compiler/rustc_mir_transform/src/lib.rs

   1 #![feature(box_patterns)]
   2 #![feature(box_syntax)]
   3 #![feature(crate_visibility_modifier)]
   4 #![feature(let_else)]
   5 #![feature(map_try_insert)]
   6 #![feature(min_specialization)]
   7 #![feature(option_get_or_insert_default)]
   8 #![feature(once_cell)]
   9 #![feature(never_type)]
  10 #![feature(trusted_step)]
  11 #![feature(try_blocks)]
  12 #![recursion_limit = "256"]
  13 #![cfg_attr(not(bootstrap), allow(rustc::potential_query_instability))]
  14
  15 #[macro_use]
  16 extern crate tracing;
  17 #[macro_use]
  18 extern crate rustc_middle;
  19
  20 use required_consts::RequiredConstsVisitor;
  21 use rustc_const_eval::util;
  22 use rustc_data_structures::fx::FxIndexSet;
  23 use rustc_data_structures::steal::Steal;
  24 use rustc_hir as hir;
  25 use rustc_hir::def_id::{DefId, LocalDefId};
  26 use rustc_hir::intravisit::{self, Visitor};
  27 use rustc_index::vec::IndexVec;
  28 use rustc_middle::mir::visit::Visitor as _;
  29 use rustc_middle::mir::{traversal, Body, ConstQualifs, MirPass, MirPhase, Promoted};
  30 use rustc_middle::ty::query::Providers;
  31 use rustc_middle::ty::{self, TyCtxt, TypeFoldable};
  32 use rustc_span::{Span, Symbol};
  33
  34 #[macro_use]
  35 mod pass_manager;
  36
  37 use pass_manager::{self as pm, Lint, MirLint, WithMinOptLevel};
  38
  39 mod abort_unwinding_calls;
  40 mod add_call_guards;
  41 mod add_moves_for_packed_drops;
  42 mod add_retag;
  43 mod check_const_item_mutation;
  44 mod check_packed_ref;
  45 pub mod check_unsafety;
  46 mod cleanup_post_borrowck;
  47 mod const_debuginfo;
  48 mod const_goto;
  49 mod const_prop;
  50 mod coverage;
  51 mod deaggregator;
  52 mod deduplicate_blocks;
  53 mod dest_prop;
  54 pub mod dump_mir;
  55 mod early_otherwise_branch;
  56 mod elaborate_drops;
  57 mod function_item_references;
  58 mod generator;
  59 mod inline;
  60 mod instcombine;
  61 mod lower_intrinsics;
  62 mod lower_slice_len;
  63 mod marker;
  64 mod match_branches;
  65 mod multiple_return_terminators;
  66 mod normalize_array_len;
  67 mod nrvo;
  68 mod remove_false_edges;
  69 mod remove_noop_landing_pads;
  70 mod remove_storage_markers;
  71 mod remove_uninit_drops;
  72 mod remove_unneeded_drops;
  73 mod remove_zsts;
  74 mod required_consts;
  75 mod reveal_all;
  76 mod separate_const_switch;
  77 mod shim;
  78 mod simplify;
  79 mod simplify_branches;
  80 mod simplify_comparison_integral;
  81 mod simplify_try;
  82 mod uninhabited_enum_branching;
  83 mod unreachable_prop;
  84
  85 use rustc_const_eval::transform::check_consts::{self, ConstCx};
  86 use rustc_const_eval::transform::promote_consts;
  87 use rustc_const_eval::transform::validate;
  88 use rustc_mir_dataflow::rustc_peek;
  89
  90 pub fn provide(providers: &mut Providers) {
  91     check_unsafety::provide(providers);
  92     check_packed_ref::provide(providers);
  93     coverage::query::provide(providers);
  94     shim::provide(providers);
  95     *providers = Providers {
  96         mir_keys,
  97         mir_const,
  98         mir_const_qualif: |tcx, def_id| {
  99             let def_id = def_id.expect_local();
 100             if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
 101                 tcx.mir_const_qualif_const_arg(def)
 102             } else {
 103                 mir_const_qualif(tcx, ty::WithOptConstParam::unknown(def_id))
 104             }
 105         },
 106         mir_const_qualif_const_arg: |tcx, (did, param_did)| {
 107             mir_const_qualif(tcx, ty::WithOptConstParam { did, const_param_did: Some(param_did) })
 108         },
 109         mir_promoted,
 110         mir_drops_elaborated_and_const_checked,
 111         mir_for_ctfe,
 112         mir_for_ctfe_of_const_arg,
 113         optimized_mir,
 114         is_mir_available,
 115         is_ctfe_mir_available: |tcx, did| is_mir_available(tcx, did),
 116         mir_callgraph_reachable: inline::cycle::mir_callgraph_reachable,
 117         mir_inliner_callees: inline::cycle::mir_inliner_callees,
 118         promoted_mir: |tcx, def_id| {
 119             let def_id = def_id.expect_local();
 120             if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
 121                 tcx.promoted_mir_of_const_arg(def)
 122             } else {
 123                 promoted_mir(tcx, ty::WithOptConstParam::unknown(def_id))
 124             }
 125         },
 126         promoted_mir_of_const_arg: |tcx, (did, param_did)| {
 127             promoted_mir(tcx, ty::WithOptConstParam { did, const_param_did: Some(param_did) })
 128         },
 129         ..*providers
 130     };
 131 }
 132
 133 fn is_mir_available(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 134     let def_id = def_id.expect_local();
 135     tcx.mir_keys(()).contains(&def_id)
 136 }
 137
 138 /// Finds the full set of `DefId`s within the current crate that have
 139 /// MIR associated with them.
 140 fn mir_keys(tcx: TyCtxt<'_>, (): ()) -> FxIndexSet<LocalDefId> {
 141     let mut set = FxIndexSet::default();
 142
 143     // All body-owners have MIR associated with them.
 144     set.extend(tcx.hir().body_owners());
 145
 146     // Additionally, tuple struct/variant constructors have MIR, but
 147     // they don't have a BodyId, so we need to build them separately.
 148     struct GatherCtors<'a, 'tcx> {
 149         tcx: TyCtxt<'tcx>,
 150         set: &'a mut FxIndexSet<LocalDefId>,
 151     }
 152     impl<'tcx> Visitor<'tcx> for GatherCtors<'_, 'tcx> {
 153         fn visit_variant_data(
 154             &mut self,
 155             v: &'tcx hir::VariantData<'tcx>,
 156             _: Symbol,
 157             _: &'tcx hir::Generics<'tcx>,
 158             _: hir::HirId,
 159             _: Span,
 160         ) {
 161             if let hir::VariantData::Tuple(_, hir_id) = *v {
 162                 self.set.insert(self.tcx.hir().local_def_id(hir_id));
 163             }
 164             intravisit::walk_struct_def(self, v)
 165         }
 166     }
 167     tcx.hir().visit_all_item_likes(&mut GatherCtors { tcx, set: &mut set }.as_deep_visitor());
 168
 169     set
 170 }
 171
 172 fn mir_const_qualif(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -> ConstQualifs {
 173     let const_kind = tcx.hir().body_const_context(def.did);
 174
 175     // No need to const-check a non-const `fn`.
 176     if const_kind.is_none() {
 177         return Default::default();
 178     }
 179
 180     // N.B., this `borrow()` is guaranteed to be valid (i.e., the value
 181     // cannot yet be stolen), because `mir_promoted()`, which steals
 182     // from `mir_const(), forces this query to execute before
 183     // performing the steal.
 184     let body = &tcx.mir_const(def).borrow();
 185
 186     if body.return_ty().references_error() {
 187         tcx.sess.delay_span_bug(body.span, "mir_const_qualif: MIR had errors");
 188         return Default::default();
 189     }
 190
 191     let ccx = check_consts::ConstCx { body, tcx, const_kind, param_env: tcx.param_env(def.did) };
 192
 193     let mut validator = check_consts::check::Checker::new(&ccx);
 194     validator.check_body();
 195
 196     // We return the qualifs in the return place for every MIR body, even though it is only used
 197     // when deciding to promote a reference to a `const` for now.
 198     validator.qualifs_in_return_place()
 199 }
 200
 201 /// Make MIR ready for const evaluation. This is run on all MIR, not just on consts!
 202 fn mir_const<'tcx>(
 203     tcx: TyCtxt<'tcx>,
 204     def: ty::WithOptConstParam<LocalDefId>,
 205 ) -> &'tcx Steal<Body<'tcx>> {
 206     if let Some(def) = def.try_upgrade(tcx) {
 207         return tcx.mir_const(def);
 208     }
 209
 210     // Unsafety check uses the raw mir, so make sure it is run.
 211     if !tcx.sess.opts.debugging_opts.thir_unsafeck {
 212         if let Some(param_did) = def.const_param_did {
 213             tcx.ensure().unsafety_check_result_for_const_arg((def.did, param_did));
 214         } else {
 215             tcx.ensure().unsafety_check_result(def.did);
 216         }
 217     }
 218
 219     let mut body = tcx.mir_built(def).steal();
 220
 221     rustc_middle::mir::dump_mir(tcx, None, "mir_map", &0, &body, |_, _| Ok(()));
 222
 223     pm::run_passes(
 224         tcx,
 225         &mut body,
 226         &[
 227             // MIR-level lints.
 228             &Lint(check_packed_ref::CheckPackedRef),
 229             &Lint(check_const_item_mutation::CheckConstItemMutation),
 230             &Lint(function_item_references::FunctionItemReferences),
 231             // What we need to do constant evaluation.
 232             &simplify::SimplifyCfg::new("initial"),
 233             &rustc_peek::SanityCheck, // Just a lint
 234             &marker::PhaseChange(MirPhase::Const),
 235         ],
 236     );
 237     tcx.alloc_steal_mir(body)
 238 }
 239
 240 /// Compute the main MIR body and the list of MIR bodies of the promoteds.
 241 fn mir_promoted<'tcx>(
 242     tcx: TyCtxt<'tcx>,
 243     def: ty::WithOptConstParam<LocalDefId>,
 244 ) -> (&'tcx Steal<Body<'tcx>>, &'tcx Steal<IndexVec<Promoted, Body<'tcx>>>) {
 245     if let Some(def) = def.try_upgrade(tcx) {
 246         return tcx.mir_promoted(def);
 247     }
 248
 249     // Ensure that we compute the `mir_const_qualif` for constants at
 250     // this point, before we steal the mir-const result.
 251     // Also this means promotion can rely on all const checks having been done.
 252     let _ = tcx.mir_const_qualif_opt_const_arg(def);
 253     let mut body = tcx.mir_const(def).steal();
 254
 255     let mut required_consts = Vec::new();
 256     let mut required_consts_visitor = RequiredConstsVisitor::new(&mut required_consts);
 257     for (bb, bb_data) in traversal::reverse_postorder(&body) {
 258         required_consts_visitor.visit_basic_block_data(bb, bb_data);
 259     }
 260     body.required_consts = required_consts;
 261
 262     // What we need to run borrowck etc.
 263     let promote_pass = promote_consts::PromoteTemps::default();
 264     pm::run_passes(
 265         tcx,
 266         &mut body,
 267         &[
 268             &promote_pass,
 269             &simplify::SimplifyCfg::new("promote-consts"),
 270             &coverage::InstrumentCoverage,
 271         ],
 272     );
 273
 274     let promoted = promote_pass.promoted_fragments.into_inner();
 275     (tcx.alloc_steal_mir(body), tcx.alloc_steal_promoted(promoted))
 276 }
 277
 278 /// Compute the MIR that is used during CTFE (and thus has no optimizations run on it)
 279 fn mir_for_ctfe<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> &'tcx Body<'tcx> {
 280     let did = def_id.expect_local();
 281     if let Some(def) = ty::WithOptConstParam::try_lookup(did, tcx) {
 282         tcx.mir_for_ctfe_of_const_arg(def)
 283     } else {
 284         tcx.arena.alloc(inner_mir_for_ctfe(tcx, ty::WithOptConstParam::unknown(did)))
 285     }
 286 }
 287
 288 /// Same as `mir_for_ctfe`, but used to get the MIR of a const generic parameter.
 289 /// The docs on `WithOptConstParam` explain this a bit more, but the TLDR is that
 290 /// we'd get cycle errors with `mir_for_ctfe`, because typeck would need to typeck
 291 /// the const parameter while type checking the main body, which in turn would try
 292 /// to type check the main body again.
 293 fn mir_for_ctfe_of_const_arg<'tcx>(
 294     tcx: TyCtxt<'tcx>,
 295     (did, param_did): (LocalDefId, DefId),
 296 ) -> &'tcx Body<'tcx> {
 297     tcx.arena.alloc(inner_mir_for_ctfe(
 298         tcx,
 299         ty::WithOptConstParam { did, const_param_did: Some(param_did) },
 300     ))
 301 }
 302
 303 fn inner_mir_for_ctfe(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -> Body<'_> {
 304     // FIXME: don't duplicate this between the optimized_mir/mir_for_ctfe queries
 305     if tcx.is_constructor(def.did.to_def_id()) {
 306         // There's no reason to run all of the MIR passes on constructors when
 307         // we can just output the MIR we want directly. This also saves const
 308         // qualification and borrow checking the trouble of special casing
 309         // constructors.
 310         return shim::build_adt_ctor(tcx, def.did.to_def_id());
 311     }
 312
 313     let context = tcx
 314         .hir()
 315         .body_const_context(def.did)
 316         .expect("mir_for_ctfe should not be used for runtime functions");
 317
 318     let mut body = tcx.mir_drops_elaborated_and_const_checked(def).borrow().clone();
 319
 320     match context {
 321         // Do not const prop functions, either they get executed at runtime or exported to metadata,
 322         // so we run const prop on them, or they don't, in which case we const evaluate some control
 323         // flow paths of the function and any errors in those paths will get emitted as const eval
 324         // errors.
 325         hir::ConstContext::ConstFn => {}
 326         // Static items always get evaluated, so we can just let const eval see if any erroneous
 327         // control flow paths get executed.
 328         hir::ConstContext::Static(_) => {}
 329         // Associated constants get const prop run so we detect common failure situations in the
 330         // crate that defined the constant.
 331         // Technically we want to not run on regular const items, but oli-obk doesn't know how to
 332         // conveniently detect that at this point without looking at the HIR.
 333         hir::ConstContext::Const => {
 334             pm::run_passes(
 335                 tcx,
 336                 &mut body,
 337                 &[&const_prop::ConstProp, &marker::PhaseChange(MirPhase::Optimization)],
 338             );
 339         }
 340     }
 341
 342     debug_assert!(!body.has_free_regions(), "Free regions in MIR for CTFE");
 343
 344     body
 345 }
 346
 347 /// Obtain just the main MIR (no promoteds) and run some cleanups on it. This also runs
 348 /// mir borrowck *before* doing so in order to ensure that borrowck can be run and doesn't
 349 /// end up missing the source MIR due to stealing happening.
 350 fn mir_drops_elaborated_and_const_checked<'tcx>(
 351     tcx: TyCtxt<'tcx>,
 352     def: ty::WithOptConstParam<LocalDefId>,
 353 ) -> &'tcx Steal<Body<'tcx>> {
 354     if let Some(def) = def.try_upgrade(tcx) {
 355         return tcx.mir_drops_elaborated_and_const_checked(def);
 356     }
 357
 358     // (Mir-)Borrowck uses `mir_promoted`, so we have to force it to
 359     // execute before we can steal.
 360     if let Some(param_did) = def.const_param_did {
 361         tcx.ensure().mir_borrowck_const_arg((def.did, param_did));
 362     } else {
 363         tcx.ensure().mir_borrowck(def.did);
 364     }
 365
 366     let is_fn_like = tcx.hir().get_by_def_id(def.did).fn_kind().is_some();
 367     if is_fn_like {
 368         let did = def.did.to_def_id();
 369         let def = ty::WithOptConstParam::unknown(did);
 370
 371         // Do not compute the mir call graph without said call graph actually being used.
 372         if inline::Inline.is_enabled(&tcx.sess) {
 373             let _ = tcx.mir_inliner_callees(ty::InstanceDef::Item(def));
 374         }
 375     }
 376
 377     let (body, _) = tcx.mir_promoted(def);
 378     let mut body = body.steal();
 379
 380     // IMPORTANT
 381     pm::run_passes(tcx, &mut body, &[&remove_false_edges::RemoveFalseEdges]);
 382
 383     // Do a little drop elaboration before const-checking if `const_precise_live_drops` is enabled.
 384     if check_consts::post_drop_elaboration::checking_enabled(&ConstCx::new(tcx, &body)) {
 385         pm::run_passes(
 386             tcx,
 387             &mut body,
 388             &[
 389                 &simplify::SimplifyCfg::new("remove-false-edges"),
 390                 &remove_uninit_drops::RemoveUninitDrops,
 391             ],
 392         );
 393         check_consts::post_drop_elaboration::check_live_drops(tcx, &body); // FIXME: make this a MIR lint
 394     }
 395
 396     run_post_borrowck_cleanup_passes(tcx, &mut body);
 397     assert!(body.phase == MirPhase::DropLowering);
 398     tcx.alloc_steal_mir(body)
 399 }
 400
 401 /// After this series of passes, no lifetime analysis based on borrowing can be done.
 402 fn run_post_borrowck_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 403     debug!("post_borrowck_cleanup({:?})", body.source.def_id());
 404
 405     let post_borrowck_cleanup: &[&dyn MirPass<'tcx>] = &[
 406         // Remove all things only needed by analysis
 407         &simplify_branches::SimplifyConstCondition::new("initial"),
 408         &remove_noop_landing_pads::RemoveNoopLandingPads,
 409         &cleanup_post_borrowck::CleanupNonCodegenStatements,
 410         &simplify::SimplifyCfg::new("early-opt"),
 411         // These next passes must be executed together
 412         &add_call_guards::CriticalCallEdges,
 413         &elaborate_drops::ElaborateDrops,
 414         // This will remove extraneous landing pads which are no longer
 415         // necessary as well as well as forcing any call in a non-unwinding
 416         // function calling a possibly-unwinding function to abort the process.
 417         &abort_unwinding_calls::AbortUnwindingCalls,
 418         // AddMovesForPackedDrops needs to run after drop
 419         // elaboration.
 420         &add_moves_for_packed_drops::AddMovesForPackedDrops,
 421         // `AddRetag` needs to run after `ElaborateDrops`. Otherwise it should run fairly late,
 422         // but before optimizations begin.
 423         &add_retag::AddRetag,
 424         &lower_intrinsics::LowerIntrinsics,
 425         &simplify::SimplifyCfg::new("elaborate-drops"),
 426         // `Deaggregator` is conceptually part of MIR building, some backends rely on it happening
 427         // and it can help optimizations.
 428         &deaggregator::Deaggregator,
 429     ];
 430
 431     pm::run_passes(tcx, body, post_borrowck_cleanup);
 432 }
 433
 434 fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 435     fn o1<T>(x: T) -> WithMinOptLevel<T> {
 436         WithMinOptLevel(1, x)
 437     }
 438
 439     // Lowering generator control-flow and variables has to happen before we do anything else
 440     // to them. We run some optimizations before that, because they may be harder to do on the state
 441     // machine than on MIR with async primitives.
 442     pm::run_passes(
 443         tcx,
 444         body,
 445         &[
 446             &reveal_all::RevealAll, // has to be done before inlining, since inlined code is in RevealAll mode.
 447             &lower_slice_len::LowerSliceLenCalls, // has to be done before inlining, otherwise actual call will be almost always inlined. Also simple, so can just do first
 448             &normalize_array_len::NormalizeArrayLen, // has to run after `slice::len` lowering
 449             &unreachable_prop::UnreachablePropagation,
 450             &uninhabited_enum_branching::UninhabitedEnumBranching,
 451             &o1(simplify::SimplifyCfg::new("after-uninhabited-enum-branching")),
 452             &inline::Inline,
 453             &generator::StateTransform,
 454         ],
 455     );
 456
 457     assert!(body.phase == MirPhase::GeneratorLowering);
 458
 459     // The main optimizations that we do on MIR.
 460     pm::run_passes(
 461         tcx,
 462         body,
 463         &[
 464             &remove_storage_markers::RemoveStorageMarkers,
 465             &remove_zsts::RemoveZsts,
 466             &const_goto::ConstGoto,
 467             &remove_unneeded_drops::RemoveUnneededDrops,
 468             &match_branches::MatchBranchSimplification,
 469             // inst combine is after MatchBranchSimplification to clean up Ne(_1, false)
 470             &multiple_return_terminators::MultipleReturnTerminators,
 471             &instcombine::InstCombine,
 472             &separate_const_switch::SeparateConstSwitch,
 473             //
 474             // FIXME(#70073): This pass is responsible for both optimization as well as some lints.
 475             &const_prop::ConstProp,
 476             //
 477             // Const-prop runs unconditionally, but doesn't mutate the MIR at mir-opt-level=0.
 478             &o1(simplify_branches::SimplifyConstCondition::new("after-const-prop")),
 479             &early_otherwise_branch::EarlyOtherwiseBranch,
 480             &simplify_comparison_integral::SimplifyComparisonIntegral,
 481             &simplify_try::SimplifyArmIdentity,
 482             &simplify_try::SimplifyBranchSame,
 483             &dest_prop::DestinationPropagation,
 484             &o1(simplify_branches::SimplifyConstCondition::new("final")),
 485             &o1(remove_noop_landing_pads::RemoveNoopLandingPads),
 486             &o1(simplify::SimplifyCfg::new("final")),
 487             &nrvo::RenameReturnPlace,
 488             &const_debuginfo::ConstDebugInfo,
 489             &simplify::SimplifyLocals,
 490             &multiple_return_terminators::MultipleReturnTerminators,
 491             &deduplicate_blocks::DeduplicateBlocks,
 492             // Some cleanup necessary at least for LLVM and potentially other codegen backends.
 493             &add_call_guards::CriticalCallEdges,
 494             &marker::PhaseChange(MirPhase::Optimization),
 495             // Dump the end result for testing and debugging purposes.
 496             &dump_mir::Marker("PreCodegen"),
 497         ],
 498     );
 499 }
 500
 501 /// Optimize the MIR and prepare it for codegen.
 502 fn optimized_mir<'tcx>(tcx: TyCtxt<'tcx>, did: DefId) -> &'tcx Body<'tcx> {
 503     let did = did.expect_local();
 504     assert_eq!(ty::WithOptConstParam::try_lookup(did, tcx), None);
 505     tcx.arena.alloc(inner_optimized_mir(tcx, did))
 506 }
 507
 508 fn inner_optimized_mir(tcx: TyCtxt<'_>, did: LocalDefId) -> Body<'_> {
 509     if tcx.is_constructor(did.to_def_id()) {
 510         // There's no reason to run all of the MIR passes on constructors when
 511         // we can just output the MIR we want directly. This also saves const
 512         // qualification and borrow checking the trouble of special casing
 513         // constructors.
 514         return shim::build_adt_ctor(tcx, did.to_def_id());
 515     }
 516
 517     match tcx.hir().body_const_context(did) {
 518         // Run the `mir_for_ctfe` query, which depends on `mir_drops_elaborated_and_const_checked`
 519         // which we are going to steal below. Thus we need to run `mir_for_ctfe` first, so it
 520         // computes and caches its result.
 521         Some(hir::ConstContext::ConstFn) => tcx.ensure().mir_for_ctfe(did),
 522         None => {}
 523         Some(other) => panic!("do not use `optimized_mir` for constants: {:?}", other),
 524     }
 525     let mut body =
 526         tcx.mir_drops_elaborated_and_const_checked(ty::WithOptConstParam::unknown(did)).steal();
 527     run_optimization_passes(tcx, &mut body);
 528
 529     debug_assert!(!body.has_free_regions(), "Free regions in optimized MIR");
 530
 531     body
 532 }
 533
 534 /// Fetch all the promoteds of an item and prepare their MIR bodies to be ready for
 535 /// constant evaluation once all substitutions become known.
 536 fn promoted_mir<'tcx>(
 537     tcx: TyCtxt<'tcx>,
 538     def: ty::WithOptConstParam<LocalDefId>,
 539 ) -> &'tcx IndexVec<Promoted, Body<'tcx>> {
 540     if tcx.is_constructor(def.did.to_def_id()) {
 541         return tcx.arena.alloc(IndexVec::new());
 542     }
 543
 544     if let Some(param_did) = def.const_param_did {
 545         tcx.ensure().mir_borrowck_const_arg((def.did, param_did));
 546     } else {
 547         tcx.ensure().mir_borrowck(def.did);
 548     }
 549     let (_, promoted) = tcx.mir_promoted(def);
 550     let mut promoted = promoted.steal();
 551
 552     for body in &mut promoted {
 553         run_post_borrowck_cleanup_passes(tcx, body);
 554     }
 555
 556     debug_assert!(!promoted.has_free_regions(), "Free regions in promoted MIR");
 557
 558     tcx.arena.alloc(promoted)
 559 }