src/librustc_mir/transform/qualify_consts.rs

   1 // Copyright 2016 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! A pass that qualifies constness of temporaries in constants,
  12 //! static initializers and functions and also drives promotion.
  13 //!
  14 //! The Qualif flags below can be used to also provide better
  15 //! diagnostics as to why a constant rvalue wasn't promoted.
  16
  17 use rustc_data_structures::bit_set::BitSet;
  18 use rustc_data_structures::indexed_vec::IndexVec;
  19 use rustc_data_structures::fx::FxHashSet;
  20 use rustc::hir;
  21 use rustc::hir::def_id::DefId;
  22 use rustc::mir::interpret::ConstValue;
  23 use rustc::traits::{self, TraitEngine};
  24 use rustc::ty::{self, TyCtxt, Ty, TypeFoldable};
  25 use rustc::ty::cast::CastTy;
  26 use rustc::ty::query::Providers;
  27 use rustc::mir::*;
  28 use rustc::mir::traversal::ReversePostorder;
  29 use rustc::mir::visit::{PlaceContext, Visitor};
  30 use rustc::middle::lang_items;
  31 use rustc_target::spec::abi::Abi;
  32 use syntax::attr;
  33 use syntax::ast::LitKind;
  34 use syntax::feature_gate::{UnstableFeatures, feature_err, emit_feature_err, GateIssue};
  35 use syntax_pos::{Span, DUMMY_SP};
  36
  37 use std::fmt;
  38 use rustc_data_structures::sync::Lrc;
  39 use std::usize;
  40
  41 use transform::{MirPass, MirSource};
  42 use super::promote_consts::{self, Candidate, TempState};
  43
  44 bitflags! {
  45     // Borrows of temporaries can be promoted only if
  46     // they have none of these qualifications, with
  47     // the exception of `STATIC_REF` (in statics only).
  48     struct Qualif: u8 {
  49         // Constant containing interior mutability (UnsafeCell).
  50         const MUTABLE_INTERIOR  = 1 << 0;
  51
  52         // Constant containing an ADT that implements Drop.
  53         const NEEDS_DROP        = 1 << 1;
  54
  55         // Function argument.
  56         const FN_ARGUMENT       = 1 << 2;
  57
  58         // Not constant at all - non-`const fn` calls, asm!,
  59         // pointer comparisons, ptr-to-int casts, etc.
  60         const NOT_CONST         = 1 << 3;
  61
  62         // Refers to temporaries which cannot be promoted as
  63         // promote_consts decided they weren't simple enough.
  64         const NOT_PROMOTABLE    = 1 << 4;
  65
  66         // Const items can only have MUTABLE_INTERIOR
  67         // and NOT_PROMOTABLE without producing an error.
  68         const CONST_ERROR       = !Qualif::MUTABLE_INTERIOR.bits &
  69                                   !Qualif::NOT_PROMOTABLE.bits;
  70     }
  71 }
  72
  73 impl<'a, 'tcx> Qualif {
  74     /// Remove flags which are impossible for the given type.
  75     fn restrict(&mut self, ty: Ty<'tcx>,
  76                 tcx: TyCtxt<'a, 'tcx, 'tcx>,
  77                 param_env: ty::ParamEnv<'tcx>) {
  78         if ty.is_freeze(tcx, param_env, DUMMY_SP) {
  79             *self = *self - Qualif::MUTABLE_INTERIOR;
  80         }
  81         if !ty.needs_drop(tcx, param_env) {
  82             *self = *self - Qualif::NEEDS_DROP;
  83         }
  84     }
  85 }
  86
  87 /// What kind of item we are in.
  88 #[derive(Copy, Clone, PartialEq, Eq)]
  89 enum Mode {
  90     Const,
  91     Static,
  92     StaticMut,
  93     ConstFn,
  94     Fn
  95 }
  96
  97 impl fmt::Display for Mode {
  98     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  99         match *self {
 100             Mode::Const => write!(f, "constant"),
 101             Mode::Static | Mode::StaticMut => write!(f, "static"),
 102             Mode::ConstFn => write!(f, "constant function"),
 103             Mode::Fn => write!(f, "function")
 104         }
 105     }
 106 }
 107
 108 struct Qualifier<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 109     mode: Mode,
 110     span: Span,
 111     def_id: DefId,
 112     mir: &'a Mir<'tcx>,
 113     rpo: ReversePostorder<'a, 'tcx>,
 114     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 115     param_env: ty::ParamEnv<'tcx>,
 116     local_qualif: IndexVec<Local, Option<Qualif>>,
 117     qualif: Qualif,
 118     const_fn_arg_vars: BitSet<Local>,
 119     temp_promotion_state: IndexVec<Local, TempState>,
 120     promotion_candidates: Vec<Candidate>
 121 }
 122
 123 impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
 124     fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 125            def_id: DefId,
 126            mir: &'a Mir<'tcx>,
 127            mode: Mode)
 128            -> Qualifier<'a, 'tcx, 'tcx> {
 129         assert!(def_id.is_local());
 130         let mut rpo = traversal::reverse_postorder(mir);
 131         let temps = promote_consts::collect_temps(mir, &mut rpo);
 132         rpo.reset();
 133
 134         let param_env = tcx.param_env(def_id);
 135
 136         let mut local_qualif = IndexVec::from_elem(None, &mir.local_decls);
 137         for arg in mir.args_iter() {
 138             let mut qualif = Qualif::NEEDS_DROP;
 139             qualif.restrict(mir.local_decls[arg].ty, tcx, param_env);
 140             local_qualif[arg] = Some(qualif);
 141         }
 142
 143         Qualifier {
 144             mode,
 145             span: mir.span,
 146             def_id,
 147             mir,
 148             rpo,
 149             tcx,
 150             param_env,
 151             local_qualif,
 152             qualif: Qualif::empty(),
 153             const_fn_arg_vars: BitSet::new_empty(mir.local_decls.len()),
 154             temp_promotion_state: temps,
 155             promotion_candidates: vec![]
 156         }
 157     }
 158
 159     // FIXME(eddyb) we could split the errors into meaningful
 160     // categories, but enabling full miri would make that
 161     // slightly pointless (even with feature-gating).
 162     fn not_const(&mut self) {
 163         self.add(Qualif::NOT_CONST);
 164         if self.mode != Mode::Fn {
 165             let mut err = struct_span_err!(
 166                 self.tcx.sess,
 167                 self.span,
 168                 E0019,
 169                 "{} contains unimplemented expression type",
 170                 self.mode
 171             );
 172             if self.tcx.sess.teach(&err.get_code().unwrap()) {
 173                 err.note("A function call isn't allowed in the const's initialization expression \
 174                           because the expression's value must be known at compile-time.");
 175                 err.note("Remember: you can't use a function call inside a const's initialization \
 176                           expression! However, you can use it anywhere else.");
 177             }
 178             err.emit();
 179         }
 180     }
 181
 182     /// Error about extra statements in a constant.
 183     fn statement_like(&mut self) {
 184         self.add(Qualif::NOT_CONST);
 185         if self.mode != Mode::Fn {
 186             let mut err = feature_err(
 187                 &self.tcx.sess.parse_sess,
 188                 "const_let",
 189                 self.span,
 190                 GateIssue::Language,
 191                 &format!("statements in {}s are unstable", self.mode),
 192             );
 193             if self.tcx.sess.teach(&err.get_code().unwrap()) {
 194                 err.note("Blocks in constants may only contain items (such as constant, function \
 195                           definition, etc...) and a tail expression.");
 196                 err.help("To avoid it, you have to replace the non-item object.");
 197             }
 198             err.emit();
 199         }
 200     }
 201
 202     /// Add the given qualification to self.qualif.
 203     fn add(&mut self, qualif: Qualif) {
 204         self.qualif = self.qualif | qualif;
 205     }
 206
 207     /// Add the given type's qualification to self.qualif.
 208     fn add_type(&mut self, ty: Ty<'tcx>) {
 209         self.add(Qualif::MUTABLE_INTERIOR | Qualif::NEEDS_DROP);
 210         self.qualif.restrict(ty, self.tcx, self.param_env);
 211     }
 212
 213     /// Within the provided closure, self.qualif will start
 214     /// out empty, and its value after the closure returns will
 215     /// be combined with the value before the call to nest.
 216     fn nest<F: FnOnce(&mut Self)>(&mut self, f: F) {
 217         let original = self.qualif;
 218         self.qualif = Qualif::empty();
 219         f(self);
 220         self.add(original);
 221     }
 222
 223     /// Assign the current qualification to the given destination.
 224     fn assign(&mut self, dest: &Place<'tcx>, location: Location) {
 225         trace!("assign: {:?}", dest);
 226         let qualif = self.qualif;
 227         let span = self.span;
 228         let store = |slot: &mut Option<Qualif>| {
 229             if slot.is_some() {
 230                 span_bug!(span, "multiple assignments to {:?}", dest);
 231             }
 232             *slot = Some(qualif);
 233         };
 234
 235         // Only handle promotable temps in non-const functions.
 236         if self.mode == Mode::Fn {
 237             if let Place::Local(index) = *dest {
 238                 if self.mir.local_kind(index) == LocalKind::Temp
 239                 && self.temp_promotion_state[index].is_promotable() {
 240                     debug!("store to promotable temp {:?} ({:?})", index, qualif);
 241                     store(&mut self.local_qualif[index]);
 242                 }
 243             }
 244             return;
 245         }
 246
 247         match *dest {
 248             Place::Local(index) if (self.mir.local_kind(index) == LocalKind::Var ||
 249                                    self.mir.local_kind(index) == LocalKind::Arg) &&
 250                                    self.tcx.sess.features_untracked().const_let => {
 251                 debug!("store to var {:?}", index);
 252                 self.local_qualif[index] = Some(self.qualif);
 253             }
 254             Place::Local(index) if self.mir.local_kind(index) == LocalKind::Temp ||
 255                                    self.mir.local_kind(index) == LocalKind::ReturnPointer => {
 256                 debug!("store to {:?} (temp or return pointer)", index);
 257                 store(&mut self.local_qualif[index])
 258             }
 259
 260             Place::Projection(box Projection {
 261                 base: Place::Local(index),
 262                 elem: ProjectionElem::Deref
 263             }) if self.mir.local_kind(index) == LocalKind::Temp
 264                && self.mir.local_decls[index].ty.is_box()
 265                && self.local_qualif[index].map_or(false, |qualif| {
 266                     qualif.contains(Qualif::NOT_CONST)
 267                }) => {
 268                 // Part of `box expr`, we should've errored
 269                 // already for the Box allocation Rvalue.
 270             }
 271
 272             // This must be an explicit assignment.
 273             _ => {
 274                 // Catch more errors in the destination.
 275                 self.visit_place(dest, PlaceContext::Store, location);
 276                 self.statement_like();
 277             }
 278         }
 279     }
 280
 281     /// Qualify a whole const, static initializer or const fn.
 282     fn qualify_const(&mut self) -> (Qualif, Lrc<BitSet<Local>>) {
 283         debug!("qualifying {} {:?}", self.mode, self.def_id);
 284
 285         let mir = self.mir;
 286
 287         let mut seen_blocks = BitSet::new_empty(mir.basic_blocks().len());
 288         let mut bb = START_BLOCK;
 289         loop {
 290             seen_blocks.insert(bb.index());
 291
 292             self.visit_basic_block_data(bb, &mir[bb]);
 293
 294             let target = match mir[bb].terminator().kind {
 295                 TerminatorKind::Goto { target } |
 296                 TerminatorKind::Drop { target, .. } |
 297                 TerminatorKind::Assert { target, .. } |
 298                 TerminatorKind::Call { destination: Some((_, target)), .. } => {
 299                     Some(target)
 300                 }
 301
 302                 // Non-terminating calls cannot produce any value.
 303                 TerminatorKind::Call { destination: None, .. } => {
 304                     break;
 305                 }
 306
 307                 TerminatorKind::SwitchInt {..} |
 308                 TerminatorKind::DropAndReplace { .. } |
 309                 TerminatorKind::Resume |
 310                 TerminatorKind::Abort |
 311                 TerminatorKind::GeneratorDrop |
 312                 TerminatorKind::Yield { .. } |
 313                 TerminatorKind::Unreachable |
 314                 TerminatorKind::FalseEdges { .. } |
 315                 TerminatorKind::FalseUnwind { .. } => None,
 316
 317                 TerminatorKind::Return => {
 318                     if !self.tcx.sess.features_untracked().const_let {
 319                         // Check for unused values. This usually means
 320                         // there are extra statements in the AST.
 321                         for temp in mir.temps_iter() {
 322                             if self.local_qualif[temp].is_none() {
 323                                 continue;
 324                             }
 325
 326                             let state = self.temp_promotion_state[temp];
 327                             if let TempState::Defined { location, uses: 0 } = state {
 328                                 let data = &mir[location.block];
 329                                 let stmt_idx = location.statement_index;
 330
 331                                 // Get the span for the initialization.
 332                                 let source_info = if stmt_idx < data.statements.len() {
 333                                     data.statements[stmt_idx].source_info
 334                                 } else {
 335                                     data.terminator().source_info
 336                                 };
 337                                 self.span = source_info.span;
 338
 339                                 // Treat this as a statement in the AST.
 340                                 self.statement_like();
 341                             }
 342                         }
 343
 344                         // Make sure there are no extra unassigned variables.
 345                         self.qualif = Qualif::NOT_CONST;
 346                         for index in mir.vars_iter() {
 347                             if !self.const_fn_arg_vars.contains(index) {
 348                                 debug!("unassigned variable {:?}", index);
 349                                 self.assign(&Place::Local(index), Location {
 350                                     block: bb,
 351                                     statement_index: usize::MAX,
 352                                 });
 353                             }
 354                         }
 355                     }
 356
 357                     break;
 358                 }
 359             };
 360
 361             match target {
 362                 // No loops allowed.
 363                 Some(target) if !seen_blocks.contains(target.index()) => {
 364                     bb = target;
 365                 }
 366                 _ => {
 367                     self.not_const();
 368                     break;
 369                 }
 370             }
 371         }
 372
 373         self.qualif = self.local_qualif[RETURN_PLACE].unwrap_or(Qualif::NOT_CONST);
 374
 375         // Account for errors in consts by using the
 376         // conservative type qualification instead.
 377         if self.qualif.intersects(Qualif::CONST_ERROR) {
 378             self.qualif = Qualif::empty();
 379             let return_ty = mir.return_ty();
 380             self.add_type(return_ty);
 381         }
 382
 383
 384         // Collect all the temps we need to promote.
 385         let mut promoted_temps = BitSet::new_empty(self.temp_promotion_state.len());
 386
 387         for candidate in &self.promotion_candidates {
 388             match *candidate {
 389                 Candidate::Ref(Location { block: bb, statement_index: stmt_idx }) => {
 390                     match self.mir[bb].statements[stmt_idx].kind {
 391                         StatementKind::Assign(_, box Rvalue::Ref(_, _, Place::Local(index))) => {
 392                             promoted_temps.insert(index);
 393                         }
 394                         _ => {}
 395                     }
 396                 }
 397                 Candidate::Argument { .. } => {}
 398             }
 399         }
 400
 401         (self.qualif, Lrc::new(promoted_temps))
 402     }
 403
 404     fn is_const_panic_fn(&self, def_id: DefId) -> bool {
 405         Some(def_id) == self.tcx.lang_items().panic_fn() ||
 406         Some(def_id) == self.tcx.lang_items().begin_panic_fn()
 407     }
 408 }
 409
 410 /// Accumulates an Rvalue or Call's effects in self.qualif.
 411 /// For functions (constant or not), it also records
 412 /// candidates for promotion in promotion_candidates.
 413 impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
 414     fn visit_local(&mut self,
 415                    &local: &Local,
 416                    _: PlaceContext<'tcx>,
 417                    _: Location) {
 418         let kind = self.mir.local_kind(local);
 419         match kind {
 420             LocalKind::ReturnPointer => {
 421                 self.not_const();
 422             }
 423             LocalKind::Var if !self.tcx.sess.features_untracked().const_let => {
 424                 if self.mode != Mode::Fn {
 425                     emit_feature_err(&self.tcx.sess.parse_sess, "const_let",
 426                                     self.span, GateIssue::Language,
 427                                     &format!("let bindings in {}s are unstable",self.mode));
 428                 }
 429                 self.add(Qualif::NOT_CONST);
 430             }
 431             LocalKind::Var |
 432             LocalKind::Arg |
 433             LocalKind::Temp => {
 434                 if let LocalKind::Arg = kind {
 435                     self.add(Qualif::FN_ARGUMENT);
 436                 }
 437
 438                 if !self.temp_promotion_state[local].is_promotable() {
 439                     self.add(Qualif::NOT_PROMOTABLE);
 440                 }
 441
 442                 if let Some(qualif) = self.local_qualif[local] {
 443                     self.add(qualif);
 444                 } else {
 445                     self.not_const();
 446                 }
 447             }
 448         }
 449     }
 450
 451     fn visit_place(&mut self,
 452                     place: &Place<'tcx>,
 453                     context: PlaceContext<'tcx>,
 454                     location: Location) {
 455         match *place {
 456             Place::Local(ref local) => self.visit_local(local, context, location),
 457             Place::Promoted(_) => bug!("promoting already promoted MIR"),
 458             Place::Static(ref global) => {
 459                 if self.tcx
 460                        .get_attrs(global.def_id)
 461                        .iter()
 462                        .any(|attr| attr.check_name("thread_local")) {
 463                     if self.mode != Mode::Fn {
 464                         span_err!(self.tcx.sess, self.span, E0625,
 465                                   "thread-local statics cannot be \
 466                                    accessed at compile-time");
 467                     }
 468                     self.add(Qualif::NOT_CONST);
 469                     return;
 470                 }
 471
 472                 // Only allow statics (not consts) to refer to other statics.
 473                 if self.mode == Mode::Static || self.mode == Mode::StaticMut {
 474                     return;
 475                 }
 476                 self.add(Qualif::NOT_CONST);
 477
 478                 if self.mode != Mode::Fn {
 479                     let mut err = struct_span_err!(self.tcx.sess, self.span, E0013,
 480                                                    "{}s cannot refer to statics, use \
 481                                                     a constant instead", self.mode);
 482                     if self.tcx.sess.teach(&err.get_code().unwrap()) {
 483                         err.note(
 484                             "Static and const variables can refer to other const variables. But a \
 485                              const variable cannot refer to a static variable."
 486                         );
 487                         err.help(
 488                             "To fix this, the value can be extracted as a const and then used."
 489                         );
 490                     }
 491                     err.emit()
 492                 }
 493             }
 494             Place::Projection(ref proj) => {
 495                 self.nest(|this| {
 496                     this.super_place(place, context, location);
 497                     match proj.elem {
 498                         ProjectionElem::Deref => {
 499                             this.add(Qualif::NOT_CONST);
 500                             let base_ty = proj.base.ty(this.mir, this.tcx).to_ty(this.tcx);
 501                             match this.mode {
 502                                 Mode::Fn => {},
 503                                 _ => {
 504                                     if let ty::RawPtr(_) = base_ty.sty {
 505                                         if !this.tcx.sess.features_untracked().const_raw_ptr_deref {
 506                                             emit_feature_err(
 507                                                 &this.tcx.sess.parse_sess, "const_raw_ptr_deref",
 508                                                 this.span, GateIssue::Language,
 509                                                 &format!(
 510                                                     "dereferencing raw pointers in {}s is unstable",
 511                                                     this.mode,
 512                                                 ),
 513                                             );
 514                                         }
 515                                     }
 516                                 }
 517                             }
 518                         }
 519
 520                         ProjectionElem::Field(..) |
 521                         ProjectionElem::Index(_) => {
 522                             let base_ty = proj.base.ty(this.mir, this.tcx).to_ty(this.tcx);
 523                             if let Some(def) = base_ty.ty_adt_def() {
 524                                 if def.is_union() {
 525                                     match this.mode {
 526                                         Mode::Fn => this.not_const(),
 527                                         Mode::ConstFn => {
 528                                             if !this.tcx.sess.features_untracked().const_fn_union {
 529                                                 emit_feature_err(
 530                                                     &this.tcx.sess.parse_sess, "const_fn_union",
 531                                                     this.span, GateIssue::Language,
 532                                                     "unions in const fn are unstable",
 533                                                 );
 534                                             }
 535                                         },
 536
 537                                         | Mode::Static
 538                                         | Mode::StaticMut
 539                                         | Mode::Const
 540                                         => {},
 541                                     }
 542                                 }
 543                             }
 544
 545                             let ty = place.ty(this.mir, this.tcx).to_ty(this.tcx);
 546                             this.qualif.restrict(ty, this.tcx, this.param_env);
 547                         }
 548
 549                         ProjectionElem::ConstantIndex {..} |
 550                         ProjectionElem::Subslice {..} |
 551                         ProjectionElem::Downcast(..) => {
 552                             this.not_const()
 553                         }
 554                     }
 555                 });
 556             }
 557         }
 558     }
 559
 560     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
 561         self.super_operand(operand, location);
 562
 563         match *operand {
 564             Operand::Copy(_) |
 565             Operand::Move(_) => {
 566                 // Mark the consumed locals to indicate later drops are noops.
 567                 if let Operand::Move(Place::Local(local)) = *operand {
 568                     self.local_qualif[local] = self.local_qualif[local].map(|q|
 569                         q - Qualif::NEEDS_DROP
 570                     );
 571                 }
 572             }
 573             Operand::Constant(ref constant) => {
 574                 if let ConstValue::Unevaluated(def_id, _) = constant.literal.val {
 575                     // Don't peek inside trait associated constants.
 576                     if self.tcx.trait_of_item(def_id).is_some() {
 577                         self.add_type(constant.literal.ty);
 578                     } else {
 579                         let (bits, _) = self.tcx.at(constant.span).mir_const_qualif(def_id);
 580
 581                         let qualif = Qualif::from_bits(bits).expect("invalid mir_const_qualif");
 582                         self.add(qualif);
 583
 584                         // Just in case the type is more specific than
 585                         // the definition, e.g. impl associated const
 586                         // with type parameters, take it into account.
 587                         self.qualif.restrict(constant.literal.ty, self.tcx, self.param_env);
 588                     }
 589                 }
 590             }
 591         }
 592     }
 593
 594     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 595         // Recurse through operands and places.
 596         if let Rvalue::Ref(region, kind, ref place) = *rvalue {
 597             let mut is_reborrow = false;
 598             if let Place::Projection(ref proj) = *place {
 599                 if let ProjectionElem::Deref = proj.elem {
 600                     let base_ty = proj.base.ty(self.mir, self.tcx).to_ty(self.tcx);
 601                     if let ty::Ref(..) = base_ty.sty {
 602                         is_reborrow = true;
 603                     }
 604                 }
 605             }
 606
 607             if is_reborrow {
 608                 self.super_place(place, PlaceContext::Borrow {
 609                     region,
 610                     kind
 611                 }, location);
 612             } else {
 613                 self.super_rvalue(rvalue, location);
 614             }
 615         } else {
 616             self.super_rvalue(rvalue, location);
 617         }
 618
 619         match *rvalue {
 620             Rvalue::Use(_) |
 621             Rvalue::Repeat(..) |
 622             Rvalue::UnaryOp(UnOp::Neg, _) |
 623             Rvalue::UnaryOp(UnOp::Not, _) |
 624             Rvalue::NullaryOp(NullOp::SizeOf, _) |
 625             Rvalue::CheckedBinaryOp(..) |
 626             Rvalue::Cast(CastKind::ReifyFnPointer, ..) |
 627             Rvalue::Cast(CastKind::UnsafeFnPointer, ..) |
 628             Rvalue::Cast(CastKind::ClosureFnPointer, ..) |
 629             Rvalue::Cast(CastKind::Unsize, ..) |
 630             Rvalue::Discriminant(..) |
 631             Rvalue::Len(_) => {}
 632
 633             Rvalue::Ref(_, kind, ref place) => {
 634                 let ty = place.ty(self.mir, self.tcx).to_ty(self.tcx);
 635
 636                 // Default to forbidding the borrow and/or its promotion,
 637                 // due to the potential for direct or interior mutability,
 638                 // and only proceed by setting `forbidden_mut` to `false`.
 639                 let mut forbidden_mut = true;
 640
 641                 if let BorrowKind::Mut { .. } = kind {
 642                     // In theory, any zero-sized value could be borrowed
 643                     // mutably without consequences. However, only &mut []
 644                     // is allowed right now, and only in functions.
 645                     if self.mode == Mode::StaticMut {
 646                         // Inside a `static mut`, &mut [...] is also allowed.
 647                         match ty.sty {
 648                             ty::Array(..) | ty::Slice(_) => forbidden_mut = false,
 649                             _ => {}
 650                         }
 651                     } else if let ty::Array(_, len) = ty.sty {
 652                         // FIXME(eddyb) the `self.mode == Mode::Fn` condition
 653                         // seems unnecessary, given that this is merely a ZST.
 654                         if len.unwrap_usize(self.tcx) == 0 && self.mode == Mode::Fn {
 655                             forbidden_mut = false;
 656                         }
 657                     }
 658
 659                     if forbidden_mut {
 660                         self.add(Qualif::NOT_CONST);
 661                         if self.mode != Mode::Fn {
 662                             let mut err = struct_span_err!(self.tcx.sess,  self.span, E0017,
 663                                                            "references in {}s may only refer \
 664                                                             to immutable values", self.mode);
 665                             err.span_label(self.span, format!("{}s require immutable values",
 666                                                                 self.mode));
 667                             if self.tcx.sess.teach(&err.get_code().unwrap()) {
 668                                 err.note("References in statics and constants may only refer to \
 669                                           immutable values.\n\n\
 670                                           Statics are shared everywhere, and if they refer to \
 671                                           mutable data one might violate memory safety since \
 672                                           holding multiple mutable references to shared data is \
 673                                           not allowed.\n\n\
 674                                           If you really want global mutable state, try using \
 675                                           static mut or a global UnsafeCell.");
 676                             }
 677                             err.emit();
 678                         }
 679                     }
 680                 } else {
 681                     // Constants cannot be borrowed if they contain interior mutability as
 682                     // it means that our "silent insertion of statics" could change
 683                     // initializer values (very bad).
 684                     if self.qualif.contains(Qualif::MUTABLE_INTERIOR) {
 685                         // A reference of a MUTABLE_INTERIOR place is instead
 686                         // NOT_CONST (see `if forbidden_mut` below), to avoid
 687                         // duplicate errors (from reborrowing, for example).
 688                         self.qualif = self.qualif - Qualif::MUTABLE_INTERIOR;
 689                         if self.mode != Mode::Fn {
 690                             span_err!(self.tcx.sess, self.span, E0492,
 691                                       "cannot borrow a constant which may contain \
 692                                        interior mutability, create a static instead");
 693                         }
 694                     } else {
 695                         // We allow immutable borrows of frozen data.
 696                         forbidden_mut = false;
 697                     }
 698                 }
 699
 700                 if forbidden_mut {
 701                     self.add(Qualif::NOT_CONST);
 702                 } else {
 703                     // We might have a candidate for promotion.
 704                     let candidate = Candidate::Ref(location);
 705                     // We can only promote interior borrows of promotable temps.
 706                     let mut place = place;
 707                     while let Place::Projection(ref proj) = *place {
 708                         if proj.elem == ProjectionElem::Deref {
 709                             break;
 710                         }
 711                         place = &proj.base;
 712                     }
 713                     if let Place::Local(local) = *place {
 714                         if self.mir.local_kind(local) == LocalKind::Temp {
 715                             if let Some(qualif) = self.local_qualif[local] {
 716                                 // `forbidden_mut` is false, so we can safely ignore
 717                                 // `MUTABLE_INTERIOR` from the local's qualifications.
 718                                 // This allows borrowing fields which don't have
 719                                 // `MUTABLE_INTERIOR`, from a type that does, e.g.:
 720                                 // `let _: &'static _ = &(Cell::new(1), 2).1;`
 721                                 if (qualif - Qualif::MUTABLE_INTERIOR).is_empty() {
 722                                     self.promotion_candidates.push(candidate);
 723                                 }
 724                             }
 725                         }
 726                     }
 727                 }
 728             }
 729
 730             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {
 731                 let operand_ty = operand.ty(self.mir, self.tcx);
 732                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 733                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 734                 match (cast_in, cast_out) {
 735                     (CastTy::Ptr(_), CastTy::Int(_)) |
 736                     (CastTy::FnPtr, CastTy::Int(_)) => {
 737                         if let Mode::Fn = self.mode {
 738                             self.add(Qualif::NOT_CONST);
 739                         } else if !self.tcx.sess.features_untracked().const_raw_ptr_to_usize_cast {
 740                             emit_feature_err(
 741                                 &self.tcx.sess.parse_sess, "const_raw_ptr_to_usize_cast",
 742                                 self.span, GateIssue::Language,
 743                                 &format!(
 744                                     "casting pointers to integers in {}s is unstable",
 745                                     self.mode,
 746                                 ),
 747                             );
 748                         }
 749                     }
 750                     _ => {}
 751                 }
 752             }
 753
 754             Rvalue::BinaryOp(op, ref lhs, _) => {
 755                 if let ty::RawPtr(_) = lhs.ty(self.mir, self.tcx).sty {
 756                     assert!(op == BinOp::Eq || op == BinOp::Ne ||
 757                             op == BinOp::Le || op == BinOp::Lt ||
 758                             op == BinOp::Ge || op == BinOp::Gt ||
 759                             op == BinOp::Offset);
 760
 761                     if let Mode::Fn = self.mode {
 762                         self.add(Qualif::NOT_CONST);
 763                     } else if !self.tcx.sess.features_untracked().const_compare_raw_pointers {
 764                         emit_feature_err(
 765                             &self.tcx.sess.parse_sess,
 766                             "const_compare_raw_pointers",
 767                             self.span,
 768                             GateIssue::Language,
 769                             &format!("comparing raw pointers inside {}", self.mode),
 770                         );
 771                     }
 772                 }
 773             }
 774
 775             Rvalue::NullaryOp(NullOp::Box, _) => {
 776                 self.add(Qualif::NOT_CONST);
 777                 if self.mode != Mode::Fn {
 778                     let mut err = struct_span_err!(self.tcx.sess, self.span, E0010,
 779                                                    "allocations are not allowed in {}s", self.mode);
 780                     err.span_label(self.span, format!("allocation not allowed in {}s", self.mode));
 781                     if self.tcx.sess.teach(&err.get_code().unwrap()) {
 782                         err.note(
 783                             "The value of statics and constants must be known at compile time, \
 784                              and they live for the entire lifetime of a program. Creating a boxed \
 785                              value allocates memory on the heap at runtime, and therefore cannot \
 786                              be done at compile time."
 787                         );
 788                     }
 789                     err.emit();
 790                 }
 791             }
 792
 793             Rvalue::Aggregate(ref kind, _) => {
 794                 if let AggregateKind::Adt(def, ..) = **kind {
 795                     if def.has_dtor(self.tcx) {
 796                         self.add(Qualif::NEEDS_DROP);
 797                     }
 798
 799                     if Some(def.did) == self.tcx.lang_items().unsafe_cell_type() {
 800                         let ty = rvalue.ty(self.mir, self.tcx);
 801                         self.add_type(ty);
 802                         assert!(self.qualif.contains(Qualif::MUTABLE_INTERIOR));
 803                     }
 804                 }
 805             }
 806         }
 807     }
 808
 809     fn visit_terminator_kind(&mut self,
 810                              bb: BasicBlock,
 811                              kind: &TerminatorKind<'tcx>,
 812                              location: Location) {
 813         if let TerminatorKind::Call { ref func, ref args, ref destination, .. } = *kind {
 814             self.visit_operand(func, location);
 815
 816             let fn_ty = func.ty(self.mir, self.tcx);
 817             let mut callee_def_id = None;
 818             let (mut is_shuffle, mut is_const_fn) = (false, None);
 819             if let ty::FnDef(def_id, _) = fn_ty.sty {
 820                 callee_def_id = Some(def_id);
 821                 match self.tcx.fn_sig(def_id).abi() {
 822                     Abi::RustIntrinsic |
 823                     Abi::PlatformIntrinsic => {
 824                         assert!(!self.tcx.is_const_fn(def_id));
 825                         match &self.tcx.item_name(def_id).as_str()[..] {
 826                             | "size_of"
 827                             | "min_align_of"
 828                             | "needs_drop"
 829                             | "type_id"
 830                             | "bswap"
 831                             | "bitreverse"
 832                             | "ctpop"
 833                             | "cttz"
 834                             | "cttz_nonzero"
 835                             | "ctlz"
 836                             | "ctlz_nonzero"
 837                             | "overflowing_add"
 838                             | "overflowing_sub"
 839                             | "overflowing_mul"
 840                             | "unchecked_shl"
 841                             | "unchecked_shr"
 842                             | "add_with_overflow"
 843                             | "sub_with_overflow"
 844                             | "mul_with_overflow" => is_const_fn = Some(def_id),
 845                             "transmute" => {
 846                                 if self.mode != Mode::Fn {
 847                                     is_const_fn = Some(def_id);
 848                                     if !self.tcx.sess.features_untracked().const_transmute {
 849                                         emit_feature_err(
 850                                             &self.tcx.sess.parse_sess, "const_transmute",
 851                                             self.span, GateIssue::Language,
 852                                             &format!("The use of std::mem::transmute() \
 853                                             is gated in {}s", self.mode));
 854                                     }
 855                                 }
 856                             }
 857
 858                             name if name.starts_with("simd_shuffle") => {
 859                                 is_shuffle = true;
 860                             }
 861
 862                             _ => {}
 863                         }
 864                     }
 865                     _ => {
 866                         if self.tcx.is_const_fn(def_id) || self.is_const_panic_fn(def_id) {
 867                             is_const_fn = Some(def_id);
 868                         }
 869                     }
 870                 }
 871             }
 872
 873             let constant_arguments = callee_def_id.and_then(|id| {
 874                 args_required_const(self.tcx, id)
 875             });
 876             for (i, arg) in args.iter().enumerate() {
 877                 self.nest(|this| {
 878                     this.visit_operand(arg, location);
 879                     if this.mode != Mode::Fn {
 880                         return
 881                     }
 882                     let candidate = Candidate::Argument { bb, index: i };
 883                     if is_shuffle && i == 2 {
 884                         if this.qualif.is_empty() {
 885                             this.promotion_candidates.push(candidate);
 886                         } else {
 887                             span_err!(this.tcx.sess, this.span, E0526,
 888                                       "shuffle indices are not constant");
 889                         }
 890                         return
 891                     }
 892
 893                     let constant_arguments = match constant_arguments.as_ref() {
 894                         Some(s) => s,
 895                         None => return,
 896                     };
 897                     if !constant_arguments.contains(&i) {
 898                         return
 899                     }
 900                     if this.qualif.is_empty() {
 901                         this.promotion_candidates.push(candidate);
 902                     } else {
 903                         this.tcx.sess.span_err(this.span,
 904                             &format!("argument {} is required to be a constant",
 905                                      i + 1));
 906                     }
 907                 });
 908             }
 909
 910             // Const fn calls.
 911             if let Some(def_id) = is_const_fn {
 912                 // check the const_panic feature gate or
 913                 // find corresponding rustc_const_unstable feature
 914                 // FIXME: cannot allow this inside `allow_internal_unstable` because that would make
 915                 // `panic!` insta stable in constants, since the macro is marked with the attr
 916                 if self.is_const_panic_fn(def_id) {
 917                     if self.mode == Mode::Fn {
 918                         // never promote panics
 919                         self.qualif = Qualif::NOT_CONST;
 920                     } else if !self.tcx.sess.features_untracked().const_panic {
 921                         // don't allow panics in constants without the feature gate
 922                         emit_feature_err(
 923                             &self.tcx.sess.parse_sess,
 924                             "const_panic",
 925                             self.span,
 926                             GateIssue::Language,
 927                             &format!("panicking in {}s is unstable", self.mode),
 928                         );
 929                     }
 930                 } else if let Some(&attr::Stability {
 931                     const_stability: Some(ref feature_name),
 932                 .. }) = self.tcx.lookup_stability(def_id) {
 933                     if
 934                         // feature-gate is not enabled,
 935                         !self.tcx.features()
 936                             .declared_lib_features
 937                             .iter()
 938                             .any(|&(ref sym, _)| sym == feature_name) &&
 939
 940                         // this doesn't come from a macro that has #[allow_internal_unstable]
 941                         !self.span.allows_unstable()
 942                     {
 943                         self.qualif = Qualif::NOT_CONST;
 944                         if self.mode != Mode::Fn {
 945                             // inside a constant environment, not having the feature gate is
 946                             // an error
 947                             let mut err = self.tcx.sess.struct_span_err(self.span,
 948                                 &format!("`{}` is not yet stable as a const fn",
 949                                         self.tcx.item_path_str(def_id)));
 950                             help!(&mut err,
 951                                 "in Nightly builds, add `#![feature({})]` \
 952                                 to the crate attributes to enable",
 953                                 feature_name);
 954                             err.emit();
 955                         }
 956                     }
 957                 }
 958             } else {
 959                 self.qualif = Qualif::NOT_CONST;
 960                 if self.mode != Mode::Fn {
 961                     // FIXME(#24111) Remove this check when const fn stabilizes
 962                     let (msg, note) = if let UnstableFeatures::Disallow =
 963                             self.tcx.sess.opts.unstable_features {
 964                         (format!("calls in {}s are limited to \
 965                                   tuple structs and tuple variants",
 966                                  self.mode),
 967                          Some("a limited form of compile-time function \
 968                                evaluation is available on a nightly \
 969                                compiler via `const fn`"))
 970                     } else {
 971                         (format!("calls in {}s are limited \
 972                                   to constant functions, \
 973                                   tuple structs and tuple variants",
 974                                  self.mode),
 975                          None)
 976                     };
 977                     let mut err = struct_span_err!(self.tcx.sess, self.span, E0015, "{}", msg);
 978                     if let Some(note) = note {
 979                         err.span_note(self.span, note);
 980                     }
 981                     err.emit();
 982                 }
 983             }
 984
 985             if let Some((ref dest, _)) = *destination {
 986                 // Avoid propagating irrelevant callee/argument qualifications.
 987                 if self.qualif.intersects(Qualif::CONST_ERROR) {
 988                     self.qualif = Qualif::NOT_CONST;
 989                 } else {
 990                     // Be conservative about the returned value of a const fn.
 991                     let tcx = self.tcx;
 992                     let ty = dest.ty(self.mir, tcx).to_ty(tcx);
 993                     self.qualif = Qualif::empty();
 994                     self.add_type(ty);
 995                 }
 996                 self.assign(dest, location);
 997             }
 998         } else if let TerminatorKind::Drop { location: ref place, .. } = *kind {
 999             self.super_terminator_kind(bb, kind, location);
1000
1001             // Deny *any* live drops anywhere other than functions.
1002             if self.mode != Mode::Fn {
1003                 // HACK(eddyb) Emulate a bit of dataflow analysis,
1004                 // conservatively, that drop elaboration will do.
1005                 let needs_drop = if let Place::Local(local) = *place {
1006                     if self.local_qualif[local].map_or(true, |q| q.contains(Qualif::NEEDS_DROP)) {
1007                         Some(self.mir.local_decls[local].source_info.span)
1008                     } else {
1009                         None
1010                     }
1011                 } else {
1012                     Some(self.span)
1013                 };
1014
1015                 if let Some(span) = needs_drop {
1016                     // Double-check the type being dropped, to minimize false positives.
1017                     let ty = place.ty(self.mir, self.tcx).to_ty(self.tcx);
1018                     if ty.needs_drop(self.tcx, self.param_env) {
1019                         struct_span_err!(self.tcx.sess, span, E0493,
1020                                          "destructors cannot be evaluated at compile-time")
1021                             .span_label(span, format!("{}s cannot evaluate destructors",
1022                                                       self.mode))
1023                             .emit();
1024                     }
1025                 }
1026             }
1027         } else {
1028             // Qualify any operands inside other terminators.
1029             self.super_terminator_kind(bb, kind, location);
1030         }
1031     }
1032
1033     fn visit_assign(&mut self,
1034                     _: BasicBlock,
1035                     dest: &Place<'tcx>,
1036                     rvalue: &Rvalue<'tcx>,
1037                     location: Location) {
1038         self.visit_rvalue(rvalue, location);
1039
1040         // Check the allowed const fn argument forms.
1041         if let (Mode::ConstFn, &Place::Local(index)) = (self.mode, dest) {
1042             if self.mir.local_kind(index) == LocalKind::Var &&
1043                self.const_fn_arg_vars.insert(index) &&
1044                !self.tcx.sess.features_untracked().const_let {
1045
1046                 // Direct use of an argument is permitted.
1047                 match *rvalue {
1048                     Rvalue::Use(Operand::Copy(Place::Local(local))) |
1049                     Rvalue::Use(Operand::Move(Place::Local(local))) => {
1050                         if self.mir.local_kind(local) == LocalKind::Arg {
1051                             return;
1052                         }
1053                     }
1054                     _ => {}
1055                 }
1056
1057                 // Avoid a generic error for other uses of arguments.
1058                 if self.qualif.contains(Qualif::FN_ARGUMENT) {
1059                     let decl = &self.mir.local_decls[index];
1060                     let mut err = feature_err(
1061                         &self.tcx.sess.parse_sess,
1062                         "const_let",
1063                         decl.source_info.span,
1064                         GateIssue::Language,
1065                         "arguments of constant functions can only be immutable by-value bindings"
1066                     );
1067                     if self.tcx.sess.teach(&err.get_code().unwrap()) {
1068                         err.note("Constant functions are not allowed to mutate anything. Thus, \
1069                                   binding to an argument with a mutable pattern is not allowed.");
1070                         err.note("Remove any mutable bindings from the argument list to fix this \
1071                                   error. In case you need to mutate the argument, try lazily \
1072                                   initializing a global variable instead of using a const fn, or \
1073                                   refactoring the code to a functional style to avoid mutation if \
1074                                   possible.");
1075                     }
1076                     err.emit();
1077                     return;
1078                 }
1079             }
1080         }
1081
1082         self.assign(dest, location);
1083     }
1084
1085     fn visit_source_info(&mut self, source_info: &SourceInfo) {
1086         self.span = source_info.span;
1087     }
1088
1089     fn visit_statement(&mut self, bb: BasicBlock, statement: &Statement<'tcx>, location: Location) {
1090         self.nest(|this| {
1091             this.visit_source_info(&statement.source_info);
1092             match statement.kind {
1093                 StatementKind::Assign(ref place, ref rvalue) => {
1094                     this.visit_assign(bb, place, rvalue, location);
1095                 }
1096                 StatementKind::FakeRead(..) |
1097                 StatementKind::SetDiscriminant { .. } |
1098                 StatementKind::StorageLive(_) |
1099                 StatementKind::StorageDead(_) |
1100                 StatementKind::InlineAsm {..} |
1101                 StatementKind::EndRegion(_) |
1102                 StatementKind::Validate(..) |
1103                 StatementKind::AscribeUserType(..) |
1104                 StatementKind::Nop => {}
1105             }
1106         });
1107     }
1108
1109     fn visit_terminator(&mut self,
1110                         bb: BasicBlock,
1111                         terminator: &Terminator<'tcx>,
1112                         location: Location) {
1113         self.nest(|this| this.super_terminator(bb, terminator, location));
1114     }
1115 }
1116
1117 pub fn provide(providers: &mut Providers) {
1118     *providers = Providers {
1119         mir_const_qualif,
1120         ..*providers
1121     };
1122 }
1123
1124 fn mir_const_qualif<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
1125                               def_id: DefId)
1126                               -> (u8, Lrc<BitSet<Local>>) {
1127     // NB: This `borrow()` is guaranteed to be valid (i.e., the value
1128     // cannot yet be stolen), because `mir_validated()`, which steals
1129     // from `mir_const(), forces this query to execute before
1130     // performing the steal.
1131     let mir = &tcx.mir_const(def_id).borrow();
1132
1133     if mir.return_ty().references_error() {
1134         tcx.sess.delay_span_bug(mir.span, "mir_const_qualif: Mir had errors");
1135         return (Qualif::NOT_CONST.bits(), Lrc::new(BitSet::new_empty(0)));
1136     }
1137
1138     let mut qualifier = Qualifier::new(tcx, def_id, mir, Mode::Const);
1139     let (qualif, promoted_temps) = qualifier.qualify_const();
1140     (qualif.bits(), promoted_temps)
1141 }
1142
1143 pub struct QualifyAndPromoteConstants;
1144
1145 impl MirPass for QualifyAndPromoteConstants {
1146     fn run_pass<'a, 'tcx>(&self,
1147                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
1148                           src: MirSource,
1149                           mir: &mut Mir<'tcx>) {
1150         // There's not really any point in promoting errorful MIR.
1151         if mir.return_ty().references_error() {
1152             tcx.sess.delay_span_bug(mir.span, "QualifyAndPromoteConstants: Mir had errors");
1153             return;
1154         }
1155
1156         if src.promoted.is_some() {
1157             return;
1158         }
1159
1160         let def_id = src.def_id;
1161         let id = tcx.hir.as_local_node_id(def_id).unwrap();
1162         let mut const_promoted_temps = None;
1163         let mode = match tcx.hir.body_owner_kind(id) {
1164             hir::BodyOwnerKind::Fn => {
1165                 if tcx.is_const_fn(def_id) {
1166                     Mode::ConstFn
1167                 } else {
1168                     Mode::Fn
1169                 }
1170             }
1171             hir::BodyOwnerKind::Const => {
1172                 const_promoted_temps = Some(tcx.mir_const_qualif(def_id).1);
1173                 Mode::Const
1174             }
1175             hir::BodyOwnerKind::Static(hir::MutImmutable) => Mode::Static,
1176             hir::BodyOwnerKind::Static(hir::MutMutable) => Mode::StaticMut,
1177         };
1178
1179         if mode == Mode::Fn || mode == Mode::ConstFn {
1180             // This is ugly because Qualifier holds onto mir,
1181             // which can't be mutated until its scope ends.
1182             let (temps, candidates) = {
1183                 let mut qualifier = Qualifier::new(tcx, def_id, mir, mode);
1184                 if mode == Mode::ConstFn {
1185                     if tcx.is_min_const_fn(def_id) {
1186                         // enforce `min_const_fn` for stable const fns
1187                         use super::qualify_min_const_fn::is_min_const_fn;
1188                         if let Err((span, err)) = is_min_const_fn(tcx, def_id, mir) {
1189                             tcx.sess.span_err(span, &err);
1190                         } else {
1191                             // this should not produce any errors, but better safe than sorry
1192                             // FIXME(#53819)
1193                             qualifier.qualify_const();
1194                         }
1195                     } else {
1196                         // Enforce a constant-like CFG for `const fn`.
1197                         qualifier.qualify_const();
1198                     }
1199                 } else {
1200                     while let Some((bb, data)) = qualifier.rpo.next() {
1201                         qualifier.visit_basic_block_data(bb, data);
1202                     }
1203                 }
1204
1205                 (qualifier.temp_promotion_state, qualifier.promotion_candidates)
1206             };
1207
1208             // Do the actual promotion, now that we know what's viable.
1209             promote_consts::promote_candidates(mir, tcx, temps, candidates);
1210         } else {
1211             let promoted_temps = if mode == Mode::Const {
1212                 // Already computed by `mir_const_qualif`.
1213                 const_promoted_temps.unwrap()
1214             } else {
1215                 Qualifier::new(tcx, def_id, mir, mode).qualify_const().1
1216             };
1217
1218             // In `const` and `static` everything without `StorageDead`
1219             // is `'static`, we don't have to create promoted MIR fragments,
1220             // just remove `Drop` and `StorageDead` on "promoted" locals.
1221             for block in mir.basic_blocks_mut() {
1222                 block.statements.retain(|statement| {
1223                     match statement.kind {
1224                         StatementKind::StorageDead(index) => {
1225                             !promoted_temps.contains(index)
1226                         }
1227                         _ => true
1228                     }
1229                 });
1230                 let terminator = block.terminator_mut();
1231                 match terminator.kind {
1232                     TerminatorKind::Drop { location: Place::Local(index), target, .. } => {
1233                         if promoted_temps.contains(index) {
1234                             terminator.kind = TerminatorKind::Goto {
1235                                 target,
1236                             };
1237                         }
1238                     }
1239                     _ => {}
1240                 }
1241             }
1242         }
1243
1244         // Statics must be Sync.
1245         if mode == Mode::Static {
1246             // `#[thread_local]` statics don't have to be `Sync`.
1247             for attr in &tcx.get_attrs(def_id)[..] {
1248                 if attr.check_name("thread_local") {
1249                     return;
1250                 }
1251             }
1252             let ty = mir.return_ty();
1253             tcx.infer_ctxt().enter(|infcx| {
1254                 let param_env = ty::ParamEnv::empty();
1255                 let cause = traits::ObligationCause::new(mir.span, id, traits::SharedStatic);
1256                 let mut fulfillment_cx = traits::FulfillmentContext::new();
1257                 fulfillment_cx.register_bound(&infcx,
1258                                               param_env,
1259                                               ty,
1260                                               tcx.require_lang_item(lang_items::SyncTraitLangItem),
1261                                               cause);
1262                 if let Err(err) = fulfillment_cx.select_all_or_error(&infcx) {
1263                     infcx.report_fulfillment_errors(&err, None, false);
1264                 }
1265             });
1266         }
1267     }
1268 }
1269
1270 fn args_required_const(tcx: TyCtxt, def_id: DefId) -> Option<FxHashSet<usize>> {
1271     let attrs = tcx.get_attrs(def_id);
1272     let attr = attrs.iter().find(|a| a.check_name("rustc_args_required_const"))?;
1273     let mut ret = FxHashSet();
1274     for meta in attr.meta_item_list()? {
1275         match meta.literal()?.node {
1276             LitKind::Int(a, _) => { ret.insert(a as usize); }
1277             _ => return None,
1278         }
1279     }
1280     Some(ret)
1281 }