1 //! Structural const qualification.
3 //! See the `Qualif` trait for more info.
5 use rustc_errors::ErrorGuaranteed;
6 use rustc_hir::LangItem;
7 use rustc_infer::infer::TyCtxtInferExt;
9 use rustc_middle::mir::*;
10 use rustc_middle::ty::{self, subst::SubstsRef, AdtDef, Ty};
11 use rustc_trait_selection::traits::{
12 self, ImplSource, Obligation, ObligationCause, SelectionContext,
17 pub fn in_any_value_of_ty<'tcx>(
18 cx: &ConstCx<'_, 'tcx>,
20 tainted_by_errors: Option<ErrorGuaranteed>,
23 has_mut_interior: HasMutInterior::in_any_value_of_ty(cx, ty),
24 needs_drop: NeedsDrop::in_any_value_of_ty(cx, ty),
25 needs_non_const_drop: NeedsNonConstDrop::in_any_value_of_ty(cx, ty),
26 custom_eq: CustomEq::in_any_value_of_ty(cx, ty),
31 /// A "qualif"(-ication) is a way to look for something "bad" in the MIR that would disqualify some
32 /// code for promotion or prevent it from evaluating at compile time.
34 /// Normally, we would determine what qualifications apply to each type and error when an illegal
35 /// operation is performed on such a type. However, this was found to be too imprecise, especially
36 /// in the presence of `enum`s. If only a single variant of an enum has a certain qualification, we
37 /// needn't reject code unless it actually constructs and operates on the qualified variant.
39 /// To accomplish this, const-checking and promotion use a value-based analysis (as opposed to a
40 /// type-based one). Qualifications propagate structurally across variables: If a local (or a
41 /// projection of a local) is assigned a qualified value, that local itself becomes qualified.
43 /// The name of the file used to debug the dataflow analysis that computes this qualif.
44 const ANALYSIS_NAME: &'static str;
46 /// Whether this `Qualif` is cleared when a local is moved from.
47 const IS_CLEARED_ON_MOVE: bool = false;
49 /// Whether this `Qualif` might be evaluated after the promotion and can encounter a promoted.
50 const ALLOW_PROMOTED: bool = false;
52 /// Extracts the field of `ConstQualifs` that corresponds to this `Qualif`.
53 fn in_qualifs(qualifs: &ConstQualifs) -> bool;
55 /// Returns `true` if *any* value of the given type could possibly have this `Qualif`.
57 /// This function determines `Qualif`s when we cannot do a value-based analysis. Since qualif
58 /// propagation is context-insensitive, this includes function arguments and values returned
59 /// from a call to another function.
61 /// It also determines the `Qualif`s for primitive types.
62 fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool;
64 /// Returns `true` if this `Qualif` is inherent to the given struct or enum.
66 /// By default, `Qualif`s propagate into ADTs in a structural way: An ADT only becomes
67 /// qualified if part of it is assigned a value with that `Qualif`. However, some ADTs *always*
68 /// have a certain `Qualif`, regardless of whether their fields have it. For example, a type
69 /// with a custom `Drop` impl is inherently `NeedsDrop`.
71 /// Returning `true` for `in_adt_inherently` but `false` for `in_any_value_of_ty` is unsound.
72 fn in_adt_inherently<'tcx>(
73 cx: &ConstCx<'_, 'tcx>,
75 substs: SubstsRef<'tcx>,
79 /// Constant containing interior mutability (`UnsafeCell<T>`).
80 /// This must be ruled out to make sure that evaluating the constant at compile-time
81 /// and at *any point* during the run-time would produce the same result. In particular,
82 /// promotion of temporaries must not change program behavior; if the promoted could be
83 /// written to, that would be a problem.
84 pub struct HasMutInterior;
86 impl Qualif for HasMutInterior {
87 const ANALYSIS_NAME: &'static str = "flow_has_mut_interior";
89 fn in_qualifs(qualifs: &ConstQualifs) -> bool {
90 qualifs.has_mut_interior
93 fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
94 !ty.is_freeze(cx.tcx, cx.param_env)
97 fn in_adt_inherently<'tcx>(
98 _cx: &ConstCx<'_, 'tcx>,
102 // Exactly one type, `UnsafeCell`, has the `HasMutInterior` qualif inherently.
103 // It arises structurally for all other types.
108 /// Constant containing an ADT that implements `Drop`.
109 /// This must be ruled out because implicit promotion would remove side-effects
110 /// that occur as part of dropping that value. N.B., the implicit promotion has
111 /// to reject const Drop implementations because even if side-effects are ruled
112 /// out through other means, the execution of the drop could diverge.
113 pub struct NeedsDrop;
115 impl Qualif for NeedsDrop {
116 const ANALYSIS_NAME: &'static str = "flow_needs_drop";
117 const IS_CLEARED_ON_MOVE: bool = true;
119 fn in_qualifs(qualifs: &ConstQualifs) -> bool {
123 fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
124 ty.needs_drop(cx.tcx, cx.param_env)
127 fn in_adt_inherently<'tcx>(
128 cx: &ConstCx<'_, 'tcx>,
136 /// Constant containing an ADT that implements non-const `Drop`.
137 /// This must be ruled out because we cannot run `Drop` during compile-time.
138 pub struct NeedsNonConstDrop;
140 impl Qualif for NeedsNonConstDrop {
141 const ANALYSIS_NAME: &'static str = "flow_needs_nonconst_drop";
142 const IS_CLEARED_ON_MOVE: bool = true;
143 const ALLOW_PROMOTED: bool = true;
145 fn in_qualifs(qualifs: &ConstQualifs) -> bool {
146 qualifs.needs_non_const_drop
149 #[instrument(level = "trace", skip(cx), ret)]
150 fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
151 // Avoid selecting for simple cases, such as builtin types.
152 if ty::util::is_trivially_const_drop(ty) {
156 let destruct = cx.tcx.require_lang_item(LangItem::Destruct, None);
158 let obligation = Obligation::new(
159 ObligationCause::dummy(),
161 ty::Binder::dummy(ty::TraitPredicate {
162 trait_ref: ty::TraitRef {
164 substs: cx.tcx.mk_substs_trait(ty, &[]),
166 constness: ty::BoundConstness::ConstIfConst,
167 polarity: ty::ImplPolarity::Positive,
171 let infcx = cx.tcx.infer_ctxt().build();
172 let mut selcx = SelectionContext::new(&infcx);
173 let Some(impl_src) = selcx.select(&obligation).ok().flatten() else {
174 // If we couldn't select a const destruct candidate, then it's bad
182 ImplSource::ConstDestruct(_) | ImplSource::Param(_, ty::BoundConstness::ConstIfConst)
184 // If our const destruct candidate is not ConstDestruct or implied by the param env,
189 if impl_src.borrow_nested_obligations().is_empty() {
193 // If we had any errors, then it's bad
194 !traits::fully_solve_obligations(&infcx, impl_src.nested_obligations()).is_empty()
197 fn in_adt_inherently<'tcx>(
198 cx: &ConstCx<'_, 'tcx>,
202 adt.has_non_const_dtor(cx.tcx)
206 /// A constant that cannot be used as part of a pattern in a `match` expression.
209 impl Qualif for CustomEq {
210 const ANALYSIS_NAME: &'static str = "flow_custom_eq";
212 fn in_qualifs(qualifs: &ConstQualifs) -> bool {
216 fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
217 // If *any* component of a composite data type does not implement `Structural{Partial,}Eq`,
218 // we know that at least some values of that type are not structural-match. I say "some"
219 // because that component may be part of an enum variant (e.g.,
220 // `Option::<NonStructuralMatchTy>::Some`), in which case some values of this type may be
221 // structural-match (`Option::None`).
222 traits::search_for_structural_match_violation(cx.body.span, cx.tcx, ty).is_some()
225 fn in_adt_inherently<'tcx>(
226 cx: &ConstCx<'_, 'tcx>,
228 substs: SubstsRef<'tcx>,
230 let ty = cx.tcx.mk_ty(ty::Adt(adt, substs));
231 !ty.is_structural_eq_shallow(cx.tcx)
235 // FIXME: Use `mir::visit::Visitor` for the `in_*` functions if/when it supports early return.
237 /// Returns `true` if this `Rvalue` contains qualif `Q`.
238 pub fn in_rvalue<'tcx, Q, F>(
239 cx: &ConstCx<'_, 'tcx>,
241 rvalue: &Rvalue<'tcx>,
245 F: FnMut(Local) -> bool,
248 Rvalue::ThreadLocalRef(_) | Rvalue::NullaryOp(..) => {
249 Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx))
252 Rvalue::Discriminant(place) | Rvalue::Len(place) => {
253 in_place::<Q, _>(cx, in_local, place.as_ref())
256 Rvalue::CopyForDeref(place) => in_place::<Q, _>(cx, in_local, place.as_ref()),
259 | Rvalue::Repeat(operand, _)
260 | Rvalue::UnaryOp(_, operand)
261 | Rvalue::Cast(_, operand, _)
262 | Rvalue::ShallowInitBox(operand, _) => in_operand::<Q, _>(cx, in_local, operand),
264 Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
265 in_operand::<Q, _>(cx, in_local, lhs) || in_operand::<Q, _>(cx, in_local, rhs)
268 Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
269 // Special-case reborrows to be more like a copy of the reference.
270 if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection() {
271 let base_ty = place_base.ty(cx.body, cx.tcx).ty;
272 if let ty::Ref(..) = base_ty.kind() {
273 return in_place::<Q, _>(cx, in_local, place_base);
277 in_place::<Q, _>(cx, in_local, place.as_ref())
280 Rvalue::Aggregate(kind, operands) => {
281 // Return early if we know that the struct or enum being constructed is always
283 if let AggregateKind::Adt(adt_did, _, substs, ..) = **kind {
284 let def = cx.tcx.adt_def(adt_did);
285 if Q::in_adt_inherently(cx, def, substs) {
288 if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) {
293 // Otherwise, proceed structurally...
294 operands.iter().any(|o| in_operand::<Q, _>(cx, in_local, o))
299 /// Returns `true` if this `Place` contains qualif `Q`.
300 pub fn in_place<'tcx, Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, place: PlaceRef<'tcx>) -> bool
303 F: FnMut(Local) -> bool,
305 let mut place = place;
306 while let Some((place_base, elem)) = place.last_projection() {
308 ProjectionElem::Index(index) if in_local(index) => return true,
310 ProjectionElem::Deref
311 | ProjectionElem::Field(_, _)
312 | ProjectionElem::OpaqueCast(_)
313 | ProjectionElem::ConstantIndex { .. }
314 | ProjectionElem::Subslice { .. }
315 | ProjectionElem::Downcast(_, _)
316 | ProjectionElem::Index(_) => {}
319 let base_ty = place_base.ty(cx.body, cx.tcx);
320 let proj_ty = base_ty.projection_ty(cx.tcx, elem).ty;
321 if !Q::in_any_value_of_ty(cx, proj_ty) {
328 assert!(place.projection.is_empty());
329 in_local(place.local)
332 /// Returns `true` if this `Operand` contains qualif `Q`.
333 pub fn in_operand<'tcx, Q, F>(
334 cx: &ConstCx<'_, 'tcx>,
336 operand: &Operand<'tcx>,
340 F: FnMut(Local) -> bool,
342 let constant = match operand {
343 Operand::Copy(place) | Operand::Move(place) => {
344 return in_place::<Q, _>(cx, in_local, place.as_ref());
347 Operand::Constant(c) => c,
350 // Check the qualifs of the value of `const` items.
351 // FIXME(valtrees): check whether const qualifs should behave the same
352 // way for type and mir constants.
353 let uneval = match constant.literal {
354 ConstantKind::Ty(ct) if matches!(ct.kind(), ty::ConstKind::Param(_)) => None,
355 ConstantKind::Ty(c) => bug!("expected ConstKind::Param here, found {:?}", c),
356 ConstantKind::Unevaluated(uv, _) => Some(uv),
357 ConstantKind::Val(..) => None,
360 if let Some(mir::UnevaluatedConst { def, substs: _, promoted }) = uneval {
361 // Use qualifs of the type for the promoted. Promoteds in MIR body should be possible
362 // only for `NeedsNonConstDrop` with precise drop checking. This is the only const
363 // check performed after the promotion. Verify that with an assertion.
364 assert!(promoted.is_none() || Q::ALLOW_PROMOTED);
366 // Don't peek inside trait associated constants.
367 if promoted.is_none() && cx.tcx.trait_of_item(def.did).is_none() {
368 assert_eq!(def.const_param_did, None, "expected associated const: {def:?}");
369 let qualifs = cx.tcx.at(constant.span).mir_const_qualif(def.did);
371 if !Q::in_qualifs(&qualifs) {
375 // Just in case the type is more specific than
376 // the definition, e.g., impl associated const
377 // with type parameters, take it into account.
381 // Otherwise use the qualifs of the type.
382 Q::in_any_value_of_ty(cx, constant.literal.ty())