compiler/rustc_mir_transform/src/lib.rs

   1 #![feature(box_patterns)]
   2 #![feature(box_syntax)]
   3 #![feature(crate_visibility_modifier)]
   4 #![feature(iter_zip)]
   5 #![feature(let_else)]
   6 #![feature(map_try_insert)]
   7 #![feature(min_specialization)]
   8 #![feature(option_get_or_insert_default)]
   9 #![feature(once_cell)]
  10 #![feature(never_type)]
  11 #![feature(trusted_step)]
  12 #![feature(try_blocks)]
  13 #![recursion_limit = "256"]
  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::FxHashSet;
  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, NestedVisitorMap, 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<'_>, (): ()) -> FxHashSet<LocalDefId> {
 141     let mut set = FxHashSet::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 FxHashSet<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         type Map = intravisit::ErasedMap<'tcx>;
 167         fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 168             NestedVisitorMap::None
 169         }
 170     }
 171     tcx.hir().visit_all_item_likes(&mut GatherCtors { tcx, set: &mut set }.as_deep_visitor());
 172
 173     set
 174 }
 175
 176 fn mir_const_qualif(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -> ConstQualifs {
 177     let const_kind = tcx.hir().body_const_context(def.did);
 178
 179     // No need to const-check a non-const `fn`.
 180     if const_kind.is_none() {
 181         return Default::default();
 182     }
 183
 184     // N.B., this `borrow()` is guaranteed to be valid (i.e., the value
 185     // cannot yet be stolen), because `mir_promoted()`, which steals
 186     // from `mir_const(), forces this query to execute before
 187     // performing the steal.
 188     let body = &tcx.mir_const(def).borrow();
 189
 190     if body.return_ty().references_error() {
 191         tcx.sess.delay_span_bug(body.span, "mir_const_qualif: MIR had errors");
 192         return Default::default();
 193     }
 194
 195     let ccx = check_consts::ConstCx { body, tcx, const_kind, param_env: tcx.param_env(def.did) };
 196
 197     let mut validator = check_consts::check::Checker::new(&ccx);
 198     validator.check_body();
 199
 200     // We return the qualifs in the return place for every MIR body, even though it is only used
 201     // when deciding to promote a reference to a `const` for now.
 202     validator.qualifs_in_return_place()
 203 }
 204
 205 /// Make MIR ready for const evaluation. This is run on all MIR, not just on consts!
 206 fn mir_const<'tcx>(
 207     tcx: TyCtxt<'tcx>,
 208     def: ty::WithOptConstParam<LocalDefId>,
 209 ) -> &'tcx Steal<Body<'tcx>> {
 210     if let Some(def) = def.try_upgrade(tcx) {
 211         return tcx.mir_const(def);
 212     }
 213
 214     // Unsafety check uses the raw mir, so make sure it is run.
 215     if !tcx.sess.opts.debugging_opts.thir_unsafeck {
 216         if let Some(param_did) = def.const_param_did {
 217             tcx.ensure().unsafety_check_result_for_const_arg((def.did, param_did));
 218         } else {
 219             tcx.ensure().unsafety_check_result(def.did);
 220         }
 221     }
 222
 223     let mut body = tcx.mir_built(def).steal();
 224
 225     rustc_middle::mir::dump_mir(tcx, None, "mir_map", &0, &body, |_, _| Ok(()));
 226
 227     pm::run_passes(
 228         tcx,
 229         &mut body,
 230         &[
 231             // MIR-level lints.
 232             &Lint(check_packed_ref::CheckPackedRef),
 233             &Lint(check_const_item_mutation::CheckConstItemMutation),
 234             &Lint(function_item_references::FunctionItemReferences),
 235             // What we need to do constant evaluation.
 236             &simplify::SimplifyCfg::new("initial"),
 237             &rustc_peek::SanityCheck, // Just a lint
 238             &marker::PhaseChange(MirPhase::Const),
 239         ],
 240     );
 241     tcx.alloc_steal_mir(body)
 242 }
 243
 244 /// Compute the main MIR body and the list of MIR bodies of the promoteds.
 245 fn mir_promoted<'tcx>(
 246     tcx: TyCtxt<'tcx>,
 247     def: ty::WithOptConstParam<LocalDefId>,
 248 ) -> (&'tcx Steal<Body<'tcx>>, &'tcx Steal<IndexVec<Promoted, Body<'tcx>>>) {
 249     if let Some(def) = def.try_upgrade(tcx) {
 250         return tcx.mir_promoted(def);
 251     }
 252
 253     // Ensure that we compute the `mir_const_qualif` for constants at
 254     // this point, before we steal the mir-const result.
 255     // Also this means promotion can rely on all const checks having been done.
 256     let _ = tcx.mir_const_qualif_opt_const_arg(def);
 257     let mut body = tcx.mir_const(def).steal();
 258
 259     let mut required_consts = Vec::new();
 260     let mut required_consts_visitor = RequiredConstsVisitor::new(&mut required_consts);
 261     for (bb, bb_data) in traversal::reverse_postorder(&body) {
 262         required_consts_visitor.visit_basic_block_data(bb, bb_data);
 263     }
 264     body.required_consts = required_consts;
 265
 266     // What we need to run borrowck etc.
 267     let promote_pass = promote_consts::PromoteTemps::default();
 268     pm::run_passes(
 269         tcx,
 270         &mut body,
 271         &[
 272             &promote_pass,
 273             &simplify::SimplifyCfg::new("promote-consts"),
 274             &coverage::InstrumentCoverage,
 275         ],
 276     );
 277
 278     let promoted = promote_pass.promoted_fragments.into_inner();
 279     (tcx.alloc_steal_mir(body), tcx.alloc_steal_promoted(promoted))
 280 }
 281
 282 /// Compute the MIR that is used during CTFE (and thus has no optimizations run on it)
 283 fn mir_for_ctfe<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> &'tcx Body<'tcx> {
 284     let did = def_id.expect_local();
 285     if let Some(def) = ty::WithOptConstParam::try_lookup(did, tcx) {
 286         tcx.mir_for_ctfe_of_const_arg(def)
 287     } else {
 288         tcx.arena.alloc(inner_mir_for_ctfe(tcx, ty::WithOptConstParam::unknown(did)))
 289     }
 290 }
 291
 292 /// Same as `mir_for_ctfe`, but used to get the MIR of a const generic parameter.
 293 /// The docs on `WithOptConstParam` explain this a bit more, but the TLDR is that
 294 /// we'd get cycle errors with `mir_for_ctfe`, because typeck would need to typeck
 295 /// the const parameter while type checking the main body, which in turn would try
 296 /// to type check the main body again.
 297 fn mir_for_ctfe_of_const_arg<'tcx>(
 298     tcx: TyCtxt<'tcx>,
 299     (did, param_did): (LocalDefId, DefId),
 300 ) -> &'tcx Body<'tcx> {
 301     tcx.arena.alloc(inner_mir_for_ctfe(
 302         tcx,
 303         ty::WithOptConstParam { did, const_param_did: Some(param_did) },
 304     ))
 305 }
 306
 307 fn inner_mir_for_ctfe(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -> Body<'_> {
 308     // FIXME: don't duplicate this between the optimized_mir/mir_for_ctfe queries
 309     if tcx.is_constructor(def.did.to_def_id()) {
 310         // There's no reason to run all of the MIR passes on constructors when
 311         // we can just output the MIR we want directly. This also saves const
 312         // qualification and borrow checking the trouble of special casing
 313         // constructors.
 314         return shim::build_adt_ctor(tcx, def.did.to_def_id());
 315     }
 316
 317     let context = tcx
 318         .hir()
 319         .body_const_context(def.did)
 320         .expect("mir_for_ctfe should not be used for runtime functions");
 321
 322     let mut body = tcx.mir_drops_elaborated_and_const_checked(def).borrow().clone();
 323
 324     match context {
 325         // Do not const prop functions, either they get executed at runtime or exported to metadata,
 326         // so we run const prop on them, or they don't, in which case we const evaluate some control
 327         // flow paths of the function and any errors in those paths will get emitted as const eval
 328         // errors.
 329         hir::ConstContext::ConstFn => {}
 330         // Static items always get evaluated, so we can just let const eval see if any erroneous
 331         // control flow paths get executed.
 332         hir::ConstContext::Static(_) => {}
 333         // Associated constants get const prop run so we detect common failure situations in the
 334         // crate that defined the constant.
 335         // Technically we want to not run on regular const items, but oli-obk doesn't know how to
 336         // conveniently detect that at this point without looking at the HIR.
 337         hir::ConstContext::Const => {
 338             pm::run_passes(
 339                 tcx,
 340                 &mut body,
 341                 &[&const_prop::ConstProp, &marker::PhaseChange(MirPhase::Optimization)],
 342             );
 343         }
 344     }
 345
 346     debug_assert!(!body.has_free_regions(tcx), "Free regions in MIR for CTFE");
 347
 348     body
 349 }
 350
 351 /// Obtain just the main MIR (no promoteds) and run some cleanups on it. This also runs
 352 /// mir borrowck *before* doing so in order to ensure that borrowck can be run and doesn't
 353 /// end up missing the source MIR due to stealing happening.
 354 fn mir_drops_elaborated_and_const_checked<'tcx>(
 355     tcx: TyCtxt<'tcx>,
 356     def: ty::WithOptConstParam<LocalDefId>,
 357 ) -> &'tcx Steal<Body<'tcx>> {
 358     if let Some(def) = def.try_upgrade(tcx) {
 359         return tcx.mir_drops_elaborated_and_const_checked(def);
 360     }
 361
 362     // (Mir-)Borrowck uses `mir_promoted`, so we have to force it to
 363     // execute before we can steal.
 364     if let Some(param_did) = def.const_param_did {
 365         tcx.ensure().mir_borrowck_const_arg((def.did, param_did));
 366     } else {
 367         tcx.ensure().mir_borrowck(def.did);
 368     }
 369
 370     let hir_id = tcx.hir().local_def_id_to_hir_id(def.did);
 371     let is_fn_like = tcx.hir().get(hir_id).fn_kind().is_some();
 372     if is_fn_like {
 373         let did = def.did.to_def_id();
 374         let def = ty::WithOptConstParam::unknown(did);
 375
 376         // Do not compute the mir call graph without said call graph actually being used.
 377         if inline::Inline.is_enabled(&tcx.sess) {
 378             let _ = tcx.mir_inliner_callees(ty::InstanceDef::Item(def));
 379         }
 380     }
 381
 382     let (body, _) = tcx.mir_promoted(def);
 383     let mut body = body.steal();
 384
 385     // IMPORTANT
 386     pm::run_passes(tcx, &mut body, &[&remove_false_edges::RemoveFalseEdges]);
 387
 388     // Do a little drop elaboration before const-checking if `const_precise_live_drops` is enabled.
 389     if check_consts::post_drop_elaboration::checking_enabled(&ConstCx::new(tcx, &body)) {
 390         pm::run_passes(
 391             tcx,
 392             &mut body,
 393             &[
 394                 &simplify::SimplifyCfg::new("remove-false-edges"),
 395                 &remove_uninit_drops::RemoveUninitDrops,
 396             ],
 397         );
 398         check_consts::post_drop_elaboration::check_live_drops(tcx, &body); // FIXME: make this a MIR lint
 399     }
 400
 401     run_post_borrowck_cleanup_passes(tcx, &mut body);
 402     assert!(body.phase == MirPhase::DropLowering);
 403     tcx.alloc_steal_mir(body)
 404 }
 405
 406 /// After this series of passes, no lifetime analysis based on borrowing can be done.
 407 fn run_post_borrowck_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 408     debug!("post_borrowck_cleanup({:?})", body.source.def_id());
 409
 410     let post_borrowck_cleanup: &[&dyn MirPass<'tcx>] = &[
 411         // Remove all things only needed by analysis
 412         &simplify_branches::SimplifyConstCondition::new("initial"),
 413         &remove_noop_landing_pads::RemoveNoopLandingPads,
 414         &cleanup_post_borrowck::CleanupNonCodegenStatements,
 415         &simplify::SimplifyCfg::new("early-opt"),
 416         // These next passes must be executed together
 417         &add_call_guards::CriticalCallEdges,
 418         &elaborate_drops::ElaborateDrops,
 419         // This will remove extraneous landing pads which are no longer
 420         // necessary as well as well as forcing any call in a non-unwinding
 421         // function calling a possibly-unwinding function to abort the process.
 422         &abort_unwinding_calls::AbortUnwindingCalls,
 423         // AddMovesForPackedDrops needs to run after drop
 424         // elaboration.
 425         &add_moves_for_packed_drops::AddMovesForPackedDrops,
 426         // `AddRetag` needs to run after `ElaborateDrops`. Otherwise it should run fairly late,
 427         // but before optimizations begin.
 428         &add_retag::AddRetag,
 429         &lower_intrinsics::LowerIntrinsics,
 430         &simplify::SimplifyCfg::new("elaborate-drops"),
 431         // `Deaggregator` is conceptually part of MIR building, some backends rely on it happening
 432         // and it can help optimizations.
 433         &deaggregator::Deaggregator,
 434     ];
 435
 436     pm::run_passes(tcx, body, post_borrowck_cleanup);
 437 }
 438
 439 fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 440     fn o1<T>(x: T) -> WithMinOptLevel<T> {
 441         WithMinOptLevel(1, x)
 442     }
 443
 444     // Lowering generator control-flow and variables has to happen before we do anything else
 445     // to them. We run some optimizations before that, because they may be harder to do on the state
 446     // machine than on MIR with async primitives.
 447     pm::run_passes(
 448         tcx,
 449         body,
 450         &[
 451             &reveal_all::RevealAll, // has to be done before inlining, since inlined code is in RevealAll mode.
 452             &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
 453             &normalize_array_len::NormalizeArrayLen, // has to run after `slice::len` lowering
 454             &unreachable_prop::UnreachablePropagation,
 455             &uninhabited_enum_branching::UninhabitedEnumBranching,
 456             &o1(simplify::SimplifyCfg::new("after-uninhabited-enum-branching")),
 457             &inline::Inline,
 458             &generator::StateTransform,
 459         ],
 460     );
 461
 462     assert!(body.phase == MirPhase::GeneratorLowering);
 463
 464     // The main optimizations that we do on MIR.
 465     pm::run_passes(
 466         tcx,
 467         body,
 468         &[
 469             &remove_storage_markers::RemoveStorageMarkers,
 470             &remove_zsts::RemoveZsts,
 471             &const_goto::ConstGoto,
 472             &remove_unneeded_drops::RemoveUnneededDrops,
 473             &match_branches::MatchBranchSimplification,
 474             // inst combine is after MatchBranchSimplification to clean up Ne(_1, false)
 475             &multiple_return_terminators::MultipleReturnTerminators,
 476             &instcombine::InstCombine,
 477             &separate_const_switch::SeparateConstSwitch,
 478             //
 479             // FIXME(#70073): This pass is responsible for both optimization as well as some lints.
 480             &const_prop::ConstProp,
 481             //
 482             // Const-prop runs unconditionally, but doesn't mutate the MIR at mir-opt-level=0.
 483             &o1(simplify_branches::SimplifyConstCondition::new("after-const-prop")),
 484             &early_otherwise_branch::EarlyOtherwiseBranch,
 485             &simplify_comparison_integral::SimplifyComparisonIntegral,
 486             &simplify_try::SimplifyArmIdentity,
 487             &simplify_try::SimplifyBranchSame,
 488             &dest_prop::DestinationPropagation,
 489             &o1(simplify_branches::SimplifyConstCondition::new("final")),
 490             &o1(remove_noop_landing_pads::RemoveNoopLandingPads),
 491             &o1(simplify::SimplifyCfg::new("final")),
 492             &nrvo::RenameReturnPlace,
 493             &const_debuginfo::ConstDebugInfo,
 494             &simplify::SimplifyLocals,
 495             &multiple_return_terminators::MultipleReturnTerminators,
 496             &deduplicate_blocks::DeduplicateBlocks,
 497             // Some cleanup necessary at least for LLVM and potentially other codegen backends.
 498             &add_call_guards::CriticalCallEdges,
 499             &marker::PhaseChange(MirPhase::Optimization),
 500             // Dump the end result for testing and debugging purposes.
 501             &dump_mir::Marker("PreCodegen"),
 502         ],
 503     );
 504 }
 505
 506 /// Optimize the MIR and prepare it for codegen.
 507 fn optimized_mir<'tcx>(tcx: TyCtxt<'tcx>, did: DefId) -> &'tcx Body<'tcx> {
 508     let did = did.expect_local();
 509     assert_eq!(ty::WithOptConstParam::try_lookup(did, tcx), None);
 510     tcx.arena.alloc(inner_optimized_mir(tcx, did))
 511 }
 512
 513 fn inner_optimized_mir(tcx: TyCtxt<'_>, did: LocalDefId) -> Body<'_> {
 514     if tcx.is_constructor(did.to_def_id()) {
 515         // There's no reason to run all of the MIR passes on constructors when
 516         // we can just output the MIR we want directly. This also saves const
 517         // qualification and borrow checking the trouble of special casing
 518         // constructors.
 519         return shim::build_adt_ctor(tcx, did.to_def_id());
 520     }
 521
 522     match tcx.hir().body_const_context(did) {
 523         // Run the `mir_for_ctfe` query, which depends on `mir_drops_elaborated_and_const_checked`
 524         // which we are going to steal below. Thus we need to run `mir_for_ctfe` first, so it
 525         // computes and caches its result.
 526         Some(hir::ConstContext::ConstFn) => tcx.ensure().mir_for_ctfe(did),
 527         None => {}
 528         Some(other) => panic!("do not use `optimized_mir` for constants: {:?}", other),
 529     }
 530     let mut body =
 531         tcx.mir_drops_elaborated_and_const_checked(ty::WithOptConstParam::unknown(did)).steal();
 532     run_optimization_passes(tcx, &mut body);
 533
 534     debug_assert!(!body.has_free_regions(tcx), "Free regions in optimized MIR");
 535
 536     body
 537 }
 538
 539 /// Fetch all the promoteds of an item and prepare their MIR bodies to be ready for
 540 /// constant evaluation once all substitutions become known.
 541 fn promoted_mir<'tcx>(
 542     tcx: TyCtxt<'tcx>,
 543     def: ty::WithOptConstParam<LocalDefId>,
 544 ) -> &'tcx IndexVec<Promoted, Body<'tcx>> {
 545     if tcx.is_constructor(def.did.to_def_id()) {
 546         return tcx.arena.alloc(IndexVec::new());
 547     }
 548
 549     if let Some(param_did) = def.const_param_did {
 550         tcx.ensure().mir_borrowck_const_arg((def.did, param_did));
 551     } else {
 552         tcx.ensure().mir_borrowck(def.did);
 553     }
 554     let (_, promoted) = tcx.mir_promoted(def);
 555     let mut promoted = promoted.steal();
 556
 557     for body in &mut promoted {
 558         run_post_borrowck_cleanup_passes(tcx, body);
 559     }
 560
 561     debug_assert!(!promoted.has_free_regions(tcx), "Free regions in promoted MIR");
 562
 563     tcx.arena.alloc(promoted)
 564 }