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_vec::{IndexVec, Idx};
  19 use rustc::dep_graph::DepNode;
  20 use rustc::hir;
  21 use rustc::hir::def_id::DefId;
  22 use rustc::hir::intravisit::FnKind;
  23 use rustc::hir::map::blocks::FnLikeNode;
  24 use rustc::traits::{self, Reveal};
  25 use rustc::ty::{self, TyCtxt, Ty};
  26 use rustc::ty::cast::CastTy;
  27 use rustc::mir::repr::*;
  28 use rustc::mir::mir_map::MirMap;
  29 use rustc::mir::traversal::{self, ReversePostorder};
  30 use rustc::mir::transform::{Pass, MirMapPass, MirPassHook, MirSource};
  31 use rustc::mir::visit::{LvalueContext, Visitor};
  32 use rustc::util::nodemap::DefIdMap;
  33 use syntax::abi::Abi;
  34 use syntax::feature_gate::UnstableFeatures;
  35 use syntax_pos::Span;
  36
  37 use std::collections::hash_map::Entry;
  38 use std::fmt;
  39
  40 use build::Location;
  41
  42 use super::promote_consts::{self, Candidate, TempState};
  43
  44 bitflags! {
  45     flags Qualif: u8 {
  46         // Const item's qualification while recursing.
  47         // Recursive consts are an error.
  48         const RECURSIVE         = 1 << 0,
  49
  50         // Constant containing interior mutability (UnsafeCell).
  51         const MUTABLE_INTERIOR  = 1 << 1,
  52
  53         // Constant containing an ADT that implements Drop.
  54         const NEEDS_DROP        = 1 << 2,
  55
  56         // Function argument.
  57         const FN_ARGUMENT       = 1 << 3,
  58
  59         // Static lvalue or move from a static.
  60         const STATIC            = 1 << 4,
  61
  62         // Reference to a static.
  63         const STATIC_REF        = 1 << 5,
  64
  65         // Not constant at all - non-`const fn` calls, asm!,
  66         // pointer comparisons, ptr-to-int casts, etc.
  67         const NOT_CONST         = 1 << 6,
  68
  69         // Refers to temporaries which cannot be promoted as
  70         // promote_consts decided they weren't simple enough.
  71         const NOT_PROMOTABLE    = 1 << 7,
  72
  73         // Borrows of temporaries can be promoted only
  74         // if they have none of the above qualifications.
  75         const NEVER_PROMOTE     = !0,
  76
  77         // Const items can only have MUTABLE_INTERIOR
  78         // and NOT_PROMOTABLE without producing an error.
  79         const CONST_ERROR       = !Qualif::MUTABLE_INTERIOR.bits &
  80                                   !Qualif::NOT_PROMOTABLE.bits
  81     }
  82 }
  83
  84 impl<'a, 'tcx> Qualif {
  85     /// Remove flags which are impossible for the given type.
  86     fn restrict(&mut self, ty: Ty<'tcx>,
  87                 tcx: TyCtxt<'a, 'tcx, 'tcx>,
  88                 param_env: &ty::ParameterEnvironment<'tcx>) {
  89         if !ty.type_contents(tcx).interior_unsafe() {
  90             *self = *self - Qualif::MUTABLE_INTERIOR;
  91         }
  92         if !tcx.type_needs_drop_given_env(ty, param_env) {
  93             *self = *self - Qualif::NEEDS_DROP;
  94         }
  95     }
  96 }
  97
  98 /// What kind of item we are in.
  99 #[derive(Copy, Clone, PartialEq, Eq)]
 100 enum Mode {
 101     Const,
 102     Static,
 103     StaticMut,
 104     ConstFn,
 105     Fn
 106 }
 107
 108 impl fmt::Display for Mode {
 109     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 110         match *self {
 111             Mode::Const => write!(f, "constant"),
 112             Mode::Static | Mode::StaticMut => write!(f, "static"),
 113             Mode::ConstFn => write!(f, "constant function"),
 114             Mode::Fn => write!(f, "function")
 115         }
 116     }
 117 }
 118
 119 fn is_const_fn(tcx: TyCtxt, def_id: DefId) -> bool {
 120     if let Some(node_id) = tcx.map.as_local_node_id(def_id) {
 121         let fn_like = FnLikeNode::from_node(tcx.map.get(node_id));
 122         match fn_like.map(|f| f.kind()) {
 123             Some(FnKind::ItemFn(_, _, _, c, _, _, _)) => {
 124                 c == hir::Constness::Const
 125             }
 126             Some(FnKind::Method(_, m, _, _)) => {
 127                 m.constness == hir::Constness::Const
 128             }
 129             _ => false
 130         }
 131     } else {
 132         tcx.sess.cstore.is_const_fn(def_id)
 133     }
 134 }
 135
 136 struct Qualifier<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 137     mode: Mode,
 138     span: Span,
 139     def_id: DefId,
 140     mir: &'a Mir<'tcx>,
 141     rpo: ReversePostorder<'a, 'tcx>,
 142     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 143     param_env: ty::ParameterEnvironment<'tcx>,
 144     qualif_map: &'a mut DefIdMap<Qualif>,
 145     mir_map: Option<&'a MirMap<'tcx>>,
 146     temp_qualif: IndexVec<Temp, Option<Qualif>>,
 147     return_qualif: Option<Qualif>,
 148     qualif: Qualif,
 149     const_fn_arg_vars: BitVector,
 150     location: Location,
 151     temp_promotion_state: IndexVec<Temp, TempState>,
 152     promotion_candidates: Vec<Candidate>
 153 }
 154
 155 impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
 156     fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 157            param_env: ty::ParameterEnvironment<'tcx>,
 158            qualif_map: &'a mut DefIdMap<Qualif>,
 159            mir_map: Option<&'a MirMap<'tcx>>,
 160            def_id: DefId,
 161            mir: &'a Mir<'tcx>,
 162            mode: Mode)
 163            -> Qualifier<'a, 'tcx, 'tcx> {
 164         let mut rpo = traversal::reverse_postorder(mir);
 165         let temps = promote_consts::collect_temps(mir, &mut rpo);
 166         rpo.reset();
 167         Qualifier {
 168             mode: mode,
 169             span: mir.span,
 170             def_id: def_id,
 171             mir: mir,
 172             rpo: rpo,
 173             tcx: tcx,
 174             param_env: param_env,
 175             qualif_map: qualif_map,
 176             mir_map: mir_map,
 177             temp_qualif: IndexVec::from_elem(None, &mir.temp_decls),
 178             return_qualif: None,
 179             qualif: Qualif::empty(),
 180             const_fn_arg_vars: BitVector::new(mir.var_decls.len()),
 181             location: Location {
 182                 block: START_BLOCK,
 183                 statement_index: 0
 184             },
 185             temp_promotion_state: temps,
 186             promotion_candidates: vec![]
 187         }
 188     }
 189
 190     // FIXME(eddyb) we could split the errors into meaningful
 191     // categories, but enabling full miri would make that
 192     // slightly pointless (even with feature-gating).
 193     fn not_const(&mut self) {
 194         self.add(Qualif::NOT_CONST);
 195         if self.mode != Mode::Fn {
 196             span_err!(self.tcx.sess, self.span, E0019,
 197                       "{} contains unimplemented expression type", self.mode);
 198         }
 199     }
 200
 201     /// Error about extra statements in a constant.
 202     fn statement_like(&mut self) {
 203         self.add(Qualif::NOT_CONST);
 204         if self.mode != Mode::Fn {
 205             span_err!(self.tcx.sess, self.span, E0016,
 206                       "blocks in {}s are limited to items and tail expressions",
 207                       self.mode);
 208         }
 209     }
 210
 211     /// Add the given qualification to self.qualif.
 212     fn add(&mut self, qualif: Qualif) {
 213         self.qualif = self.qualif | qualif;
 214     }
 215
 216     /// Add the given type's qualification to self.qualif.
 217     fn add_type(&mut self, ty: Ty<'tcx>) {
 218         self.add(Qualif::MUTABLE_INTERIOR | Qualif::NEEDS_DROP);
 219         self.qualif.restrict(ty, self.tcx, &self.param_env);
 220     }
 221
 222     /// Within the provided closure, self.qualif will start
 223     /// out empty, and its value after the closure returns will
 224     /// be combined with the value before the call to nest.
 225     fn nest<F: FnOnce(&mut Self)>(&mut self, f: F) {
 226         let original = self.qualif;
 227         self.qualif = Qualif::empty();
 228         f(self);
 229         self.add(original);
 230     }
 231
 232     /// Check for NEEDS_DROP (from an ADT or const fn call) and
 233     /// error, unless we're in a function, or the feature-gate
 234     /// for globals with destructors is enabled.
 235     fn deny_drop(&self) {
 236         if self.mode == Mode::Fn || !self.qualif.intersects(Qualif::NEEDS_DROP) {
 237             return;
 238         }
 239
 240         // Static and const fn's allow destructors, but they're feature-gated.
 241         let msg = if self.mode != Mode::Const {
 242             // Feature-gate for globals with destructors is enabled.
 243             if self.tcx.sess.features.borrow().drop_types_in_const {
 244                 return;
 245             }
 246
 247             // This comes from a macro that has #[allow_internal_unstable].
 248             if self.tcx.sess.codemap().span_allows_unstable(self.span) {
 249                 return;
 250             }
 251
 252             format!("destructors in {}s are an unstable feature",
 253                     self.mode)
 254         } else {
 255             format!("{}s are not allowed to have destructors",
 256                     self.mode)
 257         };
 258
 259         let mut err =
 260             struct_span_err!(self.tcx.sess, self.span, E0493, "{}", msg);
 261         if self.mode != Mode::Const {
 262             help!(&mut err,
 263                   "in Nightly builds, add `#![feature(drop_types_in_const)]` \
 264                    to the crate attributes to enable");
 265         }
 266         err.emit();
 267     }
 268
 269     /// Check if an Lvalue with the current qualifications could
 270     /// be consumed, by either an operand or a Deref projection.
 271     fn try_consume(&mut self) -> bool {
 272         if self.qualif.intersects(Qualif::STATIC) && self.mode != Mode::Fn {
 273             let msg = if self.mode == Mode::Static ||
 274                          self.mode == Mode::StaticMut {
 275                 "cannot refer to other statics by value, use the \
 276                  address-of operator or a constant instead"
 277             } else {
 278                 "cannot refer to statics by value, use a constant instead"
 279             };
 280             span_err!(self.tcx.sess, self.span, E0394, "{}", msg);
 281
 282             // Replace STATIC with NOT_CONST to avoid further errors.
 283             self.qualif = self.qualif - Qualif::STATIC;
 284             self.add(Qualif::NOT_CONST);
 285
 286             false
 287         } else {
 288             true
 289         }
 290     }
 291
 292     /// Assign the current qualification to the given destination.
 293     fn assign(&mut self, dest: &Lvalue<'tcx>) {
 294         let qualif = self.qualif;
 295         let span = self.span;
 296         let store = |slot: &mut Option<Qualif>| {
 297             if slot.is_some() {
 298                 span_bug!(span, "multiple assignments to {:?}", dest);
 299             }
 300             *slot = Some(qualif);
 301         };
 302
 303         // Only handle promotable temps in non-const functions.
 304         if self.mode == Mode::Fn {
 305             if let Lvalue::Temp(index) = *dest {
 306                 if self.temp_promotion_state[index].is_promotable() {
 307                     store(&mut self.temp_qualif[index]);
 308                 }
 309             }
 310             return;
 311         }
 312
 313         match *dest {
 314             Lvalue::Temp(index) => store(&mut self.temp_qualif[index]),
 315             Lvalue::ReturnPointer => store(&mut self.return_qualif),
 316
 317             Lvalue::Projection(box Projection {
 318                 base: Lvalue::Temp(index),
 319                 elem: ProjectionElem::Deref
 320             }) if self.mir.temp_decls[index].ty.is_unique()
 321                && self.temp_qualif[index].map_or(false, |qualif| {
 322                     qualif.intersects(Qualif::NOT_CONST)
 323                }) => {
 324                 // Part of `box expr`, we should've errored
 325                 // already for the Box allocation Rvalue.
 326             }
 327
 328             // This must be an explicit assignment.
 329             _ => {
 330                 // Catch more errors in the destination.
 331                 self.visit_lvalue(dest, LvalueContext::Store);
 332                 self.statement_like();
 333             }
 334         }
 335     }
 336
 337     /// Qualify a whole const, static initializer or const fn.
 338     fn qualify_const(&mut self) -> Qualif {
 339         let mir = self.mir;
 340
 341         let mut seen_blocks = BitVector::new(mir.basic_blocks().len());
 342         let mut bb = START_BLOCK;
 343         loop {
 344             seen_blocks.insert(bb.index());
 345
 346             self.visit_basic_block_data(bb, &mir[bb]);
 347
 348             let target = match mir[bb].terminator().kind {
 349                 TerminatorKind::Goto { target } |
 350                 // Drops are considered noops.
 351                 TerminatorKind::Drop { target, .. } |
 352                 TerminatorKind::Assert { target, .. } |
 353                 TerminatorKind::Call { destination: Some((_, target)), .. } => {
 354                     Some(target)
 355                 }
 356
 357                 // Non-terminating calls cannot produce any value.
 358                 TerminatorKind::Call { destination: None, .. } => {
 359                     return Qualif::empty();
 360                 }
 361
 362                 TerminatorKind::If {..} |
 363                 TerminatorKind::Switch {..} |
 364                 TerminatorKind::SwitchInt {..} |
 365                 TerminatorKind::DropAndReplace { .. } |
 366                 TerminatorKind::Resume |
 367                 TerminatorKind::Unreachable => None,
 368
 369                 TerminatorKind::Return => {
 370                     // Check for unused values. This usually means
 371                     // there are extra statements in the AST.
 372                     for temp in mir.temp_decls.indices() {
 373                         if self.temp_qualif[temp].is_none() {
 374                             continue;
 375                         }
 376
 377                         let state = self.temp_promotion_state[temp];
 378                         if let TempState::Defined { location, uses: 0 } = state {
 379                             let data = &mir[location.block];
 380                             let stmt_idx = location.statement_index;
 381
 382                             // Get the span for the initialization.
 383                             let source_info = if stmt_idx < data.statements.len() {
 384                                 data.statements[stmt_idx].source_info
 385                             } else {
 386                                 data.terminator().source_info
 387                             };
 388                             self.span = source_info.span;
 389
 390                             // Treat this as a statement in the AST.
 391                             self.statement_like();
 392                         }
 393                     }
 394
 395                     // Make sure there are no extra unassigned variables.
 396                     self.qualif = Qualif::NOT_CONST;
 397                     for index in 0..mir.var_decls.len() {
 398                         if !self.const_fn_arg_vars.contains(index) {
 399                             self.assign(&Lvalue::Var(Var::new(index)));
 400                         }
 401                     }
 402
 403                     break;
 404                 }
 405             };
 406
 407             match target {
 408                 // No loops allowed.
 409                 Some(target) if !seen_blocks.contains(target.index()) => {
 410                     bb = target;
 411                 }
 412                 _ => {
 413                     self.not_const();
 414                     break;
 415                 }
 416             }
 417         }
 418
 419         let return_ty = mir.return_ty;
 420         self.qualif = self.return_qualif.unwrap_or(Qualif::NOT_CONST);
 421
 422         match self.mode {
 423             Mode::StaticMut => {
 424                 // Check for destructors in static mut.
 425                 self.add_type(return_ty);
 426                 self.deny_drop();
 427             }
 428             _ => {
 429                 // Account for errors in consts by using the
 430                 // conservative type qualification instead.
 431                 if self.qualif.intersects(Qualif::CONST_ERROR) {
 432                     self.qualif = Qualif::empty();
 433                     self.add_type(return_ty);
 434                 }
 435             }
 436         }
 437         self.qualif
 438     }
 439 }
 440
 441 /// Accumulates an Rvalue or Call's effects in self.qualif.
 442 /// For functions (constant or not), it also records
 443 /// candidates for promotion in promotion_candidates.
 444 impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
 445     fn visit_lvalue(&mut self, lvalue: &Lvalue<'tcx>, context: LvalueContext) {
 446         match *lvalue {
 447             Lvalue::Arg(_) => {
 448                 self.add(Qualif::FN_ARGUMENT);
 449             }
 450             Lvalue::Var(_) => {
 451                 self.add(Qualif::NOT_CONST);
 452             }
 453             Lvalue::Temp(index) => {
 454                 if !self.temp_promotion_state[index].is_promotable() {
 455                     self.add(Qualif::NOT_PROMOTABLE);
 456                 }
 457
 458                 if let Some(qualif) = self.temp_qualif[index] {
 459                     self.add(qualif);
 460                 } else {
 461                     self.not_const();
 462                 }
 463             }
 464             Lvalue::Static(_) => {
 465                 self.add(Qualif::STATIC);
 466                 if self.mode == Mode::Const || self.mode == Mode::ConstFn {
 467                     span_err!(self.tcx.sess, self.span, E0013,
 468                               "{}s cannot refer to statics, use \
 469                                a constant instead", self.mode);
 470                 }
 471             }
 472             Lvalue::ReturnPointer => {
 473                 self.not_const();
 474             }
 475             Lvalue::Projection(ref proj) => {
 476                 self.nest(|this| {
 477                     this.super_lvalue(lvalue, context);
 478                     match proj.elem {
 479                         ProjectionElem::Deref => {
 480                             if !this.try_consume() {
 481                                 return;
 482                             }
 483
 484                             if this.qualif.intersects(Qualif::STATIC_REF) {
 485                                 this.qualif = this.qualif - Qualif::STATIC_REF;
 486                                 this.add(Qualif::STATIC);
 487                             }
 488
 489                             let base_ty = proj.base.ty(this.mir, this.tcx).to_ty(this.tcx);
 490                             if let ty::TyRawPtr(_) = base_ty.sty {
 491                                 this.add(Qualif::NOT_CONST);
 492                                 if this.mode != Mode::Fn {
 493                                     struct_span_err!(this.tcx.sess,
 494                                         this.span, E0396,
 495                                         "raw pointers cannot be dereferenced in {}s",
 496                                         this.mode)
 497                                     .span_label(this.span,
 498                                         &format!("dereference of raw pointer in constant"))
 499                                     .emit();
 500                                 }
 501                             }
 502                         }
 503
 504                         ProjectionElem::Field(..) |
 505                         ProjectionElem::Index(_) => {
 506                             if this.mode != Mode::Fn &&
 507                                this.qualif.intersects(Qualif::STATIC) {
 508                                 span_err!(this.tcx.sess, this.span, E0494,
 509                                           "cannot refer to the interior of another \
 510                                            static, use a constant instead");
 511                             }
 512                             let ty = lvalue.ty(this.mir, this.tcx).to_ty(this.tcx);
 513                             this.qualif.restrict(ty, this.tcx, &this.param_env);
 514                         }
 515
 516                         ProjectionElem::ConstantIndex {..} |
 517                         ProjectionElem::Subslice {..} |
 518                         ProjectionElem::Downcast(..) => {
 519                             this.not_const()
 520                         }
 521                     }
 522                 });
 523             }
 524         }
 525     }
 526
 527     fn visit_operand(&mut self, operand: &Operand<'tcx>) {
 528         match *operand {
 529             Operand::Consume(_) => {
 530                 self.nest(|this| {
 531                     this.super_operand(operand);
 532                     this.try_consume();
 533                 });
 534             }
 535             Operand::Constant(ref constant) => {
 536                 // Only functions and methods can have these types.
 537                 if let ty::TyFnDef(..) = constant.ty.sty {
 538                     return;
 539                 }
 540
 541                 if let Literal::Item { def_id, substs } = constant.literal {
 542                     // Don't peek inside generic (associated) constants.
 543                     if !substs.types.is_empty() {
 544                         self.add_type(constant.ty);
 545                     } else {
 546                         let qualif = qualify_const_item_cached(self.tcx,
 547                                                                self.qualif_map,
 548                                                                self.mir_map,
 549                                                                def_id);
 550                         self.add(qualif);
 551                     }
 552
 553                     // FIXME(eddyb) check recursive constants here,
 554                     // instead of rustc_passes::static_recursion.
 555                     if self.qualif.intersects(Qualif::RECURSIVE) {
 556                         span_bug!(constant.span,
 557                                   "recursive constant wasn't caught earlier");
 558                     }
 559
 560                     // Let `const fn` transitively have destructors,
 561                     // but they do get stopped in `const` or `static`.
 562                     if self.mode != Mode::ConstFn {
 563                         self.deny_drop();
 564                     }
 565                 }
 566             }
 567         }
 568     }
 569
 570     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>) {
 571         // Recurse through operands and lvalues.
 572         self.super_rvalue(rvalue);
 573
 574         match *rvalue {
 575             Rvalue::Use(_) |
 576             Rvalue::Repeat(..) |
 577             Rvalue::UnaryOp(..) |
 578             Rvalue::CheckedBinaryOp(..) |
 579             Rvalue::Cast(CastKind::ReifyFnPointer, _, _) |
 580             Rvalue::Cast(CastKind::UnsafeFnPointer, _, _) |
 581             Rvalue::Cast(CastKind::Unsize, _, _) => {}
 582
 583             Rvalue::Len(_) => {
 584                 // Static lvalues in consts would have errored already,
 585                 // don't treat length checks as reads from statics.
 586                 self.qualif = self.qualif - Qualif::STATIC;
 587             }
 588
 589             Rvalue::Ref(_, kind, ref lvalue) => {
 590                 // Static lvalues in consts would have errored already,
 591                 // only keep track of references to them here.
 592                 if self.qualif.intersects(Qualif::STATIC) {
 593                     self.qualif = self.qualif - Qualif::STATIC;
 594                     self.add(Qualif::STATIC_REF);
 595                 }
 596
 597                 let ty = lvalue.ty(self.mir, self.tcx).to_ty(self.tcx);
 598                 if kind == BorrowKind::Mut {
 599                     // In theory, any zero-sized value could be borrowed
 600                     // mutably without consequences. However, only &mut []
 601                     // is allowed right now, and only in functions.
 602                     let allow = if self.mode == Mode::StaticMut {
 603                         // Inside a `static mut`, &mut [...] is also allowed.
 604                         match ty.sty {
 605                             ty::TyArray(..) | ty::TySlice(_) => {
 606                                 // Mutating can expose drops, be conservative.
 607                                 self.add_type(ty);
 608                                 self.deny_drop();
 609                                 true
 610                             }
 611                             _ => false
 612                         }
 613                     } else if let ty::TyArray(_, 0) = ty.sty {
 614                         self.mode == Mode::Fn
 615                     } else {
 616                         false
 617                     };
 618
 619                     if !allow {
 620                         self.add(Qualif::NOT_CONST);
 621                         if self.mode != Mode::Fn {
 622                             struct_span_err!(self.tcx.sess,  self.span, E0017,
 623                                              "references in {}s may only refer \
 624                                               to immutable values", self.mode)
 625                                 .span_label(self.span, &format!("{}s require immutable values",
 626                                                                 self.mode))
 627                                 .emit();
 628                         }
 629                     }
 630                 } else {
 631                     // Constants cannot be borrowed if they contain interior mutability as
 632                     // it means that our "silent insertion of statics" could change
 633                     // initializer values (very bad).
 634                     if self.qualif.intersects(Qualif::MUTABLE_INTERIOR) {
 635                         // Replace MUTABLE_INTERIOR with NOT_CONST to avoid
 636                         // duplicate errors (from reborrowing, for example).
 637                         self.qualif = self.qualif - Qualif::MUTABLE_INTERIOR;
 638                         self.add(Qualif::NOT_CONST);
 639                         if self.mode != Mode::Fn {
 640                             span_err!(self.tcx.sess, self.span, E0492,
 641                                       "cannot borrow a constant which contains \
 642                                        interior mutability, create a static instead");
 643                         }
 644                     }
 645                 }
 646
 647                 // We might have a candidate for promotion.
 648                 let candidate = Candidate::Ref(self.location);
 649                 if self.mode == Mode::Fn || self.mode == Mode::ConstFn {
 650                     if !self.qualif.intersects(Qualif::NEVER_PROMOTE) {
 651                         // We can only promote direct borrows of temps.
 652                         if let Lvalue::Temp(_) = *lvalue {
 653                             self.promotion_candidates.push(candidate);
 654                         }
 655                     }
 656                 }
 657             }
 658
 659             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {
 660                 let operand_ty = operand.ty(self.mir, self.tcx);
 661                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 662                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 663                 match (cast_in, cast_out) {
 664                     (CastTy::Ptr(_), CastTy::Int(_)) |
 665                     (CastTy::FnPtr, CastTy::Int(_)) => {
 666                         self.add(Qualif::NOT_CONST);
 667                         if self.mode != Mode::Fn {
 668                             span_err!(self.tcx.sess, self.span, E0018,
 669                                       "raw pointers cannot be cast to integers in {}s",
 670                                       self.mode);
 671                         }
 672                     }
 673                     _ => {}
 674                 }
 675             }
 676
 677             Rvalue::BinaryOp(op, ref lhs, _) => {
 678                 if let ty::TyRawPtr(_) = lhs.ty(self.mir, self.tcx).sty {
 679                     assert!(op == BinOp::Eq || op == BinOp::Ne ||
 680                             op == BinOp::Le || op == BinOp::Lt ||
 681                             op == BinOp::Ge || op == BinOp::Gt);
 682
 683                     self.add(Qualif::NOT_CONST);
 684                     if self.mode != Mode::Fn {
 685                         span_err!(self.tcx.sess, self.span, E0395,
 686                                   "raw pointers cannot be compared in {}s",
 687                                   self.mode);
 688                     }
 689                 }
 690             }
 691
 692             Rvalue::Box(_) => {
 693                 self.add(Qualif::NOT_CONST);
 694                 if self.mode != Mode::Fn {
 695                     struct_span_err!(self.tcx.sess, self.span, E0010,
 696                                      "allocations are not allowed in {}s", self.mode)
 697                         .span_label(self.span, &format!("allocation not allowed in {}s", self.mode))
 698                         .emit();
 699                 }
 700             }
 701
 702             Rvalue::Aggregate(ref kind, _) => {
 703                 if let AggregateKind::Adt(def, _, _) = *kind {
 704                     if def.has_dtor() {
 705                         self.add(Qualif::NEEDS_DROP);
 706                         self.deny_drop();
 707                     }
 708
 709                     if Some(def.did) == self.tcx.lang_items.unsafe_cell_type() {
 710                         let ty = rvalue.ty(self.mir, self.tcx).unwrap();
 711                         self.add_type(ty);
 712                         assert!(self.qualif.intersects(Qualif::MUTABLE_INTERIOR));
 713                         // Even if the value inside may not need dropping,
 714                         // mutating it would change that.
 715                         if !self.qualif.intersects(Qualif::NOT_CONST) {
 716                             self.deny_drop();
 717                         }
 718                     }
 719                 }
 720             }
 721
 722             Rvalue::InlineAsm {..} => {
 723                 self.not_const();
 724             }
 725         }
 726     }
 727
 728     fn visit_terminator_kind(&mut self, bb: BasicBlock, kind: &TerminatorKind<'tcx>) {
 729         if let TerminatorKind::Call { ref func, ref args, ref destination, .. } = *kind {
 730             self.visit_operand(func);
 731
 732             let fn_ty = func.ty(self.mir, self.tcx);
 733             let (is_shuffle, is_const_fn) = match fn_ty.sty {
 734                 ty::TyFnDef(def_id, _, f) => {
 735                     (f.abi == Abi::PlatformIntrinsic &&
 736                      self.tcx.item_name(def_id).as_str().starts_with("simd_shuffle"),
 737                      is_const_fn(self.tcx, def_id))
 738                 }
 739                 _ => (false, false)
 740             };
 741
 742             for (i, arg) in args.iter().enumerate() {
 743                 self.nest(|this| {
 744                     this.visit_operand(arg);
 745                     if is_shuffle && i == 2 && this.mode == Mode::Fn {
 746                         let candidate = Candidate::ShuffleIndices(bb);
 747                         if !this.qualif.intersects(Qualif::NEVER_PROMOTE) {
 748                             this.promotion_candidates.push(candidate);
 749                         } else {
 750                             span_err!(this.tcx.sess, this.span, E0526,
 751                                       "shuffle indices are not constant");
 752                         }
 753                     }
 754                 });
 755             }
 756
 757             // Const fn calls.
 758             if is_const_fn {
 759                 // We are in a const or static initializer,
 760                 if self.mode != Mode::Fn &&
 761
 762                     // feature-gate is not enabled,
 763                     !self.tcx.sess.features.borrow().const_fn &&
 764
 765                     // this doesn't come from a crate with the feature-gate enabled,
 766                     self.def_id.is_local() &&
 767
 768                     // this doesn't come from a macro that has #[allow_internal_unstable]
 769                     !self.tcx.sess.codemap().span_allows_unstable(self.span)
 770                 {
 771                     let mut err = self.tcx.sess.struct_span_err(self.span,
 772                         "const fns are an unstable feature");
 773                     help!(&mut err,
 774                           "in Nightly builds, add `#![feature(const_fn)]` \
 775                            to the crate attributes to enable");
 776                     err.emit();
 777                 }
 778             } else {
 779                 self.qualif = Qualif::NOT_CONST;
 780                 if self.mode != Mode::Fn {
 781                     // FIXME(#24111) Remove this check when const fn stabilizes
 782                     let (msg, note) = if let UnstableFeatures::Disallow =
 783                             self.tcx.sess.opts.unstable_features {
 784                         (format!("calls in {}s are limited to \
 785                                   struct and enum constructors",
 786                                  self.mode),
 787                          Some("a limited form of compile-time function \
 788                                evaluation is available on a nightly \
 789                                compiler via `const fn`"))
 790                     } else {
 791                         (format!("calls in {}s are limited \
 792                                   to constant functions, \
 793                                   struct and enum constructors",
 794                                  self.mode),
 795                          None)
 796                     };
 797                     let mut err = struct_span_err!(self.tcx.sess, self.span, E0015, "{}", msg);
 798                     if let Some(note) = note {
 799                         err.span_note(self.span, note);
 800                     }
 801                     err.emit();
 802                 }
 803             }
 804
 805             if let Some((ref dest, _)) = *destination {
 806                 // Avoid propagating irrelevant callee/argument qualifications.
 807                 if self.qualif.intersects(Qualif::CONST_ERROR) {
 808                     self.qualif = Qualif::NOT_CONST;
 809                 } else {
 810                     // Be conservative about the returned value of a const fn.
 811                     let tcx = self.tcx;
 812                     let ty = dest.ty(self.mir, tcx).to_ty(tcx);
 813                     self.qualif = Qualif::empty();
 814                     self.add_type(ty);
 815
 816                     // Let `const fn` transitively have destructors,
 817                     // but they do get stopped in `const` or `static`.
 818                     if self.mode != Mode::ConstFn {
 819                         self.deny_drop();
 820                     }
 821                 }
 822                 self.assign(dest);
 823             }
 824         } else {
 825             // Qualify any operands inside other terminators.
 826             self.super_terminator_kind(bb, kind);
 827         }
 828     }
 829
 830     fn visit_assign(&mut self, _: BasicBlock, dest: &Lvalue<'tcx>, rvalue: &Rvalue<'tcx>) {
 831         self.visit_rvalue(rvalue);
 832
 833         // Check the allowed const fn argument forms.
 834         if let (Mode::ConstFn, &Lvalue::Var(index)) = (self.mode, dest) {
 835             if self.const_fn_arg_vars.insert(index.index()) {
 836                 // Direct use of an argument is permitted.
 837                 if let Rvalue::Use(Operand::Consume(Lvalue::Arg(_))) = *rvalue {
 838                     return;
 839                 }
 840
 841                 // Avoid a generic error for other uses of arguments.
 842                 if self.qualif.intersects(Qualif::FN_ARGUMENT) {
 843                     let decl = &self.mir.var_decls[index];
 844                     span_err!(self.tcx.sess, decl.source_info.span, E0022,
 845                               "arguments of constant functions can only \
 846                                be immutable by-value bindings");
 847                     return;
 848                 }
 849             }
 850         }
 851
 852         self.assign(dest);
 853     }
 854
 855     fn visit_source_info(&mut self, source_info: &SourceInfo) {
 856         self.span = source_info.span;
 857     }
 858
 859     fn visit_statement(&mut self, bb: BasicBlock, statement: &Statement<'tcx>) {
 860         assert_eq!(self.location.block, bb);
 861         self.nest(|this| {
 862             this.visit_source_info(&statement.source_info);
 863             match statement.kind {
 864                 StatementKind::Assign(ref lvalue, ref rvalue) => {
 865                     this.visit_assign(bb, lvalue, rvalue);
 866                 }
 867                 StatementKind::SetDiscriminant { .. } |
 868                 StatementKind::StorageLive(_) |
 869                 StatementKind::StorageDead(_) => {}
 870             }
 871         });
 872         self.location.statement_index += 1;
 873     }
 874
 875     fn visit_terminator(&mut self, bb: BasicBlock, terminator: &Terminator<'tcx>) {
 876         assert_eq!(self.location.block, bb);
 877         self.nest(|this| this.super_terminator(bb, terminator));
 878     }
 879
 880     fn visit_basic_block_data(&mut self, bb: BasicBlock, data: &BasicBlockData<'tcx>) {
 881         self.location.statement_index = 0;
 882         self.location.block = bb;
 883         self.super_basic_block_data(bb, data);
 884     }
 885 }
 886
 887 fn qualify_const_item_cached<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 888                                        qualif_map: &mut DefIdMap<Qualif>,
 889                                        mir_map: Option<&MirMap<'tcx>>,
 890                                        def_id: DefId)
 891                                        -> Qualif {
 892     match qualif_map.entry(def_id) {
 893         Entry::Occupied(entry) => return *entry.get(),
 894         Entry::Vacant(entry) => {
 895             // Guard against `const` recursion.
 896             entry.insert(Qualif::RECURSIVE);
 897         }
 898     }
 899
 900     let extern_mir;
 901     let param_env_and_mir = if def_id.is_local() {
 902         mir_map.and_then(|map| map.map.get(&def_id)).map(|mir| {
 903             let node_id = tcx.map.as_local_node_id(def_id).unwrap();
 904             (ty::ParameterEnvironment::for_item(tcx, node_id), mir)
 905         })
 906     } else if let Some(mir) = tcx.sess.cstore.maybe_get_item_mir(tcx, def_id) {
 907         // These should only be monomorphic constants.
 908         extern_mir = mir;
 909         Some((tcx.empty_parameter_environment(), &extern_mir))
 910     } else {
 911         None
 912     };
 913
 914     let (param_env, mir) = param_env_and_mir.unwrap_or_else(|| {
 915         bug!("missing constant MIR for {}", tcx.item_path_str(def_id))
 916     });
 917
 918     let mut qualifier = Qualifier::new(tcx, param_env, qualif_map, mir_map,
 919                                        def_id, mir, Mode::Const);
 920     let qualif = qualifier.qualify_const();
 921     qualifier.qualif_map.insert(def_id, qualif);
 922     qualif
 923 }
 924
 925 pub struct QualifyAndPromoteConstants;
 926
 927 impl Pass for QualifyAndPromoteConstants {}
 928
 929 impl<'tcx> MirMapPass<'tcx> for QualifyAndPromoteConstants {
 930     fn run_pass<'a>(&mut self,
 931                     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 932                     map: &mut MirMap<'tcx>,
 933                     hooks: &mut [Box<for<'s> MirPassHook<'s>>]) {
 934         let mut qualif_map = DefIdMap();
 935
 936         // First, visit `const` items, potentially recursing, to get
 937         // accurate MUTABLE_INTERIOR and NEEDS_DROP qualifications.
 938         let keys = map.map.keys();
 939         for &def_id in &keys {
 940             let _task = tcx.dep_graph.in_task(DepNode::Mir(def_id));
 941             let id = tcx.map.as_local_node_id(def_id).unwrap();
 942             let src = MirSource::from_node(tcx, id);
 943             if let MirSource::Const(_) = src {
 944                 qualify_const_item_cached(tcx, &mut qualif_map, Some(map), def_id);
 945             }
 946         }
 947
 948         // Then, handle everything else, without recursing,
 949         // as the MIR map is not shared, since promotion
 950         // in functions (including `const fn`) mutates it.
 951         for &def_id in &keys {
 952             let _task = tcx.dep_graph.in_task(DepNode::Mir(def_id));
 953             let id = tcx.map.as_local_node_id(def_id).unwrap();
 954             let src = MirSource::from_node(tcx, id);
 955             let mode = match src {
 956                 MirSource::Fn(_) => {
 957                     if is_const_fn(tcx, def_id) {
 958                         Mode::ConstFn
 959                     } else {
 960                         Mode::Fn
 961                     }
 962                 }
 963                 MirSource::Const(_) => continue,
 964                 MirSource::Static(_, hir::MutImmutable) => Mode::Static,
 965                 MirSource::Static(_, hir::MutMutable) => Mode::StaticMut,
 966                 MirSource::Promoted(..) => bug!()
 967             };
 968             let param_env = ty::ParameterEnvironment::for_item(tcx, id);
 969
 970             let mir = map.map.get_mut(&def_id).unwrap();
 971             for hook in &mut *hooks {
 972                 hook.on_mir_pass(tcx, src, mir, self, false);
 973             }
 974
 975             if mode == Mode::Fn || mode == Mode::ConstFn {
 976                 // This is ugly because Qualifier holds onto mir,
 977                 // which can't be mutated until its scope ends.
 978                 let (temps, candidates) = {
 979                     let mut qualifier = Qualifier::new(tcx, param_env, &mut qualif_map,
 980                                                        None, def_id, mir, mode);
 981                     if mode == Mode::ConstFn {
 982                         // Enforce a constant-like CFG for `const fn`.
 983                         qualifier.qualify_const();
 984                     } else {
 985                         while let Some((bb, data)) = qualifier.rpo.next() {
 986                             qualifier.visit_basic_block_data(bb, data);
 987                         }
 988                     }
 989
 990                     (qualifier.temp_promotion_state,
 991                      qualifier.promotion_candidates)
 992                 };
 993
 994                 // Do the actual promotion, now that we know what's viable.
 995                 promote_consts::promote_candidates(mir, tcx, temps, candidates);
 996             } else {
 997                 let mut qualifier = Qualifier::new(tcx, param_env, &mut qualif_map,
 998                                                    None, def_id, mir, mode);
 999                 qualifier.qualify_const();
1000             }
1001
1002             for hook in &mut *hooks {
1003                 hook.on_mir_pass(tcx, src, mir, self, true);
1004             }
1005
1006             // Statics must be Sync.
1007             if mode == Mode::Static {
1008                 let ty = mir.return_ty;
1009                 tcx.infer_ctxt(None, None, Reveal::NotSpecializable).enter(|infcx| {
1010                     let cause = traits::ObligationCause::new(mir.span, id, traits::SharedStatic);
1011                     let mut fulfillment_cx = traits::FulfillmentContext::new();
1012                     fulfillment_cx.register_builtin_bound(&infcx, ty, ty::BoundSync, cause);
1013                     if let Err(err) = fulfillment_cx.select_all_or_error(&infcx) {
1014                         infcx.report_fulfillment_errors(&err);
1015                     }
1016
1017                     if let Err(errors) = fulfillment_cx.select_rfc1592_obligations(&infcx) {
1018                         infcx.report_fulfillment_errors_as_warnings(&errors, id);
1019                     }
1020                 });
1021             }
1022         }
1023     }
1024 }