From 698f8c2f01ea549d77d7dc3338a12e04c11057b9 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Wed, 17 Apr 2024 14:02:58 +0200 Subject: Adding upstream version 1.64.0+dfsg1. Signed-off-by: Daniel Baumann --- .../src/transform/check_consts/qualifs.rs | 384 +++++++++++++++++++++ 1 file changed, 384 insertions(+) create mode 100644 compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs (limited to 'compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs') diff --git a/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs b/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs new file mode 100644 index 000000000..c8a63c9c3 --- /dev/null +++ b/compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs @@ -0,0 +1,384 @@ +//! 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_infer::traits::TraitEngine; +use rustc_middle::mir::*; +use rustc_middle::ty::{self, subst::SubstsRef, AdtDef, Ty}; +use rustc_span::DUMMY_SP; +use rustc_trait_selection::traits::{ + self, ImplSource, Obligation, ObligationCause, SelectionContext, TraitEngineExt, +}; + +use super::ConstCx; + +pub fn in_any_value_of_ty<'tcx>( + cx: &ConstCx<'_, 'tcx>, + ty: Ty<'tcx>, + tainted_by_errors: Option, +) -> ConstQualifs { + ConstQualifs { + has_mut_interior: HasMutInterior::in_any_value_of_ty(cx, ty), + needs_drop: NeedsDrop::in_any_value_of_ty(cx, ty), + needs_non_const_drop: NeedsNonConstDrop::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>, + substs: SubstsRef<'tcx>, + ) -> bool; +} + +/// Constant containing interior mutability (`UnsafeCell`). +/// 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.at(DUMMY_SP), cx.param_env) + } + + fn in_adt_inherently<'tcx>( + _cx: &ConstCx<'_, 'tcx>, + adt: AdtDef<'tcx>, + _: SubstsRef<'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>, + _: SubstsRef<'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 + } + + 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; + } + + let destruct = cx.tcx.require_lang_item(LangItem::Destruct, None); + + let obligation = Obligation::new( + ObligationCause::dummy(), + cx.param_env, + ty::Binder::dummy(ty::TraitPredicate { + trait_ref: ty::TraitRef { + def_id: destruct, + substs: cx.tcx.mk_substs_trait(ty, &[]), + }, + constness: ty::BoundConstness::ConstIfConst, + polarity: ty::ImplPolarity::Positive, + }), + ); + + cx.tcx.infer_ctxt().enter(|infcx| { + 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; + }; + + if !matches!( + impl_src, + ImplSource::ConstDestruct(_) + | ImplSource::Param(_, ty::BoundConstness::ConstIfConst) + ) { + // 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 successfully found one, then select all of the predicates + // implied by our const drop impl. + let mut fcx = >::new(cx.tcx); + for nested in impl_src.nested_obligations() { + fcx.register_predicate_obligation(&infcx, nested); + } + + // If we had any errors, then it's bad + !fcx.select_all_or_error(&infcx).is_empty() + }) + } + + fn in_adt_inherently<'tcx>( + cx: &ConstCx<'_, 'tcx>, + adt: AdtDef<'tcx>, + _: SubstsRef<'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::::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>, + adt: AdtDef<'tcx>, + substs: SubstsRef<'tcx>, + ) -> bool { + let ty = cx.tcx.mk_ty(ty::Adt(adt, substs)); + !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::(cx, in_local, place.as_ref()) + } + + Rvalue::CopyForDeref(place) => in_place::(cx, in_local, place.as_ref()), + + Rvalue::Use(operand) + | Rvalue::Repeat(operand, _) + | Rvalue::UnaryOp(_, operand) + | Rvalue::Cast(_, operand, _) + | Rvalue::ShallowInitBox(operand, _) => in_operand::(cx, in_local, operand), + + Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => { + in_operand::(cx, in_local, lhs) || in_operand::(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::(cx, in_local, place_base); + } + } + + in_place::(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, _, substs, ..) = **kind { + let def = cx.tcx.adt_def(adt_did); + if Q::in_adt_inherently(cx, def, substs) { + 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::(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::Field(_, _) + | 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::(cx, in_local, place.as_ref()); + } + + Operand::Constant(c) => c, + }; + + // Check the qualifs of the value of `const` items. + if let Some(ct) = constant.literal.const_for_ty() { + if let ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs: _, promoted }) = ct.kind() + { + // 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.did).is_none() { + let qualifs = if let Some((did, param_did)) = def.as_const_arg() { + cx.tcx.at(constant.span).mir_const_qualif_const_arg((did, param_did)) + } else { + cx.tcx.at(constant.span).mir_const_qualif(def.did) + }; + + 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.literal.ty()) +} -- cgit v1.2.3