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 rustc_data_structures::fx::FxIndexMap;
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};
46 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
47 pub fn closure_analyze(&self, body: &'tcx hir::Body<'tcx>) {
48 InferBorrowKindVisitor { fcx: self }.visit_body(body);
50 // it's our job to process these.
51 assert!(self.deferred_call_resolutions.borrow().is_empty());
55 struct InferBorrowKindVisitor<'a, 'tcx> {
56 fcx: &'a FnCtxt<'a, 'tcx>,
59 impl<'a, 'tcx> Visitor<'tcx> for InferBorrowKindVisitor<'a, 'tcx> {
60 type Map = intravisit::ErasedMap<'tcx>;
62 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
63 NestedVisitorMap::None
66 fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
67 if let hir::ExprKind::Closure(cc, _, body_id, _, _) = expr.kind {
68 let body = self.fcx.tcx.hir().body(body_id);
69 self.visit_body(body);
70 self.fcx.analyze_closure(expr.hir_id, expr.span, body, cc);
73 intravisit::walk_expr(self, expr);
77 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
78 /// Analysis starting point.
81 closure_hir_id: hir::HirId,
84 capture_clause: hir::CaptureBy,
86 debug!("analyze_closure(id={:?}, body.id={:?})", closure_hir_id, body.id());
88 // Extract the type of the closure.
89 let ty = self.node_ty(closure_hir_id);
90 let (closure_def_id, substs) = match ty.kind {
91 ty::Closure(def_id, substs) => (def_id, UpvarSubsts::Closure(substs)),
92 ty::Generator(def_id, substs, _) => (def_id, UpvarSubsts::Generator(substs)),
94 // #51714: skip analysis when we have already encountered type errors
100 "type of closure expr {:?} is not a closure {:?}",
107 let infer_kind = if let UpvarSubsts::Closure(closure_substs) = substs {
108 self.closure_kind(closure_substs).is_none().then_some(closure_substs)
113 if let Some(upvars) = self.tcx.upvars_mentioned(closure_def_id) {
114 let mut closure_captures: FxIndexMap<hir::HirId, ty::UpvarId> =
115 FxIndexMap::with_capacity_and_hasher(upvars.len(), Default::default());
116 for (&var_hir_id, _) in upvars.iter() {
117 let upvar_id = ty::UpvarId {
118 var_path: ty::UpvarPath { hir_id: var_hir_id },
119 closure_expr_id: closure_def_id.expect_local(),
121 debug!("seed upvar_id {:?}", upvar_id);
122 // Adding the upvar Id to the list of Upvars, which will be added
123 // to the map for the closure at the end of the for loop.
124 closure_captures.insert(var_hir_id, upvar_id);
126 let capture_kind = match capture_clause {
127 hir::CaptureBy::Value => ty::UpvarCapture::ByValue,
128 hir::CaptureBy::Ref => {
129 let origin = UpvarRegion(upvar_id, span);
130 let upvar_region = self.next_region_var(origin);
132 ty::UpvarBorrow { kind: ty::ImmBorrow, region: upvar_region };
133 ty::UpvarCapture::ByRef(upvar_borrow)
137 self.tables.borrow_mut().upvar_capture_map.insert(upvar_id, capture_kind);
139 // Add the vector of upvars to the map keyed with the closure id.
140 // This gives us an easier access to them without having to call
141 // tcx.upvars again..
142 if !closure_captures.is_empty() {
143 self.tables.borrow_mut().closure_captures.insert(closure_def_id, closure_captures);
147 let body_owner_def_id = self.tcx.hir().body_owner_def_id(body.id());
148 assert_eq!(body_owner_def_id.to_def_id(), closure_def_id);
149 let mut delegate = InferBorrowKind {
152 current_closure_kind: ty::ClosureKind::LATTICE_BOTTOM,
153 current_origin: None,
154 adjust_upvar_captures: ty::UpvarCaptureMap::default(),
156 euv::ExprUseVisitor::new(
161 &self.tables.borrow(),
165 if let Some(closure_substs) = infer_kind {
166 // Unify the (as yet unbound) type variable in the closure
167 // substs with the kind we inferred.
168 let inferred_kind = delegate.current_closure_kind;
169 let closure_kind_ty = closure_substs.as_closure().kind_ty();
170 self.demand_eqtype(span, inferred_kind.to_ty(self.tcx), closure_kind_ty);
172 // If we have an origin, store it.
173 if let Some(origin) = delegate.current_origin {
174 self.tables.borrow_mut().closure_kind_origins_mut().insert(closure_hir_id, origin);
178 self.tables.borrow_mut().upvar_capture_map.extend(delegate.adjust_upvar_captures);
180 // Now that we've analyzed the closure, we know how each
181 // variable is borrowed, and we know what traits the closure
182 // implements (Fn vs FnMut etc). We now have some updates to do
183 // with that information.
185 // Note that no closure type C may have an upvar of type C
186 // (though it may reference itself via a trait object). This
187 // results from the desugaring of closures to a struct like
188 // `Foo<..., UV0...UVn>`. If one of those upvars referenced
189 // C, then the type would have infinite size (and the
190 // inference algorithm will reject it).
192 // Equate the type variables for the upvars with the actual types.
193 let final_upvar_tys = self.final_upvar_tys(closure_hir_id);
195 "analyze_closure: id={:?} substs={:?} final_upvar_tys={:?}",
196 closure_hir_id, substs, final_upvar_tys
198 for (upvar_ty, final_upvar_ty) in substs.upvar_tys().zip(final_upvar_tys) {
199 self.demand_suptype(span, upvar_ty, final_upvar_ty);
202 // If we are also inferred the closure kind here,
203 // process any deferred resolutions.
204 let deferred_call_resolutions = self.remove_deferred_call_resolutions(closure_def_id);
205 for deferred_call_resolution in deferred_call_resolutions {
206 deferred_call_resolution.resolve(self);
210 // Returns a list of `Ty`s for each upvar.
211 fn final_upvar_tys(&self, closure_id: hir::HirId) -> Vec<Ty<'tcx>> {
212 // Presently an unboxed closure type cannot "escape" out of a
213 // function, so we will only encounter ones that originated in the
214 // local crate or were inlined into it along with some function.
215 // This may change if abstract return types of some sort are
218 let closure_def_id = tcx.hir().local_def_id(closure_id);
220 tcx.upvars_mentioned(closure_def_id)
223 upvars.iter().map(|(&var_hir_id, _)| {
224 let upvar_ty = self.node_ty(var_hir_id);
225 let upvar_id = ty::UpvarId {
226 var_path: ty::UpvarPath { hir_id: var_hir_id },
227 closure_expr_id: closure_def_id,
229 let capture = self.tables.borrow().upvar_capture(upvar_id);
231 debug!("var_id={:?} upvar_ty={:?} capture={:?}", var_hir_id, upvar_ty, capture);
234 ty::UpvarCapture::ByValue => upvar_ty,
235 ty::UpvarCapture::ByRef(borrow) => tcx.mk_ref(
237 ty::TypeAndMut { ty: upvar_ty, mutbl: borrow.kind.to_mutbl_lossy() },
246 struct InferBorrowKind<'a, 'tcx> {
247 fcx: &'a FnCtxt<'a, 'tcx>,
249 // The def-id of the closure whose kind and upvar accesses are being inferred.
250 closure_def_id: DefId,
252 // The kind that we have inferred that the current closure
253 // requires. Note that we *always* infer a minimal kind, even if
254 // we don't always *use* that in the final result (i.e., sometimes
255 // we've taken the closure kind from the expectations instead, and
256 // for generators we don't even implement the closure traits
258 current_closure_kind: ty::ClosureKind,
260 // If we modified `current_closure_kind`, this field contains a `Some()` with the
261 // variable access that caused us to do so.
262 current_origin: Option<(Span, Symbol)>,
264 // For each upvar that we access, we track the minimal kind of
265 // access we need (ref, ref mut, move, etc).
266 adjust_upvar_captures: ty::UpvarCaptureMap<'tcx>,
269 impl<'a, 'tcx> InferBorrowKind<'a, 'tcx> {
270 fn adjust_upvar_borrow_kind_for_consume(
272 place_with_id: &PlaceWithHirId<'tcx>,
273 mode: euv::ConsumeMode,
276 "adjust_upvar_borrow_kind_for_consume(place_with_id={:?}, mode={:?})",
280 // we only care about moves
288 let tcx = self.fcx.tcx;
289 let upvar_id = if let PlaceBase::Upvar(upvar_id) = place_with_id.place.base {
295 debug!("adjust_upvar_borrow_kind_for_consume: upvar={:?}", upvar_id);
297 // To move out of an upvar, this must be a FnOnce closure
298 self.adjust_closure_kind(
299 upvar_id.closure_expr_id,
300 ty::ClosureKind::FnOnce,
301 tcx.hir().span(place_with_id.hir_id),
302 var_name(tcx, upvar_id.var_path.hir_id),
305 self.adjust_upvar_captures.insert(upvar_id, ty::UpvarCapture::ByValue);
308 /// Indicates that `place_with_id` is being directly mutated (e.g., assigned
309 /// to). If the place is based on a by-ref upvar, this implies that
310 /// the upvar must be borrowed using an `&mut` borrow.
311 fn adjust_upvar_borrow_kind_for_mut(&mut self, place_with_id: &PlaceWithHirId<'tcx>) {
312 debug!("adjust_upvar_borrow_kind_for_mut(place_with_id={:?})", place_with_id);
314 if let PlaceBase::Upvar(upvar_id) = place_with_id.place.base {
315 let mut borrow_kind = ty::MutBorrow;
316 for pointer_ty in place_with_id.place.deref_tys() {
317 match pointer_ty.kind {
318 // Raw pointers don't inherit mutability.
319 ty::RawPtr(_) => return,
320 // assignment to deref of an `&mut`
321 // borrowed pointer implies that the
322 // pointer itself must be unique, but not
323 // necessarily *mutable*
324 ty::Ref(.., hir::Mutability::Mut) => borrow_kind = ty::UniqueImmBorrow,
328 self.adjust_upvar_deref(
330 self.fcx.tcx.hir().span(place_with_id.hir_id),
336 fn adjust_upvar_borrow_kind_for_unique(&mut self, place_with_id: &PlaceWithHirId<'tcx>) {
337 debug!("adjust_upvar_borrow_kind_for_unique(place_with_id={:?})", place_with_id);
339 if let PlaceBase::Upvar(upvar_id) = place_with_id.place.base {
340 if place_with_id.place.deref_tys().any(ty::TyS::is_unsafe_ptr) {
341 // Raw pointers don't inherit mutability.
344 // for a borrowed pointer to be unique, its base must be unique
345 self.adjust_upvar_deref(
347 self.fcx.tcx.hir().span(place_with_id.hir_id),
353 fn adjust_upvar_deref(
355 upvar_id: ty::UpvarId,
357 borrow_kind: ty::BorrowKind,
359 assert!(match borrow_kind {
360 ty::MutBorrow => true,
361 ty::UniqueImmBorrow => true,
363 // imm borrows never require adjusting any kinds, so we don't wind up here
364 ty::ImmBorrow => false,
367 let tcx = self.fcx.tcx;
369 // if this is an implicit deref of an
370 // upvar, then we need to modify the
371 // borrow_kind of the upvar to make sure it
372 // is inferred to mutable if necessary
373 self.adjust_upvar_borrow_kind(upvar_id, borrow_kind);
375 // also need to be in an FnMut closure since this is not an ImmBorrow
376 self.adjust_closure_kind(
377 upvar_id.closure_expr_id,
378 ty::ClosureKind::FnMut,
380 var_name(tcx, upvar_id.var_path.hir_id),
384 /// We infer the borrow_kind with which to borrow upvars in a stack closure.
385 /// The borrow_kind basically follows a lattice of `imm < unique-imm < mut`,
386 /// moving from left to right as needed (but never right to left).
387 /// Here the argument `mutbl` is the borrow_kind that is required by
388 /// some particular use.
389 fn adjust_upvar_borrow_kind(&mut self, upvar_id: ty::UpvarId, kind: ty::BorrowKind) {
390 let upvar_capture = self
391 .adjust_upvar_captures
394 .unwrap_or_else(|| self.fcx.tables.borrow().upvar_capture(upvar_id));
396 "adjust_upvar_borrow_kind(upvar_id={:?}, upvar_capture={:?}, kind={:?})",
397 upvar_id, upvar_capture, kind
400 match upvar_capture {
401 ty::UpvarCapture::ByValue => {
402 // Upvar is already by-value, the strongest criteria.
404 ty::UpvarCapture::ByRef(mut upvar_borrow) => {
405 match (upvar_borrow.kind, kind) {
407 (ty::ImmBorrow, ty::UniqueImmBorrow | ty::MutBorrow)
408 | (ty::UniqueImmBorrow, ty::MutBorrow) => {
409 upvar_borrow.kind = kind;
410 self.adjust_upvar_captures
411 .insert(upvar_id, ty::UpvarCapture::ByRef(upvar_borrow));
414 (ty::ImmBorrow, ty::ImmBorrow)
415 | (ty::UniqueImmBorrow, ty::ImmBorrow | ty::UniqueImmBorrow)
416 | (ty::MutBorrow, _) => {}
422 fn adjust_closure_kind(
424 closure_id: LocalDefId,
425 new_kind: ty::ClosureKind,
430 "adjust_closure_kind(closure_id={:?}, new_kind={:?}, upvar_span={:?}, var_name={})",
431 closure_id, new_kind, upvar_span, var_name
434 // Is this the closure whose kind is currently being inferred?
435 if closure_id.to_def_id() != self.closure_def_id {
436 debug!("adjust_closure_kind: not current closure");
440 // closures start out as `Fn`.
441 let existing_kind = self.current_closure_kind;
444 "adjust_closure_kind: closure_id={:?}, existing_kind={:?}, new_kind={:?}",
445 closure_id, existing_kind, new_kind
448 match (existing_kind, new_kind) {
449 (ty::ClosureKind::Fn, ty::ClosureKind::Fn)
450 | (ty::ClosureKind::FnMut, ty::ClosureKind::Fn | ty::ClosureKind::FnMut)
451 | (ty::ClosureKind::FnOnce, _) => {
455 (ty::ClosureKind::Fn, ty::ClosureKind::FnMut | ty::ClosureKind::FnOnce)
456 | (ty::ClosureKind::FnMut, ty::ClosureKind::FnOnce) => {
457 // new kind is stronger than the old kind
458 self.current_closure_kind = new_kind;
459 self.current_origin = Some((upvar_span, var_name));
465 impl<'a, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'tcx> {
466 fn consume(&mut self, place_with_id: &PlaceWithHirId<'tcx>, mode: euv::ConsumeMode) {
467 debug!("consume(place_with_id={:?},mode={:?})", place_with_id, mode);
468 self.adjust_upvar_borrow_kind_for_consume(place_with_id, mode);
471 fn borrow(&mut self, place_with_id: &PlaceWithHirId<'tcx>, bk: ty::BorrowKind) {
472 debug!("borrow(place_with_id={:?}, bk={:?})", place_with_id, bk);
476 ty::UniqueImmBorrow => {
477 self.adjust_upvar_borrow_kind_for_unique(place_with_id);
480 self.adjust_upvar_borrow_kind_for_mut(place_with_id);
485 fn mutate(&mut self, assignee_place: &PlaceWithHirId<'tcx>) {
486 debug!("mutate(assignee_place={:?})", assignee_place);
488 self.adjust_upvar_borrow_kind_for_mut(assignee_place);
492 fn var_name(tcx: TyCtxt<'_>, var_hir_id: hir::HirId) -> Symbol {
493 tcx.hir().name(var_hir_id)