1 //! ### Inferring borrow kinds for upvars
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:
9 //! ty::ImmBorrow -> ty::UniqueImmBorrow -> ty::MutBorrow
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
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.)
35 use crate::expr_use_visitor as euv;
36 use crate::mem_categorization as mc;
37 use crate::mem_categorization::PlaceBase;
38 use rustc::hir::map::Map;
39 use rustc::infer::UpvarRegion;
40 use rustc::ty::{self, Ty, TyCtxt, UpvarSubsts};
41 use rustc_data_structures::fx::FxIndexMap;
43 use rustc_hir::def_id::DefId;
44 use rustc_hir::def_id::LocalDefId;
45 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
49 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
50 pub fn closure_analyze(&self, body: &'tcx hir::Body<'tcx>) {
51 InferBorrowKindVisitor { fcx: self }.visit_body(body);
53 // it's our job to process these.
54 assert!(self.deferred_call_resolutions.borrow().is_empty());
58 struct InferBorrowKindVisitor<'a, 'tcx> {
59 fcx: &'a FnCtxt<'a, 'tcx>,
62 impl<'a, 'tcx> Visitor<'tcx> for InferBorrowKindVisitor<'a, 'tcx> {
65 fn nested_visit_map(&mut self) -> NestedVisitorMap<'_, Self::Map> {
66 NestedVisitorMap::None
69 fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
70 if let hir::ExprKind::Closure(cc, _, body_id, _, _) = expr.kind {
71 let body = self.fcx.tcx.hir().body(body_id);
72 self.visit_body(body);
73 self.fcx.analyze_closure(expr.hir_id, expr.span, body, cc);
76 intravisit::walk_expr(self, expr);
80 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
81 /// Analysis starting point.
84 closure_hir_id: hir::HirId,
87 capture_clause: hir::CaptureBy,
89 debug!("analyze_closure(id={:?}, body.id={:?})", closure_hir_id, body.id());
91 // Extract the type of the closure.
92 let ty = self.node_ty(closure_hir_id);
93 let (closure_def_id, substs) = match ty.kind {
94 ty::Closure(def_id, substs) => (def_id, UpvarSubsts::Closure(substs)),
95 ty::Generator(def_id, substs, _) => (def_id, UpvarSubsts::Generator(substs)),
97 // #51714: skip analysis when we have already encountered type errors
103 "type of closure expr {:?} is not a closure {:?}",
110 let infer_kind = if let UpvarSubsts::Closure(closure_substs) = substs {
111 self.closure_kind(closure_def_id, closure_substs).is_none().then_some(closure_substs)
116 if let Some(upvars) = self.tcx.upvars(closure_def_id) {
117 let mut upvar_list: FxIndexMap<hir::HirId, ty::UpvarId> =
118 FxIndexMap::with_capacity_and_hasher(upvars.len(), Default::default());
119 for (&var_hir_id, _) in upvars.iter() {
120 let upvar_id = ty::UpvarId {
121 var_path: ty::UpvarPath { hir_id: var_hir_id },
122 closure_expr_id: LocalDefId::from_def_id(closure_def_id),
124 debug!("seed upvar_id {:?}", upvar_id);
125 // Adding the upvar Id to the list of Upvars, which will be added
126 // to the map for the closure at the end of the for loop.
127 upvar_list.insert(var_hir_id, upvar_id);
129 let capture_kind = match capture_clause {
130 hir::CaptureBy::Value => ty::UpvarCapture::ByValue,
131 hir::CaptureBy::Ref => {
132 let origin = UpvarRegion(upvar_id, span);
133 let upvar_region = self.next_region_var(origin);
135 ty::UpvarBorrow { kind: ty::ImmBorrow, region: upvar_region };
136 ty::UpvarCapture::ByRef(upvar_borrow)
140 self.tables.borrow_mut().upvar_capture_map.insert(upvar_id, capture_kind);
142 // Add the vector of upvars to the map keyed with the closure id.
143 // This gives us an easier access to them without having to call
144 // tcx.upvars again..
145 if !upvar_list.is_empty() {
146 self.tables.borrow_mut().upvar_list.insert(closure_def_id, upvar_list);
150 let body_owner_def_id = self.tcx.hir().body_owner_def_id(body.id());
151 assert_eq!(body_owner_def_id, closure_def_id);
152 let mut delegate = InferBorrowKind {
155 current_closure_kind: ty::ClosureKind::LATTICE_BOTTOM,
156 current_origin: None,
157 adjust_upvar_captures: ty::UpvarCaptureMap::default(),
159 euv::ExprUseVisitor::new(
164 &self.tables.borrow(),
168 if let Some(closure_substs) = infer_kind {
169 // Unify the (as yet unbound) type variable in the closure
170 // substs with the kind we inferred.
171 let inferred_kind = delegate.current_closure_kind;
172 let closure_kind_ty = closure_substs.as_closure().kind_ty(closure_def_id, self.tcx);
173 self.demand_eqtype(span, inferred_kind.to_ty(self.tcx), closure_kind_ty);
175 // If we have an origin, store it.
176 if let Some(origin) = delegate.current_origin {
177 self.tables.borrow_mut().closure_kind_origins_mut().insert(closure_hir_id, origin);
181 self.tables.borrow_mut().upvar_capture_map.extend(delegate.adjust_upvar_captures);
183 // Now that we've analyzed the closure, we know how each
184 // variable is borrowed, and we know what traits the closure
185 // implements (Fn vs FnMut etc). We now have some updates to do
186 // with that information.
188 // Note that no closure type C may have an upvar of type C
189 // (though it may reference itself via a trait object). This
190 // results from the desugaring of closures to a struct like
191 // `Foo<..., UV0...UVn>`. If one of those upvars referenced
192 // C, then the type would have infinite size (and the
193 // inference algorithm will reject it).
195 // Equate the type variables for the upvars with the actual types.
196 let final_upvar_tys = self.final_upvar_tys(closure_hir_id);
198 "analyze_closure: id={:?} substs={:?} final_upvar_tys={:?}",
199 closure_hir_id, substs, final_upvar_tys
201 for (upvar_ty, final_upvar_ty) in
202 substs.upvar_tys(closure_def_id, self.tcx).zip(final_upvar_tys)
204 self.demand_suptype(span, upvar_ty, final_upvar_ty);
207 // If we are also inferred the closure kind here,
208 // process any deferred resolutions.
209 let deferred_call_resolutions = self.remove_deferred_call_resolutions(closure_def_id);
210 for deferred_call_resolution in deferred_call_resolutions {
211 deferred_call_resolution.resolve(self);
215 // Returns a list of `Ty`s for each upvar.
216 fn final_upvar_tys(&self, closure_id: hir::HirId) -> Vec<Ty<'tcx>> {
217 // Presently an unboxed closure type cannot "escape" out of a
218 // function, so we will only encounter ones that originated in the
219 // local crate or were inlined into it along with some function.
220 // This may change if abstract return types of some sort are
223 let closure_def_id = tcx.hir().local_def_id(closure_id);
225 tcx.upvars(closure_def_id)
228 upvars.iter().map(|(&var_hir_id, _)| {
229 let upvar_ty = self.node_ty(var_hir_id);
230 let upvar_id = ty::UpvarId {
231 var_path: ty::UpvarPath { hir_id: var_hir_id },
232 closure_expr_id: LocalDefId::from_def_id(closure_def_id),
234 let capture = self.tables.borrow().upvar_capture(upvar_id);
236 debug!("var_id={:?} upvar_ty={:?} capture={:?}", var_hir_id, upvar_ty, capture);
239 ty::UpvarCapture::ByValue => upvar_ty,
240 ty::UpvarCapture::ByRef(borrow) => tcx.mk_ref(
242 ty::TypeAndMut { ty: upvar_ty, mutbl: borrow.kind.to_mutbl_lossy() },
251 struct InferBorrowKind<'a, 'tcx> {
252 fcx: &'a FnCtxt<'a, 'tcx>,
254 // The def-id of the closure whose kind and upvar accesses are being inferred.
255 closure_def_id: DefId,
257 // The kind that we have inferred that the current closure
258 // requires. Note that we *always* infer a minimal kind, even if
259 // we don't always *use* that in the final result (i.e., sometimes
260 // we've taken the closure kind from the expectations instead, and
261 // for generators we don't even implement the closure traits
263 current_closure_kind: ty::ClosureKind,
265 // If we modified `current_closure_kind`, this field contains a `Some()` with the
266 // variable access that caused us to do so.
267 current_origin: Option<(Span, ast::Name)>,
269 // For each upvar that we access, we track the minimal kind of
270 // access we need (ref, ref mut, move, etc).
271 adjust_upvar_captures: ty::UpvarCaptureMap<'tcx>,
274 impl<'a, 'tcx> InferBorrowKind<'a, 'tcx> {
275 fn adjust_upvar_borrow_kind_for_consume(
277 place: &mc::Place<'tcx>,
278 mode: euv::ConsumeMode,
280 debug!("adjust_upvar_borrow_kind_for_consume(place={:?}, mode={:?})", place, mode);
282 // we only care about moves
290 let tcx = self.fcx.tcx;
291 let upvar_id = if let PlaceBase::Upvar(upvar_id) = place.base {
297 debug!("adjust_upvar_borrow_kind_for_consume: upvar={:?}", upvar_id);
299 // To move out of an upvar, this must be a FnOnce closure
300 self.adjust_closure_kind(
301 upvar_id.closure_expr_id,
302 ty::ClosureKind::FnOnce,
304 var_name(tcx, upvar_id.var_path.hir_id),
307 self.adjust_upvar_captures.insert(upvar_id, ty::UpvarCapture::ByValue);
310 /// Indicates that `place` is being directly mutated (e.g., assigned
311 /// to). If the place is based on a by-ref upvar, this implies that
312 /// the upvar must be borrowed using an `&mut` borrow.
313 fn adjust_upvar_borrow_kind_for_mut(&mut self, place: &mc::Place<'tcx>) {
314 debug!("adjust_upvar_borrow_kind_for_mut(place={:?})", place);
316 if let PlaceBase::Upvar(upvar_id) = place.base {
317 let mut borrow_kind = ty::MutBorrow;
318 for pointer_ty in place.deref_tys() {
319 match pointer_ty.kind {
320 // Raw pointers don't inherit mutability.
321 ty::RawPtr(_) => return,
322 // assignment to deref of an `&mut`
323 // borrowed pointer implies that the
324 // pointer itself must be unique, but not
325 // necessarily *mutable*
326 ty::Ref(.., hir::Mutability::Mut) => borrow_kind = ty::UniqueImmBorrow,
330 self.adjust_upvar_deref(upvar_id, place.span, borrow_kind);
334 fn adjust_upvar_borrow_kind_for_unique(&mut self, place: &mc::Place<'tcx>) {
335 debug!("adjust_upvar_borrow_kind_for_unique(place={:?})", place);
337 if let PlaceBase::Upvar(upvar_id) = place.base {
338 if place.deref_tys().any(ty::TyS::is_unsafe_ptr) {
339 // Raw pointers don't inherit mutability.
342 // for a borrowed pointer to be unique, its base must be unique
343 self.adjust_upvar_deref(upvar_id, place.span, ty::UniqueImmBorrow);
347 fn adjust_upvar_deref(
349 upvar_id: ty::UpvarId,
351 borrow_kind: ty::BorrowKind,
353 assert!(match borrow_kind {
354 ty::MutBorrow => true,
355 ty::UniqueImmBorrow => true,
357 // imm borrows never require adjusting any kinds, so we don't wind up here
358 ty::ImmBorrow => false,
361 let tcx = self.fcx.tcx;
363 // if this is an implicit deref of an
364 // upvar, then we need to modify the
365 // borrow_kind of the upvar to make sure it
366 // is inferred to mutable if necessary
367 self.adjust_upvar_borrow_kind(upvar_id, borrow_kind);
369 // also need to be in an FnMut closure since this is not an ImmBorrow
370 self.adjust_closure_kind(
371 upvar_id.closure_expr_id,
372 ty::ClosureKind::FnMut,
374 var_name(tcx, upvar_id.var_path.hir_id),
378 /// We infer the borrow_kind with which to borrow upvars in a stack closure.
379 /// The borrow_kind basically follows a lattice of `imm < unique-imm < mut`,
380 /// moving from left to right as needed (but never right to left).
381 /// Here the argument `mutbl` is the borrow_kind that is required by
382 /// some particular use.
383 fn adjust_upvar_borrow_kind(&mut self, upvar_id: ty::UpvarId, kind: ty::BorrowKind) {
384 let upvar_capture = self
385 .adjust_upvar_captures
388 .unwrap_or_else(|| self.fcx.tables.borrow().upvar_capture(upvar_id));
390 "adjust_upvar_borrow_kind(upvar_id={:?}, upvar_capture={:?}, kind={:?})",
391 upvar_id, upvar_capture, kind
394 match upvar_capture {
395 ty::UpvarCapture::ByValue => {
396 // Upvar is already by-value, the strongest criteria.
398 ty::UpvarCapture::ByRef(mut upvar_borrow) => {
399 match (upvar_borrow.kind, kind) {
401 (ty::ImmBorrow, ty::UniqueImmBorrow)
402 | (ty::ImmBorrow, ty::MutBorrow)
403 | (ty::UniqueImmBorrow, ty::MutBorrow) => {
404 upvar_borrow.kind = kind;
405 self.adjust_upvar_captures
406 .insert(upvar_id, ty::UpvarCapture::ByRef(upvar_borrow));
409 (ty::ImmBorrow, ty::ImmBorrow)
410 | (ty::UniqueImmBorrow, ty::ImmBorrow)
411 | (ty::UniqueImmBorrow, ty::UniqueImmBorrow)
412 | (ty::MutBorrow, _) => {}
418 fn adjust_closure_kind(
420 closure_id: LocalDefId,
421 new_kind: ty::ClosureKind,
426 "adjust_closure_kind(closure_id={:?}, new_kind={:?}, upvar_span={:?}, var_name={})",
427 closure_id, new_kind, upvar_span, var_name
430 // Is this the closure whose kind is currently being inferred?
431 if closure_id.to_def_id() != self.closure_def_id {
432 debug!("adjust_closure_kind: not current closure");
436 // closures start out as `Fn`.
437 let existing_kind = self.current_closure_kind;
440 "adjust_closure_kind: closure_id={:?}, existing_kind={:?}, new_kind={:?}",
441 closure_id, existing_kind, new_kind
444 match (existing_kind, new_kind) {
445 (ty::ClosureKind::Fn, ty::ClosureKind::Fn)
446 | (ty::ClosureKind::FnMut, ty::ClosureKind::Fn)
447 | (ty::ClosureKind::FnMut, ty::ClosureKind::FnMut)
448 | (ty::ClosureKind::FnOnce, _) => {
452 (ty::ClosureKind::Fn, ty::ClosureKind::FnMut)
453 | (ty::ClosureKind::Fn, ty::ClosureKind::FnOnce)
454 | (ty::ClosureKind::FnMut, ty::ClosureKind::FnOnce) => {
455 // new kind is stronger than the old kind
456 self.current_closure_kind = new_kind;
457 self.current_origin = Some((upvar_span, var_name));
463 impl<'a, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'tcx> {
464 fn consume(&mut self, place: &mc::Place<'tcx>, mode: euv::ConsumeMode) {
465 debug!("consume(place={:?},mode={:?})", place, mode);
466 self.adjust_upvar_borrow_kind_for_consume(place, mode);
469 fn borrow(&mut self, place: &mc::Place<'tcx>, bk: ty::BorrowKind) {
470 debug!("borrow(place={:?}, bk={:?})", place, bk);
474 ty::UniqueImmBorrow => {
475 self.adjust_upvar_borrow_kind_for_unique(place);
478 self.adjust_upvar_borrow_kind_for_mut(place);
483 fn mutate(&mut self, assignee_place: &mc::Place<'tcx>) {
484 debug!("mutate(assignee_place={:?})", assignee_place);
486 self.adjust_upvar_borrow_kind_for_mut(assignee_place);
490 fn var_name(tcx: TyCtxt<'_>, var_hir_id: hir::HirId) -> ast::Name {
491 tcx.hir().name(var_hir_id)