rust/clippy_utils/src/qualify_min_const_fn.rs

401 lines
16 KiB
Rust

// This code used to be a part of `rustc` but moved to Clippy as a result of
// https://github.com/rust-lang/rust/issues/76618. Because of that, it contains unused code and some
// of terminologies might not be relevant in the context of Clippy. Note that its behavior might
// differ from the time of `rustc` even if the name stays the same.
use crate::msrvs::Msrv;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_middle::mir::{
Body, CastKind, NonDivergingIntrinsic, NullOp, Operand, Place, ProjectionElem, Rvalue, Statement, StatementKind,
Terminator, TerminatorKind,
};
use rustc_middle::ty::subst::GenericArgKind;
use rustc_middle::ty::{self, adjustment::PointerCast, Ty, TyCtxt};
use rustc_semver::RustcVersion;
use rustc_span::symbol::sym;
use rustc_span::Span;
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();
let mut current = def_id;
loop {
let predicates = tcx.predicates_of(current);
for (predicate, _) in predicates.predicates {
match predicate.kind().skip_binder() {
ty::PredicateKind::Clause(
ty::Clause::RegionOutlives(_)
| ty::Clause::TypeOutlives(_)
| ty::Clause::Projection(_)
| ty::Clause::Trait(..)
| ty::Clause::ConstArgHasType(..),
)
| ty::PredicateKind::WellFormed(_)
| ty::PredicateKind::ConstEvaluatable(..)
| ty::PredicateKind::ConstEquate(..)
| ty::PredicateKind::TypeWellFormedFromEnv(..) => continue,
ty::PredicateKind::AliasRelate(..) => panic!("alias relate predicate on function: {predicate:#?}"),
ty::PredicateKind::ObjectSafe(_) => panic!("object safe predicate on function: {predicate:#?}"),
ty::PredicateKind::ClosureKind(..) => panic!("closure kind predicate on function: {predicate:#?}"),
ty::PredicateKind::Subtype(_) => panic!("subtype predicate on function: {predicate:#?}"),
ty::PredicateKind::Coerce(_) => panic!("coerce predicate on function: {predicate:#?}"),
ty::PredicateKind::Ambiguous => panic!("ambiguous predicate on function: {predicate:#?}"),
}
}
match predicates.parent {
Some(parent) => current = parent,
None => break,
}
}
for local in &body.local_decls {
check_ty(tcx, local.ty, local.source_info.span)?;
}
// impl trait is gone in MIR, so check the return type manually
check_ty(
tcx,
tcx.fn_sig(def_id).subst_identity().output().skip_binder(),
body.local_decls.iter().next().unwrap().source_info.span,
)?;
for bb in body.basic_blocks.iter() {
check_terminator(tcx, body, bb.terminator(), msrv)?;
for stmt in &bb.statements {
check_statement(tcx, body, def_id, stmt)?;
}
}
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,
// No constraints on lifetimes or constants, except potentially
// constants' types, but `walk` will get to them as well.
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.iter() {
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,
) -> 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::AddressOf(_, place) => {
check_place(tcx, *place, span, body)
},
Rvalue::CopyForDeref(place) => check_place(tcx, *place, span, body),
Rvalue::Repeat(operand, _)
| Rvalue::Use(operand)
| Rvalue::Cast(
CastKind::PointerFromExposedAddress
| CastKind::IntToInt
| CastKind::FloatToInt
| CastKind::IntToFloat
| CastKind::FloatToFloat
| CastKind::FnPtrToPtr
| CastKind::PtrToPtr
| CastKind::Pointer(PointerCast::MutToConstPointer | PointerCast::ArrayToPointer),
operand,
_,
) => check_operand(tcx, operand, span, body),
Rvalue::Cast(
CastKind::Pointer(
PointerCast::UnsafeFnPointer | PointerCast::ClosureFnPointer(_) | PointerCast::ReifyFnPointer,
),
_,
_,
) => Err((span, "function pointer casts are not allowed in const fn".into())),
Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), op, cast_ty) => {
let pointee_ty = if let Some(deref_ty) = cast_ty.builtin_deref(true) {
deref_ty.ty
} else {
// We cannot allow this for now.
return Err((span, "unsizing casts are only allowed for references right now".into()));
};
let unsized_ty = tcx.struct_tail_erasing_lifetimes(pointee_ty, tcx.param_env(def_id));
if let ty::Slice(_) | ty::Str = unsized_ty.kind() {
check_operand(tcx, op, span, body)?;
// Casting/coercing things to slices is fine.
Ok(())
} else {
// We just can't allow trait objects until we have figured out trait method calls.
Err((span, "unsizing casts are not allowed in const fn".into()))
}
},
Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => {
Err((span, "casting pointers to ints is unstable in const fn".into()))
},
Rvalue::Cast(CastKind::DynStar, _, _) => {
// FIXME(dyn-star)
unimplemented!()
},
Rvalue::Cast(CastKind::Transmute, _, _) => Err((
span,
"transmute can attempt to turn pointers into integers, so is unstable in const fn".into(),
)),
// binops are fine on integers
Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
check_operand(tcx, lhs, span, body)?;
check_operand(tcx, rhs, span, body)?;
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(_), _) | 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)
} 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)?;
}
Ok(())
},
}
}
fn check_statement<'tcx>(
tcx: TyCtxt<'tcx>,
body: &Body<'tcx>,
def_id: DefId,
statement: &Statement<'tcx>,
) -> McfResult {
let span = statement.source_info.span;
match &statement.kind {
StatementKind::Assign(box (place, rval)) => {
check_place(tcx, *place, span, body)?;
check_rvalue(tcx, body, def_id, rval, span)
},
StatementKind::FakeRead(box (_, place)) => check_place(tcx, *place, span, body),
// just an assignment
StatementKind::SetDiscriminant { place, .. } | StatementKind::Deinit(place) => {
check_place(tcx, **place, span, body)
},
StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(op)) => check_operand(tcx, op, span, body),
StatementKind::Intrinsic(box NonDivergingIntrinsic::CopyNonOverlapping(
rustc_middle::mir::CopyNonOverlapping { dst, src, count },
)) => {
check_operand(tcx, dst, span, body)?;
check_operand(tcx, src, span, body)?;
check_operand(tcx, count, span, body)
},
// These are all NOPs
StatementKind::StorageLive(_)
| StatementKind::StorageDead(_)
| StatementKind::Retag { .. }
| StatementKind::AscribeUserType(..)
| StatementKind::PlaceMention(..)
| StatementKind::Coverage(..)
| StatementKind::ConstEvalCounter
| StatementKind::Nop => Ok(()),
}
}
fn check_operand<'tcx>(tcx: TyCtxt<'tcx>, operand: &Operand<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
match operand {
Operand::Move(place) | Operand::Copy(place) => check_place(tcx, *place, span, body),
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>) -> McfResult {
let mut cursor = place.projection.as_ref();
while let [ref proj_base @ .., elem] = *cursor {
cursor = proj_base;
match elem {
ProjectionElem::Field(..) => {
let base_ty = Place::ty_from(place.local, proj_base, body, tcx).ty;
if let Some(def) = base_ty.ty_adt_def() {
// No union field accesses in `const fn`
if def.is_union() {
return Err((span, "accessing union fields is unstable".into()));
}
}
},
ProjectionElem::ConstantIndex { .. }
| ProjectionElem::OpaqueCast(..)
| ProjectionElem::Downcast(..)
| ProjectionElem::Subslice { .. }
| ProjectionElem::Deref
| 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::Resume
| TerminatorKind::Terminate
| TerminatorKind::Unreachable => Ok(()),
TerminatorKind::Drop { place, .. } => check_place(tcx, *place, span, body),
TerminatorKind::SwitchInt { discr, targets: _ } => check_operand(tcx, discr, span, body),
TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
Err((span, "const fn generators are unstable".into()))
},
TerminatorKind::Call {
func,
args,
from_hir_call: _,
destination: _,
target: _,
unwind: _,
fn_span: _,
} => {
let fn_ty = func.ty(body, tcx);
if let ty::FnDef(fn_def_id, _) = *fn_ty.kind() {
if !is_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(),
));
}
// HACK: This is to "unstabilize" the `transmute` intrinsic
// within const fns. `transmute` is allowed in all other const contexts.
// This won't really scale to more intrinsics or functions. Let's allow const
// transmutes in const fn before we add more hacks to this.
if tcx.is_intrinsic(fn_def_id) && tcx.item_name(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)?;
for arg in args {
check_operand(tcx, arg, span, body)?;
}
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),
TerminatorKind::InlineAsm { .. } => Err((span, "cannot use inline assembly in const fn".into())),
}
}
fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: &Msrv) -> bool {
tcx.is_const_fn(def_id)
&& tcx.lookup_const_stability(def_id).map_or(true, |const_stab| {
if let rustc_attr::StabilityLevel::Stable { since, .. } = const_stab.level {
// Checking MSRV is manually necessary because `rustc` has no such concept. This entire
// function could be removed if `rustc` provided a MSRV-aware version of `is_const_fn`.
// as a part of an unimplemented MSRV check https://github.com/rust-lang/rust/issues/65262.
// HACK(nilstrieb): CURRENT_RUSTC_VERSION can return versions like 1.66.0-dev. `rustc-semver`
// doesn't accept the `-dev` version number so we have to strip it
// off.
let short_version = since
.as_str()
.split('-')
.next()
.expect("rustc_attr::StabilityLevel::Stable::since` is empty");
let since = rustc_span::Symbol::intern(short_version);
msrv.meets(RustcVersion::parse(since.as_str()).unwrap_or_else(|err| {
panic!("`rustc_attr::StabilityLevel::Stable::since` is ill-formatted: `{since}`, {err:?}")
}))
} else {
// Unstable const fn with the feature enabled.
msrv.current().is_none()
}
})
}