compiler/rustc_mir_transform/src/lib.rs

   1 #![allow(rustc::potential_query_instability)]
   2 #![feature(box_patterns)]
   3 #![feature(let_chains)]
   4 #![feature(let_else)]
   5 #![feature(map_try_insert)]
   6 #![feature(min_specialization)]
   7 #![feature(never_type)]
   8 #![feature(once_cell)]
   9 #![feature(option_get_or_insert_default)]
  10 #![feature(trusted_step)]
  11 #![feature(try_blocks)]
  12 #![feature(yeet_expr)]
  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 dead_store_elimination;
  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_box_derefs;
  61 mod elaborate_drops;
  62 mod ffi_unwind_calls;
  63 mod function_item_references;
  64 mod generator;
  65 mod inline;
  66 mod instcombine;
  67 mod lower_intrinsics;
  68 mod lower_slice_len;
  69 mod marker;
  70 mod match_branches;
  71 mod multiple_return_terminators;
  72 mod normalize_array_len;
  73 mod nrvo;
  74 // This pass is public to allow external drivers to perform MIR cleanup
  75 pub mod remove_false_edges;
  76 mod remove_noop_landing_pads;
  77 mod remove_storage_markers;
  78 mod remove_uninit_drops;
  79 mod remove_unneeded_drops;
  80 mod remove_zsts;
  81 mod required_consts;
  82 mod reveal_all;
  83 mod separate_const_switch;
  84 mod shim;
  85 // This pass is public to allow external drivers to perform MIR cleanup
  86 pub mod simplify;
  87 mod simplify_branches;
  88 mod simplify_comparison_integral;
  89 mod simplify_try;
  90 mod uninhabited_enum_branching;
  91 mod unreachable_prop;
  92
  93 use rustc_const_eval::transform::check_consts::{self, ConstCx};
  94 use rustc_const_eval::transform::promote_consts;
  95 use rustc_const_eval::transform::validate;
  96 use rustc_mir_dataflow::rustc_peek;
  97
  98 pub fn provide(providers: &mut Providers) {
  99     check_unsafety::provide(providers);
 100     check_packed_ref::provide(providers);
 101     coverage::query::provide(providers);
 102     ffi_unwind_calls::provide(providers);
 103     shim::provide(providers);
 104     *providers = Providers {
 105         mir_keys,
 106         mir_const,
 107         mir_const_qualif: |tcx, def_id| {
 108             let def_id = def_id.expect_local();
 109             if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
 110                 tcx.mir_const_qualif_const_arg(def)
 111             } else {
 112                 mir_const_qualif(tcx, ty::WithOptConstParam::unknown(def_id))
 113             }
 114         },
 115         mir_const_qualif_const_arg: |tcx, (did, param_did)| {
 116             mir_const_qualif(tcx, ty::WithOptConstParam { did, const_param_did: Some(param_did) })
 117         },
 118         mir_promoted,
 119         mir_drops_elaborated_and_const_checked,
 120         mir_for_ctfe,
 121         mir_for_ctfe_of_const_arg,
 122         optimized_mir,
 123         is_mir_available,
 124         is_ctfe_mir_available: |tcx, did| is_mir_available(tcx, did),
 125         mir_callgraph_reachable: inline::cycle::mir_callgraph_reachable,
 126         mir_inliner_callees: inline::cycle::mir_inliner_callees,
 127         promoted_mir: |tcx, def_id| {
 128             let def_id = def_id.expect_local();
 129             if let Some(def) = ty::WithOptConstParam::try_lookup(def_id, tcx) {
 130                 tcx.promoted_mir_of_const_arg(def)
 131             } else {
 132                 promoted_mir(tcx, ty::WithOptConstParam::unknown(def_id))
 133             }
 134         },
 135         promoted_mir_of_const_arg: |tcx, (did, param_did)| {
 136             promoted_mir(tcx, ty::WithOptConstParam { did, const_param_did: Some(param_did) })
 137         },
 138         ..*providers
 139     };
 140 }
 141
 142 fn is_mir_available(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 143     let def_id = def_id.expect_local();
 144     tcx.mir_keys(()).contains(&def_id)
 145 }
 146
 147 /// Finds the full set of `DefId`s within the current crate that have
 148 /// MIR associated with them.
 149 fn mir_keys(tcx: TyCtxt<'_>, (): ()) -> FxIndexSet<LocalDefId> {
 150     let mut set = FxIndexSet::default();
 151
 152     // All body-owners have MIR associated with them.
 153     set.extend(tcx.hir().body_owners());
 154
 155     // Additionally, tuple struct/variant constructors have MIR, but
 156     // they don't have a BodyId, so we need to build them separately.
 157     struct GatherCtors<'a, 'tcx> {
 158         tcx: TyCtxt<'tcx>,
 159         set: &'a mut FxIndexSet<LocalDefId>,
 160     }
 161     impl<'tcx> Visitor<'tcx> for GatherCtors<'_, 'tcx> {
 162         fn visit_variant_data(
 163             &mut self,
 164             v: &'tcx hir::VariantData<'tcx>,
 165             _: Symbol,
 166             _: &'tcx hir::Generics<'tcx>,
 167             _: hir::HirId,
 168             _: Span,
 169         ) {
 170             if let hir::VariantData::Tuple(_, hir_id) = *v {
 171                 self.set.insert(self.tcx.hir().local_def_id(hir_id));
 172             }
 173             intravisit::walk_struct_def(self, v)
 174         }
 175     }
 176     tcx.hir().deep_visit_all_item_likes(&mut GatherCtors { tcx, set: &mut set });
 177
 178     set
 179 }
 180
 181 fn mir_const_qualif(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -> ConstQualifs {
 182     let const_kind = tcx.hir().body_const_context(def.did);
 183
 184     // No need to const-check a non-const `fn`.
 185     if const_kind.is_none() {
 186         return Default::default();
 187     }
 188
 189     // N.B., this `borrow()` is guaranteed to be valid (i.e., the value
 190     // cannot yet be stolen), because `mir_promoted()`, which steals
 191     // from `mir_const(), forces this query to execute before
 192     // performing the steal.
 193     let body = &tcx.mir_const(def).borrow();
 194
 195     if body.return_ty().references_error() {
 196         tcx.sess.delay_span_bug(body.span, "mir_const_qualif: MIR had errors");
 197         return Default::default();
 198     }
 199
 200     let ccx = check_consts::ConstCx { body, tcx, const_kind, param_env: tcx.param_env(def.did) };
 201
 202     let mut validator = check_consts::check::Checker::new(&ccx);
 203     validator.check_body();
 204
 205     // We return the qualifs in the return place for every MIR body, even though it is only used
 206     // when deciding to promote a reference to a `const` for now.
 207     validator.qualifs_in_return_place()
 208 }
 209
 210 /// Make MIR ready for const evaluation. This is run on all MIR, not just on consts!
 211 fn mir_const<'tcx>(
 212     tcx: TyCtxt<'tcx>,
 213     def: ty::WithOptConstParam<LocalDefId>,
 214 ) -> &'tcx Steal<Body<'tcx>> {
 215     if let Some(def) = def.try_upgrade(tcx) {
 216         return tcx.mir_const(def);
 217     }
 218
 219     // Unsafety check uses the raw mir, so make sure it is run.
 220     if !tcx.sess.opts.debugging_opts.thir_unsafeck {
 221         if let Some(param_did) = def.const_param_did {
 222             tcx.ensure().unsafety_check_result_for_const_arg((def.did, param_did));
 223         } else {
 224             tcx.ensure().unsafety_check_result(def.did);
 225         }
 226     }
 227
 228     // has_ffi_unwind_calls query uses the raw mir, so make sure it is run.
 229     tcx.ensure().has_ffi_unwind_calls(def.did);
 230
 231     let mut body = tcx.mir_built(def).steal();
 232
 233     rustc_middle::mir::dump_mir(tcx, None, "mir_map", &0, &body, |_, _| Ok(()));
 234
 235     pm::run_passes(
 236         tcx,
 237         &mut body,
 238         &[
 239             // MIR-level lints.
 240             &Lint(check_packed_ref::CheckPackedRef),
 241             &Lint(check_const_item_mutation::CheckConstItemMutation),
 242             &Lint(function_item_references::FunctionItemReferences),
 243             // What we need to do constant evaluation.
 244             &simplify::SimplifyCfg::new("initial"),
 245             &rustc_peek::SanityCheck, // Just a lint
 246             &marker::PhaseChange(MirPhase::Const),
 247         ],
 248     );
 249     tcx.alloc_steal_mir(body)
 250 }
 251
 252 /// Compute the main MIR body and the list of MIR bodies of the promoteds.
 253 fn mir_promoted<'tcx>(
 254     tcx: TyCtxt<'tcx>,
 255     def: ty::WithOptConstParam<LocalDefId>,
 256 ) -> (&'tcx Steal<Body<'tcx>>, &'tcx Steal<IndexVec<Promoted, Body<'tcx>>>) {
 257     if let Some(def) = def.try_upgrade(tcx) {
 258         return tcx.mir_promoted(def);
 259     }
 260
 261     // Ensure that we compute the `mir_const_qualif` for constants at
 262     // this point, before we steal the mir-const result.
 263     // Also this means promotion can rely on all const checks having been done.
 264     let const_qualifs = tcx.mir_const_qualif_opt_const_arg(def);
 265     let mut body = tcx.mir_const(def).steal();
 266     if let Some(error_reported) = const_qualifs.tainted_by_errors {
 267         body.tainted_by_errors = Some(error_reported);
 268     }
 269
 270     let mut required_consts = Vec::new();
 271     let mut required_consts_visitor = RequiredConstsVisitor::new(&mut required_consts);
 272     for (bb, bb_data) in traversal::reverse_postorder(&body) {
 273         required_consts_visitor.visit_basic_block_data(bb, bb_data);
 274     }
 275     body.required_consts = required_consts;
 276
 277     // What we need to run borrowck etc.
 278     let promote_pass = promote_consts::PromoteTemps::default();
 279     pm::run_passes(
 280         tcx,
 281         &mut body,
 282         &[
 283             &promote_pass,
 284             &simplify::SimplifyCfg::new("promote-consts"),
 285             &coverage::InstrumentCoverage,
 286         ],
 287     );
 288
 289     let promoted = promote_pass.promoted_fragments.into_inner();
 290     (tcx.alloc_steal_mir(body), tcx.alloc_steal_promoted(promoted))
 291 }
 292
 293 /// Compute the MIR that is used during CTFE (and thus has no optimizations run on it)
 294 fn mir_for_ctfe<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> &'tcx Body<'tcx> {
 295     let did = def_id.expect_local();
 296     if let Some(def) = ty::WithOptConstParam::try_lookup(did, tcx) {
 297         tcx.mir_for_ctfe_of_const_arg(def)
 298     } else {
 299         tcx.arena.alloc(inner_mir_for_ctfe(tcx, ty::WithOptConstParam::unknown(did)))
 300     }
 301 }
 302
 303 /// Same as `mir_for_ctfe`, but used to get the MIR of a const generic parameter.
 304 /// The docs on `WithOptConstParam` explain this a bit more, but the TLDR is that
 305 /// we'd get cycle errors with `mir_for_ctfe`, because typeck would need to typeck
 306 /// the const parameter while type checking the main body, which in turn would try
 307 /// to type check the main body again.
 308 fn mir_for_ctfe_of_const_arg<'tcx>(
 309     tcx: TyCtxt<'tcx>,
 310     (did, param_did): (LocalDefId, DefId),
 311 ) -> &'tcx Body<'tcx> {
 312     tcx.arena.alloc(inner_mir_for_ctfe(
 313         tcx,
 314         ty::WithOptConstParam { did, const_param_did: Some(param_did) },
 315     ))
 316 }
 317
 318 fn inner_mir_for_ctfe(tcx: TyCtxt<'_>, def: ty::WithOptConstParam<LocalDefId>) -> Body<'_> {
 319     // FIXME: don't duplicate this between the optimized_mir/mir_for_ctfe queries
 320     if tcx.is_constructor(def.did.to_def_id()) {
 321         // There's no reason to run all of the MIR passes on constructors when
 322         // we can just output the MIR we want directly. This also saves const
 323         // qualification and borrow checking the trouble of special casing
 324         // constructors.
 325         return shim::build_adt_ctor(tcx, def.did.to_def_id());
 326     }
 327
 328     let context = tcx
 329         .hir()
 330         .body_const_context(def.did)
 331         .expect("mir_for_ctfe should not be used for runtime functions");
 332
 333     let mut body = tcx.mir_drops_elaborated_and_const_checked(def).borrow().clone();
 334
 335     match context {
 336         // Do not const prop functions, either they get executed at runtime or exported to metadata,
 337         // so we run const prop on them, or they don't, in which case we const evaluate some control
 338         // flow paths of the function and any errors in those paths will get emitted as const eval
 339         // errors.
 340         hir::ConstContext::ConstFn => {}
 341         // Static items always get evaluated, so we can just let const eval see if any erroneous
 342         // control flow paths get executed.
 343         hir::ConstContext::Static(_) => {}
 344         // Associated constants get const prop run so we detect common failure situations in the
 345         // crate that defined the constant.
 346         // Technically we want to not run on regular const items, but oli-obk doesn't know how to
 347         // conveniently detect that at this point without looking at the HIR.
 348         hir::ConstContext::Const => {
 349             pm::run_passes(
 350                 tcx,
 351                 &mut body,
 352                 &[&const_prop::ConstProp, &marker::PhaseChange(MirPhase::Optimized)],
 353             );
 354         }
 355     }
 356
 357     debug_assert!(!body.has_free_regions(), "Free regions in MIR for CTFE");
 358
 359     body
 360 }
 361
 362 /// Obtain just the main MIR (no promoteds) and run some cleanups on it. This also runs
 363 /// mir borrowck *before* doing so in order to ensure that borrowck can be run and doesn't
 364 /// end up missing the source MIR due to stealing happening.
 365 fn mir_drops_elaborated_and_const_checked<'tcx>(
 366     tcx: TyCtxt<'tcx>,
 367     def: ty::WithOptConstParam<LocalDefId>,
 368 ) -> &'tcx Steal<Body<'tcx>> {
 369     if let Some(def) = def.try_upgrade(tcx) {
 370         return tcx.mir_drops_elaborated_and_const_checked(def);
 371     }
 372
 373     let mir_borrowck = tcx.mir_borrowck_opt_const_arg(def);
 374
 375     let is_fn_like = tcx.def_kind(def.did).is_fn_like();
 376     if is_fn_like {
 377         let did = def.did.to_def_id();
 378         let def = ty::WithOptConstParam::unknown(did);
 379
 380         // Do not compute the mir call graph without said call graph actually being used.
 381         if inline::Inline.is_enabled(&tcx.sess) {
 382             let _ = tcx.mir_inliner_callees(ty::InstanceDef::Item(def));
 383         }
 384     }
 385
 386     let (body, _) = tcx.mir_promoted(def);
 387     let mut body = body.steal();
 388     if let Some(error_reported) = mir_borrowck.tainted_by_errors {
 389         body.tainted_by_errors = Some(error_reported);
 390     }
 391
 392     // IMPORTANT
 393     pm::run_passes(tcx, &mut body, &[&remove_false_edges::RemoveFalseEdges]);
 394
 395     // Do a little drop elaboration before const-checking if `const_precise_live_drops` is enabled.
 396     if check_consts::post_drop_elaboration::checking_enabled(&ConstCx::new(tcx, &body)) {
 397         pm::run_passes(
 398             tcx,
 399             &mut body,
 400             &[
 401                 &simplify::SimplifyCfg::new("remove-false-edges"),
 402                 &remove_uninit_drops::RemoveUninitDrops,
 403             ],
 404         );
 405         check_consts::post_drop_elaboration::check_live_drops(tcx, &body); // FIXME: make this a MIR lint
 406     }
 407
 408     run_post_borrowck_cleanup_passes(tcx, &mut body);
 409     assert!(body.phase == MirPhase::Deaggregated);
 410     tcx.alloc_steal_mir(body)
 411 }
 412
 413 /// After this series of passes, no lifetime analysis based on borrowing can be done.
 414 fn run_post_borrowck_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 415     debug!("post_borrowck_cleanup({:?})", body.source.def_id());
 416
 417     let post_borrowck_cleanup: &[&dyn MirPass<'tcx>] = &[
 418         // Remove all things only needed by analysis
 419         &simplify_branches::SimplifyConstCondition::new("initial"),
 420         &remove_noop_landing_pads::RemoveNoopLandingPads,
 421         &cleanup_post_borrowck::CleanupNonCodegenStatements,
 422         &simplify::SimplifyCfg::new("early-opt"),
 423         // These next passes must be executed together
 424         &add_call_guards::CriticalCallEdges,
 425         &elaborate_drops::ElaborateDrops,
 426         // This will remove extraneous landing pads which are no longer
 427         // necessary as well as well as forcing any call in a non-unwinding
 428         // function calling a possibly-unwinding function to abort the process.
 429         &abort_unwinding_calls::AbortUnwindingCalls,
 430         // AddMovesForPackedDrops needs to run after drop
 431         // elaboration.
 432         &add_moves_for_packed_drops::AddMovesForPackedDrops,
 433         // `AddRetag` needs to run after `ElaborateDrops`. Otherwise it should run fairly late,
 434         // but before optimizations begin.
 435         &deref_separator::Derefer,
 436         &elaborate_box_derefs::ElaborateBoxDerefs,
 437         &add_retag::AddRetag,
 438         &lower_intrinsics::LowerIntrinsics,
 439         &simplify::SimplifyCfg::new("elaborate-drops"),
 440         // `Deaggregator` is conceptually part of MIR building, some backends rely on it happening
 441         // and it can help optimizations.
 442         &deaggregator::Deaggregator,
 443         &Lint(const_prop_lint::ConstProp),
 444     ];
 445
 446     pm::run_passes(tcx, body, post_borrowck_cleanup);
 447 }
 448
 449 fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 450     fn o1<T>(x: T) -> WithMinOptLevel<T> {
 451         WithMinOptLevel(1, x)
 452     }
 453
 454     // Lowering generator control-flow and variables has to happen before we do anything else
 455     // to them. We run some optimizations before that, because they may be harder to do on the state
 456     // machine than on MIR with async primitives.
 457     pm::run_passes(
 458         tcx,
 459         body,
 460         &[
 461             &reveal_all::RevealAll, // has to be done before inlining, since inlined code is in RevealAll mode.
 462             &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
 463             &normalize_array_len::NormalizeArrayLen, // has to run after `slice::len` lowering
 464             &unreachable_prop::UnreachablePropagation,
 465             &uninhabited_enum_branching::UninhabitedEnumBranching,
 466             &o1(simplify::SimplifyCfg::new("after-uninhabited-enum-branching")),
 467             &inline::Inline,
 468             &generator::StateTransform,
 469         ],
 470     );
 471
 472     assert!(body.phase == MirPhase::GeneratorsLowered);
 473
 474     // The main optimizations that we do on MIR.
 475     pm::run_passes(
 476         tcx,
 477         body,
 478         &[
 479             &remove_storage_markers::RemoveStorageMarkers,
 480             &remove_zsts::RemoveZsts,
 481             &const_goto::ConstGoto,
 482             &remove_unneeded_drops::RemoveUnneededDrops,
 483             &match_branches::MatchBranchSimplification,
 484             // inst combine is after MatchBranchSimplification to clean up Ne(_1, false)
 485             &multiple_return_terminators::MultipleReturnTerminators,
 486             &instcombine::InstCombine,
 487             &separate_const_switch::SeparateConstSwitch,
 488             //
 489             // FIXME(#70073): This pass is responsible for both optimization as well as some lints.
 490             &const_prop::ConstProp,
 491             //
 492             // Const-prop runs unconditionally, but doesn't mutate the MIR at mir-opt-level=0.
 493             &const_debuginfo::ConstDebugInfo,
 494             &o1(simplify_branches::SimplifyConstCondition::new("after-const-prop")),
 495             &early_otherwise_branch::EarlyOtherwiseBranch,
 496             &simplify_comparison_integral::SimplifyComparisonIntegral,
 497             &simplify_try::SimplifyArmIdentity,
 498             &simplify_try::SimplifyBranchSame,
 499             &dead_store_elimination::DeadStoreElimination,
 500             &dest_prop::DestinationPropagation,
 501             &o1(simplify_branches::SimplifyConstCondition::new("final")),
 502             &o1(remove_noop_landing_pads::RemoveNoopLandingPads),
 503             &o1(simplify::SimplifyCfg::new("final")),
 504             &nrvo::RenameReturnPlace,
 505             &simplify::SimplifyLocals,
 506             &multiple_return_terminators::MultipleReturnTerminators,
 507             &deduplicate_blocks::DeduplicateBlocks,
 508             // Some cleanup necessary at least for LLVM and potentially other codegen backends.
 509             &add_call_guards::CriticalCallEdges,
 510             &marker::PhaseChange(MirPhase::Optimized),
 511             // Dump the end result for testing and debugging purposes.
 512             &dump_mir::Marker("PreCodegen"),
 513         ],
 514     );
 515 }
 516
 517 /// Optimize the MIR and prepare it for codegen.
 518 fn optimized_mir<'tcx>(tcx: TyCtxt<'tcx>, did: DefId) -> &'tcx Body<'tcx> {
 519     let did = did.expect_local();
 520     assert_eq!(ty::WithOptConstParam::try_lookup(did, tcx), None);
 521     tcx.arena.alloc(inner_optimized_mir(tcx, did))
 522 }
 523
 524 fn inner_optimized_mir(tcx: TyCtxt<'_>, did: LocalDefId) -> Body<'_> {
 525     if tcx.is_constructor(did.to_def_id()) {
 526         // There's no reason to run all of the MIR passes on constructors when
 527         // we can just output the MIR we want directly. This also saves const
 528         // qualification and borrow checking the trouble of special casing
 529         // constructors.
 530         return shim::build_adt_ctor(tcx, did.to_def_id());
 531     }
 532
 533     match tcx.hir().body_const_context(did) {
 534         // Run the `mir_for_ctfe` query, which depends on `mir_drops_elaborated_and_const_checked`
 535         // which we are going to steal below. Thus we need to run `mir_for_ctfe` first, so it
 536         // computes and caches its result.
 537         Some(hir::ConstContext::ConstFn) => tcx.ensure().mir_for_ctfe(did),
 538         None => {}
 539         Some(other) => panic!("do not use `optimized_mir` for constants: {:?}", other),
 540     }
 541     debug!("about to call mir_drops_elaborated...");
 542     let mut body =
 543         tcx.mir_drops_elaborated_and_const_checked(ty::WithOptConstParam::unknown(did)).steal();
 544     debug!("body: {:#?}", body);
 545     run_optimization_passes(tcx, &mut body);
 546
 547     debug_assert!(!body.has_free_regions(), "Free regions in optimized MIR");
 548
 549     body
 550 }
 551
 552 /// Fetch all the promoteds of an item and prepare their MIR bodies to be ready for
 553 /// constant evaluation once all substitutions become known.
 554 fn promoted_mir<'tcx>(
 555     tcx: TyCtxt<'tcx>,
 556     def: ty::WithOptConstParam<LocalDefId>,
 557 ) -> &'tcx IndexVec<Promoted, Body<'tcx>> {
 558     if tcx.is_constructor(def.did.to_def_id()) {
 559         return tcx.arena.alloc(IndexVec::new());
 560     }
 561
 562     let tainted_by_errors = tcx.mir_borrowck_opt_const_arg(def).tainted_by_errors;
 563     let mut promoted = tcx.mir_promoted(def).1.steal();
 564
 565     for body in &mut promoted {
 566         if let Some(error_reported) = tainted_by_errors {
 567             body.tainted_by_errors = Some(error_reported);
 568         }
 569         run_post_borrowck_cleanup_passes(tcx, body);
 570     }
 571
 572     debug_assert!(!promoted.has_free_regions(), "Free regions in promoted MIR");
 573
 574     tcx.arena.alloc(promoted)
 575 }