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Commit | Line | Data |
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ba9703b0 XL |
1 | //! Structural const qualification. |
2 | //! | |
3 | //! See the `Qualif` trait for more info. | |
e74abb32 | 4 | |
fc512014 | 5 | use rustc_errors::ErrorReported; |
c295e0f8 | 6 | use rustc_infer::infer::TyCtxtInferExt; |
5099ac24 | 7 | use rustc_infer::traits::TraitEngine; |
ba9703b0 | 8 | use rustc_middle::mir::*; |
f9f354fc | 9 | use rustc_middle::ty::{self, subst::SubstsRef, AdtDef, Ty}; |
dfeec247 | 10 | use rustc_span::DUMMY_SP; |
c295e0f8 | 11 | use rustc_trait_selection::traits::{ |
5099ac24 | 12 | self, FulfillmentContext, ImplSource, Obligation, ObligationCause, SelectionContext, |
c295e0f8 | 13 | }; |
e74abb32 | 14 | |
f9f354fc | 15 | use super::ConstCx; |
e74abb32 | 16 | |
a2a8927a | 17 | pub fn in_any_value_of_ty<'tcx>( |
fc512014 XL |
18 | cx: &ConstCx<'_, 'tcx>, |
19 | ty: Ty<'tcx>, | |
5099ac24 | 20 | tainted_by_errors: Option<ErrorReported>, |
fc512014 | 21 | ) -> ConstQualifs { |
60c5eb7d XL |
22 | ConstQualifs { |
23 | has_mut_interior: HasMutInterior::in_any_value_of_ty(cx, ty), | |
3c0e092e XL |
24 | needs_drop: NeedsDrop::in_any_value_of_ty(cx, ty), |
25 | needs_non_const_drop: NeedsNonConstDrop::in_any_value_of_ty(cx, ty), | |
f9f354fc | 26 | custom_eq: CustomEq::in_any_value_of_ty(cx, ty), |
5099ac24 | 27 | tainted_by_errors, |
e74abb32 XL |
28 | } |
29 | } | |
30 | ||
31 | /// A "qualif"(-ication) is a way to look for something "bad" in the MIR that would disqualify some | |
ba9703b0 | 32 | /// code for promotion or prevent it from evaluating at compile time. |
e74abb32 | 33 | /// |
ba9703b0 XL |
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 | |
cdc7bbd5 | 37 | /// needn't reject code unless it actually constructs and operates on the qualified variant. |
ba9703b0 XL |
38 | /// |
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 | |
cdc7bbd5 | 41 | /// projection of a local) is assigned a qualified value, that local itself becomes qualified. |
e74abb32 | 42 | pub trait Qualif { |
e74abb32 XL |
43 | /// The name of the file used to debug the dataflow analysis that computes this qualif. |
44 | const ANALYSIS_NAME: &'static str; | |
45 | ||
46 | /// Whether this `Qualif` is cleared when a local is moved from. | |
47 | const IS_CLEARED_ON_MOVE: bool = false; | |
48 | ||
3c0e092e XL |
49 | /// Whether this `Qualif` might be evaluated after the promotion and can encounter a promoted. |
50 | const ALLOW_PROMOTED: bool = false; | |
51 | ||
ba9703b0 | 52 | /// Extracts the field of `ConstQualifs` that corresponds to this `Qualif`. |
60c5eb7d XL |
53 | fn in_qualifs(qualifs: &ConstQualifs) -> bool; |
54 | ||
ba9703b0 XL |
55 | /// Returns `true` if *any* value of the given type could possibly have this `Qualif`. |
56 | /// | |
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. | |
60 | /// | |
61 | /// It also determines the `Qualif`s for primitive types. | |
a2a8927a | 62 | fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool; |
ba9703b0 XL |
63 | |
64 | /// Returns `true` if this `Qualif` is inherent to the given struct or enum. | |
65 | /// | |
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`. | |
70 | /// | |
71 | /// Returning `true` for `in_adt_inherently` but `false` for `in_any_value_of_ty` is unsound. | |
a2a8927a | 72 | fn in_adt_inherently<'tcx>( |
f9f354fc XL |
73 | cx: &ConstCx<'_, 'tcx>, |
74 | adt: &'tcx AdtDef, | |
75 | substs: SubstsRef<'tcx>, | |
76 | ) -> bool; | |
e74abb32 XL |
77 | } |
78 | ||
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; | |
85 | ||
86 | impl Qualif for HasMutInterior { | |
e74abb32 XL |
87 | const ANALYSIS_NAME: &'static str = "flow_has_mut_interior"; |
88 | ||
60c5eb7d XL |
89 | fn in_qualifs(qualifs: &ConstQualifs) -> bool { |
90 | qualifs.has_mut_interior | |
91 | } | |
92 | ||
a2a8927a | 93 | fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool { |
f035d41b | 94 | !ty.is_freeze(cx.tcx.at(DUMMY_SP), cx.param_env) |
e74abb32 XL |
95 | } |
96 | ||
a2a8927a XL |
97 | fn in_adt_inherently<'tcx>( |
98 | cx: &ConstCx<'_, 'tcx>, | |
99 | adt: &'tcx AdtDef, | |
100 | _: SubstsRef<'tcx>, | |
101 | ) -> bool { | |
ba9703b0 XL |
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() | |
e74abb32 XL |
105 | } |
106 | } | |
107 | ||
108 | /// Constant containing an ADT that implements `Drop`. | |
3c0e092e XL |
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; | |
114 | ||
115 | impl Qualif for NeedsDrop { | |
116 | const ANALYSIS_NAME: &'static str = "flow_needs_drop"; | |
117 | const IS_CLEARED_ON_MOVE: bool = true; | |
118 | ||
119 | fn in_qualifs(qualifs: &ConstQualifs) -> bool { | |
120 | qualifs.needs_drop | |
121 | } | |
122 | ||
a2a8927a | 123 | fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool { |
3c0e092e XL |
124 | ty.needs_drop(cx.tcx, cx.param_env) |
125 | } | |
126 | ||
a2a8927a XL |
127 | fn in_adt_inherently<'tcx>( |
128 | cx: &ConstCx<'_, 'tcx>, | |
129 | adt: &'tcx AdtDef, | |
130 | _: SubstsRef<'tcx>, | |
131 | ) -> bool { | |
3c0e092e XL |
132 | adt.has_dtor(cx.tcx) |
133 | } | |
134 | } | |
135 | ||
136 | /// Constant containing an ADT that implements non-const `Drop`. | |
137 | /// This must be ruled out because we cannot run `Drop` during compile-time. | |
c295e0f8 | 138 | pub struct NeedsNonConstDrop; |
e74abb32 | 139 | |
c295e0f8 XL |
140 | impl Qualif for NeedsNonConstDrop { |
141 | const ANALYSIS_NAME: &'static str = "flow_needs_nonconst_drop"; | |
e74abb32 | 142 | const IS_CLEARED_ON_MOVE: bool = true; |
3c0e092e | 143 | const ALLOW_PROMOTED: bool = true; |
e74abb32 | 144 | |
60c5eb7d | 145 | fn in_qualifs(qualifs: &ConstQualifs) -> bool { |
3c0e092e | 146 | qualifs.needs_non_const_drop |
60c5eb7d XL |
147 | } |
148 | ||
5099ac24 FG |
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) { | |
152 | return false; | |
c295e0f8 XL |
153 | } |
154 | ||
3c0e092e | 155 | let Some(drop_trait) = cx.tcx.lang_items().drop_trait() else { |
c295e0f8 XL |
156 | // there is no way to define a type that needs non-const drop |
157 | // without having the lang item present. | |
158 | return false; | |
159 | }; | |
5099ac24 | 160 | |
c295e0f8 XL |
161 | let obligation = Obligation::new( |
162 | ObligationCause::dummy(), | |
163 | cx.param_env, | |
164 | ty::Binder::dummy(ty::TraitPredicate { | |
5099ac24 FG |
165 | trait_ref: ty::TraitRef { |
166 | def_id: drop_trait, | |
167 | substs: cx.tcx.mk_substs_trait(ty, &[]), | |
168 | }, | |
c295e0f8 | 169 | constness: ty::BoundConstness::ConstIfConst, |
3c0e092e | 170 | polarity: ty::ImplPolarity::Positive, |
c295e0f8 XL |
171 | }), |
172 | ); | |
173 | ||
5099ac24 | 174 | cx.tcx.infer_ctxt().enter(|infcx| { |
a2a8927a | 175 | let mut selcx = SelectionContext::new(&infcx); |
5099ac24 FG |
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 | |
178 | return true; | |
179 | }; | |
180 | ||
181 | if !matches!( | |
182 | impl_src, | |
3c0e092e | 183 | ImplSource::ConstDrop(_) | ImplSource::Param(_, ty::BoundConstness::ConstIfConst) |
5099ac24 FG |
184 | ) { |
185 | // If our const drop candidate is not ConstDrop or implied by the param env, | |
186 | // then it's bad | |
187 | return true; | |
188 | } | |
189 | ||
190 | if impl_src.borrow_nested_obligations().is_empty() { | |
191 | return false; | |
192 | } | |
193 | ||
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); | |
199 | } | |
200 | ||
201 | // If we had any errors, then it's bad | |
202 | !fcx.select_all_or_error(&infcx).is_empty() | |
203 | }) | |
e74abb32 XL |
204 | } |
205 | ||
a2a8927a XL |
206 | fn in_adt_inherently<'tcx>( |
207 | cx: &ConstCx<'_, 'tcx>, | |
208 | adt: &'tcx AdtDef, | |
209 | _: SubstsRef<'tcx>, | |
210 | ) -> bool { | |
c295e0f8 | 211 | adt.has_non_const_dtor(cx.tcx) |
ba9703b0 XL |
212 | } |
213 | } | |
214 | ||
f9f354fc XL |
215 | /// A constant that cannot be used as part of a pattern in a `match` expression. |
216 | pub struct CustomEq; | |
217 | ||
218 | impl Qualif for CustomEq { | |
219 | const ANALYSIS_NAME: &'static str = "flow_custom_eq"; | |
220 | ||
221 | fn in_qualifs(qualifs: &ConstQualifs) -> bool { | |
222 | qualifs.custom_eq | |
223 | } | |
224 | ||
a2a8927a | 225 | fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool { |
f9f354fc XL |
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`). | |
5099ac24 | 231 | traits::search_for_structural_match_violation(cx.body.span, cx.tcx, ty).is_some() |
f9f354fc XL |
232 | } |
233 | ||
a2a8927a | 234 | fn in_adt_inherently<'tcx>( |
f9f354fc XL |
235 | cx: &ConstCx<'_, 'tcx>, |
236 | adt: &'tcx AdtDef, | |
237 | substs: SubstsRef<'tcx>, | |
238 | ) -> bool { | |
239 | let ty = cx.tcx.mk_ty(ty::Adt(adt, substs)); | |
f035d41b | 240 | !ty.is_structural_eq_shallow(cx.tcx) |
f9f354fc XL |
241 | } |
242 | } | |
243 | ||
ba9703b0 XL |
244 | // FIXME: Use `mir::visit::Visitor` for the `in_*` functions if/when it supports early return. |
245 | ||
246 | /// Returns `true` if this `Rvalue` contains qualif `Q`. | |
a2a8927a XL |
247 | pub fn in_rvalue<'tcx, Q, F>( |
248 | cx: &ConstCx<'_, 'tcx>, | |
249 | in_local: &mut F, | |
250 | rvalue: &Rvalue<'tcx>, | |
251 | ) -> bool | |
ba9703b0 XL |
252 | where |
253 | Q: Qualif, | |
254 | F: FnMut(Local) -> bool, | |
255 | { | |
256 | match rvalue { | |
f9f354fc XL |
257 | Rvalue::ThreadLocalRef(_) | Rvalue::NullaryOp(..) => { |
258 | Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) | |
259 | } | |
ba9703b0 XL |
260 | |
261 | Rvalue::Discriminant(place) | Rvalue::Len(place) => { | |
262 | in_place::<Q, _>(cx, in_local, place.as_ref()) | |
263 | } | |
264 | ||
265 | Rvalue::Use(operand) | |
266 | | Rvalue::Repeat(operand, _) | |
267 | | Rvalue::UnaryOp(_, operand) | |
c295e0f8 XL |
268 | | Rvalue::Cast(_, operand, _) |
269 | | Rvalue::ShallowInitBox(operand, _) => in_operand::<Q, _>(cx, in_local, operand), | |
ba9703b0 | 270 | |
6a06907d | 271 | Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => { |
ba9703b0 XL |
272 | in_operand::<Q, _>(cx, in_local, lhs) || in_operand::<Q, _>(cx, in_local, rhs) |
273 | } | |
274 | ||
275 | Rvalue::Ref(_, _, place) | Rvalue::AddressOf(_, place) => { | |
276 | // Special-case reborrows to be more like a copy of the reference. | |
5869c6ff XL |
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; | |
1b1a35ee | 279 | if let ty::Ref(..) = base_ty.kind() { |
5869c6ff | 280 | return in_place::<Q, _>(cx, in_local, place_base); |
ba9703b0 XL |
281 | } |
282 | } | |
283 | ||
284 | in_place::<Q, _>(cx, in_local, place.as_ref()) | |
285 | } | |
286 | ||
287 | Rvalue::Aggregate(kind, operands) => { | |
288 | // Return early if we know that the struct or enum being constructed is always | |
289 | // qualified. | |
a2a8927a XL |
290 | if let AggregateKind::Adt(adt_did, _, substs, ..) = **kind { |
291 | let def = cx.tcx.adt_def(adt_did); | |
f9f354fc | 292 | if Q::in_adt_inherently(cx, def, substs) { |
e74abb32 XL |
293 | return true; |
294 | } | |
3c0e092e XL |
295 | if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) { |
296 | return true; | |
297 | } | |
e74abb32 | 298 | } |
ba9703b0 XL |
299 | |
300 | // Otherwise, proceed structurally... | |
301 | operands.iter().any(|o| in_operand::<Q, _>(cx, in_local, o)) | |
e74abb32 | 302 | } |
ba9703b0 XL |
303 | } |
304 | } | |
e74abb32 | 305 | |
ba9703b0 | 306 | /// Returns `true` if this `Place` contains qualif `Q`. |
a2a8927a | 307 | pub fn in_place<'tcx, Q, F>(cx: &ConstCx<'_, 'tcx>, in_local: &mut F, place: PlaceRef<'tcx>) -> bool |
ba9703b0 XL |
308 | where |
309 | Q: Qualif, | |
310 | F: FnMut(Local) -> bool, | |
311 | { | |
5869c6ff XL |
312 | let mut place = place; |
313 | while let Some((place_base, elem)) = place.last_projection() { | |
314 | match elem { | |
ba9703b0 XL |
315 | ProjectionElem::Index(index) if in_local(index) => return true, |
316 | ||
317 | ProjectionElem::Deref | |
318 | | ProjectionElem::Field(_, _) | |
319 | | ProjectionElem::ConstantIndex { .. } | |
320 | | ProjectionElem::Subslice { .. } | |
321 | | ProjectionElem::Downcast(_, _) | |
322 | | ProjectionElem::Index(_) => {} | |
323 | } | |
324 | ||
5869c6ff XL |
325 | let base_ty = place_base.ty(cx.body, cx.tcx); |
326 | let proj_ty = base_ty.projection_ty(cx.tcx, elem).ty; | |
ba9703b0 XL |
327 | if !Q::in_any_value_of_ty(cx, proj_ty) { |
328 | return false; | |
329 | } | |
330 | ||
5869c6ff | 331 | place = place_base; |
ba9703b0 XL |
332 | } |
333 | ||
5869c6ff | 334 | assert!(place.projection.is_empty()); |
ba9703b0 XL |
335 | in_local(place.local) |
336 | } | |
337 | ||
338 | /// Returns `true` if this `Operand` contains qualif `Q`. | |
a2a8927a XL |
339 | pub fn in_operand<'tcx, Q, F>( |
340 | cx: &ConstCx<'_, 'tcx>, | |
341 | in_local: &mut F, | |
342 | operand: &Operand<'tcx>, | |
343 | ) -> bool | |
ba9703b0 XL |
344 | where |
345 | Q: Qualif, | |
346 | F: FnMut(Local) -> bool, | |
347 | { | |
348 | let constant = match operand { | |
349 | Operand::Copy(place) | Operand::Move(place) => { | |
350 | return in_place::<Q, _>(cx, in_local, place.as_ref()); | |
351 | } | |
352 | ||
353 | Operand::Constant(c) => c, | |
354 | }; | |
355 | ||
356 | // Check the qualifs of the value of `const` items. | |
6a06907d | 357 | if let Some(ct) = constant.literal.const_for_ty() { |
5099ac24 | 358 | if let ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs: _, promoted }) = ct.val() { |
3c0e092e XL |
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); | |
6a06907d | 363 | // Don't peek inside trait associated constants. |
3c0e092e | 364 | if promoted.is_none() && cx.tcx.trait_of_item(def.did).is_none() { |
6a06907d XL |
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)) | |
367 | } else { | |
368 | cx.tcx.at(constant.span).mir_const_qualif(def.did) | |
369 | }; | |
370 | ||
371 | if !Q::in_qualifs(&qualifs) { | |
372 | return false; | |
373 | } | |
ba9703b0 | 374 | |
6a06907d XL |
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. | |
378 | } | |
ba9703b0 | 379 | } |
e74abb32 | 380 | } |
ba9703b0 | 381 | // Otherwise use the qualifs of the type. |
6a06907d | 382 | Q::in_any_value_of_ty(cx, constant.literal.ty()) |
e74abb32 | 383 | } |