src/librustc_mir/transform/qualify_consts.rs

   1 //! A pass that qualifies constness of temporaries in constants,
   2 //! static initializers and functions and also drives promotion.
   3 //!
   4 //! The Qualif flags below can be used to also provide better
   5 //! diagnostics as to why a constant rvalue wasn't promoted.
   6
   7 use rustc_data_structures::bit_set::BitSet;
   8 use rustc_data_structures::indexed_vec::IndexVec;
   9 use rustc_data_structures::fx::FxHashSet;
  10 use rustc_target::spec::abi::Abi;
  11 use rustc::hir;
  12 use rustc::hir::def_id::DefId;
  13 use rustc::traits::{self, TraitEngine};
  14 use rustc::ty::{self, TyCtxt, Ty, TypeFoldable};
  15 use rustc::ty::cast::CastTy;
  16 use rustc::ty::query::Providers;
  17 use rustc::mir::*;
  18 use rustc::mir::interpret::ConstValue;
  19 use rustc::mir::traversal::ReversePostorder;
  20 use rustc::mir::visit::{PlaceContext, Visitor, MutatingUseContext, NonMutatingUseContext};
  21 use rustc::middle::lang_items;
  22 use rustc::session::config::nightly_options;
  23 use syntax::ast::LitKind;
  24 use syntax::feature_gate::{emit_feature_err, GateIssue};
  25 use syntax::symbol::sym;
  26 use syntax_pos::{Span, DUMMY_SP};
  27
  28 use std::borrow::Cow;
  29 use std::cell::Cell;
  30 use std::fmt;
  31 use std::ops::{Deref, Index, IndexMut};
  32 use std::usize;
  33
  34 use rustc::hir::HirId;
  35 use crate::transform::{MirPass, MirSource};
  36 use super::promote_consts::{self, Candidate, TempState};
  37 use crate::transform::check_consts::ops::{self, NonConstOp};
  38
  39 /// What kind of item we are in.
  40 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
  41 enum Mode {
  42     /// A `static` item.
  43     Static,
  44     /// A `static mut` item.
  45     StaticMut,
  46     /// A `const fn` item.
  47     ConstFn,
  48     /// A `const` item or an anonymous constant (e.g. in array lengths).
  49     Const,
  50     /// Other type of `fn`.
  51     NonConstFn,
  52 }
  53
  54 impl Mode {
  55     /// Determine whether we have to do full const-checking because syntactically, we
  56     /// are required to be "const".
  57     #[inline]
  58     fn requires_const_checking(self) -> bool {
  59         self != Mode::NonConstFn
  60     }
  61 }
  62
  63 impl fmt::Display for Mode {
  64     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  65         match *self {
  66             Mode::Const => write!(f, "constant"),
  67             Mode::Static | Mode::StaticMut => write!(f, "static"),
  68             Mode::ConstFn => write!(f, "constant function"),
  69             Mode::NonConstFn => write!(f, "function")
  70         }
  71     }
  72 }
  73
  74 const QUALIF_COUNT: usize = 4;
  75
  76 // FIXME(eddyb) once we can use const generics, replace this array with
  77 // something like `IndexVec` but for fixed-size arrays (`IndexArray`?).
  78 #[derive(Copy, Clone, Default)]
  79 struct PerQualif<T>([T; QUALIF_COUNT]);
  80
  81 impl<T: Clone> PerQualif<T> {
  82     fn new(x: T) -> Self {
  83         PerQualif([x.clone(), x.clone(), x.clone(), x])
  84     }
  85 }
  86
  87 impl<T> PerQualif<T> {
  88     fn as_mut(&mut self) -> PerQualif<&mut T> {
  89         let [x0, x1, x2, x3] = &mut self.0;
  90         PerQualif([x0, x1, x2, x3])
  91     }
  92
  93     fn zip<U>(self, other: PerQualif<U>) -> PerQualif<(T, U)> {
  94         let [x0, x1, x2, x3] = self.0;
  95         let [y0, y1, y2, y3] = other.0;
  96         PerQualif([(x0, y0), (x1, y1), (x2, y2), (x3, y3)])
  97     }
  98 }
  99
 100 impl PerQualif<bool> {
 101     fn encode_to_bits(self) -> u8 {
 102         self.0.iter().enumerate().fold(0, |bits, (i, &qualif)| {
 103             bits | ((qualif as u8) << i)
 104         })
 105     }
 106
 107     fn decode_from_bits(bits: u8) -> Self {
 108         let mut qualifs = Self::default();
 109         for (i, qualif) in qualifs.0.iter_mut().enumerate() {
 110             *qualif = (bits & (1 << i)) != 0;
 111         }
 112         qualifs
 113     }
 114 }
 115
 116 impl<Q: Qualif, T> Index<Q> for PerQualif<T> {
 117     type Output = T;
 118
 119     fn index(&self, _: Q) -> &T {
 120         &self.0[Q::IDX]
 121     }
 122 }
 123
 124 impl<Q: Qualif, T> IndexMut<Q> for PerQualif<T> {
 125     fn index_mut(&mut self, _: Q) -> &mut T {
 126         &mut self.0[Q::IDX]
 127     }
 128 }
 129
 130 struct ConstCx<'a, 'tcx> {
 131     tcx: TyCtxt<'tcx>,
 132     param_env: ty::ParamEnv<'tcx>,
 133     mode: Mode,
 134     body: &'a Body<'tcx>,
 135
 136     per_local: PerQualif<BitSet<Local>>,
 137 }
 138
 139 impl<'a, 'tcx> ConstCx<'a, 'tcx> {
 140     fn is_const_panic_fn(&self, def_id: DefId) -> bool {
 141         Some(def_id) == self.tcx.lang_items().panic_fn() ||
 142         Some(def_id) == self.tcx.lang_items().begin_panic_fn()
 143     }
 144 }
 145
 146 #[derive(Copy, Clone, Debug)]
 147 enum ValueSource<'a, 'tcx> {
 148     Rvalue(&'a Rvalue<'tcx>),
 149     DropAndReplace(&'a Operand<'tcx>),
 150     Call {
 151         callee: &'a Operand<'tcx>,
 152         args: &'a [Operand<'tcx>],
 153         return_ty: Ty<'tcx>,
 154     },
 155 }
 156
 157 /// A "qualif"(-ication) is a way to look for something "bad" in the MIR that would disqualify some
 158 /// code for promotion or prevent it from evaluating at compile time. So `return true` means
 159 /// "I found something bad, no reason to go on searching". `false` is only returned if we
 160 /// definitely cannot find anything bad anywhere.
 161 ///
 162 /// The default implementations proceed structurally.
 163 trait Qualif {
 164     const IDX: usize;
 165
 166     /// Return the qualification that is (conservatively) correct for any value
 167     /// of the type, or `None` if the qualification is not value/type-based.
 168     fn in_any_value_of_ty(_cx: &ConstCx<'_, 'tcx>, _ty: Ty<'tcx>) -> Option<bool> {
 169         None
 170     }
 171
 172     /// Return a mask for the qualification, given a type. This is `false` iff
 173     /// no value of that type can have the qualification.
 174     fn mask_for_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
 175         Self::in_any_value_of_ty(cx, ty).unwrap_or(true)
 176     }
 177
 178     fn in_local(cx: &ConstCx<'_, '_>, local: Local) -> bool {
 179         cx.per_local.0[Self::IDX].contains(local)
 180     }
 181
 182     fn in_static(_cx: &ConstCx<'_, 'tcx>, _static: &Static<'tcx>) -> bool {
 183         // FIXME(eddyb) should we do anything here for value properties?
 184         false
 185     }
 186
 187     fn in_projection_structurally(
 188         cx: &ConstCx<'_, 'tcx>,
 189         place: PlaceRef<'_, 'tcx>,
 190     ) -> bool {
 191         if let [proj_base @ .., elem] = place.projection {
 192             let base_qualif = Self::in_place(cx, PlaceRef {
 193                 base: place.base,
 194                 projection: proj_base,
 195             });
 196             let qualif = base_qualif && Self::mask_for_ty(
 197                 cx,
 198                 Place::ty_from(place.base, proj_base, cx.body, cx.tcx)
 199                     .projection_ty(cx.tcx, elem)
 200                     .ty,
 201             );
 202             match elem {
 203                 ProjectionElem::Deref |
 204                 ProjectionElem::Subslice { .. } |
 205                 ProjectionElem::Field(..) |
 206                 ProjectionElem::ConstantIndex { .. } |
 207                 ProjectionElem::Downcast(..) => qualif,
 208
 209                 ProjectionElem::Index(local) => qualif || Self::in_local(cx, *local),
 210             }
 211         } else {
 212             bug!("This should be called if projection is not empty");
 213         }
 214     }
 215
 216     fn in_projection(
 217         cx: &ConstCx<'_, 'tcx>,
 218         place: PlaceRef<'_, 'tcx>,
 219     ) -> bool {
 220         Self::in_projection_structurally(cx, place)
 221     }
 222
 223     fn in_place(cx: &ConstCx<'_, 'tcx>, place: PlaceRef<'_, 'tcx>) -> bool {
 224         match place {
 225             PlaceRef {
 226                 base: PlaceBase::Local(local),
 227                 projection: [],
 228             } => Self::in_local(cx, *local),
 229             PlaceRef {
 230                 base: PlaceBase::Static(box Static {
 231                     kind: StaticKind::Promoted(..),
 232                     ..
 233                 }),
 234                 projection: [],
 235             } => bug!("qualifying already promoted MIR"),
 236             PlaceRef {
 237                 base: PlaceBase::Static(static_),
 238                 projection: [],
 239             } => {
 240                 Self::in_static(cx, static_)
 241             },
 242             PlaceRef {
 243                 base: _,
 244                 projection: [.., _],
 245             } => Self::in_projection(cx, place),
 246         }
 247     }
 248
 249     fn in_operand(cx: &ConstCx<'_, 'tcx>, operand: &Operand<'tcx>) -> bool {
 250         match *operand {
 251             Operand::Copy(ref place) |
 252             Operand::Move(ref place) => Self::in_place(cx, place.as_ref()),
 253
 254             Operand::Constant(ref constant) => {
 255                 if let ConstValue::Unevaluated(def_id, _) = constant.literal.val {
 256                     // Don't peek inside trait associated constants.
 257                     if cx.tcx.trait_of_item(def_id).is_some() {
 258                         Self::in_any_value_of_ty(cx, constant.literal.ty).unwrap_or(false)
 259                     } else {
 260                         let (bits, _) = cx.tcx.at(constant.span).mir_const_qualif(def_id);
 261
 262                         let qualif = PerQualif::decode_from_bits(bits).0[Self::IDX];
 263
 264                         // Just in case the type is more specific than
 265                         // the definition, e.g., impl associated const
 266                         // with type parameters, take it into account.
 267                         qualif && Self::mask_for_ty(cx, constant.literal.ty)
 268                     }
 269                 } else {
 270                     false
 271                 }
 272             }
 273         }
 274     }
 275
 276     fn in_rvalue_structurally(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
 277         match *rvalue {
 278             Rvalue::NullaryOp(..) => false,
 279
 280             Rvalue::Discriminant(ref place) |
 281             Rvalue::Len(ref place) => Self::in_place(cx, place.as_ref()),
 282
 283             Rvalue::Use(ref operand) |
 284             Rvalue::Repeat(ref operand, _) |
 285             Rvalue::UnaryOp(_, ref operand) |
 286             Rvalue::Cast(_, ref operand, _) => Self::in_operand(cx, operand),
 287
 288             Rvalue::BinaryOp(_, ref lhs, ref rhs) |
 289             Rvalue::CheckedBinaryOp(_, ref lhs, ref rhs) => {
 290                 Self::in_operand(cx, lhs) || Self::in_operand(cx, rhs)
 291             }
 292
 293             Rvalue::Ref(_, _, ref place) => {
 294                 // Special-case reborrows to be more like a copy of the reference.
 295                 if let box [proj_base @ .., elem] = &place.projection {
 296                     if ProjectionElem::Deref == *elem {
 297                         let base_ty = Place::ty_from(&place.base, proj_base, cx.body, cx.tcx).ty;
 298                         if let ty::Ref(..) = base_ty.kind {
 299                             return Self::in_place(cx, PlaceRef {
 300                                 base: &place.base,
 301                                 projection: proj_base,
 302                             });
 303                         }
 304                     }
 305                 }
 306
 307                 Self::in_place(cx, place.as_ref())
 308             }
 309
 310             Rvalue::Aggregate(_, ref operands) => {
 311                 operands.iter().any(|o| Self::in_operand(cx, o))
 312             }
 313         }
 314     }
 315
 316     fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
 317         Self::in_rvalue_structurally(cx, rvalue)
 318     }
 319
 320     fn in_call(
 321         cx: &ConstCx<'_, 'tcx>,
 322         _callee: &Operand<'tcx>,
 323         _args: &[Operand<'tcx>],
 324         return_ty: Ty<'tcx>,
 325     ) -> bool {
 326         // Be conservative about the returned value of a const fn.
 327         Self::in_any_value_of_ty(cx, return_ty).unwrap_or(false)
 328     }
 329
 330     fn in_value(cx: &ConstCx<'_, 'tcx>, source: ValueSource<'_, 'tcx>) -> bool {
 331         match source {
 332             ValueSource::Rvalue(rvalue) => Self::in_rvalue(cx, rvalue),
 333             ValueSource::DropAndReplace(source) => Self::in_operand(cx, source),
 334             ValueSource::Call { callee, args, return_ty } => {
 335                 Self::in_call(cx, callee, args, return_ty)
 336             }
 337         }
 338     }
 339 }
 340
 341 /// Constant containing interior mutability (`UnsafeCell<T>`).
 342 /// This must be ruled out to make sure that evaluating the constant at compile-time
 343 /// and at *any point* during the run-time would produce the same result. In particular,
 344 /// promotion of temporaries must not change program behavior; if the promoted could be
 345 /// written to, that would be a problem.
 346 struct HasMutInterior;
 347
 348 impl Qualif for HasMutInterior {
 349     const IDX: usize = 0;
 350
 351     fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> Option<bool> {
 352         Some(!ty.is_freeze(cx.tcx, cx.param_env, DUMMY_SP))
 353     }
 354
 355     fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
 356         match *rvalue {
 357             // Returning `true` for `Rvalue::Ref` indicates the borrow isn't
 358             // allowed in constants (and the `Checker` will error), and/or it
 359             // won't be promoted, due to `&mut ...` or interior mutability.
 360             Rvalue::Ref(_, kind, ref place) => {
 361                 let ty = place.ty(cx.body, cx.tcx).ty;
 362
 363                 if let BorrowKind::Mut { .. } = kind {
 364                     // In theory, any zero-sized value could be borrowed
 365                     // mutably without consequences. However, only &mut []
 366                     // is allowed right now, and only in functions.
 367                     if cx.mode == Mode::StaticMut {
 368                         // Inside a `static mut`, &mut [...] is also allowed.
 369                         match ty.kind {
 370                             ty::Array(..) | ty::Slice(_) => {}
 371                             _ => return true,
 372                         }
 373                     } else if let ty::Array(_, len) = ty.kind {
 374                         // FIXME(eddyb) the `cx.mode == Mode::NonConstFn` condition
 375                         // seems unnecessary, given that this is merely a ZST.
 376                         match len.try_eval_usize(cx.tcx, cx.param_env) {
 377                             Some(0) if cx.mode == Mode::NonConstFn => {},
 378                             _ => return true,
 379                         }
 380                     } else {
 381                         return true;
 382                     }
 383                 }
 384             }
 385
 386             Rvalue::Aggregate(ref kind, _) => {
 387                 if let AggregateKind::Adt(def, ..) = **kind {
 388                     if Some(def.did) == cx.tcx.lang_items().unsafe_cell_type() {
 389                         let ty = rvalue.ty(cx.body, cx.tcx);
 390                         assert_eq!(Self::in_any_value_of_ty(cx, ty), Some(true));
 391                         return true;
 392                     }
 393                 }
 394             }
 395
 396             _ => {}
 397         }
 398
 399         Self::in_rvalue_structurally(cx, rvalue)
 400     }
 401 }
 402
 403 /// Constant containing an ADT that implements `Drop`.
 404 /// This must be ruled out (a) because we cannot run `Drop` during compile-time
 405 /// as that might not be a `const fn`, and (b) because implicit promotion would
 406 /// remove side-effects that occur as part of dropping that value.
 407 struct NeedsDrop;
 408
 409 impl Qualif for NeedsDrop {
 410     const IDX: usize = 1;
 411
 412     fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> Option<bool> {
 413         Some(ty.needs_drop(cx.tcx, cx.param_env))
 414     }
 415
 416     fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
 417         if let Rvalue::Aggregate(ref kind, _) = *rvalue {
 418             if let AggregateKind::Adt(def, ..) = **kind {
 419                 if def.has_dtor(cx.tcx) {
 420                     return true;
 421                 }
 422             }
 423         }
 424
 425         Self::in_rvalue_structurally(cx, rvalue)
 426     }
 427 }
 428
 429 /// Not promotable at all - non-`const fn` calls, `asm!`,
 430 /// pointer comparisons, ptr-to-int casts, etc.
 431 /// Inside a const context all constness rules apply, so promotion simply has to follow the regular
 432 /// constant rules (modulo interior mutability or `Drop` rules which are handled `HasMutInterior`
 433 /// and `NeedsDrop` respectively). Basically this duplicates the checks that the const-checking
 434 /// visitor enforces by emitting errors when working in const context.
 435 struct IsNotPromotable;
 436
 437 impl Qualif for IsNotPromotable {
 438     const IDX: usize = 2;
 439
 440     fn in_static(cx: &ConstCx<'_, 'tcx>, static_: &Static<'tcx>) -> bool {
 441         match static_.kind {
 442             StaticKind::Promoted(_, _) => unreachable!(),
 443             StaticKind::Static => {
 444                 // Only allow statics (not consts) to refer to other statics.
 445                 let allowed = cx.mode == Mode::Static || cx.mode == Mode::StaticMut;
 446
 447                 !allowed ||
 448                     cx.tcx.get_attrs(static_.def_id).iter().any(
 449                         |attr| attr.check_name(sym::thread_local)
 450                     )
 451             }
 452         }
 453     }
 454
 455     fn in_projection(
 456         cx: &ConstCx<'_, 'tcx>,
 457         place: PlaceRef<'_, 'tcx>,
 458     ) -> bool {
 459         if let [proj_base @ .., elem] = place.projection {
 460             match elem {
 461                 ProjectionElem::Deref |
 462                 ProjectionElem::Downcast(..) => return true,
 463
 464                 ProjectionElem::ConstantIndex {..} |
 465                 ProjectionElem::Subslice {..} |
 466                 ProjectionElem::Index(_) => {}
 467
 468                 ProjectionElem::Field(..) => {
 469                     if cx.mode == Mode::NonConstFn {
 470                         let base_ty = Place::ty_from(place.base, proj_base, cx.body, cx.tcx).ty;
 471                         if let Some(def) = base_ty.ty_adt_def() {
 472                             // No promotion of union field accesses.
 473                             if def.is_union() {
 474                                 return true;
 475                             }
 476                         }
 477                     }
 478                 }
 479             }
 480
 481             Self::in_projection_structurally(cx, place)
 482         } else {
 483             bug!("This should be called if projection is not empty");
 484         }
 485     }
 486
 487     fn in_rvalue(cx: &ConstCx<'_, 'tcx>, rvalue: &Rvalue<'tcx>) -> bool {
 488         match *rvalue {
 489             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) if cx.mode == Mode::NonConstFn => {
 490                 let operand_ty = operand.ty(cx.body, cx.tcx);
 491                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 492                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 493                 match (cast_in, cast_out) {
 494                     (CastTy::Ptr(_), CastTy::Int(_)) |
 495                     (CastTy::FnPtr, CastTy::Int(_)) => {
 496                         // in normal functions, mark such casts as not promotable
 497                         return true;
 498                     }
 499                     _ => {}
 500                 }
 501             }
 502
 503             Rvalue::BinaryOp(op, ref lhs, _) if cx.mode == Mode::NonConstFn => {
 504                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(cx.body, cx.tcx).kind {
 505                     assert!(op == BinOp::Eq || op == BinOp::Ne ||
 506                             op == BinOp::Le || op == BinOp::Lt ||
 507                             op == BinOp::Ge || op == BinOp::Gt ||
 508                             op == BinOp::Offset);
 509
 510                     // raw pointer operations are not allowed inside promoteds
 511                     return true;
 512                 }
 513             }
 514
 515             Rvalue::NullaryOp(NullOp::Box, _) => return true,
 516
 517             _ => {}
 518         }
 519
 520         Self::in_rvalue_structurally(cx, rvalue)
 521     }
 522
 523     fn in_call(
 524         cx: &ConstCx<'_, 'tcx>,
 525         callee: &Operand<'tcx>,
 526         args: &[Operand<'tcx>],
 527         _return_ty: Ty<'tcx>,
 528     ) -> bool {
 529         let fn_ty = callee.ty(cx.body, cx.tcx);
 530         match fn_ty.kind {
 531             ty::FnDef(def_id, _) => {
 532                 match cx.tcx.fn_sig(def_id).abi() {
 533                     Abi::RustIntrinsic |
 534                     Abi::PlatformIntrinsic => {
 535                         assert!(!cx.tcx.is_const_fn(def_id));
 536                         match &cx.tcx.item_name(def_id).as_str()[..] {
 537                             | "size_of"
 538                             | "min_align_of"
 539                             | "needs_drop"
 540                             | "type_id"
 541                             | "bswap"
 542                             | "bitreverse"
 543                             | "ctpop"
 544                             | "cttz"
 545                             | "cttz_nonzero"
 546                             | "ctlz"
 547                             | "ctlz_nonzero"
 548                             | "wrapping_add"
 549                             | "wrapping_sub"
 550                             | "wrapping_mul"
 551                             | "unchecked_shl"
 552                             | "unchecked_shr"
 553                             | "rotate_left"
 554                             | "rotate_right"
 555                             | "add_with_overflow"
 556                             | "sub_with_overflow"
 557                             | "mul_with_overflow"
 558                             | "saturating_add"
 559                             | "saturating_sub"
 560                             | "transmute"
 561                             | "simd_insert"
 562                             | "simd_extract"
 563                             => return true,
 564
 565                             _ => {}
 566                         }
 567                     }
 568                     _ => {
 569                         let is_const_fn =
 570                             cx.tcx.is_const_fn(def_id) ||
 571                             cx.tcx.is_unstable_const_fn(def_id).is_some() ||
 572                             cx.is_const_panic_fn(def_id);
 573                         if !is_const_fn {
 574                             return true;
 575                         }
 576                     }
 577                 }
 578             }
 579             _ => return true,
 580         }
 581
 582         Self::in_operand(cx, callee) || args.iter().any(|arg| Self::in_operand(cx, arg))
 583     }
 584 }
 585
 586 /// Refers to temporaries which cannot be promoted *implicitly*.
 587 /// Explicit promotion happens e.g. for constant arguments declared via `rustc_args_required_const`.
 588 /// Implicit promotion has almost the same rules, except that disallows `const fn` except for
 589 /// those marked `#[rustc_promotable]`. This is to avoid changing a legitimate run-time operation
 590 /// into a failing compile-time operation e.g. due to addresses being compared inside the function.
 591 struct IsNotImplicitlyPromotable;
 592
 593 impl Qualif for IsNotImplicitlyPromotable {
 594     const IDX: usize = 3;
 595
 596     fn in_call(
 597         cx: &ConstCx<'_, 'tcx>,
 598         callee: &Operand<'tcx>,
 599         args: &[Operand<'tcx>],
 600         _return_ty: Ty<'tcx>,
 601     ) -> bool {
 602         if cx.mode == Mode::NonConstFn {
 603             if let ty::FnDef(def_id, _) = callee.ty(cx.body, cx.tcx).kind {
 604                 // Never promote runtime `const fn` calls of
 605                 // functions without `#[rustc_promotable]`.
 606                 if !cx.tcx.is_promotable_const_fn(def_id) {
 607                     return true;
 608                 }
 609             }
 610         }
 611
 612         Self::in_operand(cx, callee) || args.iter().any(|arg| Self::in_operand(cx, arg))
 613     }
 614 }
 615
 616 // Ensure the `IDX` values are sequential (`0..QUALIF_COUNT`).
 617 macro_rules! static_assert_seq_qualifs {
 618     ($i:expr => $first:ident $(, $rest:ident)*) => {
 619         static_assert!({
 620             static_assert_seq_qualifs!($i + 1 => $($rest),*);
 621
 622             $first::IDX == $i
 623         });
 624     };
 625     ($i:expr =>) => {
 626         static_assert!(QUALIF_COUNT == $i);
 627     };
 628 }
 629 static_assert_seq_qualifs!(
 630     0 => HasMutInterior, NeedsDrop, IsNotPromotable, IsNotImplicitlyPromotable
 631 );
 632
 633 impl ConstCx<'_, 'tcx> {
 634     fn qualifs_in_any_value_of_ty(&self, ty: Ty<'tcx>) -> PerQualif<bool> {
 635         let mut qualifs = PerQualif::default();
 636         qualifs[HasMutInterior] = HasMutInterior::in_any_value_of_ty(self, ty).unwrap_or(false);
 637         qualifs[NeedsDrop] = NeedsDrop::in_any_value_of_ty(self, ty).unwrap_or(false);
 638         qualifs[IsNotPromotable] = IsNotPromotable::in_any_value_of_ty(self, ty).unwrap_or(false);
 639         qualifs[IsNotImplicitlyPromotable] =
 640             IsNotImplicitlyPromotable::in_any_value_of_ty(self, ty).unwrap_or(false);
 641         qualifs
 642     }
 643
 644     fn qualifs_in_local(&self, local: Local) -> PerQualif<bool> {
 645         let mut qualifs = PerQualif::default();
 646         qualifs[HasMutInterior] = HasMutInterior::in_local(self, local);
 647         qualifs[NeedsDrop] = NeedsDrop::in_local(self, local);
 648         qualifs[IsNotPromotable] = IsNotPromotable::in_local(self, local);
 649         qualifs[IsNotImplicitlyPromotable] = IsNotImplicitlyPromotable::in_local(self, local);
 650         qualifs
 651     }
 652
 653     fn qualifs_in_value(&self, source: ValueSource<'_, 'tcx>) -> PerQualif<bool> {
 654         let mut qualifs = PerQualif::default();
 655         qualifs[HasMutInterior] = HasMutInterior::in_value(self, source);
 656         qualifs[NeedsDrop] = NeedsDrop::in_value(self, source);
 657         qualifs[IsNotPromotable] = IsNotPromotable::in_value(self, source);
 658         qualifs[IsNotImplicitlyPromotable] = IsNotImplicitlyPromotable::in_value(self, source);
 659         qualifs
 660     }
 661 }
 662
 663 /// Checks MIR for being admissible as a compile-time constant, using `ConstCx`
 664 /// for value qualifications, and accumulates writes of
 665 /// rvalue/call results to locals, in `local_qualif`.
 666 /// It also records candidates for promotion in `promotion_candidates`,
 667 /// both in functions and const/static items.
 668 struct Checker<'a, 'tcx> {
 669     cx: ConstCx<'a, 'tcx>,
 670
 671     span: Span,
 672     def_id: DefId,
 673     rpo: ReversePostorder<'a, 'tcx>,
 674
 675     temp_promotion_state: IndexVec<Local, TempState>,
 676     promotion_candidates: Vec<Candidate>,
 677
 678     /// If `true`, do not emit errors to the user, merely collect them in `errors`.
 679     suppress_errors: bool,
 680     errors: Vec<(Span, String)>,
 681 }
 682
 683 macro_rules! unleash_miri {
 684     ($this:expr) => {{
 685         if $this.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
 686             if $this.mode.requires_const_checking() && !$this.suppress_errors {
 687                 $this.tcx.sess.span_warn($this.span, "skipping const checks");
 688             }
 689             return;
 690         }
 691     }}
 692 }
 693
 694 impl Deref for Checker<'a, 'tcx> {
 695     type Target = ConstCx<'a, 'tcx>;
 696
 697     fn deref(&self) -> &Self::Target {
 698         &self.cx
 699     }
 700 }
 701
 702 impl<'a, 'tcx> Checker<'a, 'tcx> {
 703     fn new(tcx: TyCtxt<'tcx>, def_id: DefId, body: &'a Body<'tcx>, mode: Mode) -> Self {
 704         assert!(def_id.is_local());
 705         let mut rpo = traversal::reverse_postorder(body);
 706         let temps = promote_consts::collect_temps(body, &mut rpo);
 707         rpo.reset();
 708
 709         let param_env = tcx.param_env(def_id);
 710
 711         let mut cx = ConstCx {
 712             tcx,
 713             param_env,
 714             mode,
 715             body,
 716             per_local: PerQualif::new(BitSet::new_empty(body.local_decls.len())),
 717         };
 718
 719         for (local, decl) in body.local_decls.iter_enumerated() {
 720             if let LocalKind::Arg = body.local_kind(local) {
 721                 let qualifs = cx.qualifs_in_any_value_of_ty(decl.ty);
 722                 for (per_local, qualif) in &mut cx.per_local.as_mut().zip(qualifs).0 {
 723                     if *qualif {
 724                         per_local.insert(local);
 725                     }
 726                 }
 727             }
 728             if !temps[local].is_promotable() {
 729                 cx.per_local[IsNotPromotable].insert(local);
 730             }
 731             if let LocalKind::Var = body.local_kind(local) {
 732                 // Sanity check to prevent implicit and explicit promotion of
 733                 // named locals
 734                 assert!(cx.per_local[IsNotPromotable].contains(local));
 735             }
 736         }
 737
 738         Checker {
 739             cx,
 740             span: body.span,
 741             def_id,
 742             rpo,
 743             temp_promotion_state: temps,
 744             promotion_candidates: vec![],
 745             errors: vec![],
 746             suppress_errors: false,
 747         }
 748     }
 749
 750     // FIXME(eddyb) we could split the errors into meaningful
 751     // categories, but enabling full miri would make that
 752     // slightly pointless (even with feature-gating).
 753     fn not_const(&mut self, op: impl NonConstOp) {
 754         unleash_miri!(self);
 755         if self.mode.requires_const_checking() && !self.suppress_errors {
 756             self.record_error(op);
 757             let mut err = struct_span_err!(
 758                 self.tcx.sess,
 759                 self.span,
 760                 E0019,
 761                 "{} contains unimplemented expression type",
 762                 self.mode
 763             );
 764             if self.tcx.sess.teach(&err.get_code().unwrap()) {
 765                 err.note("A function call isn't allowed in the const's initialization expression \
 766                           because the expression's value must be known at compile-time.");
 767                 err.note("Remember: you can't use a function call inside a const's initialization \
 768                           expression! However, you can use it anywhere else.");
 769             }
 770             err.emit();
 771         }
 772     }
 773
 774     fn record_error(&mut self, op: impl NonConstOp) {
 775         self.record_error_spanned(op, self.span);
 776     }
 777
 778     fn record_error_spanned(&mut self, op: impl NonConstOp, span: Span) {
 779         self.errors.push((span, format!("{:?}", op)));
 780     }
 781
 782     /// Assigns an rvalue/call qualification to the given destination.
 783     fn assign(&mut self, dest: &Place<'tcx>, source: ValueSource<'_, 'tcx>, location: Location) {
 784         trace!("assign: {:?} <- {:?}", dest, source);
 785
 786         let mut qualifs = self.qualifs_in_value(source);
 787
 788         match source {
 789             ValueSource::Rvalue(&Rvalue::Ref(_, kind, ref place)) => {
 790                 // Getting `true` from `HasMutInterior::in_rvalue` means
 791                 // the borrowed place is disallowed from being borrowed,
 792                 // due to either a mutable borrow (with some exceptions),
 793                 // or an shared borrow of a value with interior mutability.
 794                 // Then `HasMutInterior` is replaced with `IsNotPromotable`,
 795                 // to avoid duplicate errors (e.g. from reborrowing).
 796                 if qualifs[HasMutInterior] {
 797                     qualifs[HasMutInterior] = false;
 798                     qualifs[IsNotPromotable] = true;
 799
 800                     debug!("suppress_errors: {}", self.suppress_errors);
 801                     if self.mode.requires_const_checking() && !self.suppress_errors {
 802                         if !self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
 803                             self.record_error(ops::MutBorrow(kind));
 804                             if let BorrowKind::Mut { .. } = kind {
 805                                 let mut err = struct_span_err!(self.tcx.sess,  self.span, E0017,
 806                                                                "references in {}s may only refer \
 807                                                                 to immutable values", self.mode);
 808                                 err.span_label(self.span, format!("{}s require immutable values",
 809                                                                     self.mode));
 810                                 if self.tcx.sess.teach(&err.get_code().unwrap()) {
 811                                     err.note("References in statics and constants may only refer \
 812                                               to immutable values.\n\n\
 813                                               Statics are shared everywhere, and if they refer to \
 814                                               mutable data one might violate memory safety since \
 815                                               holding multiple mutable references to shared data \
 816                                               is not allowed.\n\n\
 817                                               If you really want global mutable state, try using \
 818                                               static mut or a global UnsafeCell.");
 819                                 }
 820                                 err.emit();
 821                             } else {
 822                                 span_err!(self.tcx.sess, self.span, E0492,
 823                                           "cannot borrow a constant which may contain \
 824                                            interior mutability, create a static instead");
 825                             }
 826                         }
 827                     }
 828                 } else if let BorrowKind::Mut { .. } | BorrowKind::Shared = kind {
 829                     // Don't promote BorrowKind::Shallow borrows, as they don't
 830                     // reach codegen.
 831
 832                     // We might have a candidate for promotion.
 833                     let candidate = Candidate::Ref(location);
 834                     // Start by traversing to the "base", with non-deref projections removed.
 835                     let deref_proj =
 836                         place.projection.iter().rev().find(|&elem| *elem == ProjectionElem::Deref);
 837
 838                     debug!(
 839                         "qualify_consts: promotion candidate: place={:?} {:?}",
 840                         place.base, deref_proj
 841                     );
 842                     // We can only promote interior borrows of promotable temps (non-temps
 843                     // don't get promoted anyway).
 844                     // (If we bailed out of the loop due to a `Deref` above, we will definitely
 845                     // not enter the conditional here.)
 846                     if let (PlaceBase::Local(local), None) = (&place.base, deref_proj) {
 847                         if self.body.local_kind(*local) == LocalKind::Temp {
 848                             debug!("qualify_consts: promotion candidate: local={:?}", local);
 849                             // The borrowed place doesn't have `HasMutInterior`
 850                             // (from `in_rvalue`), so we can safely ignore
 851                             // `HasMutInterior` from the local's qualifications.
 852                             // This allows borrowing fields which don't have
 853                             // `HasMutInterior`, from a type that does, e.g.:
 854                             // `let _: &'static _ = &(Cell::new(1), 2).1;`
 855                             let mut local_qualifs = self.qualifs_in_local(*local);
 856                             // Any qualifications, except HasMutInterior (see above), disqualify
 857                             // from promotion.
 858                             // This is, in particular, the "implicit promotion" version of
 859                             // the check making sure that we don't run drop glue during const-eval.
 860                             local_qualifs[HasMutInterior] = false;
 861                             if !local_qualifs.0.iter().any(|&qualif| qualif) {
 862                                 debug!("qualify_consts: promotion candidate: {:?}", candidate);
 863                                 self.promotion_candidates.push(candidate);
 864                             }
 865                         }
 866                     }
 867                 }
 868             },
 869             ValueSource::Rvalue(&Rvalue::Repeat(ref operand, _)) => {
 870                 let candidate = Candidate::Repeat(location);
 871                 let not_promotable = IsNotImplicitlyPromotable::in_operand(self, operand) ||
 872                                      IsNotPromotable::in_operand(self, operand);
 873                 debug!("assign: self.def_id={:?} operand={:?}", self.def_id, operand);
 874                 if !not_promotable && self.tcx.features().const_in_array_repeat_expressions {
 875                     debug!("assign: candidate={:?}", candidate);
 876                     self.promotion_candidates.push(candidate);
 877                 }
 878             },
 879             _ => {},
 880         }
 881
 882         let mut dest_projection = &dest.projection[..];
 883         let index = loop {
 884             match (&dest.base, dest_projection) {
 885                 // We treat all locals equal in constants
 886                 (&PlaceBase::Local(index), []) => break index,
 887                 // projections are transparent for assignments
 888                 // we qualify the entire destination at once, even if just a field would have
 889                 // stricter qualification
 890                 (base, [proj_base @ .., _]) => {
 891                     // Catch more errors in the destination. `visit_place` also checks various
 892                     // projection rules like union field access and raw pointer deref
 893                     let context = PlaceContext::MutatingUse(MutatingUseContext::Store);
 894                     self.visit_place_base(base, context, location);
 895                     self.visit_projection(base, dest_projection, context, location);
 896                     dest_projection = proj_base;
 897                 },
 898                 (&PlaceBase::Static(box Static {
 899                     kind: StaticKind::Promoted(..),
 900                     ..
 901                 }), []) => bug!("promoteds don't exist yet during promotion"),
 902                 (&PlaceBase::Static(box Static{ kind: _, .. }), []) => {
 903                     // Catch more errors in the destination. `visit_place` also checks that we
 904                     // do not try to access statics from constants or try to mutate statics
 905                     let context = PlaceContext::MutatingUse(MutatingUseContext::Store);
 906                     self.visit_place_base(&dest.base, context, location);
 907                     return;
 908                 }
 909             }
 910         };
 911
 912         let kind = self.body.local_kind(index);
 913         debug!("store to {:?} {:?}", kind, index);
 914
 915         // Only handle promotable temps in non-const functions.
 916         if self.mode == Mode::NonConstFn {
 917             if kind != LocalKind::Temp ||
 918                !self.temp_promotion_state[index].is_promotable() {
 919                 return;
 920             }
 921         }
 922
 923         // this is overly restrictive, because even full assignments do not clear the qualif
 924         // While we could special case full assignments, this would be inconsistent with
 925         // aggregates where we overwrite all fields via assignments, which would not get
 926         // that feature.
 927         for (per_local, qualif) in &mut self.cx.per_local.as_mut().zip(qualifs).0 {
 928             if *qualif {
 929                 per_local.insert(index);
 930             }
 931         }
 932
 933         // Ensure the `IsNotPromotable` qualification is preserved.
 934         // NOTE(eddyb) this is actually unnecessary right now, as
 935         // we never replace the local's qualif, but we might in
 936         // the future, and so it serves to catch changes that unset
 937         // important bits (in which case, asserting `contains` could
 938         // be replaced with calling `insert` to re-set the bit).
 939         if kind == LocalKind::Temp {
 940             if !self.temp_promotion_state[index].is_promotable() {
 941                 assert!(self.cx.per_local[IsNotPromotable].contains(index));
 942             }
 943         }
 944     }
 945
 946     /// Check a whole const, static initializer or const fn.
 947     fn check_const(&mut self) -> (u8, &'tcx BitSet<Local>) {
 948         use crate::transform::check_consts as new_checker;
 949
 950         debug!("const-checking {} {:?}", self.mode, self.def_id);
 951
 952         // FIXME: Also use the new validator when features that require it (e.g. `const_if`) are
 953         // enabled.
 954         let use_new_validator = self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you;
 955         if use_new_validator {
 956             debug!("Using dataflow-based const validator");
 957         }
 958
 959         let item = new_checker::Item::new(self.tcx, self.def_id, self.body);
 960         let mut validator = new_checker::validation::Validator::new(&item);
 961
 962         validator.suppress_errors = !use_new_validator;
 963         self.suppress_errors = use_new_validator;
 964
 965         let body = self.body;
 966
 967         let mut seen_blocks = BitSet::new_empty(body.basic_blocks().len());
 968         let mut bb = START_BLOCK;
 969         loop {
 970             seen_blocks.insert(bb.index());
 971
 972             self.visit_basic_block_data(bb, &body[bb]);
 973             validator.visit_basic_block_data(bb, &body[bb]);
 974
 975             let target = match body[bb].terminator().kind {
 976                 TerminatorKind::Goto { target } |
 977                 TerminatorKind::FalseUnwind { real_target: target, .. } |
 978                 TerminatorKind::Drop { target, .. } |
 979                 TerminatorKind::DropAndReplace { target, .. } |
 980                 TerminatorKind::Assert { target, .. } |
 981                 TerminatorKind::Call { destination: Some((_, target)), .. } => {
 982                     Some(target)
 983                 }
 984
 985                 // Non-terminating calls cannot produce any value.
 986                 TerminatorKind::Call { destination: None, .. } => {
 987                     break;
 988                 }
 989
 990                 TerminatorKind::SwitchInt {..} |
 991                 TerminatorKind::Resume |
 992                 TerminatorKind::Abort |
 993                 TerminatorKind::GeneratorDrop |
 994                 TerminatorKind::Yield { .. } |
 995                 TerminatorKind::Unreachable |
 996                 TerminatorKind::FalseEdges { .. } => None,
 997
 998                 TerminatorKind::Return => {
 999                     break;
1000                 }
1001             };
1002
1003             match target {
1004                 // No loops allowed.
1005                 Some(target) if !seen_blocks.contains(target.index()) => {
1006                     bb = target;
1007                 }
1008                 _ => {
1009                     self.not_const(ops::Loop);
1010                     validator.check_op(ops::Loop);
1011                     break;
1012                 }
1013             }
1014         }
1015
1016         // The new validation pass should agree with the old when running on simple const bodies
1017         // (e.g. no `if` or `loop`).
1018         if !use_new_validator {
1019             let mut new_errors = validator.take_errors();
1020
1021             // FIXME: each checker sometimes emits the same error with the same span twice in a row.
1022             self.errors.dedup();
1023             new_errors.dedup();
1024
1025             if self.errors != new_errors {
1026                 error!("old validator: {:?}", self.errors);
1027                 error!("new validator: {:?}", new_errors);
1028
1029                 // ICE on nightly if the validators do not emit exactly the same errors.
1030                 // Users can supress this panic with an unstable compiler flag (hopefully after
1031                 // filing an issue).
1032                 let opts = &self.tcx.sess.opts;
1033                 let trigger_ice = opts.unstable_features.is_nightly_build()
1034                     && !opts.debugging_opts.suppress_const_validation_back_compat_ice;
1035
1036                 if trigger_ice {
1037                     span_bug!(
1038                         body.span,
1039                         "{}",
1040                         VALIDATOR_MISMATCH_ERR,
1041                     );
1042                 }
1043             }
1044         }
1045
1046         // Collect all the temps we need to promote.
1047         let mut promoted_temps = BitSet::new_empty(self.temp_promotion_state.len());
1048
1049         debug!("qualify_const: promotion_candidates={:?}", self.promotion_candidates);
1050         for candidate in &self.promotion_candidates {
1051             match *candidate {
1052                 Candidate::Repeat(Location { block: bb, statement_index: stmt_idx }) => {
1053                     if let StatementKind::Assign(box(_, Rvalue::Repeat(
1054                         Operand::Move(Place {
1055                             base: PlaceBase::Local(index),
1056                             projection: box [],
1057                         }),
1058                         _
1059                     ))) = self.body[bb].statements[stmt_idx].kind {
1060                         promoted_temps.insert(index);
1061                     }
1062                 }
1063                 Candidate::Ref(Location { block: bb, statement_index: stmt_idx }) => {
1064                     if let StatementKind::Assign(
1065                         box(
1066                             _,
1067                             Rvalue::Ref(_, _, Place {
1068                                 base: PlaceBase::Local(index),
1069                                 projection: box [],
1070                             })
1071                         )
1072                     ) = self.body[bb].statements[stmt_idx].kind {
1073                         promoted_temps.insert(index);
1074                     }
1075                 }
1076                 Candidate::Argument { .. } => {}
1077             }
1078         }
1079
1080         let mut qualifs = self.qualifs_in_local(RETURN_PLACE);
1081
1082         // Account for errors in consts by using the
1083         // conservative type qualification instead.
1084         if qualifs[IsNotPromotable] {
1085             qualifs = self.qualifs_in_any_value_of_ty(body.return_ty());
1086         }
1087
1088         (qualifs.encode_to_bits(), self.tcx.arena.alloc(promoted_temps))
1089     }
1090 }
1091
1092 impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
1093     fn visit_place_base(
1094         &mut self,
1095         place_base: &PlaceBase<'tcx>,
1096         context: PlaceContext,
1097         location: Location,
1098     ) {
1099         self.super_place_base(place_base, context, location);
1100         match place_base {
1101             PlaceBase::Local(_) => {}
1102             PlaceBase::Static(box Static{ kind: StaticKind::Promoted(_, _), .. }) => {
1103                 unreachable!()
1104             }
1105             PlaceBase::Static(box Static{ kind: StaticKind::Static, def_id, .. }) => {
1106                 if self.tcx
1107                         .get_attrs(*def_id)
1108                         .iter()
1109                         .any(|attr| attr.check_name(sym::thread_local)) {
1110                     if self.mode.requires_const_checking() && !self.suppress_errors {
1111                         self.record_error(ops::ThreadLocalAccess);
1112                         span_err!(self.tcx.sess, self.span, E0625,
1113                                     "thread-local statics cannot be \
1114                                     accessed at compile-time");
1115                     }
1116                     return;
1117                 }
1118
1119                 // Only allow statics (not consts) to refer to other statics.
1120                 if self.mode == Mode::Static || self.mode == Mode::StaticMut {
1121                     if self.mode == Mode::Static
1122                         && context.is_mutating_use()
1123                         && !self.suppress_errors
1124                     {
1125                         // this is not strictly necessary as miri will also bail out
1126                         // For interior mutability we can't really catch this statically as that
1127                         // goes through raw pointers and intermediate temporaries, so miri has
1128                         // to catch this anyway
1129                         self.tcx.sess.span_err(
1130                             self.span,
1131                             "cannot mutate statics in the initializer of another static",
1132                         );
1133                     }
1134                     return;
1135                 }
1136                 unleash_miri!(self);
1137
1138                 if self.mode.requires_const_checking() && !self.suppress_errors {
1139                     self.record_error(ops::StaticAccess);
1140                     let mut err = struct_span_err!(self.tcx.sess, self.span, E0013,
1141                                                     "{}s cannot refer to statics, use \
1142                                                     a constant instead", self.mode);
1143                     if self.tcx.sess.teach(&err.get_code().unwrap()) {
1144                         err.note(
1145                             "Static and const variables can refer to other const variables. \
1146                                 But a const variable cannot refer to a static variable."
1147                         );
1148                         err.help(
1149                             "To fix this, the value can be extracted as a const and then used."
1150                         );
1151                     }
1152                     err.emit()
1153                 }
1154             }
1155         }
1156     }
1157
1158     fn visit_projection(
1159         &mut self,
1160         place_base: &PlaceBase<'tcx>,
1161         proj: &[PlaceElem<'tcx>],
1162         context: PlaceContext,
1163         location: Location,
1164     ) {
1165         debug!(
1166             "visit_place_projection: proj={:?} context={:?} location={:?}",
1167             proj, context, location,
1168         );
1169         self.super_projection(place_base, proj, context, location);
1170
1171         if let [proj_base @ .., elem] = proj {
1172             match elem {
1173                 ProjectionElem::Deref => {
1174                     if context.is_mutating_use() {
1175                         // `not_const` errors out in const contexts
1176                         self.not_const(ops::MutDeref)
1177                     }
1178                     let base_ty = Place::ty_from(place_base, proj_base, self.body, self.tcx).ty;
1179                     match self.mode {
1180                         Mode::NonConstFn => {}
1181                         _ if self.suppress_errors => {}
1182                         _ => {
1183                             if let ty::RawPtr(_) = base_ty.kind {
1184                                 if !self.tcx.features().const_raw_ptr_deref {
1185                                     self.record_error(ops::RawPtrDeref);
1186                                     emit_feature_err(
1187                                         &self.tcx.sess.parse_sess, sym::const_raw_ptr_deref,
1188                                         self.span, GateIssue::Language,
1189                                         &format!(
1190                                             "dereferencing raw pointers in {}s is unstable",
1191                                             self.mode,
1192                                         ),
1193                                     );
1194                                 }
1195                             }
1196                         }
1197                     }
1198                 }
1199
1200                 ProjectionElem::ConstantIndex {..} |
1201                 ProjectionElem::Subslice {..} |
1202                 ProjectionElem::Field(..) |
1203                 ProjectionElem::Index(_) => {
1204                     let base_ty = Place::ty_from(place_base, proj_base, self.body, self.tcx).ty;
1205                     if let Some(def) = base_ty.ty_adt_def() {
1206                         if def.is_union() {
1207                             match self.mode {
1208                                 Mode::ConstFn => {
1209                                     if !self.tcx.features().const_fn_union
1210                                         && !self.suppress_errors
1211                                     {
1212                                         self.record_error(ops::UnionAccess);
1213                                         emit_feature_err(
1214                                             &self.tcx.sess.parse_sess, sym::const_fn_union,
1215                                             self.span, GateIssue::Language,
1216                                             "unions in const fn are unstable",
1217                                         );
1218                                     }
1219                                 },
1220
1221                                 | Mode::NonConstFn
1222                                 | Mode::Static
1223                                 | Mode::StaticMut
1224                                 | Mode::Const
1225                                 => {},
1226                             }
1227                         }
1228                     }
1229                 }
1230
1231                 ProjectionElem::Downcast(..) => {
1232                     self.not_const(ops::Downcast)
1233                 }
1234             }
1235         }
1236     }
1237
1238     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
1239         debug!("visit_operand: operand={:?} location={:?}", operand, location);
1240         self.super_operand(operand, location);
1241
1242         match *operand {
1243             Operand::Move(ref place) => {
1244                 // Mark the consumed locals to indicate later drops are noops.
1245                 if let Place {
1246                     base: PlaceBase::Local(local),
1247                     projection: box [],
1248                 } = *place {
1249                     self.cx.per_local[NeedsDrop].remove(local);
1250                 }
1251             }
1252             Operand::Copy(_) |
1253             Operand::Constant(_) => {}
1254         }
1255     }
1256
1257     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
1258         debug!("visit_rvalue: rvalue={:?} location={:?}", rvalue, location);
1259
1260         // Check nested operands and places.
1261         if let Rvalue::Ref(_, kind, ref place) = *rvalue {
1262             // Special-case reborrows.
1263             let mut reborrow_place = None;
1264             if let box [proj_base @ .., elem] = &place.projection {
1265                 if *elem == ProjectionElem::Deref {
1266                     let base_ty = Place::ty_from(&place.base, proj_base, self.body, self.tcx).ty;
1267                     if let ty::Ref(..) = base_ty.kind {
1268                         reborrow_place = Some(proj_base);
1269                     }
1270                 }
1271             }
1272
1273             if let Some(proj) = reborrow_place {
1274                 let ctx = match kind {
1275                     BorrowKind::Shared => PlaceContext::NonMutatingUse(
1276                         NonMutatingUseContext::SharedBorrow,
1277                     ),
1278                     BorrowKind::Shallow => PlaceContext::NonMutatingUse(
1279                         NonMutatingUseContext::ShallowBorrow,
1280                     ),
1281                     BorrowKind::Unique => PlaceContext::NonMutatingUse(
1282                         NonMutatingUseContext::UniqueBorrow,
1283                     ),
1284                     BorrowKind::Mut { .. } => PlaceContext::MutatingUse(
1285                         MutatingUseContext::Borrow,
1286                     ),
1287                 };
1288                 self.visit_place_base(&place.base, ctx, location);
1289                 self.visit_projection(&place.base, proj, ctx, location);
1290             } else {
1291                 self.super_rvalue(rvalue, location);
1292             }
1293         } else {
1294             self.super_rvalue(rvalue, location);
1295         }
1296
1297         match *rvalue {
1298             Rvalue::Use(_) |
1299             Rvalue::Repeat(..) |
1300             Rvalue::UnaryOp(UnOp::Neg, _) |
1301             Rvalue::UnaryOp(UnOp::Not, _) |
1302             Rvalue::NullaryOp(NullOp::SizeOf, _) |
1303             Rvalue::CheckedBinaryOp(..) |
1304             Rvalue::Cast(CastKind::Pointer(_), ..) |
1305             Rvalue::Discriminant(..) |
1306             Rvalue::Len(_) |
1307             Rvalue::Ref(..) |
1308             Rvalue::Aggregate(..) => {}
1309
1310             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {
1311                 let operand_ty = operand.ty(self.body, self.tcx);
1312                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
1313                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
1314                 match (cast_in, cast_out) {
1315                     (CastTy::Ptr(_), CastTy::Int(_)) |
1316                     (CastTy::FnPtr, CastTy::Int(_)) if self.mode != Mode::NonConstFn => {
1317                         unleash_miri!(self);
1318                         if !self.tcx.features().const_raw_ptr_to_usize_cast
1319                             && !self.suppress_errors
1320                         {
1321                             // in const fn and constants require the feature gate
1322                             // FIXME: make it unsafe inside const fn and constants
1323                             self.record_error(ops::RawPtrToIntCast);
1324                             emit_feature_err(
1325                                 &self.tcx.sess.parse_sess, sym::const_raw_ptr_to_usize_cast,
1326                                 self.span, GateIssue::Language,
1327                                 &format!(
1328                                     "casting pointers to integers in {}s is unstable",
1329                                     self.mode,
1330                                 ),
1331                             );
1332                         }
1333                     }
1334                     _ => {}
1335                 }
1336             }
1337
1338             Rvalue::BinaryOp(op, ref lhs, _) => {
1339                 if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind {
1340                     assert!(op == BinOp::Eq || op == BinOp::Ne ||
1341                             op == BinOp::Le || op == BinOp::Lt ||
1342                             op == BinOp::Ge || op == BinOp::Gt ||
1343                             op == BinOp::Offset);
1344
1345                     unleash_miri!(self);
1346                     if self.mode.requires_const_checking() &&
1347                         !self.tcx.features().const_compare_raw_pointers &&
1348                         !self.suppress_errors
1349                     {
1350                         self.record_error(ops::RawPtrComparison);
1351                         // require the feature gate inside constants and const fn
1352                         // FIXME: make it unsafe to use these operations
1353                         emit_feature_err(
1354                             &self.tcx.sess.parse_sess,
1355                             sym::const_compare_raw_pointers,
1356                             self.span,
1357                             GateIssue::Language,
1358                             &format!("comparing raw pointers inside {}", self.mode),
1359                         );
1360                     }
1361                 }
1362             }
1363
1364             Rvalue::NullaryOp(NullOp::Box, _) => {
1365                 unleash_miri!(self);
1366                 if self.mode.requires_const_checking() && !self.suppress_errors {
1367                     self.record_error(ops::HeapAllocation);
1368                     let mut err = struct_span_err!(self.tcx.sess, self.span, E0010,
1369                                                    "allocations are not allowed in {}s", self.mode);
1370                     err.span_label(self.span, format!("allocation not allowed in {}s", self.mode));
1371                     if self.tcx.sess.teach(&err.get_code().unwrap()) {
1372                         err.note(
1373                             "The value of statics and constants must be known at compile time, \
1374                              and they live for the entire lifetime of a program. Creating a boxed \
1375                              value allocates memory on the heap at runtime, and therefore cannot \
1376                              be done at compile time."
1377                         );
1378                     }
1379                     err.emit();
1380                 }
1381             }
1382         }
1383     }
1384
1385     fn visit_terminator_kind(&mut self,
1386                              kind: &TerminatorKind<'tcx>,
1387                              location: Location) {
1388         debug!("visit_terminator_kind: kind={:?} location={:?}", kind, location);
1389         if let TerminatorKind::Call { ref func, ref args, ref destination, .. } = *kind {
1390             if let Some((ref dest, _)) = *destination {
1391                 self.assign(dest, ValueSource::Call {
1392                     callee: func,
1393                     args,
1394                     return_ty: dest.ty(self.body, self.tcx).ty,
1395                 }, location);
1396             }
1397
1398             let fn_ty = func.ty(self.body, self.tcx);
1399             let mut callee_def_id = None;
1400             let mut is_shuffle = false;
1401             match fn_ty.kind {
1402                 ty::FnDef(def_id, _) => {
1403                     callee_def_id = Some(def_id);
1404                     match self.tcx.fn_sig(def_id).abi() {
1405                         Abi::RustIntrinsic |
1406                         Abi::PlatformIntrinsic => {
1407                             assert!(!self.tcx.is_const_fn(def_id));
1408                             match &self.tcx.item_name(def_id).as_str()[..] {
1409                                 // special intrinsic that can be called diretly without an intrinsic
1410                                 // feature gate needs a language feature gate
1411                                 "transmute" => {
1412                                     if self.mode.requires_const_checking()
1413                                         && !self.suppress_errors
1414                                     {
1415                                         // const eval transmute calls only with the feature gate
1416                                         if !self.tcx.features().const_transmute {
1417                                             self.record_error(ops::Transmute);
1418                                             emit_feature_err(
1419                                                 &self.tcx.sess.parse_sess, sym::const_transmute,
1420                                                 self.span, GateIssue::Language,
1421                                                 &format!("The use of std::mem::transmute() \
1422                                                 is gated in {}s", self.mode));
1423                                         }
1424                                     }
1425                                 }
1426
1427                                 name if name.starts_with("simd_shuffle") => {
1428                                     is_shuffle = true;
1429                                 }
1430
1431                                 // no need to check feature gates, intrinsics are only callable
1432                                 // from the libstd or with forever unstable feature gates
1433                                 _ => {}
1434                             }
1435                         }
1436                         _ => {
1437                             // In normal functions no calls are feature-gated.
1438                             if self.mode.requires_const_checking() {
1439                                 let unleash_miri = self
1440                                     .tcx
1441                                     .sess
1442                                     .opts
1443                                     .debugging_opts
1444                                     .unleash_the_miri_inside_of_you;
1445                                 if self.tcx.is_const_fn(def_id)
1446                                     || unleash_miri
1447                                     || self.suppress_errors
1448                                 {
1449                                     // stable const fns or unstable const fns
1450                                     // with their feature gate active
1451                                     // FIXME(eddyb) move stability checks from `is_const_fn` here.
1452                                 } else if self.is_const_panic_fn(def_id) {
1453                                     // Check the const_panic feature gate.
1454                                     // FIXME: cannot allow this inside `allow_internal_unstable`
1455                                     // because that would make `panic!` insta stable in constants,
1456                                     // since the macro is marked with the attribute.
1457                                     if !self.tcx.features().const_panic {
1458                                         // Don't allow panics in constants without the feature gate.
1459                                         self.record_error(ops::Panic);
1460                                         emit_feature_err(
1461                                             &self.tcx.sess.parse_sess,
1462                                             sym::const_panic,
1463                                             self.span,
1464                                             GateIssue::Language,
1465                                             &format!("panicking in {}s is unstable", self.mode),
1466                                         );
1467                                     }
1468                                 } else if let Some(feature)
1469                                               = self.tcx.is_unstable_const_fn(def_id) {
1470                                     // Check `#[unstable]` const fns or `#[rustc_const_unstable]`
1471                                     // functions without the feature gate active in this crate in
1472                                     // order to report a better error message than the one below.
1473                                     if !self.span.allows_unstable(feature) {
1474                                         self.record_error(ops::FnCallUnstable(def_id, feature));
1475                                         let mut err = self.tcx.sess.struct_span_err(self.span,
1476                                             &format!("`{}` is not yet stable as a const fn",
1477                                                     self.tcx.def_path_str(def_id)));
1478                                         if nightly_options::is_nightly_build() {
1479                                             help!(&mut err,
1480                                                   "add `#![feature({})]` to the \
1481                                                    crate attributes to enable",
1482                                                   feature);
1483                                         }
1484                                         err.emit();
1485                                     }
1486                                 } else {
1487                                     self.record_error(ops::FnCallNonConst(def_id));
1488                                     let mut err = struct_span_err!(
1489                                         self.tcx.sess,
1490                                         self.span,
1491                                         E0015,
1492                                         "calls in {}s are limited to constant functions, \
1493                                          tuple structs and tuple variants",
1494                                         self.mode,
1495                                     );
1496                                     err.emit();
1497                                 }
1498                             }
1499                         }
1500                     }
1501                 }
1502                 ty::FnPtr(_) => {
1503                     unleash_miri!(self);
1504                     if self.mode.requires_const_checking() && !self.suppress_errors {
1505                         self.record_error(ops::FnCallIndirect);
1506                         let mut err = self.tcx.sess.struct_span_err(
1507                             self.span,
1508                             "function pointers are not allowed in const fn"
1509                         );
1510                         err.emit();
1511                     }
1512                 }
1513                 _ => {
1514                     self.not_const(ops::FnCallOther);
1515                 }
1516             }
1517
1518             // No need to do anything in constants and statics, as everything is "constant" anyway
1519             // so promotion would be useless.
1520             if self.mode != Mode::Static && self.mode != Mode::Const {
1521                 let constant_args = callee_def_id.and_then(|id| {
1522                     args_required_const(self.tcx, id)
1523                 }).unwrap_or_default();
1524                 for (i, arg) in args.iter().enumerate() {
1525                     if !(is_shuffle && i == 2 || constant_args.contains(&i)) {
1526                         continue;
1527                     }
1528
1529                     let candidate = Candidate::Argument { bb: location.block, index: i };
1530                     // Since the argument is required to be constant,
1531                     // we care about constness, not promotability.
1532                     // If we checked for promotability, we'd miss out on
1533                     // the results of function calls (which are never promoted
1534                     // in runtime code).
1535                     // This is not a problem, because the argument explicitly
1536                     // requests constness, in contrast to regular promotion
1537                     // which happens even without the user requesting it.
1538                     // We can error out with a hard error if the argument is not
1539                     // constant here.
1540                     if !IsNotPromotable::in_operand(self, arg) {
1541                         debug!("visit_terminator_kind: candidate={:?}", candidate);
1542                         self.promotion_candidates.push(candidate);
1543                     } else {
1544                         if is_shuffle {
1545                             span_err!(self.tcx.sess, self.span, E0526,
1546                                       "shuffle indices are not constant");
1547                         } else {
1548                             self.tcx.sess.span_err(self.span,
1549                                 &format!("argument {} is required to be a constant",
1550                                          i + 1));
1551                         }
1552                     }
1553                 }
1554             }
1555
1556             // Check callee and argument operands.
1557             self.visit_operand(func, location);
1558             for arg in args {
1559                 self.visit_operand(arg, location);
1560             }
1561         } else if let TerminatorKind::Drop {
1562             location: ref place, ..
1563         } | TerminatorKind::DropAndReplace {
1564             location: ref place, ..
1565         } = *kind {
1566             match *kind {
1567                 TerminatorKind::DropAndReplace { .. } => {}
1568                 _ => self.super_terminator_kind(kind, location),
1569             }
1570
1571             // Deny *any* live drops anywhere other than functions.
1572             if self.mode.requires_const_checking() && !self.suppress_errors {
1573                 unleash_miri!(self);
1574                 // HACK(eddyb): emulate a bit of dataflow analysis,
1575                 // conservatively, that drop elaboration will do.
1576                 let needs_drop = if let Place {
1577                     base: PlaceBase::Local(local),
1578                     projection: box [],
1579                 } = *place {
1580                     if NeedsDrop::in_local(self, local) {
1581                         Some(self.body.local_decls[local].source_info.span)
1582                     } else {
1583                         None
1584                     }
1585                 } else {
1586                     Some(self.span)
1587                 };
1588
1589                 if let Some(span) = needs_drop {
1590                     // Double-check the type being dropped, to minimize false positives.
1591                     let ty = place.ty(self.body, self.tcx).ty;
1592                     if ty.needs_drop(self.tcx, self.param_env) {
1593                         self.record_error_spanned(ops::LiveDrop, span);
1594                         struct_span_err!(self.tcx.sess, span, E0493,
1595                                          "destructors cannot be evaluated at compile-time")
1596                             .span_label(span, format!("{}s cannot evaluate destructors",
1597                                                       self.mode))
1598                             .emit();
1599                     }
1600                 }
1601             }
1602
1603             match *kind {
1604                 TerminatorKind::DropAndReplace { ref value, .. } => {
1605                     self.assign(place, ValueSource::DropAndReplace(value), location);
1606                     self.visit_operand(value, location);
1607                 }
1608                 _ => {}
1609             }
1610         } else {
1611             // Qualify any operands inside other terminators.
1612             self.super_terminator_kind(kind, location);
1613         }
1614     }
1615
1616     fn visit_assign(&mut self,
1617                     dest: &Place<'tcx>,
1618                     rvalue: &Rvalue<'tcx>,
1619                     location: Location) {
1620         debug!("visit_assign: dest={:?} rvalue={:?} location={:?}", dest, rvalue, location);
1621         self.assign(dest, ValueSource::Rvalue(rvalue), location);
1622
1623         self.visit_rvalue(rvalue, location);
1624     }
1625
1626     fn visit_source_info(&mut self, source_info: &SourceInfo) {
1627         debug!("visit_source_info: source_info={:?}", source_info);
1628         self.span = source_info.span;
1629     }
1630
1631     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
1632         debug!("visit_statement: statement={:?} location={:?}", statement, location);
1633         match statement.kind {
1634             StatementKind::Assign(..) => {
1635                 self.super_statement(statement, location);
1636             }
1637             StatementKind::FakeRead(FakeReadCause::ForMatchedPlace, _) => {
1638                 self.not_const(ops::IfOrMatch);
1639             }
1640             // FIXME(eddyb) should these really do nothing?
1641             StatementKind::FakeRead(..) |
1642             StatementKind::SetDiscriminant { .. } |
1643             StatementKind::StorageLive(_) |
1644             StatementKind::StorageDead(_) |
1645             StatementKind::InlineAsm {..} |
1646             StatementKind::Retag { .. } |
1647             StatementKind::AscribeUserType(..) |
1648             StatementKind::Nop => {}
1649         }
1650     }
1651 }
1652
1653 pub fn provide(providers: &mut Providers<'_>) {
1654     *providers = Providers {
1655         mir_const_qualif,
1656         ..*providers
1657     };
1658 }
1659
1660 fn mir_const_qualif(tcx: TyCtxt<'_>, def_id: DefId) -> (u8, &BitSet<Local>) {
1661     // N.B., this `borrow()` is guaranteed to be valid (i.e., the value
1662     // cannot yet be stolen), because `mir_validated()`, which steals
1663     // from `mir_const(), forces this query to execute before
1664     // performing the steal.
1665     let body = &tcx.mir_const(def_id).borrow();
1666
1667     if body.return_ty().references_error() {
1668         tcx.sess.delay_span_bug(body.span, "mir_const_qualif: MIR had errors");
1669         return (1 << IsNotPromotable::IDX, tcx.arena.alloc(BitSet::new_empty(0)));
1670     }
1671
1672     Checker::new(tcx, def_id, body, Mode::Const).check_const()
1673 }
1674
1675 pub struct QualifyAndPromoteConstants<'tcx> {
1676     pub promoted: Cell<IndexVec<Promoted, Body<'tcx>>>,
1677 }
1678
1679 impl<'tcx> Default for QualifyAndPromoteConstants<'tcx> {
1680     fn default() -> Self {
1681         QualifyAndPromoteConstants {
1682             promoted: Cell::new(IndexVec::new()),
1683         }
1684     }
1685 }
1686
1687 impl<'tcx> MirPass<'tcx> for QualifyAndPromoteConstants<'tcx> {
1688     fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut Body<'tcx>) {
1689         // There's not really any point in promoting errorful MIR.
1690         if body.return_ty().references_error() {
1691             tcx.sess.delay_span_bug(body.span, "QualifyAndPromoteConstants: MIR had errors");
1692             return;
1693         }
1694
1695         if src.promoted.is_some() {
1696             return;
1697         }
1698
1699         let def_id = src.def_id();
1700         let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
1701
1702         let mode = determine_mode(tcx, hir_id, def_id);
1703
1704         debug!("run_pass: mode={:?}", mode);
1705         if let Mode::NonConstFn | Mode::ConstFn = mode {
1706             // This is ugly because Checker holds onto mir,
1707             // which can't be mutated until its scope ends.
1708             let (temps, candidates) = {
1709                 let mut checker = Checker::new(tcx, def_id, body, mode);
1710                 if let Mode::ConstFn = mode {
1711                     if tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
1712                         checker.check_const();
1713                     } else if tcx.is_min_const_fn(def_id) {
1714                         // Enforce `min_const_fn` for stable `const fn`s.
1715                         use super::qualify_min_const_fn::is_min_const_fn;
1716                         if let Err((span, err)) = is_min_const_fn(tcx, def_id, body) {
1717                             error_min_const_fn_violation(tcx, span, err);
1718                         } else {
1719                             // this should not produce any errors, but better safe than sorry
1720                             // FIXME(#53819)
1721                             checker.check_const();
1722                         }
1723                     } else {
1724                         // Enforce a constant-like CFG for `const fn`.
1725                         checker.check_const();
1726                     }
1727                 } else {
1728                     while let Some((bb, data)) = checker.rpo.next() {
1729                         checker.visit_basic_block_data(bb, data);
1730                     }
1731                 }
1732
1733                 (checker.temp_promotion_state, checker.promotion_candidates)
1734             };
1735
1736             // Do the actual promotion, now that we know what's viable.
1737             self.promoted.set(
1738                 promote_consts::promote_candidates(def_id, body, tcx, temps, candidates)
1739             );
1740         } else {
1741             check_short_circuiting_in_const_local(tcx, body, mode);
1742
1743             let promoted_temps = match mode {
1744                 Mode::Const => tcx.mir_const_qualif(def_id).1,
1745                 _ => Checker::new(tcx, def_id, body, mode).check_const().1,
1746             };
1747             remove_drop_and_storage_dead_on_promoted_locals(body, promoted_temps);
1748         }
1749
1750         if mode == Mode::Static && !tcx.has_attr(def_id, sym::thread_local) {
1751             // `static`s (not `static mut`s) which are not `#[thread_local]` must be `Sync`.
1752             check_static_is_sync(tcx, body, hir_id);
1753         }
1754     }
1755 }
1756
1757 fn determine_mode(tcx: TyCtxt<'_>, hir_id: HirId, def_id: DefId) -> Mode {
1758     match tcx.hir().body_owner_kind(hir_id) {
1759         hir::BodyOwnerKind::Closure => Mode::NonConstFn,
1760         hir::BodyOwnerKind::Fn if tcx.is_const_fn(def_id) => Mode::ConstFn,
1761         hir::BodyOwnerKind::Fn => Mode::NonConstFn,
1762         hir::BodyOwnerKind::Const => Mode::Const,
1763         hir::BodyOwnerKind::Static(hir::MutImmutable) => Mode::Static,
1764         hir::BodyOwnerKind::Static(hir::MutMutable) => Mode::StaticMut,
1765     }
1766 }
1767
1768 fn error_min_const_fn_violation(tcx: TyCtxt<'_>, span: Span, msg: Cow<'_, str>) {
1769     struct_span_err!(tcx.sess, span, E0723, "{}", msg)
1770         .note("for more information, see issue https://github.com/rust-lang/rust/issues/57563")
1771         .help("add `#![feature(const_fn)]` to the crate attributes to enable")
1772         .emit();
1773 }
1774
1775 fn check_short_circuiting_in_const_local(tcx: TyCtxt<'_>, body: &mut Body<'tcx>, mode: Mode) {
1776     if body.control_flow_destroyed.is_empty() {
1777         return;
1778     }
1779
1780     let mut locals = body.vars_iter();
1781     if let Some(local) = locals.next() {
1782         let span = body.local_decls[local].source_info.span;
1783         let mut error = tcx.sess.struct_span_err(
1784             span,
1785             &format!(
1786                 "new features like let bindings are not permitted in {}s \
1787                 which also use short circuiting operators",
1788                 mode,
1789             ),
1790         );
1791         for (span, kind) in body.control_flow_destroyed.iter() {
1792             error.span_note(
1793                 *span,
1794                 &format!("use of {} here does not actually short circuit due to \
1795                 the const evaluator presently not being able to do control flow. \
1796                 See https://github.com/rust-lang/rust/issues/49146 for more \
1797                 information.", kind),
1798             );
1799         }
1800         for local in locals {
1801             let span = body.local_decls[local].source_info.span;
1802             error.span_note(span, "more locals defined here");
1803         }
1804         error.emit();
1805     }
1806 }
1807
1808 /// In `const` and `static` everything without `StorageDead`
1809 /// is `'static`, we don't have to create promoted MIR fragments,
1810 /// just remove `Drop` and `StorageDead` on "promoted" locals.
1811 fn remove_drop_and_storage_dead_on_promoted_locals(
1812     body: &mut Body<'tcx>,
1813     promoted_temps: &BitSet<Local>,
1814 ) {
1815     debug!("run_pass: promoted_temps={:?}", promoted_temps);
1816
1817     for block in body.basic_blocks_mut() {
1818         block.statements.retain(|statement| {
1819             match statement.kind {
1820                 StatementKind::StorageDead(index) => !promoted_temps.contains(index),
1821                 _ => true
1822             }
1823         });
1824         let terminator = block.terminator_mut();
1825         match terminator.kind {
1826             TerminatorKind::Drop {
1827                 location: Place {
1828                     base: PlaceBase::Local(index),
1829                     projection: box [],
1830                 },
1831                 target,
1832                 ..
1833             } if promoted_temps.contains(index) => {
1834                 terminator.kind = TerminatorKind::Goto { target };
1835             }
1836             _ => {}
1837         }
1838     }
1839 }
1840
1841 fn check_static_is_sync(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, hir_id: HirId) {
1842     let ty = body.return_ty();
1843     tcx.infer_ctxt().enter(|infcx| {
1844         let cause = traits::ObligationCause::new(body.span, hir_id, traits::SharedStatic);
1845         let mut fulfillment_cx = traits::FulfillmentContext::new();
1846         let sync_def_id = tcx.require_lang_item(lang_items::SyncTraitLangItem, Some(body.span));
1847         fulfillment_cx.register_bound(&infcx, ty::ParamEnv::empty(), ty, sync_def_id, cause);
1848         if let Err(err) = fulfillment_cx.select_all_or_error(&infcx) {
1849             infcx.report_fulfillment_errors(&err, None, false);
1850         }
1851     });
1852 }
1853
1854 fn args_required_const(tcx: TyCtxt<'_>, def_id: DefId) -> Option<FxHashSet<usize>> {
1855     let attrs = tcx.get_attrs(def_id);
1856     let attr = attrs.iter().find(|a| a.check_name(sym::rustc_args_required_const))?;
1857     let mut ret = FxHashSet::default();
1858     for meta in attr.meta_item_list()? {
1859         match meta.literal()?.kind {
1860             LitKind::Int(a, _) => { ret.insert(a as usize); }
1861             _ => return None,
1862         }
1863     }
1864     Some(ret)
1865 }
1866
1867 const VALIDATOR_MISMATCH_ERR: &str =
1868     r"Disagreement between legacy and dataflow-based const validators.
1869     After filing an issue, use `-Zsuppress-const-validation-back-compat-ice` to compile your code.";