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//! Structural const qualification.
//!
//! See the `Qualif` trait for more info.
use rustc_errors::ErrorGuaranteed;
use rustc_hir::LangItem;
use rustc_infer::infer::TyCtxtInferExt;
use rustc_middle::mir;
use rustc_middle::mir::*;
use rustc_middle::traits::BuiltinImplSource;
use rustc_middle::ty::{self, AdtDef, GenericArgsRef, Ty};
use rustc_trait_selection::traits::{
self, ImplSource, Obligation, ObligationCause, ObligationCtxt, SelectionContext,
};
use super::ConstCx;
pub fn in_any_value_of_ty<'tcx>(
cx: &ConstCx<'_, 'tcx>,
ty: Ty<'tcx>,
tainted_by_errors: Option<ErrorGuaranteed>,
) -> ConstQualifs {
ConstQualifs {
has_mut_interior: HasMutInterior::in_any_value_of_ty(cx, ty),
needs_drop: NeedsDrop::in_any_value_of_ty(cx, ty),
// FIXME(effects)
needs_non_const_drop: NeedsDrop::in_any_value_of_ty(cx, ty),
custom_eq: CustomEq::in_any_value_of_ty(cx, ty),
tainted_by_errors,
}
}
/// A "qualif"(-ication) is a way to look for something "bad" in the MIR that would disqualify some
/// code for promotion or prevent it from evaluating at compile time.
///
/// Normally, we would determine what qualifications apply to each type and error when an illegal
/// operation is performed on such a type. However, this was found to be too imprecise, especially
/// in the presence of `enum`s. If only a single variant of an enum has a certain qualification, we
/// needn't reject code unless it actually constructs and operates on the qualified variant.
///
/// To accomplish this, const-checking and promotion use a value-based analysis (as opposed to a
/// type-based one). Qualifications propagate structurally across variables: If a local (or a
/// projection of a local) is assigned a qualified value, that local itself becomes qualified.
pub trait Qualif {
/// The name of the file used to debug the dataflow analysis that computes this qualif.
const ANALYSIS_NAME: &'static str;
/// Whether this `Qualif` is cleared when a local is moved from.
const IS_CLEARED_ON_MOVE: bool = false;
/// Whether this `Qualif` might be evaluated after the promotion and can encounter a promoted.
const ALLOW_PROMOTED: bool = false;
/// Extracts the field of `ConstQualifs` that corresponds to this `Qualif`.
fn in_qualifs(qualifs: &ConstQualifs) -> bool;
/// Returns `true` if *any* value of the given type could possibly have this `Qualif`.
///
/// This function determines `Qualif`s when we cannot do a value-based analysis. Since qualif
/// propagation is context-insensitive, this includes function arguments and values returned
/// from a call to another function.
///
/// It also determines the `Qualif`s for primitive types.
fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool;
/// Returns `true` if this `Qualif` is inherent to the given struct or enum.
///
/// By default, `Qualif`s propagate into ADTs in a structural way: An ADT only becomes
/// qualified if part of it is assigned a value with that `Qualif`. However, some ADTs *always*
/// have a certain `Qualif`, regardless of whether their fields have it. For example, a type
/// with a custom `Drop` impl is inherently `NeedsDrop`.
///
/// Returning `true` for `in_adt_inherently` but `false` for `in_any_value_of_ty` is unsound.
fn in_adt_inherently<'tcx>(
cx: &ConstCx<'_, 'tcx>,
adt: AdtDef<'tcx>,
args: GenericArgsRef<'tcx>,
) -> bool;
}
/// Constant containing interior mutability (`UnsafeCell<T>`).
/// This must be ruled out to make sure that evaluating the constant at compile-time
/// and at *any point* during the run-time would produce the same result. In particular,
/// promotion of temporaries must not change program behavior; if the promoted could be
/// written to, that would be a problem.
pub struct HasMutInterior;
impl Qualif for HasMutInterior {
const ANALYSIS_NAME: &'static str = "flow_has_mut_interior";
fn in_qualifs(qualifs: &ConstQualifs) -> bool {
qualifs.has_mut_interior
}
fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
!ty.is_freeze(cx.tcx, cx.param_env)
}
fn in_adt_inherently<'tcx>(
_cx: &ConstCx<'_, 'tcx>,
adt: AdtDef<'tcx>,
_: GenericArgsRef<'tcx>,
) -> bool {
// Exactly one type, `UnsafeCell`, has the `HasMutInterior` qualif inherently.
// It arises structurally for all other types.
adt.is_unsafe_cell()
}
}
/// Constant containing an ADT that implements `Drop`.
/// This must be ruled out because implicit promotion would remove side-effects
/// that occur as part of dropping that value. N.B., the implicit promotion has
/// to reject const Drop implementations because even if side-effects are ruled
/// out through other means, the execution of the drop could diverge.
pub struct NeedsDrop;
impl Qualif for NeedsDrop {
const ANALYSIS_NAME: &'static str = "flow_needs_drop";
const IS_CLEARED_ON_MOVE: bool = true;
fn in_qualifs(qualifs: &ConstQualifs) -> bool {
qualifs.needs_drop
}
fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
ty.needs_drop(cx.tcx, cx.param_env)
}
fn in_adt_inherently<'tcx>(
cx: &ConstCx<'_, 'tcx>,
adt: AdtDef<'tcx>,
_: GenericArgsRef<'tcx>,
) -> bool {
adt.has_dtor(cx.tcx)
}
}
/// Constant containing an ADT that implements non-const `Drop`.
/// This must be ruled out because we cannot run `Drop` during compile-time.
pub struct NeedsNonConstDrop;
impl Qualif for NeedsNonConstDrop {
const ANALYSIS_NAME: &'static str = "flow_needs_nonconst_drop";
const IS_CLEARED_ON_MOVE: bool = true;
const ALLOW_PROMOTED: bool = true;
fn in_qualifs(qualifs: &ConstQualifs) -> bool {
qualifs.needs_non_const_drop
}
#[instrument(level = "trace", skip(cx), ret)]
fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
// Avoid selecting for simple cases, such as builtin types.
if ty::util::is_trivially_const_drop(ty) {
return false;
}
// FIXME(effects) constness
let obligation = Obligation::new(
cx.tcx,
ObligationCause::dummy_with_span(cx.body.span),
cx.param_env,
ty::TraitRef::from_lang_item(cx.tcx, LangItem::Destruct, cx.body.span, [ty]),
);
let infcx = cx.tcx.infer_ctxt().build();
let mut selcx = SelectionContext::new(&infcx);
let Some(impl_src) = selcx.select(&obligation).ok().flatten() else {
// If we couldn't select a const destruct candidate, then it's bad
return true;
};
trace!(?impl_src);
if !matches!(
impl_src,
ImplSource::Builtin(BuiltinImplSource::Misc, _) | ImplSource::Param(_)
) {
// If our const destruct candidate is not ConstDestruct or implied by the param env,
// then it's bad
return true;
}
if impl_src.borrow_nested_obligations().is_empty() {
return false;
}
// If we had any errors, then it's bad
let ocx = ObligationCtxt::new(&infcx);
ocx.register_obligations(impl_src.nested_obligations());
let errors = ocx.select_all_or_error();
!errors.is_empty()
}
fn in_adt_inherently<'tcx>(
cx: &ConstCx<'_, 'tcx>,
adt: AdtDef<'tcx>,
_: GenericArgsRef<'tcx>,
) -> bool {
adt.has_non_const_dtor(cx.tcx)
}
}
/// A constant that cannot be used as part of a pattern in a `match` expression.
pub struct CustomEq;
impl Qualif for CustomEq {
const ANALYSIS_NAME: &'static str = "flow_custom_eq";
fn in_qualifs(qualifs: &ConstQualifs) -> bool {
qualifs.custom_eq
}
fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
// If *any* component of a composite data type does not implement `Structural{Partial,}Eq`,
// we know that at least some values of that type are not structural-match. I say "some"
// because that component may be part of an enum variant (e.g.,
// `Option::<NonStructuralMatchTy>::Some`), in which case some values of this type may be
// structural-match (`Option::None`).
traits::search_for_structural_match_violation(cx.body.span, cx.tcx, ty).is_some()
}
fn in_adt_inherently<'tcx>(
cx: &ConstCx<'_, 'tcx>,
def: AdtDef<'tcx>,
args: GenericArgsRef<'tcx>,
) -> bool {
let ty = Ty::new_adt(cx.tcx, def, args);
!ty.is_structural_eq_shallow(cx.tcx)
}
}
// FIXME: Use `mir::visit::Visitor` for the `in_*` functions if/when it supports early return.
/// Returns `true` if this `Rvalue` contains qualif `Q`.
pub fn in_rvalue<'tcx, Q, F>(
cx: &ConstCx<'_, 'tcx>,
in_local: &mut F,
rvalue: &Rvalue<'tcx>,
) -> bool
where
Q: Qualif,
F: FnMut(Local) -> bool,
{
match rvalue {
Rvalue::ThreadLocalRef(_) | Rvalue::NullaryOp(..) => {
Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx))
}
Rvalue::Discriminant(place) | Rvalue::Len(place) => {
in_place::<Q, _>(cx, in_local, place.as_ref())
}
Rvalue::CopyForDeref(place) => in_place::<Q, _>(cx, in_local, place.as_ref()),
Rvalue::Use(operand)
| Rvalue::Repeat(operand, _)
| Rvalue::UnaryOp(_, operand)
| Rvalue::Cast(_, operand, _)
| Rvalue::ShallowInitBox(operand, _) => in_operand::<Q, _>(cx, in_local, operand),
Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
in_operand::<Q, _>(cx, in_local, lhs) || in_operand::<Q, _>(cx, in_local, rhs)
}
Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
// Special-case reborrows to be more like a copy of the reference.
if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection() {
let base_ty = place_base.ty(cx.body, cx.tcx).ty;
if let ty::Ref(..) = base_ty.kind() {
return in_place::<Q, _>(cx, in_local, place_base);
}
}
in_place::<Q, _>(cx, in_local, place.as_ref())
}
Rvalue::Aggregate(kind, operands) => {
// Return early if we know that the struct or enum being constructed is always
// qualified.
if let AggregateKind::Adt(adt_did, _, args, ..) = **kind {
let def = cx.tcx.adt_def(adt_did);
if Q::in_adt_inherently(cx, def, args) {
return true;
}
if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) {
return true;
}
}
// Otherwise, proceed structurally...
operands.iter().any(|o| in_operand::<Q, _>(cx, in_local, o))
}
}
}
/// Returns `true` if this `Place` contains qualif `Q`.
pub fn in_place<'tcx, Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, place: PlaceRef<'tcx>) -> bool
where
Q: Qualif,
F: FnMut(Local) -> bool,
{
let mut place = place;
while let Some((place_base, elem)) = place.last_projection() {
match elem {
ProjectionElem::Index(index) if in_local(index) => return true,
ProjectionElem::Deref
| ProjectionElem::Subtype(_)
| ProjectionElem::Field(_, _)
| ProjectionElem::OpaqueCast(_)
| ProjectionElem::ConstantIndex { .. }
| ProjectionElem::Subslice { .. }
| ProjectionElem::Downcast(_, _)
| ProjectionElem::Index(_) => {}
}
let base_ty = place_base.ty(cx.body, cx.tcx);
let proj_ty = base_ty.projection_ty(cx.tcx, elem).ty;
if !Q::in_any_value_of_ty(cx, proj_ty) {
return false;
}
place = place_base;
}
assert!(place.projection.is_empty());
in_local(place.local)
}
/// Returns `true` if this `Operand` contains qualif `Q`.
pub fn in_operand<'tcx, Q, F>(
cx: &ConstCx<'_, 'tcx>,
in_local: &mut F,
operand: &Operand<'tcx>,
) -> bool
where
Q: Qualif,
F: FnMut(Local) -> bool,
{
let constant = match operand {
Operand::Copy(place) | Operand::Move(place) => {
return in_place::<Q, _>(cx, in_local, place.as_ref());
}
Operand::Constant(c) => c,
};
// Check the qualifs of the value of `const` items.
let uneval = match constant.const_ {
Const::Ty(ct)
if matches!(
ct.kind(),
ty::ConstKind::Param(_) | ty::ConstKind::Error(_) | ty::ConstKind::Value(_)
) =>
{
None
}
Const::Ty(c) => {
bug!("expected ConstKind::Param or ConstKind::Value here, found {:?}", c)
}
Const::Unevaluated(uv, _) => Some(uv),
Const::Val(..) => None,
};
if let Some(mir::UnevaluatedConst { def, args: _, promoted }) = uneval {
// Use qualifs of the type for the promoted. Promoteds in MIR body should be possible
// only for `NeedsNonConstDrop` with precise drop checking. This is the only const
// check performed after the promotion. Verify that with an assertion.
assert!(promoted.is_none() || Q::ALLOW_PROMOTED);
// Don't peek inside trait associated constants.
if promoted.is_none() && cx.tcx.trait_of_item(def).is_none() {
let qualifs = cx.tcx.at(constant.span).mir_const_qualif(def);
if !Q::in_qualifs(&qualifs) {
return false;
}
// Just in case the type is more specific than
// the definition, e.g., impl associated const
// with type parameters, take it into account.
}
}
// Otherwise use the qualifs of the type.
Q::in_any_value_of_ty(cx, constant.const_.ty())
}