(substs)),
  93             ty::Generator(def_id, substs, _) => (def_id, UpvarSubsts::Generator(substs)),
  94             ty::Error(_) => {
  95                 // #51714: skip analysis when we have already encountered type errors
  96                 return;
  97             }
  98             _ => {
  99                 span_bug!(
 100                     span,
 101                     "type of closure expr {:?} is not a closure {:?}",
 102                     closure_hir_id,
 103                     ty
 104                 );
 105             }
 106         };
 107
 108         let infer_kind = if let UpvarSubsts::Closure(closure_substs) = substs {
 109             self.closure_kind(closure_substs).is_none().then_some(closure_substs)
 110         } else {
 111             None
 112         };
 113
 114         if let Some(upvars) = self.tcx.upvars_mentioned(closure_def_id) {
 115             let mut closure_captures: FxIndexMap<hir::HirId, ty::UpvarId> =
 116                 FxIndexMap::with_capacity_and_hasher(upvars.len(), Default::default());
 117             for (&var_hir_id, _) in upvars.iter() {
 118                 let upvar_id = ty::UpvarId {
 119                     var_path: ty::UpvarPath { hir_id: var_hir_id },
 120                     closure_expr_id: closure_def_id.expect_local(),
 121                 };
 122                 debug!("seed upvar_id {:?}", upvar_id);
 123                 // Adding the upvar Id to the list of Upvars, which will be added
 124                 // to the map for the closure at the end of the for loop.
 125                 closure_captures.insert(var_hir_id, upvar_id);
 126
 127                 let capture_kind = match capture_clause {
 128                     hir::CaptureBy::Value => ty::UpvarCapture::ByValue(None),
 129                     hir::CaptureBy::Ref => {
 130                         let origin = UpvarRegion(upvar_id, span);
 131                         let upvar_region = self.next_region_var(origin);
 132                         let upvar_borrow =
 133                             ty::UpvarBorrow { kind: ty::ImmBorrow, region: upvar_region };
 134                         ty::UpvarCapture::ByRef(upvar_borrow)
 135                     }
 136                 };
 137
 138                 self.typeck_results.borrow_mut().upvar_capture_map.insert(upvar_id, capture_kind);
 139             }
 140             // Add the vector of upvars to the map keyed with the closure id.
 141             // This gives us an easier access to them without having to call
 142             // tcx.upvars again..
 143             if !closure_captures.is_empty() {
 144                 self.typeck_results
 145                     .borrow_mut()
 146                     .closure_captures
 147                     .insert(closure_def_id, closure_captures);
 148             }
 149         }
 150
 151         let body_owner_def_id = self.tcx.hir().body_owner_def_id(body.id());
 152         assert_eq!(body_owner_def_id.to_def_id(), closure_def_id);
 153         let mut delegate = InferBorrowKind {
 154             fcx: self,
 155             closure_def_id,
 156             current_closure_kind: ty::ClosureKind::LATTICE_BOTTOM,
 157             current_origin: None,
 158             adjust_upvar_captures: ty::UpvarCaptureMap::default(),
 159         };
 160         euv::ExprUseVisitor::new(
 161             &mut delegate,
 162             &self.infcx,
 163             body_owner_def_id,
 164             self.param_env,
 165             &self.typeck_results.borrow(),
 166         )
 167         .consume_body(body);
 168
 169         if let Some(closure_substs) = infer_kind {
 170             // Unify the (as yet unbound) type variable in the closure
 171             // substs with the kind we inferred.
 172             let inferred_kind = delegate.current_closure_kind;
 173             let closure_kind_ty = closure_substs.as_closure().kind_ty();
 174             self.demand_eqtype(span, inferred_kind.to_ty(self.tcx), closure_kind_ty);
 175
 176             // If we have an origin, store it.
 177             if let Some(origin) = delegate.current_origin {
 178                 self.typeck_results
 179                     .borrow_mut()
 180                     .closure_kind_origins_mut()
 181                     .insert(closure_hir_id, origin);
 182             }
 183         }
 184
 185         self.typeck_results.borrow_mut().upvar_capture_map.extend(delegate.adjust_upvar_captures);
 186
 187         // Now that we've analyzed the closure, we know how each
 188         // variable is borrowed, and we know what traits the closure
 189         // implements (Fn vs FnMut etc). We now have some updates to do
 190         // with that information.
 191         //
 192         // Note that no closure type C may have an upvar of type C
 193         // (though it may reference itself via a trait object). This
 194         // results from the desugaring of closures to a struct like
 195         // `Foo<..., UV0...UVn>`. If one of those upvars referenced
 196         // C, then the type would have infinite size (and the
 197         // inference algorithm will reject it).
 198
 199         // Equate the type variables for the upvars with the actual types.
 200         let final_upvar_tys = self.final_upvar_tys(closure_hir_id);
 201         debug!(
 202             "analyze_closure: id={:?} substs={:?} final_upvar_tys={:?}",
 203             closure_hir_id, substs, final_upvar_tys
 204         );
 205
 206         // Build a tuple (U0..Un) of the final upvar types U0..Un
 207         // and unify the upvar tupe type in the closure with it:
 208         let final_tupled_upvars_type = self.tcx.mk_tup(final_upvar_tys.iter());
 209         self.demand_suptype(span, substs.tupled_upvars_ty(), final_tupled_upvars_type);
 210
 211         // If we are also inferred the closure kind here,
 212         // process any deferred resolutions.
 213         let deferred_call_resolutions = self.remove_deferred_call_resolutions(closure_def_id);
 214         for deferred_call_resolution in deferred_call_resolutions {
 215             deferred_call_resolution.resolve(self);
 216         }
 217     }
 218
 219     // Returns a list of `Ty`s for each upvar.
 220     fn final_upvar_tys(&self, closure_id: hir::HirId) -> Vec<Ty<'tcx>> {
 221         // Presently an unboxed closure type cannot "escape" out of a
 222         // function, so we will only encounter ones that originated in the
 223         // local crate or were inlined into it along with some function.
 224         // This may change if abstract return types of some sort are
 225         // implemented.
 226         let tcx = self.tcx;
 227         let closure_def_id = tcx.hir().local_def_id(closure_id);
 228
 229         tcx.upvars_mentioned(closure_def_id)
 230             .iter()
 231             .flat_map(|upvars| {
 232                 upvars.iter().map(|(&var_hir_id, _)| {
 233                     let upvar_ty = self.node_ty(var_hir_id);
 234                     let upvar_id = ty::UpvarId {
 235                         var_path: ty::UpvarPath { hir_id: var_hir_id },
 236                         closure_expr_id: closure_def_id,
 237                     };
 238                     let capture = self.typeck_results.borrow().upvar_capture(upvar_id);
 239
 240                     debug!("var_id={:?} upvar_ty={:?} capture={:?}", var_hir_id, upvar_ty, capture);
 241
 242                     match capture {
 243                         ty::UpvarCapture::ByValue(_) => upvar_ty,
 244                         ty::UpvarCapture::ByRef(borrow) => tcx.mk_ref(
 245                             borrow.region,
 246                             ty::TypeAndMut { ty: upvar_ty, mutbl: borrow.kind.to_mutbl_lossy() },
 247                         ),
 248                     }
 249                 })
 250             })
 251             .collect()
 252     }
 253 }
 254
 255 struct InferBorrowKind<'a, 'tcx> {
 256     fcx: &'a FnCtxt<'a, 'tcx>,
 257
 258     // The def-id of the closure whose kind and upvar accesses are being inferred.
 259     closure_def_id: DefId,
 260
 261     // The kind that we have inferred that the current closure
 262     // requires. Note that we *always* infer a minimal kind, even if
 263     // we don't always *use* that in the final result (i.e., sometimes
 264     // we've taken the closure kind from the expectations instead, and
 265     // for generators we don't even implement the closure traits
 266     // really).
 267     current_closure_kind: ty::ClosureKind,
 268
 269     // If we modified `current_closure_kind`, this field contains a `Some()` with the
 270     // variable access that caused us to do so.
 271     current_origin: Option<(Span, Symbol)>,
 272
 273     // For each upvar that we access, we track the minimal kind of
 274     // access we need (ref, ref mut, move, etc).
 275     adjust_upvar_captures: ty::UpvarCaptureMap<'tcx>,
 276 }
 277
 278 impl<'a, 'tcx> InferBorrowKind<'a, 'tcx> {
 279     fn adjust_upvar_borrow_kind_for_consume(
 280         &mut self,
 281         place_with_id: &PlaceWithHirId<'tcx>,
 282         diag_expr_id: hir::HirId,
 283         mode: euv::ConsumeMode,
 284     ) {
 285         debug!(
 286             "adjust_upvar_borrow_kind_for_consume(place_with_id={:?}, diag_expr_id={:?}, mode={:?})",
 287             place_with_id, diag_expr_id, mode
 288         );
 289
 290         // we only care about moves
 291         match mode {
 292             euv::Copy => {
 293                 return;
 294             }
 295             euv::Move => {}
 296         }
 297
 298         let tcx = self.fcx.tcx;
 299         let upvar_id = if let PlaceBase::Upvar(upvar_id) = place_with_id.place.base {
 300             upvar_id
 301         } else {
 302             return;
 303         };
 304
 305         debug!("adjust_upvar_borrow_kind_for_consume: upvar={:?}", upvar_id);
 306
 307         let usage_span = tcx.hir().span(diag_expr_id);
 308
 309         // To move out of an upvar, this must be a FnOnce closure
 310         self.adjust_closure_kind(
 311             upvar_id.closure_expr_id,
 312             ty::ClosureKind::FnOnce,
 313             usage_span,
 314             var_name(tcx, upvar_id.var_path.hir_id),
 315         );
 316
 317         let new_capture = ty::UpvarCapture::ByValue(Some(usage_span));
 318         match self.adjust_upvar_captures.entry(upvar_id) {
 319             Entry::Occupied(mut e) => {
 320                 match e.get() {
 321                     // We always overwrite `ByRef`, since we require
 322                     // that the upvar be available by value.
 323                     //
 324                     // If we had a previous by-value usage without a specific
 325                     // span, use ours instead. Otherwise, keep the first span
 326                     // we encountered, since there isn't an obviously better one.
 327                     ty::UpvarCapture::ByRef(_) | ty::UpvarCapture::ByValue(None) => {
 328                         e.insert(new_capture);
 329                     }
 330                     _ => {}
 331                 }
 332             }
 333             Entry::Vacant(e) => {
 334                 e.insert(new_capture);
 335             }
 336         }
 337     }
 338
 339     /// Indicates that `place_with_id` is being directly mutated (e.g., assigned
 340     /// to). If the place is based on a by-ref upvar, this implies that
 341     /// the upvar must be borrowed using an `&mut` borrow.
 342     fn adjust_upvar_borrow_kind_for_mut(
 343         &mut self,
 344         place_with_id: &PlaceWithHirId<'tcx>,
 345         diag_expr_id: hir::HirId,
 346     ) {
 347         debug!(
 348             "adjust_upvar_borrow_kind_for_mut(place_with_id={:?}, diag_expr_id={:?})",
 349             place_with_id, diag_expr_id
 350         );
 351
 352         if let PlaceBase::Upvar(upvar_id) = place_with_id.place.base {
 353             let mut borrow_kind = ty::MutBorrow;
 354             for pointer_ty in place_with_id.place.deref_tys() {
 355                 match pointer_ty.kind() {
 356                     // Raw pointers don't inherit mutability.
 357                     ty::RawPtr(_) => return,
 358                     // assignment to deref of an `&mut`
 359                     // borrowed pointer implies that the
 360                     // pointer itself must be unique, but not
 361                     // necessarily *mutable*
 362                     ty::Ref(.., hir::Mutability::Mut) => borrow_kind = ty::UniqueImmBorrow,
 363                     _ => (),
 364                 }
 365             }
 366             self.adjust_upvar_deref(upvar_id, self.fcx.tcx.hir().span(diag_expr_id), borrow_kind);
 367         }
 368     }
 369
 370     fn adjust_upvar_borrow_kind_for_unique(
 371         &mut self,
 372         place_with_id: &PlaceWithHirId<'tcx>,
 373         diag_expr_id: hir::HirId,
 374     ) {
 375         debug!(
 376             "adjust_upvar_borrow_kind_for_unique(place_with_id={:?}, diag_expr_id={:?})",
 377             place_with_id, diag_expr_id
 378         );
 379
 380         if let PlaceBase::Upvar(upvar_id) = place_with_id.place.base {
 381             if place_with_id.place.deref_tys().any(ty::TyS::is_unsafe_ptr) {
 382                 // Raw pointers don't inherit mutability.
 383                 return;
 384             }
 385             // for a borrowed pointer to be unique, its base must be unique
 386             self.adjust_upvar_deref(
 387                 upvar_id,
 388                 self.fcx.tcx.hir().span(diag_expr_id),
 389                 ty::UniqueImmBorrow,
 390             );
 391         }
 392     }
 393
 394     fn adjust_upvar_deref(
 395         &mut self,
 396         upvar_id: ty::UpvarId,
 397         place_span: Span,
 398         borrow_kind: ty::BorrowKind,
 399     ) {
 400         assert!(match borrow_kind {
 401             ty::MutBorrow => true,
 402             ty::UniqueImmBorrow => true,
 403
 404             // imm borrows never require adjusting any kinds, so we don't wind up here
 405             ty::ImmBorrow => false,
 406         });
 407
 408         let tcx = self.fcx.tcx;
 409
 410         // if this is an implicit deref of an
 411         // upvar, then we need to modify the
 412         // borrow_kind of the upvar to make sure it
 413         // is inferred to mutable if necessary
 414         self.adjust_upvar_borrow_kind(upvar_id, borrow_kind);
 415
 416         // also need to be in an FnMut closure since this is not an ImmBorrow
 417         self.adjust_closure_kind(
 418             upvar_id.closure_expr_id,
 419             ty::ClosureKind::FnMut,
 420             place_span,
 421             var_name(tcx, upvar_id.var_path.hir_id),
 422         );
 423     }
 424
 425     /// We infer the borrow_kind with which to borrow upvars in a stack closure.
 426     /// The borrow_kind basically follows a lattice of `imm < unique-imm < mut`,
 427     /// moving from left to right as needed (but never right to left).
 428     /// Here the argument `mutbl` is the borrow_kind that is required by
 429     /// some particular use.
 430     fn adjust_upvar_borrow_kind(&mut self, upvar_id: ty::UpvarId, kind: ty::BorrowKind) {
 431         let upvar_capture = self
 432             .adjust_upvar_captures
 433             .get(&upvar_id)
 434             .copied()
 435             .unwrap_or_else(|| self.fcx.typeck_results.borrow().upvar_capture(upvar_id));
 436         debug!(
 437             "adjust_upvar_borrow_kind(upvar_id={:?}, upvar_capture={:?}, kind={:?})",
 438             upvar_id, upvar_capture, kind
 439         );
 440
 441         match upvar_capture {
 442             ty::UpvarCapture::ByValue(_) => {
 443                 // Upvar is already by-value, the strongest criteria.
 444             }
 445             ty::UpvarCapture::ByRef(mut upvar_borrow) => {
 446                 match (upvar_borrow.kind, kind) {
 447                     // Take RHS:
 448                     (ty::ImmBorrow, ty::UniqueImmBorrow | ty::MutBorrow)
 449                     | (ty::UniqueImmBorrow, ty::MutBorrow) => {
 450                         upvar_borrow.kind = kind;
 451                         self.adjust_upvar_captures
 452                             .insert(upvar_id, ty::UpvarCapture::ByRef(upvar_borrow));
 453                     }
 454                     // Take LHS:
 455                     (ty::ImmBorrow, ty::ImmBorrow)
 456                     | (ty::UniqueImmBorrow, ty::ImmBorrow | ty::UniqueImmBorrow)
 457                     | (ty::MutBorrow, _) => {}
 458                 }
 459             }
 460         }
 461     }
 462
 463     fn adjust_closure_kind(
 464         &mut self,
 465         closure_id: LocalDefId,
 466         new_kind: ty::ClosureKind,
 467         upvar_span: Span,
 468         var_name: Symbol,
 469     ) {
 470         debug!(
 471             "adjust_closure_kind(closure_id={:?}, new_kind={:?}, upvar_span={:?}, var_name={})",
 472             closure_id, new_kind, upvar_span, var_name
 473         );
 474
 475         // Is this the closure whose kind is currently being inferred?
 476         if closure_id.to_def_id() != self.closure_def_id {
 477             debug!("adjust_closure_kind: not current closure");
 478             return;
 479         }
 480
 481         // closures start out as `Fn`.
 482         let existing_kind = self.current_closure_kind;
 483
 484         debug!(
 485             "adjust_closure_kind: closure_id={:?}, existing_kind={:?}, new_kind={:?}",
 486             closure_id, existing_kind, new_kind
 487         );
 488
 489         match (existing_kind, new_kind) {
 490             (ty::ClosureKind::Fn, ty::ClosureKind::Fn)
 491             | (ty::ClosureKind::FnMut, ty::ClosureKind::Fn | ty::ClosureKind::FnMut)
 492             | (ty::ClosureKind::FnOnce, _) => {
 493                 // no change needed
 494             }
 495
 496             (ty::ClosureKind::Fn, ty::ClosureKind::FnMut | ty::ClosureKind::FnOnce)
 497             | (ty::ClosureKind::FnMut, ty::ClosureKind::FnOnce) => {
 498                 // new kind is stronger than the old kind
 499                 self.current_closure_kind = new_kind;
 500                 self.current_origin = Some((upvar_span, var_name));
 501             }
 502         }
 503     }
 504 }
 505
 506 impl<'a, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'tcx> {
 507     fn consume(
 508         &mut self,
 509         place_with_id: &PlaceWithHirId<'tcx>,
 510         diag_expr_id: hir::HirId,
 511         mode: euv::ConsumeMode,
 512     ) {
 513         debug!(
 514             "consume(place_with_id={:?}, diag_expr_id={:?}, mode={:?})",
 515             place_with_id, diag_expr_id, mode
 516         );
 517         self.adjust_upvar_borrow_kind_for_consume(&place_with_id, diag_expr_id, mode);
 518     }
 519
 520     fn borrow(
 521         &mut self,
 522         place_with_id: &PlaceWithHirId<'tcx>,
 523         diag_expr_id: hir::HirId,
 524         bk: ty::BorrowKind,
 525     ) {
 526         debug!(
 527             "borrow(place_with_id={:?}, diag_expr_id={:?}, bk={:?})",
 528             place_with_id, diag_expr_id, bk
 529         );
 530
 531         match bk {
 532             ty::ImmBorrow => {}
 533             ty::UniqueImmBorrow => {
 534                 self.adjust_upvar_borrow_kind_for_unique(&place_with_id, diag_expr_id);
 535             }
 536             ty::MutBorrow => {
 537                 self.adjust_upvar_borrow_kind_for_mut(&place_with_id, diag_expr_id);
 538             }
 539         }
 540     }
 541
 542     fn mutate(&mut self, assignee_place: &PlaceWithHirId<'tcx>, diag_expr_id: hir::HirId) {
 543         debug!("mutate(assignee_place={:?}, diag_expr_id={:?})", assignee_place, diag_expr_id);
 544         self.adjust_upvar_borrow_kind_for_mut(assignee_place, diag_expr_id);
 545     }
 546 }
 547
 548 fn var_name(tcx: TyCtxt<'_>, var_hir_id: hir::HirId) -> Symbol {
 549     tcx.hir().name(var_hir_id)
 550 }