compiler/rustc_mir_transform/src/lib.rs

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