compiler/rustc_mir_transform/src/lib.rs

   1 #![allow(rustc::potential_query_instability)]
   2 #![feature(box_patterns)]
   3 #![feature(box_syntax)]
   4 #![feature(crate_visibility_modifier)]
   5 #![feature(let_chains)]
   6 #![feature(let_else)]
   7 #![feature(map_try_insert)]
   8 #![feature(min_specialization)]
   9 #![feature(never_type)]
  10 #![feature(once_cell)]
  11 #![feature(option_get_or_insert_default)]
  12 #![feature(trusted_step)]
  13 #![feature(try_blocks)]
  14 #![recursion_limit = "256"]
  15
  16 #[macro_use]
  17 extern crate tracing;
  18 #[macro_use]
  19 extern crate rustc_middle;
  20
  21 use required_consts::RequiredConstsVisitor;
  22 use rustc_const_eval::util;
  23 use rustc_data_structures::fx::FxIndexSet;
  24 use rustc_data_structures::steal::Steal;
  25 use rustc_hir as hir;
  26 use rustc_hir::def_id::{DefId, LocalDefId};
  27 use rustc_hir::intravisit::{self, Visitor};
  28 use rustc_index::vec::IndexVec;
  29 use rustc_middle::mir::visit::Visitor as _;
  30 use rustc_middle::mir::{traversal, Body, ConstQualifs, MirPass, MirPhase, Promoted};
  31 use rustc_middle::ty::query::Providers;
  32 use rustc_middle::ty::{self, TyCtxt, TypeFoldable};
  33 use rustc_span::{Span, Symbol};
  34
  35 #[macro_use]
  36 mod pass_manager;
  37
  38 use pass_manager::{self as pm, Lint, MirLint, WithMinOptLevel};
  39
  40 mod abort_unwinding_calls;
  41 mod add_call_guards;
  42 mod add_moves_for_packed_drops;
  43 mod add_retag;
  44 mod check_const_item_mutation;
  45 mod check_packed_ref;
  46 pub mod check_unsafety;
  47 // This pass is public to allow external drivers to perform MIR cleanup
  48 pub mod cleanup_post_borrowck;
  49 mod const_debuginfo;
  50 mod const_goto;
  51 mod const_prop;
  52 mod const_prop_lint;
  53 mod coverage;
  54 mod deaggregator;
  55 mod deduplicate_blocks;
  56 mod deref_separator;
  57 mod dest_prop;
  58 pub mod dump_mir;
  59 mod early_otherwise_branch;
  60 mod elaborate_drops;
  61 mod function_item_references;
  62 mod generator;
  63 mod inline;
  64 mod instcombine;
  65 mod lower_intrinsics;
  66 mod lower_slice_len;
  67 mod marker;
  68 mod match_branches;
  69 mod multiple_return_terminators;
  70 mod normalize_array_len;
  71 mod nrvo;
  72 // This pass is public to allow external drivers to perform MIR cleanup
  73 pub mod remove_false_edges;
  74 mod remove_noop_landing_pads;
  75 mod remove_storage_markers;
  76 mod remove_uninit_drops;
  77 mod remove_unneeded_drops;
  78 mod remove_zsts;
  79 mod required_consts;
  80 mod reveal_all;
  81 mod separate_const_switch;
  82 mod shim;
  83 // This pass is public to allow external drivers to perform MIR cleanup
  84 pub mod simplify;
  85 mod simplify_branches;
  86 mod simplify_comparison_integral;
  87 mod simplify_try;
  88 mod uninhabited_enum_branching;
  89 mod unreachable_prop;
  90
  91 use rustc_const_eval::transform::check_consts::{self, ConstCx};
  92 use rustc_const_eval::transform::promote_consts;
  93 use rustc_const_eval::transform::validate;
  94 use rustc_mir_dataflow::rustc_peek;
  95
  96 pub fn provide(providers: &mut Providers) {
  97     check_unsafety::provide(providers);
  98     check_packed_ref::provide(providers);
  99     coverage::query::provide(providers);
 100     shim::provide(providers);
 101     *providers = Providers {
 102         mir_keys,
 103         mir_const,
 104         mir_const_qualif: |tcx, def_id| {
 105             let def_id = def_id.expect_local();
 106             if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
 107                 tcx.mir_const_qualif_const_arg(def)
 108             } else {
 109                 mir_const_qualif(tcx, ty::WithOptConstParam::unknown(def_id))
 110             }
 111         },
 112         mir_const_qualif_const_arg: |tcx, (did, param_did)| {
 113             mir_const_qualif(tcx, ty::WithOptConstParam { did, const_param_did: Some(param_did) })
 114         },
 115         mir_promoted,
 116         mir_drops_elaborated_and_const_checked,
 117         mir_for_ctfe,
 118         mir_for_ctfe_of_const_arg,
 119         optimized_mir,
 120         is_mir_available,
 121         is_ctfe_mir_available: |tcx, did| is_mir_available(tcx, did),
 122         mir_callgraph_reachable: inline::cycle::mir_callgraph_reachable,
 123         mir_inliner_callees: inline::cycle::mir_inliner_callees,
 124         promoted_mir: |tcx, def_id| {
 125             let def_id = def_id.expect_local();
 126             if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
 127                 tcx.promoted_mir_of_const_arg(def)
 128             } else {
 129                 promoted_mir(tcx, ty::WithOptConstParam::unknown(def_id))
 130             }
 131         },
 132         promoted_mir_of_const_arg: |tcx, (did, param_did)| {
 133             promoted_mir(tcx, ty::WithOptConstParam { did, const_param_did: Some(param_did) })
 134         },
 135         ..*providers
 136     };
 137 }
 138
 139 fn is_mir_available(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 140     let def_id = def_id.expect_local();
 141     tcx.mir_keys(()).contains(&def_id)
 142 }
 143
 144 /// Finds the full set of `DefId`s within the current crate that have
 145 /// MIR associated with them.
 146 fn mir_keys(tcx: TyCtxt<'_>, (): ()) -> FxIndexSet<LocalDefId> {
 147     let mut set = FxIndexSet::default();
 148
 149     // All body-owners have MIR associated with them.
 150     set.extend(tcx.hir().body_owners());
 151
 152     // Additionally, tuple struct/variant constructors have MIR, but
 153     // they don't have a BodyId, so we need to build them separately.
 154     struct GatherCtors<'a, 'tcx> {
 155         tcx: TyCtxt<'tcx>,
 156         set: &'a mut FxIndexSet<LocalDefId>,
 157     }
 158     impl<'tcx> Visitor<'tcx> for GatherCtors<'_, 'tcx> {
 159         fn visit_variant_data(
 160             &mut self,
 161             v: &'tcx hir::VariantData<'tcx>,
 162             _: Symbol,
 163             _: &'tcx hir::Generics<'tcx>,
 164             _: hir::HirId,
 165             _: Span,
 166         ) {
 167             if let hir::VariantData::Tuple(_, hir_id) = *v {
 168                 self.set.insert(self.tcx.hir().local_def_id(hir_id));
 169             }
 170             intravisit::walk_struct_def(self, v)
 171         }
 172     }
 173     tcx.hir().visit_all_item_likes(&mut GatherCtors { tcx, set: &mut set }.as_deep_visitor());
 174
 175     set
 176 }
 177
 178 fn mir_const_qualif(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -> ConstQualifs {
 179     let const_kind = tcx.hir().body_const_context(def.did);
 180
 181     // No need to const-check a non-const `fn`.
 182     if const_kind.is_none() {
 183         return Default::default();
 184     }
 185
 186     // N.B., this `borrow()` is guaranteed to be valid (i.e., the value
 187     // cannot yet be stolen), because `mir_promoted()`, which steals
 188     // from `mir_const(), forces this query to execute before
 189     // performing the steal.
 190     let body = &tcx.mir_const(def).borrow();
 191
 192     if body.return_ty().references_error() {
 193         tcx.sess.delay_span_bug(body.span, "mir_const_qualif: MIR had errors");
 194         return Default::default();
 195     }
 196
 197     let ccx = check_consts::ConstCx { body, tcx, const_kind, param_env: tcx.param_env(def.did) };
 198
 199     let mut validator = check_consts::check::Checker::new(&ccx);
 200     validator.check_body();
 201
 202     // We return the qualifs in the return place for every MIR body, even though it is only used
 203     // when deciding to promote a reference to a `const` for now.
 204     validator.qualifs_in_return_place()
 205 }
 206
 207 /// Make MIR ready for const evaluation. This is run on all MIR, not just on consts!
 208 fn mir_const<'tcx>(
 209     tcx: TyCtxt<'tcx>,
 210     def: ty::WithOptConstParam<LocalDefId>,
 211 ) -> &'tcx Steal<Body<'tcx>> {
 212     if let Some(def) = def.try_upgrade(tcx) {
 213         return tcx.mir_const(def);
 214     }
 215
 216     // Unsafety check uses the raw mir, so make sure it is run.
 217     if !tcx.sess.opts.debugging_opts.thir_unsafeck {
 218         if let Some(param_did) = def.const_param_did {
 219             tcx.ensure().unsafety_check_result_for_const_arg((def.did, param_did));
 220         } else {
 221             tcx.ensure().unsafety_check_result(def.did);
 222         }
 223     }
 224
 225     let mut body = tcx.mir_built(def).steal();
 226
 227     rustc_middle::mir::dump_mir(tcx, None, "mir_map", &0, &body, |_, _| Ok(()));
 228
 229     pm::run_passes(
 230         tcx,
 231         &mut body,
 232         &[
 233             // MIR-level lints.
 234             &Lint(check_packed_ref::CheckPackedRef),
 235             &Lint(check_const_item_mutation::CheckConstItemMutation),
 236             &Lint(function_item_references::FunctionItemReferences),
 237             // What we need to do constant evaluation.
 238             &simplify::SimplifyCfg::new("initial"),
 239             &rustc_peek::SanityCheck, // Just a lint
 240             &marker::PhaseChange(MirPhase::Const),
 241         ],
 242     );
 243     tcx.alloc_steal_mir(body)
 244 }
 245
 246 /// Compute the main MIR body and the list of MIR bodies of the promoteds.
 247 fn mir_promoted<'tcx>(
 248     tcx: TyCtxt<'tcx>,
 249     def: ty::WithOptConstParam<LocalDefId>,
 250 ) -> (&'tcx Steal<Body<'tcx>>, &'tcx Steal<IndexVec<Promoted, Body<'tcx>>>) {
 251     if let Some(def) = def.try_upgrade(tcx) {
 252         return tcx.mir_promoted(def);
 253     }
 254
 255     // Ensure that we compute the `mir_const_qualif` for constants at
 256     // this point, before we steal the mir-const result.
 257     // Also this means promotion can rely on all const checks having been done.
 258     let const_qualifs = tcx.mir_const_qualif_opt_const_arg(def);
 259     let mut body = tcx.mir_const(def).steal();
 260     if let Some(error_reported) = const_qualifs.tainted_by_errors {
 261         body.tainted_by_errors = Some(error_reported);
 262     }
 263
 264     let mut required_consts = Vec::new();
 265     let mut required_consts_visitor = RequiredConstsVisitor::new(&mut required_consts);
 266     for (bb, bb_data) in traversal::reverse_postorder(&body) {
 267         required_consts_visitor.visit_basic_block_data(bb, bb_data);
 268     }
 269     body.required_consts = required_consts;
 270
 271     // What we need to run borrowck etc.
 272     let promote_pass = promote_consts::PromoteTemps::default();
 273     pm::run_passes(
 274         tcx,
 275         &mut body,
 276         &[
 277             &promote_pass,
 278             &simplify::SimplifyCfg::new("promote-consts"),
 279             &coverage::InstrumentCoverage,
 280         ],
 281     );
 282
 283     let promoted = promote_pass.promoted_fragments.into_inner();
 284     (tcx.alloc_steal_mir(body), tcx.alloc_steal_promoted(promoted))
 285 }
 286
 287 /// Compute the MIR that is used during CTFE (and thus has no optimizations run on it)
 288 fn mir_for_ctfe<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> &'tcx Body<'tcx> {
 289     let did = def_id.expect_local();
 290     if let Some(def) = ty::WithOptConstParam::try_lookup(did, tcx) {
 291         tcx.mir_for_ctfe_of_const_arg(def)
 292     } else {
 293         tcx.arena.alloc(inner_mir_for_ctfe(tcx, ty::WithOptConstParam::unknown(did)))
 294     }
 295 }
 296
 297 /// Same as `mir_for_ctfe`, but used to get the MIR of a const generic parameter.
 298 /// The docs on `WithOptConstParam` explain this a bit more, but the TLDR is that
 299 /// we'd get cycle errors with `mir_for_ctfe`, because typeck would need to typeck
 300 /// the const parameter while type checking the main body, which in turn would try
 301 /// to type check the main body again.
 302 fn mir_for_ctfe_of_const_arg<'tcx>(
 303     tcx: TyCtxt<'tcx>,
 304     (did, param_did): (LocalDefId, DefId),
 305 ) -> &'tcx Body<'tcx> {
 306     tcx.arena.alloc(inner_mir_for_ctfe(
 307         tcx,
 308         ty::WithOptConstParam { did, const_param_did: Some(param_did) },
 309     ))
 310 }
 311
 312 fn inner_mir_for_ctfe(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -> Body<'_> {
 313     // FIXME: don't duplicate this between the optimized_mir/mir_for_ctfe queries
 314     if tcx.is_constructor(def.did.to_def_id()) {
 315         // There's no reason to run all of the MIR passes on constructors when
 316         // we can just output the MIR we want directly. This also saves const
 317         // qualification and borrow checking the trouble of special casing
 318         // constructors.
 319         return shim::build_adt_ctor(tcx, def.did.to_def_id());
 320     }
 321
 322     let context = tcx
 323         .hir()
 324         .body_const_context(def.did)
 325         .expect("mir_for_ctfe should not be used for runtime functions");
 326
 327     let mut body = tcx.mir_drops_elaborated_and_const_checked(def).borrow().clone();
 328
 329     match context {
 330         // Do not const prop functions, either they get executed at runtime or exported to metadata,
 331         // so we run const prop on them, or they don't, in which case we const evaluate some control
 332         // flow paths of the function and any errors in those paths will get emitted as const eval
 333         // errors.
 334         hir::ConstContext::ConstFn => {}
 335         // Static items always get evaluated, so we can just let const eval see if any erroneous
 336         // control flow paths get executed.
 337         hir::ConstContext::Static(_) => {}
 338         // Associated constants get const prop run so we detect common failure situations in the
 339         // crate that defined the constant.
 340         // Technically we want to not run on regular const items, but oli-obk doesn't know how to
 341         // conveniently detect that at this point without looking at the HIR.
 342         hir::ConstContext::Const => {
 343             pm::run_passes(
 344                 tcx,
 345                 &mut body,
 346                 &[&const_prop::ConstProp, &marker::PhaseChange(MirPhase::Optimized)],
 347             );
 348         }
 349     }
 350
 351     debug_assert!(!body.has_free_regions(), "Free regions in MIR for CTFE");
 352
 353     body
 354 }
 355
 356 /// Obtain just the main MIR (no promoteds) and run some cleanups on it. This also runs
 357 /// mir borrowck *before* doing so in order to ensure that borrowck can be run and doesn't
 358 /// end up missing the source MIR due to stealing happening.
 359 fn mir_drops_elaborated_and_const_checked<'tcx>(
 360     tcx: TyCtxt<'tcx>,
 361     def: ty::WithOptConstParam<LocalDefId>,
 362 ) -> &'tcx Steal<Body<'tcx>> {
 363     if let Some(def) = def.try_upgrade(tcx) {
 364         return tcx.mir_drops_elaborated_and_const_checked(def);
 365     }
 366
 367     let mir_borrowck = tcx.mir_borrowck_opt_const_arg(def);
 368
 369     let is_fn_like = tcx.def_kind(def.did).is_fn_like();
 370     if is_fn_like {
 371         let did = def.did.to_def_id();
 372         let def = ty::WithOptConstParam::unknown(did);
 373
 374         // Do not compute the mir call graph without said call graph actually being used.
 375         if inline::Inline.is_enabled(&tcx.sess) {
 376             let _ = tcx.mir_inliner_callees(ty::InstanceDef::Item(def));
 377         }
 378     }
 379
 380     let (body, _) = tcx.mir_promoted(def);
 381     let mut body = body.steal();
 382     if let Some(error_reported) = mir_borrowck.tainted_by_errors {
 383         body.tainted_by_errors = Some(error_reported);
 384     }
 385
 386     // IMPORTANT
 387     pm::run_passes(tcx, &mut body, &[&remove_false_edges::RemoveFalseEdges]);
 388
 389     // Do a little drop elaboration before const-checking if `const_precise_live_drops` is enabled.
 390     if check_consts::post_drop_elaboration::checking_enabled(&ConstCx::new(tcx, &body)) {
 391         pm::run_passes(
 392             tcx,
 393             &mut body,
 394             &[
 395                 &simplify::SimplifyCfg::new("remove-false-edges"),
 396                 &remove_uninit_drops::RemoveUninitDrops,
 397             ],
 398         );
 399         check_consts::post_drop_elaboration::check_live_drops(tcx, &body); // FIXME: make this a MIR lint
 400     }
 401
 402     run_post_borrowck_cleanup_passes(tcx, &mut body);
 403     assert!(body.phase == MirPhase::Deaggregated);
 404     tcx.alloc_steal_mir(body)
 405 }
 406
 407 /// After this series of passes, no lifetime analysis based on borrowing can be done.
 408 fn run_post_borrowck_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 409     debug!("post_borrowck_cleanup({:?})", body.source.def_id());
 410
 411     let post_borrowck_cleanup: &[&dyn MirPass<'tcx>] = &[
 412         // Remove all things only needed by analysis
 413         &simplify_branches::SimplifyConstCondition::new("initial"),
 414         &remove_noop_landing_pads::RemoveNoopLandingPads,
 415         &cleanup_post_borrowck::CleanupNonCodegenStatements,
 416         &simplify::SimplifyCfg::new("early-opt"),
 417         // These next passes must be executed together
 418         &add_call_guards::CriticalCallEdges,
 419         &elaborate_drops::ElaborateDrops,
 420         // This will remove extraneous landing pads which are no longer
 421         // necessary as well as well as forcing any call in a non-unwinding
 422         // function calling a possibly-unwinding function to abort the process.
 423         &abort_unwinding_calls::AbortUnwindingCalls,
 424         // AddMovesForPackedDrops needs to run after drop
 425         // elaboration.
 426         &add_moves_for_packed_drops::AddMovesForPackedDrops,
 427         // `AddRetag` needs to run after `ElaborateDrops`. Otherwise it should run fairly late,
 428         // but before optimizations begin.
 429         &add_retag::AddRetag,
 430         &lower_intrinsics::LowerIntrinsics,
 431         &simplify::SimplifyCfg::new("elaborate-drops"),
 432         // `Deaggregator` is conceptually part of MIR building, some backends rely on it happening
 433         // and it can help optimizations.
 434         &deaggregator::Deaggregator,
 435         &deref_separator::Derefer,
 436         &Lint(const_prop_lint::ConstProp),
 437     ];
 438
 439     pm::run_passes(tcx, body, post_borrowck_cleanup);
 440 }
 441
 442 fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 443     fn o1<T>(x: T) -> WithMinOptLevel<T> {
 444         WithMinOptLevel(1, x)
 445     }
 446
 447     // Lowering generator control-flow and variables has to happen before we do anything else
 448     // to them. We run some optimizations before that, because they may be harder to do on the state
 449     // machine than on MIR with async primitives.
 450     pm::run_passes(
 451         tcx,
 452         body,
 453         &[
 454             &reveal_all::RevealAll, // has to be done before inlining, since inlined code is in RevealAll mode.
 455             &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
 456             &normalize_array_len::NormalizeArrayLen, // has to run after `slice::len` lowering
 457             &unreachable_prop::UnreachablePropagation,
 458             &uninhabited_enum_branching::UninhabitedEnumBranching,
 459             &o1(simplify::SimplifyCfg::new("after-uninhabited-enum-branching")),
 460             &inline::Inline,
 461             &generator::StateTransform,
 462         ],
 463     );
 464
 465     assert!(body.phase == MirPhase::GeneratorsLowered);
 466
 467     // The main optimizations that we do on MIR.
 468     pm::run_passes(
 469         tcx,
 470         body,
 471         &[
 472             &remove_storage_markers::RemoveStorageMarkers,
 473             &remove_zsts::RemoveZsts,
 474             &const_goto::ConstGoto,
 475             &remove_unneeded_drops::RemoveUnneededDrops,
 476             &match_branches::MatchBranchSimplification,
 477             // inst combine is after MatchBranchSimplification to clean up Ne(_1, false)
 478             &multiple_return_terminators::MultipleReturnTerminators,
 479             &instcombine::InstCombine,
 480             &separate_const_switch::SeparateConstSwitch,
 481             //
 482             // FIXME(#70073): This pass is responsible for both optimization as well as some lints.
 483             &const_prop::ConstProp,
 484             //
 485             // Const-prop runs unconditionally, but doesn't mutate the MIR at mir-opt-level=0.
 486             &o1(simplify_branches::SimplifyConstCondition::new("after-const-prop")),
 487             &early_otherwise_branch::EarlyOtherwiseBranch,
 488             &simplify_comparison_integral::SimplifyComparisonIntegral,
 489             &simplify_try::SimplifyArmIdentity,
 490             &simplify_try::SimplifyBranchSame,
 491             &dest_prop::DestinationPropagation,
 492             &o1(simplify_branches::SimplifyConstCondition::new("final")),
 493             &o1(remove_noop_landing_pads::RemoveNoopLandingPads),
 494             &o1(simplify::SimplifyCfg::new("final")),
 495             &nrvo::RenameReturnPlace,
 496             &const_debuginfo::ConstDebugInfo,
 497             &simplify::SimplifyLocals,
 498             &multiple_return_terminators::MultipleReturnTerminators,
 499             &deduplicate_blocks::DeduplicateBlocks,
 500             // Some cleanup necessary at least for LLVM and potentially other codegen backends.
 501             &add_call_guards::CriticalCallEdges,
 502             &marker::PhaseChange(MirPhase::Optimized),
 503             // Dump the end result for testing and debugging purposes.
 504             &dump_mir::Marker("PreCodegen"),
 505         ],
 506     );
 507 }
 508
 509 /// Optimize the MIR and prepare it for codegen.
 510 fn optimized_mir<'tcx>(tcx: TyCtxt<'tcx>, did: DefId) -> &'tcx Body<'tcx> {
 511     let did = did.expect_local();
 512     assert_eq!(ty::WithOptConstParam::try_lookup(did, tcx), None);
 513     tcx.arena.alloc(inner_optimized_mir(tcx, did))
 514 }
 515
 516 fn inner_optimized_mir(tcx: TyCtxt<'_>, did: LocalDefId) -> Body<'_> {
 517     if tcx.is_constructor(did.to_def_id()) {
 518         // There's no reason to run all of the MIR passes on constructors when
 519         // we can just output the MIR we want directly. This also saves const
 520         // qualification and borrow checking the trouble of special casing
 521         // constructors.
 522         return shim::build_adt_ctor(tcx, did.to_def_id());
 523     }
 524
 525     match tcx.hir().body_const_context(did) {
 526         // Run the `mir_for_ctfe` query, which depends on `mir_drops_elaborated_and_const_checked`
 527         // which we are going to steal below. Thus we need to run `mir_for_ctfe` first, so it
 528         // computes and caches its result.
 529         Some(hir::ConstContext::ConstFn) => tcx.ensure().mir_for_ctfe(did),
 530         None => {}
 531         Some(other) => panic!("do not use `optimized_mir` for constants: {:?}", other),
 532     }
 533     let mut body =
 534         tcx.mir_drops_elaborated_and_const_checked(ty::WithOptConstParam::unknown(did)).steal();
 535     run_optimization_passes(tcx, &mut body);
 536
 537     debug_assert!(!body.has_free_regions(), "Free regions in optimized MIR");
 538
 539     body
 540 }
 541
 542 /// Fetch all the promoteds of an item and prepare their MIR bodies to be ready for
 543 /// constant evaluation once all substitutions become known.
 544 fn promoted_mir<'tcx>(
 545     tcx: TyCtxt<'tcx>,
 546     def: ty::WithOptConstParam<LocalDefId>,
 547 ) -> &'tcx IndexVec<Promoted, Body<'tcx>> {
 548     if tcx.is_constructor(def.did.to_def_id()) {
 549         return tcx.arena.alloc(IndexVec::new());
 550     }
 551
 552     let tainted_by_errors = tcx.mir_borrowck_opt_const_arg(def).tainted_by_errors;
 553     let mut promoted = tcx.mir_promoted(def).1.steal();
 554
 555     for body in &mut promoted {
 556         if let Some(error_reported) = tainted_by_errors {
 557             body.tainted_by_errors = Some(error_reported);
 558         }
 559         run_post_borrowck_cleanup_passes(tcx, body);
 560     }
 561
 562     debug_assert!(!promoted.has_free_regions(), "Free regions in promoted MIR");
 563
 564     tcx.arena.alloc(promoted)
 565 }