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 #![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 // This pass is public to allow external drivers to perform MIR cleanup
  47 pub mod cleanup_post_borrowck;
  48 mod const_debuginfo;
  49 mod const_goto;
  50 mod const_prop;
  51 mod coverage;
  52 mod deaggregator;
  53 mod deduplicate_blocks;
  54 mod dest_prop;
  55 pub mod dump_mir;
  56 mod early_otherwise_branch;
  57 mod elaborate_drops;
  58 mod function_item_references;
  59 mod generator;
  60 mod inline;
  61 mod instcombine;
  62 mod lower_intrinsics;
  63 mod lower_slice_len;
  64 mod marker;
  65 mod match_branches;
  66 mod multiple_return_terminators;
  67 mod normalize_array_len;
  68 mod nrvo;
  69 // This pass is public to allow external drivers to perform MIR cleanup
  70 pub mod remove_false_edges;
  71 mod remove_noop_landing_pads;
  72 mod remove_storage_markers;
  73 mod remove_uninit_drops;
  74 mod remove_unneeded_drops;
  75 mod remove_zsts;
  76 mod required_consts;
  77 mod reveal_all;
  78 mod separate_const_switch;
  79 mod shim;
  80 // This pass is public to allow external drivers to perform MIR cleanup
  81 pub mod simplify;
  82 mod simplify_branches;
  83 mod simplify_comparison_integral;
  84 mod simplify_try;
  85 mod uninhabited_enum_branching;
  86 mod unreachable_prop;
  87
  88 use rustc_const_eval::transform::check_consts::{self, ConstCx};
  89 use rustc_const_eval::transform::promote_consts;
  90 use rustc_const_eval::transform::validate;
  91 use rustc_mir_dataflow::rustc_peek;
  92
  93 pub fn provide(providers: &mut Providers) {
  94     check_unsafety::provide(providers);
  95     check_packed_ref::provide(providers);
  96     coverage::query::provide(providers);
  97     shim::provide(providers);
  98     *providers = Providers {
  99         mir_keys,
 100         mir_const,
 101         mir_const_qualif: |tcx, def_id| {
 102             let def_id = def_id.expect_local();
 103             if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
 104                 tcx.mir_const_qualif_const_arg(def)
 105             } else {
 106                 mir_const_qualif(tcx, ty::WithOptConstParam::unknown(def_id))
 107             }
 108         },
 109         mir_const_qualif_const_arg: |tcx, (did, param_did)| {
 110             mir_const_qualif(tcx, ty::WithOptConstParam { did, const_param_did: Some(param_did) })
 111         },
 112         mir_promoted,
 113         mir_drops_elaborated_and_const_checked,
 114         mir_for_ctfe,
 115         mir_for_ctfe_of_const_arg,
 116         optimized_mir,
 117         is_mir_available,
 118         is_ctfe_mir_available: |tcx, did| is_mir_available(tcx, did),
 119         mir_callgraph_reachable: inline::cycle::mir_callgraph_reachable,
 120         mir_inliner_callees: inline::cycle::mir_inliner_callees,
 121         promoted_mir: |tcx, def_id| {
 122             let def_id = def_id.expect_local();
 123             if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
 124                 tcx.promoted_mir_of_const_arg(def)
 125             } else {
 126                 promoted_mir(tcx, ty::WithOptConstParam::unknown(def_id))
 127             }
 128         },
 129         promoted_mir_of_const_arg: |tcx, (did, param_did)| {
 130             promoted_mir(tcx, ty::WithOptConstParam { did, const_param_did: Some(param_did) })
 131         },
 132         ..*providers
 133     };
 134 }
 135
 136 fn is_mir_available(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 137     let def_id = def_id.expect_local();
 138     tcx.mir_keys(()).contains(&def_id)
 139 }
 140
 141 /// Finds the full set of `DefId`s within the current crate that have
 142 /// MIR associated with them.
 143 fn mir_keys(tcx: TyCtxt<'_>, (): ()) -> FxIndexSet<LocalDefId> {
 144     let mut set = FxIndexSet::default();
 145
 146     // All body-owners have MIR associated with them.
 147     set.extend(tcx.hir().body_owners());
 148
 149     // Additionally, tuple struct/variant constructors have MIR, but
 150     // they don't have a BodyId, so we need to build them separately.
 151     struct GatherCtors<'a, 'tcx> {
 152         tcx: TyCtxt<'tcx>,
 153         set: &'a mut FxIndexSet<LocalDefId>,
 154     }
 155     impl<'tcx> Visitor<'tcx> for GatherCtors<'_, 'tcx> {
 156         fn visit_variant_data(
 157             &mut self,
 158             v: &'tcx hir::VariantData<'tcx>,
 159             _: Symbol,
 160             _: &'tcx hir::Generics<'tcx>,
 161             _: hir::HirId,
 162             _: Span,
 163         ) {
 164             if let hir::VariantData::Tuple(_, hir_id) = *v {
 165                 self.set.insert(self.tcx.hir().local_def_id(hir_id));
 166             }
 167             intravisit::walk_struct_def(self, v)
 168         }
 169     }
 170     tcx.hir().visit_all_item_likes(&mut GatherCtors { tcx, set: &mut set }.as_deep_visitor());
 171
 172     set
 173 }
 174
 175 fn mir_const_qualif(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -> ConstQualifs {
 176     let const_kind = tcx.hir().body_const_context(def.did);
 177
 178     // No need to const-check a non-const `fn`.
 179     if const_kind.is_none() {
 180         return Default::default();
 181     }
 182
 183     // N.B., this `borrow()` is guaranteed to be valid (i.e., the value
 184     // cannot yet be stolen), because `mir_promoted()`, which steals
 185     // from `mir_const(), forces this query to execute before
 186     // performing the steal.
 187     let body = &tcx.mir_const(def).borrow();
 188
 189     if body.return_ty().references_error() {
 190         tcx.sess.delay_span_bug(body.span, "mir_const_qualif: MIR had errors");
 191         return Default::default();
 192     }
 193
 194     let ccx = check_consts::ConstCx { body, tcx, const_kind, param_env: tcx.param_env(def.did) };
 195
 196     let mut validator = check_consts::check::Checker::new(&ccx);
 197     validator.check_body();
 198
 199     // We return the qualifs in the return place for every MIR body, even though it is only used
 200     // when deciding to promote a reference to a `const` for now.
 201     validator.qualifs_in_return_place()
 202 }
 203
 204 /// Make MIR ready for const evaluation. This is run on all MIR, not just on consts!
 205 fn mir_const<'tcx>(
 206     tcx: TyCtxt<'tcx>,
 207     def: ty::WithOptConstParam<LocalDefId>,
 208 ) -> &'tcx Steal<Body<'tcx>> {
 209     if let Some(def) = def.try_upgrade(tcx) {
 210         return tcx.mir_const(def);
 211     }
 212
 213     // Unsafety check uses the raw mir, so make sure it is run.
 214     if !tcx.sess.opts.debugging_opts.thir_unsafeck {
 215         if let Some(param_did) = def.const_param_did {
 216             tcx.ensure().unsafety_check_result_for_const_arg((def.did, param_did));
 217         } else {
 218             tcx.ensure().unsafety_check_result(def.did);
 219         }
 220     }
 221
 222     let mut body = tcx.mir_built(def).steal();
 223
 224     rustc_middle::mir::dump_mir(tcx, None, "mir_map", &0, &body, |_, _| Ok(()));
 225
 226     pm::run_passes(
 227         tcx,
 228         &mut body,
 229         &[
 230             // MIR-level lints.
 231             &Lint(check_packed_ref::CheckPackedRef),
 232             &Lint(check_const_item_mutation::CheckConstItemMutation),
 233             &Lint(function_item_references::FunctionItemReferences),
 234             // What we need to do constant evaluation.
 235             &simplify::SimplifyCfg::new("initial"),
 236             &rustc_peek::SanityCheck, // Just a lint
 237             &marker::PhaseChange(MirPhase::Const),
 238         ],
 239     );
 240     tcx.alloc_steal_mir(body)
 241 }
 242
 243 /// Compute the main MIR body and the list of MIR bodies of the promoteds.
 244 fn mir_promoted<'tcx>(
 245     tcx: TyCtxt<'tcx>,
 246     def: ty::WithOptConstParam<LocalDefId>,
 247 ) -> (&'tcx Steal<Body<'tcx>>, &'tcx Steal<IndexVec<Promoted, Body<'tcx>>>) {
 248     if let Some(def) = def.try_upgrade(tcx) {
 249         return tcx.mir_promoted(def);
 250     }
 251
 252     // Ensure that we compute the `mir_const_qualif` for constants at
 253     // this point, before we steal the mir-const result.
 254     // Also this means promotion can rely on all const checks having been done.
 255     let const_qualifs = tcx.mir_const_qualif_opt_const_arg(def);
 256     let mut body = tcx.mir_const(def).steal();
 257     if let Some(error_reported) = const_qualifs.tainted_by_errors {
 258         body.tainted_by_errors = Some(error_reported);
 259     }
 260
 261     let mut required_consts = Vec::new();
 262     let mut required_consts_visitor = RequiredConstsVisitor::new(&mut required_consts);
 263     for (bb, bb_data) in traversal::reverse_postorder(&body) {
 264         required_consts_visitor.visit_basic_block_data(bb, bb_data);
 265     }
 266     body.required_consts = required_consts;
 267
 268     // What we need to run borrowck etc.
 269     let promote_pass = promote_consts::PromoteTemps::default();
 270     pm::run_passes(
 271         tcx,
 272         &mut body,
 273         &[
 274             &promote_pass,
 275             &simplify::SimplifyCfg::new("promote-consts"),
 276             &coverage::InstrumentCoverage,
 277         ],
 278     );
 279
 280     let promoted = promote_pass.promoted_fragments.into_inner();
 281     (tcx.alloc_steal_mir(body), tcx.alloc_steal_promoted(promoted))
 282 }
 283
 284 /// Compute the MIR that is used during CTFE (and thus has no optimizations run on it)
 285 fn mir_for_ctfe<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> &'tcx Body<'tcx> {
 286     let did = def_id.expect_local();
 287     if let Some(def) = ty::WithOptConstParam::try_lookup(did, tcx) {
 288         tcx.mir_for_ctfe_of_const_arg(def)
 289     } else {
 290         tcx.arena.alloc(inner_mir_for_ctfe(tcx, ty::WithOptConstParam::unknown(did)))
 291     }
 292 }
 293
 294 /// Same as `mir_for_ctfe`, but used to get the MIR of a const generic parameter.
 295 /// The docs on `WithOptConstParam` explain this a bit more, but the TLDR is that
 296 /// we'd get cycle errors with `mir_for_ctfe`, because typeck would need to typeck
 297 /// the const parameter while type checking the main body, which in turn would try
 298 /// to type check the main body again.
 299 fn mir_for_ctfe_of_const_arg<'tcx>(
 300     tcx: TyCtxt<'tcx>,
 301     (did, param_did): (LocalDefId, DefId),
 302 ) -> &'tcx Body<'tcx> {
 303     tcx.arena.alloc(inner_mir_for_ctfe(
 304         tcx,
 305         ty::WithOptConstParam { did, const_param_did: Some(param_did) },
 306     ))
 307 }
 308
 309 fn inner_mir_for_ctfe(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -> Body<'_> {
 310     // FIXME: don't duplicate this between the optimized_mir/mir_for_ctfe queries
 311     if tcx.is_constructor(def.did.to_def_id()) {
 312         // There's no reason to run all of the MIR passes on constructors when
 313         // we can just output the MIR we want directly. This also saves const
 314         // qualification and borrow checking the trouble of special casing
 315         // constructors.
 316         return shim::build_adt_ctor(tcx, def.did.to_def_id());
 317     }
 318
 319     let context = tcx
 320         .hir()
 321         .body_const_context(def.did)
 322         .expect("mir_for_ctfe should not be used for runtime functions");
 323
 324     let mut body = tcx.mir_drops_elaborated_and_const_checked(def).borrow().clone();
 325
 326     match context {
 327         // Do not const prop functions, either they get executed at runtime or exported to metadata,
 328         // so we run const prop on them, or they don't, in which case we const evaluate some control
 329         // flow paths of the function and any errors in those paths will get emitted as const eval
 330         // errors.
 331         hir::ConstContext::ConstFn => {}
 332         // Static items always get evaluated, so we can just let const eval see if any erroneous
 333         // control flow paths get executed.
 334         hir::ConstContext::Static(_) => {}
 335         // Associated constants get const prop run so we detect common failure situations in the
 336         // crate that defined the constant.
 337         // Technically we want to not run on regular const items, but oli-obk doesn't know how to
 338         // conveniently detect that at this point without looking at the HIR.
 339         hir::ConstContext::Const => {
 340             pm::run_passes(
 341                 tcx,
 342                 &mut body,
 343                 &[&const_prop::ConstProp, &marker::PhaseChange(MirPhase::Optimization)],
 344             );
 345         }
 346     }
 347
 348     debug_assert!(!body.has_free_regions(), "Free regions in MIR for CTFE");
 349
 350     body
 351 }
 352
 353 /// Obtain just the main MIR (no promoteds) and run some cleanups on it. This also runs
 354 /// mir borrowck *before* doing so in order to ensure that borrowck can be run and doesn't
 355 /// end up missing the source MIR due to stealing happening.
 356 fn mir_drops_elaborated_and_const_checked<'tcx>(
 357     tcx: TyCtxt<'tcx>,
 358     def: ty::WithOptConstParam<LocalDefId>,
 359 ) -> &'tcx Steal<Body<'tcx>> {
 360     if let Some(def) = def.try_upgrade(tcx) {
 361         return tcx.mir_drops_elaborated_and_const_checked(def);
 362     }
 363
 364     let mir_borrowck = tcx.mir_borrowck_opt_const_arg(def);
 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     if let Some(error_reported) = mir_borrowck.tainted_by_errors {
 380         body.tainted_by_errors = Some(error_reported);
 381     }
 382
 383     // IMPORTANT
 384     pm::run_passes(tcx, &mut body, &[&remove_false_edges::RemoveFalseEdges]);
 385
 386     // Do a little drop elaboration before const-checking if `const_precise_live_drops` is enabled.
 387     if check_consts::post_drop_elaboration::checking_enabled(&ConstCx::new(tcx, &body)) {
 388         pm::run_passes(
 389             tcx,
 390             &mut body,
 391             &[
 392                 &simplify::SimplifyCfg::new("remove-false-edges"),
 393                 &remove_uninit_drops::RemoveUninitDrops,
 394             ],
 395         );
 396         check_consts::post_drop_elaboration::check_live_drops(tcx, &body); // FIXME: make this a MIR lint
 397     }
 398
 399     run_post_borrowck_cleanup_passes(tcx, &mut body);
 400     assert!(body.phase == MirPhase::DropLowering);
 401     tcx.alloc_steal_mir(body)
 402 }
 403
 404 /// After this series of passes, no lifetime analysis based on borrowing can be done.
 405 fn run_post_borrowck_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 406     debug!("post_borrowck_cleanup({:?})", body.source.def_id());
 407
 408     let post_borrowck_cleanup: &[&dyn MirPass<'tcx>] = &[
 409         // Remove all things only needed by analysis
 410         &simplify_branches::SimplifyConstCondition::new("initial"),
 411         &remove_noop_landing_pads::RemoveNoopLandingPads,
 412         &cleanup_post_borrowck::CleanupNonCodegenStatements,
 413         &simplify::SimplifyCfg::new("early-opt"),
 414         // These next passes must be executed together
 415         &add_call_guards::CriticalCallEdges,
 416         &elaborate_drops::ElaborateDrops,
 417         // This will remove extraneous landing pads which are no longer
 418         // necessary as well as well as forcing any call in a non-unwinding
 419         // function calling a possibly-unwinding function to abort the process.
 420         &abort_unwinding_calls::AbortUnwindingCalls,
 421         // AddMovesForPackedDrops needs to run after drop
 422         // elaboration.
 423         &add_moves_for_packed_drops::AddMovesForPackedDrops,
 424         // `AddRetag` needs to run after `ElaborateDrops`. Otherwise it should run fairly late,
 425         // but before optimizations begin.
 426         &add_retag::AddRetag,
 427         &lower_intrinsics::LowerIntrinsics,
 428         &simplify::SimplifyCfg::new("elaborate-drops"),
 429         // `Deaggregator` is conceptually part of MIR building, some backends rely on it happening
 430         // and it can help optimizations.
 431         &deaggregator::Deaggregator,
 432     ];
 433
 434     pm::run_passes(tcx, body, post_borrowck_cleanup);
 435 }
 436
 437 fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 438     fn o1<T>(x: T) -> WithMinOptLevel<T> {
 439         WithMinOptLevel(1, x)
 440     }
 441
 442     // Lowering generator control-flow and variables has to happen before we do anything else
 443     // to them. We run some optimizations before that, because they may be harder to do on the state
 444     // machine than on MIR with async primitives.
 445     pm::run_passes(
 446         tcx,
 447         body,
 448         &[
 449             &reveal_all::RevealAll, // has to be done before inlining, since inlined code is in RevealAll mode.
 450             &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
 451             &normalize_array_len::NormalizeArrayLen, // has to run after `slice::len` lowering
 452             &unreachable_prop::UnreachablePropagation,
 453             &uninhabited_enum_branching::UninhabitedEnumBranching,
 454             &o1(simplify::SimplifyCfg::new("after-uninhabited-enum-branching")),
 455             &inline::Inline,
 456             &generator::StateTransform,
 457         ],
 458     );
 459
 460     assert!(body.phase == MirPhase::GeneratorLowering);
 461
 462     // The main optimizations that we do on MIR.
 463     pm::run_passes(
 464         tcx,
 465         body,
 466         &[
 467             &remove_storage_markers::RemoveStorageMarkers,
 468             &remove_zsts::RemoveZsts,
 469             &const_goto::ConstGoto,
 470             &remove_unneeded_drops::RemoveUnneededDrops,
 471             &match_branches::MatchBranchSimplification,
 472             // inst combine is after MatchBranchSimplification to clean up Ne(_1, false)
 473             &multiple_return_terminators::MultipleReturnTerminators,
 474             &instcombine::InstCombine,
 475             &separate_const_switch::SeparateConstSwitch,
 476             //
 477             // FIXME(#70073): This pass is responsible for both optimization as well as some lints.
 478             &const_prop::ConstProp,
 479             //
 480             // Const-prop runs unconditionally, but doesn't mutate the MIR at mir-opt-level=0.
 481             &o1(simplify_branches::SimplifyConstCondition::new("after-const-prop")),
 482             &early_otherwise_branch::EarlyOtherwiseBranch,
 483             &simplify_comparison_integral::SimplifyComparisonIntegral,
 484             &simplify_try::SimplifyArmIdentity,
 485             &simplify_try::SimplifyBranchSame,
 486             &dest_prop::DestinationPropagation,
 487             &o1(simplify_branches::SimplifyConstCondition::new("final")),
 488             &o1(remove_noop_landing_pads::RemoveNoopLandingPads),
 489             &o1(simplify::SimplifyCfg::new("final")),
 490             &nrvo::RenameReturnPlace,
 491             &const_debuginfo::ConstDebugInfo,
 492             &simplify::SimplifyLocals,
 493             &multiple_return_terminators::MultipleReturnTerminators,
 494             &deduplicate_blocks::DeduplicateBlocks,
 495             // Some cleanup necessary at least for LLVM and potentially other codegen backends.
 496             &add_call_guards::CriticalCallEdges,
 497             &marker::PhaseChange(MirPhase::Optimization),
 498             // Dump the end result for testing and debugging purposes.
 499             &dump_mir::Marker("PreCodegen"),
 500         ],
 501     );
 502 }
 503
 504 /// Optimize the MIR and prepare it for codegen.
 505 fn optimized_mir<'tcx>(tcx: TyCtxt<'tcx>, did: DefId) -> &'tcx Body<'tcx> {
 506     let did = did.expect_local();
 507     assert_eq!(ty::WithOptConstParam::try_lookup(did, tcx), None);
 508     tcx.arena.alloc(inner_optimized_mir(tcx, did))
 509 }
 510
 511 fn inner_optimized_mir(tcx: TyCtxt<'_>, did: LocalDefId) -> Body<'_> {
 512     if tcx.is_constructor(did.to_def_id()) {
 513         // There's no reason to run all of the MIR passes on constructors when
 514         // we can just output the MIR we want directly. This also saves const
 515         // qualification and borrow checking the trouble of special casing
 516         // constructors.
 517         return shim::build_adt_ctor(tcx, did.to_def_id());
 518     }
 519
 520     match tcx.hir().body_const_context(did) {
 521         // Run the `mir_for_ctfe` query, which depends on `mir_drops_elaborated_and_const_checked`
 522         // which we are going to steal below. Thus we need to run `mir_for_ctfe` first, so it
 523         // computes and caches its result.
 524         Some(hir::ConstContext::ConstFn) => tcx.ensure().mir_for_ctfe(did),
 525         None => {}
 526         Some(other) => panic!("do not use `optimized_mir` for constants: {:?}", other),
 527     }
 528     let mut body =
 529         tcx.mir_drops_elaborated_and_const_checked(ty::WithOptConstParam::unknown(did)).steal();
 530     run_optimization_passes(tcx, &mut body);
 531
 532     debug_assert!(!body.has_free_regions(), "Free regions in optimized MIR");
 533
 534     body
 535 }
 536
 537 /// Fetch all the promoteds of an item and prepare their MIR bodies to be ready for
 538 /// constant evaluation once all substitutions become known.
 539 fn promoted_mir<'tcx>(
 540     tcx: TyCtxt<'tcx>,
 541     def: ty::WithOptConstParam<LocalDefId>,
 542 ) -> &'tcx IndexVec<Promoted, Body<'tcx>> {
 543     if tcx.is_constructor(def.did.to_def_id()) {
 544         return tcx.arena.alloc(IndexVec::new());
 545     }
 546
 547     let tainted_by_errors = tcx.mir_borrowck_opt_const_arg(def).tainted_by_errors;
 548     let mut promoted = tcx.mir_promoted(def).1.steal();
 549
 550     for body in &mut promoted {
 551         if let Some(error_reported) = tainted_by_errors {
 552             body.tainted_by_errors = Some(error_reported);
 553         }
 554         run_post_borrowck_cleanup_passes(tcx, body);
 555     }
 556
 557     debug_assert!(!promoted.has_free_regions(), "Free regions in promoted MIR");
 558
 559     tcx.arena.alloc(promoted)
 560 }