clippy_utils/
qualify_min_const_fn.rs

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.
5
6use crate::msrvs::{self, Msrv};
7use hir::LangItem;
8use rustc_const_eval::check_consts::ConstCx;
9use rustc_hir as hir;
10use rustc_hir::def_id::DefId;
11use rustc_hir::{RustcVersion, StableSince};
12use rustc_infer::infer::TyCtxtInferExt;
13use rustc_infer::traits::Obligation;
14use rustc_lint::LateContext;
15use rustc_middle::mir::{
16    Body, CastKind, NonDivergingIntrinsic, Operand, Place, ProjectionElem, Rvalue, Statement, StatementKind,
17    Terminator, TerminatorKind,
18};
19use rustc_middle::traits::{BuiltinImplSource, ImplSource, ObligationCause};
20use rustc_middle::ty::adjustment::PointerCoercion;
21use rustc_middle::ty::{self, GenericArgKind, Instance, TraitRef, Ty, TyCtxt};
22use rustc_span::Span;
23use rustc_span::symbol::sym;
24use rustc_trait_selection::traits::{ObligationCtxt, SelectionContext};
25use std::borrow::Cow;
26
27type McfResult = Result<(), (Span, Cow<'static, str>)>;
28
29pub fn is_min_const_fn<'tcx>(cx: &LateContext<'tcx>, body: &Body<'tcx>, msrv: Msrv) -> McfResult {
30    let def_id = body.source.def_id();
31
32    for local in &body.local_decls {
33        check_ty(cx, local.ty, local.source_info.span, msrv)?;
34    }
35    if !msrv.meets(cx, msrvs::CONST_FN_TRAIT_BOUND)
36        && let Some(sized_did) = cx.tcx.lang_items().sized_trait()
37        && let Some(meta_sized_did) = cx.tcx.lang_items().meta_sized_trait()
38        && cx.tcx.param_env(def_id).caller_bounds().iter().any(|bound| {
39            bound.as_trait_clause().is_some_and(|clause| {
40                let did = clause.def_id();
41                did != sized_did && did != meta_sized_did
42            })
43        })
44    {
45        return Err((
46            body.span,
47            "non-`Sized` trait clause before `const_fn_trait_bound` is stabilized".into(),
48        ));
49    }
50    // impl trait is gone in MIR, so check the return type manually
51    check_ty(
52        cx,
53        cx.tcx.fn_sig(def_id).instantiate_identity().skip_norm_wip().output().skip_binder(),
54        body.local_decls.iter().next().unwrap().source_info.span,
55        msrv,
56    )?;
57
58    for bb in &*body.basic_blocks {
59        // Cleanup blocks are ignored entirely by const eval, so we can too:
60        // https://github.com/rust-lang/rust/blob/1dea922ea6e74f99a0e97de5cdb8174e4dea0444/compiler/rustc_const_eval/src/transform/check_consts/check.rs#L382
61        if !bb.is_cleanup {
62            check_terminator(cx, body, bb.terminator(), msrv)?;
63            for stmt in &bb.statements {
64                check_statement(cx, body, def_id, stmt, msrv)?;
65            }
66        }
67    }
68    Ok(())
69}
70
71fn check_ty<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, span: Span, msrv: Msrv) -> McfResult {
72    for arg in ty.walk() {
73        let ty = match arg.kind() {
74            GenericArgKind::Type(ty) => ty,
75
76            // No constraints on lifetimes or constants, except potentially
77            // constants' types, but `walk` will get to them as well.
78            GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => continue,
79        };
80
81        match ty.kind() {
82            ty::Ref(_, _, hir::Mutability::Mut) if !msrv.meets(cx, msrvs::CONST_MUT_REFS) => {
83                return Err((span, "mutable references in const fn are unstable".into()));
84            },
85            ty::Alias(ty::AliasTy {
86                kind: ty::Opaque { .. },
87                ..
88            }) => return Err((span, "`impl Trait` in const fn is unstable".into())),
89            ty::FnPtr(..) => {
90                return Err((span, "function pointers in const fn are unstable".into()));
91            },
92            ty::Dynamic(preds, _) => {
93                for pred in *preds {
94                    match pred.skip_binder() {
95                        ty::ExistentialPredicate::AutoTrait(_) | ty::ExistentialPredicate::Projection(_) => {
96                            return Err((
97                                span,
98                                "trait bounds other than `Sized` \
99                                 on const fn parameters are unstable"
100                                    .into(),
101                            ));
102                        },
103                        ty::ExistentialPredicate::Trait(trait_ref) => {
104                            if Some(trait_ref.def_id) != cx.tcx.lang_items().sized_trait() {
105                                return Err((
106                                    span,
107                                    "trait bounds other than `Sized` \
108                                     on const fn parameters are unstable"
109                                        .into(),
110                                ));
111                            }
112                        },
113                    }
114                }
115            },
116            _ => {},
117        }
118    }
119    Ok(())
120}
121
122fn check_rvalue<'tcx>(
123    cx: &LateContext<'tcx>,
124    body: &Body<'tcx>,
125    def_id: DefId,
126    rvalue: &Rvalue<'tcx>,
127    span: Span,
128    msrv: Msrv,
129) -> McfResult {
130    match rvalue {
131        Rvalue::ThreadLocalRef(_) => Err((span, "cannot access thread local storage in const fn".into())),
132        Rvalue::Discriminant(place) | Rvalue::Ref(_, _, place) | Rvalue::RawPtr(_, place) => {
133            check_place(cx, *place, span, body, msrv)
134        },
135        Rvalue::CopyForDeref(place) => check_place(cx, *place, span, body, msrv),
136        Rvalue::Repeat(operand, _)
137        | Rvalue::Use(operand)
138        | Rvalue::WrapUnsafeBinder(operand, _)
139        | Rvalue::Cast(
140            CastKind::PointerWithExposedProvenance
141            | CastKind::IntToInt
142            | CastKind::FloatToInt
143            | CastKind::IntToFloat
144            | CastKind::FloatToFloat
145            | CastKind::FnPtrToPtr
146            | CastKind::PtrToPtr
147            | CastKind::PointerCoercion(PointerCoercion::MutToConstPointer | PointerCoercion::ArrayToPointer, _)
148            | CastKind::Subtype,
149            operand,
150            _,
151        ) => check_operand(cx, operand, span, body, msrv),
152        Rvalue::Cast(
153            CastKind::PointerCoercion(
154                PointerCoercion::UnsafeFnPointer
155                | PointerCoercion::ClosureFnPointer(_)
156                | PointerCoercion::ReifyFnPointer(_),
157                _,
158            ),
159            _,
160            _,
161        ) => Err((span, "function pointer casts are not allowed in const fn".into())),
162        Rvalue::Cast(CastKind::PointerCoercion(PointerCoercion::Unsize, _), op, cast_ty) => {
163            let Some(pointee_ty) = cast_ty.builtin_deref(true) else {
164                // We cannot allow this for now.
165                return Err((span, "unsizing casts are only allowed for references right now".into()));
166            };
167            let unsized_ty = cx
168                .tcx
169                .struct_tail_for_codegen(pointee_ty, ty::TypingEnv::post_analysis(cx.tcx, def_id));
170            if let ty::Slice(_) | ty::Str = unsized_ty.kind() {
171                check_operand(cx, op, span, body, msrv)?;
172                // Casting/coercing things to slices is fine.
173                Ok(())
174            } else {
175                // We just can't allow trait objects until we have figured out trait method calls.
176                Err((span, "unsizing casts are not allowed in const fn".into()))
177            }
178        },
179        Rvalue::Cast(CastKind::PointerExposeProvenance, _, _) => {
180            Err((span, "casting pointers to ints is unstable in const fn".into()))
181        },
182        Rvalue::Cast(CastKind::Transmute, _, _) => Err((
183            span,
184            "transmute can attempt to turn pointers into integers, so is unstable in const fn".into(),
185        )),
186        // binops are fine on integers
187        Rvalue::BinaryOp(_, box (lhs, rhs)) => {
188            check_operand(cx, lhs, span, body, msrv)?;
189            check_operand(cx, rhs, span, body, msrv)?;
190            let ty = lhs.ty(body, cx.tcx);
191            if ty.is_integral() || ty.is_bool() || ty.is_char() {
192                Ok(())
193            } else {
194                Err((
195                    span,
196                    "only int, `bool` and `char` operations are stable in const fn".into(),
197                ))
198            }
199        },
200        Rvalue::UnaryOp(_, operand) => {
201            let ty = operand.ty(body, cx.tcx);
202            if ty.is_integral() || ty.is_bool() {
203                check_operand(cx, operand, span, body, msrv)
204            } else {
205                Err((span, "only int and `bool` operations are stable in const fn".into()))
206            }
207        },
208        Rvalue::Aggregate(_, operands) => {
209            for operand in operands {
210                check_operand(cx, operand, span, body, msrv)?;
211            }
212            Ok(())
213        },
214    }
215}
216
217fn check_statement<'tcx>(
218    cx: &LateContext<'tcx>,
219    body: &Body<'tcx>,
220    def_id: DefId,
221    statement: &Statement<'tcx>,
222    msrv: Msrv,
223) -> McfResult {
224    let span = statement.source_info.span;
225    match &statement.kind {
226        StatementKind::Assign(box (place, rval)) => {
227            check_place(cx, *place, span, body, msrv)?;
228            check_rvalue(cx, body, def_id, rval, span, msrv)
229        },
230
231        StatementKind::FakeRead(box (_, place)) => check_place(cx, *place, span, body, msrv),
232        // just an assignment
233        StatementKind::SetDiscriminant { place, .. } => check_place(cx, **place, span, body, msrv),
234
235        StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(op)) => check_operand(cx, op, span, body, msrv),
236
237        StatementKind::Intrinsic(box NonDivergingIntrinsic::CopyNonOverlapping(
238            rustc_middle::mir::CopyNonOverlapping { dst, src, count },
239        )) => {
240            check_operand(cx, dst, span, body, msrv)?;
241            check_operand(cx, src, span, body, msrv)?;
242            check_operand(cx, count, span, body, msrv)
243        },
244        // These are all NOPs
245        StatementKind::StorageLive(_)
246        | StatementKind::StorageDead(_)
247        | StatementKind::Retag { .. }
248        | StatementKind::AscribeUserType(..)
249        | StatementKind::PlaceMention(..)
250        | StatementKind::Coverage(..)
251        | StatementKind::ConstEvalCounter
252        | StatementKind::BackwardIncompatibleDropHint { .. }
253        | StatementKind::Nop => Ok(()),
254    }
255}
256
257fn check_operand<'tcx>(
258    cx: &LateContext<'tcx>,
259    operand: &Operand<'tcx>,
260    span: Span,
261    body: &Body<'tcx>,
262    msrv: Msrv,
263) -> McfResult {
264    match operand {
265        Operand::Move(place) => {
266            if !place.projection.as_ref().is_empty()
267                && !is_ty_const_destruct(cx.tcx, place.ty(&body.local_decls, cx.tcx).ty, body)
268            {
269                return Err((
270                    span,
271                    "cannot drop locals with a non constant destructor in const fn".into(),
272                ));
273            }
274
275            check_place(cx, *place, span, body, msrv)
276        },
277        Operand::Copy(place) => check_place(cx, *place, span, body, msrv),
278        Operand::Constant(c) => match c.check_static_ptr(cx.tcx) {
279            Some(_) => Err((span, "cannot access `static` items in const fn".into())),
280            None => Ok(()),
281        },
282        Operand::RuntimeChecks(..) => Ok(()),
283    }
284}
285
286fn check_place<'tcx>(
287    cx: &LateContext<'tcx>,
288    place: Place<'tcx>,
289    span: Span,
290    body: &Body<'tcx>,
291    msrv: Msrv,
292) -> McfResult {
293    for (base, elem) in place.as_ref().iter_projections() {
294        match elem {
295            ProjectionElem::Field(..) => {
296                if base.ty(body, cx.tcx).ty.is_union() && !msrv.meets(cx, msrvs::CONST_FN_UNION) {
297                    return Err((span, "accessing union fields is unstable".into()));
298                }
299            },
300            ProjectionElem::Deref => match base.ty(body, cx.tcx).ty.kind() {
301                ty::RawPtr(_, hir::Mutability::Mut) => {
302                    return Err((span, "dereferencing raw mut pointer in const fn is unstable".into()));
303                },
304                ty::RawPtr(_, hir::Mutability::Not) if !msrv.meets(cx, msrvs::CONST_RAW_PTR_DEREF) => {
305                    return Err((span, "dereferencing raw const pointer in const fn is unstable".into()));
306                },
307                _ => (),
308            },
309            ProjectionElem::ConstantIndex { .. }
310            | ProjectionElem::OpaqueCast(..)
311            | ProjectionElem::Downcast(..)
312            | ProjectionElem::Subslice { .. }
313            | ProjectionElem::Index(_)
314            | ProjectionElem::UnwrapUnsafeBinder(_) => {},
315        }
316    }
317
318    Ok(())
319}
320
321fn check_terminator<'tcx>(
322    cx: &LateContext<'tcx>,
323    body: &Body<'tcx>,
324    terminator: &Terminator<'tcx>,
325    msrv: Msrv,
326) -> McfResult {
327    let span = terminator.source_info.span;
328    match &terminator.kind {
329        TerminatorKind::FalseEdge { .. }
330        | TerminatorKind::FalseUnwind { .. }
331        | TerminatorKind::Goto { .. }
332        | TerminatorKind::Return
333        | TerminatorKind::UnwindResume
334        | TerminatorKind::UnwindTerminate(_)
335        | TerminatorKind::Unreachable => Ok(()),
336        TerminatorKind::Drop { place, .. } => {
337            if !is_ty_const_destruct(cx.tcx, place.ty(&body.local_decls, cx.tcx).ty, body) {
338                return Err((
339                    span,
340                    "cannot drop locals with a non constant destructor in const fn".into(),
341                ));
342            }
343            Ok(())
344        },
345        TerminatorKind::SwitchInt { discr, targets: _ } => check_operand(cx, discr, span, body, msrv),
346        TerminatorKind::CoroutineDrop | TerminatorKind::Yield { .. } => {
347            Err((span, "const fn coroutines are unstable".into()))
348        },
349        TerminatorKind::Call {
350            func,
351            args,
352            call_source: _,
353            destination: _,
354            target: _,
355            unwind: _,
356            fn_span: _,
357        }
358        | TerminatorKind::TailCall { func, args, fn_span: _ } => {
359            let fn_ty = func.ty(body, cx.tcx);
360            if let ty::FnDef(fn_def_id, fn_substs) = fn_ty.kind() {
361                // FIXME: when analyzing a function with generic parameters, we may not have enough information to
362                // resolve to an instance. However, we could check if a host effect predicate can guarantee that
363                // this can be made a `const` call.
364                let fn_def_id = match Instance::try_resolve(cx.tcx, cx.typing_env(), *fn_def_id, fn_substs) {
365                    Ok(Some(fn_inst)) => fn_inst.def_id(),
366                    Ok(None) => return Err((span, format!("cannot resolve instance for {func:?}").into())),
367                    Err(_) => return Err((span, format!("error during instance resolution of {func:?}").into())),
368                };
369                if !is_stable_const_fn(cx, fn_def_id, msrv) {
370                    return Err((
371                        span,
372                        format!(
373                            "can only call other `const fn` within a `const fn`, \
374                             but `{func:?}` is not stable as `const fn`",
375                        )
376                        .into(),
377                    ));
378                }
379
380                // HACK: This is to "unstabilize" the `transmute` intrinsic
381                // within const fns. `transmute` is allowed in all other const contexts.
382                // This won't really scale to more intrinsics or functions. Let's allow const
383                // transmutes in const fn before we add more hacks to this.
384                if cx.tcx.is_intrinsic(fn_def_id, sym::transmute) {
385                    return Err((
386                        span,
387                        "can only call `transmute` from const items, not `const fn`".into(),
388                    ));
389                }
390
391                check_operand(cx, func, span, body, msrv)?;
392
393                for arg in args {
394                    check_operand(cx, &arg.node, span, body, msrv)?;
395                }
396                Ok(())
397            } else {
398                Err((span, "can only call other const fns within const fn".into()))
399            }
400        },
401        TerminatorKind::Assert {
402            cond,
403            expected: _,
404            msg: _,
405            target: _,
406            unwind: _,
407        } => check_operand(cx, cond, span, body, msrv),
408        TerminatorKind::InlineAsm { .. } => Err((span, "cannot use inline assembly in const fn".into())),
409    }
410}
411
412/// Checks if the given `def_id` is a stable const fn, in respect to the given MSRV.
413pub fn is_stable_const_fn(cx: &LateContext<'_>, def_id: DefId, msrv: Msrv) -> bool {
414    cx.tcx.is_const_fn(def_id)
415        && cx
416            .tcx
417            .lookup_const_stability(def_id)
418            .or_else(|| {
419                cx.tcx
420                    .trait_of_assoc(def_id)
421                    .and_then(|trait_def_id| cx.tcx.lookup_const_stability(trait_def_id))
422            })
423            .is_none_or(|const_stab| {
424                if let rustc_hir::StabilityLevel::Stable { since, .. } = const_stab.level {
425                    // Checking MSRV is manually necessary because `rustc` has no such concept. This entire
426                    // function could be removed if `rustc` provided a MSRV-aware version of `is_stable_const_fn`.
427                    // as a part of an unimplemented MSRV check https://github.com/rust-lang/rust/issues/65262.
428
429                    let const_stab_rust_version = match since {
430                        StableSince::Version(version) => version,
431                        StableSince::Current => RustcVersion::CURRENT,
432                        StableSince::Err(_) => return false,
433                    };
434
435                    msrv.meets(cx, const_stab_rust_version)
436                } else {
437                    // Unstable const fn, check if the feature is enabled.
438                    cx.tcx.features().enabled(const_stab.feature) && msrv.current(cx).is_none()
439                }
440            })
441}
442
443fn is_ty_const_destruct<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, body: &Body<'tcx>) -> bool {
444    // FIXME(const_trait_impl, fee1-dead) revert to const destruct once it works again
445    #[expect(unused)]
446    fn is_ty_const_destruct_unused<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, body: &Body<'tcx>) -> bool {
447        // If this doesn't need drop at all, then don't select `[const] Destruct`.
448        if !ty.needs_drop(tcx, body.typing_env(tcx)) {
449            return false;
450        }
451
452        let (infcx, param_env) = tcx.infer_ctxt().build_with_typing_env(body.typing_env(tcx));
453        // FIXME(const_trait_impl) constness
454        let obligation = Obligation::new(
455            tcx,
456            ObligationCause::dummy_with_span(body.span),
457            param_env,
458            TraitRef::new(tcx, tcx.require_lang_item(LangItem::Destruct, body.span), [ty]),
459        );
460
461        let mut selcx = SelectionContext::new(&infcx);
462        let Some(impl_src) = selcx.select(&obligation).ok().flatten() else {
463            return false;
464        };
465
466        if !matches!(
467            impl_src,
468            ImplSource::Builtin(BuiltinImplSource::Misc, _) | ImplSource::Param(_)
469        ) {
470            return false;
471        }
472
473        let ocx = ObligationCtxt::new(&infcx);
474        ocx.register_obligations(impl_src.nested_obligations());
475        ocx.evaluate_obligations_error_on_ambiguity().is_empty()
476    }
477
478    !ty.needs_drop(tcx, ConstCx::new(tcx, body).typing_env)
479}
clippy_utils/qualify_min_const_fn.rs

clippy_utils/
qualify_min_const_fn.rs
