compiler/rustc_const_eval/src/transform/check_consts/check.rs

   1 //! The `Visitor` responsible for actually checking a `mir::Body` for invalid operations.
   2
   3 use rustc_errors::{Applicability, Diagnostic, ErrorReported};
   4 use rustc_hir as hir;
   5 use rustc_hir::def_id::DefId;
   6 use rustc_index::bit_set::BitSet;
   7 use rustc_infer::infer::TyCtxtInferExt;
   8 use rustc_infer::traits::{ImplSource, Obligation, ObligationCause};
   9 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
  10 use rustc_middle::mir::*;
  11 use rustc_middle::ty::cast::CastTy;
  12 use rustc_middle::ty::subst::{GenericArgKind, InternalSubsts};
  13 use rustc_middle::ty::{self, adjustment::PointerCast, Instance, InstanceDef, Ty, TyCtxt};
  14 use rustc_middle::ty::{Binder, TraitPredicate, TraitRef, TypeFoldable};
  15 use rustc_mir_dataflow::{self, Analysis};
  16 use rustc_span::{sym, Span, Symbol};
  17 use rustc_trait_selection::traits::SelectionContext;
  18
  19 use std::mem;
  20 use std::ops::Deref;
  21
  22 use super::ops::{self, NonConstOp, Status};
  23 use super::qualifs::{self, CustomEq, HasMutInterior, NeedsDrop, NeedsNonConstDrop};
  24 use super::resolver::FlowSensitiveAnalysis;
  25 use super::{ConstCx, Qualif};
  26 use crate::const_eval::is_unstable_const_fn;
  27
  28 type QualifResults<'mir, 'tcx, Q> =
  29     rustc_mir_dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'mir, 'mir, 'tcx, Q>>;
  30
  31 #[derive(Default)]
  32 pub struct Qualifs<'mir, 'tcx> {
  33     has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
  34     needs_drop: Option<QualifResults<'mir, 'tcx, NeedsDrop>>,
  35     needs_non_const_drop: Option<QualifResults<'mir, 'tcx, NeedsNonConstDrop>>,
  36 }
  37
  38 impl<'mir, 'tcx> Qualifs<'mir, 'tcx> {
  39     /// Returns `true` if `local` is `NeedsDrop` at the given `Location`.
  40     ///
  41     /// Only updates the cursor if absolutely necessary
  42     pub fn needs_drop(
  43         &mut self,
  44         ccx: &'mir ConstCx<'mir, 'tcx>,
  45         local: Local,
  46         location: Location,
  47     ) -> bool {
  48         let ty = ccx.body.local_decls[local].ty;
  49         // Peeking into opaque types causes cycles if the current function declares said opaque
  50         // type. Thus we avoid short circuiting on the type and instead run the more expensive
  51         // analysis that looks at the actual usage within this function
  52         if !ty.has_opaque_types() && !NeedsDrop::in_any_value_of_ty(ccx, ty) {
  53             return false;
  54         }
  55
  56         let needs_drop = self.needs_drop.get_or_insert_with(|| {
  57             let ConstCx { tcx, body, .. } = *ccx;
  58
  59             FlowSensitiveAnalysis::new(NeedsDrop, ccx)
  60                 .into_engine(tcx, &body)
  61                 .iterate_to_fixpoint()
  62                 .into_results_cursor(&body)
  63         });
  64
  65         needs_drop.seek_before_primary_effect(location);
  66         needs_drop.get().contains(local)
  67     }
  68
  69     /// Returns `true` if `local` is `NeedsNonConstDrop` at the given `Location`.
  70     ///
  71     /// Only updates the cursor if absolutely necessary
  72     pub fn needs_non_const_drop(
  73         &mut self,
  74         ccx: &'mir ConstCx<'mir, 'tcx>,
  75         local: Local,
  76         location: Location,
  77     ) -> bool {
  78         let ty = ccx.body.local_decls[local].ty;
  79         if !NeedsNonConstDrop::in_any_value_of_ty(ccx, ty) {
  80             return false;
  81         }
  82
  83         let needs_non_const_drop = self.needs_non_const_drop.get_or_insert_with(|| {
  84             let ConstCx { tcx, body, .. } = *ccx;
  85
  86             FlowSensitiveAnalysis::new(NeedsNonConstDrop, ccx)
  87                 .into_engine(tcx, &body)
  88                 .iterate_to_fixpoint()
  89                 .into_results_cursor(&body)
  90         });
  91
  92         needs_non_const_drop.seek_before_primary_effect(location);
  93         needs_non_const_drop.get().contains(local)
  94     }
  95
  96     /// Returns `true` if `local` is `HasMutInterior` at the given `Location`.
  97     ///
  98     /// Only updates the cursor if absolutely necessary.
  99     pub fn has_mut_interior(
 100         &mut self,
 101         ccx: &'mir ConstCx<'mir, 'tcx>,
 102         local: Local,
 103         location: Location,
 104     ) -> bool {
 105         let ty = ccx.body.local_decls[local].ty;
 106         // Peeking into opaque types causes cycles if the current function declares said opaque
 107         // type. Thus we avoid short circuiting on the type and instead run the more expensive
 108         // analysis that looks at the actual usage within this function
 109         if !ty.has_opaque_types() && !HasMutInterior::in_any_value_of_ty(ccx, ty) {
 110             return false;
 111         }
 112
 113         let has_mut_interior = self.has_mut_interior.get_or_insert_with(|| {
 114             let ConstCx { tcx, body, .. } = *ccx;
 115
 116             FlowSensitiveAnalysis::new(HasMutInterior, ccx)
 117                 .into_engine(tcx, &body)
 118                 .iterate_to_fixpoint()
 119                 .into_results_cursor(&body)
 120         });
 121
 122         has_mut_interior.seek_before_primary_effect(location);
 123         has_mut_interior.get().contains(local)
 124     }
 125
 126     fn in_return_place(
 127         &mut self,
 128         ccx: &'mir ConstCx<'mir, 'tcx>,
 129         error_occured: Option<ErrorReported>,
 130     ) -> ConstQualifs {
 131         // Find the `Return` terminator if one exists.
 132         //
 133         // If no `Return` terminator exists, this MIR is divergent. Just return the conservative
 134         // qualifs for the return type.
 135         let return_block = ccx
 136             .body
 137             .basic_blocks()
 138             .iter_enumerated()
 139             .find(|(_, block)| matches!(block.terminator().kind, TerminatorKind::Return))
 140             .map(|(bb, _)| bb);
 141
 142         let return_block = match return_block {
 143             None => return qualifs::in_any_value_of_ty(ccx, ccx.body.return_ty(), error_occured),
 144             Some(bb) => bb,
 145         };
 146
 147         let return_loc = ccx.body.terminator_loc(return_block);
 148
 149         let custom_eq = match ccx.const_kind() {
 150             // We don't care whether a `const fn` returns a value that is not structurally
 151             // matchable. Functions calls are opaque and always use type-based qualification, so
 152             // this value should never be used.
 153             hir::ConstContext::ConstFn => true,
 154
 155             // If we know that all values of the return type are structurally matchable, there's no
 156             // need to run dataflow.
 157             // Opaque types do not participate in const generics or pattern matching, so we can safely count them out.
 158             _ if ccx.body.return_ty().has_opaque_types()
 159                 || !CustomEq::in_any_value_of_ty(ccx, ccx.body.return_ty()) =>
 160             {
 161                 false
 162             }
 163
 164             hir::ConstContext::Const | hir::ConstContext::Static(_) => {
 165                 let mut cursor = FlowSensitiveAnalysis::new(CustomEq, ccx)
 166                     .into_engine(ccx.tcx, &ccx.body)
 167                     .iterate_to_fixpoint()
 168                     .into_results_cursor(&ccx.body);
 169
 170                 cursor.seek_after_primary_effect(return_loc);
 171                 cursor.get().contains(RETURN_PLACE)
 172             }
 173         };
 174
 175         ConstQualifs {
 176             needs_drop: self.needs_drop(ccx, RETURN_PLACE, return_loc),
 177             needs_non_const_drop: self.needs_non_const_drop(ccx, RETURN_PLACE, return_loc),
 178             has_mut_interior: self.has_mut_interior(ccx, RETURN_PLACE, return_loc),
 179             custom_eq,
 180             error_occured,
 181         }
 182     }
 183 }
 184
 185 pub struct Checker<'mir, 'tcx> {
 186     ccx: &'mir ConstCx<'mir, 'tcx>,
 187     qualifs: Qualifs<'mir, 'tcx>,
 188
 189     /// The span of the current statement.
 190     span: Span,
 191
 192     /// A set that stores for each local whether it has a `StorageDead` for it somewhere.
 193     local_has_storage_dead: Option<BitSet<Local>>,
 194
 195     error_emitted: Option<ErrorReported>,
 196     secondary_errors: Vec<Diagnostic>,
 197 }
 198
 199 impl<'mir, 'tcx> Deref for Checker<'mir, 'tcx> {
 200     type Target = ConstCx<'mir, 'tcx>;
 201
 202     fn deref(&self) -> &Self::Target {
 203         &self.ccx
 204     }
 205 }
 206
 207 impl<'mir, 'tcx> Checker<'mir, 'tcx> {
 208     pub fn new(ccx: &'mir ConstCx<'mir, 'tcx>) -> Self {
 209         Checker {
 210             span: ccx.body.span,
 211             ccx,
 212             qualifs: Default::default(),
 213             local_has_storage_dead: None,
 214             error_emitted: None,
 215             secondary_errors: Vec::new(),
 216         }
 217     }
 218
 219     pub fn check_body(&mut self) {
 220         let ConstCx { tcx, body, .. } = *self.ccx;
 221         let def_id = self.ccx.def_id();
 222
 223         // `async` functions cannot be `const fn`. This is checked during AST lowering, so there's
 224         // no need to emit duplicate errors here.
 225         if is_async_fn(self.ccx) || body.generator.is_some() {
 226             tcx.sess.delay_span_bug(body.span, "`async` functions cannot be `const fn`");
 227             return;
 228         }
 229
 230         // The local type and predicate checks are not free and only relevant for `const fn`s.
 231         if self.const_kind() == hir::ConstContext::ConstFn {
 232             // Prevent const trait methods from being annotated as `stable`.
 233             // FIXME: Do this as part of stability checking.
 234             if self.is_const_stable_const_fn() {
 235                 if crate::const_eval::is_parent_const_impl_raw(tcx, def_id) {
 236                     self.ccx
 237                         .tcx
 238                         .sess
 239                         .struct_span_err(self.span, "trait methods cannot be stable const fn")
 240                         .emit();
 241                 }
 242             }
 243
 244             self.check_item_predicates();
 245
 246             for (idx, local) in body.local_decls.iter_enumerated() {
 247                 // Handle the return place below.
 248                 if idx == RETURN_PLACE || local.internal {
 249                     continue;
 250                 }
 251
 252                 self.span = local.source_info.span;
 253                 self.check_local_or_return_ty(local.ty, idx);
 254             }
 255
 256             // impl trait is gone in MIR, so check the return type of a const fn by its signature
 257             // instead of the type of the return place.
 258             self.span = body.local_decls[RETURN_PLACE].source_info.span;
 259             let return_ty = tcx.fn_sig(def_id).output();
 260             self.check_local_or_return_ty(return_ty.skip_binder(), RETURN_PLACE);
 261         }
 262
 263         if !tcx.has_attr(def_id.to_def_id(), sym::rustc_do_not_const_check) {
 264             self.visit_body(&body);
 265         }
 266
 267         // If we got through const-checking without emitting any "primary" errors, emit any
 268         // "secondary" errors if they occurred.
 269         let secondary_errors = mem::take(&mut self.secondary_errors);
 270         if self.error_emitted.is_none() {
 271             for error in secondary_errors {
 272                 self.tcx.sess.diagnostic().emit_diagnostic(&error);
 273             }
 274         } else {
 275             assert!(self.tcx.sess.has_errors());
 276         }
 277     }
 278
 279     fn local_has_storage_dead(&mut self, local: Local) -> bool {
 280         let ccx = self.ccx;
 281         self.local_has_storage_dead
 282             .get_or_insert_with(|| {
 283                 struct StorageDeads {
 284                     locals: BitSet<Local>,
 285                 }
 286                 impl<'tcx> Visitor<'tcx> for StorageDeads {
 287                     fn visit_statement(&mut self, stmt: &Statement<'tcx>, _: Location) {
 288                         if let StatementKind::StorageDead(l) = stmt.kind {
 289                             self.locals.insert(l);
 290                         }
 291                     }
 292                 }
 293                 let mut v = StorageDeads { locals: BitSet::new_empty(ccx.body.local_decls.len()) };
 294                 v.visit_body(ccx.body);
 295                 v.locals
 296             })
 297             .contains(local)
 298     }
 299
 300     pub fn qualifs_in_return_place(&mut self) -> ConstQualifs {
 301         self.qualifs.in_return_place(self.ccx, self.error_emitted)
 302     }
 303
 304     /// Emits an error if an expression cannot be evaluated in the current context.
 305     pub fn check_op(&mut self, op: impl NonConstOp) {
 306         self.check_op_spanned(op, self.span);
 307     }
 308
 309     /// Emits an error at the given `span` if an expression cannot be evaluated in the current
 310     /// context.
 311     pub fn check_op_spanned<O: NonConstOp>(&mut self, op: O, span: Span) {
 312         let gate = match op.status_in_item(self.ccx) {
 313             Status::Allowed => return,
 314
 315             Status::Unstable(gate) if self.tcx.features().enabled(gate) => {
 316                 let unstable_in_stable = self.ccx.is_const_stable_const_fn()
 317                     && !super::rustc_allow_const_fn_unstable(
 318                         self.tcx,
 319                         self.def_id().to_def_id(),
 320                         gate,
 321                     );
 322                 if unstable_in_stable {
 323                     emit_unstable_in_stable_error(self.ccx, span, gate);
 324                 }
 325
 326                 return;
 327             }
 328
 329             Status::Unstable(gate) => Some(gate),
 330             Status::Forbidden => None,
 331         };
 332
 333         if self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
 334             self.tcx.sess.miri_unleashed_feature(span, gate);
 335             return;
 336         }
 337
 338         let mut err = op.build_error(self.ccx, span);
 339         assert!(err.is_error());
 340
 341         match op.importance() {
 342             ops::DiagnosticImportance::Primary => {
 343                 self.error_emitted = Some(ErrorReported);
 344                 err.emit();
 345             }
 346
 347             ops::DiagnosticImportance::Secondary => err.buffer(&mut self.secondary_errors),
 348         }
 349     }
 350
 351     fn check_static(&mut self, def_id: DefId, span: Span) {
 352         if self.tcx.is_thread_local_static(def_id) {
 353             self.tcx.sess.delay_span_bug(span, "tls access is checked in `Rvalue::ThreadLocalRef");
 354         }
 355         self.check_op_spanned(ops::StaticAccess, span)
 356     }
 357
 358     fn check_local_or_return_ty(&mut self, ty: Ty<'tcx>, local: Local) {
 359         let kind = self.body.local_kind(local);
 360
 361         for ty in ty.walk() {
 362             let ty = match ty.unpack() {
 363                 GenericArgKind::Type(ty) => ty,
 364
 365                 // No constraints on lifetimes or constants, except potentially
 366                 // constants' types, but `walk` will get to them as well.
 367                 GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => continue,
 368             };
 369
 370             match *ty.kind() {
 371                 ty::Ref(_, _, hir::Mutability::Mut) => self.check_op(ops::ty::MutRef(kind)),
 372                 ty::Opaque(..) => self.check_op(ops::ty::ImplTrait),
 373                 ty::FnPtr(..) => self.check_op(ops::ty::FnPtr(kind)),
 374
 375                 ty::Dynamic(preds, _) => {
 376                     for pred in preds.iter() {
 377                         match pred.skip_binder() {
 378                             ty::ExistentialPredicate::AutoTrait(_)
 379                             | ty::ExistentialPredicate::Projection(_) => {
 380                                 self.check_op(ops::ty::DynTrait(kind))
 381                             }
 382                             ty::ExistentialPredicate::Trait(trait_ref) => {
 383                                 if Some(trait_ref.def_id) != self.tcx.lang_items().sized_trait() {
 384                                     self.check_op(ops::ty::DynTrait(kind))
 385                                 }
 386                             }
 387                         }
 388                     }
 389                 }
 390                 _ => {}
 391             }
 392         }
 393     }
 394
 395     fn check_item_predicates(&mut self) {
 396         let ConstCx { tcx, .. } = *self.ccx;
 397
 398         let mut current = self.def_id().to_def_id();
 399         loop {
 400             let predicates = tcx.predicates_of(current);
 401             for (predicate, _) in predicates.predicates {
 402                 match predicate.kind().skip_binder() {
 403                     ty::PredicateKind::RegionOutlives(_)
 404                     | ty::PredicateKind::TypeOutlives(_)
 405                     | ty::PredicateKind::WellFormed(_)
 406                     | ty::PredicateKind::Projection(_)
 407                     | ty::PredicateKind::ConstEvaluatable(..)
 408                     | ty::PredicateKind::ConstEquate(..)
 409                     | ty::PredicateKind::OpaqueType(..)
 410                     | ty::PredicateKind::TypeWellFormedFromEnv(..) => continue,
 411                     ty::PredicateKind::ObjectSafe(_) => {
 412                         bug!("object safe predicate on function: {:#?}", predicate)
 413                     }
 414                     ty::PredicateKind::ClosureKind(..) => {
 415                         bug!("closure kind predicate on function: {:#?}", predicate)
 416                     }
 417                     ty::PredicateKind::Subtype(_) | ty::PredicateKind::Coerce(_) => {
 418                         bug!("subtype/coerce predicate on function: {:#?}", predicate)
 419                     }
 420                     ty::PredicateKind::Trait(pred) => {
 421                         if Some(pred.def_id()) == tcx.lang_items().sized_trait() {
 422                             continue;
 423                         }
 424                         match pred.self_ty().kind() {
 425                             ty::Param(p) => {
 426                                 let generics = tcx.generics_of(current);
 427                                 let def = generics.type_param(p, tcx);
 428                                 let span = tcx.def_span(def.def_id);
 429
 430                                 // These are part of the function signature, so treat them like
 431                                 // arguments when determining importance.
 432                                 let kind = LocalKind::Arg;
 433
 434                                 self.check_op_spanned(ops::ty::TraitBound(kind), span);
 435                             }
 436                             // other kinds of bounds are either tautologies
 437                             // or cause errors in other passes
 438                             _ => continue,
 439                         }
 440                     }
 441                 }
 442             }
 443             match predicates.parent {
 444                 Some(parent) => current = parent,
 445                 None => break,
 446             }
 447         }
 448     }
 449
 450     fn check_mut_borrow(&mut self, local: Local, kind: hir::BorrowKind) {
 451         match self.const_kind() {
 452             // In a const fn all borrows are transient or point to the places given via
 453             // references in the arguments (so we already checked them with
 454             // TransientMutBorrow/MutBorrow as appropriate).
 455             // The borrow checker guarantees that no new non-transient borrows are created.
 456             // NOTE: Once we have heap allocations during CTFE we need to figure out
 457             // how to prevent `const fn` to create long-lived allocations that point
 458             // to mutable memory.
 459             hir::ConstContext::ConstFn => self.check_op(ops::TransientMutBorrow(kind)),
 460             _ => {
 461                 // Locals with StorageDead do not live beyond the evaluation and can
 462                 // thus safely be borrowed without being able to be leaked to the final
 463                 // value of the constant.
 464                 if self.local_has_storage_dead(local) {
 465                     self.check_op(ops::TransientMutBorrow(kind));
 466                 } else {
 467                     self.check_op(ops::MutBorrow(kind));
 468                 }
 469             }
 470         }
 471     }
 472 }
 473
 474 impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
 475     fn visit_basic_block_data(&mut self, bb: BasicBlock, block: &BasicBlockData<'tcx>) {
 476         trace!("visit_basic_block_data: bb={:?} is_cleanup={:?}", bb, block.is_cleanup);
 477
 478         // We don't const-check basic blocks on the cleanup path since we never unwind during
 479         // const-eval: a panic causes an immediate compile error. In other words, cleanup blocks
 480         // are unreachable during const-eval.
 481         //
 482         // We can't be more conservative (e.g., by const-checking cleanup blocks anyways) because
 483         // locals that would never be dropped during normal execution are sometimes dropped during
 484         // unwinding, which means backwards-incompatible live-drop errors.
 485         if block.is_cleanup {
 486             return;
 487         }
 488
 489         self.super_basic_block_data(bb, block);
 490     }
 491
 492     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
 493         trace!("visit_rvalue: rvalue={:?} location={:?}", rvalue, location);
 494
 495         // Special-case reborrows to be more like a copy of a reference.
 496         match *rvalue {
 497             Rvalue::Ref(_, kind, place) => {
 498                 if let Some(reborrowed_place_ref) = place_as_reborrow(self.tcx, self.body, place) {
 499                     let ctx = match kind {
 500                         BorrowKind::Shared => {
 501                             PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
 502                         }
 503                         BorrowKind::Shallow => {
 504                             PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
 505                         }
 506                         BorrowKind::Unique => {
 507                             PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
 508                         }
 509                         BorrowKind::Mut { .. } => {
 510                             PlaceContext::MutatingUse(MutatingUseContext::Borrow)
 511                         }
 512                     };
 513                     self.visit_local(&reborrowed_place_ref.local, ctx, location);
 514                     self.visit_projection(reborrowed_place_ref, ctx, location);
 515                     return;
 516                 }
 517             }
 518             Rvalue::AddressOf(mutbl, place) => {
 519                 if let Some(reborrowed_place_ref) = place_as_reborrow(self.tcx, self.body, place) {
 520                     let ctx = match mutbl {
 521                         Mutability::Not => {
 522                             PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
 523                         }
 524                         Mutability::Mut => PlaceContext::MutatingUse(MutatingUseContext::AddressOf),
 525                     };
 526                     self.visit_local(&reborrowed_place_ref.local, ctx, location);
 527                     self.visit_projection(reborrowed_place_ref, ctx, location);
 528                     return;
 529                 }
 530             }
 531             _ => {}
 532         }
 533
 534         self.super_rvalue(rvalue, location);
 535
 536         match *rvalue {
 537             Rvalue::ThreadLocalRef(_) => self.check_op(ops::ThreadLocalAccess),
 538
 539             Rvalue::Use(_)
 540             | Rvalue::Repeat(..)
 541             | Rvalue::Discriminant(..)
 542             | Rvalue::Len(_)
 543             | Rvalue::Aggregate(..) => {}
 544
 545             Rvalue::Ref(_, kind @ BorrowKind::Mut { .. }, ref place)
 546             | Rvalue::Ref(_, kind @ BorrowKind::Unique, ref place) => {
 547                 let ty = place.ty(self.body, self.tcx).ty;
 548                 let is_allowed = match ty.kind() {
 549                     // Inside a `static mut`, `&mut [...]` is allowed.
 550                     ty::Array(..) | ty::Slice(_)
 551                         if self.const_kind() == hir::ConstContext::Static(hir::Mutability::Mut) =>
 552                     {
 553                         true
 554                     }
 555
 556                     // FIXME(ecstaticmorse): We could allow `&mut []` inside a const context given
 557                     // that this is merely a ZST and it is already eligible for promotion.
 558                     // This may require an RFC?
 559                     /*
 560                     ty::Array(_, len) if len.try_eval_usize(cx.tcx, cx.param_env) == Some(0)
 561                         => true,
 562                     */
 563                     _ => false,
 564                 };
 565
 566                 if !is_allowed {
 567                     if let BorrowKind::Mut { .. } = kind {
 568                         self.check_mut_borrow(place.local, hir::BorrowKind::Ref)
 569                     } else {
 570                         self.check_op(ops::CellBorrow);
 571                     }
 572                 }
 573             }
 574
 575             Rvalue::AddressOf(Mutability::Mut, ref place) => {
 576                 self.check_mut_borrow(place.local, hir::BorrowKind::Raw)
 577             }
 578
 579             Rvalue::Ref(_, BorrowKind::Shared | BorrowKind::Shallow, ref place)
 580             | Rvalue::AddressOf(Mutability::Not, ref place) => {
 581                 let borrowed_place_has_mut_interior = qualifs::in_place::<HasMutInterior, _>(
 582                     &self.ccx,
 583                     &mut |local| self.qualifs.has_mut_interior(self.ccx, local, location),
 584                     place.as_ref(),
 585                 );
 586
 587                 if borrowed_place_has_mut_interior {
 588                     match self.const_kind() {
 589                         // In a const fn all borrows are transient or point to the places given via
 590                         // references in the arguments (so we already checked them with
 591                         // TransientCellBorrow/CellBorrow as appropriate).
 592                         // The borrow checker guarantees that no new non-transient borrows are created.
 593                         // NOTE: Once we have heap allocations during CTFE we need to figure out
 594                         // how to prevent `const fn` to create long-lived allocations that point
 595                         // to (interior) mutable memory.
 596                         hir::ConstContext::ConstFn => self.check_op(ops::TransientCellBorrow),
 597                         _ => {
 598                             // Locals with StorageDead are definitely not part of the final constant value, and
 599                             // it is thus inherently safe to permit such locals to have their
 600                             // address taken as we can't end up with a reference to them in the
 601                             // final value.
 602                             // Note: This is only sound if every local that has a `StorageDead` has a
 603                             // `StorageDead` in every control flow path leading to a `return` terminator.
 604                             if self.local_has_storage_dead(place.local) {
 605                                 self.check_op(ops::TransientCellBorrow);
 606                             } else {
 607                                 self.check_op(ops::CellBorrow);
 608                             }
 609                         }
 610                     }
 611                 }
 612             }
 613
 614             Rvalue::Cast(
 615                 CastKind::Pointer(PointerCast::MutToConstPointer | PointerCast::ArrayToPointer),
 616                 _,
 617                 _,
 618             ) => {}
 619
 620             Rvalue::Cast(
 621                 CastKind::Pointer(
 622                     PointerCast::UnsafeFnPointer
 623                     | PointerCast::ClosureFnPointer(_)
 624                     | PointerCast::ReifyFnPointer,
 625                 ),
 626                 _,
 627                 _,
 628             ) => self.check_op(ops::FnPtrCast),
 629
 630             Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), _, _) => {
 631                 // Nothing to check here (`check_local_or_return_ty` ensures no trait objects occur
 632                 // in the type of any local, which also excludes casts).
 633             }
 634
 635             Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {
 636                 let operand_ty = operand.ty(self.body, self.tcx);
 637                 let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
 638                 let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 639
 640                 if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
 641                     self.check_op(ops::RawPtrToIntCast);
 642                 }
 643             }
 644
 645             Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf, _) => {}
 646             Rvalue::ShallowInitBox(_, _) => {}
 647
 648             Rvalue::UnaryOp(_, ref operand) => {
 649                 let ty = operand.ty(self.body, self.tcx);
 650                 if is_int_bool_or_char(ty) {
 651                     // Int, bool, and char operations are fine.
 652                 } else if ty.is_floating_point() {
 653                     self.check_op(ops::FloatingPointOp);
 654                 } else {
 655                     span_bug!(self.span, "non-primitive type in `Rvalue::UnaryOp`: {:?}", ty);
 656                 }
 657             }
 658
 659             Rvalue::BinaryOp(op, box (ref lhs, ref rhs))
 660             | Rvalue::CheckedBinaryOp(op, box (ref lhs, ref rhs)) => {
 661                 let lhs_ty = lhs.ty(self.body, self.tcx);
 662                 let rhs_ty = rhs.ty(self.body, self.tcx);
 663
 664                 if is_int_bool_or_char(lhs_ty) && is_int_bool_or_char(rhs_ty) {
 665                     // Int, bool, and char operations are fine.
 666                 } else if lhs_ty.is_fn_ptr() || lhs_ty.is_unsafe_ptr() {
 667                     assert_eq!(lhs_ty, rhs_ty);
 668                     assert!(
 669                         op == BinOp::Eq
 670                             || op == BinOp::Ne
 671                             || op == BinOp::Le
 672                             || op == BinOp::Lt
 673                             || op == BinOp::Ge
 674                             || op == BinOp::Gt
 675                             || op == BinOp::Offset
 676                     );
 677
 678                     self.check_op(ops::RawPtrComparison);
 679                 } else if lhs_ty.is_floating_point() || rhs_ty.is_floating_point() {
 680                     self.check_op(ops::FloatingPointOp);
 681                 } else {
 682                     span_bug!(
 683                         self.span,
 684                         "non-primitive type in `Rvalue::BinaryOp`: {:?} ⚬ {:?}",
 685                         lhs_ty,
 686                         rhs_ty
 687                     );
 688                 }
 689             }
 690         }
 691     }
 692
 693     fn visit_operand(&mut self, op: &Operand<'tcx>, location: Location) {
 694         self.super_operand(op, location);
 695         if let Operand::Constant(c) = op {
 696             if let Some(def_id) = c.check_static_ptr(self.tcx) {
 697                 self.check_static(def_id, self.span);
 698             }
 699         }
 700     }
 701     fn visit_projection_elem(
 702         &mut self,
 703         place_local: Local,
 704         proj_base: &[PlaceElem<'tcx>],
 705         elem: PlaceElem<'tcx>,
 706         context: PlaceContext,
 707         location: Location,
 708     ) {
 709         trace!(
 710             "visit_projection_elem: place_local={:?} proj_base={:?} elem={:?} \
 711             context={:?} location={:?}",
 712             place_local,
 713             proj_base,
 714             elem,
 715             context,
 716             location,
 717         );
 718
 719         self.super_projection_elem(place_local, proj_base, elem, context, location);
 720
 721         match elem {
 722             ProjectionElem::Deref => {
 723                 let base_ty = Place::ty_from(place_local, proj_base, self.body, self.tcx).ty;
 724                 if base_ty.is_unsafe_ptr() {
 725                     if proj_base.is_empty() {
 726                         let decl = &self.body.local_decls[place_local];
 727                         if let Some(box LocalInfo::StaticRef { def_id, .. }) = decl.local_info {
 728                             let span = decl.source_info.span;
 729                             self.check_static(def_id, span);
 730                             return;
 731                         }
 732                     }
 733
 734                     // `*const T` is stable, `*mut T` is not
 735                     if !base_ty.is_mutable_ptr() {
 736                         return;
 737                     }
 738
 739                     self.check_op(ops::RawMutPtrDeref);
 740                 }
 741
 742                 if context.is_mutating_use() {
 743                     self.check_op(ops::MutDeref);
 744                 }
 745             }
 746
 747             ProjectionElem::ConstantIndex { .. }
 748             | ProjectionElem::Downcast(..)
 749             | ProjectionElem::Subslice { .. }
 750             | ProjectionElem::Field(..)
 751             | ProjectionElem::Index(_) => {}
 752         }
 753     }
 754
 755     fn visit_source_info(&mut self, source_info: &SourceInfo) {
 756         trace!("visit_source_info: source_info={:?}", source_info);
 757         self.span = source_info.span;
 758     }
 759
 760     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
 761         trace!("visit_statement: statement={:?} location={:?}", statement, location);
 762
 763         self.super_statement(statement, location);
 764
 765         match statement.kind {
 766             StatementKind::Assign(..)
 767             | StatementKind::SetDiscriminant { .. }
 768             | StatementKind::FakeRead(..)
 769             | StatementKind::StorageLive(_)
 770             | StatementKind::StorageDead(_)
 771             | StatementKind::Retag { .. }
 772             | StatementKind::AscribeUserType(..)
 773             | StatementKind::Coverage(..)
 774             | StatementKind::CopyNonOverlapping(..)
 775             | StatementKind::Nop => {}
 776         }
 777     }
 778
 779     #[instrument(level = "debug", skip(self))]
 780     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
 781         use rustc_target::spec::abi::Abi::RustIntrinsic;
 782
 783         self.super_terminator(terminator, location);
 784
 785         match &terminator.kind {
 786             TerminatorKind::Call { func, args, .. } => {
 787                 let ConstCx { tcx, body, param_env, .. } = *self.ccx;
 788                 let caller = self.def_id().to_def_id();
 789
 790                 let fn_ty = func.ty(body, tcx);
 791
 792                 let (mut callee, mut substs) = match *fn_ty.kind() {
 793                     ty::FnDef(def_id, substs) => (def_id, substs),
 794
 795                     ty::FnPtr(_) => {
 796                         self.check_op(ops::FnCallIndirect);
 797                         return;
 798                     }
 799                     _ => {
 800                         span_bug!(terminator.source_info.span, "invalid callee of type {:?}", fn_ty)
 801                     }
 802                 };
 803
 804                 let mut nonconst_call_permission = false;
 805
 806                 // Attempting to call a trait method?
 807                 if let Some(trait_id) = tcx.trait_of_item(callee) {
 808                     trace!("attempting to call a trait method");
 809                     if !self.tcx.features().const_trait_impl {
 810                         self.check_op(ops::FnCallNonConst(Some((callee, substs))));
 811                         return;
 812                     }
 813
 814                     let trait_ref = TraitRef::from_method(tcx, trait_id, substs);
 815                     let obligation = Obligation::new(
 816                         ObligationCause::dummy(),
 817                         param_env,
 818                         Binder::dummy(TraitPredicate {
 819                             trait_ref,
 820                             constness: ty::BoundConstness::NotConst,
 821                             polarity: ty::ImplPolarity::Positive,
 822                         }),
 823                     );
 824
 825                     let implsrc = tcx.infer_ctxt().enter(|infcx| {
 826                         let mut selcx = SelectionContext::new(&infcx);
 827                         selcx.select(&obligation)
 828                     });
 829
 830                     match implsrc {
 831                         Ok(Some(ImplSource::Param(_, ty::BoundConstness::ConstIfConst))) => {
 832                             debug!(
 833                                 "const_trait_impl: provided {:?} via where-clause in {:?}",
 834                                 trait_ref, param_env
 835                             );
 836                             return;
 837                         }
 838                         Ok(Some(ImplSource::UserDefined(data))) => {
 839                             if let hir::Constness::NotConst = tcx.impl_constness(data.impl_def_id) {
 840                                 self.check_op(ops::FnCallNonConst(None));
 841                                 return;
 842                             }
 843                             let callee_name = tcx.item_name(callee);
 844                             if let Some(&did) = tcx
 845                                 .associated_item_def_ids(data.impl_def_id)
 846                                 .iter()
 847                                 .find(|did| tcx.item_name(**did) == callee_name)
 848                             {
 849                                 // using internal substs is ok here, since this is only
 850                                 // used for the `resolve` call below
 851                                 substs = InternalSubsts::identity_for_item(tcx, did);
 852                                 callee = did;
 853                             }
 854                         }
 855                         _ if !tcx.is_const_fn_raw(callee) => {
 856                             // At this point, it is only legal when the caller is marked with
 857                             // #[default_method_body_is_const], and the callee is in the same
 858                             // trait.
 859                             let callee_trait = tcx.trait_of_item(callee);
 860                             if callee_trait.is_some() {
 861                                 if tcx.has_attr(caller, sym::default_method_body_is_const) {
 862                                     if tcx.trait_of_item(caller) == callee_trait {
 863                                         nonconst_call_permission = true;
 864                                     }
 865                                 }
 866                             }
 867
 868                             if !nonconst_call_permission {
 869                                 self.check_op(ops::FnCallNonConst(None));
 870                                 return;
 871                             }
 872                         }
 873                         _ => {}
 874                     }
 875
 876                     // Resolve a trait method call to its concrete implementation, which may be in a
 877                     // `const` trait impl.
 878                     let instance = Instance::resolve(tcx, param_env, callee, substs);
 879                     debug!("Resolving ({:?}) -> {:?}", callee, instance);
 880                     if let Ok(Some(func)) = instance {
 881                         if let InstanceDef::Item(def) = func.def {
 882                             callee = def.did;
 883                         }
 884                     }
 885                 }
 886
 887                 // At this point, we are calling a function, `callee`, whose `DefId` is known...
 888
 889                 // `begin_panic` and `panic_display` are generic functions that accept
 890                 // types other than str. Check to enforce that only str can be used in
 891                 // const-eval.
 892
 893                 // const-eval of the `begin_panic` fn assumes the argument is `&str`
 894                 if Some(callee) == tcx.lang_items().begin_panic_fn() {
 895                     match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
 896                         ty::Ref(_, ty, _) if ty.is_str() => return,
 897                         _ => self.check_op(ops::PanicNonStr),
 898                     }
 899                 }
 900
 901                 // const-eval of the `panic_display` fn assumes the argument is `&&str`
 902                 if Some(callee) == tcx.lang_items().panic_display() {
 903                     match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
 904                         ty::Ref(_, ty, _) if matches!(ty.kind(), ty::Ref(_, ty, _) if ty.is_str()) =>
 905                         {
 906                             return;
 907                         }
 908                         _ => self.check_op(ops::PanicNonStr),
 909                     }
 910                 }
 911
 912                 if Some(callee) == tcx.lang_items().exchange_malloc_fn() {
 913                     self.check_op(ops::HeapAllocation);
 914                     return;
 915                 }
 916
 917                 // `async` blocks get lowered to `std::future::from_generator(/* a closure */)`.
 918                 let is_async_block = Some(callee) == tcx.lang_items().from_generator_fn();
 919                 if is_async_block {
 920                     let kind = hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block);
 921                     self.check_op(ops::Generator(kind));
 922                     return;
 923                 }
 924
 925                 let is_intrinsic = tcx.fn_sig(callee).abi() == RustIntrinsic;
 926
 927                 if !tcx.is_const_fn_raw(callee) {
 928                     if tcx.trait_of_item(callee).is_some() {
 929                         if tcx.has_attr(callee, sym::default_method_body_is_const) {
 930                             // To get to here we must have already found a const impl for the
 931                             // trait, but for it to still be non-const can be that the impl is
 932                             // using default method bodies.
 933                             nonconst_call_permission = true;
 934                         }
 935                     }
 936
 937                     if !nonconst_call_permission {
 938                         self.check_op(ops::FnCallNonConst(None));
 939                         return;
 940                     }
 941                 }
 942
 943                 // If the `const fn` we are trying to call is not const-stable, ensure that we have
 944                 // the proper feature gate enabled.
 945                 if let Some(gate) = is_unstable_const_fn(tcx, callee) {
 946                     trace!(?gate, "calling unstable const fn");
 947                     if self.span.allows_unstable(gate) {
 948                         return;
 949                     }
 950
 951                     // Calling an unstable function *always* requires that the corresponding gate
 952                     // be enabled, even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
 953                     if !tcx.features().declared_lib_features.iter().any(|&(sym, _)| sym == gate) {
 954                         self.check_op(ops::FnCallUnstable(callee, Some(gate)));
 955                         return;
 956                     }
 957
 958                     // If this crate is not using stability attributes, or the caller is not claiming to be a
 959                     // stable `const fn`, that is all that is required.
 960                     if !self.ccx.is_const_stable_const_fn() {
 961                         trace!("crate not using stability attributes or caller not stably const");
 962                         return;
 963                     }
 964
 965                     // Otherwise, we are something const-stable calling a const-unstable fn.
 966
 967                     if super::rustc_allow_const_fn_unstable(tcx, caller, gate) {
 968                         trace!("rustc_allow_const_fn_unstable gate active");
 969                         return;
 970                     }
 971
 972                     self.check_op(ops::FnCallUnstable(callee, Some(gate)));
 973                     return;
 974                 }
 975
 976                 // FIXME(ecstaticmorse); For compatibility, we consider `unstable` callees that
 977                 // have no `rustc_const_stable` attributes to be const-unstable as well. This
 978                 // should be fixed later.
 979                 let callee_is_unstable_unmarked = tcx.lookup_const_stability(callee).is_none()
 980                     && tcx.lookup_stability(callee).map_or(false, |s| s.level.is_unstable());
 981                 if callee_is_unstable_unmarked {
 982                     trace!("callee_is_unstable_unmarked");
 983                     // We do not use `const` modifiers for intrinsic "functions", as intrinsics are
 984                     // `extern` funtions, and these have no way to get marked `const`. So instead we
 985                     // use `rustc_const_(un)stable` attributes to mean that the intrinsic is `const`
 986                     if self.ccx.is_const_stable_const_fn() || is_intrinsic {
 987                         self.check_op(ops::FnCallUnstable(callee, None));
 988                         return;
 989                     }
 990                 }
 991                 trace!("permitting call");
 992             }
 993
 994             // Forbid all `Drop` terminators unless the place being dropped is a local with no
 995             // projections that cannot be `NeedsNonConstDrop`.
 996             TerminatorKind::Drop { place: dropped_place, .. }
 997             | TerminatorKind::DropAndReplace { place: dropped_place, .. } => {
 998                 // If we are checking live drops after drop-elaboration, don't emit duplicate
 999                 // errors here.
1000                 if super::post_drop_elaboration::checking_enabled(self.ccx) {
1001                     return;
1002                 }
1003
1004                 let mut err_span = self.span;
1005                 let ty_of_dropped_place = dropped_place.ty(self.body, self.tcx).ty;
1006
1007                 let ty_needs_non_const_drop =
1008                     qualifs::NeedsNonConstDrop::in_any_value_of_ty(self.ccx, ty_of_dropped_place);
1009
1010                 debug!(?ty_of_dropped_place, ?ty_needs_non_const_drop);
1011
1012                 if !ty_needs_non_const_drop {
1013                     return;
1014                 }
1015
1016                 let needs_non_const_drop = if let Some(local) = dropped_place.as_local() {
1017                     // Use the span where the local was declared as the span of the drop error.
1018                     err_span = self.body.local_decls[local].source_info.span;
1019                     self.qualifs.needs_non_const_drop(self.ccx, local, location)
1020                 } else {
1021                     true
1022                 };
1023
1024                 if needs_non_const_drop {
1025                     self.check_op_spanned(
1026                         ops::LiveDrop { dropped_at: Some(terminator.source_info.span) },
1027                         err_span,
1028                     );
1029                 }
1030             }
1031
1032             TerminatorKind::InlineAsm { .. } => self.check_op(ops::InlineAsm),
1033
1034             TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
1035                 self.check_op(ops::Generator(hir::GeneratorKind::Gen))
1036             }
1037
1038             TerminatorKind::Abort => {
1039                 // Cleanup blocks are skipped for const checking (see `visit_basic_block_data`).
1040                 span_bug!(self.span, "`Abort` terminator outside of cleanup block")
1041             }
1042
1043             TerminatorKind::Assert { .. }
1044             | TerminatorKind::FalseEdge { .. }
1045             | TerminatorKind::FalseUnwind { .. }
1046             | TerminatorKind::Goto { .. }
1047             | TerminatorKind::Resume
1048             | TerminatorKind::Return
1049             | TerminatorKind::SwitchInt { .. }
1050             | TerminatorKind::Unreachable => {}
1051         }
1052     }
1053 }
1054
1055 fn place_as_reborrow<'tcx>(
1056     tcx: TyCtxt<'tcx>,
1057     body: &Body<'tcx>,
1058     place: Place<'tcx>,
1059 ) -> Option<PlaceRef<'tcx>> {
1060     match place.as_ref().last_projection() {
1061         Some((place_base, ProjectionElem::Deref)) => {
1062             // A borrow of a `static` also looks like `&(*_1)` in the MIR, but `_1` is a `const`
1063             // that points to the allocation for the static. Don't treat these as reborrows.
1064             if body.local_decls[place_base.local].is_ref_to_static() {
1065                 None
1066             } else {
1067                 // Ensure the type being derefed is a reference and not a raw pointer.
1068                 // This is sufficient to prevent an access to a `static mut` from being marked as a
1069                 // reborrow, even if the check above were to disappear.
1070                 let inner_ty = place_base.ty(body, tcx).ty;
1071
1072                 if let ty::Ref(..) = inner_ty.kind() {
1073                     return Some(place_base);
1074                 } else {
1075                     return None;
1076                 }
1077             }
1078         }
1079         _ => None,
1080     }
1081 }
1082
1083 fn is_int_bool_or_char(ty: Ty<'_>) -> bool {
1084     ty.is_bool() || ty.is_integral() || ty.is_char()
1085 }
1086
1087 fn is_async_fn(ccx: &ConstCx<'_, '_>) -> bool {
1088     ccx.fn_sig().map_or(false, |sig| sig.header.asyncness == hir::IsAsync::Async)
1089 }
1090
1091 fn emit_unstable_in_stable_error(ccx: &ConstCx<'_, '_>, span: Span, gate: Symbol) {
1092     let attr_span = ccx.fn_sig().map_or(ccx.body.span, |sig| sig.span.shrink_to_lo());
1093
1094     ccx.tcx
1095         .sess
1096         .struct_span_err(
1097             span,
1098             &format!("const-stable function cannot use `#[feature({})]`", gate.as_str()),
1099         )
1100         .span_suggestion(
1101             attr_span,
1102             "if it is not part of the public API, make this function unstably const",
1103             concat!(r#"#[rustc_const_unstable(feature = "...", issue = "...")]"#, '\n').to_owned(),
1104             Applicability::HasPlaceholders,
1105         )
1106         .span_suggestion(
1107             attr_span,
1108             "otherwise `#[rustc_allow_const_fn_unstable]` can be used to bypass stability checks",
1109             format!("#[rustc_allow_const_fn_unstable({})]\n", gate),
1110             Applicability::MaybeIncorrect,
1111         )
1112         .emit();
1113 }