use crate::msrvs::{self, Msrv};
use hir::LangItem;
use rustc_attr::StableSince;
use rustc_const_eval::check_consts::ConstCx;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_infer::infer::TyCtxtInferExt;
use rustc_infer::traits::Obligation;
use rustc_middle::mir::{
Body, CastKind, NonDivergingIntrinsic, NullOp, Operand, Place, ProjectionElem, Rvalue, Statement, StatementKind,
Terminator, TerminatorKind,
};
use rustc_middle::traits::{BuiltinImplSource, ImplSource, ObligationCause};
use rustc_middle::ty::adjustment::PointerCoercion;
use rustc_middle::ty::{self, GenericArgKind, TraitRef, Ty, TyCtxt};
use rustc_span::Span;
use rustc_span::symbol::sym;
use rustc_trait_selection::traits::{ObligationCtxt, SelectionContext};
use std::borrow::Cow;
type McfResult = Result<(), (Span, Cow<'static, str>)>;
pub fn is_min_const_fn<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, msrv: &Msrv) -> McfResult {
let def_id = body.source.def_id();
for local in &body.local_decls {
check_ty(tcx, local.ty, local.source_info.span)?;
}
check_ty(
tcx,
tcx.fn_sig(def_id).instantiate_identity().output().skip_binder(),
body.local_decls.iter().next().unwrap().source_info.span,
)?;
for bb in &*body.basic_blocks {
if !bb.is_cleanup {
check_terminator(tcx, body, bb.terminator(), msrv)?;
for stmt in &bb.statements {
check_statement(tcx, body, def_id, stmt, msrv)?;
}
}
}
Ok(())
}
fn check_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span) -> McfResult {
for arg in ty.walk() {
let ty = match arg.unpack() {
GenericArgKind::Type(ty) => ty,
GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => continue,
};
match ty.kind() {
ty::Ref(_, _, hir::Mutability::Mut) => {
return Err((span, "mutable references in const fn are unstable".into()));
},
ty::Alias(ty::Opaque, ..) => return Err((span, "`impl Trait` in const fn is unstable".into())),
ty::FnPtr(..) => {
return Err((span, "function pointers in const fn are unstable".into()));
},
ty::Dynamic(preds, _, _) => {
for pred in *preds {
match pred.skip_binder() {
ty::ExistentialPredicate::AutoTrait(_) | ty::ExistentialPredicate::Projection(_) => {
return Err((
span,
"trait bounds other than `Sized` \
on const fn parameters are unstable"
.into(),
));
},
ty::ExistentialPredicate::Trait(trait_ref) => {
if Some(trait_ref.def_id) != tcx.lang_items().sized_trait() {
return Err((
span,
"trait bounds other than `Sized` \
on const fn parameters are unstable"
.into(),
));
}
},
}
}
},
_ => {},
}
}
Ok(())
}
fn check_rvalue<'tcx>(
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,
def_id: DefId,
rvalue: &Rvalue<'tcx>,
span: Span,
msrv: &Msrv,
) -> McfResult {
match rvalue {
Rvalue::ThreadLocalRef(_) => Err((span, "cannot access thread local storage in const fn".into())),
Rvalue::Len(place) | Rvalue::Discriminant(place) | Rvalue::Ref(_, _, place) | Rvalue::RawPtr(_, place) => {
check_place(tcx, *place, span, body, msrv)
},
Rvalue::CopyForDeref(place) => check_place(tcx, *place, span, body, msrv),
Rvalue::Repeat(operand, _)
| Rvalue::Use(operand)
| Rvalue::Cast(
CastKind::PointerWithExposedProvenance
| CastKind::IntToInt
| CastKind::FloatToInt
| CastKind::IntToFloat
| CastKind::FloatToFloat
| CastKind::FnPtrToPtr
| CastKind::PtrToPtr
| CastKind::PointerCoercion(PointerCoercion::MutToConstPointer | PointerCoercion::ArrayToPointer, _),
operand,
_,
) => check_operand(tcx, operand, span, body, msrv),
Rvalue::Cast(
CastKind::PointerCoercion(
PointerCoercion::UnsafeFnPointer
| PointerCoercion::ClosureFnPointer(_)
| PointerCoercion::ReifyFnPointer,
_,
),
_,
_,
) => Err((span, "function pointer casts are not allowed in const fn".into())),
Rvalue::Cast(CastKind::PointerCoercion(PointerCoercion::Unsize, _), op, cast_ty) => {
let Some(pointee_ty) = cast_ty.builtin_deref(true) else {
return Err((span, "unsizing casts are only allowed for references right now".into()));
};
let unsized_ty = tcx.struct_tail_for_codegen(pointee_ty, ty::TypingEnv::post_analysis(tcx, def_id));
if let ty::Slice(_) | ty::Str = unsized_ty.kind() {
check_operand(tcx, op, span, body, msrv)?;
Ok(())
} else {
Err((span, "unsizing casts are not allowed in const fn".into()))
}
},
Rvalue::Cast(CastKind::PointerExposeProvenance, _, _) => {
Err((span, "casting pointers to ints is unstable in const fn".into()))
},
Rvalue::Cast(CastKind::PointerCoercion(PointerCoercion::DynStar, _), _, _) => {
unimplemented!()
},
Rvalue::Cast(CastKind::Transmute, _, _) => Err((
span,
"transmute can attempt to turn pointers into integers, so is unstable in const fn".into(),
)),
Rvalue::BinaryOp(_, box (lhs, rhs)) => {
check_operand(tcx, lhs, span, body, msrv)?;
check_operand(tcx, rhs, span, body, msrv)?;
let ty = lhs.ty(body, tcx);
if ty.is_integral() || ty.is_bool() || ty.is_char() {
Ok(())
} else {
Err((
span,
"only int, `bool` and `char` operations are stable in const fn".into(),
))
}
},
Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf | NullOp::OffsetOf(_) | NullOp::UbChecks, _)
| Rvalue::ShallowInitBox(_, _) => Ok(()),
Rvalue::UnaryOp(_, operand) => {
let ty = operand.ty(body, tcx);
if ty.is_integral() || ty.is_bool() {
check_operand(tcx, operand, span, body, msrv)
} else {
Err((span, "only int and `bool` operations are stable in const fn".into()))
}
},
Rvalue::Aggregate(_, operands) => {
for operand in operands {
check_operand(tcx, operand, span, body, msrv)?;
}
Ok(())
},
}
}
fn check_statement<'tcx>(
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,
def_id: DefId,
statement: &Statement<'tcx>,
msrv: &Msrv,
) -> McfResult {
let span = statement.source_info.span;
match &statement.kind {
StatementKind::Assign(box (place, rval)) => {
check_place(tcx, *place, span, body, msrv)?;
check_rvalue(tcx, body, def_id, rval, span, msrv)
},
StatementKind::FakeRead(box (_, place)) => check_place(tcx, *place, span, body, msrv),
StatementKind::SetDiscriminant { place, .. } | StatementKind::Deinit(place) => {
check_place(tcx, **place, span, body, msrv)
},
StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(op)) => check_operand(tcx, op, span, body, msrv),
StatementKind::Intrinsic(box NonDivergingIntrinsic::CopyNonOverlapping(
rustc_middle::mir::CopyNonOverlapping { dst, src, count },
)) => {
check_operand(tcx, dst, span, body, msrv)?;
check_operand(tcx, src, span, body, msrv)?;
check_operand(tcx, count, span, body, msrv)
},
StatementKind::StorageLive(_)
| StatementKind::StorageDead(_)
| StatementKind::Retag { .. }
| StatementKind::AscribeUserType(..)
| StatementKind::PlaceMention(..)
| StatementKind::Coverage(..)
| StatementKind::ConstEvalCounter
| StatementKind::BackwardIncompatibleDropHint { .. }
| StatementKind::Nop => Ok(()),
}
}
fn check_operand<'tcx>(
tcx: TyCtxt<'tcx>,
operand: &Operand<'tcx>,
span: Span,
body: &Body<'tcx>,
msrv: &Msrv,
) -> McfResult {
match operand {
Operand::Move(place) => {
if !place.projection.as_ref().is_empty()
&& !is_ty_const_destruct(tcx, place.ty(&body.local_decls, tcx).ty, body)
{
return Err((
span,
"cannot drop locals with a non constant destructor in const fn".into(),
));
}
check_place(tcx, *place, span, body, msrv)
},
Operand::Copy(place) => check_place(tcx, *place, span, body, msrv),
Operand::Constant(c) => match c.check_static_ptr(tcx) {
Some(_) => Err((span, "cannot access `static` items in const fn".into())),
None => Ok(()),
},
}
}
fn check_place<'tcx>(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &Body<'tcx>, msrv: &Msrv) -> McfResult {
for (base, elem) in place.as_ref().iter_projections() {
match elem {
ProjectionElem::Field(..) => {
if base.ty(body, tcx).ty.is_union() && !msrv.meets(msrvs::CONST_FN_UNION) {
return Err((span, "accessing union fields is unstable".into()));
}
},
ProjectionElem::Deref => match base.ty(body, tcx).ty.kind() {
ty::RawPtr(_, hir::Mutability::Mut) => {
return Err((span, "dereferencing raw mut pointer in const fn is unstable".into()));
},
ty::RawPtr(_, hir::Mutability::Not) if !msrv.meets(msrvs::CONST_RAW_PTR_DEREF) => {
return Err((span, "dereferencing raw const pointer in const fn is unstable".into()));
},
_ => (),
},
ProjectionElem::ConstantIndex { .. }
| ProjectionElem::OpaqueCast(..)
| ProjectionElem::Downcast(..)
| ProjectionElem::Subslice { .. }
| ProjectionElem::Subtype(_)
| ProjectionElem::Index(_) => {},
}
}
Ok(())
}
fn check_terminator<'tcx>(
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,
terminator: &Terminator<'tcx>,
msrv: &Msrv,
) -> McfResult {
let span = terminator.source_info.span;
match &terminator.kind {
TerminatorKind::FalseEdge { .. }
| TerminatorKind::FalseUnwind { .. }
| TerminatorKind::Goto { .. }
| TerminatorKind::Return
| TerminatorKind::UnwindResume
| TerminatorKind::UnwindTerminate(_)
| TerminatorKind::Unreachable => Ok(()),
TerminatorKind::Drop { place, .. } => {
if !is_ty_const_destruct(tcx, place.ty(&body.local_decls, tcx).ty, body) {
return Err((
span,
"cannot drop locals with a non constant destructor in const fn".into(),
));
}
Ok(())
},
TerminatorKind::SwitchInt { discr, targets: _ } => check_operand(tcx, discr, span, body, msrv),
TerminatorKind::CoroutineDrop | TerminatorKind::Yield { .. } => {
Err((span, "const fn coroutines are unstable".into()))
},
TerminatorKind::Call {
func,
args,
call_source: _,
destination: _,
target: _,
unwind: _,
fn_span: _,
}
| TerminatorKind::TailCall { func, args, fn_span: _ } => {
let fn_ty = func.ty(body, tcx);
if let ty::FnDef(fn_def_id, _) = *fn_ty.kind() {
if !is_stable_const_fn(tcx, fn_def_id, msrv) {
return Err((
span,
format!(
"can only call other `const fn` within a `const fn`, \
but `{func:?}` is not stable as `const fn`",
)
.into(),
));
}
if tcx.is_intrinsic(fn_def_id, sym::transmute) {
return Err((
span,
"can only call `transmute` from const items, not `const fn`".into(),
));
}
check_operand(tcx, func, span, body, msrv)?;
for arg in args {
check_operand(tcx, &arg.node, span, body, msrv)?;
}
Ok(())
} else {
Err((span, "can only call other const fns within const fn".into()))
}
},
TerminatorKind::Assert {
cond,
expected: _,
msg: _,
target: _,
unwind: _,
} => check_operand(tcx, cond, span, body, msrv),
TerminatorKind::InlineAsm { .. } => Err((span, "cannot use inline assembly in const fn".into())),
}
}
fn is_stable_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: &Msrv) -> bool {
tcx.is_const_fn(def_id)
&& tcx.lookup_const_stability(def_id).is_none_or(|const_stab| {
if let rustc_attr::StabilityLevel::Stable { since, .. } = const_stab.level {
let const_stab_rust_version = match since {
StableSince::Version(version) => version,
StableSince::Current => rustc_session::RustcVersion::CURRENT,
StableSince::Err => return false,
};
msrv.meets(const_stab_rust_version)
} else {
tcx.features().enabled(const_stab.feature) && msrv.current().is_none()
}
})
}
fn is_ty_const_destruct<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, body: &Body<'tcx>) -> bool {
#[expect(unused)]
fn is_ty_const_destruct_unused<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, body: &Body<'tcx>) -> bool {
if !ty.needs_drop(tcx, body.typing_env(tcx)) {
return false;
}
let (infcx, param_env) = tcx.infer_ctxt().build_with_typing_env(body.typing_env(tcx));
let obligation = Obligation::new(
tcx,
ObligationCause::dummy_with_span(body.span),
param_env,
TraitRef::new(tcx, tcx.require_lang_item(LangItem::Destruct, Some(body.span)), [ty]),
);
let mut selcx = SelectionContext::new(&infcx);
let Some(impl_src) = selcx.select(&obligation).ok().flatten() else {
return false;
};
if !matches!(
impl_src,
ImplSource::Builtin(BuiltinImplSource::Misc, _) | ImplSource::Param(_)
) {
return false;
}
let ocx = ObligationCtxt::new(&infcx);
ocx.register_obligations(impl_src.nested_obligations());
ocx.select_all_or_error().is_empty()
}
!ty.needs_drop(tcx, ConstCx::new(tcx, body).typing_env)
}