1 //! Structural const qualification.
3 //! See the `Qualif` trait for more info.
5 use rustc_errors::ErrorReported;
6 use rustc_infer::infer::TyCtxtInferExt;
7 use rustc_infer::traits::TraitEngine;
8 use rustc_middle::mir::*;
9 use rustc_middle::ty::{self, subst::SubstsRef, AdtDef, Ty};
10 use rustc_span::DUMMY_SP;
11 use rustc_trait_selection::traits::{
12 self, FulfillmentContext, ImplSource, Obligation, ObligationCause, SelectionContext,
17 pub fn in_any_value_of_ty<'tcx>(
18 cx: &ConstCx<'_, 'tcx>,
20 error_occured: Option<ErrorReported>,
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-insenstive, 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.at(DUMMY_SP), 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.
104 Some(adt.did) == cx.tcx.lang_items().unsafe_cell_type()
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 fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
150 // Avoid selecting for simple cases, such as builtin types.
151 if ty::util::is_trivially_const_drop(ty) {
155 let Some(drop_trait) = cx.tcx.lang_items().drop_trait() else {
156 // there is no way to define a type that needs non-const drop
157 // without having the lang item present.
161 let obligation = Obligation::new(
162 ObligationCause::dummy(),
164 ty::Binder::dummy(ty::TraitPredicate {
165 trait_ref: ty::TraitRef {
167 substs: cx.tcx.mk_substs_trait(ty, &[]),
169 constness: ty::BoundConstness::ConstIfConst,
170 polarity: ty::ImplPolarity::Positive,
174 cx.tcx.infer_ctxt().enter(|infcx| {
175 let mut selcx = SelectionContext::new(&infcx);
176 let Some(impl_src) = selcx.select(&obligation).ok().flatten() else {
177 // If we couldn't select a const drop candidate, then it's bad
183 ImplSource::ConstDrop(_) | ImplSource::Param(_, ty::BoundConstness::ConstIfConst)
185 // If our const drop candidate is not ConstDrop or implied by the param env,
190 if impl_src.borrow_nested_obligations().is_empty() {
194 // If we successfully found one, then select all of the predicates
195 // implied by our const drop impl.
196 let mut fcx = FulfillmentContext::new();
197 for nested in impl_src.nested_obligations() {
198 fcx.register_predicate_obligation(&infcx, nested);
201 // If we had any errors, then it's bad
202 !fcx.select_all_or_error(&infcx).is_empty()
206 fn in_adt_inherently<'tcx>(
207 cx: &ConstCx<'_, 'tcx>,
211 adt.has_non_const_dtor(cx.tcx)
215 /// A constant that cannot be used as part of a pattern in a `match` expression.
218 impl Qualif for CustomEq {
219 const ANALYSIS_NAME: &'static str = "flow_custom_eq";
221 fn in_qualifs(qualifs: &ConstQualifs) -> bool {
225 fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
226 // If *any* component of a composite data type does not implement `Structural{Partial,}Eq`,
227 // we know that at least some values of that type are not structural-match. I say "some"
228 // because that component may be part of an enum variant (e.g.,
229 // `Option::<NonStructuralMatchTy>::Some`), in which case some values of this type may be
230 // structural-match (`Option::None`).
231 traits::search_for_structural_match_violation(cx.body.span, cx.tcx, ty).is_some()
234 fn in_adt_inherently<'tcx>(
235 cx: &ConstCx<'_, 'tcx>,
237 substs: SubstsRef<'tcx>,
239 let ty = cx.tcx.mk_ty(ty::Adt(adt, substs));
240 !ty.is_structural_eq_shallow(cx.tcx)
244 // FIXME: Use `mir::visit::Visitor` for the `in_*` functions if/when it supports early return.
246 /// Returns `true` if this `Rvalue` contains qualif `Q`.
247 pub fn in_rvalue<'tcx, Q, F>(
248 cx: &ConstCx<'_, 'tcx>,
250 rvalue: &Rvalue<'tcx>,
254 F: FnMut(Local) -> bool,
257 Rvalue::ThreadLocalRef(_) | Rvalue::NullaryOp(..) => {
258 Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx))
261 Rvalue::Discriminant(place) | Rvalue::Len(place) => {
262 in_place::<Q, _>(cx, in_local, place.as_ref())
266 | Rvalue::Repeat(operand, _)
267 | Rvalue::UnaryOp(_, operand)
268 | Rvalue::Cast(_, operand, _)
269 | Rvalue::ShallowInitBox(operand, _) => in_operand::<Q, _>(cx, in_local, operand),
271 Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
272 in_operand::<Q, _>(cx, in_local, lhs) || in_operand::<Q, _>(cx, in_local, rhs)
275 Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => {
276 // Special-case reborrows to be more like a copy of the reference.
277 if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection() {
278 let base_ty = place_base.ty(cx.body, cx.tcx).ty;
279 if let ty::Ref(..) = base_ty.kind() {
280 return in_place::<Q, _>(cx, in_local, place_base);
284 in_place::<Q, _>(cx, in_local, place.as_ref())
287 Rvalue::Aggregate(kind, operands) => {
288 // Return early if we know that the struct or enum being constructed is always
290 if let AggregateKind::Adt(adt_did, _, substs, ..) = **kind {
291 let def = cx.tcx.adt_def(adt_did);
292 if Q::in_adt_inherently(cx, def, substs) {
295 if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) {
300 // Otherwise, proceed structurally...
301 operands.iter().any(|o| in_operand::<Q, _>(cx, in_local, o))
306 /// Returns `true` if this `Place` contains qualif `Q`.
307 pub fn in_place<'tcx, Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, place: PlaceRef<'tcx>) -> bool
310 F: FnMut(Local) -> bool,
312 let mut place = place;
313 while let Some((place_base, elem)) = place.last_projection() {
315 ProjectionElem::Index(index) if in_local(index) => return true,
317 ProjectionElem::Deref
318 | ProjectionElem::Field(_, _)
319 | ProjectionElem::ConstantIndex { .. }
320 | ProjectionElem::Subslice { .. }
321 | ProjectionElem::Downcast(_, _)
322 | ProjectionElem::Index(_) => {}
325 let base_ty = place_base.ty(cx.body, cx.tcx);
326 let proj_ty = base_ty.projection_ty(cx.tcx, elem).ty;
327 if !Q::in_any_value_of_ty(cx, proj_ty) {
334 assert!(place.projection.is_empty());
335 in_local(place.local)
338 /// Returns `true` if this `Operand` contains qualif `Q`.
339 pub fn in_operand<'tcx, Q, F>(
340 cx: &ConstCx<'_, 'tcx>,
342 operand: &Operand<'tcx>,
346 F: FnMut(Local) -> bool,
348 let constant = match operand {
349 Operand::Copy(place) | Operand::Move(place) => {
350 return in_place::<Q, _>(cx, in_local, place.as_ref());
353 Operand::Constant(c) => c,
356 // Check the qualifs of the value of `const` items.
357 if let Some(ct) = constant.literal.const_for_ty() {
358 if let ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs: _, promoted }) = ct.val {
359 // Use qualifs of the type for the promoted. Promoteds in MIR body should be possible
360 // only for `NeedsNonConstDrop` with precise drop checking. This is the only const
361 // check performed after the promotion. Verify that with an assertion.
362 assert!(promoted.is_none() || Q::ALLOW_PROMOTED);
363 // Don't peek inside trait associated constants.
364 if promoted.is_none() && cx.tcx.trait_of_item(def.did).is_none() {
365 let qualifs = if let Some((did, param_did)) = def.as_const_arg() {
366 cx.tcx.at(constant.span).mir_const_qualif_const_arg((did, param_did))
368 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())