compiler/rustc_borrowck/src/lib.rs

   1 //! This query borrow-checks the MIR to (further) ensure it is not broken.
   2
   3 #![feature(bool_to_option)]
   4 #![feature(box_patterns)]
   5 #![feature(crate_visibility_modifier)]
   6 #![feature(let_else)]
   7 #![feature(min_specialization)]
   8 #![feature(stmt_expr_attributes)]
   9 #![feature(trusted_step)]
  10 #![feature(try_blocks)]
  11 #![recursion_limit = "256"]
  12 #![cfg_attr(not(bootstrap), allow(rustc::potential_query_instability))]
  13
  14 #[macro_use]
  15 extern crate rustc_middle;
  16 #[macro_use]
  17 extern crate tracing;
  18
  19 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  20 use rustc_data_structures::graph::dominators::Dominators;
  21 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorReported};
  22 use rustc_hir as hir;
  23 use rustc_hir::def_id::LocalDefId;
  24 use rustc_hir::Node;
  25 use rustc_index::bit_set::BitSet;
  26 use rustc_index::vec::IndexVec;
  27 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
  28 use rustc_middle::mir::{
  29     traversal, Body, ClearCrossCrate, Local, Location, Mutability, Operand, Place, PlaceElem,
  30     PlaceRef, VarDebugInfoContents,
  31 };
  32 use rustc_middle::mir::{AggregateKind, BasicBlock, BorrowCheckResult, BorrowKind};
  33 use rustc_middle::mir::{Field, ProjectionElem, Promoted, Rvalue, Statement, StatementKind};
  34 use rustc_middle::mir::{InlineAsmOperand, Terminator, TerminatorKind};
  35 use rustc_middle::ty::query::Providers;
  36 use rustc_middle::ty::{self, CapturedPlace, ParamEnv, RegionVid, TyCtxt};
  37 use rustc_session::lint::builtin::{MUTABLE_BORROW_RESERVATION_CONFLICT, UNUSED_MUT};
  38 use rustc_span::{Span, Symbol, DUMMY_SP};
  39
  40 use either::Either;
  41 use smallvec::SmallVec;
  42 use std::cell::RefCell;
  43 use std::collections::BTreeMap;
  44 use std::mem;
  45 use std::rc::Rc;
  46
  47 use rustc_mir_dataflow::impls::{
  48     EverInitializedPlaces, MaybeInitializedPlaces, MaybeUninitializedPlaces,
  49 };
  50 use rustc_mir_dataflow::move_paths::{InitIndex, MoveOutIndex, MovePathIndex};
  51 use rustc_mir_dataflow::move_paths::{InitLocation, LookupResult, MoveData, MoveError};
  52 use rustc_mir_dataflow::Analysis;
  53 use rustc_mir_dataflow::MoveDataParamEnv;
  54
  55 use self::diagnostics::{AccessKind, RegionName};
  56 use self::location::LocationTable;
  57 use self::prefixes::PrefixSet;
  58 use facts::AllFacts;
  59
  60 use self::path_utils::*;
  61
  62 pub mod borrow_set;
  63 mod borrowck_errors;
  64 mod constraint_generation;
  65 mod constraints;
  66 mod dataflow;
  67 mod def_use;
  68 mod diagnostics;
  69 mod facts;
  70 mod invalidation;
  71 mod location;
  72 mod member_constraints;
  73 mod nll;
  74 mod path_utils;
  75 mod place_ext;
  76 mod places_conflict;
  77 mod prefixes;
  78 mod region_infer;
  79 mod renumber;
  80 mod type_check;
  81 mod universal_regions;
  82 mod used_muts;
  83
  84 // A public API provided for the Rust compiler consumers.
  85 pub mod consumers;
  86
  87 use borrow_set::{BorrowData, BorrowSet};
  88 use dataflow::{BorrowIndex, BorrowckFlowState as Flows, BorrowckResults, Borrows};
  89 use nll::{PoloniusOutput, ToRegionVid};
  90 use place_ext::PlaceExt;
  91 use places_conflict::{places_conflict, PlaceConflictBias};
  92 use region_infer::RegionInferenceContext;
  93
  94 // FIXME(eddyb) perhaps move this somewhere more centrally.
  95 #[derive(Debug)]
  96 struct Upvar<'tcx> {
  97     place: CapturedPlace<'tcx>,
  98
  99     /// If true, the capture is behind a reference.
 100     by_ref: bool,
 101 }
 102
 103 const DEREF_PROJECTION: &[PlaceElem<'_>; 1] = &[ProjectionElem::Deref];
 104
 105 pub fn provide(providers: &mut Providers) {
 106     *providers = Providers {
 107         mir_borrowck: |tcx, did| {
 108             if let Some(def) = ty::WithOptConstParam::try_lookup(did, tcx) {
 109                 tcx.mir_borrowck_const_arg(def)
 110             } else {
 111                 mir_borrowck(tcx, ty::WithOptConstParam::unknown(did))
 112             }
 113         },
 114         mir_borrowck_const_arg: |tcx, (did, param_did)| {
 115             mir_borrowck(tcx, ty::WithOptConstParam { did, const_param_did: Some(param_did) })
 116         },
 117         ..*providers
 118     };
 119 }
 120
 121 fn mir_borrowck<'tcx>(
 122     tcx: TyCtxt<'tcx>,
 123     def: ty::WithOptConstParam<LocalDefId>,
 124 ) -> &'tcx BorrowCheckResult<'tcx> {
 125     let (input_body, promoted) = tcx.mir_promoted(def);
 126     debug!("run query mir_borrowck: {}", tcx.def_path_str(def.did.to_def_id()));
 127
 128     let opt_closure_req = tcx.infer_ctxt().with_opaque_type_inference(def.did).enter(|infcx| {
 129         let input_body: &Body<'_> = &input_body.borrow();
 130         let promoted: &IndexVec<_, _> = &promoted.borrow();
 131         do_mir_borrowck(&infcx, input_body, promoted, false).0
 132     });
 133     debug!("mir_borrowck done");
 134
 135     tcx.arena.alloc(opt_closure_req)
 136 }
 137
 138 /// Perform the actual borrow checking.
 139 ///
 140 /// If `return_body_with_facts` is true, then return the body with non-erased
 141 /// region ids on which the borrow checking was performed together with Polonius
 142 /// facts.
 143 #[instrument(skip(infcx, input_body, input_promoted), level = "debug")]
 144 fn do_mir_borrowck<'a, 'tcx>(
 145     infcx: &InferCtxt<'a, 'tcx>,
 146     input_body: &Body<'tcx>,
 147     input_promoted: &IndexVec<Promoted, Body<'tcx>>,
 148     return_body_with_facts: bool,
 149 ) -> (BorrowCheckResult<'tcx>, Option<Box<BodyWithBorrowckFacts<'tcx>>>) {
 150     let def = input_body.source.with_opt_param().as_local().unwrap();
 151
 152     debug!(?def);
 153
 154     let tcx = infcx.tcx;
 155     let param_env = tcx.param_env(def.did);
 156     let id = tcx.hir().local_def_id_to_hir_id(def.did);
 157
 158     let mut local_names = IndexVec::from_elem(None, &input_body.local_decls);
 159     for var_debug_info in &input_body.var_debug_info {
 160         if let VarDebugInfoContents::Place(place) = var_debug_info.value {
 161             if let Some(local) = place.as_local() {
 162                 if let Some(prev_name) = local_names[local] {
 163                     if var_debug_info.name != prev_name {
 164                         span_bug!(
 165                             var_debug_info.source_info.span,
 166                             "local {:?} has many names (`{}` vs `{}`)",
 167                             local,
 168                             prev_name,
 169                             var_debug_info.name
 170                         );
 171                     }
 172                 }
 173                 local_names[local] = Some(var_debug_info.name);
 174             }
 175         }
 176     }
 177
 178     // Gather the upvars of a closure, if any.
 179     let tables = tcx.typeck_opt_const_arg(def);
 180     if let Some(ErrorReported) = tables.tainted_by_errors {
 181         infcx.set_tainted_by_errors();
 182     }
 183     let upvars: Vec<_> = tables
 184         .closure_min_captures_flattened(def.did.to_def_id())
 185         .map(|captured_place| {
 186             let capture = captured_place.info.capture_kind;
 187             let by_ref = match capture {
 188                 ty::UpvarCapture::ByValue => false,
 189                 ty::UpvarCapture::ByRef(..) => true,
 190             };
 191             Upvar { place: captured_place.clone(), by_ref }
 192         })
 193         .collect();
 194
 195     // Replace all regions with fresh inference variables. This
 196     // requires first making our own copy of the MIR. This copy will
 197     // be modified (in place) to contain non-lexical lifetimes. It
 198     // will have a lifetime tied to the inference context.
 199     let mut body_owned = input_body.clone();
 200     let mut promoted = input_promoted.clone();
 201     let free_regions =
 202         nll::replace_regions_in_mir(infcx, param_env, &mut body_owned, &mut promoted);
 203     let body = &body_owned; // no further changes
 204
 205     let location_table_owned = LocationTable::new(body);
 206     let location_table = &location_table_owned;
 207
 208     let mut errors_buffer = Vec::new();
 209     let (move_data, move_errors): (MoveData<'tcx>, Vec<(Place<'tcx>, MoveError<'tcx>)>) =
 210         match MoveData::gather_moves(&body, tcx, param_env) {
 211             Ok(move_data) => (move_data, Vec::new()),
 212             Err((move_data, move_errors)) => (move_data, move_errors),
 213         };
 214     let promoted_errors = promoted
 215         .iter_enumerated()
 216         .map(|(idx, body)| (idx, MoveData::gather_moves(&body, tcx, param_env)));
 217
 218     let mdpe = MoveDataParamEnv { move_data, param_env };
 219
 220     let mut flow_inits = MaybeInitializedPlaces::new(tcx, &body, &mdpe)
 221         .into_engine(tcx, &body)
 222         .pass_name("borrowck")
 223         .iterate_to_fixpoint()
 224         .into_results_cursor(&body);
 225
 226     let locals_are_invalidated_at_exit = tcx.hir().body_owner_kind(id).is_fn_or_closure();
 227     let borrow_set =
 228         Rc::new(BorrowSet::build(tcx, body, locals_are_invalidated_at_exit, &mdpe.move_data));
 229
 230     let use_polonius = return_body_with_facts || infcx.tcx.sess.opts.debugging_opts.polonius;
 231
 232     // Compute non-lexical lifetimes.
 233     let nll::NllOutput {
 234         regioncx,
 235         opaque_type_values,
 236         polonius_input,
 237         polonius_output,
 238         opt_closure_req,
 239         nll_errors,
 240     } = nll::compute_regions(
 241         infcx,
 242         free_regions,
 243         body,
 244         &promoted,
 245         location_table,
 246         param_env,
 247         &mut flow_inits,
 248         &mdpe.move_data,
 249         &borrow_set,
 250         &upvars,
 251         use_polonius,
 252     );
 253
 254     // Dump MIR results into a file, if that is enabled. This let us
 255     // write unit-tests, as well as helping with debugging.
 256     nll::dump_mir_results(infcx, &body, &regioncx, &opt_closure_req);
 257
 258     // We also have a `#[rustc_regions]` annotation that causes us to dump
 259     // information.
 260     nll::dump_annotation(
 261         infcx,
 262         &body,
 263         &regioncx,
 264         &opt_closure_req,
 265         &opaque_type_values,
 266         &mut errors_buffer,
 267     );
 268
 269     // The various `flow_*` structures can be large. We drop `flow_inits` here
 270     // so it doesn't overlap with the others below. This reduces peak memory
 271     // usage significantly on some benchmarks.
 272     drop(flow_inits);
 273
 274     let regioncx = Rc::new(regioncx);
 275
 276     let flow_borrows = Borrows::new(tcx, body, &regioncx, &borrow_set)
 277         .into_engine(tcx, body)
 278         .pass_name("borrowck")
 279         .iterate_to_fixpoint();
 280     let flow_uninits = MaybeUninitializedPlaces::new(tcx, body, &mdpe)
 281         .into_engine(tcx, body)
 282         .pass_name("borrowck")
 283         .iterate_to_fixpoint();
 284     let flow_ever_inits = EverInitializedPlaces::new(tcx, body, &mdpe)
 285         .into_engine(tcx, body)
 286         .pass_name("borrowck")
 287         .iterate_to_fixpoint();
 288
 289     let movable_generator = !matches!(
 290         tcx.hir().get(id),
 291         Node::Expr(&hir::Expr {
 292             kind: hir::ExprKind::Closure(.., Some(hir::Movability::Static)),
 293             ..
 294         })
 295     );
 296
 297     for (idx, move_data_results) in promoted_errors {
 298         let promoted_body = &promoted[idx];
 299
 300         if let Err((move_data, move_errors)) = move_data_results {
 301             let mut promoted_mbcx = MirBorrowckCtxt {
 302                 infcx,
 303                 param_env,
 304                 body: promoted_body,
 305                 move_data: &move_data,
 306                 location_table, // no need to create a real one for the promoted, it is not used
 307                 movable_generator,
 308                 fn_self_span_reported: Default::default(),
 309                 locals_are_invalidated_at_exit,
 310                 access_place_error_reported: Default::default(),
 311                 reservation_error_reported: Default::default(),
 312                 reservation_warnings: Default::default(),
 313                 move_error_reported: BTreeMap::new(),
 314                 uninitialized_error_reported: Default::default(),
 315                 errors_buffer,
 316                 tainted_by_errors: false,
 317                 regioncx: regioncx.clone(),
 318                 used_mut: Default::default(),
 319                 used_mut_upvars: SmallVec::new(),
 320                 borrow_set: Rc::clone(&borrow_set),
 321                 dominators: Dominators::dummy(), // not used
 322                 upvars: Vec::new(),
 323                 local_names: IndexVec::from_elem(None, &promoted_body.local_decls),
 324                 region_names: RefCell::default(),
 325                 next_region_name: RefCell::new(1),
 326                 polonius_output: None,
 327             };
 328             promoted_mbcx.report_move_errors(move_errors);
 329             errors_buffer = promoted_mbcx.errors_buffer;
 330         };
 331     }
 332
 333     let dominators = body.dominators();
 334
 335     let mut mbcx = MirBorrowckCtxt {
 336         infcx,
 337         param_env,
 338         body,
 339         move_data: &mdpe.move_data,
 340         location_table,
 341         movable_generator,
 342         locals_are_invalidated_at_exit,
 343         fn_self_span_reported: Default::default(),
 344         access_place_error_reported: Default::default(),
 345         reservation_error_reported: Default::default(),
 346         reservation_warnings: Default::default(),
 347         move_error_reported: BTreeMap::new(),
 348         uninitialized_error_reported: Default::default(),
 349         errors_buffer,
 350         tainted_by_errors: false,
 351         regioncx: Rc::clone(&regioncx),
 352         used_mut: Default::default(),
 353         used_mut_upvars: SmallVec::new(),
 354         borrow_set: Rc::clone(&borrow_set),
 355         dominators,
 356         upvars,
 357         local_names,
 358         region_names: RefCell::default(),
 359         next_region_name: RefCell::new(1),
 360         polonius_output,
 361     };
 362
 363     // Compute and report region errors, if any.
 364     mbcx.report_region_errors(nll_errors);
 365
 366     let results = BorrowckResults {
 367         ever_inits: flow_ever_inits,
 368         uninits: flow_uninits,
 369         borrows: flow_borrows,
 370     };
 371
 372     mbcx.report_move_errors(move_errors);
 373
 374     rustc_mir_dataflow::visit_results(
 375         body,
 376         traversal::reverse_postorder(body).map(|(bb, _)| bb),
 377         &results,
 378         &mut mbcx,
 379     );
 380
 381     // Convert any reservation warnings into lints.
 382     let reservation_warnings = mem::take(&mut mbcx.reservation_warnings);
 383     for (_, (place, span, location, bk, borrow)) in reservation_warnings {
 384         let mut initial_diag = mbcx.report_conflicting_borrow(location, (place, span), bk, &borrow);
 385
 386         let scope = mbcx.body.source_info(location).scope;
 387         let lint_root = match &mbcx.body.source_scopes[scope].local_data {
 388             ClearCrossCrate::Set(data) => data.lint_root,
 389             _ => id,
 390         };
 391
 392         // Span and message don't matter; we overwrite them below anyway
 393         mbcx.infcx.tcx.struct_span_lint_hir(
 394             MUTABLE_BORROW_RESERVATION_CONFLICT,
 395             lint_root,
 396             DUMMY_SP,
 397             |lint| {
 398                 let mut diag = lint.build("");
 399
 400                 diag.message = initial_diag.styled_message().clone();
 401                 diag.span = initial_diag.span.clone();
 402
 403                 mbcx.buffer_error(diag);
 404             },
 405         );
 406         initial_diag.cancel();
 407     }
 408
 409     // For each non-user used mutable variable, check if it's been assigned from
 410     // a user-declared local. If so, then put that local into the used_mut set.
 411     // Note that this set is expected to be small - only upvars from closures
 412     // would have a chance of erroneously adding non-user-defined mutable vars
 413     // to the set.
 414     let temporary_used_locals: FxHashSet<Local> = mbcx
 415         .used_mut
 416         .iter()
 417         .filter(|&local| !mbcx.body.local_decls[*local].is_user_variable())
 418         .cloned()
 419         .collect();
 420     // For the remaining unused locals that are marked as mutable, we avoid linting any that
 421     // were never initialized. These locals may have been removed as unreachable code; or will be
 422     // linted as unused variables.
 423     let unused_mut_locals =
 424         mbcx.body.mut_vars_iter().filter(|local| !mbcx.used_mut.contains(local)).collect();
 425     mbcx.gather_used_muts(temporary_used_locals, unused_mut_locals);
 426
 427     debug!("mbcx.used_mut: {:?}", mbcx.used_mut);
 428     let used_mut = std::mem::take(&mut mbcx.used_mut);
 429     for local in mbcx.body.mut_vars_and_args_iter().filter(|local| !used_mut.contains(local)) {
 430         let local_decl = &mbcx.body.local_decls[local];
 431         let lint_root = match &mbcx.body.source_scopes[local_decl.source_info.scope].local_data {
 432             ClearCrossCrate::Set(data) => data.lint_root,
 433             _ => continue,
 434         };
 435
 436         // Skip over locals that begin with an underscore or have no name
 437         match mbcx.local_names[local] {
 438             Some(name) => {
 439                 if name.as_str().starts_with('_') {
 440                     continue;
 441                 }
 442             }
 443             None => continue,
 444         }
 445
 446         let span = local_decl.source_info.span;
 447         if span.desugaring_kind().is_some() {
 448             // If the `mut` arises as part of a desugaring, we should ignore it.
 449             continue;
 450         }
 451
 452         tcx.struct_span_lint_hir(UNUSED_MUT, lint_root, span, |lint| {
 453             let mut_span = tcx.sess.source_map().span_until_non_whitespace(span);
 454             lint.build("variable does not need to be mutable")
 455                 .span_suggestion_short(
 456                     mut_span,
 457                     "remove this `mut`",
 458                     String::new(),
 459                     Applicability::MachineApplicable,
 460                 )
 461                 .emit();
 462         })
 463     }
 464
 465     // Buffer any move errors that we collected and de-duplicated.
 466     for (_, (_, diag)) in std::mem::take(&mut mbcx.move_error_reported) {
 467         mbcx.buffer_error(diag);
 468     }
 469
 470     if !mbcx.errors_buffer.is_empty() {
 471         mbcx.errors_buffer.sort_by_key(|diag| diag.sort_span);
 472
 473         for diag in mbcx.errors_buffer.drain(..) {
 474             mbcx.infcx.tcx.sess.diagnostic().emit_diagnostic(&diag);
 475         }
 476     }
 477
 478     let result = BorrowCheckResult {
 479         concrete_opaque_types: opaque_type_values,
 480         closure_requirements: opt_closure_req,
 481         used_mut_upvars: mbcx.used_mut_upvars,
 482         tainted_by_errors: mbcx.tainted_by_errors,
 483     };
 484
 485     let body_with_facts = if return_body_with_facts {
 486         let output_facts = mbcx.polonius_output.expect("Polonius output was not computed");
 487         Some(Box::new(BodyWithBorrowckFacts {
 488             body: body_owned,
 489             input_facts: *polonius_input.expect("Polonius input facts were not generated"),
 490             output_facts,
 491             location_table: location_table_owned,
 492         }))
 493     } else {
 494         None
 495     };
 496
 497     debug!("do_mir_borrowck: result = {:#?}", result);
 498
 499     (result, body_with_facts)
 500 }
 501
 502 /// A `Body` with information computed by the borrow checker. This struct is
 503 /// intended to be consumed by compiler consumers.
 504 ///
 505 /// We need to include the MIR body here because the region identifiers must
 506 /// match the ones in the Polonius facts.
 507 pub struct BodyWithBorrowckFacts<'tcx> {
 508     /// A mir body that contains region identifiers.
 509     pub body: Body<'tcx>,
 510     /// Polonius input facts.
 511     pub input_facts: AllFacts,
 512     /// Polonius output facts.
 513     pub output_facts: Rc<self::nll::PoloniusOutput>,
 514     /// The table that maps Polonius points to locations in the table.
 515     pub location_table: LocationTable,
 516 }
 517
 518 struct MirBorrowckCtxt<'cx, 'tcx> {
 519     infcx: &'cx InferCtxt<'cx, 'tcx>,
 520     param_env: ParamEnv<'tcx>,
 521     body: &'cx Body<'tcx>,
 522     move_data: &'cx MoveData<'tcx>,
 523
 524     /// Map from MIR `Location` to `LocationIndex`; created
 525     /// when MIR borrowck begins.
 526     location_table: &'cx LocationTable,
 527
 528     movable_generator: bool,
 529     /// This keeps track of whether local variables are free-ed when the function
 530     /// exits even without a `StorageDead`, which appears to be the case for
 531     /// constants.
 532     ///
 533     /// I'm not sure this is the right approach - @eddyb could you try and
 534     /// figure this out?
 535     locals_are_invalidated_at_exit: bool,
 536     /// This field keeps track of when borrow errors are reported in the access_place function
 537     /// so that there is no duplicate reporting. This field cannot also be used for the conflicting
 538     /// borrow errors that is handled by the `reservation_error_reported` field as the inclusion
 539     /// of the `Span` type (while required to mute some errors) stops the muting of the reservation
 540     /// errors.
 541     access_place_error_reported: FxHashSet<(Place<'tcx>, Span)>,
 542     /// This field keeps track of when borrow conflict errors are reported
 543     /// for reservations, so that we don't report seemingly duplicate
 544     /// errors for corresponding activations.
 545     //
 546     // FIXME: ideally this would be a set of `BorrowIndex`, not `Place`s,
 547     // but it is currently inconvenient to track down the `BorrowIndex`
 548     // at the time we detect and report a reservation error.
 549     reservation_error_reported: FxHashSet<Place<'tcx>>,
 550     /// This fields keeps track of the `Span`s that we have
 551     /// used to report extra information for `FnSelfUse`, to avoid
 552     /// unnecessarily verbose errors.
 553     fn_self_span_reported: FxHashSet<Span>,
 554     /// Migration warnings to be reported for #56254. We delay reporting these
 555     /// so that we can suppress the warning if there's a corresponding error
 556     /// for the activation of the borrow.
 557     reservation_warnings:
 558         FxHashMap<BorrowIndex, (Place<'tcx>, Span, Location, BorrowKind, BorrowData<'tcx>)>,
 559     /// This field keeps track of move errors that are to be reported for given move indices.
 560     ///
 561     /// There are situations where many errors can be reported for a single move out (see #53807)
 562     /// and we want only the best of those errors.
 563     ///
 564     /// The `report_use_of_moved_or_uninitialized` function checks this map and replaces the
 565     /// diagnostic (if there is one) if the `Place` of the error being reported is a prefix of the
 566     /// `Place` of the previous most diagnostic. This happens instead of buffering the error. Once
 567     /// all move errors have been reported, any diagnostics in this map are added to the buffer
 568     /// to be emitted.
 569     ///
 570     /// `BTreeMap` is used to preserve the order of insertions when iterating. This is necessary
 571     /// when errors in the map are being re-added to the error buffer so that errors with the
 572     /// same primary span come out in a consistent order.
 573     move_error_reported: BTreeMap<Vec<MoveOutIndex>, (PlaceRef<'tcx>, DiagnosticBuilder<'cx>)>,
 574     /// This field keeps track of errors reported in the checking of uninitialized variables,
 575     /// so that we don't report seemingly duplicate errors.
 576     uninitialized_error_reported: FxHashSet<PlaceRef<'tcx>>,
 577     /// Errors to be reported buffer
 578     errors_buffer: Vec<Diagnostic>,
 579     /// Set to true if we emit an error during borrowck
 580     tainted_by_errors: bool,
 581     /// This field keeps track of all the local variables that are declared mut and are mutated.
 582     /// Used for the warning issued by an unused mutable local variable.
 583     used_mut: FxHashSet<Local>,
 584     /// If the function we're checking is a closure, then we'll need to report back the list of
 585     /// mutable upvars that have been used. This field keeps track of them.
 586     used_mut_upvars: SmallVec<[Field; 8]>,
 587     /// Region inference context. This contains the results from region inference and lets us e.g.
 588     /// find out which CFG points are contained in each borrow region.
 589     regioncx: Rc<RegionInferenceContext<'tcx>>,
 590
 591     /// The set of borrows extracted from the MIR
 592     borrow_set: Rc<BorrowSet<'tcx>>,
 593
 594     /// Dominators for MIR
 595     dominators: Dominators<BasicBlock>,
 596
 597     /// Information about upvars not necessarily preserved in types or MIR
 598     upvars: Vec<Upvar<'tcx>>,
 599
 600     /// Names of local (user) variables (extracted from `var_debug_info`).
 601     local_names: IndexVec<Local, Option<Symbol>>,
 602
 603     /// Record the region names generated for each region in the given
 604     /// MIR def so that we can reuse them later in help/error messages.
 605     region_names: RefCell<FxHashMap<RegionVid, RegionName>>,
 606
 607     /// The counter for generating new region names.
 608     next_region_name: RefCell<usize>,
 609
 610     /// Results of Polonius analysis.
 611     polonius_output: Option<Rc<PoloniusOutput>>,
 612 }
 613
 614 // Check that:
 615 // 1. assignments are always made to mutable locations (FIXME: does that still really go here?)
 616 // 2. loans made in overlapping scopes do not conflict
 617 // 3. assignments do not affect things loaned out as immutable
 618 // 4. moves do not affect things loaned out in any way
 619 impl<'cx, 'tcx> rustc_mir_dataflow::ResultsVisitor<'cx, 'tcx> for MirBorrowckCtxt<'cx, 'tcx> {
 620     type FlowState = Flows<'cx, 'tcx>;
 621
 622     fn visit_statement_before_primary_effect(
 623         &mut self,
 624         flow_state: &Flows<'cx, 'tcx>,
 625         stmt: &'cx Statement<'tcx>,
 626         location: Location,
 627     ) {
 628         debug!("MirBorrowckCtxt::process_statement({:?}, {:?}): {:?}", location, stmt, flow_state);
 629         let span = stmt.source_info.span;
 630
 631         self.check_activations(location, span, flow_state);
 632
 633         match &stmt.kind {
 634             StatementKind::Assign(box (lhs, ref rhs)) => {
 635                 self.consume_rvalue(location, (rhs, span), flow_state);
 636
 637                 self.mutate_place(location, (*lhs, span), Shallow(None), flow_state);
 638             }
 639             StatementKind::FakeRead(box (_, ref place)) => {
 640                 // Read for match doesn't access any memory and is used to
 641                 // assert that a place is safe and live. So we don't have to
 642                 // do any checks here.
 643                 //
 644                 // FIXME: Remove check that the place is initialized. This is
 645                 // needed for now because matches don't have never patterns yet.
 646                 // So this is the only place we prevent
 647                 //      let x: !;
 648                 //      match x {};
 649                 // from compiling.
 650                 self.check_if_path_or_subpath_is_moved(
 651                     location,
 652                     InitializationRequiringAction::Use,
 653                     (place.as_ref(), span),
 654                     flow_state,
 655                 );
 656             }
 657             StatementKind::SetDiscriminant { place, variant_index: _ } => {
 658                 self.mutate_place(location, (**place, span), Shallow(None), flow_state);
 659             }
 660             StatementKind::CopyNonOverlapping(box rustc_middle::mir::CopyNonOverlapping {
 661                 ..
 662             }) => {
 663                 span_bug!(
 664                     span,
 665                     "Unexpected CopyNonOverlapping, should only appear after lower_intrinsics",
 666                 )
 667             }
 668             StatementKind::Nop
 669             | StatementKind::Coverage(..)
 670             | StatementKind::AscribeUserType(..)
 671             | StatementKind::Retag { .. }
 672             | StatementKind::StorageLive(..) => {
 673                 // `Nop`, `AscribeUserType`, `Retag`, and `StorageLive` are irrelevant
 674                 // to borrow check.
 675             }
 676             StatementKind::StorageDead(local) => {
 677                 self.access_place(
 678                     location,
 679                     (Place::from(*local), span),
 680                     (Shallow(None), Write(WriteKind::StorageDeadOrDrop)),
 681                     LocalMutationIsAllowed::Yes,
 682                     flow_state,
 683                 );
 684             }
 685         }
 686     }
 687
 688     fn visit_terminator_before_primary_effect(
 689         &mut self,
 690         flow_state: &Flows<'cx, 'tcx>,
 691         term: &'cx Terminator<'tcx>,
 692         loc: Location,
 693     ) {
 694         debug!("MirBorrowckCtxt::process_terminator({:?}, {:?}): {:?}", loc, term, flow_state);
 695         let span = term.source_info.span;
 696
 697         self.check_activations(loc, span, flow_state);
 698
 699         match term.kind {
 700             TerminatorKind::SwitchInt { ref discr, switch_ty: _, targets: _ } => {
 701                 self.consume_operand(loc, (discr, span), flow_state);
 702             }
 703             TerminatorKind::Drop { place, target: _, unwind: _ } => {
 704                 debug!(
 705                     "visit_terminator_drop \
 706                      loc: {:?} term: {:?} place: {:?} span: {:?}",
 707                     loc, term, place, span
 708                 );
 709
 710                 self.access_place(
 711                     loc,
 712                     (place, span),
 713                     (AccessDepth::Drop, Write(WriteKind::StorageDeadOrDrop)),
 714                     LocalMutationIsAllowed::Yes,
 715                     flow_state,
 716                 );
 717             }
 718             TerminatorKind::DropAndReplace {
 719                 place: drop_place,
 720                 value: ref new_value,
 721                 target: _,
 722                 unwind: _,
 723             } => {
 724                 self.mutate_place(loc, (drop_place, span), Deep, flow_state);
 725                 self.consume_operand(loc, (new_value, span), flow_state);
 726             }
 727             TerminatorKind::Call {
 728                 ref func,
 729                 ref args,
 730                 ref destination,
 731                 cleanup: _,
 732                 from_hir_call: _,
 733                 fn_span: _,
 734             } => {
 735                 self.consume_operand(loc, (func, span), flow_state);
 736                 for arg in args {
 737                     self.consume_operand(loc, (arg, span), flow_state);
 738                 }
 739                 if let Some((dest, _ /*bb*/)) = *destination {
 740                     self.mutate_place(loc, (dest, span), Deep, flow_state);
 741                 }
 742             }
 743             TerminatorKind::Assert { ref cond, expected: _, ref msg, target: _, cleanup: _ } => {
 744                 self.consume_operand(loc, (cond, span), flow_state);
 745                 use rustc_middle::mir::AssertKind;
 746                 if let AssertKind::BoundsCheck { ref len, ref index } = *msg {
 747                     self.consume_operand(loc, (len, span), flow_state);
 748                     self.consume_operand(loc, (index, span), flow_state);
 749                 }
 750             }
 751
 752             TerminatorKind::Yield { ref value, resume: _, resume_arg, drop: _ } => {
 753                 self.consume_operand(loc, (value, span), flow_state);
 754                 self.mutate_place(loc, (resume_arg, span), Deep, flow_state);
 755             }
 756
 757             TerminatorKind::InlineAsm {
 758                 template: _,
 759                 ref operands,
 760                 options: _,
 761                 line_spans: _,
 762                 destination: _,
 763                 cleanup: _,
 764             } => {
 765                 for op in operands {
 766                     match *op {
 767                         InlineAsmOperand::In { reg: _, ref value } => {
 768                             self.consume_operand(loc, (value, span), flow_state);
 769                         }
 770                         InlineAsmOperand::Out { reg: _, late: _, place, .. } => {
 771                             if let Some(place) = place {
 772                                 self.mutate_place(loc, (place, span), Shallow(None), flow_state);
 773                             }
 774                         }
 775                         InlineAsmOperand::InOut { reg: _, late: _, ref in_value, out_place } => {
 776                             self.consume_operand(loc, (in_value, span), flow_state);
 777                             if let Some(out_place) = out_place {
 778                                 self.mutate_place(
 779                                     loc,
 780                                     (out_place, span),
 781                                     Shallow(None),
 782                                     flow_state,
 783                                 );
 784                             }
 785                         }
 786                         InlineAsmOperand::Const { value: _ }
 787                         | InlineAsmOperand::SymFn { value: _ }
 788                         | InlineAsmOperand::SymStatic { def_id: _ } => {}
 789                     }
 790                 }
 791             }
 792
 793             TerminatorKind::Goto { target: _ }
 794             | TerminatorKind::Abort
 795             | TerminatorKind::Unreachable
 796             | TerminatorKind::Resume
 797             | TerminatorKind::Return
 798             | TerminatorKind::GeneratorDrop
 799             | TerminatorKind::FalseEdge { real_target: _, imaginary_target: _ }
 800             | TerminatorKind::FalseUnwind { real_target: _, unwind: _ } => {
 801                 // no data used, thus irrelevant to borrowck
 802             }
 803         }
 804     }
 805
 806     fn visit_terminator_after_primary_effect(
 807         &mut self,
 808         flow_state: &Flows<'cx, 'tcx>,
 809         term: &'cx Terminator<'tcx>,
 810         loc: Location,
 811     ) {
 812         let span = term.source_info.span;
 813
 814         match term.kind {
 815             TerminatorKind::Yield { value: _, resume: _, resume_arg: _, drop: _ } => {
 816                 if self.movable_generator {
 817                     // Look for any active borrows to locals
 818                     let borrow_set = self.borrow_set.clone();
 819                     for i in flow_state.borrows.iter() {
 820                         let borrow = &borrow_set[i];
 821                         self.check_for_local_borrow(borrow, span);
 822                     }
 823                 }
 824             }
 825
 826             TerminatorKind::Resume | TerminatorKind::Return | TerminatorKind::GeneratorDrop => {
 827                 // Returning from the function implicitly kills storage for all locals and statics.
 828                 // Often, the storage will already have been killed by an explicit
 829                 // StorageDead, but we don't always emit those (notably on unwind paths),
 830                 // so this "extra check" serves as a kind of backup.
 831                 let borrow_set = self.borrow_set.clone();
 832                 for i in flow_state.borrows.iter() {
 833                     let borrow = &borrow_set[i];
 834                     self.check_for_invalidation_at_exit(loc, borrow, span);
 835                 }
 836             }
 837
 838             TerminatorKind::Abort
 839             | TerminatorKind::Assert { .. }
 840             | TerminatorKind::Call { .. }
 841             | TerminatorKind::Drop { .. }
 842             | TerminatorKind::DropAndReplace { .. }
 843             | TerminatorKind::FalseEdge { real_target: _, imaginary_target: _ }
 844             | TerminatorKind::FalseUnwind { real_target: _, unwind: _ }
 845             | TerminatorKind::Goto { .. }
 846             | TerminatorKind::SwitchInt { .. }
 847             | TerminatorKind::Unreachable
 848             | TerminatorKind::InlineAsm { .. } => {}
 849         }
 850     }
 851 }
 852
 853 use self::AccessDepth::{Deep, Shallow};
 854 use self::ReadOrWrite::{Activation, Read, Reservation, Write};
 855
 856 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 857 enum ArtificialField {
 858     ArrayLength,
 859     ShallowBorrow,
 860 }
 861
 862 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 863 enum AccessDepth {
 864     /// From the RFC: "A *shallow* access means that the immediate
 865     /// fields reached at P are accessed, but references or pointers
 866     /// found within are not dereferenced. Right now, the only access
 867     /// that is shallow is an assignment like `x = ...;`, which would
 868     /// be a *shallow write* of `x`."
 869     Shallow(Option<ArtificialField>),
 870
 871     /// From the RFC: "A *deep* access means that all data reachable
 872     /// through the given place may be invalidated or accesses by
 873     /// this action."
 874     Deep,
 875
 876     /// Access is Deep only when there is a Drop implementation that
 877     /// can reach the data behind the reference.
 878     Drop,
 879 }
 880
 881 /// Kind of access to a value: read or write
 882 /// (For informational purposes only)
 883 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 884 enum ReadOrWrite {
 885     /// From the RFC: "A *read* means that the existing data may be
 886     /// read, but will not be changed."
 887     Read(ReadKind),
 888
 889     /// From the RFC: "A *write* means that the data may be mutated to
 890     /// new values or otherwise invalidated (for example, it could be
 891     /// de-initialized, as in a move operation).
 892     Write(WriteKind),
 893
 894     /// For two-phase borrows, we distinguish a reservation (which is treated
 895     /// like a Read) from an activation (which is treated like a write), and
 896     /// each of those is furthermore distinguished from Reads/Writes above.
 897     Reservation(WriteKind),
 898     Activation(WriteKind, BorrowIndex),
 899 }
 900
 901 /// Kind of read access to a value
 902 /// (For informational purposes only)
 903 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 904 enum ReadKind {
 905     Borrow(BorrowKind),
 906     Copy,
 907 }
 908
 909 /// Kind of write access to a value
 910 /// (For informational purposes only)
 911 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 912 enum WriteKind {
 913     StorageDeadOrDrop,
 914     MutableBorrow(BorrowKind),
 915     Mutate,
 916     Move,
 917 }
 918
 919 /// When checking permissions for a place access, this flag is used to indicate that an immutable
 920 /// local place can be mutated.
 921 //
 922 // FIXME: @nikomatsakis suggested that this flag could be removed with the following modifications:
 923 // - Merge `check_access_permissions()` and `check_if_reassignment_to_immutable_state()`.
 924 // - Split `is_mutable()` into `is_assignable()` (can be directly assigned) and
 925 //   `is_declared_mutable()`.
 926 // - Take flow state into consideration in `is_assignable()` for local variables.
 927 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 928 enum LocalMutationIsAllowed {
 929     Yes,
 930     /// We want use of immutable upvars to cause a "write to immutable upvar"
 931     /// error, not an "reassignment" error.
 932     ExceptUpvars,
 933     No,
 934 }
 935
 936 #[derive(Copy, Clone, Debug)]
 937 enum InitializationRequiringAction {
 938     Borrow,
 939     MatchOn,
 940     Use,
 941     Assignment,
 942     PartialAssignment,
 943 }
 944
 945 struct RootPlace<'tcx> {
 946     place_local: Local,
 947     place_projection: &'tcx [PlaceElem<'tcx>],
 948     is_local_mutation_allowed: LocalMutationIsAllowed,
 949 }
 950
 951 impl InitializationRequiringAction {
 952     fn as_noun(self) -> &'static str {
 953         match self {
 954             InitializationRequiringAction::Borrow => "borrow",
 955             InitializationRequiringAction::MatchOn => "use", // no good noun
 956             InitializationRequiringAction::Use => "use",
 957             InitializationRequiringAction::Assignment => "assign",
 958             InitializationRequiringAction::PartialAssignment => "assign to part",
 959         }
 960     }
 961
 962     fn as_verb_in_past_tense(self) -> &'static str {
 963         match self {
 964             InitializationRequiringAction::Borrow => "borrowed",
 965             InitializationRequiringAction::MatchOn => "matched on",
 966             InitializationRequiringAction::Use => "used",
 967             InitializationRequiringAction::Assignment => "assigned",
 968             InitializationRequiringAction::PartialAssignment => "partially assigned",
 969         }
 970     }
 971 }
 972
 973 impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 974     fn body(&self) -> &'cx Body<'tcx> {
 975         self.body
 976     }
 977
 978     /// Checks an access to the given place to see if it is allowed. Examines the set of borrows
 979     /// that are in scope, as well as which paths have been initialized, to ensure that (a) the
 980     /// place is initialized and (b) it is not borrowed in some way that would prevent this
 981     /// access.
 982     ///
 983     /// Returns `true` if an error is reported.
 984     fn access_place(
 985         &mut self,
 986         location: Location,
 987         place_span: (Place<'tcx>, Span),
 988         kind: (AccessDepth, ReadOrWrite),
 989         is_local_mutation_allowed: LocalMutationIsAllowed,
 990         flow_state: &Flows<'cx, 'tcx>,
 991     ) {
 992         let (sd, rw) = kind;
 993
 994         if let Activation(_, borrow_index) = rw {
 995             if self.reservation_error_reported.contains(&place_span.0) {
 996                 debug!(
 997                     "skipping access_place for activation of invalid reservation \
 998                      place: {:?} borrow_index: {:?}",
 999                     place_span.0, borrow_index
1000                 );
1001                 return;
1002             }
1003         }
1004
1005         // Check is_empty() first because it's the common case, and doing that
1006         // way we avoid the clone() call.
1007         if !self.access_place_error_reported.is_empty()
1008             && self.access_place_error_reported.contains(&(place_span.0, place_span.1))
1009         {
1010             debug!(
1011                 "access_place: suppressing error place_span=`{:?}` kind=`{:?}`",
1012                 place_span, kind
1013             );
1014             return;
1015         }
1016
1017         let mutability_error = self.check_access_permissions(
1018             place_span,
1019             rw,
1020             is_local_mutation_allowed,
1021             flow_state,
1022             location,
1023         );
1024         let conflict_error =
1025             self.check_access_for_conflict(location, place_span, sd, rw, flow_state);
1026
1027         if let (Activation(_, borrow_idx), true) = (kind.1, conflict_error) {
1028             // Suppress this warning when there's an error being emitted for the
1029             // same borrow: fixing the error is likely to fix the warning.
1030             self.reservation_warnings.remove(&borrow_idx);
1031         }
1032
1033         if conflict_error || mutability_error {
1034             debug!("access_place: logging error place_span=`{:?}` kind=`{:?}`", place_span, kind);
1035
1036             self.set_tainted_by_errors();
1037             self.access_place_error_reported.insert((place_span.0, place_span.1));
1038         }
1039     }
1040
1041     fn check_access_for_conflict(
1042         &mut self,
1043         location: Location,
1044         place_span: (Place<'tcx>, Span),
1045         sd: AccessDepth,
1046         rw: ReadOrWrite,
1047         flow_state: &Flows<'cx, 'tcx>,
1048     ) -> bool {
1049         debug!(
1050             "check_access_for_conflict(location={:?}, place_span={:?}, sd={:?}, rw={:?})",
1051             location, place_span, sd, rw,
1052         );
1053
1054         let mut error_reported = false;
1055         let tcx = self.infcx.tcx;
1056         let body = self.body;
1057         let borrow_set = self.borrow_set.clone();
1058
1059         // Use polonius output if it has been enabled.
1060         let polonius_output = self.polonius_output.clone();
1061         let borrows_in_scope = if let Some(polonius) = &polonius_output {
1062             let location = self.location_table.start_index(location);
1063             Either::Left(polonius.errors_at(location).iter().copied())
1064         } else {
1065             Either::Right(flow_state.borrows.iter())
1066         };
1067
1068         each_borrow_involving_path(
1069             self,
1070             tcx,
1071             body,
1072             location,
1073             (sd, place_span.0),
1074             &borrow_set,
1075             borrows_in_scope,
1076             |this, borrow_index, borrow| match (rw, borrow.kind) {
1077                 // Obviously an activation is compatible with its own
1078                 // reservation (or even prior activating uses of same
1079                 // borrow); so don't check if they interfere.
1080                 //
1081                 // NOTE: *reservations* do conflict with themselves;
1082                 // thus aren't injecting unsoundenss w/ this check.)
1083                 (Activation(_, activating), _) if activating == borrow_index => {
1084                     debug!(
1085                         "check_access_for_conflict place_span: {:?} sd: {:?} rw: {:?} \
1086                          skipping {:?} b/c activation of same borrow_index",
1087                         place_span,
1088                         sd,
1089                         rw,
1090                         (borrow_index, borrow),
1091                     );
1092                     Control::Continue
1093                 }
1094
1095                 (Read(_), BorrowKind::Shared | BorrowKind::Shallow)
1096                 | (
1097                     Read(ReadKind::Borrow(BorrowKind::Shallow)),
1098                     BorrowKind::Unique | BorrowKind::Mut { .. },
1099                 ) => Control::Continue,
1100
1101                 (Write(WriteKind::Move), BorrowKind::Shallow) => {
1102                     // Handled by initialization checks.
1103                     Control::Continue
1104                 }
1105
1106                 (Read(kind), BorrowKind::Unique | BorrowKind::Mut { .. }) => {
1107                     // Reading from mere reservations of mutable-borrows is OK.
1108                     if !is_active(&this.dominators, borrow, location) {
1109                         assert!(allow_two_phase_borrow(borrow.kind));
1110                         return Control::Continue;
1111                     }
1112
1113                     error_reported = true;
1114                     match kind {
1115                         ReadKind::Copy => {
1116                             let err = this
1117                                 .report_use_while_mutably_borrowed(location, place_span, borrow);
1118                             this.buffer_error(err);
1119                         }
1120                         ReadKind::Borrow(bk) => {
1121                             let err =
1122                                 this.report_conflicting_borrow(location, place_span, bk, borrow);
1123                             this.buffer_error(err);
1124                         }
1125                     }
1126                     Control::Break
1127                 }
1128
1129                 (
1130                     Reservation(WriteKind::MutableBorrow(bk)),
1131                     BorrowKind::Shallow | BorrowKind::Shared,
1132                 ) if { tcx.migrate_borrowck() && this.borrow_set.contains(&location) } => {
1133                     let bi = this.borrow_set.get_index_of(&location).unwrap();
1134                     debug!(
1135                         "recording invalid reservation of place: {:?} with \
1136                          borrow index {:?} as warning",
1137                         place_span.0, bi,
1138                     );
1139                     // rust-lang/rust#56254 - This was previously permitted on
1140                     // the 2018 edition so we emit it as a warning. We buffer
1141                     // these sepately so that we only emit a warning if borrow
1142                     // checking was otherwise successful.
1143                     this.reservation_warnings
1144                         .insert(bi, (place_span.0, place_span.1, location, bk, borrow.clone()));
1145
1146                     // Don't suppress actual errors.
1147                     Control::Continue
1148                 }
1149
1150                 (Reservation(kind) | Activation(kind, _) | Write(kind), _) => {
1151                     match rw {
1152                         Reservation(..) => {
1153                             debug!(
1154                                 "recording invalid reservation of \
1155                                  place: {:?}",
1156                                 place_span.0
1157                             );
1158                             this.reservation_error_reported.insert(place_span.0);
1159                         }
1160                         Activation(_, activating) => {
1161                             debug!(
1162                                 "observing check_place for activation of \
1163                                  borrow_index: {:?}",
1164                                 activating
1165                             );
1166                         }
1167                         Read(..) | Write(..) => {}
1168                     }
1169
1170                     error_reported = true;
1171                     match kind {
1172                         WriteKind::MutableBorrow(bk) => {
1173                             let err =
1174                                 this.report_conflicting_borrow(location, place_span, bk, borrow);
1175                             this.buffer_error(err);
1176                         }
1177                         WriteKind::StorageDeadOrDrop => this
1178                             .report_borrowed_value_does_not_live_long_enough(
1179                                 location,
1180                                 borrow,
1181                                 place_span,
1182                                 Some(kind),
1183                             ),
1184                         WriteKind::Mutate => {
1185                             this.report_illegal_mutation_of_borrowed(location, place_span, borrow)
1186                         }
1187                         WriteKind::Move => {
1188                             this.report_move_out_while_borrowed(location, place_span, borrow)
1189                         }
1190                     }
1191                     Control::Break
1192                 }
1193             },
1194         );
1195
1196         error_reported
1197     }
1198
1199     fn mutate_place(
1200         &mut self,
1201         location: Location,
1202         place_span: (Place<'tcx>, Span),
1203         kind: AccessDepth,
1204         flow_state: &Flows<'cx, 'tcx>,
1205     ) {
1206         // Write of P[i] or *P requires P init'd.
1207         self.check_if_assigned_path_is_moved(location, place_span, flow_state);
1208
1209         // Special case: you can assign an immutable local variable
1210         // (e.g., `x = ...`) so long as it has never been initialized
1211         // before (at this point in the flow).
1212         if let Some(local) = place_span.0.as_local() {
1213             if let Mutability::Not = self.body.local_decls[local].mutability {
1214                 // check for reassignments to immutable local variables
1215                 self.check_if_reassignment_to_immutable_state(
1216                     location, local, place_span, flow_state,
1217                 );
1218                 return;
1219             }
1220         }
1221
1222         // Otherwise, use the normal access permission rules.
1223         self.access_place(
1224             location,
1225             place_span,
1226             (kind, Write(WriteKind::Mutate)),
1227             LocalMutationIsAllowed::No,
1228             flow_state,
1229         );
1230     }
1231
1232     fn consume_rvalue(
1233         &mut self,
1234         location: Location,
1235         (rvalue, span): (&'cx Rvalue<'tcx>, Span),
1236         flow_state: &Flows<'cx, 'tcx>,
1237     ) {
1238         match *rvalue {
1239             Rvalue::Ref(_ /*rgn*/, bk, place) => {
1240                 let access_kind = match bk {
1241                     BorrowKind::Shallow => {
1242                         (Shallow(Some(ArtificialField::ShallowBorrow)), Read(ReadKind::Borrow(bk)))
1243                     }
1244                     BorrowKind::Shared => (Deep, Read(ReadKind::Borrow(bk))),
1245                     BorrowKind::Unique | BorrowKind::Mut { .. } => {
1246                         let wk = WriteKind::MutableBorrow(bk);
1247                         if allow_two_phase_borrow(bk) {
1248                             (Deep, Reservation(wk))
1249                         } else {
1250                             (Deep, Write(wk))
1251                         }
1252                     }
1253                 };
1254
1255                 self.access_place(
1256                     location,
1257                     (place, span),
1258                     access_kind,
1259                     LocalMutationIsAllowed::No,
1260                     flow_state,
1261                 );
1262
1263                 let action = if bk == BorrowKind::Shallow {
1264                     InitializationRequiringAction::MatchOn
1265                 } else {
1266                     InitializationRequiringAction::Borrow
1267                 };
1268
1269                 self.check_if_path_or_subpath_is_moved(
1270                     location,
1271                     action,
1272                     (place.as_ref(), span),
1273                     flow_state,
1274                 );
1275             }
1276
1277             Rvalue::AddressOf(mutability, place) => {
1278                 let access_kind = match mutability {
1279                     Mutability::Mut => (
1280                         Deep,
1281                         Write(WriteKind::MutableBorrow(BorrowKind::Mut {
1282                             allow_two_phase_borrow: false,
1283                         })),
1284                     ),
1285                     Mutability::Not => (Deep, Read(ReadKind::Borrow(BorrowKind::Shared))),
1286                 };
1287
1288                 self.access_place(
1289                     location,
1290                     (place, span),
1291                     access_kind,
1292                     LocalMutationIsAllowed::No,
1293                     flow_state,
1294                 );
1295
1296                 self.check_if_path_or_subpath_is_moved(
1297                     location,
1298                     InitializationRequiringAction::Borrow,
1299                     (place.as_ref(), span),
1300                     flow_state,
1301                 );
1302             }
1303
1304             Rvalue::ThreadLocalRef(_) => {}
1305
1306             Rvalue::Use(ref operand)
1307             | Rvalue::Repeat(ref operand, _)
1308             | Rvalue::UnaryOp(_ /*un_op*/, ref operand)
1309             | Rvalue::Cast(_ /*cast_kind*/, ref operand, _ /*ty*/)
1310             | Rvalue::ShallowInitBox(ref operand, _ /*ty*/) => {
1311                 self.consume_operand(location, (operand, span), flow_state)
1312             }
1313
1314             Rvalue::Len(place) | Rvalue::Discriminant(place) => {
1315                 let af = match *rvalue {
1316                     Rvalue::Len(..) => Some(ArtificialField::ArrayLength),
1317                     Rvalue::Discriminant(..) => None,
1318                     _ => unreachable!(),
1319                 };
1320                 self.access_place(
1321                     location,
1322                     (place, span),
1323                     (Shallow(af), Read(ReadKind::Copy)),
1324                     LocalMutationIsAllowed::No,
1325                     flow_state,
1326                 );
1327                 self.check_if_path_or_subpath_is_moved(
1328                     location,
1329                     InitializationRequiringAction::Use,
1330                     (place.as_ref(), span),
1331                     flow_state,
1332                 );
1333             }
1334
1335             Rvalue::BinaryOp(_bin_op, box (ref operand1, ref operand2))
1336             | Rvalue::CheckedBinaryOp(_bin_op, box (ref operand1, ref operand2)) => {
1337                 self.consume_operand(location, (operand1, span), flow_state);
1338                 self.consume_operand(location, (operand2, span), flow_state);
1339             }
1340
1341             Rvalue::NullaryOp(_op, _ty) => {
1342                 // nullary ops take no dynamic input; no borrowck effect.
1343             }
1344
1345             Rvalue::Aggregate(ref aggregate_kind, ref operands) => {
1346                 // We need to report back the list of mutable upvars that were
1347                 // moved into the closure and subsequently used by the closure,
1348                 // in order to populate our used_mut set.
1349                 match **aggregate_kind {
1350                     AggregateKind::Closure(def_id, _) | AggregateKind::Generator(def_id, _, _) => {
1351                         let BorrowCheckResult { used_mut_upvars, .. } =
1352                             self.infcx.tcx.mir_borrowck(def_id.expect_local());
1353                         debug!("{:?} used_mut_upvars={:?}", def_id, used_mut_upvars);
1354                         for field in used_mut_upvars {
1355                             self.propagate_closure_used_mut_upvar(&operands[field.index()]);
1356                         }
1357                     }
1358                     AggregateKind::Adt(..)
1359                     | AggregateKind::Array(..)
1360                     | AggregateKind::Tuple { .. } => (),
1361                 }
1362
1363                 for operand in operands {
1364                     self.consume_operand(location, (operand, span), flow_state);
1365                 }
1366             }
1367         }
1368     }
1369
1370     fn propagate_closure_used_mut_upvar(&mut self, operand: &Operand<'tcx>) {
1371         let propagate_closure_used_mut_place = |this: &mut Self, place: Place<'tcx>| {
1372             // We have three possibilities here:
1373             // a. We are modifying something through a mut-ref
1374             // b. We are modifying something that is local to our parent
1375             // c. Current body is a nested closure, and we are modifying path starting from
1376             //    a Place captured by our parent closure.
1377
1378             // Handle (c), the path being modified is exactly the path captured by our parent
1379             if let Some(field) = this.is_upvar_field_projection(place.as_ref()) {
1380                 this.used_mut_upvars.push(field);
1381                 return;
1382             }
1383
1384             for (place_ref, proj) in place.iter_projections().rev() {
1385                 // Handle (a)
1386                 if proj == ProjectionElem::Deref {
1387                     match place_ref.ty(this.body(), this.infcx.tcx).ty.kind() {
1388                         // We aren't modifying a variable directly
1389                         ty::Ref(_, _, hir::Mutability::Mut) => return,
1390
1391                         _ => {}
1392                     }
1393                 }
1394
1395                 // Handle (c)
1396                 if let Some(field) = this.is_upvar_field_projection(place_ref) {
1397                     this.used_mut_upvars.push(field);
1398                     return;
1399                 }
1400             }
1401
1402             // Handle(b)
1403             this.used_mut.insert(place.local);
1404         };
1405
1406         // This relies on the current way that by-value
1407         // captures of a closure are copied/moved directly
1408         // when generating MIR.
1409         match *operand {
1410             Operand::Move(place) | Operand::Copy(place) => {
1411                 match place.as_local() {
1412                     Some(local) if !self.body.local_decls[local].is_user_variable() => {
1413                         if self.body.local_decls[local].ty.is_mutable_ptr() {
1414                             // The variable will be marked as mutable by the borrow.
1415                             return;
1416                         }
1417                         // This is an edge case where we have a `move` closure
1418                         // inside a non-move closure, and the inner closure
1419                         // contains a mutation:
1420                         //
1421                         // let mut i = 0;
1422                         // || { move || { i += 1; }; };
1423                         //
1424                         // In this case our usual strategy of assuming that the
1425                         // variable will be captured by mutable reference is
1426                         // wrong, since `i` can be copied into the inner
1427                         // closure from a shared reference.
1428                         //
1429                         // As such we have to search for the local that this
1430                         // capture comes from and mark it as being used as mut.
1431
1432                         let temp_mpi = self.move_data.rev_lookup.find_local(local);
1433                         let init = if let [init_index] = *self.move_data.init_path_map[temp_mpi] {
1434                             &self.move_data.inits[init_index]
1435                         } else {
1436                             bug!("temporary should be initialized exactly once")
1437                         };
1438
1439                         let InitLocation::Statement(loc) = init.location else {
1440                             bug!("temporary initialized in arguments")
1441                         };
1442
1443                         let body = self.body;
1444                         let bbd = &body[loc.block];
1445                         let stmt = &bbd.statements[loc.statement_index];
1446                         debug!("temporary assigned in: stmt={:?}", stmt);
1447
1448                         if let StatementKind::Assign(box (_, Rvalue::Ref(_, _, source))) = stmt.kind
1449                         {
1450                             propagate_closure_used_mut_place(self, source);
1451                         } else {
1452                             bug!(
1453                                 "closures should only capture user variables \
1454                                  or references to user variables"
1455                             );
1456                         }
1457                     }
1458                     _ => propagate_closure_used_mut_place(self, place),
1459                 }
1460             }
1461             Operand::Constant(..) => {}
1462         }
1463     }
1464
1465     fn consume_operand(
1466         &mut self,
1467         location: Location,
1468         (operand, span): (&'cx Operand<'tcx>, Span),
1469         flow_state: &Flows<'cx, 'tcx>,
1470     ) {
1471         match *operand {
1472             Operand::Copy(place) => {
1473                 // copy of place: check if this is "copy of frozen path"
1474                 // (FIXME: see check_loans.rs)
1475                 self.access_place(
1476                     location,
1477                     (place, span),
1478                     (Deep, Read(ReadKind::Copy)),
1479                     LocalMutationIsAllowed::No,
1480                     flow_state,
1481                 );
1482
1483                 // Finally, check if path was already moved.
1484                 self.check_if_path_or_subpath_is_moved(
1485                     location,
1486                     InitializationRequiringAction::Use,
1487                     (place.as_ref(), span),
1488                     flow_state,
1489                 );
1490             }
1491             Operand::Move(place) => {
1492                 // move of place: check if this is move of already borrowed path
1493                 self.access_place(
1494                     location,
1495                     (place, span),
1496                     (Deep, Write(WriteKind::Move)),
1497                     LocalMutationIsAllowed::Yes,
1498                     flow_state,
1499                 );
1500
1501                 // Finally, check if path was already moved.
1502                 self.check_if_path_or_subpath_is_moved(
1503                     location,
1504                     InitializationRequiringAction::Use,
1505                     (place.as_ref(), span),
1506                     flow_state,
1507                 );
1508             }
1509             Operand::Constant(_) => {}
1510         }
1511     }
1512
1513     /// Checks whether a borrow of this place is invalidated when the function
1514     /// exits
1515     fn check_for_invalidation_at_exit(
1516         &mut self,
1517         location: Location,
1518         borrow: &BorrowData<'tcx>,
1519         span: Span,
1520     ) {
1521         debug!("check_for_invalidation_at_exit({:?})", borrow);
1522         let place = borrow.borrowed_place;
1523         let mut root_place = PlaceRef { local: place.local, projection: &[] };
1524
1525         // FIXME(nll-rfc#40): do more precise destructor tracking here. For now
1526         // we just know that all locals are dropped at function exit (otherwise
1527         // we'll have a memory leak) and assume that all statics have a destructor.
1528         //
1529         // FIXME: allow thread-locals to borrow other thread locals?
1530
1531         let (might_be_alive, will_be_dropped) =
1532             if self.body.local_decls[root_place.local].is_ref_to_thread_local() {
1533                 // Thread-locals might be dropped after the function exits
1534                 // We have to dereference the outer reference because
1535                 // borrows don't conflict behind shared references.
1536                 root_place.projection = DEREF_PROJECTION;
1537                 (true, true)
1538             } else {
1539                 (false, self.locals_are_invalidated_at_exit)
1540             };
1541
1542         if !will_be_dropped {
1543             debug!("place_is_invalidated_at_exit({:?}) - won't be dropped", place);
1544             return;
1545         }
1546
1547         let sd = if might_be_alive { Deep } else { Shallow(None) };
1548
1549         if places_conflict::borrow_conflicts_with_place(
1550             self.infcx.tcx,
1551             &self.body,
1552             place,
1553             borrow.kind,
1554             root_place,
1555             sd,
1556             places_conflict::PlaceConflictBias::Overlap,
1557         ) {
1558             debug!("check_for_invalidation_at_exit({:?}): INVALID", place);
1559             // FIXME: should be talking about the region lifetime instead
1560             // of just a span here.
1561             let span = self.infcx.tcx.sess.source_map().end_point(span);
1562             self.report_borrowed_value_does_not_live_long_enough(
1563                 location,
1564                 borrow,
1565                 (place, span),
1566                 None,
1567             )
1568         }
1569     }
1570
1571     /// Reports an error if this is a borrow of local data.
1572     /// This is called for all Yield expressions on movable generators
1573     fn check_for_local_borrow(&mut self, borrow: &BorrowData<'tcx>, yield_span: Span) {
1574         debug!("check_for_local_borrow({:?})", borrow);
1575
1576         if borrow_of_local_data(borrow.borrowed_place) {
1577             let err = self.cannot_borrow_across_generator_yield(
1578                 self.retrieve_borrow_spans(borrow).var_or_use(),
1579                 yield_span,
1580             );
1581
1582             self.buffer_error(err);
1583         }
1584     }
1585
1586     fn check_activations(&mut self, location: Location, span: Span, flow_state: &Flows<'cx, 'tcx>) {
1587         // Two-phase borrow support: For each activation that is newly
1588         // generated at this statement, check if it interferes with
1589         // another borrow.
1590         let borrow_set = self.borrow_set.clone();
1591         for &borrow_index in borrow_set.activations_at_location(location) {
1592             let borrow = &borrow_set[borrow_index];
1593
1594             // only mutable borrows should be 2-phase
1595             assert!(match borrow.kind {
1596                 BorrowKind::Shared | BorrowKind::Shallow => false,
1597                 BorrowKind::Unique | BorrowKind::Mut { .. } => true,
1598             });
1599
1600             self.access_place(
1601                 location,
1602                 (borrow.borrowed_place, span),
1603                 (Deep, Activation(WriteKind::MutableBorrow(borrow.kind), borrow_index)),
1604                 LocalMutationIsAllowed::No,
1605                 flow_state,
1606             );
1607             // We do not need to call `check_if_path_or_subpath_is_moved`
1608             // again, as we already called it when we made the
1609             // initial reservation.
1610         }
1611     }
1612
1613     fn check_if_reassignment_to_immutable_state(
1614         &mut self,
1615         location: Location,
1616         local: Local,
1617         place_span: (Place<'tcx>, Span),
1618         flow_state: &Flows<'cx, 'tcx>,
1619     ) {
1620         debug!("check_if_reassignment_to_immutable_state({:?})", local);
1621
1622         // Check if any of the initializiations of `local` have happened yet:
1623         if let Some(init_index) = self.is_local_ever_initialized(local, flow_state) {
1624             // And, if so, report an error.
1625             let init = &self.move_data.inits[init_index];
1626             let span = init.span(&self.body);
1627             self.report_illegal_reassignment(location, place_span, span, place_span.0);
1628         }
1629     }
1630
1631     fn check_if_full_path_is_moved(
1632         &mut self,
1633         location: Location,
1634         desired_action: InitializationRequiringAction,
1635         place_span: (PlaceRef<'tcx>, Span),
1636         flow_state: &Flows<'cx, 'tcx>,
1637     ) {
1638         let maybe_uninits = &flow_state.uninits;
1639
1640         // Bad scenarios:
1641         //
1642         // 1. Move of `a.b.c`, use of `a.b.c`
1643         // 2. Move of `a.b.c`, use of `a.b.c.d` (without first reinitializing `a.b.c.d`)
1644         // 3. Uninitialized `(a.b.c: &_)`, use of `*a.b.c`; note that with
1645         //    partial initialization support, one might have `a.x`
1646         //    initialized but not `a.b`.
1647         //
1648         // OK scenarios:
1649         //
1650         // 4. Move of `a.b.c`, use of `a.b.d`
1651         // 5. Uninitialized `a.x`, initialized `a.b`, use of `a.b`
1652         // 6. Copied `(a.b: &_)`, use of `*(a.b).c`; note that `a.b`
1653         //    must have been initialized for the use to be sound.
1654         // 7. Move of `a.b.c` then reinit of `a.b.c.d`, use of `a.b.c.d`
1655
1656         // The dataflow tracks shallow prefixes distinctly (that is,
1657         // field-accesses on P distinctly from P itself), in order to
1658         // track substructure initialization separately from the whole
1659         // structure.
1660         //
1661         // E.g., when looking at (*a.b.c).d, if the closest prefix for
1662         // which we have a MovePath is `a.b`, then that means that the
1663         // initialization state of `a.b` is all we need to inspect to
1664         // know if `a.b.c` is valid (and from that we infer that the
1665         // dereference and `.d` access is also valid, since we assume
1666         // `a.b.c` is assigned a reference to an initialized and
1667         // well-formed record structure.)
1668
1669         // Therefore, if we seek out the *closest* prefix for which we
1670         // have a MovePath, that should capture the initialization
1671         // state for the place scenario.
1672         //
1673         // This code covers scenarios 1, 2, and 3.
1674
1675         debug!("check_if_full_path_is_moved place: {:?}", place_span.0);
1676         let (prefix, mpi) = self.move_path_closest_to(place_span.0);
1677         if maybe_uninits.contains(mpi) {
1678             self.report_use_of_moved_or_uninitialized(
1679                 location,
1680                 desired_action,
1681                 (prefix, place_span.0, place_span.1),
1682                 mpi,
1683             );
1684         } // Only query longest prefix with a MovePath, not further
1685         // ancestors; dataflow recurs on children when parents
1686         // move (to support partial (re)inits).
1687         //
1688         // (I.e., querying parents breaks scenario 7; but may want
1689         // to do such a query based on partial-init feature-gate.)
1690     }
1691
1692     /// Subslices correspond to multiple move paths, so we iterate through the
1693     /// elements of the base array. For each element we check
1694     ///
1695     /// * Does this element overlap with our slice.
1696     /// * Is any part of it uninitialized.
1697     fn check_if_subslice_element_is_moved(
1698         &mut self,
1699         location: Location,
1700         desired_action: InitializationRequiringAction,
1701         place_span: (PlaceRef<'tcx>, Span),
1702         maybe_uninits: &BitSet<MovePathIndex>,
1703         from: u64,
1704         to: u64,
1705     ) {
1706         if let Some(mpi) = self.move_path_for_place(place_span.0) {
1707             let move_paths = &self.move_data.move_paths;
1708
1709             let root_path = &move_paths[mpi];
1710             for (child_mpi, child_move_path) in root_path.children(move_paths) {
1711                 let last_proj = child_move_path.place.projection.last().unwrap();
1712                 if let ProjectionElem::ConstantIndex { offset, from_end, .. } = last_proj {
1713                     debug_assert!(!from_end, "Array constant indexing shouldn't be `from_end`.");
1714
1715                     if (from..to).contains(offset) {
1716                         let uninit_child =
1717                             self.move_data.find_in_move_path_or_its_descendants(child_mpi, |mpi| {
1718                                 maybe_uninits.contains(mpi)
1719                             });
1720
1721                         if let Some(uninit_child) = uninit_child {
1722                             self.report_use_of_moved_or_uninitialized(
1723                                 location,
1724                                 desired_action,
1725                                 (place_span.0, place_span.0, place_span.1),
1726                                 uninit_child,
1727                             );
1728                             return; // don't bother finding other problems.
1729                         }
1730                     }
1731                 }
1732             }
1733         }
1734     }
1735
1736     fn check_if_path_or_subpath_is_moved(
1737         &mut self,
1738         location: Location,
1739         desired_action: InitializationRequiringAction,
1740         place_span: (PlaceRef<'tcx>, Span),
1741         flow_state: &Flows<'cx, 'tcx>,
1742     ) {
1743         let maybe_uninits = &flow_state.uninits;
1744
1745         // Bad scenarios:
1746         //
1747         // 1. Move of `a.b.c`, use of `a` or `a.b`
1748         //    partial initialization support, one might have `a.x`
1749         //    initialized but not `a.b`.
1750         // 2. All bad scenarios from `check_if_full_path_is_moved`
1751         //
1752         // OK scenarios:
1753         //
1754         // 3. Move of `a.b.c`, use of `a.b.d`
1755         // 4. Uninitialized `a.x`, initialized `a.b`, use of `a.b`
1756         // 5. Copied `(a.b: &_)`, use of `*(a.b).c`; note that `a.b`
1757         //    must have been initialized for the use to be sound.
1758         // 6. Move of `a.b.c` then reinit of `a.b.c.d`, use of `a.b.c.d`
1759
1760         self.check_if_full_path_is_moved(location, desired_action, place_span, flow_state);
1761
1762         if let Some((place_base, ProjectionElem::Subslice { from, to, from_end: false })) =
1763             place_span.0.last_projection()
1764         {
1765             let place_ty = place_base.ty(self.body(), self.infcx.tcx);
1766             if let ty::Array(..) = place_ty.ty.kind() {
1767                 self.check_if_subslice_element_is_moved(
1768                     location,
1769                     desired_action,
1770                     (place_base, place_span.1),
1771                     maybe_uninits,
1772                     from,
1773                     to,
1774                 );
1775                 return;
1776             }
1777         }
1778
1779         // A move of any shallow suffix of `place` also interferes
1780         // with an attempt to use `place`. This is scenario 3 above.
1781         //
1782         // (Distinct from handling of scenarios 1+2+4 above because
1783         // `place` does not interfere with suffixes of its prefixes,
1784         // e.g., `a.b.c` does not interfere with `a.b.d`)
1785         //
1786         // This code covers scenario 1.
1787
1788         debug!("check_if_path_or_subpath_is_moved place: {:?}", place_span.0);
1789         if let Some(mpi) = self.move_path_for_place(place_span.0) {
1790             let uninit_mpi = self
1791                 .move_data
1792                 .find_in_move_path_or_its_descendants(mpi, |mpi| maybe_uninits.contains(mpi));
1793
1794             if let Some(uninit_mpi) = uninit_mpi {
1795                 self.report_use_of_moved_or_uninitialized(
1796                     location,
1797                     desired_action,
1798                     (place_span.0, place_span.0, place_span.1),
1799                     uninit_mpi,
1800                 );
1801                 return; // don't bother finding other problems.
1802             }
1803         }
1804     }
1805
1806     /// Currently MoveData does not store entries for all places in
1807     /// the input MIR. For example it will currently filter out
1808     /// places that are Copy; thus we do not track places of shared
1809     /// reference type. This routine will walk up a place along its
1810     /// prefixes, searching for a foundational place that *is*
1811     /// tracked in the MoveData.
1812     ///
1813     /// An Err result includes a tag indicated why the search failed.
1814     /// Currently this can only occur if the place is built off of a
1815     /// static variable, as we do not track those in the MoveData.
1816     fn move_path_closest_to(&mut self, place: PlaceRef<'tcx>) -> (PlaceRef<'tcx>, MovePathIndex) {
1817         match self.move_data.rev_lookup.find(place) {
1818             LookupResult::Parent(Some(mpi)) | LookupResult::Exact(mpi) => {
1819                 (self.move_data.move_paths[mpi].place.as_ref(), mpi)
1820             }
1821             LookupResult::Parent(None) => panic!("should have move path for every Local"),
1822         }
1823     }
1824
1825     fn move_path_for_place(&mut self, place: PlaceRef<'tcx>) -> Option<MovePathIndex> {
1826         // If returns None, then there is no move path corresponding
1827         // to a direct owner of `place` (which means there is nothing
1828         // that borrowck tracks for its analysis).
1829
1830         match self.move_data.rev_lookup.find(place) {
1831             LookupResult::Parent(_) => None,
1832             LookupResult::Exact(mpi) => Some(mpi),
1833         }
1834     }
1835
1836     fn check_if_assigned_path_is_moved(
1837         &mut self,
1838         location: Location,
1839         (place, span): (Place<'tcx>, Span),
1840         flow_state: &Flows<'cx, 'tcx>,
1841     ) {
1842         debug!("check_if_assigned_path_is_moved place: {:?}", place);
1843
1844         // None case => assigning to `x` does not require `x` be initialized.
1845         for (place_base, elem) in place.iter_projections().rev() {
1846             match elem {
1847                 ProjectionElem::Index(_/*operand*/) |
1848                 ProjectionElem::ConstantIndex { .. } |
1849                 // assigning to P[i] requires P to be valid.
1850                 ProjectionElem::Downcast(_/*adt_def*/, _/*variant_idx*/) =>
1851                 // assigning to (P->variant) is okay if assigning to `P` is okay
1852                 //
1853                 // FIXME: is this true even if P is an adt with a dtor?
1854                 { }
1855
1856                 // assigning to (*P) requires P to be initialized
1857                 ProjectionElem::Deref => {
1858                     self.check_if_full_path_is_moved(
1859                         location, InitializationRequiringAction::Use,
1860                         (place_base, span), flow_state);
1861                     // (base initialized; no need to
1862                     // recur further)
1863                     break;
1864                 }
1865
1866                 ProjectionElem::Subslice { .. } => {
1867                     panic!("we don't allow assignments to subslices, location: {:?}",
1868                            location);
1869                 }
1870
1871                 ProjectionElem::Field(..) => {
1872                     // if type of `P` has a dtor, then
1873                     // assigning to `P.f` requires `P` itself
1874                     // be already initialized
1875                     let tcx = self.infcx.tcx;
1876                     let base_ty = place_base.ty(self.body(), tcx).ty;
1877                     match base_ty.kind() {
1878                         ty::Adt(def, _) if def.has_dtor(tcx) => {
1879                             self.check_if_path_or_subpath_is_moved(
1880                                 location, InitializationRequiringAction::Assignment,
1881                                 (place_base, span), flow_state);
1882
1883                             // (base initialized; no need to
1884                             // recur further)
1885                             break;
1886                         }
1887
1888                         // Once `let s; s.x = V; read(s.x);`,
1889                         // is allowed, remove this match arm.
1890                         ty::Adt(..) | ty::Tuple(..) => {
1891                             check_parent_of_field(self, location, place_base, span, flow_state);
1892
1893                             // rust-lang/rust#21232, #54499, #54986: during period where we reject
1894                             // partial initialization, do not complain about unnecessary `mut` on
1895                             // an attempt to do a partial initialization.
1896                             self.used_mut.insert(place.local);
1897                         }
1898
1899                         _ => {}
1900                     }
1901                 }
1902             }
1903         }
1904
1905         fn check_parent_of_field<'cx, 'tcx>(
1906             this: &mut MirBorrowckCtxt<'cx, 'tcx>,
1907             location: Location,
1908             base: PlaceRef<'tcx>,
1909             span: Span,
1910             flow_state: &Flows<'cx, 'tcx>,
1911         ) {
1912             // rust-lang/rust#21232: Until Rust allows reads from the
1913             // initialized parts of partially initialized structs, we
1914             // will, starting with the 2018 edition, reject attempts
1915             // to write to structs that are not fully initialized.
1916             //
1917             // In other words, *until* we allow this:
1918             //
1919             // 1. `let mut s; s.x = Val; read(s.x);`
1920             //
1921             // we will for now disallow this:
1922             //
1923             // 2. `let mut s; s.x = Val;`
1924             //
1925             // and also this:
1926             //
1927             // 3. `let mut s = ...; drop(s); s.x=Val;`
1928             //
1929             // This does not use check_if_path_or_subpath_is_moved,
1930             // because we want to *allow* reinitializations of fields:
1931             // e.g., want to allow
1932             //
1933             // `let mut s = ...; drop(s.x); s.x=Val;`
1934             //
1935             // This does not use check_if_full_path_is_moved on
1936             // `base`, because that would report an error about the
1937             // `base` as a whole, but in this scenario we *really*
1938             // want to report an error about the actual thing that was
1939             // moved, which may be some prefix of `base`.
1940
1941             // Shallow so that we'll stop at any dereference; we'll
1942             // report errors about issues with such bases elsewhere.
1943             let maybe_uninits = &flow_state.uninits;
1944
1945             // Find the shortest uninitialized prefix you can reach
1946             // without going over a Deref.
1947             let mut shortest_uninit_seen = None;
1948             for prefix in this.prefixes(base, PrefixSet::Shallow) {
1949                 let mpi = match this.move_path_for_place(prefix) {
1950                     Some(mpi) => mpi,
1951                     None => continue,
1952                 };
1953
1954                 if maybe_uninits.contains(mpi) {
1955                     debug!(
1956                         "check_parent_of_field updating shortest_uninit_seen from {:?} to {:?}",
1957                         shortest_uninit_seen,
1958                         Some((prefix, mpi))
1959                     );
1960                     shortest_uninit_seen = Some((prefix, mpi));
1961                 } else {
1962                     debug!("check_parent_of_field {:?} is definitely initialized", (prefix, mpi));
1963                 }
1964             }
1965
1966             if let Some((prefix, mpi)) = shortest_uninit_seen {
1967                 // Check for a reassignment into an uninitialized field of a union (for example,
1968                 // after a move out). In this case, do not report an error here. There is an
1969                 // exception, if this is the first assignment into the union (that is, there is
1970                 // no move out from an earlier location) then this is an attempt at initialization
1971                 // of the union - we should error in that case.
1972                 let tcx = this.infcx.tcx;
1973                 if base.ty(this.body(), tcx).ty.is_union() {
1974                     if this.move_data.path_map[mpi].iter().any(|moi| {
1975                         this.move_data.moves[*moi].source.is_predecessor_of(location, this.body)
1976                     }) {
1977                         return;
1978                     }
1979                 }
1980
1981                 this.report_use_of_moved_or_uninitialized(
1982                     location,
1983                     InitializationRequiringAction::PartialAssignment,
1984                     (prefix, base, span),
1985                     mpi,
1986                 );
1987             }
1988         }
1989     }
1990
1991     /// Checks the permissions for the given place and read or write kind
1992     ///
1993     /// Returns `true` if an error is reported.
1994     fn check_access_permissions(
1995         &mut self,
1996         (place, span): (Place<'tcx>, Span),
1997         kind: ReadOrWrite,
1998         is_local_mutation_allowed: LocalMutationIsAllowed,
1999         flow_state: &Flows<'cx, 'tcx>,
2000         location: Location,
2001     ) -> bool {
2002         debug!(
2003             "check_access_permissions({:?}, {:?}, is_local_mutation_allowed: {:?})",
2004             place, kind, is_local_mutation_allowed
2005         );
2006
2007         let error_access;
2008         let the_place_err;
2009
2010         match kind {
2011             Reservation(WriteKind::MutableBorrow(
2012                 borrow_kind @ (BorrowKind::Unique | BorrowKind::Mut { .. }),
2013             ))
2014             | Write(WriteKind::MutableBorrow(
2015                 borrow_kind @ (BorrowKind::Unique | BorrowKind::Mut { .. }),
2016             )) => {
2017                 let is_local_mutation_allowed = match borrow_kind {
2018                     BorrowKind::Unique => LocalMutationIsAllowed::Yes,
2019                     BorrowKind::Mut { .. } => is_local_mutation_allowed,
2020                     BorrowKind::Shared | BorrowKind::Shallow => unreachable!(),
2021                 };
2022                 match self.is_mutable(place.as_ref(), is_local_mutation_allowed) {
2023                     Ok(root_place) => {
2024                         self.add_used_mut(root_place, flow_state);
2025                         return false;
2026                     }
2027                     Err(place_err) => {
2028                         error_access = AccessKind::MutableBorrow;
2029                         the_place_err = place_err;
2030                     }
2031                 }
2032             }
2033             Reservation(WriteKind::Mutate) | Write(WriteKind::Mutate) => {
2034                 match self.is_mutable(place.as_ref(), is_local_mutation_allowed) {
2035                     Ok(root_place) => {
2036                         self.add_used_mut(root_place, flow_state);
2037                         return false;
2038                     }
2039                     Err(place_err) => {
2040                         error_access = AccessKind::Mutate;
2041                         the_place_err = place_err;
2042                     }
2043                 }
2044             }
2045
2046             Reservation(
2047                 WriteKind::Move
2048                 | WriteKind::StorageDeadOrDrop
2049                 | WriteKind::MutableBorrow(BorrowKind::Shared)
2050                 | WriteKind::MutableBorrow(BorrowKind::Shallow),
2051             )
2052             | Write(
2053                 WriteKind::Move
2054                 | WriteKind::StorageDeadOrDrop
2055                 | WriteKind::MutableBorrow(BorrowKind::Shared)
2056                 | WriteKind::MutableBorrow(BorrowKind::Shallow),
2057             ) => {
2058                 if let (Err(_), true) = (
2059                     self.is_mutable(place.as_ref(), is_local_mutation_allowed),
2060                     self.errors_buffer.is_empty(),
2061                 ) {
2062                     // rust-lang/rust#46908: In pure NLL mode this code path should be
2063                     // unreachable, but we use `delay_span_bug` because we can hit this when
2064                     // dereferencing a non-Copy raw pointer *and* have `-Ztreat-err-as-bug`
2065                     // enabled. We don't want to ICE for that case, as other errors will have
2066                     // been emitted (#52262).
2067                     self.infcx.tcx.sess.delay_span_bug(
2068                         span,
2069                         &format!(
2070                             "Accessing `{:?}` with the kind `{:?}` shouldn't be possible",
2071                             place, kind,
2072                         ),
2073                     );
2074                 }
2075                 return false;
2076             }
2077             Activation(..) => {
2078                 // permission checks are done at Reservation point.
2079                 return false;
2080             }
2081             Read(
2082                 ReadKind::Borrow(
2083                     BorrowKind::Unique
2084                     | BorrowKind::Mut { .. }
2085                     | BorrowKind::Shared
2086                     | BorrowKind::Shallow,
2087                 )
2088                 | ReadKind::Copy,
2089             ) => {
2090                 // Access authorized
2091                 return false;
2092             }
2093         }
2094
2095         // rust-lang/rust#21232, #54986: during period where we reject
2096         // partial initialization, do not complain about mutability
2097         // errors except for actual mutation (as opposed to an attempt
2098         // to do a partial initialization).
2099         let previously_initialized =
2100             self.is_local_ever_initialized(place.local, flow_state).is_some();
2101
2102         // at this point, we have set up the error reporting state.
2103         if previously_initialized {
2104             self.report_mutability_error(place, span, the_place_err, error_access, location);
2105             true
2106         } else {
2107             false
2108         }
2109     }
2110
2111     fn is_local_ever_initialized(
2112         &self,
2113         local: Local,
2114         flow_state: &Flows<'cx, 'tcx>,
2115     ) -> Option<InitIndex> {
2116         let mpi = self.move_data.rev_lookup.find_local(local);
2117         let ii = &self.move_data.init_path_map[mpi];
2118         for &index in ii {
2119             if flow_state.ever_inits.contains(index) {
2120                 return Some(index);
2121             }
2122         }
2123         None
2124     }
2125
2126     /// Adds the place into the used mutable variables set
2127     fn add_used_mut(&mut self, root_place: RootPlace<'tcx>, flow_state: &Flows<'cx, 'tcx>) {
2128         match root_place {
2129             RootPlace { place_local: local, place_projection: [], is_local_mutation_allowed } => {
2130                 // If the local may have been initialized, and it is now currently being
2131                 // mutated, then it is justified to be annotated with the `mut`
2132                 // keyword, since the mutation may be a possible reassignment.
2133                 if is_local_mutation_allowed != LocalMutationIsAllowed::Yes
2134                     && self.is_local_ever_initialized(local, flow_state).is_some()
2135                 {
2136                     self.used_mut.insert(local);
2137                 }
2138             }
2139             RootPlace {
2140                 place_local: _,
2141                 place_projection: _,
2142                 is_local_mutation_allowed: LocalMutationIsAllowed::Yes,
2143             } => {}
2144             RootPlace {
2145                 place_local,
2146                 place_projection: place_projection @ [.., _],
2147                 is_local_mutation_allowed: _,
2148             } => {
2149                 if let Some(field) = self.is_upvar_field_projection(PlaceRef {
2150                     local: place_local,
2151                     projection: place_projection,
2152                 }) {
2153                     self.used_mut_upvars.push(field);
2154                 }
2155             }
2156         }
2157     }
2158
2159     /// Whether this value can be written or borrowed mutably.
2160     /// Returns the root place if the place passed in is a projection.
2161     fn is_mutable(
2162         &self,
2163         place: PlaceRef<'tcx>,
2164         is_local_mutation_allowed: LocalMutationIsAllowed,
2165     ) -> Result<RootPlace<'tcx>, PlaceRef<'tcx>> {
2166         debug!("is_mutable: place={:?}, is_local...={:?}", place, is_local_mutation_allowed);
2167         match place.last_projection() {
2168             None => {
2169                 let local = &self.body.local_decls[place.local];
2170                 match local.mutability {
2171                     Mutability::Not => match is_local_mutation_allowed {
2172                         LocalMutationIsAllowed::Yes => Ok(RootPlace {
2173                             place_local: place.local,
2174                             place_projection: place.projection,
2175                             is_local_mutation_allowed: LocalMutationIsAllowed::Yes,
2176                         }),
2177                         LocalMutationIsAllowed::ExceptUpvars => Ok(RootPlace {
2178                             place_local: place.local,
2179                             place_projection: place.projection,
2180                             is_local_mutation_allowed: LocalMutationIsAllowed::ExceptUpvars,
2181                         }),
2182                         LocalMutationIsAllowed::No => Err(place),
2183                     },
2184                     Mutability::Mut => Ok(RootPlace {
2185                         place_local: place.local,
2186                         place_projection: place.projection,
2187                         is_local_mutation_allowed,
2188                     }),
2189                 }
2190             }
2191             Some((place_base, elem)) => {
2192                 match elem {
2193                     ProjectionElem::Deref => {
2194                         let base_ty = place_base.ty(self.body(), self.infcx.tcx).ty;
2195
2196                         // Check the kind of deref to decide
2197                         match base_ty.kind() {
2198                             ty::Ref(_, _, mutbl) => {
2199                                 match mutbl {
2200                                     // Shared borrowed data is never mutable
2201                                     hir::Mutability::Not => Err(place),
2202                                     // Mutably borrowed data is mutable, but only if we have a
2203                                     // unique path to the `&mut`
2204                                     hir::Mutability::Mut => {
2205                                         let mode = match self.is_upvar_field_projection(place) {
2206                                             Some(field) if self.upvars[field.index()].by_ref => {
2207                                                 is_local_mutation_allowed
2208                                             }
2209                                             _ => LocalMutationIsAllowed::Yes,
2210                                         };
2211
2212                                         self.is_mutable(place_base, mode)
2213                                     }
2214                                 }
2215                             }
2216                             ty::RawPtr(tnm) => {
2217                                 match tnm.mutbl {
2218                                     // `*const` raw pointers are not mutable
2219                                     hir::Mutability::Not => Err(place),
2220                                     // `*mut` raw pointers are always mutable, regardless of
2221                                     // context. The users have to check by themselves.
2222                                     hir::Mutability::Mut => Ok(RootPlace {
2223                                         place_local: place.local,
2224                                         place_projection: place.projection,
2225                                         is_local_mutation_allowed,
2226                                     }),
2227                                 }
2228                             }
2229                             // `Box<T>` owns its content, so mutable if its location is mutable
2230                             _ if base_ty.is_box() => {
2231                                 self.is_mutable(place_base, is_local_mutation_allowed)
2232                             }
2233                             // Deref should only be for reference, pointers or boxes
2234                             _ => bug!("Deref of unexpected type: {:?}", base_ty),
2235                         }
2236                     }
2237                     // All other projections are owned by their base path, so mutable if
2238                     // base path is mutable
2239                     ProjectionElem::Field(..)
2240                     | ProjectionElem::Index(..)
2241                     | ProjectionElem::ConstantIndex { .. }
2242                     | ProjectionElem::Subslice { .. }
2243                     | ProjectionElem::Downcast(..) => {
2244                         let upvar_field_projection = self.is_upvar_field_projection(place);
2245                         if let Some(field) = upvar_field_projection {
2246                             let upvar = &self.upvars[field.index()];
2247                             debug!(
2248                                 "is_mutable: upvar.mutability={:?} local_mutation_is_allowed={:?} \
2249                                  place={:?}, place_base={:?}",
2250                                 upvar, is_local_mutation_allowed, place, place_base
2251                             );
2252                             match (upvar.place.mutability, is_local_mutation_allowed) {
2253                                 (
2254                                     Mutability::Not,
2255                                     LocalMutationIsAllowed::No
2256                                     | LocalMutationIsAllowed::ExceptUpvars,
2257                                 ) => Err(place),
2258                                 (Mutability::Not, LocalMutationIsAllowed::Yes)
2259                                 | (Mutability::Mut, _) => {
2260                                     // Subtle: this is an upvar
2261                                     // reference, so it looks like
2262                                     // `self.foo` -- we want to double
2263                                     // check that the location `*self`
2264                                     // is mutable (i.e., this is not a
2265                                     // `Fn` closure).  But if that
2266                                     // check succeeds, we want to
2267                                     // *blame* the mutability on
2268                                     // `place` (that is,
2269                                     // `self.foo`). This is used to
2270                                     // propagate the info about
2271                                     // whether mutability declarations
2272                                     // are used outwards, so that we register
2273                                     // the outer variable as mutable. Otherwise a
2274                                     // test like this fails to record the `mut`
2275                                     // as needed:
2276                                     //
2277                                     // ```
2278                                     // fn foo<F: FnOnce()>(_f: F) { }
2279                                     // fn main() {
2280                                     //     let var = Vec::new();
2281                                     //     foo(move || {
2282                                     //         var.push(1);
2283                                     //     });
2284                                     // }
2285                                     // ```
2286                                     let _ =
2287                                         self.is_mutable(place_base, is_local_mutation_allowed)?;
2288                                     Ok(RootPlace {
2289                                         place_local: place.local,
2290                                         place_projection: place.projection,
2291                                         is_local_mutation_allowed,
2292                                     })
2293                                 }
2294                             }
2295                         } else {
2296                             self.is_mutable(place_base, is_local_mutation_allowed)
2297                         }
2298                     }
2299                 }
2300             }
2301         }
2302     }
2303
2304     /// If `place` is a field projection, and the field is being projected from a closure type,
2305     /// then returns the index of the field being projected. Note that this closure will always
2306     /// be `self` in the current MIR, because that is the only time we directly access the fields
2307     /// of a closure type.
2308     fn is_upvar_field_projection(&self, place_ref: PlaceRef<'tcx>) -> Option<Field> {
2309         path_utils::is_upvar_field_projection(self.infcx.tcx, &self.upvars, place_ref, self.body())
2310     }
2311
2312     pub fn buffer_error(&mut self, t: DiagnosticBuilder<'_>) {
2313         self.tainted_by_errors = true;
2314         t.buffer(&mut self.errors_buffer);
2315     }
2316
2317     pub fn set_tainted_by_errors(&mut self) {
2318         self.tainted_by_errors = true;
2319     }
2320 }
2321
2322 /// The degree of overlap between 2 places for borrow-checking.
2323 enum Overlap {
2324     /// The places might partially overlap - in this case, we give
2325     /// up and say that they might conflict. This occurs when
2326     /// different fields of a union are borrowed. For example,
2327     /// if `u` is a union, we have no way of telling how disjoint
2328     /// `u.a.x` and `a.b.y` are.
2329     Arbitrary,
2330     /// The places have the same type, and are either completely disjoint
2331     /// or equal - i.e., they can't "partially" overlap as can occur with
2332     /// unions. This is the "base case" on which we recur for extensions
2333     /// of the place.
2334     EqualOrDisjoint,
2335     /// The places are disjoint, so we know all extensions of them
2336     /// will also be disjoint.
2337     Disjoint,
2338 }