// Copyright 2017 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 or the MIT license // , at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! This query borrow-checks the MIR to (further) ensure it is not broken. use rustc::hir; use rustc::hir::def_id::DefId; use rustc::hir::map::definitions::DefPathData; use rustc::infer::InferCtxt; use rustc::ty::{self, ParamEnv, TyCtxt}; use rustc::ty::maps::Providers; use rustc::mir::{AssertMessage, BasicBlock, BorrowKind, Local, Location, Place}; use rustc::mir::{Mir, Mutability, Operand, Projection, ProjectionElem, Rvalue}; use rustc::mir::{Field, Statement, StatementKind, Terminator, TerminatorKind}; use rustc::mir::ClosureRegionRequirements; use rustc_data_structures::fx::FxHashSet; use rustc_data_structures::indexed_set::IdxSetBuf; use rustc_data_structures::indexed_vec::Idx; use syntax::ast; use syntax_pos::Span; use dataflow::{do_dataflow, DebugFormatted}; use dataflow::FlowAtLocation; use dataflow::MoveDataParamEnv; use dataflow::{DataflowAnalysis, DataflowResultsConsumer}; use dataflow::{MaybeInitializedLvals, MaybeUninitializedLvals}; use dataflow::{EverInitializedLvals, MovingOutStatements}; use dataflow::{Borrows, BorrowData, ReserveOrActivateIndex}; use dataflow::{ActiveBorrows, Reservations}; use dataflow::indexes::{BorrowIndex}; use dataflow::move_paths::{IllegalMoveOriginKind, MoveError}; use dataflow::move_paths::{HasMoveData, LookupResult, MoveData, MovePathIndex}; use util::borrowck_errors::{BorrowckErrors, Origin}; use std::iter; use self::flows::Flows; use self::prefixes::PrefixSet; use self::MutateMode::{JustWrite, WriteAndRead}; mod error_reporting; mod flows; mod prefixes; use std::borrow::Cow; pub(crate) mod nll; pub fn provide(providers: &mut Providers) { *providers = Providers { mir_borrowck, ..*providers }; } fn mir_borrowck<'a, 'tcx>( tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId, ) -> Option> { let input_mir = tcx.mir_validated(def_id); debug!("run query mir_borrowck: {}", tcx.item_path_str(def_id)); if { !tcx.has_attr(def_id, "rustc_mir_borrowck") && !tcx.sess.opts.borrowck_mode.use_mir() && !tcx.sess.opts.debugging_opts.nll } { return None; } let opt_closure_req = tcx.infer_ctxt().enter(|infcx| { let input_mir: &Mir = &input_mir.borrow(); do_mir_borrowck(&infcx, input_mir, def_id) }); debug!("mir_borrowck done"); opt_closure_req } fn do_mir_borrowck<'a, 'gcx, 'tcx>( infcx: &InferCtxt<'a, 'gcx, 'tcx>, input_mir: &Mir<'gcx>, def_id: DefId, ) -> Option> { let tcx = infcx.tcx; let attributes = tcx.get_attrs(def_id); let param_env = tcx.param_env(def_id); let id = tcx.hir .as_local_node_id(def_id) .expect("do_mir_borrowck: non-local DefId"); // Make our own copy of the MIR. This copy will be modified (in place) to // contain non-lexical lifetimes. It will have a lifetime tied // to the inference context. let mut mir: Mir<'tcx> = input_mir.clone(); let free_regions = if !tcx.sess.opts.debugging_opts.nll { None } else { let mir = &mut mir; // Replace all regions with fresh inference variables. Some(nll::replace_regions_in_mir(infcx, def_id, param_env, mir)) }; let mir = &mir; let move_data: MoveData<'tcx> = match MoveData::gather_moves(mir, tcx) { Ok(move_data) => move_data, Err((move_data, move_errors)) => { for move_error in move_errors { let (span, kind): (Span, IllegalMoveOriginKind) = match move_error { MoveError::UnionMove { .. } => { unimplemented!("dont know how to report union move errors yet.") } MoveError::IllegalMove { cannot_move_out_of: o, } => (o.span, o.kind), }; let origin = Origin::Mir; let mut err = match kind { IllegalMoveOriginKind::Static => { tcx.cannot_move_out_of(span, "static item", origin) } IllegalMoveOriginKind::BorrowedContent => { tcx.cannot_move_out_of(span, "borrowed content", origin) } IllegalMoveOriginKind::InteriorOfTypeWithDestructor { container_ty: ty } => { tcx.cannot_move_out_of_interior_of_drop(span, ty, origin) } IllegalMoveOriginKind::InteriorOfSliceOrArray { ty, is_index } => { tcx.cannot_move_out_of_interior_noncopy(span, ty, is_index, origin) } }; err.emit(); } move_data } }; let mdpe = MoveDataParamEnv { move_data: move_data, param_env: param_env, }; let body_id = match tcx.def_key(def_id).disambiguated_data.data { DefPathData::StructCtor | DefPathData::EnumVariant(_) => None, _ => Some(tcx.hir.body_owned_by(id)) }; let dead_unwinds = IdxSetBuf::new_empty(mir.basic_blocks().len()); let mut flow_inits = FlowAtLocation::new(do_dataflow( tcx, mir, id, &attributes, &dead_unwinds, MaybeInitializedLvals::new(tcx, mir, &mdpe), |bd, i| DebugFormatted::new(&bd.move_data().move_paths[i]), )); let flow_uninits = FlowAtLocation::new(do_dataflow( tcx, mir, id, &attributes, &dead_unwinds, MaybeUninitializedLvals::new(tcx, mir, &mdpe), |bd, i| DebugFormatted::new(&bd.move_data().move_paths[i]), )); let flow_move_outs = FlowAtLocation::new(do_dataflow( tcx, mir, id, &attributes, &dead_unwinds, MovingOutStatements::new(tcx, mir, &mdpe), |bd, i| DebugFormatted::new(&bd.move_data().moves[i]), )); let flow_ever_inits = FlowAtLocation::new(do_dataflow( tcx, mir, id, &attributes, &dead_unwinds, EverInitializedLvals::new(tcx, mir, &mdpe), |bd, i| DebugFormatted::new(&bd.move_data().inits[i]), )); // If we are in non-lexical mode, compute the non-lexical lifetimes. let (opt_regioncx, opt_closure_req) = if let Some(free_regions) = free_regions { let (regioncx, opt_closure_req) = nll::compute_regions( infcx, def_id, free_regions, mir, param_env, &mut flow_inits, &mdpe.move_data, ); (Some(regioncx), opt_closure_req) } else { assert!(!tcx.sess.opts.debugging_opts.nll); (None, None) }; let flow_inits = flow_inits; // remove mut let mut mbcx = MirBorrowckCtxt { tcx: tcx, mir: mir, node_id: id, move_data: &mdpe.move_data, param_env: param_env, locals_are_invalidated_at_exit: match tcx.hir.body_owner_kind(id) { hir::BodyOwnerKind::Const | hir::BodyOwnerKind::Static(_) => false, hir::BodyOwnerKind::Fn => true, }, storage_dead_or_drop_error_reported_l: FxHashSet(), storage_dead_or_drop_error_reported_s: FxHashSet(), reservation_error_reported: FxHashSet(), }; let borrows = Borrows::new(tcx, mir, opt_regioncx, def_id, body_id); let flow_reservations = do_dataflow( tcx, mir, id, &attributes, &dead_unwinds, Reservations::new(borrows), |rs, i| { // In principle we could make the dataflow ensure that // only reservation bits show up, and assert so here. // // In practice it is easier to be looser; in particular, // it is okay for the kill-sets to hold activation bits. DebugFormatted::new(&(i.kind(), rs.location(i))) }); let flow_active_borrows = { let reservations_on_entry = flow_reservations.0.sets.entry_set_state(); let reservations = flow_reservations.0.operator; let a = DataflowAnalysis::new_with_entry_sets(mir, &dead_unwinds, Cow::Borrowed(reservations_on_entry), ActiveBorrows::new(reservations)); let results = a.run(tcx, id, &attributes, |ab, i| DebugFormatted::new(&(i.kind(), ab.location(i)))); FlowAtLocation::new(results) }; let mut state = Flows::new( flow_active_borrows, flow_inits, flow_uninits, flow_move_outs, flow_ever_inits, ); mbcx.analyze_results(&mut state); // entry point for DataflowResultsConsumer opt_closure_req } #[allow(dead_code)] pub struct MirBorrowckCtxt<'cx, 'gcx: 'tcx, 'tcx: 'cx> { tcx: TyCtxt<'cx, 'gcx, 'tcx>, mir: &'cx Mir<'tcx>, node_id: ast::NodeId, move_data: &'cx MoveData<'tcx>, param_env: ParamEnv<'gcx>, /// This keeps track of whether local variables are free-ed when the function /// exits even without a `StorageDead`, which appears to be the case for /// constants. /// /// I'm not sure this is the right approach - @eddyb could you try and /// figure this out? locals_are_invalidated_at_exit: bool, /// This field keeps track of when storage dead or drop errors are reported /// in order to stop duplicate error reporting and identify the conditions required /// for a "temporary value dropped here while still borrowed" error. See #45360. storage_dead_or_drop_error_reported_l: FxHashSet, /// Same as the above, but for statics (thread-locals) storage_dead_or_drop_error_reported_s: FxHashSet, /// This field keeps track of when borrow conflict errors are reported /// for reservations, so that we don't report seemingly duplicate /// errors for corresponding activations /// /// FIXME: Ideally this would be a set of BorrowIndex, not Places, /// but it is currently inconvenient to track down the BorrowIndex /// at the time we detect and report a reservation error. reservation_error_reported: FxHashSet>, } // Check that: // 1. assignments are always made to mutable locations (FIXME: does that still really go here?) // 2. loans made in overlapping scopes do not conflict // 3. assignments do not affect things loaned out as immutable // 4. moves do not affect things loaned out in any way impl<'cx, 'gcx, 'tcx> DataflowResultsConsumer<'cx, 'tcx> for MirBorrowckCtxt<'cx, 'gcx, 'tcx> { type FlowState = Flows<'cx, 'gcx, 'tcx>; fn mir(&self) -> &'cx Mir<'tcx> { self.mir } fn visit_block_entry(&mut self, bb: BasicBlock, flow_state: &Self::FlowState) { debug!("MirBorrowckCtxt::process_block({:?}): {}", bb, flow_state); } fn visit_statement_entry( &mut self, location: Location, stmt: &Statement<'tcx>, flow_state: &Self::FlowState, ) { debug!( "MirBorrowckCtxt::process_statement({:?}, {:?}): {}", location, stmt, flow_state ); let span = stmt.source_info.span; self.check_activations(location, span, flow_state); match stmt.kind { StatementKind::Assign(ref lhs, ref rhs) => { // NOTE: NLL RFC calls for *shallow* write; using Deep // for short-term compat w/ AST-borrowck. Also, switch // to shallow requires to dataflow: "if this is an // assignment `place = `, then any loan for some // path P of which `place` is a prefix is killed." self.mutate_place( ContextKind::AssignLhs.new(location), (lhs, span), Deep, JustWrite, flow_state, ); self.consume_rvalue( ContextKind::AssignRhs.new(location), (rhs, span), location, flow_state, ); } StatementKind::SetDiscriminant { ref place, variant_index: _, } => { self.mutate_place( ContextKind::SetDiscrim.new(location), (place, span), Shallow(Some(ArtificialField::Discriminant)), JustWrite, flow_state, ); } StatementKind::InlineAsm { ref asm, ref outputs, ref inputs, } => { let context = ContextKind::InlineAsm.new(location); for (o, output) in asm.outputs.iter().zip(outputs) { if o.is_indirect { // FIXME(eddyb) indirect inline asm outputs should // be encoeded through MIR place derefs instead. self.access_place( context, (output, span), (Deep, Read(ReadKind::Copy)), LocalMutationIsAllowed::No, flow_state, ); self.check_if_path_is_moved( context, InitializationRequiringAction::Use, (output, span), flow_state, ); } else { self.mutate_place( context, (output, span), Deep, if o.is_rw { WriteAndRead } else { JustWrite }, flow_state, ); } } for input in inputs { self.consume_operand(context, (input, span), flow_state); } } StatementKind::EndRegion(ref _rgn) => { // ignored when consuming results (update to // flow_state already handled). } StatementKind::Nop | StatementKind::Validate(..) | StatementKind::StorageLive(..) => { // `Nop`, `Validate`, and `StorageLive` are irrelevant // to borrow check. } StatementKind::StorageDead(local) => { self.access_place( ContextKind::StorageDead.new(location), (&Place::Local(local), span), (Shallow(None), Write(WriteKind::StorageDeadOrDrop)), LocalMutationIsAllowed::Yes, flow_state, ); } } } fn visit_terminator_entry( &mut self, location: Location, term: &Terminator<'tcx>, flow_state: &Self::FlowState, ) { let loc = location; debug!( "MirBorrowckCtxt::process_terminator({:?}, {:?}): {}", location, term, flow_state ); let span = term.source_info.span; self.check_activations(location, span, flow_state); match term.kind { TerminatorKind::SwitchInt { ref discr, switch_ty: _, values: _, targets: _, } => { self.consume_operand(ContextKind::SwitchInt.new(loc), (discr, span), flow_state); } TerminatorKind::Drop { location: ref drop_place, target: _, unwind: _, } => { self.access_place( ContextKind::Drop.new(loc), (drop_place, span), (Deep, Write(WriteKind::StorageDeadOrDrop)), LocalMutationIsAllowed::Yes, flow_state, ); } TerminatorKind::DropAndReplace { location: ref drop_place, value: ref new_value, target: _, unwind: _, } => { self.mutate_place( ContextKind::DropAndReplace.new(loc), (drop_place, span), Deep, JustWrite, flow_state, ); self.consume_operand( ContextKind::DropAndReplace.new(loc), (new_value, span), flow_state, ); } TerminatorKind::Call { ref func, ref args, ref destination, cleanup: _, } => { self.consume_operand(ContextKind::CallOperator.new(loc), (func, span), flow_state); for arg in args { self.consume_operand( ContextKind::CallOperand.new(loc), (arg, span), flow_state, ); } if let Some((ref dest, _ /*bb*/)) = *destination { self.mutate_place( ContextKind::CallDest.new(loc), (dest, span), Deep, JustWrite, flow_state, ); } } TerminatorKind::Assert { ref cond, expected: _, ref msg, target: _, cleanup: _, } => { self.consume_operand(ContextKind::Assert.new(loc), (cond, span), flow_state); match *msg { AssertMessage::BoundsCheck { ref len, ref index } => { self.consume_operand(ContextKind::Assert.new(loc), (len, span), flow_state); self.consume_operand( ContextKind::Assert.new(loc), (index, span), flow_state, ); } AssertMessage::Math(_ /*const_math_err*/) => {} AssertMessage::GeneratorResumedAfterReturn => {} AssertMessage::GeneratorResumedAfterPanic => {} } } TerminatorKind::Yield { ref value, resume: _, drop: _, } => { self.consume_operand(ContextKind::Yield.new(loc), (value, span), flow_state); } TerminatorKind::Resume | TerminatorKind::Return | TerminatorKind::GeneratorDrop => { // Returning from the function implicitly kills storage for all locals and statics. // Often, the storage will already have been killed by an explicit // StorageDead, but we don't always emit those (notably on unwind paths), // so this "extra check" serves as a kind of backup. let domain = flow_state.borrows.operator(); let data = domain.borrows(); flow_state.borrows.with_elems_outgoing(|borrows| { for i in borrows { let borrow = &data[i.borrow_index()]; let context = ContextKind::StorageDead.new(loc); self.check_for_invalidation_at_exit(context, borrow, span, flow_state); } }); } TerminatorKind::Goto { target: _ } | TerminatorKind::Unreachable | TerminatorKind::FalseEdges { .. } => { // no data used, thus irrelevant to borrowck } } } } #[derive(Copy, Clone, PartialEq, Eq, Debug)] enum MutateMode { JustWrite, WriteAndRead, } #[derive(Copy, Clone, PartialEq, Eq, Debug)] enum Control { Continue, Break, } use self::ShallowOrDeep::{Deep, Shallow}; use self::ReadOrWrite::{Activation, Read, Reservation, Write}; #[derive(Copy, Clone, PartialEq, Eq, Debug)] enum ArtificialField { Discriminant, ArrayLength, } #[derive(Copy, Clone, PartialEq, Eq, Debug)] enum ShallowOrDeep { /// From the RFC: "A *shallow* access means that the immediate /// fields reached at LV are accessed, but references or pointers /// found within are not dereferenced. Right now, the only access /// that is shallow is an assignment like `x = ...;`, which would /// be a *shallow write* of `x`." Shallow(Option), /// From the RFC: "A *deep* access means that all data reachable /// through the given place may be invalidated or accesses by /// this action." Deep, } /// Kind of access to a value: read or write /// (For informational purposes only) #[derive(Copy, Clone, PartialEq, Eq, Debug)] enum ReadOrWrite { /// From the RFC: "A *read* means that the existing data may be /// read, but will not be changed." Read(ReadKind), /// From the RFC: "A *write* means that the data may be mutated to /// new values or otherwise invalidated (for example, it could be /// de-initialized, as in a move operation). Write(WriteKind), /// For two-phase borrows, we distinguish a reservation (which is treated /// like a Read) from an activation (which is treated like a write), and /// each of those is furthermore distinguished from Reads/Writes above. Reservation(WriteKind), Activation(WriteKind, BorrowIndex), } /// Kind of read access to a value /// (For informational purposes only) #[derive(Copy, Clone, PartialEq, Eq, Debug)] enum ReadKind { Borrow(BorrowKind), Copy, } /// Kind of write access to a value /// (For informational purposes only) #[derive(Copy, Clone, PartialEq, Eq, Debug)] enum WriteKind { StorageDeadOrDrop, MutableBorrow(BorrowKind), Mutate, Move, } /// When checking permissions for a place access, this flag is used to indicate that an immutable /// local place can be mutated. /// /// FIXME: @nikomatsakis suggested that this flag could be removed with the following modifications: /// - Merge `check_access_permissions()` and `check_if_reassignment_to_immutable_state()` /// - Split `is_mutable()` into `is_assignable()` (can be directly assigned) and /// `is_declared_mutable()` /// - Take flow state into consideration in `is_assignable()` for local variables #[derive(Copy, Clone, PartialEq, Eq, Debug)] enum LocalMutationIsAllowed { Yes, /// We want use of immutable upvars to cause a "write to immutable upvar" /// error, not an "reassignment" error. ExceptUpvars, No } struct AccessErrorsReported { mutability_error: bool, #[allow(dead_code)] conflict_error: bool } #[derive(Copy, Clone)] enum InitializationRequiringAction { Update, Borrow, Use, Assignment, } impl InitializationRequiringAction { fn as_noun(self) -> &'static str { match self { InitializationRequiringAction::Update => "update", InitializationRequiringAction::Borrow => "borrow", InitializationRequiringAction::Use => "use", InitializationRequiringAction::Assignment => "assign", } } fn as_verb_in_past_tense(self) -> &'static str { match self { InitializationRequiringAction::Update => "updated", InitializationRequiringAction::Borrow => "borrowed", InitializationRequiringAction::Use => "used", InitializationRequiringAction::Assignment => "assigned", } } } impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> { /// Checks an access to the given place to see if it is allowed. Examines the set of borrows /// that are in scope, as well as which paths have been initialized, to ensure that (a) the /// place is initialized and (b) it is not borrowed in some way that would prevent this /// access. /// /// Returns true if an error is reported, false otherwise. fn access_place( &mut self, context: Context, place_span: (&Place<'tcx>, Span), kind: (ShallowOrDeep, ReadOrWrite), is_local_mutation_allowed: LocalMutationIsAllowed, flow_state: &Flows<'cx, 'gcx, 'tcx>, ) -> AccessErrorsReported { let (sd, rw) = kind; if let Activation(_, borrow_index) = rw { if self.reservation_error_reported.contains(&place_span.0) { debug!("skipping access_place for activation of invalid reservation \ place: {:?} borrow_index: {:?}", place_span.0, borrow_index); return AccessErrorsReported { mutability_error: false, conflict_error: true }; } } let mutability_error = self.check_access_permissions(place_span, rw, is_local_mutation_allowed); let conflict_error = self.check_access_for_conflict(context, place_span, sd, rw, flow_state); AccessErrorsReported { mutability_error, conflict_error } } fn check_access_for_conflict( &mut self, context: Context, place_span: (&Place<'tcx>, Span), sd: ShallowOrDeep, rw: ReadOrWrite, flow_state: &Flows<'cx, 'gcx, 'tcx>, ) -> bool { let mut error_reported = false; self.each_borrow_involving_path( context, (sd, place_span.0), flow_state, |this, index, borrow| match (rw, borrow.kind) { // Obviously an activation is compatible with its own // reservation (or even prior activating uses of same // borrow); so don't check if they interfere. // // NOTE: *reservations* do conflict with themselves; // thus aren't injecting unsoundenss w/ this check.) (Activation(_, activating), _) if activating == index.borrow_index() => { debug!("check_access_for_conflict place_span: {:?} sd: {:?} rw: {:?} \ skipping {:?} b/c activation of same borrow_index: {:?}", place_span, sd, rw, (index, borrow), index.borrow_index()); Control::Continue } (Read(_), BorrowKind::Shared) | (Reservation(..), BorrowKind::Shared) => Control::Continue, (Read(kind), BorrowKind::Unique) | (Read(kind), BorrowKind::Mut) => { // Reading from mere reservations of mutable-borrows is OK. if this.tcx.sess.opts.debugging_opts.two_phase_borrows && index.is_reservation() { return Control::Continue; } match kind { ReadKind::Copy => { error_reported = true; this.report_use_while_mutably_borrowed(context, place_span, borrow) } ReadKind::Borrow(bk) => { let end_issued_loan_span = flow_state .borrows .operator() .opt_region_end_span(&borrow.region); error_reported = true; this.report_conflicting_borrow( context, place_span, bk, &borrow, end_issued_loan_span, ) } } Control::Break } (Reservation(kind), BorrowKind::Unique) | (Reservation(kind), BorrowKind::Mut) | (Activation(kind, _), _) | (Write(kind), _) => { match rw { Reservation(_) => { debug!("recording invalid reservation of \ place: {:?}", place_span.0); this.reservation_error_reported.insert(place_span.0.clone()); } Activation(_, activating) => { debug!("observing check_place for activation of \ borrow_index: {:?}", activating); } Read(..) | Write(..) => {} } match kind { WriteKind::MutableBorrow(bk) => { let end_issued_loan_span = flow_state .borrows .operator() .opt_region_end_span(&borrow.region); error_reported = true; this.report_conflicting_borrow( context, place_span, bk, &borrow, end_issued_loan_span, ) } WriteKind::StorageDeadOrDrop => { error_reported = true; this.report_borrowed_value_does_not_live_long_enough( context, borrow, place_span.1, flow_state.borrows.operator()); } WriteKind::Mutate => { error_reported = true; this.report_illegal_mutation_of_borrowed(context, place_span, borrow) } WriteKind::Move => { error_reported = true; this.report_move_out_while_borrowed(context, place_span, &borrow) } } Control::Break } }, ); error_reported } fn mutate_place( &mut self, context: Context, place_span: (&Place<'tcx>, Span), kind: ShallowOrDeep, mode: MutateMode, flow_state: &Flows<'cx, 'gcx, 'tcx>, ) { // Write of P[i] or *P, or WriteAndRead of any P, requires P init'd. match mode { MutateMode::WriteAndRead => { self.check_if_path_is_moved( context, InitializationRequiringAction::Update, place_span, flow_state, ); } MutateMode::JustWrite => { self.check_if_assigned_path_is_moved(context, place_span, flow_state); } } let errors_reported = self.access_place( context, place_span, (kind, Write(WriteKind::Mutate)), // We want immutable upvars to cause an "assignment to immutable var" // error, not an "reassignment of immutable var" error, because the // latter can't find a good previous assignment span. // // There's probably a better way to do this. LocalMutationIsAllowed::ExceptUpvars, flow_state, ); if !errors_reported.mutability_error { // check for reassignments to immutable local variables self.check_if_reassignment_to_immutable_state(context, place_span, flow_state); } } fn consume_rvalue( &mut self, context: Context, (rvalue, span): (&Rvalue<'tcx>, Span), _location: Location, flow_state: &Flows<'cx, 'gcx, 'tcx>, ) { match *rvalue { Rvalue::Ref(_ /*rgn*/, bk, ref place) => { let access_kind = match bk { BorrowKind::Shared => (Deep, Read(ReadKind::Borrow(bk))), BorrowKind::Unique | BorrowKind::Mut => { let wk = WriteKind::MutableBorrow(bk); if self.tcx.sess.opts.debugging_opts.two_phase_borrows { (Deep, Reservation(wk)) } else { (Deep, Write(wk)) } } }; self.access_place( context, (place, span), access_kind, LocalMutationIsAllowed::No, flow_state, ); self.check_if_path_is_moved( context, InitializationRequiringAction::Borrow, (place, span), flow_state, ); } Rvalue::Use(ref operand) | Rvalue::Repeat(ref operand, _) | Rvalue::UnaryOp(_ /*un_op*/, ref operand) | Rvalue::Cast(_ /*cast_kind*/, ref operand, _ /*ty*/) => { self.consume_operand(context, (operand, span), flow_state) } Rvalue::Len(ref place) | Rvalue::Discriminant(ref place) => { let af = match *rvalue { Rvalue::Len(..) => ArtificialField::ArrayLength, Rvalue::Discriminant(..) => ArtificialField::Discriminant, _ => unreachable!(), }; self.access_place( context, (place, span), (Shallow(Some(af)), Read(ReadKind::Copy)), LocalMutationIsAllowed::No, flow_state, ); self.check_if_path_is_moved( context, InitializationRequiringAction::Use, (place, span), flow_state, ); } Rvalue::BinaryOp(_bin_op, ref operand1, ref operand2) | Rvalue::CheckedBinaryOp(_bin_op, ref operand1, ref operand2) => { self.consume_operand(context, (operand1, span), flow_state); self.consume_operand(context, (operand2, span), flow_state); } Rvalue::NullaryOp(_op, _ty) => { // nullary ops take no dynamic input; no borrowck effect. // // FIXME: is above actually true? Do we want to track // the fact that uninitialized data can be created via // `NullOp::Box`? } Rvalue::Aggregate(ref _aggregate_kind, ref operands) => for operand in operands { self.consume_operand(context, (operand, span), flow_state); }, } } fn consume_operand( &mut self, context: Context, (operand, span): (&Operand<'tcx>, Span), flow_state: &Flows<'cx, 'gcx, 'tcx>, ) { match *operand { Operand::Copy(ref place) => { // copy of place: check if this is "copy of frozen path" // (FIXME: see check_loans.rs) self.access_place( context, (place, span), (Deep, Read(ReadKind::Copy)), LocalMutationIsAllowed::No, flow_state, ); // Finally, check if path was already moved. self.check_if_path_is_moved( context, InitializationRequiringAction::Use, (place, span), flow_state, ); } Operand::Move(ref place) => { // move of place: check if this is move of already borrowed path self.access_place( context, (place, span), (Deep, Write(WriteKind::Move)), LocalMutationIsAllowed::Yes, flow_state, ); // Finally, check if path was already moved. self.check_if_path_is_moved( context, InitializationRequiringAction::Use, (place, span), flow_state, ); } Operand::Constant(_) => {} } } /// Returns whether a borrow of this place is invalidated when the function /// exits fn check_for_invalidation_at_exit(&mut self, context: Context, borrow: &BorrowData<'tcx>, span: Span, flow_state: &Flows<'cx, 'gcx, 'tcx>) { debug!("check_for_invalidation_at_exit({:?})", borrow); let place = &borrow.borrowed_place; let root_place = self.prefixes(place, PrefixSet::All).last().unwrap(); // FIXME(nll-rfc#40): do more precise destructor tracking here. For now // we just know that all locals are dropped at function exit (otherwise // we'll have a memory leak) and assume that all statics have a destructor. // // FIXME: allow thread-locals to borrow other thread locals? let (might_be_alive, will_be_dropped) = match root_place { Place::Static(statik) => { // Thread-locals might be dropped after the function exits, but // "true" statics will never be. let is_thread_local = self.tcx .get_attrs(statik.def_id) .iter() .any(|attr| attr.check_name("thread_local")); (true, is_thread_local) } Place::Local(_) => { // Locals are always dropped at function exit, and if they // have a destructor it would've been called already. (false, self.locals_are_invalidated_at_exit) } Place::Projection(..) => { bug!("root of {:?} is a projection ({:?})?", place, root_place) } }; if !will_be_dropped { debug!( "place_is_invalidated_at_exit({:?}) - won't be dropped", place ); return; } // FIXME: replace this with a proper borrow_conflicts_with_place when // that is merged. let sd = if might_be_alive { Deep } else { Shallow(None) }; if self.places_conflict(place, root_place, sd) { debug!("check_for_invalidation_at_exit({:?}): INVALID", place); // FIXME: should be talking about the region lifetime instead // of just a span here. self.report_borrowed_value_does_not_live_long_enough( context, borrow, span.end_point(), flow_state.borrows.operator() ) } } fn check_activations(&mut self, location: Location, span: Span, flow_state: &Flows<'cx, 'gcx, 'tcx>) { if !self.tcx.sess.opts.debugging_opts.two_phase_borrows { return; } // Two-phase borrow support: For each activation that is newly // generated at this statement, check if it interferes with // another borrow. let domain = flow_state.borrows.operator(); let data = domain.borrows(); flow_state.borrows.each_gen_bit(|gen| { if gen.is_activation() { let borrow_index = gen.borrow_index(); let borrow = &data[borrow_index]; // currently the flow analysis registers // activations for both mutable and immutable // borrows. So make sure we are talking about a // mutable borrow before we check it. match borrow.kind { BorrowKind::Shared => return, BorrowKind::Unique | BorrowKind::Mut => {} } self.access_place(ContextKind::Activation.new(location), (&borrow.borrowed_place, span), (Deep, Activation(WriteKind::MutableBorrow(borrow.kind), borrow_index)), LocalMutationIsAllowed::No, flow_state); // We do not need to call `check_if_path_is_moved` // again, as we already called it when we made the // initial reservation. } }); } } impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> { fn check_if_reassignment_to_immutable_state( &mut self, context: Context, (place, span): (&Place<'tcx>, Span), flow_state: &Flows<'cx, 'gcx, 'tcx>, ) { debug!("check_if_reassignment_to_immutable_state({:?})", place); // determine if this path has a non-mut owner (and thus needs checking). if let Ok(()) = self.is_mutable(place, LocalMutationIsAllowed::No) { return; } debug!("check_if_reassignment_to_immutable_state({:?}) - is an imm local", place); for i in flow_state.ever_inits.elems_incoming() { let init = self.move_data.inits[i]; let init_place = &self.move_data.move_paths[init.path].place; if self.places_conflict(&init_place, place, Deep) { self.report_illegal_reassignment(context, (place, span), init.span); break; } } } fn check_if_path_is_moved( &mut self, context: Context, desired_action: InitializationRequiringAction, place_span: (&Place<'tcx>, Span), flow_state: &Flows<'cx, 'gcx, 'tcx>, ) { // FIXME: analogous code in check_loans first maps `place` to // its base_path ... but is that what we want here? let place = self.base_path(place_span.0); let maybe_uninits = &flow_state.uninits; let curr_move_outs = &flow_state.move_outs; // Bad scenarios: // // 1. Move of `a.b.c`, use of `a.b.c` // 2. Move of `a.b.c`, use of `a.b.c.d` (without first reinitializing `a.b.c.d`) // 3. Move of `a.b.c`, use of `a` or `a.b` // 4. Uninitialized `(a.b.c: &_)`, use of `*a.b.c`; note that with // partial initialization support, one might have `a.x` // initialized but not `a.b`. // // OK scenarios: // // 5. Move of `a.b.c`, use of `a.b.d` // 6. Uninitialized `a.x`, initialized `a.b`, use of `a.b` // 7. Copied `(a.b: &_)`, use of `*(a.b).c`; note that `a.b` // must have been initialized for the use to be sound. // 8. Move of `a.b.c` then reinit of `a.b.c.d`, use of `a.b.c.d` // The dataflow tracks shallow prefixes distinctly (that is, // field-accesses on P distinctly from P itself), in order to // track substructure initialization separately from the whole // structure. // // E.g., when looking at (*a.b.c).d, if the closest prefix for // which we have a MovePath is `a.b`, then that means that the // initialization state of `a.b` is all we need to inspect to // know if `a.b.c` is valid (and from that we infer that the // dereference and `.d` access is also valid, since we assume // `a.b.c` is assigned a reference to a initialized and // well-formed record structure.) // Therefore, if we seek out the *closest* prefix for which we // have a MovePath, that should capture the initialization // state for the place scenario. // // This code covers scenarios 1, 2, and 4. debug!("check_if_path_is_moved part1 place: {:?}", place); match self.move_path_closest_to(place) { Ok(mpi) => { if maybe_uninits.contains(&mpi) { self.report_use_of_moved_or_uninitialized( context, desired_action, place_span, mpi, curr_move_outs, ); return; // don't bother finding other problems. } } Err(NoMovePathFound::ReachedStatic) => { // Okay: we do not build MoveData for static variables } // Only query longest prefix with a MovePath, not further // ancestors; dataflow recurs on children when parents // move (to support partial (re)inits). // // (I.e. querying parents breaks scenario 8; but may want // to do such a query based on partial-init feature-gate.) } // A move of any shallow suffix of `place` also interferes // with an attempt to use `place`. This is scenario 3 above. // // (Distinct from handling of scenarios 1+2+4 above because // `place` does not interfere with suffixes of its prefixes, // e.g. `a.b.c` does not interfere with `a.b.d`) debug!("check_if_path_is_moved part2 place: {:?}", place); if let Some(mpi) = self.move_path_for_place(place) { if let Some(child_mpi) = maybe_uninits.has_any_child_of(mpi) { self.report_use_of_moved_or_uninitialized( context, desired_action, place_span, child_mpi, curr_move_outs, ); return; // don't bother finding other problems. } } } /// Currently MoveData does not store entries for all places in /// the input MIR. For example it will currently filter out /// places that are Copy; thus we do not track places of shared /// reference type. This routine will walk up a place along its /// prefixes, searching for a foundational place that *is* /// tracked in the MoveData. /// /// An Err result includes a tag indicated why the search failed. /// Currenly this can only occur if the place is built off of a /// static variable, as we do not track those in the MoveData. fn move_path_closest_to( &mut self, place: &Place<'tcx>, ) -> Result { let mut last_prefix = place; for prefix in self.prefixes(place, PrefixSet::All) { if let Some(mpi) = self.move_path_for_place(prefix) { return Ok(mpi); } last_prefix = prefix; } match *last_prefix { Place::Local(_) => panic!("should have move path for every Local"), Place::Projection(_) => panic!("PrefixSet::All meant dont stop for Projection"), Place::Static(_) => return Err(NoMovePathFound::ReachedStatic), } } fn move_path_for_place(&mut self, place: &Place<'tcx>) -> Option { // If returns None, then there is no move path corresponding // to a direct owner of `place` (which means there is nothing // that borrowck tracks for its analysis). match self.move_data.rev_lookup.find(place) { LookupResult::Parent(_) => None, LookupResult::Exact(mpi) => Some(mpi), } } fn check_if_assigned_path_is_moved( &mut self, context: Context, (place, span): (&Place<'tcx>, Span), flow_state: &Flows<'cx, 'gcx, 'tcx>, ) { // recur down place; dispatch to check_if_path_is_moved when necessary let mut place = place; loop { match *place { Place::Local(_) | Place::Static(_) => { // assigning to `x` does not require `x` be initialized. break; } Place::Projection(ref proj) => { let Projection { ref base, ref elem } = **proj; match *elem { ProjectionElem::Deref | // assigning to *P requires `P` initialized. ProjectionElem::Index(_/*operand*/) | ProjectionElem::ConstantIndex { .. } | // assigning to P[i] requires `P` initialized. ProjectionElem::Downcast(_/*adt_def*/, _/*variant_idx*/) => // assigning to (P->variant) is okay if assigning to `P` is okay // // FIXME: is this true even if P is a adt with a dtor? { } ProjectionElem::Subslice { .. } => { panic!("we dont allow assignments to subslices, context: {:?}", context); } ProjectionElem::Field(..) => { // if type of `P` has a dtor, then // assigning to `P.f` requires `P` itself // be already initialized let tcx = self.tcx; match base.ty(self.mir, tcx).to_ty(tcx).sty { ty::TyAdt(def, _) if def.has_dtor(tcx) => { // FIXME: analogous code in // check_loans.rs first maps // `base` to its base_path. self.check_if_path_is_moved( context, InitializationRequiringAction::Assignment, (base, span), flow_state); // (base initialized; no need to // recur further) break; } _ => {} } } } place = base; continue; } } } } /// Check the permissions for the given place and read or write kind /// /// Returns true if an error is reported, false otherwise. fn check_access_permissions( &self, (place, span): (&Place<'tcx>, Span), kind: ReadOrWrite, is_local_mutation_allowed: LocalMutationIsAllowed, ) -> bool { debug!( "check_access_permissions({:?}, {:?}, {:?})", place, kind, is_local_mutation_allowed ); let mut error_reported = false; match kind { Reservation(WriteKind::MutableBorrow(BorrowKind::Unique)) | Write(WriteKind::MutableBorrow(BorrowKind::Unique)) => { if let Err(_place_err) = self.is_mutable(place, LocalMutationIsAllowed::Yes) { span_bug!(span, "&unique borrow for {:?} should not fail", place); } } Reservation(WriteKind::MutableBorrow(BorrowKind::Mut)) | Write(WriteKind::MutableBorrow(BorrowKind::Mut)) => if let Err(place_err) = self.is_mutable(place, is_local_mutation_allowed) { error_reported = true; let item_msg = match self.describe_place(place) { Some(name) => format!("immutable item `{}`", name), None => "immutable item".to_owned(), }; let mut err = self.tcx .cannot_borrow_path_as_mutable(span, &item_msg, Origin::Mir); err.span_label(span, "cannot borrow as mutable"); if place != place_err { if let Some(name) = self.describe_place(place_err) { err.note(&format!("Value not mutable causing this error: `{}`", name)); } } err.emit(); }, Reservation(WriteKind::Mutate) | Write(WriteKind::Mutate) => { if let Err(place_err) = self.is_mutable(place, is_local_mutation_allowed) { error_reported = true; let item_msg = match self.describe_place(place) { Some(name) => format!("immutable item `{}`", name), None => "immutable item".to_owned(), }; let mut err = self.tcx.cannot_assign(span, &item_msg, Origin::Mir); err.span_label(span, "cannot mutate"); if place != place_err { if let Some(name) = self.describe_place(place_err) { err.note(&format!("Value not mutable causing this error: `{}`", name)); } } err.emit(); } } Reservation(WriteKind::Move) | Reservation(WriteKind::StorageDeadOrDrop) | Reservation(WriteKind::MutableBorrow(BorrowKind::Shared)) | Write(WriteKind::Move) | Write(WriteKind::StorageDeadOrDrop) | Write(WriteKind::MutableBorrow(BorrowKind::Shared)) => { if let Err(_place_err) = self.is_mutable(place, is_local_mutation_allowed) { self.tcx.sess.delay_span_bug( span, &format!( "Accessing `{:?}` with the kind `{:?}` shouldn't be possible", place, kind ), ); } } Activation(..) => {} // permission checks are done at Reservation point. Read(ReadKind::Borrow(BorrowKind::Unique)) | Read(ReadKind::Borrow(BorrowKind::Mut)) | Read(ReadKind::Borrow(BorrowKind::Shared)) | Read(ReadKind::Copy) => {} // Access authorized } error_reported } /// Can this value be written or borrowed mutably fn is_mutable<'d>( &self, place: &'d Place<'tcx>, is_local_mutation_allowed: LocalMutationIsAllowed, ) -> Result<(), &'d Place<'tcx>> { match *place { Place::Local(local) => { let local = &self.mir.local_decls[local]; match local.mutability { Mutability::Not => match is_local_mutation_allowed { LocalMutationIsAllowed::Yes | LocalMutationIsAllowed::ExceptUpvars => Ok(()), LocalMutationIsAllowed::No => Err(place), }, Mutability::Mut => Ok(()), } } Place::Static(ref static_) => if !self.tcx.is_static_mut(static_.def_id) { Err(place) } else { Ok(()) }, Place::Projection(ref proj) => { match proj.elem { ProjectionElem::Deref => { let base_ty = proj.base.ty(self.mir, self.tcx).to_ty(self.tcx); // Check the kind of deref to decide match base_ty.sty { ty::TyRef(_, tnm) => { match tnm.mutbl { // Shared borrowed data is never mutable hir::MutImmutable => Err(place), // Mutably borrowed data is mutable, but only if we have a // unique path to the `&mut` hir::MutMutable => { let mode = match self.is_upvar_field_projection(&proj.base) { Some(field) if { self.mir.upvar_decls[field.index()].by_ref } => is_local_mutation_allowed, _ => LocalMutationIsAllowed::Yes }; self.is_mutable(&proj.base, mode) } } } ty::TyRawPtr(tnm) => { match tnm.mutbl { // `*const` raw pointers are not mutable hir::MutImmutable => return Err(place), // `*mut` raw pointers are always mutable, regardless of context // The users have to check by themselve. hir::MutMutable => return Ok(()), } } // `Box` owns its content, so mutable if its location is mutable _ if base_ty.is_box() => { self.is_mutable(&proj.base, is_local_mutation_allowed) } // Deref should only be for reference, pointers or boxes _ => bug!("Deref of unexpected type: {:?}", base_ty), } } // All other projections are owned by their base path, so mutable if // base path is mutable ProjectionElem::Field(..) | ProjectionElem::Index(..) | ProjectionElem::ConstantIndex { .. } | ProjectionElem::Subslice { .. } | ProjectionElem::Downcast(..) => { if let Some(field) = self.is_upvar_field_projection(place) { let decl = &self.mir.upvar_decls[field.index()]; debug!("decl.mutability={:?} local_mutation_is_allowed={:?} place={:?}", decl, is_local_mutation_allowed, place); match (decl.mutability, is_local_mutation_allowed) { (Mutability::Not, LocalMutationIsAllowed::No) | (Mutability::Not, LocalMutationIsAllowed::ExceptUpvars) => Err(place), (Mutability::Not, LocalMutationIsAllowed::Yes) | (Mutability::Mut, _) => self.is_mutable(&proj.base, is_local_mutation_allowed) } } else { self.is_mutable(&proj.base, is_local_mutation_allowed) } } } } } } /// If this is a field projection, and the field is being projected from a closure type, /// then returns the index of the field being projected. Note that this closure will always /// be `self` in the current MIR, because that is the only time we directly access the fields /// of a closure type. fn is_upvar_field_projection(&self, place: &Place<'tcx>) -> Option { match *place { Place::Projection(ref proj) => match proj.elem { ProjectionElem::Field(field, _ty) => { let is_projection_from_ty_closure = proj.base .ty(self.mir, self.tcx) .to_ty(self.tcx) .is_closure(); if is_projection_from_ty_closure { Some(field) } else { None } } _ => None, }, _ => None, } } } #[derive(Copy, Clone, PartialEq, Eq, Debug)] enum NoMovePathFound { ReachedStatic, } /// The degree of overlap between 2 places for borrow-checking. enum Overlap { /// The places might partially overlap - in this case, we give /// up and say that they might conflict. This occurs when /// different fields of a union are borrowed. For example, /// if `u` is a union, we have no way of telling how disjoint /// `u.a.x` and `a.b.y` are. Arbitrary, /// The places have the same type, and are either completely disjoint /// or equal - i.e. they can't "partially" overlap as can occur with /// unions. This is the "base case" on which we recur for extensions /// of the place. EqualOrDisjoint, /// The places are disjoint, so we know all extensions of them /// will also be disjoint. Disjoint, } impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> { // Given that the bases of `elem1` and `elem2` are always either equal // or disjoint (and have the same type!), return the overlap situation // between `elem1` and `elem2`. fn place_element_conflict(&self, elem1: &Place<'tcx>, elem2: &Place<'tcx>) -> Overlap { match (elem1, elem2) { (Place::Local(l1), Place::Local(l2)) => { if l1 == l2 { // the same local - base case, equal debug!("place_element_conflict: DISJOINT-OR-EQ-LOCAL"); Overlap::EqualOrDisjoint } else { // different locals - base case, disjoint debug!("place_element_conflict: DISJOINT-LOCAL"); Overlap::Disjoint } } (Place::Static(static1), Place::Static(static2)) => { if static1.def_id != static2.def_id { debug!("place_element_conflict: DISJOINT-STATIC"); Overlap::Disjoint } else if self.tcx.is_static_mut(static1.def_id) { // We ignore mutable statics - they can only be unsafe code. debug!("place_element_conflict: IGNORE-STATIC-MUT"); Overlap::Disjoint } else { debug!("place_element_conflict: DISJOINT-OR-EQ-STATIC"); Overlap::EqualOrDisjoint } } (Place::Local(_), Place::Static(_)) | (Place::Static(_), Place::Local(_)) => { debug!("place_element_conflict: DISJOINT-STATIC-LOCAL"); Overlap::Disjoint } (Place::Projection(pi1), Place::Projection(pi2)) => { match (&pi1.elem, &pi2.elem) { (ProjectionElem::Deref, ProjectionElem::Deref) => { // derefs (e.g. `*x` vs. `*x`) - recur. debug!("place_element_conflict: DISJOINT-OR-EQ-DEREF"); Overlap::EqualOrDisjoint } (ProjectionElem::Field(f1, _), ProjectionElem::Field(f2, _)) => { if f1 == f2 { // same field (e.g. `a.y` vs. `a.y`) - recur. debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD"); Overlap::EqualOrDisjoint } else { let ty = pi1.base.ty(self.mir, self.tcx).to_ty(self.tcx); match ty.sty { ty::TyAdt(def, _) if def.is_union() => { // Different fields of a union, we are basically stuck. debug!("place_element_conflict: STUCK-UNION"); Overlap::Arbitrary } _ => { // Different fields of a struct (`a.x` vs. `a.y`). Disjoint! debug!("place_element_conflict: DISJOINT-FIELD"); Overlap::Disjoint } } } } (ProjectionElem::Downcast(_, v1), ProjectionElem::Downcast(_, v2)) => { // different variants are treated as having disjoint fields, // even if they occupy the same "space", because it's // impossible for 2 variants of the same enum to exist // (and therefore, to be borrowed) at the same time. // // Note that this is different from unions - we *do* allow // this code to compile: // // ``` // fn foo(x: &mut Result) { // let mut v = None; // if let Ok(ref mut a) = *x { // v = Some(a); // } // // here, you would *think* that the // // *entirety* of `x` would be borrowed, // // but in fact only the `Ok` variant is, // // so the `Err` variant is *entirely free*: // if let Err(ref mut a) = *x { // v = Some(a); // } // drop(v); // } // ``` if v1 == v2 { debug!("place_element_conflict: DISJOINT-OR-EQ-FIELD"); Overlap::EqualOrDisjoint } else { debug!("place_element_conflict: DISJOINT-FIELD"); Overlap::Disjoint } } (ProjectionElem::Index(..), ProjectionElem::Index(..)) | (ProjectionElem::Index(..), ProjectionElem::ConstantIndex { .. }) | (ProjectionElem::Index(..), ProjectionElem::Subslice { .. }) | (ProjectionElem::ConstantIndex { .. }, ProjectionElem::Index(..)) | (ProjectionElem::ConstantIndex { .. }, ProjectionElem::ConstantIndex { .. }) | (ProjectionElem::ConstantIndex { .. }, ProjectionElem::Subslice { .. }) | (ProjectionElem::Subslice { .. }, ProjectionElem::Index(..)) | (ProjectionElem::Subslice { .. }, ProjectionElem::ConstantIndex { .. }) | (ProjectionElem::Subslice { .. }, ProjectionElem::Subslice { .. }) => { // Array indexes (`a[0]` vs. `a[i]`). These can either be disjoint // (if the indexes differ) or equal (if they are the same), so this // is the recursive case that gives "equal *or* disjoint" its meaning. // // Note that by construction, MIR at borrowck can't subdivide // `Subslice` accesses (e.g. `a[2..3][i]` will never be present) - they // are only present in slice patterns, and we "merge together" nested // slice patterns. That means we don't have to think about these. It's // probably a good idea to assert this somewhere, but I'm too lazy. // // FIXME(#8636) we might want to return Disjoint if // both projections are constant and disjoint. debug!("place_element_conflict: DISJOINT-OR-EQ-ARRAY"); Overlap::EqualOrDisjoint } (ProjectionElem::Deref, _) | (ProjectionElem::Field(..), _) | (ProjectionElem::Index(..), _) | (ProjectionElem::ConstantIndex { .. }, _) | (ProjectionElem::Subslice { .. }, _) | (ProjectionElem::Downcast(..), _) => { bug!("mismatched projections in place_element_conflict: {:?} and {:?}", elem1, elem2) } } } (Place::Projection(_), _) | (_, Place::Projection(_)) => { bug!("unexpected elements in place_element_conflict: {:?} and {:?}", elem1, elem2) } } } /// Returns whether an access of kind `access` to `access_place` conflicts with /// a borrow/full access to `borrow_place` (for deep accesses to mutable /// locations, this function is symmetric between `borrow_place` & `access_place`). fn places_conflict(&mut self, borrow_place: &Place<'tcx>, access_place: &Place<'tcx>, access: ShallowOrDeep) -> bool { debug!("places_conflict({:?},{:?},{:?})", borrow_place, access_place, access); // Return all the prefixes of `place` in reverse order, including // downcasts. fn place_elements<'a, 'tcx>(place: &'a Place<'tcx>) -> Vec<&'a Place<'tcx>> { let mut result = vec![]; let mut place = place; loop { result.push(place); match place { Place::Projection(interior) => { place = &interior.base; } Place::Local(_) | Place::Static(_) => { result.reverse(); return result; } } } } let borrow_components = place_elements(borrow_place); let access_components = place_elements(access_place); debug!("places_conflict: components {:?} / {:?}", borrow_components, access_components); let borrow_components = borrow_components.into_iter() .map(Some).chain(iter::repeat(None)); let access_components = access_components.into_iter() .map(Some).chain(iter::repeat(None)); // The borrowck rules for proving disjointness are applied from the "root" of the // borrow forwards, iterating over "similar" projections in lockstep until // we can prove overlap one way or another. Essentially, we treat `Overlap` as // a monoid and report a conflict if the product ends up not being `Disjoint`. // // At each step, if we didn't run out of borrow or place, we know that our elements // have the same type, and that they only overlap if they are the identical. // // For example, if we are comparing these: // BORROW: (*x1[2].y).z.a // ACCESS: (*x1[i].y).w.b // // Then our steps are: // x1 | x1 -- places are the same // x1[2] | x1[i] -- equal or disjoint (disjoint if indexes differ) // x1[2].y | x1[i].y -- equal or disjoint // *x1[2].y | *x1[i].y -- equal or disjoint // (*x1[2].y).z | (*x1[i].y).w -- we are disjoint and don't need to check more! // // Because `zip` does potentially bad things to the iterator inside, this loop // also handles the case where the access might be a *prefix* of the borrow, e.g. // // BORROW: (*x1[2].y).z.a // ACCESS: x1[i].y // // Then our steps are: // x1 | x1 -- places are the same // x1[2] | x1[i] -- equal or disjoint (disjoint if indexes differ) // x1[2].y | x1[i].y -- equal or disjoint // // -- here we run out of access - the borrow can access a part of it. If this // is a full deep access, then we *know* the borrow conflicts with it. However, // if the access is shallow, then we can proceed: // // x1[2].y | (*x1[i].y) -- a deref! the access can't get past this, so we // are disjoint // // Our invariant is, that at each step of the iteration: // - If we didn't run out of access to match, our borrow and access are comparable // and either equal or disjoint. // - If we did run out of accesss, the borrow can access a part of it. for (borrow_c, access_c) in borrow_components.zip(access_components) { // loop invariant: borrow_c is always either equal to access_c or disjoint from it. debug!("places_conflict: {:?} vs. {:?}", borrow_c, access_c); match (borrow_c, access_c) { (None, _) => { // If we didn't run out of access, the borrow can access all of our // place (e.g. a borrow of `a.b` with an access to `a.b.c`), // so we have a conflict. // // If we did, then we still know that the borrow can access a *part* // of our place that our access cares about (a borrow of `a.b.c` // with an access to `a.b`), so we still have a conflict. // // FIXME: Differs from AST-borrowck; includes drive-by fix // to #38899. Will probably need back-compat mode flag. debug!("places_conflict: full borrow, CONFLICT"); return true; } (Some(borrow_c), None) => { // We know that the borrow can access a part of our place. This // is a conflict if that is a part our access cares about. let (base, elem) = match borrow_c { Place::Projection(box Projection { base, elem }) => (base, elem), _ => bug!("place has no base?") }; let base_ty = base.ty(self.mir, self.tcx).to_ty(self.tcx); match (elem, &base_ty.sty, access) { (_, _, Shallow(Some(ArtificialField::Discriminant))) | (_, _, Shallow(Some(ArtificialField::ArrayLength))) => { // The discriminant and array length are like // additional fields on the type; they do not // overlap any existing data there. Furthermore, // they cannot actually be a prefix of any // borrowed place (at least in MIR as it is // currently.) // // e.g. a (mutable) borrow of `a[5]` while we read the // array length of `a`. debug!("places_conflict: implicit field"); return false; } (ProjectionElem::Deref, _, Shallow(None)) => { // e.g. a borrow of `*x.y` while we shallowly access `x.y` or some // prefix thereof - the shallow access can't touch anything behind // the pointer. debug!("places_conflict: shallow access behind ptr"); return false; } (ProjectionElem::Deref, ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }), _) => { // the borrow goes through a dereference of a shared reference. // // I'm not sure why we are tracking these borrows - shared // references can *always* be aliased, which means the // permission check already account for this borrow. debug!("places_conflict: behind a shared ref"); return false; } (ProjectionElem::Deref, _, Deep) | (ProjectionElem::Field { .. }, _, _) | (ProjectionElem::Index { ..}, _, _) | (ProjectionElem::ConstantIndex { .. }, _, _) | (ProjectionElem::Subslice { .. }, _, _) | (ProjectionElem::Downcast { .. }, _, _) => { // Recursive case. This can still be disjoint on a // further iteration if this a shallow access and // there's a deref later on, e.g. a borrow // of `*x.y` while accessing `x`. } } } (Some(borrow_c), Some(access_c)) => { match self.place_element_conflict(&borrow_c, access_c) { Overlap::Arbitrary => { // We have encountered different fields of potentially // the same union - the borrow now partially overlaps. // // There is no *easy* way of comparing the fields // further on, because they might have different types // (e.g. borrows of `u.a.0` and `u.b.y` where `.0` and // `.y` come from different structs). // // We could try to do some things here - e.g. count // dereferences - but that's probably not a good // idea, at least for now, so just give up and // report a conflict. This is unsafe code anyway so // the user could always use raw pointers. debug!("places_conflict: arbitrary -> conflict"); return true; } Overlap::EqualOrDisjoint => { // This is the recursive case - proceed to the next element. } Overlap::Disjoint => { // We have proven the borrow disjoint - further // projections will remain disjoint. debug!("places_conflict: disjoint"); return false; } } } } } unreachable!("iter::repeat returned None") } /// This function iterates over all of the current borrows /// (represented by 1-bits in `flow_state.borrows`) that conflict /// with an access to a place, invoking the `op` callback for each /// one. /// /// "Current borrow" here means a borrow that reaches the point in /// the control-flow where the access occurs. /// /// The borrow's phase is represented by the ReserveOrActivateIndex /// passed to the callback: one can call `is_reservation()` and /// `is_activation()` to determine what phase the borrow is /// currently in, when such distinction matters. fn each_borrow_involving_path( &mut self, _context: Context, access_place: (ShallowOrDeep, &Place<'tcx>), flow_state: &Flows<'cx, 'gcx, 'tcx>, mut op: F, ) where F: FnMut(&mut Self, ReserveOrActivateIndex, &BorrowData<'tcx>) -> Control, { let (access, place) = access_place; // FIXME: analogous code in check_loans first maps `place` to // its base_path. let data = flow_state.borrows.operator().borrows(); // check for loan restricting path P being used. Accounts for // borrows of P, P.a.b, etc. let mut elems_incoming = flow_state.borrows.elems_incoming(); while let Some(i) = elems_incoming.next() { let borrowed = &data[i.borrow_index()]; if self.places_conflict(&borrowed.borrowed_place, place, access) { let ctrl = op(self, i, borrowed); if ctrl == Control::Break { return; } } } } } impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> { // FIXME (#16118): function intended to allow the borrow checker // to be less precise in its handling of Box while still allowing // moves out of a Box. They should be removed when/if we stop // treating Box specially (e.g. when/if DerefMove is added...) fn base_path<'d>(&self, place: &'d Place<'tcx>) -> &'d Place<'tcx> { //! Returns the base of the leftmost (deepest) dereference of an //! Box in `place`. If there is no dereference of an Box //! in `place`, then it just returns `place` itself. let mut cursor = place; let mut deepest = place; loop { let proj = match *cursor { Place::Local(..) | Place::Static(..) => return deepest, Place::Projection(ref proj) => proj, }; if proj.elem == ProjectionElem::Deref && place.ty(self.mir, self.tcx).to_ty(self.tcx).is_box() { deepest = &proj.base; } cursor = &proj.base; } } } #[derive(Copy, Clone, PartialEq, Eq, Debug)] struct Context { kind: ContextKind, loc: Location, } #[derive(Copy, Clone, PartialEq, Eq, Debug)] enum ContextKind { Activation, AssignLhs, AssignRhs, SetDiscrim, InlineAsm, SwitchInt, Drop, DropAndReplace, CallOperator, CallOperand, CallDest, Assert, Yield, StorageDead, } impl ContextKind { fn new(self, loc: Location) -> Context { Context { kind: self, loc: loc, } } }