1 use rustc_attr as attr;
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};
9 use rustc_target::spec::abi::Abi::RustIntrinsic;
12 type McfResult = Result<(), (Span, Cow<'static, str>)>;
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()));
23 let mut current = def_id;
25 let predicates = tcx.predicates_of(current);
26 for (predicate, _) in predicates.predicates {
27 match predicate.skip_binders() {
28 ty::PredicateAtom::RegionOutlives(_)
29 | ty::PredicateAtom::TypeOutlives(_)
30 | ty::PredicateAtom::WellFormed(_)
31 | ty::PredicateAtom::Projection(_)
32 | ty::PredicateAtom::ConstEvaluatable(..)
33 | ty::PredicateAtom::ConstEquate(..) => continue,
34 ty::PredicateAtom::ObjectSafe(_) => {
35 bug!("object safe predicate on function: {:#?}", predicate)
37 ty::PredicateAtom::ClosureKind(..) => {
38 bug!("closure kind predicate on function: {:#?}", predicate)
40 ty::PredicateAtom::Subtype(_) => {
41 bug!("subtype predicate on function: {:#?}", predicate)
43 ty::PredicateAtom::Trait(pred, constness) => {
44 if Some(pred.def_id()) == tcx.lang_items().sized_trait() {
47 match pred.self_ty().kind {
49 // Allow `T: ?const Trait`
50 if constness == hir::Constness::NotConst
51 && feature_allowed(tcx, def_id, sym::const_trait_bound_opt_out)
56 let generics = tcx.generics_of(current);
57 let def = generics.type_param(p, tcx);
58 let span = tcx.def_span(def.def_id);
61 "trait bounds other than `Sized` \
62 on const fn parameters are unstable"
66 // other kinds of bounds are either tautologies
67 // or cause errors in other passes
73 match predicates.parent {
74 Some(parent) => current = parent,
79 for local in &body.local_decls {
80 check_ty(tcx, local.ty, local.source_info.span, def_id)?;
82 // impl trait is gone in MIR, so check the return type manually
85 tcx.fn_sig(def_id).output().skip_binder(),
86 body.local_decls.iter().next().unwrap().source_info.span,
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)?;
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,
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,
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()));
115 ty::Opaque(..) => return Err((span, "`impl Trait` in const fn is unstable".into())),
117 if !tcx.const_fn_is_allowed_fn_ptr(fn_def_id) {
118 return Err((span, "function pointers in const fn are unstable".into()));
121 ty::Dynamic(preds, _) => {
122 for pred in preds.iter() {
123 match pred.skip_binder() {
124 ty::ExistentialPredicate::AutoTrait(_)
125 | ty::ExistentialPredicate::Projection(_) => {
128 "trait bounds other than `Sized` \
129 on const fn parameters are unstable"
133 ty::ExistentialPredicate::Trait(trait_ref) => {
134 if Some(trait_ref.def_id) != tcx.lang_items().sized_trait() {
137 "trait bounds other than `Sized` \
138 on const fn parameters are unstable"
156 rvalue: &Rvalue<'tcx>,
160 Rvalue::ThreadLocalRef(_) => {
161 Err((span, "cannot access thread local storage in const fn".into()))
163 Rvalue::Repeat(operand, _) | Rvalue::Use(operand) => {
164 check_operand(tcx, operand, span, def_id, body)
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()))
178 _ => check_operand(tcx, operand, span, def_id, body),
182 CastKind::Pointer(PointerCast::MutToConstPointer | PointerCast::ArrayToPointer),
185 ) => check_operand(tcx, operand, span, def_id, body),
188 PointerCast::UnsafeFnPointer
189 | PointerCast::ClosureFnPointer(_)
190 | PointerCast::ReifyFnPointer,
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) {
199 // We cannot allow this for now.
202 "unsizing casts are only allowed for references right now".into(),
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.
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()))
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() {
223 Err((span, "only int, `bool` and `char` operations are stable in const fn".into()))
226 Rvalue::NullaryOp(NullOp::SizeOf, _) => Ok(()),
227 Rvalue::NullaryOp(NullOp::Box, _) => {
228 Err((span, "heap allocations are not allowed in const fn".into()))
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)
235 Err((span, "only int and `bool` operations are stable in const fn".into()))
238 Rvalue::Aggregate(_, operands) => {
239 for operand in operands {
240 check_operand(tcx, operand, span, def_id, body)?;
251 statement: &Statement<'tcx>,
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)
260 StatementKind::FakeRead(_, place) => check_place(tcx, **place, span, def_id, body),
262 // just an assignment
263 StatementKind::SetDiscriminant { place, .. } => {
264 check_place(tcx, **place, span, def_id, body)
267 StatementKind::LlvmInlineAsm { .. } => {
268 Err((span, "cannot use inline assembly in const fn".into()))
271 // These are all NOPs
272 StatementKind::StorageLive(_)
273 | StatementKind::StorageDead(_)
274 | StatementKind::Retag { .. }
275 | StatementKind::AscribeUserType(..)
276 | StatementKind::Nop => Ok(()),
282 operand: &Operand<'tcx>,
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())),
303 let mut cursor = place.projection.as_ref();
304 while let &[ref proj_base @ .., elem] = cursor {
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`
312 if !feature_allowed(tcx, def_id, sym::const_fn_union) {
313 return Err((span, "accessing union fields is unstable".into()));
318 ProjectionElem::ConstantIndex { .. }
319 | ProjectionElem::Downcast(..)
320 | ProjectionElem::Subslice { .. }
321 | ProjectionElem::Deref
322 | ProjectionElem::Index(_) => {}
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) {
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) {
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.sess, &tcx.get_attrs(def_id))
346 .map_or(false, |mut features| features.any(|name| name == feature_gate))
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) {
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) {
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.sess, &tcx.get_attrs(def_id))
366 .map_or(false, |mut features| features.any(|name| name == feature_gate))
371 body: &'a Body<'tcx>,
373 terminator: &Terminator<'tcx>,
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(()),
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)
390 TerminatorKind::SwitchInt { discr, switch_ty: _, values: _, targets: _ } => {
391 check_operand(tcx, discr, span, def_id, body)
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()))
399 TerminatorKind::Call {
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)
414 span.allows_unstable(feature)
415 || lib_feature_allowed(tcx, def_id, feature)
422 "can only call other `const fn` within a `const fn`, \
423 but `{:?}` is not stable as `const fn`",
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)
440 "can only call `transmute` from const items, not `const fn`".into(),
444 check_operand(tcx, func, span, fn_def_id, body)?;
447 check_operand(tcx, arg, span, fn_def_id, body)?;
451 Err((span, "can only call other const fns within const fn".into()))
455 TerminatorKind::Assert { cond, expected: _, msg: _, target: _, cleanup: _ } => {
456 check_operand(tcx, cond, span, def_id, body)
459 TerminatorKind::InlineAsm { .. } => {
460 Err((span, "cannot use inline assembly in const fn".into()))