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::Repeat(operand, _) | Rvalue::Use(operand) => check_operand(tcx, operand, span, body),
125 Rvalue::Len(place) | Rvalue::Discriminant(place) | Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
126 check_place(tcx, *place, span, body)
128 Rvalue::Cast(CastKind::PointerAddress, _, _) => {
129 Err((span, "casting pointers to ints is unstable in const fn".into()))
131 Rvalue::Cast(CastKind::Misc, operand, _) => {
132 check_operand(tcx, operand, span, body)
134 Rvalue::Cast(CastKind::Pointer(PointerCast::MutToConstPointer | PointerCast::ArrayToPointer), operand, _) => {
135 check_operand(tcx, operand, span, body)
139 PointerCast::UnsafeFnPointer | PointerCast::ClosureFnPointer(_) | PointerCast::ReifyFnPointer,
143 ) => Err((span, "function pointer casts are not allowed in const fn".into())),
144 Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), op, cast_ty) => {
145 let pointee_ty = if let Some(deref_ty) = cast_ty.builtin_deref(true) {
148 // We cannot allow this for now.
149 return Err((span, "unsizing casts are only allowed for references right now".into()));
151 let unsized_ty = tcx.struct_tail_erasing_lifetimes(pointee_ty, tcx.param_env(def_id));
152 if let ty::Slice(_) | ty::Str = unsized_ty.kind() {
153 check_operand(tcx, op, span, body)?;
154 // Casting/coercing things to slices is fine.
157 // We just can't allow trait objects until we have figured out trait method calls.
158 Err((span, "unsizing casts are not allowed in const fn".into()))
161 // binops are fine on integers
162 Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
163 check_operand(tcx, lhs, span, body)?;
164 check_operand(tcx, rhs, span, body)?;
165 let ty = lhs.ty(body, tcx);
166 if ty.is_integral() || ty.is_bool() || ty.is_char() {
171 "only int, `bool` and `char` operations are stable in const fn".into(),
175 Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf, _) | Rvalue::ShallowInitBox(_, _) => Ok(()),
176 Rvalue::UnaryOp(_, operand) => {
177 let ty = operand.ty(body, tcx);
178 if ty.is_integral() || ty.is_bool() {
179 check_operand(tcx, operand, span, body)
181 Err((span, "only int and `bool` operations are stable in const fn".into()))
184 Rvalue::Aggregate(_, operands) => {
185 for operand in operands {
186 check_operand(tcx, operand, span, body)?;
193 fn check_statement<'tcx>(
197 statement: &Statement<'tcx>,
199 let span = statement.source_info.span;
200 match &statement.kind {
201 StatementKind::Assign(box (place, rval)) => {
202 check_place(tcx, *place, span, body)?;
203 check_rvalue(tcx, body, def_id, rval, span)
206 StatementKind::FakeRead(box (_, place)) => check_place(tcx, *place, span, body),
207 // just an assignment
208 StatementKind::SetDiscriminant { place, .. } | StatementKind::Deinit(place) => {
209 check_place(tcx, **place, span, body)
212 StatementKind::CopyNonOverlapping(box rustc_middle::mir::CopyNonOverlapping { dst, src, count }) => {
213 check_operand(tcx, dst, span, body)?;
214 check_operand(tcx, src, span, body)?;
215 check_operand(tcx, count, span, body)
217 // These are all NOPs
218 StatementKind::StorageLive(_)
219 | StatementKind::StorageDead(_)
220 | StatementKind::Retag { .. }
221 | StatementKind::AscribeUserType(..)
222 | StatementKind::Coverage(..)
223 | StatementKind::Nop => Ok(()),
227 fn check_operand<'tcx>(tcx: TyCtxt<'tcx>, operand: &Operand<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
229 Operand::Move(place) | Operand::Copy(place) => check_place(tcx, *place, span, body),
230 Operand::Constant(c) => match c.check_static_ptr(tcx) {
231 Some(_) => Err((span, "cannot access `static` items in const fn".into())),
237 fn check_place<'tcx>(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
238 let mut cursor = place.projection.as_ref();
239 while let [ref proj_base @ .., elem] = *cursor {
242 ProjectionElem::Field(..) => {
243 let base_ty = Place::ty_from(place.local, proj_base, body, tcx).ty;
244 if let Some(def) = base_ty.ty_adt_def() {
245 // No union field accesses in `const fn`
247 return Err((span, "accessing union fields is unstable".into()));
251 ProjectionElem::ConstantIndex { .. }
252 | ProjectionElem::Downcast(..)
253 | ProjectionElem::Subslice { .. }
254 | ProjectionElem::Deref
255 | ProjectionElem::Index(_) => {},
262 fn check_terminator<'a, 'tcx>(
264 body: &'a Body<'tcx>,
265 terminator: &Terminator<'tcx>,
266 msrv: Option<RustcVersion>,
268 let span = terminator.source_info.span;
269 match &terminator.kind {
270 TerminatorKind::FalseEdge { .. }
271 | TerminatorKind::FalseUnwind { .. }
272 | TerminatorKind::Goto { .. }
273 | TerminatorKind::Return
274 | TerminatorKind::Resume
275 | TerminatorKind::Unreachable => Ok(()),
277 TerminatorKind::Drop { place, .. } => check_place(tcx, *place, span, body),
278 TerminatorKind::DropAndReplace { place, value, .. } => {
279 check_place(tcx, *place, span, body)?;
280 check_operand(tcx, value, span, body)
283 TerminatorKind::SwitchInt {
287 } => check_operand(tcx, discr, span, body),
289 TerminatorKind::Abort => Err((span, "abort is not stable in const fn".into())),
290 TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
291 Err((span, "const fn generators are unstable".into()))
294 TerminatorKind::Call {
303 let fn_ty = func.ty(body, tcx);
304 if let ty::FnDef(fn_def_id, _) = *fn_ty.kind() {
305 if !is_const_fn(tcx, fn_def_id, msrv) {
309 "can only call other `const fn` within a `const fn`, \
310 but `{:?}` is not stable as `const fn`",
317 // HACK: This is to "unstabilize" the `transmute` intrinsic
318 // within const fns. `transmute` is allowed in all other const contexts.
319 // This won't really scale to more intrinsics or functions. Let's allow const
320 // transmutes in const fn before we add more hacks to this.
321 if tcx.is_intrinsic(fn_def_id) && tcx.item_name(fn_def_id) == sym::transmute {
324 "can only call `transmute` from const items, not `const fn`".into(),
328 check_operand(tcx, func, span, body)?;
331 check_operand(tcx, arg, span, body)?;
335 Err((span, "can only call other const fns within const fn".into()))
339 TerminatorKind::Assert {
345 } => check_operand(tcx, cond, span, body),
347 TerminatorKind::InlineAsm { .. } => Err((span, "cannot use inline assembly in const fn".into())),
351 fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: Option<RustcVersion>) -> bool {
352 tcx.is_const_fn(def_id)
353 && tcx.lookup_const_stability(def_id).map_or(true, |const_stab| {
354 if let rustc_attr::StabilityLevel::Stable { since } = const_stab.level {
355 // Checking MSRV is manually necessary because `rustc` has no such concept. This entire
356 // function could be removed if `rustc` provided a MSRV-aware version of `is_const_fn`.
357 // as a part of an unimplemented MSRV check https://github.com/rust-lang/rust/issues/65262.
360 RustcVersion::parse(since.as_str())
361 .expect("`rustc_attr::StabilityLevel::Stable::since` is ill-formatted"),
364 // Unstable const fn with the feature enabled.