1 // This code used to be a part of `rustc` but moved to Clippy as a result of
2 // https://github.com/rust-lang/rust/issues/76618. Because of that, it contains unused code and some
3 // of terminologies might not be relevant in the context of Clippy. Note that its behavior might
4 // differ from the time of `rustc` even if the name stays the same.
7 use rustc_hir::def_id::DefId;
8 use rustc_middle::mir::{
9 Body, CastKind, NullOp, Operand, Place, ProjectionElem, Rvalue, Statement, StatementKind, Terminator,
12 use rustc_middle::ty::subst::GenericArgKind;
13 use rustc_middle::ty::{self, adjustment::PointerCast, Ty, TyCtxt};
14 use rustc_semver::RustcVersion;
15 use rustc_span::symbol::sym;
19 type McfResult = Result<(), (Span, Cow<'static, str>)>;
21 pub fn is_min_const_fn<'a, 'tcx>(tcx: TyCtxt<'tcx>, body: &'a Body<'tcx>, msrv: Option<RustcVersion>) -> McfResult {
22 let def_id = body.source.def_id();
23 let mut current = def_id;
25 let predicates = tcx.predicates_of(current);
26 for (predicate, _) in predicates.predicates {
27 match predicate.kind().skip_binder() {
28 ty::PredicateKind::RegionOutlives(_)
29 | ty::PredicateKind::TypeOutlives(_)
30 | ty::PredicateKind::WellFormed(_)
31 | ty::PredicateKind::Projection(_)
32 | ty::PredicateKind::ConstEvaluatable(..)
33 | ty::PredicateKind::ConstEquate(..)
34 | ty::PredicateKind::Trait(..)
35 | ty::PredicateKind::TypeWellFormedFromEnv(..) => continue,
36 ty::PredicateKind::ObjectSafe(_) => panic!("object safe predicate on function: {:#?}", predicate),
37 ty::PredicateKind::ClosureKind(..) => panic!("closure kind predicate on function: {:#?}", predicate),
38 ty::PredicateKind::Subtype(_) => panic!("subtype predicate on function: {:#?}", predicate),
39 ty::PredicateKind::Coerce(_) => panic!("coerce predicate on function: {:#?}", predicate),
42 match predicates.parent {
43 Some(parent) => current = parent,
48 for local in &body.local_decls {
49 check_ty(tcx, local.ty, local.source_info.span)?;
51 // impl trait is gone in MIR, so check the return type manually
54 tcx.fn_sig(def_id).output().skip_binder(),
55 body.local_decls.iter().next().unwrap().source_info.span,
58 for bb in body.basic_blocks() {
59 check_terminator(tcx, body, bb.terminator(), msrv)?;
60 for stmt in &bb.statements {
61 check_statement(tcx, body, def_id, stmt)?;
67 fn check_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span) -> McfResult {
68 for arg in ty.walk() {
69 let ty = match arg.unpack() {
70 GenericArgKind::Type(ty) => ty,
72 // No constraints on lifetimes or constants, except potentially
73 // constants' types, but `walk` will get to them as well.
74 GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => continue,
78 ty::Ref(_, _, hir::Mutability::Mut) => {
79 return Err((span, "mutable references in const fn are unstable".into()));
81 ty::Opaque(..) => return Err((span, "`impl Trait` in const fn is unstable".into())),
83 return Err((span, "function pointers in const fn are unstable".into()));
85 ty::Dynamic(preds, _) => {
86 for pred in preds.iter() {
87 match pred.skip_binder() {
88 ty::ExistentialPredicate::AutoTrait(_) | ty::ExistentialPredicate::Projection(_) => {
91 "trait bounds other than `Sized` \
92 on const fn parameters are unstable"
96 ty::ExistentialPredicate::Trait(trait_ref) => {
97 if Some(trait_ref.def_id) != tcx.lang_items().sized_trait() {
100 "trait bounds other than `Sized` \
101 on const fn parameters are unstable"
115 fn check_rvalue<'tcx>(
119 rvalue: &Rvalue<'tcx>,
123 Rvalue::ThreadLocalRef(_) => Err((span, "cannot access thread local storage in const fn".into())),
124 Rvalue::Len(place) | Rvalue::Discriminant(place) | Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
125 check_place(tcx, *place, span, body)
127 Rvalue::Repeat(operand, _)
128 | Rvalue::Use(operand)
130 CastKind::PointerFromExposedAddress
132 | CastKind::Pointer(PointerCast::MutToConstPointer | PointerCast::ArrayToPointer),
135 ) => check_operand(tcx, operand, span, body),
138 PointerCast::UnsafeFnPointer | PointerCast::ClosureFnPointer(_) | PointerCast::ReifyFnPointer,
142 ) => Err((span, "function pointer casts are not allowed in const fn".into())),
143 Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), op, cast_ty) => {
144 let pointee_ty = if let Some(deref_ty) = cast_ty.builtin_deref(true) {
147 // We cannot allow this for now.
148 return Err((span, "unsizing casts are only allowed for references right now".into()));
150 let unsized_ty = tcx.struct_tail_erasing_lifetimes(pointee_ty, tcx.param_env(def_id));
151 if let ty::Slice(_) | ty::Str = unsized_ty.kind() {
152 check_operand(tcx, op, span, body)?;
153 // Casting/coercing things to slices is fine.
156 // We just can't allow trait objects until we have figured out trait method calls.
157 Err((span, "unsizing casts are not allowed in const fn".into()))
160 Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => {
161 Err((span, "casting pointers to ints is unstable in const fn".into()))
163 // binops are fine on integers
164 Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
165 check_operand(tcx, lhs, span, body)?;
166 check_operand(tcx, rhs, span, body)?;
167 let ty = lhs.ty(body, tcx);
168 if ty.is_integral() || ty.is_bool() || ty.is_char() {
173 "only int, `bool` and `char` operations are stable in const fn".into(),
177 Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf, _) | Rvalue::ShallowInitBox(_, _) => Ok(()),
178 Rvalue::UnaryOp(_, operand) => {
179 let ty = operand.ty(body, tcx);
180 if ty.is_integral() || ty.is_bool() {
181 check_operand(tcx, operand, span, body)
183 Err((span, "only int and `bool` operations are stable in const fn".into()))
186 Rvalue::Aggregate(_, operands) => {
187 for operand in operands {
188 check_operand(tcx, operand, span, body)?;
195 fn check_statement<'tcx>(
199 statement: &Statement<'tcx>,
201 let span = statement.source_info.span;
202 match &statement.kind {
203 StatementKind::Assign(box (place, rval)) => {
204 check_place(tcx, *place, span, body)?;
205 check_rvalue(tcx, body, def_id, rval, span)
208 StatementKind::FakeRead(box (_, place)) => check_place(tcx, *place, span, body),
209 // just an assignment
210 StatementKind::SetDiscriminant { place, .. } | StatementKind::Deinit(place) => {
211 check_place(tcx, **place, span, body)
214 StatementKind::CopyNonOverlapping(box rustc_middle::mir::CopyNonOverlapping { dst, src, count }) => {
215 check_operand(tcx, dst, span, body)?;
216 check_operand(tcx, src, span, body)?;
217 check_operand(tcx, count, span, body)
219 // These are all NOPs
220 StatementKind::StorageLive(_)
221 | StatementKind::StorageDead(_)
222 | StatementKind::Retag { .. }
223 | StatementKind::AscribeUserType(..)
224 | StatementKind::Coverage(..)
225 | StatementKind::Nop => Ok(()),
229 fn check_operand<'tcx>(tcx: TyCtxt<'tcx>, operand: &Operand<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
231 Operand::Move(place) | Operand::Copy(place) => check_place(tcx, *place, span, body),
232 Operand::Constant(c) => match c.check_static_ptr(tcx) {
233 Some(_) => Err((span, "cannot access `static` items in const fn".into())),
239 fn check_place<'tcx>(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
240 let mut cursor = place.projection.as_ref();
241 while let [ref proj_base @ .., elem] = *cursor {
244 ProjectionElem::Field(..) => {
245 let base_ty = Place::ty_from(place.local, proj_base, body, tcx).ty;
246 if let Some(def) = base_ty.ty_adt_def() {
247 // No union field accesses in `const fn`
249 return Err((span, "accessing union fields is unstable".into()));
253 ProjectionElem::ConstantIndex { .. }
254 | ProjectionElem::Downcast(..)
255 | ProjectionElem::Subslice { .. }
256 | ProjectionElem::Deref
257 | ProjectionElem::Index(_) => {},
264 fn check_terminator<'a, 'tcx>(
266 body: &'a Body<'tcx>,
267 terminator: &Terminator<'tcx>,
268 msrv: Option<RustcVersion>,
270 let span = terminator.source_info.span;
271 match &terminator.kind {
272 TerminatorKind::FalseEdge { .. }
273 | TerminatorKind::FalseUnwind { .. }
274 | TerminatorKind::Goto { .. }
275 | TerminatorKind::Return
276 | TerminatorKind::Resume
277 | TerminatorKind::Unreachable => Ok(()),
279 TerminatorKind::Drop { place, .. } => check_place(tcx, *place, span, body),
280 TerminatorKind::DropAndReplace { place, value, .. } => {
281 check_place(tcx, *place, span, body)?;
282 check_operand(tcx, value, span, body)
285 TerminatorKind::SwitchInt {
289 } => check_operand(tcx, discr, span, body),
291 TerminatorKind::Abort => Err((span, "abort is not stable in const fn".into())),
292 TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
293 Err((span, "const fn generators are unstable".into()))
296 TerminatorKind::Call {
305 let fn_ty = func.ty(body, tcx);
306 if let ty::FnDef(fn_def_id, _) = *fn_ty.kind() {
307 if !is_const_fn(tcx, fn_def_id, msrv) {
311 "can only call other `const fn` within a `const fn`, \
312 but `{:?}` is not stable as `const fn`",
319 // HACK: This is to "unstabilize" the `transmute` intrinsic
320 // within const fns. `transmute` is allowed in all other const contexts.
321 // This won't really scale to more intrinsics or functions. Let's allow const
322 // transmutes in const fn before we add more hacks to this.
323 if tcx.is_intrinsic(fn_def_id) && tcx.item_name(fn_def_id) == sym::transmute {
326 "can only call `transmute` from const items, not `const fn`".into(),
330 check_operand(tcx, func, span, body)?;
333 check_operand(tcx, arg, span, body)?;
337 Err((span, "can only call other const fns within const fn".into()))
341 TerminatorKind::Assert {
347 } => check_operand(tcx, cond, span, body),
349 TerminatorKind::InlineAsm { .. } => Err((span, "cannot use inline assembly in const fn".into())),
353 fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: Option<RustcVersion>) -> bool {
354 tcx.is_const_fn(def_id)
355 && tcx.lookup_const_stability(def_id).map_or(true, |const_stab| {
356 if let rustc_attr::StabilityLevel::Stable { since, .. } = const_stab.level {
357 // Checking MSRV is manually necessary because `rustc` has no such concept. This entire
358 // function could be removed if `rustc` provided a MSRV-aware version of `is_const_fn`.
359 // as a part of an unimplemented MSRV check https://github.com/rust-lang/rust/issues/65262.
362 RustcVersion::parse(since.as_str())
363 .expect("`rustc_attr::StabilityLevel::Stable::since` is ill-formatted"),
366 // Unstable const fn with the feature enabled.