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::bitvec::BitVector;
  18 use rustc_data_structures::indexed_set::IdxSetBuf;
  19 use rustc_data_structures::indexed_vec::{IndexVec, Idx};
  20 use rustc_data_structures::fx::FxHashSet;
  21 use rustc::hir;
  22 use rustc::hir::def_id::DefId;
  23 use rustc::middle::const_val::ConstVal;
  24 use rustc::traits::{self, Reveal};
  25 use rustc::ty::{self, TyCtxt, Ty, TypeFoldable};
  26 use rustc::ty::cast::CastTy;
  27 use rustc::ty::maps::Providers;
  28 use rustc::mir::*;
  29 use rustc::mir::traversal::ReversePostorder;
  30 use rustc::mir::visit::{PlaceContext, Visitor};
  31 use rustc::middle::lang_items;
  32 use syntax::abi::Abi;
  33 use syntax::attr;
  34 use syntax::ast::LitKind;
  35 use syntax::feature_gate::UnstableFeatures;
  36 use syntax_pos::{Span, DUMMY_SP};
  37
  38 use std::fmt;
  39 use std::rc::Rc;
  40 use std::usize;
  41
  42 use transform::{MirPass, MirSource};
  43 use super::promote_consts::{self, Candidate, TempState};
  44
  45 bitflags! {
  46     // Borrows of temporaries can be promoted only if
  47     // they have none of these qualifications, with
  48     // the exception of `STATIC_REF` (in statics only).
  49     struct Qualif: u8 {
  50         // Constant containing interior mutability (UnsafeCell).
  51         const MUTABLE_INTERIOR  = 1 << 0;
  52
  53         // Constant containing an ADT that implements Drop.
  54         const NEEDS_DROP        = 1 << 1;
  55
  56         // Function argument.
  57         const FN_ARGUMENT       = 1 << 2;
  58
  59         // Static place or move from a static.
  60         const STATIC            = 1 << 3;
  61
  62         // Reference to a static.
  63         const STATIC_REF        = 1 << 4;
  64
  65         // Not constant at all - non-`const fn` calls, asm!,
  66         // pointer comparisons, ptr-to-int casts, etc.
  67         const NOT_CONST         = 1 << 5;
  68
  69         // Refers to temporaries which cannot be promoted as
  70         // promote_consts decided they weren't simple enough.
  71         const NOT_PROMOTABLE    = 1 << 6;
  72
  73         // Const items can only have MUTABLE_INTERIOR
  74         // and NOT_PROMOTABLE without producing an error.
  75         const CONST_ERROR       = !Qualif::MUTABLE_INTERIOR.bits &
  76                                   !Qualif::NOT_PROMOTABLE.bits;
  77     }
  78 }
  79
  80 impl<'a, 'tcx> Qualif {
  81     /// Remove flags which are impossible for the given type.
  82     fn restrict(&mut self, ty: Ty<'tcx>,
  83                 tcx: TyCtxt<'a, 'tcx, 'tcx>,
  84                 param_env: ty::ParamEnv<'tcx>) {
  85         if ty.is_freeze(tcx, param_env, DUMMY_SP) {
  86             *self = *self - Qualif::MUTABLE_INTERIOR;
  87         }
  88         if !ty.needs_drop(tcx, param_env) {
  89             *self = *self - Qualif::NEEDS_DROP;
  90         }
  91     }
  92 }
  93
  94 /// What kind of item we are in.
  95 #[derive(Copy, Clone, PartialEq, Eq)]
  96 enum Mode {
  97     Const,
  98     Static,
  99     StaticMut,
 100     ConstFn,
 101     Fn
 102 }
 103
 104 impl fmt::Display for Mode {
 105     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 106         match *self {
 107             Mode::Const => write!(f, "constant"),
 108             Mode::Static | Mode::StaticMut => write!(f, "static"),
 109             Mode::ConstFn => write!(f, "constant function"),
 110             Mode::Fn => write!(f, "function")
 111         }
 112     }
 113 }
 114
 115 struct Qualifier<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 116     mode: Mode,
 117     span: Span,
 118     def_id: DefId,
 119     mir: &'a Mir<'tcx>,
 120     rpo: ReversePostorder<'a, 'tcx>,
 121     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 122     param_env: ty::ParamEnv<'tcx>,
 123     temp_qualif: IndexVec<Local, Option<Qualif>>,
 124     return_qualif: Option<Qualif>,
 125     qualif: Qualif,
 126     const_fn_arg_vars: BitVector,
 127     temp_promotion_state: IndexVec<Local, TempState>,
 128     promotion_candidates: Vec<Candidate>
 129 }
 130
 131 impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
 132     fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 133            def_id: DefId,
 134            mir: &'a Mir<'tcx>,
 135            mode: Mode)
 136            -> Qualifier<'a, 'tcx, 'tcx> {
 137         let mut rpo = traversal::reverse_postorder(mir);
 138         let temps = promote_consts::collect_temps(mir, &mut rpo);
 139         rpo.reset();
 140
 141         let param_env = tcx.param_env(def_id);
 142
 143         let mut temp_qualif = IndexVec::from_elem(None, &mir.local_decls);
 144         for arg in mir.args_iter() {
 145             let mut qualif = Qualif::NEEDS_DROP;
 146             qualif.restrict(mir.local_decls[arg].ty, tcx, param_env);
 147             temp_qualif[arg] = Some(qualif);
 148         }
 149
 150         Qualifier {
 151             mode,
 152             span: mir.span,
 153             def_id,
 154             mir,
 155             rpo,
 156             tcx,
 157             param_env,
 158             temp_qualif,
 159             return_qualif: None,
 160             qualif: Qualif::empty(),
 161             const_fn_arg_vars: BitVector::new(mir.local_decls.len()),
 162             temp_promotion_state: temps,
 163             promotion_candidates: vec![]
 164         }
 165     }
 166
 167     // FIXME(eddyb) we could split the errors into meaningful
 168     // categories, but enabling full miri would make that
 169     // slightly pointless (even with feature-gating).
 170     fn not_const(&mut self) {
 171         self.add(Qualif::NOT_CONST);
 172         if self.mode != Mode::Fn {
 173             let mut err = struct_span_err!(
 174                 self.tcx.sess,
 175                 self.span,
 176                 E0019,
 177                 "{} contains unimplemented expression type",
 178                 self.mode
 179             );
 180             if self.tcx.sess.teach(&err.get_code().unwrap()) {
 181                 err.note("A function call isn't allowed in the const's initialization expression \
 182                           because the expression's value must be known at compile-time.");
 183                 err.note("Remember: you can't use a function call inside a const's initialization \
 184                           expression! However, you can use it anywhere else.");
 185             }
 186             err.emit();
 187         }
 188     }
 189
 190     /// Error about extra statements in a constant.
 191     fn statement_like(&mut self) {
 192         self.add(Qualif::NOT_CONST);
 193         if self.mode != Mode::Fn {
 194             let mut err = struct_span_err!(
 195                 self.tcx.sess,
 196                 self.span,
 197                 E0016,
 198                 "blocks in {}s are limited to items and tail expressions",
 199                 self.mode
 200             );
 201             if self.tcx.sess.teach(&err.get_code().unwrap()) {
 202                 err.note("Blocks in constants may only contain items (such as constant, function \
 203                           definition, etc...) and a tail expression.");
 204                 err.help("To avoid it, you have to replace the non-item object.");
 205             }
 206             err.emit();
 207         }
 208     }
 209
 210     /// Add the given qualification to self.qualif.
 211     fn add(&mut self, qualif: Qualif) {
 212         self.qualif = self.qualif | qualif;
 213     }
 214
 215     /// Add the given type's qualification to self.qualif.
 216     fn add_type(&mut self, ty: Ty<'tcx>) {
 217         self.add(Qualif::MUTABLE_INTERIOR | Qualif::NEEDS_DROP);
 218         self.qualif.restrict(ty, self.tcx, self.param_env);
 219     }
 220
 221     /// Within the provided closure, self.qualif will start
 222     /// out empty, and its value after the closure returns will
 223     /// be combined with the value before the call to nest.
 224     fn nest<F: FnOnce(&mut Self)>(&mut self, f: F) {
 225         let original = self.qualif;
 226         self.qualif = Qualif::empty();
 227         f(self);
 228         self.add(original);
 229     }
 230
 231     /// Check if a Local with the current qualifications is promotable.
 232     fn can_promote(&mut self) -> bool {
 233         // References to statics are allowed, but only in other statics.
 234         if self.mode == Mode::Static || self.mode == Mode::StaticMut {
 235             (self.qualif - Qualif::STATIC_REF).is_empty()
 236         } else {
 237             self.qualif.is_empty()
 238         }
 239     }
 240
 241     /// Check if a Place with the current qualifications could
 242     /// be consumed, by either an operand or a Deref projection.
 243     fn try_consume(&mut self) -> bool {
 244         if self.qualif.intersects(Qualif::STATIC) && self.mode != Mode::Fn {
 245             let msg = if self.mode == Mode::Static ||
 246                          self.mode == Mode::StaticMut {
 247                 "cannot refer to other statics by value, use the \
 248                  address-of operator or a constant instead"
 249             } else {
 250                 "cannot refer to statics by value, use a constant instead"
 251             };
 252             struct_span_err!(self.tcx.sess, self.span, E0394, "{}", msg)
 253                 .span_label(self.span, "referring to another static by value")
 254                 .note("use the address-of operator or a constant instead")
 255                 .emit();
 256
 257             // Replace STATIC with NOT_CONST to avoid further errors.
 258             self.qualif = self.qualif - Qualif::STATIC;
 259             self.add(Qualif::NOT_CONST);
 260
 261             false
 262         } else {
 263             true
 264         }
 265     }
 266
 267     /// Assign the current qualification to the given destination.
 268     fn assign(&mut self, dest: &Place<'tcx>, location: Location) {
 269         let qualif = self.qualif;
 270         let span = self.span;
 271         let store = |slot: &mut Option<Qualif>| {
 272             if slot.is_some() {
 273                 span_bug!(span, "multiple assignments to {:?}", dest);
 274             }
 275             *slot = Some(qualif);
 276         };
 277
 278         // Only handle promotable temps in non-const functions.
 279         if self.mode == Mode::Fn {
 280             if let Place::Local(index) = *dest {
 281                 if self.mir.local_kind(index) == LocalKind::Temp
 282                 && self.temp_promotion_state[index].is_promotable() {
 283                     debug!("store to promotable temp {:?}", index);
 284                     store(&mut self.temp_qualif[index]);
 285                 }
 286             }
 287             return;
 288         }
 289
 290         match *dest {
 291             Place::Local(index) if self.mir.local_kind(index) == LocalKind::Temp => {
 292                 debug!("store to temp {:?}", index);
 293                 store(&mut self.temp_qualif[index])
 294             }
 295             Place::Local(index) if self.mir.local_kind(index) == LocalKind::ReturnPointer => {
 296                 debug!("store to return place {:?}", index);
 297                 store(&mut self.return_qualif)
 298             }
 299
 300             Place::Projection(box Projection {
 301                 base: Place::Local(index),
 302                 elem: ProjectionElem::Deref
 303             }) if self.mir.local_kind(index) == LocalKind::Temp
 304                && self.mir.local_decls[index].ty.is_box()
 305                && self.temp_qualif[index].map_or(false, |qualif| {
 306                     qualif.intersects(Qualif::NOT_CONST)
 307                }) => {
 308                 // Part of `box expr`, we should've errored
 309                 // already for the Box allocation Rvalue.
 310             }
 311
 312             // This must be an explicit assignment.
 313             _ => {
 314                 // Catch more errors in the destination.
 315                 self.visit_place(dest, PlaceContext::Store, location);
 316                 self.statement_like();
 317             }
 318         }
 319     }
 320
 321     /// Qualify a whole const, static initializer or const fn.
 322     fn qualify_const(&mut self) -> (Qualif, Rc<IdxSetBuf<Local>>) {
 323         debug!("qualifying {} {:?}", self.mode, self.def_id);
 324
 325         let mir = self.mir;
 326
 327         let mut seen_blocks = BitVector::new(mir.basic_blocks().len());
 328         let mut bb = START_BLOCK;
 329         loop {
 330             seen_blocks.insert(bb.index());
 331
 332             self.visit_basic_block_data(bb, &mir[bb]);
 333
 334             let target = match mir[bb].terminator().kind {
 335                 TerminatorKind::Goto { target } |
 336                 TerminatorKind::Drop { target, .. } |
 337                 TerminatorKind::Assert { target, .. } |
 338                 TerminatorKind::Call { destination: Some((_, target)), .. } => {
 339                     Some(target)
 340                 }
 341
 342                 // Non-terminating calls cannot produce any value.
 343                 TerminatorKind::Call { destination: None, .. } => {
 344                     break;
 345                 }
 346
 347                 TerminatorKind::SwitchInt {..} |
 348                 TerminatorKind::DropAndReplace { .. } |
 349                 TerminatorKind::Resume |
 350                 TerminatorKind::Abort |
 351                 TerminatorKind::GeneratorDrop |
 352                 TerminatorKind::Yield { .. } |
 353                 TerminatorKind::Unreachable |
 354                 TerminatorKind::FalseEdges { .. } |
 355                 TerminatorKind::FalseUnwind { .. } => None,
 356
 357                 TerminatorKind::Return => {
 358                     // Check for unused values. This usually means
 359                     // there are extra statements in the AST.
 360                     for temp in mir.temps_iter() {
 361                         if self.temp_qualif[temp].is_none() {
 362                             continue;
 363                         }
 364
 365                         let state = self.temp_promotion_state[temp];
 366                         if let TempState::Defined { location, uses: 0 } = state {
 367                             let data = &mir[location.block];
 368                             let stmt_idx = location.statement_index;
 369
 370                             // Get the span for the initialization.
 371                             let source_info = if stmt_idx < data.statements.len() {
 372                                 data.statements[stmt_idx].source_info
 373                             } else {
 374                                 data.terminator().source_info
 375                             };
 376                             self.span = source_info.span;
 377
 378                             // Treat this as a statement in the AST.
 379                             self.statement_like();
 380                         }
 381                     }
 382
 383                     // Make sure there are no extra unassigned variables.
 384                     self.qualif = Qualif::NOT_CONST;
 385                     for index in mir.vars_iter() {
 386                         if !self.const_fn_arg_vars.contains(index.index()) {
 387                             debug!("unassigned variable {:?}", index);
 388                             self.assign(&Place::Local(index), Location {
 389                                 block: bb,
 390                                 statement_index: usize::MAX,
 391                             });
 392                         }
 393                     }
 394
 395                     break;
 396                 }
 397             };
 398
 399             match target {
 400                 // No loops allowed.
 401                 Some(target) if !seen_blocks.contains(target.index()) => {
 402                     bb = target;
 403                 }
 404                 _ => {
 405                     self.not_const();
 406                     break;
 407                 }
 408             }
 409         }
 410
 411         self.qualif = self.return_qualif.unwrap_or(Qualif::NOT_CONST);
 412
 413         // Account for errors in consts by using the
 414         // conservative type qualification instead.
 415         if self.qualif.intersects(Qualif::CONST_ERROR) {
 416             self.qualif = Qualif::empty();
 417             let return_ty = mir.return_ty();
 418             self.add_type(return_ty);
 419         }
 420
 421
 422         // Collect all the temps we need to promote.
 423         let mut promoted_temps = IdxSetBuf::new_empty(self.temp_promotion_state.len());
 424
 425         for candidate in &self.promotion_candidates {
 426             match *candidate {
 427                 Candidate::Ref(Location { block: bb, statement_index: stmt_idx }) => {
 428                     match self.mir[bb].statements[stmt_idx].kind {
 429                         StatementKind::Assign(_, Rvalue::Ref(_, _, Place::Local(index))) => {
 430                             promoted_temps.add(&index);
 431                         }
 432                         _ => {}
 433                     }
 434                 }
 435                 Candidate::Argument { .. } => {}
 436             }
 437         }
 438
 439         (self.qualif, Rc::new(promoted_temps))
 440     }
 441 }
 442
 443 /// Accumulates an Rvalue or Call's effects in self.qualif.
 444 /// For functions (constant or not), it also records
 445 /// candidates for promotion in promotion_candidates.
 446 impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
 447     fn visit_local(&mut self,
 448                    &local: &Local,
 449                    _: PlaceContext<'tcx>,
 450                    _: Location) {
 451         let kind = self.mir.local_kind(local);
 452         match kind {
 453             LocalKind::ReturnPointer => {
 454                 self.not_const();
 455             }
 456             LocalKind::Var => {
 457                 self.add(Qualif::NOT_CONST);
 458             }
 459             LocalKind::Arg |
 460             LocalKind::Temp => {
 461                 if let LocalKind::Arg = kind {
 462                     self.add(Qualif::FN_ARGUMENT);
 463                 }
 464
 465                 if !self.temp_promotion_state[local].is_promotable() {
 466                     self.add(Qualif::NOT_PROMOTABLE);
 467                 }
 468
 469                 if let Some(qualif) = self.temp_qualif[local] {
 470                     self.add(qualif);
 471                 } else {
 472                     self.not_const();
 473                 }
 474             }
 475         }
 476     }
 477
 478     fn visit_place(&mut self,
 479                     place: &Place<'tcx>,
 480                     context: PlaceContext<'tcx>,
 481                     location: Location) {
 482         match *place {
 483             Place::Local(ref local) => self.visit_local(local, context, location),
 484             Place::Static(ref global) => {
 485                 self.add(Qualif::STATIC);
 486
 487                 if self.mode != Mode::Fn {
 488                     for attr in &self.tcx.get_attrs(global.def_id)[..] {
 489                         if attr.check_name("thread_local") {
 490                             span_err!(self.tcx.sess, self.span, E0625,
 491                                       "thread-local statics cannot be \
 492                                        accessed at compile-time");
 493                             self.add(Qualif::NOT_CONST);
 494                             return;
 495                         }
 496                     }
 497                 }
 498
 499                 if self.mode == Mode::Const || self.mode == Mode::ConstFn {
 500                     let mut err = struct_span_err!(self.tcx.sess, self.span, E0013,
 501                                                    "{}s cannot refer to statics, use \
 502                                                     a constant instead", self.mode);
 503                     if self.tcx.sess.teach(&err.get_code().unwrap()) {
 504                         err.note(
 505                             "Static and const variables can refer to other const variables. But a \
 506                              const variable cannot refer to a static variable."
 507                         );
 508                         err.help(
 509                             "To fix this, the value can be extracted as a const and then used."
 510                         );
 511                     }
 512                     err.emit()
 513                 }
 514             }
 515             Place::Projection(ref proj) => {
 516                 self.nest(|this| {
 517                     this.super_place(place, context, location);
 518                     match proj.elem {
 519                         ProjectionElem::Deref => {
 520                             if !this.try_consume() {
 521                                 return;
 522                             }
 523
 524                             if this.qualif.intersects(Qualif::STATIC_REF) {
 525                                 this.qualif = this.qualif - Qualif::STATIC_REF;
 526                                 this.add(Qualif::STATIC);
 527                             }
 528
 529                             let base_ty = proj.base.ty(this.mir, this.tcx).to_ty(this.tcx);
 530                             if let ty::TyRawPtr(_) = base_ty.sty {
 531                                 this.add(Qualif::NOT_CONST);
 532                                 if this.mode != Mode::Fn {
 533                                     let mut err = struct_span_err!(
 534                                         this.tcx.sess,
 535                                         this.span,
 536                                         E0396,
 537                                         "raw pointers cannot be dereferenced in {}s",
 538                                         this.mode
 539                                     );
 540                                     err.span_label(this.span,
 541                                                    "dereference of raw pointer in constant");
 542                                     if this.tcx.sess.teach(&err.get_code().unwrap()) {
 543                                         err.note(
 544                                             "The value behind a raw pointer can't be determined \
 545                                              at compile-time (or even link-time), which means it \
 546                                              can't be used in a constant expression."
 547                                         );
 548                                         err.help("A possible fix is to dereference your pointer \
 549                                                   at some point in run-time.");
 550                                     }
 551                                     err.emit();
 552                                 }
 553                             }
 554                         }
 555
 556                         ProjectionElem::Field(..) |
 557                         ProjectionElem::Index(_) => {
 558                             if this.mode != Mode::Fn &&
 559                                this.qualif.intersects(Qualif::STATIC) {
 560                                 span_err!(this.tcx.sess, this.span, E0494,
 561                                           "cannot refer to the interior of another \
 562                                            static, use a constant instead");
 563                             }
 564                             let ty = place.ty(this.mir, this.tcx).to_ty(this.tcx);
 565                             this.qualif.restrict(ty, this.tcx, this.param_env);
 566                         }
 567
 568                         ProjectionElem::ConstantIndex {..} |
 569                         ProjectionElem::Subslice {..} |
 570                         ProjectionElem::Downcast(..) => {
 571                             this.not_const()
 572                         }
 573                     }
 574                 });
 575             }
 576         }
 577     }
 578
 579     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
 580         match *operand {
 581             Operand::Copy(_) |
 582             Operand::Move(_) => {
 583                 self.nest(|this| {
 584                     this.super_operand(operand, location);
 585                     this.try_consume();
 586                 });
 587
 588                 // Mark the consumed locals to indicate later drops are noops.
 589                 if let Operand::Move(Place::Local(local)) = *operand {
 590                     self.temp_qualif[local] = self.temp_qualif[local].map(|q|
 591                         q - Qualif::NEEDS_DROP
 592                     );
 593                 }
 594             }
 595             Operand::Constant(ref constant) => {
 596                 if let Literal::Value {
 597                     value: &ty::Const { val: ConstVal::Unevaluated(def_id, _), ty }
 598                 } = constant.literal {
 599                     // Don't peek inside trait associated constants.
 600                     if self.tcx.trait_of_item(def_id).is_some() {
 601                         self.add_type(ty);
 602                     } else {
 603                         let (bits, _) = self.tcx.at(constant.span).mir_const_qualif(def_id);
 604
 605                         let qualif = Qualif::from_bits(bits).expect("invalid mir_const_qualif");
 606                         self.add(qualif);
 607
 608                         // Just in case the type is more specific than
 609                         // the definition, e.g. impl associated const
 610                         // with type parameters, take it into account.
 611                         self.qualif.restrict(ty, self.tcx, self.param_env);
 612                     }
 613                 }
 614             }
 615         }
 616     }
 617
 618     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 619         // Recurse through operands and places.
 620         self.super_rvalue(rvalue, location);
 621
 622         match *rvalue {
 623             Rvalue::Use(_) |
 624             Rvalue::Repeat(..) |
 625             Rvalue::UnaryOp(UnOp::Neg, _) |
 626             Rvalue::UnaryOp(UnOp::Not, _) |
 627             Rvalue::NullaryOp(NullOp::SizeOf, _) |
 628             Rvalue::CheckedBinaryOp(..) |
 629             Rvalue::Cast(CastKind::ReifyFnPointer, ..) |
 630             Rvalue::Cast(CastKind::UnsafeFnPointer, ..) |
 631             Rvalue::Cast(CastKind::ClosureFnPointer, ..) |
 632             Rvalue::Cast(CastKind::Unsize, ..) |
 633             Rvalue::Discriminant(..) => {}
 634
 635             Rvalue::Len(_) => {
 636                 // Static places in consts would have errored already,
 637                 // don't treat length checks as reads from statics.
 638                 self.qualif = self.qualif - Qualif::STATIC;
 639             }
 640
 641             Rvalue::Ref(_, kind, ref place) => {
 642                 // Static places in consts would have errored already,
 643                 // only keep track of references to them here.
 644                 if self.qualif.intersects(Qualif::STATIC) {
 645                     self.qualif = self.qualif - Qualif::STATIC;
 646                     self.add(Qualif::STATIC_REF);
 647                 }
 648
 649                 let ty = place.ty(self.mir, self.tcx).to_ty(self.tcx);
 650                 if let BorrowKind::Mut { .. } = kind {
 651                     // In theory, any zero-sized value could be borrowed
 652                     // mutably without consequences. However, only &mut []
 653                     // is allowed right now, and only in functions.
 654                     let allow = if self.mode == Mode::StaticMut {
 655                         // Inside a `static mut`, &mut [...] is also allowed.
 656                         match ty.sty {
 657                             ty::TyArray(..) | ty::TySlice(_) => true,
 658                             _ => false
 659                         }
 660                     } else if let ty::TyArray(_, len) = ty.sty {
 661                         len.val.to_const_int().unwrap().to_u64().unwrap() == 0 &&
 662                             self.mode == Mode::Fn
 663                     } else {
 664                         false
 665                     };
 666
 667                     if !allow {
 668                         self.add(Qualif::NOT_CONST);
 669                         if self.mode != Mode::Fn {
 670                             let mut err = struct_span_err!(self.tcx.sess,  self.span, E0017,
 671                                                            "references in {}s may only refer \
 672                                                             to immutable values", self.mode);
 673                             err.span_label(self.span, format!("{}s require immutable values",
 674                                                                 self.mode));
 675                             if self.tcx.sess.teach(&err.get_code().unwrap()) {
 676                                 err.note("References in statics and constants may only refer to \
 677                                           immutable values.\n\n\
 678                                           Statics are shared everywhere, and if they refer to \
 679                                           mutable data one might violate memory safety since \
 680                                           holding multiple mutable references to shared data is \
 681                                           not allowed.\n\n\
 682                                           If you really want global mutable state, try using \
 683                                           static mut or a global UnsafeCell.");
 684                             }
 685                             err.emit();
 686                         }
 687                     }
 688                 } else {
 689                     // Constants cannot be borrowed if they contain interior mutability as
 690                     // it means that our "silent insertion of statics" could change
 691                     // initializer values (very bad).
 692                     if self.qualif.intersects(Qualif::MUTABLE_INTERIOR) {
 693                         // Replace MUTABLE_INTERIOR with NOT_CONST to avoid
 694                         // duplicate errors (from reborrowing, for example).
 695                         self.qualif = self.qualif - Qualif::MUTABLE_INTERIOR;
 696                         self.add(Qualif::NOT_CONST);
 697                         if self.mode != Mode::Fn {
 698                             span_err!(self.tcx.sess, self.span, E0492,
 699                                       "cannot borrow a constant which may contain \
 700                                        interior mutability, create a static instead");
 701                         }
 702                     }
 703                 }
 704
 705                 // We might have a candidate for promotion.
 706                 let candidate = Candidate::Ref(location);
 707                 if self.can_promote() {
 708                     // We can only promote direct borrows of temps.
 709                     if let Place::Local(local) = *place {
 710                         if self.mir.local_kind(local) == LocalKind::Temp {
 711                             self.promotion_candidates.push(candidate);
 712                         }
 713                     }
 714                 }
 715             }
 716
 717             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {
 718                 let operand_ty = operand.ty(self.mir, self.tcx);
 719                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 720                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 721                 match (cast_in, cast_out) {
 722                     (CastTy::Ptr(_), CastTy::Int(_)) |
 723                     (CastTy::FnPtr, CastTy::Int(_)) => {
 724                         self.add(Qualif::NOT_CONST);
 725                         if self.mode != Mode::Fn {
 726                             let mut err = struct_span_err!(
 727                                 self.tcx.sess,
 728                                 self.span,
 729                                 E0018,
 730                                 "raw pointers cannot be cast to integers in {}s",
 731                                 self.mode
 732                             );
 733                             if self.tcx.sess.teach(&err.get_code().unwrap()) {
 734                                 err.note("\
 735 The value of static and constant integers must be known at compile time. You can't cast a pointer \
 736 to an integer because the address of a pointer can vary.
 737
 738 For example, if you write:
 739
 740 ```
 741 static MY_STATIC: u32 = 42;
 742 static MY_STATIC_ADDR: usize = &MY_STATIC as *const _ as usize;
 743 static WHAT: usize = (MY_STATIC_ADDR^17) + MY_STATIC_ADDR;
 744 ```
 745
 746 Then `MY_STATIC_ADDR` would contain the address of `MY_STATIC`. However, the address can change \
 747 when the program is linked, as well as change between different executions due to ASLR, and many \
 748 linkers would not be able to calculate the value of `WHAT`.
 749
 750 On the other hand, static and constant pointers can point either to a known numeric address or to \
 751 the address of a symbol.
 752
 753 ```
 754 static MY_STATIC: u32 = 42;
 755 static MY_STATIC_ADDR: &'static u32 = &MY_STATIC;
 756 const CONST_ADDR: *const u8 = 0x5f3759df as *const u8;
 757 ```
 758
 759 This does not pose a problem by itself because they can't be accessed directly.");
 760                             }
 761                             err.emit();
 762                         }
 763                     }
 764                     _ => {}
 765                 }
 766             }
 767
 768             Rvalue::BinaryOp(op, ref lhs, _) => {
 769                 if let ty::TyRawPtr(_) = lhs.ty(self.mir, self.tcx).sty {
 770                     assert!(op == BinOp::Eq || op == BinOp::Ne ||
 771                             op == BinOp::Le || op == BinOp::Lt ||
 772                             op == BinOp::Ge || op == BinOp::Gt ||
 773                             op == BinOp::Offset);
 774
 775                     self.add(Qualif::NOT_CONST);
 776                     if self.mode != Mode::Fn {
 777                         struct_span_err!(
 778                             self.tcx.sess, self.span, E0395,
 779                             "raw pointers cannot be compared in {}s",
 780                             self.mode)
 781                         .span_label(
 782                             self.span,
 783                             "comparing raw pointers in static")
 784                         .emit();
 785                     }
 786                 }
 787             }
 788
 789             Rvalue::NullaryOp(NullOp::Box, _) => {
 790                 self.add(Qualif::NOT_CONST);
 791                 if self.mode != Mode::Fn {
 792                     let mut err = struct_span_err!(self.tcx.sess, self.span, E0010,
 793                                                    "allocations are not allowed in {}s", self.mode);
 794                     err.span_label(self.span, format!("allocation not allowed in {}s", self.mode));
 795                     if self.tcx.sess.teach(&err.get_code().unwrap()) {
 796                         err.note(
 797                             "The value of statics and constants must be known at compile time, \
 798                              and they live for the entire lifetime of a program. Creating a boxed \
 799                              value allocates memory on the heap at runtime, and therefore cannot \
 800                              be done at compile time."
 801                         );
 802                     }
 803                     err.emit();
 804                 }
 805             }
 806
 807             Rvalue::Aggregate(ref kind, _) => {
 808                 if let AggregateKind::Adt(def, ..) = **kind {
 809                     if def.has_dtor(self.tcx) {
 810                         self.add(Qualif::NEEDS_DROP);
 811                     }
 812
 813                     if Some(def.did) == self.tcx.lang_items().unsafe_cell_type() {
 814                         let ty = rvalue.ty(self.mir, self.tcx);
 815                         self.add_type(ty);
 816                         assert!(self.qualif.intersects(Qualif::MUTABLE_INTERIOR));
 817                     }
 818                 }
 819             }
 820         }
 821     }
 822
 823     fn visit_terminator_kind(&mut self,
 824                              bb: BasicBlock,
 825                              kind: &TerminatorKind<'tcx>,
 826                              location: Location) {
 827         if let TerminatorKind::Call { ref func, ref args, ref destination, .. } = *kind {
 828             self.visit_operand(func, location);
 829
 830             let fn_ty = func.ty(self.mir, self.tcx);
 831             let mut callee_def_id = None;
 832             let (mut is_shuffle, mut is_const_fn) = (false, None);
 833             if let ty::TyFnDef(def_id, _) = fn_ty.sty {
 834                 callee_def_id = Some(def_id);
 835                 match self.tcx.fn_sig(def_id).abi() {
 836                     Abi::RustIntrinsic |
 837                     Abi::PlatformIntrinsic => {
 838                         assert!(!self.tcx.is_const_fn(def_id));
 839                         match &self.tcx.item_name(def_id)[..] {
 840                             "size_of" | "min_align_of" | "type_id" => is_const_fn = Some(def_id),
 841
 842                             name if name.starts_with("simd_shuffle") => {
 843                                 is_shuffle = true;
 844                             }
 845
 846                             _ => {}
 847                         }
 848                     }
 849                     _ => {
 850                         if self.tcx.is_const_fn(def_id) {
 851                             is_const_fn = Some(def_id);
 852                         }
 853                     }
 854                 }
 855             }
 856
 857             let constant_arguments = callee_def_id.and_then(|id| {
 858                 args_required_const(self.tcx, id)
 859             });
 860             for (i, arg) in args.iter().enumerate() {
 861                 self.nest(|this| {
 862                     this.visit_operand(arg, location);
 863                     if this.mode != Mode::Fn {
 864                         return
 865                     }
 866                     let candidate = Candidate::Argument { bb, index: i };
 867                     if is_shuffle && i == 2 {
 868                         if this.can_promote() {
 869                             this.promotion_candidates.push(candidate);
 870                         } else {
 871                             span_err!(this.tcx.sess, this.span, E0526,
 872                                       "shuffle indices are not constant");
 873                         }
 874                         return
 875                     }
 876
 877                     let constant_arguments = match constant_arguments.as_ref() {
 878                         Some(s) => s,
 879                         None => return,
 880                     };
 881                     if !constant_arguments.contains(&i) {
 882                         return
 883                     }
 884                     if this.can_promote() {
 885                         this.promotion_candidates.push(candidate);
 886                     } else {
 887                         this.tcx.sess.span_err(this.span,
 888                             &format!("argument {} is required to be a constant",
 889                                      i + 1));
 890                     }
 891                 });
 892             }
 893
 894             // Const fn calls.
 895             if let Some(def_id) = is_const_fn {
 896                 // find corresponding rustc_const_unstable feature
 897                 if let Some(&attr::Stability {
 898                     rustc_const_unstable: Some(attr::RustcConstUnstable {
 899                         feature: ref feature_name
 900                     }),
 901                 .. }) = self.tcx.lookup_stability(def_id) {
 902
 903                     // We are in a const or static initializer,
 904                     if self.mode != Mode::Fn &&
 905
 906                         // feature-gate is not enabled,
 907                         !self.tcx.sess.features.borrow()
 908                             .declared_lib_features
 909                             .iter()
 910                             .any(|&(ref sym, _)| sym == feature_name) &&
 911
 912                         // this doesn't come from a crate with the feature-gate enabled,
 913                         self.def_id.is_local() &&
 914
 915                         // this doesn't come from a macro that has #[allow_internal_unstable]
 916                         !self.span.allows_unstable()
 917                     {
 918                         let mut err = self.tcx.sess.struct_span_err(self.span,
 919                             &format!("`{}` is not yet stable as a const fn",
 920                                      self.tcx.item_path_str(def_id)));
 921                         help!(&mut err,
 922                               "in Nightly builds, add `#![feature({})]` \
 923                                to the crate attributes to enable",
 924                               feature_name);
 925                         err.emit();
 926                     }
 927                 }
 928             } else {
 929                 self.qualif = Qualif::NOT_CONST;
 930                 if self.mode != Mode::Fn {
 931                     // FIXME(#24111) Remove this check when const fn stabilizes
 932                     let (msg, note) = if let UnstableFeatures::Disallow =
 933                             self.tcx.sess.opts.unstable_features {
 934                         (format!("calls in {}s are limited to \
 935                                   struct and enum constructors",
 936                                  self.mode),
 937                          Some("a limited form of compile-time function \
 938                                evaluation is available on a nightly \
 939                                compiler via `const fn`"))
 940                     } else {
 941                         (format!("calls in {}s are limited \
 942                                   to constant functions, \
 943                                   struct and enum constructors",
 944                                  self.mode),
 945                          None)
 946                     };
 947                     let mut err = struct_span_err!(self.tcx.sess, self.span, E0015, "{}", msg);
 948                     if let Some(note) = note {
 949                         err.span_note(self.span, note);
 950                     }
 951                     err.emit();
 952                 }
 953             }
 954
 955             if let Some((ref dest, _)) = *destination {
 956                 // Avoid propagating irrelevant callee/argument qualifications.
 957                 if self.qualif.intersects(Qualif::CONST_ERROR) {
 958                     self.qualif = Qualif::NOT_CONST;
 959                 } else {
 960                     // Be conservative about the returned value of a const fn.
 961                     let tcx = self.tcx;
 962                     let ty = dest.ty(self.mir, tcx).to_ty(tcx);
 963                     self.qualif = Qualif::empty();
 964                     self.add_type(ty);
 965                 }
 966                 self.assign(dest, location);
 967             }
 968         } else if let TerminatorKind::Drop { location: ref place, .. } = *kind {
 969             self.super_terminator_kind(bb, kind, location);
 970
 971             // Deny *any* live drops anywhere other than functions.
 972             if self.mode != Mode::Fn {
 973                 // HACK(eddyb) Emulate a bit of dataflow analysis,
 974                 // conservatively, that drop elaboration will do.
 975                 let needs_drop = if let Place::Local(local) = *place {
 976                     if self.temp_qualif[local].map_or(true, |q| q.intersects(Qualif::NEEDS_DROP)) {
 977                         Some(self.mir.local_decls[local].source_info.span)
 978                     } else {
 979                         None
 980                     }
 981                 } else {
 982                     Some(self.span)
 983                 };
 984
 985                 if let Some(span) = needs_drop {
 986                     // Double-check the type being dropped, to minimize false positives.
 987                     let ty = place.ty(self.mir, self.tcx).to_ty(self.tcx);
 988                     if ty.needs_drop(self.tcx, self.param_env) {
 989                         struct_span_err!(self.tcx.sess, span, E0493,
 990                                          "destructors cannot be evaluated at compile-time")
 991                             .span_label(span, format!("{}s cannot evaluate destructors",
 992                                                       self.mode))
 993                             .emit();
 994                     }
 995                 }
 996             }
 997         } else {
 998             // Qualify any operands inside other terminators.
 999             self.super_terminator_kind(bb, kind, location);
1000         }
1001     }
1002
1003     fn visit_assign(&mut self,
1004                     _: BasicBlock,
1005                     dest: &Place<'tcx>,
1006                     rvalue: &Rvalue<'tcx>,
1007                     location: Location) {
1008         self.visit_rvalue(rvalue, location);
1009
1010         // Check the allowed const fn argument forms.
1011         if let (Mode::ConstFn, &Place::Local(index)) = (self.mode, dest) {
1012             if self.mir.local_kind(index) == LocalKind::Var &&
1013                self.const_fn_arg_vars.insert(index.index()) {
1014
1015                 // Direct use of an argument is permitted.
1016                 match *rvalue {
1017                     Rvalue::Use(Operand::Copy(Place::Local(local))) |
1018                     Rvalue::Use(Operand::Move(Place::Local(local))) => {
1019                         if self.mir.local_kind(local) == LocalKind::Arg {
1020                             return;
1021                         }
1022                     }
1023                     _ => {}
1024                 }
1025
1026                 // Avoid a generic error for other uses of arguments.
1027                 if self.qualif.intersects(Qualif::FN_ARGUMENT) {
1028                     let decl = &self.mir.local_decls[index];
1029                     let mut err = struct_span_err!(
1030                         self.tcx.sess,
1031                         decl.source_info.span,
1032                         E0022,
1033                         "arguments of constant functions can only be immutable by-value bindings"
1034                     );
1035                     if self.tcx.sess.teach(&err.get_code().unwrap()) {
1036                         err.note("Constant functions are not allowed to mutate anything. Thus, \
1037                                   binding to an argument with a mutable pattern is not allowed.");
1038                         err.note("Remove any mutable bindings from the argument list to fix this \
1039                                   error. In case you need to mutate the argument, try lazily \
1040                                   initializing a global variable instead of using a const fn, or \
1041                                   refactoring the code to a functional style to avoid mutation if \
1042                                   possible.");
1043                     }
1044                     err.emit();
1045                     return;
1046                 }
1047             }
1048         }
1049
1050         self.assign(dest, location);
1051     }
1052
1053     fn visit_source_info(&mut self, source_info: &SourceInfo) {
1054         self.span = source_info.span;
1055     }
1056
1057     fn visit_statement(&mut self, bb: BasicBlock, statement: &Statement<'tcx>, location: Location) {
1058         self.nest(|this| {
1059             this.visit_source_info(&statement.source_info);
1060             match statement.kind {
1061                 StatementKind::Assign(ref place, ref rvalue) => {
1062                     this.visit_assign(bb, place, rvalue, location);
1063                 }
1064                 StatementKind::SetDiscriminant { .. } |
1065                 StatementKind::StorageLive(_) |
1066                 StatementKind::StorageDead(_) |
1067                 StatementKind::InlineAsm {..} |
1068                 StatementKind::EndRegion(_) |
1069                 StatementKind::Validate(..) |
1070                 StatementKind::Nop => {}
1071             }
1072         });
1073     }
1074
1075     fn visit_terminator(&mut self,
1076                         bb: BasicBlock,
1077                         terminator: &Terminator<'tcx>,
1078                         location: Location) {
1079         self.nest(|this| this.super_terminator(bb, terminator, location));
1080     }
1081 }
1082
1083 pub fn provide(providers: &mut Providers) {
1084     *providers = Providers {
1085         mir_const_qualif,
1086         ..*providers
1087     };
1088 }
1089
1090 fn mir_const_qualif<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
1091                               def_id: DefId)
1092                               -> (u8, Rc<IdxSetBuf<Local>>) {
1093     // NB: This `borrow()` is guaranteed to be valid (i.e., the value
1094     // cannot yet be stolen), because `mir_validated()`, which steals
1095     // from `mir_const(), forces this query to execute before
1096     // performing the steal.
1097     let mir = &tcx.mir_const(def_id).borrow();
1098
1099     if mir.return_ty().references_error() {
1100         tcx.sess.delay_span_bug(mir.span, "mir_const_qualif: Mir had errors");
1101         return (Qualif::NOT_CONST.bits(), Rc::new(IdxSetBuf::new_empty(0)));
1102     }
1103
1104     let mut qualifier = Qualifier::new(tcx, def_id, mir, Mode::Const);
1105     let (qualif, promoted_temps) = qualifier.qualify_const();
1106     (qualif.bits(), promoted_temps)
1107 }
1108
1109 pub struct QualifyAndPromoteConstants;
1110
1111 impl MirPass for QualifyAndPromoteConstants {
1112     fn run_pass<'a, 'tcx>(&self,
1113                           tcx: TyCtxt<'a, 'tcx, 'tcx>,
1114                           src: MirSource,
1115                           mir: &mut Mir<'tcx>) {
1116         // There's not really any point in promoting errorful MIR.
1117         if mir.return_ty().references_error() {
1118             tcx.sess.delay_span_bug(mir.span, "QualifyAndPromoteConstants: Mir had errors");
1119             return;
1120         }
1121
1122         if src.promoted.is_some() {
1123             return;
1124         }
1125
1126         let def_id = src.def_id;
1127         let id = tcx.hir.as_local_node_id(def_id).unwrap();
1128         let mut const_promoted_temps = None;
1129         let mode = match tcx.hir.body_owner_kind(id) {
1130             hir::BodyOwnerKind::Fn => {
1131                 if tcx.is_const_fn(def_id) {
1132                     Mode::ConstFn
1133                 } else {
1134                     Mode::Fn
1135                 }
1136             }
1137             hir::BodyOwnerKind::Const => {
1138                 const_promoted_temps = Some(tcx.mir_const_qualif(def_id).1);
1139                 Mode::Const
1140             }
1141             hir::BodyOwnerKind::Static(hir::MutImmutable) => Mode::Static,
1142             hir::BodyOwnerKind::Static(hir::MutMutable) => Mode::StaticMut,
1143         };
1144
1145         if mode == Mode::Fn || mode == Mode::ConstFn {
1146             // This is ugly because Qualifier holds onto mir,
1147             // which can't be mutated until its scope ends.
1148             let (temps, candidates) = {
1149                 let mut qualifier = Qualifier::new(tcx, def_id, mir, mode);
1150                 if mode == Mode::ConstFn {
1151                     // Enforce a constant-like CFG for `const fn`.
1152                     qualifier.qualify_const();
1153                 } else {
1154                     while let Some((bb, data)) = qualifier.rpo.next() {
1155                         qualifier.visit_basic_block_data(bb, data);
1156                     }
1157                 }
1158
1159                 (qualifier.temp_promotion_state, qualifier.promotion_candidates)
1160             };
1161
1162             // Do the actual promotion, now that we know what's viable.
1163             promote_consts::promote_candidates(mir, tcx, temps, candidates);
1164         } else {
1165             let promoted_temps = if mode == Mode::Const {
1166                 // Already computed by `mir_const_qualif`.
1167                 const_promoted_temps.unwrap()
1168             } else {
1169                 Qualifier::new(tcx, def_id, mir, mode).qualify_const().1
1170             };
1171
1172             // In `const` and `static` everything without `StorageDead`
1173             // is `'static`, we don't have to create promoted MIR fragments,
1174             // just remove `Drop` and `StorageDead` on "promoted" locals.
1175             for block in mir.basic_blocks_mut() {
1176                 block.statements.retain(|statement| {
1177                     match statement.kind {
1178                         StatementKind::StorageDead(index) => {
1179                             !promoted_temps.contains(&index)
1180                         }
1181                         _ => true
1182                     }
1183                 });
1184                 let terminator = block.terminator_mut();
1185                 match terminator.kind {
1186                     TerminatorKind::Drop { location: Place::Local(index), target, .. } => {
1187                         if promoted_temps.contains(&index) {
1188                             terminator.kind = TerminatorKind::Goto {
1189                                 target,
1190                             };
1191                         }
1192                     }
1193                     _ => {}
1194                 }
1195             }
1196         }
1197
1198         // Statics must be Sync.
1199         if mode == Mode::Static {
1200             // `#[thread_local]` statics don't have to be `Sync`.
1201             for attr in &tcx.get_attrs(def_id)[..] {
1202                 if attr.check_name("thread_local") {
1203                     return;
1204                 }
1205             }
1206             let ty = mir.return_ty();
1207             tcx.infer_ctxt().enter(|infcx| {
1208                 let param_env = ty::ParamEnv::empty(Reveal::UserFacing);
1209                 let cause = traits::ObligationCause::new(mir.span, id, traits::SharedStatic);
1210                 let mut fulfillment_cx = traits::FulfillmentContext::new();
1211                 fulfillment_cx.register_bound(&infcx,
1212                                               param_env,
1213                                               ty,
1214                                               tcx.require_lang_item(lang_items::SyncTraitLangItem),
1215                                               cause);
1216                 if let Err(err) = fulfillment_cx.select_all_or_error(&infcx) {
1217                     infcx.report_fulfillment_errors(&err, None);
1218                 }
1219             });
1220         }
1221     }
1222 }
1223
1224 fn args_required_const(tcx: TyCtxt, def_id: DefId) -> Option<FxHashSet<usize>> {
1225     let attrs = tcx.get_attrs(def_id);
1226     let attr = attrs.iter().find(|a| a.check_name("rustc_args_required_const"))?;
1227     let mut ret = FxHashSet();
1228     for meta in attr.meta_item_list()? {
1229         match meta.literal()?.node {
1230             LitKind::Int(a, _) => { ret.insert(a as usize); }
1231             _ => return None,
1232         }
1233     }
1234     Some(ret)
1235 }