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_data_structures::fx::FxIndexMap;
40 use rustc_hir::def_id::DefId;
41 use rustc_hir::def_id::LocalDefId;
42 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
43 use rustc_infer::infer::UpvarRegion;
44 use rustc_middle::ty::{self, Ty, TyCtxt, UpvarSubsts};
45 use rustc_span::{Span, Symbol};
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);
51 // it's our job to process these.
52 assert!(self.deferred_call_resolutions.borrow().is_empty());
56 struct InferBorrowKindVisitor<'a, 'tcx> {
57 fcx: &'a FnCtxt<'a, 'tcx>,
60 impl<'a, 'tcx> Visitor<'tcx> for InferBorrowKindVisitor<'a, 'tcx> {
61 type Map = intravisit::ErasedMap<'tcx>;
63 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
64 NestedVisitorMap::None
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);
74 intravisit::walk_expr(self, expr);
78 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
79 /// Analysis starting point.
82 closure_hir_id: hir::HirId,
85 capture_clause: hir::CaptureBy,
87 debug!("analyze_closure(id={:?}, body.id={:?})", closure_hir_id, body.id());
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)),
95 // #51714: skip analysis when we have already encountered type errors
101 "type of closure expr {:?} is not a closure {:?}",
108 let infer_kind = if let UpvarSubsts::Closure(closure_substs) = substs {
109 self.closure_kind(closure_substs).is_none().then_some(closure_substs)
114 if let Some(upvars) = self.tcx.upvars_mentioned(closure_def_id) {
115 let mut upvar_list: 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(),
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 upvar_list.insert(var_hir_id, upvar_id);
127 let capture_kind = match capture_clause {
128 hir::CaptureBy::Value => ty::UpvarCapture::ByValue,
129 hir::CaptureBy::Ref => {
130 let origin = UpvarRegion(upvar_id, span);
131 let upvar_region = self.next_region_var(origin);
133 ty::UpvarBorrow { kind: ty::ImmBorrow, region: upvar_region };
134 ty::UpvarCapture::ByRef(upvar_borrow)
138 self.tables.borrow_mut().upvar_capture_map.insert(upvar_id, capture_kind);
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 !upvar_list.is_empty() {
144 self.tables.borrow_mut().upvar_list.insert(closure_def_id, upvar_list);
148 let body_owner_def_id = self.tcx.hir().body_owner_def_id(body.id());
149 assert_eq!(body_owner_def_id.to_def_id(), closure_def_id);
150 let mut delegate = InferBorrowKind {
153 current_closure_kind: ty::ClosureKind::LATTICE_BOTTOM,
154 current_origin: None,
155 adjust_upvar_captures: ty::UpvarCaptureMap::default(),
157 euv::ExprUseVisitor::new(
162 &self.tables.borrow(),
166 if let Some(closure_substs) = infer_kind {
167 // Unify the (as yet unbound) type variable in the closure
168 // substs with the kind we inferred.
169 let inferred_kind = delegate.current_closure_kind;
170 let closure_kind_ty = closure_substs.as_closure().kind_ty();
171 self.demand_eqtype(span, inferred_kind.to_ty(self.tcx), closure_kind_ty);
173 // If we have an origin, store it.
174 if let Some(origin) = delegate.current_origin {
175 self.tables.borrow_mut().closure_kind_origins_mut().insert(closure_hir_id, origin);
179 self.tables.borrow_mut().upvar_capture_map.extend(delegate.adjust_upvar_captures);
181 // Now that we've analyzed the closure, we know how each
182 // variable is borrowed, and we know what traits the closure
183 // implements (Fn vs FnMut etc). We now have some updates to do
184 // with that information.
186 // Note that no closure type C may have an upvar of type C
187 // (though it may reference itself via a trait object). This
188 // results from the desugaring of closures to a struct like
189 // `Foo<..., UV0...UVn>`. If one of those upvars referenced
190 // C, then the type would have infinite size (and the
191 // inference algorithm will reject it).
193 // Equate the type variables for the upvars with the actual types.
194 let final_upvar_tys = self.final_upvar_tys(closure_hir_id);
196 "analyze_closure: id={:?} substs={:?} final_upvar_tys={:?}",
197 closure_hir_id, substs, final_upvar_tys
199 for (upvar_ty, final_upvar_ty) in substs.upvar_tys().zip(final_upvar_tys) {
200 self.demand_suptype(span, upvar_ty, final_upvar_ty);
203 // If we are also inferred the closure kind here,
204 // process any deferred resolutions.
205 let deferred_call_resolutions = self.remove_deferred_call_resolutions(closure_def_id);
206 for deferred_call_resolution in deferred_call_resolutions {
207 deferred_call_resolution.resolve(self);
211 // Returns a list of `Ty`s for each upvar.
212 fn final_upvar_tys(&self, closure_id: hir::HirId) -> Vec<Ty<'tcx>> {
213 // Presently an unboxed closure type cannot "escape" out of a
214 // function, so we will only encounter ones that originated in the
215 // local crate or were inlined into it along with some function.
216 // This may change if abstract return types of some sort are
219 let closure_def_id = tcx.hir().local_def_id(closure_id);
221 tcx.upvars_mentioned(closure_def_id)
224 upvars.iter().map(|(&var_hir_id, _)| {
225 let upvar_ty = self.node_ty(var_hir_id);
226 let upvar_id = ty::UpvarId {
227 var_path: ty::UpvarPath { hir_id: var_hir_id },
228 closure_expr_id: closure_def_id,
230 let capture = self.tables.borrow().upvar_capture(upvar_id);
232 debug!("var_id={:?} upvar_ty={:?} capture={:?}", var_hir_id, upvar_ty, capture);
235 ty::UpvarCapture::ByValue => upvar_ty,
236 ty::UpvarCapture::ByRef(borrow) => tcx.mk_ref(
238 ty::TypeAndMut { ty: upvar_ty, mutbl: borrow.kind.to_mutbl_lossy() },
247 struct InferBorrowKind<'a, 'tcx> {
248 fcx: &'a FnCtxt<'a, 'tcx>,
250 // The def-id of the closure whose kind and upvar accesses are being inferred.
251 closure_def_id: DefId,
253 // The kind that we have inferred that the current closure
254 // requires. Note that we *always* infer a minimal kind, even if
255 // we don't always *use* that in the final result (i.e., sometimes
256 // we've taken the closure kind from the expectations instead, and
257 // for generators we don't even implement the closure traits
259 current_closure_kind: ty::ClosureKind,
261 // If we modified `current_closure_kind`, this field contains a `Some()` with the
262 // variable access that caused us to do so.
263 current_origin: Option<(Span, Symbol)>,
265 // For each upvar that we access, we track the minimal kind of
266 // access we need (ref, ref mut, move, etc).
267 adjust_upvar_captures: ty::UpvarCaptureMap<'tcx>,
270 impl<'a, 'tcx> InferBorrowKind<'a, 'tcx> {
271 fn adjust_upvar_borrow_kind_for_consume(
273 place: &mc::Place<'tcx>,
274 mode: euv::ConsumeMode,
276 debug!("adjust_upvar_borrow_kind_for_consume(place={:?}, mode={:?})", place, mode);
278 // we only care about moves
286 let tcx = self.fcx.tcx;
287 let upvar_id = if let PlaceBase::Upvar(upvar_id) = place.base {
293 debug!("adjust_upvar_borrow_kind_for_consume: upvar={:?}", upvar_id);
295 // To move out of an upvar, this must be a FnOnce closure
296 self.adjust_closure_kind(
297 upvar_id.closure_expr_id,
298 ty::ClosureKind::FnOnce,
300 var_name(tcx, upvar_id.var_path.hir_id),
303 self.adjust_upvar_captures.insert(upvar_id, ty::UpvarCapture::ByValue);
306 /// Indicates that `place` is being directly mutated (e.g., assigned
307 /// to). If the place is based on a by-ref upvar, this implies that
308 /// the upvar must be borrowed using an `&mut` borrow.
309 fn adjust_upvar_borrow_kind_for_mut(&mut self, place: &mc::Place<'tcx>) {
310 debug!("adjust_upvar_borrow_kind_for_mut(place={:?})", place);
312 if let PlaceBase::Upvar(upvar_id) = place.base {
313 let mut borrow_kind = ty::MutBorrow;
314 for pointer_ty in place.deref_tys() {
315 match pointer_ty.kind {
316 // Raw pointers don't inherit mutability.
317 ty::RawPtr(_) => return,
318 // assignment to deref of an `&mut`
319 // borrowed pointer implies that the
320 // pointer itself must be unique, but not
321 // necessarily *mutable*
322 ty::Ref(.., hir::Mutability::Mut) => borrow_kind = ty::UniqueImmBorrow,
326 self.adjust_upvar_deref(upvar_id, place.span, borrow_kind);
330 fn adjust_upvar_borrow_kind_for_unique(&mut self, place: &mc::Place<'tcx>) {
331 debug!("adjust_upvar_borrow_kind_for_unique(place={:?})", place);
333 if let PlaceBase::Upvar(upvar_id) = place.base {
334 if place.deref_tys().any(ty::TyS::is_unsafe_ptr) {
335 // Raw pointers don't inherit mutability.
338 // for a borrowed pointer to be unique, its base must be unique
339 self.adjust_upvar_deref(upvar_id, place.span, ty::UniqueImmBorrow);
343 fn adjust_upvar_deref(
345 upvar_id: ty::UpvarId,
347 borrow_kind: ty::BorrowKind,
349 assert!(match borrow_kind {
350 ty::MutBorrow => true,
351 ty::UniqueImmBorrow => true,
353 // imm borrows never require adjusting any kinds, so we don't wind up here
354 ty::ImmBorrow => false,
357 let tcx = self.fcx.tcx;
359 // if this is an implicit deref of an
360 // upvar, then we need to modify the
361 // borrow_kind of the upvar to make sure it
362 // is inferred to mutable if necessary
363 self.adjust_upvar_borrow_kind(upvar_id, borrow_kind);
365 // also need to be in an FnMut closure since this is not an ImmBorrow
366 self.adjust_closure_kind(
367 upvar_id.closure_expr_id,
368 ty::ClosureKind::FnMut,
370 var_name(tcx, upvar_id.var_path.hir_id),
374 /// We infer the borrow_kind with which to borrow upvars in a stack closure.
375 /// The borrow_kind basically follows a lattice of `imm < unique-imm < mut`,
376 /// moving from left to right as needed (but never right to left).
377 /// Here the argument `mutbl` is the borrow_kind that is required by
378 /// some particular use.
379 fn adjust_upvar_borrow_kind(&mut self, upvar_id: ty::UpvarId, kind: ty::BorrowKind) {
380 let upvar_capture = self
381 .adjust_upvar_captures
384 .unwrap_or_else(|| self.fcx.tables.borrow().upvar_capture(upvar_id));
386 "adjust_upvar_borrow_kind(upvar_id={:?}, upvar_capture={:?}, kind={:?})",
387 upvar_id, upvar_capture, kind
390 match upvar_capture {
391 ty::UpvarCapture::ByValue => {
392 // Upvar is already by-value, the strongest criteria.
394 ty::UpvarCapture::ByRef(mut upvar_borrow) => {
395 match (upvar_borrow.kind, kind) {
397 (ty::ImmBorrow, ty::UniqueImmBorrow | ty::MutBorrow)
398 | (ty::UniqueImmBorrow, ty::MutBorrow) => {
399 upvar_borrow.kind = kind;
400 self.adjust_upvar_captures
401 .insert(upvar_id, ty::UpvarCapture::ByRef(upvar_borrow));
404 (ty::ImmBorrow, ty::ImmBorrow)
405 | (ty::UniqueImmBorrow, ty::ImmBorrow | ty::UniqueImmBorrow)
406 | (ty::MutBorrow, _) => {}
412 fn adjust_closure_kind(
414 closure_id: LocalDefId,
415 new_kind: ty::ClosureKind,
420 "adjust_closure_kind(closure_id={:?}, new_kind={:?}, upvar_span={:?}, var_name={})",
421 closure_id, new_kind, upvar_span, var_name
424 // Is this the closure whose kind is currently being inferred?
425 if closure_id.to_def_id() != self.closure_def_id {
426 debug!("adjust_closure_kind: not current closure");
430 // closures start out as `Fn`.
431 let existing_kind = self.current_closure_kind;
434 "adjust_closure_kind: closure_id={:?}, existing_kind={:?}, new_kind={:?}",
435 closure_id, existing_kind, new_kind
438 match (existing_kind, new_kind) {
439 (ty::ClosureKind::Fn, ty::ClosureKind::Fn)
440 | (ty::ClosureKind::FnMut, ty::ClosureKind::Fn | ty::ClosureKind::FnMut)
441 | (ty::ClosureKind::FnOnce, _) => {
445 (ty::ClosureKind::Fn, ty::ClosureKind::FnMut | ty::ClosureKind::FnOnce)
446 | (ty::ClosureKind::FnMut, ty::ClosureKind::FnOnce) => {
447 // new kind is stronger than the old kind
448 self.current_closure_kind = new_kind;
449 self.current_origin = Some((upvar_span, var_name));
455 impl<'a, 'tcx> euv::Delegate<'tcx> for InferBorrowKind<'a, 'tcx> {
456 fn consume(&mut self, place: &mc::Place<'tcx>, mode: euv::ConsumeMode) {
457 debug!("consume(place={:?},mode={:?})", place, mode);
458 self.adjust_upvar_borrow_kind_for_consume(place, mode);
461 fn borrow(&mut self, place: &mc::Place<'tcx>, bk: ty::BorrowKind) {
462 debug!("borrow(place={:?}, bk={:?})", place, bk);
466 ty::UniqueImmBorrow => {
467 self.adjust_upvar_borrow_kind_for_unique(place);
470 self.adjust_upvar_borrow_kind_for_mut(place);
475 fn mutate(&mut self, assignee_place: &mc::Place<'tcx>) {
476 debug!("mutate(assignee_place={:?})", assignee_place);
478 self.adjust_upvar_borrow_kind_for_mut(assignee_place);
482 fn var_name(tcx: TyCtxt<'_>, var_hir_id: hir::HirId) -> Symbol {
483 tcx.hir().name(var_hir_id)