src/librustc_mir/transform/qualify_min_const_fn.rs

   1 use rustc_attr as attr;
   2 use rustc_hir as hir;
   3 use rustc_hir::def_id::DefId;
   4 use rustc_middle::mir::*;
   5 use rustc_middle::ty::subst::GenericArgKind;
   6 use rustc_middle::ty::{self, adjustment::PointerCast, Ty, TyCtxt};
   7 use rustc_span::symbol::{sym, Symbol};
   8 use rustc_span::Span;
   9 use rustc_target::spec::abi::Abi::RustIntrinsic;
  10 use std::borrow::Cow;
  11
  12 type McfResult = Result<(), (Span, Cow<'static, str>)>;
  13
  14 pub fn is_min_const_fn(tcx: TyCtxt<'tcx>, def_id: DefId, body: &'a Body<'tcx>) -> McfResult {
  15     // Prevent const trait methods from being annotated as `stable`.
  16     if tcx.features().staged_api {
  17         let hir_id = tcx.hir().as_local_hir_id(def_id.expect_local());
  18         if crate::const_eval::is_parent_const_impl_raw(tcx, hir_id) {
  19             return Err((body.span, "trait methods cannot be stable const fn".into()));
  20         }
  21     }
  22
  23     let mut current = def_id;
  24     loop {
  25         let predicates = tcx.predicates_of(current);
  26         for (predicate, _) in predicates.predicates {
  27             match predicate.kind() {
  28                 ty::PredicateKind::RegionOutlives(_)
  29                 | ty::PredicateKind::TypeOutlives(_)
  30                 | ty::PredicateKind::WellFormed(_)
  31                 | ty::PredicateKind::Projection(_)
  32                 | ty::PredicateKind::ConstEvaluatable(..)
  33                 | ty::PredicateKind::ConstEquate(..) => continue,
  34                 ty::PredicateKind::ObjectSafe(_) => {
  35                     bug!("object safe predicate on function: {:#?}", predicate)
  36                 }
  37                 ty::PredicateKind::ClosureKind(..) => {
  38                     bug!("closure kind predicate on function: {:#?}", predicate)
  39                 }
  40                 ty::PredicateKind::Subtype(_) => {
  41                     bug!("subtype predicate on function: {:#?}", predicate)
  42                 }
  43                 &ty::PredicateKind::Trait(pred, constness) => {
  44                     if Some(pred.def_id()) == tcx.lang_items().sized_trait() {
  45                         continue;
  46                     }
  47                     match pred.skip_binder().self_ty().kind {
  48                         ty::Param(ref p) => {
  49                             // Allow `T: ?const Trait`
  50                             if constness == hir::Constness::NotConst
  51                                 && feature_allowed(tcx, def_id, sym::const_trait_bound_opt_out)
  52                             {
  53                                 continue;
  54                             }
  55
  56                             let generics = tcx.generics_of(current);
  57                             let def = generics.type_param(p, tcx);
  58                             let span = tcx.def_span(def.def_id);
  59                             return Err((
  60                                 span,
  61                                 "trait bounds other than `Sized` \
  62                                  on const fn parameters are unstable"
  63                                     .into(),
  64                             ));
  65                         }
  66                         // other kinds of bounds are either tautologies
  67                         // or cause errors in other passes
  68                         _ => continue,
  69                     }
  70                 }
  71             }
  72         }
  73         match predicates.parent {
  74             Some(parent) => current = parent,
  75             None => break,
  76         }
  77     }
  78
  79     for local in &body.local_decls {
  80         check_ty(tcx, local.ty, local.source_info.span, def_id)?;
  81     }
  82     // impl trait is gone in MIR, so check the return type manually
  83     check_ty(
  84         tcx,
  85         tcx.fn_sig(def_id).output().skip_binder(),
  86         body.local_decls.iter().next().unwrap().source_info.span,
  87         def_id,
  88     )?;
  89
  90     for bb in body.basic_blocks() {
  91         check_terminator(tcx, body, def_id, bb.terminator())?;
  92         for stmt in &bb.statements {
  93             check_statement(tcx, body, def_id, stmt)?;
  94         }
  95     }
  96     Ok(())
  97 }
  98
  99 fn check_ty(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span, fn_def_id: DefId) -> McfResult {
 100     for arg in ty.walk() {
 101         let ty = match arg.unpack() {
 102             GenericArgKind::Type(ty) => ty,
 103
 104             // No constraints on lifetimes or constants, except potentially
 105             // constants' types, but `walk` will get to them as well.
 106             GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => continue,
 107         };
 108
 109         match ty.kind {
 110             ty::Ref(_, _, hir::Mutability::Mut) => {
 111                 if !feature_allowed(tcx, fn_def_id, sym::const_mut_refs) {
 112                     return Err((span, "mutable references in const fn are unstable".into()));
 113                 }
 114             }
 115             ty::Opaque(..) => return Err((span, "`impl Trait` in const fn is unstable".into())),
 116             ty::FnPtr(..) => {
 117                 if !tcx.const_fn_is_allowed_fn_ptr(fn_def_id) {
 118                     return Err((span, "function pointers in const fn are unstable".into()));
 119                 }
 120             }
 121             ty::Dynamic(preds, _) => {
 122                 for pred in preds.iter() {
 123                     match pred.skip_binder() {
 124                         ty::ExistentialPredicate::AutoTrait(_)
 125                         | ty::ExistentialPredicate::Projection(_) => {
 126                             return Err((
 127                                 span,
 128                                 "trait bounds other than `Sized` \
 129                                  on const fn parameters are unstable"
 130                                     .into(),
 131                             ));
 132                         }
 133                         ty::ExistentialPredicate::Trait(trait_ref) => {
 134                             if Some(trait_ref.def_id) != tcx.lang_items().sized_trait() {
 135                                 return Err((
 136                                     span,
 137                                     "trait bounds other than `Sized` \
 138                                      on const fn parameters are unstable"
 139                                         .into(),
 140                                 ));
 141                             }
 142                         }
 143                     }
 144                 }
 145             }
 146             _ => {}
 147         }
 148     }
 149     Ok(())
 150 }
 151
 152 fn check_rvalue(
 153     tcx: TyCtxt<'tcx>,
 154     body: &Body<'tcx>,
 155     def_id: DefId,
 156     rvalue: &Rvalue<'tcx>,
 157     span: Span,
 158 ) -> McfResult {
 159     match rvalue {
 160         Rvalue::ThreadLocalRef(_) => {
 161             Err((span, "cannot access thread local storage in const fn".into()))
 162         }
 163         Rvalue::Repeat(operand, _) | Rvalue::Use(operand) => {
 164             check_operand(tcx, operand, span, def_id, body)
 165         }
 166         Rvalue::Len(place)
 167         | Rvalue::Discriminant(place)
 168         | Rvalue::Ref(_, _, place)
 169         | Rvalue::AddressOf(_, place) => check_place(tcx, *place, span, def_id, body),
 170         Rvalue::Cast(CastKind::Misc, operand, cast_ty) => {
 171             use rustc_middle::ty::cast::CastTy;
 172             let cast_in = CastTy::from_ty(operand.ty(body, tcx)).expect("bad input type for cast");
 173             let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 174             match (cast_in, cast_out) {
 175                 (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) => {
 176                     Err((span, "casting pointers to ints is unstable in const fn".into()))
 177                 }
 178                 _ => check_operand(tcx, operand, span, def_id, body),
 179             }
 180         }
 181         Rvalue::Cast(
 182             CastKind::Pointer(PointerCast::MutToConstPointer | PointerCast::ArrayToPointer),
 183             operand,
 184             _,
 185         ) => check_operand(tcx, operand, span, def_id, body),
 186         Rvalue::Cast(
 187             CastKind::Pointer(
 188                 PointerCast::UnsafeFnPointer
 189                 | PointerCast::ClosureFnPointer(_)
 190                 | PointerCast::ReifyFnPointer,
 191             ),
 192             _,
 193             _,
 194         ) => Err((span, "function pointer casts are not allowed in const fn".into())),
 195         Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), op, cast_ty) => {
 196             let pointee_ty = if let Some(deref_ty) = cast_ty.builtin_deref(true) {
 197                 deref_ty.ty
 198             } else {
 199                 // We cannot allow this for now.
 200                 return Err((
 201                     span,
 202                     "unsizing casts are only allowed for references right now".into(),
 203                 ));
 204             };
 205             let unsized_ty = tcx.struct_tail_erasing_lifetimes(pointee_ty, tcx.param_env(def_id));
 206             if let ty::Slice(_) | ty::Str = unsized_ty.kind {
 207                 check_operand(tcx, op, span, def_id, body)?;
 208                 // Casting/coercing things to slices is fine.
 209                 Ok(())
 210             } else {
 211                 // We just can't allow trait objects until we have figured out trait method calls.
 212                 Err((span, "unsizing casts are not allowed in const fn".into()))
 213             }
 214         }
 215         // binops are fine on integers
 216         Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
 217             check_operand(tcx, lhs, span, def_id, body)?;
 218             check_operand(tcx, rhs, span, def_id, body)?;
 219             let ty = lhs.ty(body, tcx);
 220             if ty.is_integral() || ty.is_bool() || ty.is_char() {
 221                 Ok(())
 222             } else {
 223                 Err((span, "only int, `bool` and `char` operations are stable in const fn".into()))
 224             }
 225         }
 226         Rvalue::NullaryOp(NullOp::SizeOf, _) => Ok(()),
 227         Rvalue::NullaryOp(NullOp::Box, _) => {
 228             Err((span, "heap allocations are not allowed in const fn".into()))
 229         }
 230         Rvalue::UnaryOp(_, operand) => {
 231             let ty = operand.ty(body, tcx);
 232             if ty.is_integral() || ty.is_bool() {
 233                 check_operand(tcx, operand, span, def_id, body)
 234             } else {
 235                 Err((span, "only int and `bool` operations are stable in const fn".into()))
 236             }
 237         }
 238         Rvalue::Aggregate(_, operands) => {
 239             for operand in operands {
 240                 check_operand(tcx, operand, span, def_id, body)?;
 241             }
 242             Ok(())
 243         }
 244     }
 245 }
 246
 247 fn check_statement(
 248     tcx: TyCtxt<'tcx>,
 249     body: &Body<'tcx>,
 250     def_id: DefId,
 251     statement: &Statement<'tcx>,
 252 ) -> McfResult {
 253     let span = statement.source_info.span;
 254     match &statement.kind {
 255         StatementKind::Assign(box (place, rval)) => {
 256             check_place(tcx, *place, span, def_id, body)?;
 257             check_rvalue(tcx, body, def_id, rval, span)
 258         }
 259
 260         StatementKind::FakeRead(_, place) => check_place(tcx, **place, span, def_id, body),
 261
 262         // just an assignment
 263         StatementKind::SetDiscriminant { place, .. } => {
 264             check_place(tcx, **place, span, def_id, body)
 265         }
 266
 267         StatementKind::LlvmInlineAsm { .. } => {
 268             Err((span, "cannot use inline assembly in const fn".into()))
 269         }
 270
 271         // These are all NOPs
 272         StatementKind::StorageLive(_)
 273         | StatementKind::StorageDead(_)
 274         | StatementKind::Retag { .. }
 275         | StatementKind::AscribeUserType(..)
 276         | StatementKind::Nop => Ok(()),
 277     }
 278 }
 279
 280 fn check_operand(
 281     tcx: TyCtxt<'tcx>,
 282     operand: &Operand<'tcx>,
 283     span: Span,
 284     def_id: DefId,
 285     body: &Body<'tcx>,
 286 ) -> McfResult {
 287     match operand {
 288         Operand::Move(place) | Operand::Copy(place) => check_place(tcx, *place, span, def_id, body),
 289         Operand::Constant(c) => match c.check_static_ptr(tcx) {
 290             Some(_) => Err((span, "cannot access `static` items in const fn".into())),
 291             None => Ok(()),
 292         },
 293     }
 294 }
 295
 296 fn check_place(
 297     tcx: TyCtxt<'tcx>,
 298     place: Place<'tcx>,
 299     span: Span,
 300     def_id: DefId,
 301     body: &Body<'tcx>,
 302 ) -> McfResult {
 303     let mut cursor = place.projection.as_ref();
 304     while let &[ref proj_base @ .., elem] = cursor {
 305         cursor = proj_base;
 306         match elem {
 307             ProjectionElem::Field(..) => {
 308                 let base_ty = Place::ty_from(place.local, &proj_base, body, tcx).ty;
 309                 if let Some(def) = base_ty.ty_adt_def() {
 310                     // No union field accesses in `const fn`
 311                     if def.is_union() {
 312                         if !feature_allowed(tcx, def_id, sym::const_fn_union) {
 313                             return Err((span, "accessing union fields is unstable".into()));
 314                         }
 315                     }
 316                 }
 317             }
 318             ProjectionElem::ConstantIndex { .. }
 319             | ProjectionElem::Downcast(..)
 320             | ProjectionElem::Subslice { .. }
 321             | ProjectionElem::Deref
 322             | ProjectionElem::Index(_) => {}
 323         }
 324     }
 325
 326     Ok(())
 327 }
 328
 329 /// Returns `true` if the given feature gate is allowed within the function with the given `DefId`.
 330 fn feature_allowed(tcx: TyCtxt<'tcx>, def_id: DefId, feature_gate: Symbol) -> bool {
 331     // All features require that the corresponding gate be enabled,
 332     // even if the function has `#[allow_internal_unstable(the_gate)]`.
 333     if !tcx.features().enabled(feature_gate) {
 334         return false;
 335     }
 336
 337     // If this crate is not using stability attributes, or this function is not claiming to be a
 338     // stable `const fn`, that is all that is required.
 339     if !tcx.features().staged_api || tcx.has_attr(def_id, sym::rustc_const_unstable) {
 340         return true;
 341     }
 342
 343     // However, we cannot allow stable `const fn`s to use unstable features without an explicit
 344     // opt-in via `allow_internal_unstable`.
 345     attr::allow_internal_unstable(&tcx.get_attrs(def_id), &tcx.sess.diagnostic())
 346         .map_or(false, |mut features| features.any(|name| name == feature_gate))
 347 }
 348
 349 /// Returns `true` if the given library feature gate is allowed within the function with the given `DefId`.
 350 pub fn lib_feature_allowed(tcx: TyCtxt<'tcx>, def_id: DefId, feature_gate: Symbol) -> bool {
 351     // All features require that the corresponding gate be enabled,
 352     // even if the function has `#[allow_internal_unstable(the_gate)]`.
 353     if !tcx.features().declared_lib_features.iter().any(|&(sym, _)| sym == feature_gate) {
 354         return false;
 355     }
 356
 357     // If this crate is not using stability attributes, or this function is not claiming to be a
 358     // stable `const fn`, that is all that is required.
 359     if !tcx.features().staged_api || tcx.has_attr(def_id, sym::rustc_const_unstable) {
 360         return true;
 361     }
 362
 363     // However, we cannot allow stable `const fn`s to use unstable features without an explicit
 364     // opt-in via `allow_internal_unstable`.
 365     attr::allow_internal_unstable(&tcx.get_attrs(def_id), &tcx.sess.diagnostic())
 366         .map_or(false, |mut features| features.any(|name| name == feature_gate))
 367 }
 368
 369 fn check_terminator(
 370     tcx: TyCtxt<'tcx>,
 371     body: &'a Body<'tcx>,
 372     def_id: DefId,
 373     terminator: &Terminator<'tcx>,
 374 ) -> McfResult {
 375     let span = terminator.source_info.span;
 376     match &terminator.kind {
 377         TerminatorKind::FalseEdge { .. }
 378         | TerminatorKind::FalseUnwind { .. }
 379         | TerminatorKind::Goto { .. }
 380         | TerminatorKind::Return
 381         | TerminatorKind::Resume
 382         | TerminatorKind::Unreachable => Ok(()),
 383
 384         TerminatorKind::Drop { place, .. } => check_place(tcx, *place, span, def_id, body),
 385         TerminatorKind::DropAndReplace { place, value, .. } => {
 386             check_place(tcx, *place, span, def_id, body)?;
 387             check_operand(tcx, value, span, def_id, body)
 388         }
 389
 390         TerminatorKind::SwitchInt { discr, switch_ty: _, values: _, targets: _ } => {
 391             check_operand(tcx, discr, span, def_id, body)
 392         }
 393
 394         TerminatorKind::Abort => Err((span, "abort is not stable in const fn".into())),
 395         TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
 396             Err((span, "const fn generators are unstable".into()))
 397         }
 398
 399         TerminatorKind::Call {
 400             func,
 401             args,
 402             from_hir_call: _,
 403             destination: _,
 404             cleanup: _,
 405             fn_span: _,
 406         } => {
 407             let fn_ty = func.ty(body, tcx);
 408             if let ty::FnDef(fn_def_id, _) = fn_ty.kind {
 409                 // Allow unstable const if we opt in by using #[allow_internal_unstable]
 410                 // on function or macro declaration.
 411                 if !crate::const_eval::is_min_const_fn(tcx, fn_def_id)
 412                     && !crate::const_eval::is_unstable_const_fn(tcx, fn_def_id)
 413                         .map(|feature| {
 414                             span.allows_unstable(feature)
 415                                 || lib_feature_allowed(tcx, def_id, feature)
 416                         })
 417                         .unwrap_or(false)
 418                 {
 419                     return Err((
 420                         span,
 421                         format!(
 422                             "can only call other `const fn` within a `const fn`, \
 423                              but `{:?}` is not stable as `const fn`",
 424                             func,
 425                         )
 426                         .into(),
 427                     ));
 428                 }
 429
 430                 // HACK: This is to "unstabilize" the `transmute` intrinsic
 431                 // within const fns. `transmute` is allowed in all other const contexts.
 432                 // This won't really scale to more intrinsics or functions. Let's allow const
 433                 // transmutes in const fn before we add more hacks to this.
 434                 if tcx.fn_sig(fn_def_id).abi() == RustIntrinsic
 435                     && tcx.item_name(fn_def_id) == sym::transmute
 436                     && !feature_allowed(tcx, def_id, sym::const_fn_transmute)
 437                 {
 438                     return Err((
 439                         span,
 440                         "can only call `transmute` from const items, not `const fn`".into(),
 441                     ));
 442                 }
 443
 444                 check_operand(tcx, func, span, fn_def_id, body)?;
 445
 446                 for arg in args {
 447                     check_operand(tcx, arg, span, fn_def_id, body)?;
 448                 }
 449                 Ok(())
 450             } else {
 451                 Err((span, "can only call other const fns within const fn".into()))
 452             }
 453         }
 454
 455         TerminatorKind::Assert { cond, expected: _, msg: _, target: _, cleanup: _ } => {
 456             check_operand(tcx, cond, span, def_id, body)
 457         }
 458
 459         TerminatorKind::InlineAsm { .. } => {
 460             Err((span, "cannot use inline assembly in const fn".into()))
 461         }
 462     }
 463 }