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ba9703b0 XL |
1 | //! # Minimal Specialization |
2 | //! | |
3 | //! This module contains the checks for sound specialization used when the | |
4 | //! `min_specialization` feature is enabled. This requires that the impl is | |
5 | //! *always applicable*. | |
6 | //! | |
7 | //! If `impl1` specializes `impl2` then `impl1` is always applicable if we know | |
8 | //! that all the bounds of `impl2` are satisfied, and all of the bounds of | |
9 | //! `impl1` are satisfied for some choice of lifetimes then we know that | |
10 | //! `impl1` applies for any choice of lifetimes. | |
11 | //! | |
12 | //! ## Basic approach | |
13 | //! | |
14 | //! To enforce this requirement on specializations we take the following | |
15 | //! approach: | |
16 | //! | |
17 | //! 1. Match up the substs for `impl2` so that the implemented trait and | |
18 | //! self-type match those for `impl1`. | |
19 | //! 2. Check for any direct use of `'static` in the substs of `impl2`. | |
20 | //! 3. Check that all of the generic parameters of `impl1` occur at most once | |
21 | //! in the *unconstrained* substs for `impl2`. A parameter is constrained if | |
22 | //! its value is completely determined by an associated type projection | |
23 | //! predicate. | |
24 | //! 4. Check that all predicates on `impl1` either exist on `impl2` (after | |
25 | //! matching substs), or are well-formed predicates for the trait's type | |
26 | //! arguments. | |
27 | //! | |
28 | //! ## Example | |
29 | //! | |
30 | //! Suppose we have the following always applicable impl: | |
31 | //! | |
04454e1e | 32 | //! ```ignore (illustrative) |
ba9703b0 XL |
33 | //! impl<T> SpecExtend<T> for std::vec::IntoIter<T> { /* specialized impl */ } |
34 | //! impl<T, I: Iterator<Item=T>> SpecExtend<T> for I { /* default impl */ } | |
35 | //! ``` | |
36 | //! | |
37 | //! We get that the subst for `impl2` are `[T, std::vec::IntoIter<T>]`. `T` is | |
38 | //! constrained to be `<I as Iterator>::Item`, so we check only | |
39 | //! `std::vec::IntoIter<T>` for repeated parameters, which it doesn't have. The | |
40 | //! predicates of `impl1` are only `T: Sized`, which is also a predicate of | |
41 | //! `impl2`. So this specialization is sound. | |
42 | //! | |
43 | //! ## Extensions | |
44 | //! | |
45 | //! Unfortunately not all specializations in the standard library are allowed | |
46 | //! by this. So there are two extensions to these rules that allow specializing | |
47 | //! on some traits: that is, using them as bounds on the specializing impl, | |
48 | //! even when they don't occur in the base impl. | |
49 | //! | |
50 | //! ### rustc_specialization_trait | |
51 | //! | |
52 | //! If a trait is always applicable, then it's sound to specialize on it. We | |
53 | //! check trait is always applicable in the same way as impls, except that step | |
54 | //! 4 is now "all predicates on `impl1` are always applicable". We require that | |
55 | //! `specialization` or `min_specialization` is enabled to implement these | |
56 | //! traits. | |
57 | //! | |
58 | //! ### rustc_unsafe_specialization_marker | |
59 | //! | |
60 | //! There are also some specialization on traits with no methods, including the | |
61 | //! stable `FusedIterator` trait. We allow marking marker traits with an | |
62 | //! unstable attribute that means we ignore them in point 3 of the checks | |
63 | //! above. This is unsound, in the sense that the specialized impl may be used | |
64 | //! when it doesn't apply, but we allow it in the short term since it can't | |
65 | //! cause use after frees with purely safe code in the same way as specializing | |
66 | //! on traits with methods can. | |
67 | ||
68 | use crate::constrained_generic_params as cgp; | |
04454e1e | 69 | use crate::errors::SubstsOnOverriddenImpl; |
ba9703b0 XL |
70 | |
71 | use rustc_data_structures::fx::FxHashSet; | |
487cf647 | 72 | use rustc_hir as hir; |
f9f354fc | 73 | use rustc_hir::def_id::{DefId, LocalDefId}; |
ba9703b0 | 74 | use rustc_infer::infer::outlives::env::OutlivesEnvironment; |
f2b60f7d | 75 | use rustc_infer::infer::TyCtxtInferExt; |
ba9703b0 | 76 | use rustc_infer::traits::specialization_graph::Node; |
ba9703b0 XL |
77 | use rustc_middle::ty::subst::{GenericArg, InternalSubsts, SubstsRef}; |
78 | use rustc_middle::ty::trait_def::TraitSpecializationKind; | |
064997fb | 79 | use rustc_middle::ty::{self, TyCtxt, TypeVisitable}; |
ba9703b0 | 80 | use rustc_span::Span; |
2b03887a | 81 | use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt; |
f2b60f7d FG |
82 | use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _; |
83 | use rustc_trait_selection::traits::{self, translate_substs, wf, ObligationCtxt}; | |
ba9703b0 | 84 | |
064997fb | 85 | pub(super) fn check_min_specialization(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) { |
ba9703b0 | 86 | if let Some(node) = parent_specialization_node(tcx, impl_def_id) { |
f2b60f7d | 87 | check_always_applicable(tcx, impl_def_id, node); |
ba9703b0 XL |
88 | } |
89 | } | |
90 | ||
064997fb | 91 | fn parent_specialization_node(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId) -> Option<Node> { |
ba9703b0 | 92 | let trait_ref = tcx.impl_trait_ref(impl1_def_id)?; |
f25598a0 | 93 | let trait_def = tcx.trait_def(trait_ref.skip_binder().def_id); |
ba9703b0 | 94 | |
064997fb | 95 | let impl2_node = trait_def.ancestors(tcx, impl1_def_id.to_def_id()).ok()?.nth(1)?; |
ba9703b0 XL |
96 | |
97 | let always_applicable_trait = | |
98 | matches!(trait_def.specialization_kind, TraitSpecializationKind::AlwaysApplicable); | |
99 | if impl2_node.is_from_trait() && !always_applicable_trait { | |
100 | // Implementing a normal trait isn't a specialization. | |
101 | return None; | |
102 | } | |
103 | Some(impl2_node) | |
104 | } | |
105 | ||
106 | /// Check that `impl1` is a sound specialization | |
487cf647 | 107 | #[instrument(level = "debug", skip(tcx))] |
f2b60f7d FG |
108 | fn check_always_applicable(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node: Node) { |
109 | if let Some((impl1_substs, impl2_substs)) = get_impl_substs(tcx, impl1_def_id, impl2_node) { | |
ba9703b0 | 110 | let impl2_def_id = impl2_node.def_id(); |
487cf647 | 111 | debug!(?impl2_def_id, ?impl2_substs); |
ba9703b0 | 112 | |
ba9703b0 XL |
113 | let parent_substs = if impl2_node.is_from_trait() { |
114 | impl2_substs.to_vec() | |
115 | } else { | |
116 | unconstrained_parent_impl_substs(tcx, impl2_def_id, impl2_substs) | |
117 | }; | |
118 | ||
064997fb | 119 | let span = tcx.def_span(impl1_def_id); |
487cf647 | 120 | check_constness(tcx, impl1_def_id, impl2_node, span); |
ba9703b0 XL |
121 | check_static_lifetimes(tcx, &parent_substs, span); |
122 | check_duplicate_params(tcx, impl1_substs, &parent_substs, span); | |
f2b60f7d | 123 | check_predicates(tcx, impl1_def_id, impl1_substs, impl2_node, impl2_substs, span); |
ba9703b0 XL |
124 | } |
125 | } | |
126 | ||
487cf647 FG |
127 | /// Check that the specializing impl `impl1` is at least as const as the base |
128 | /// impl `impl2` | |
129 | fn check_constness(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node: Node, span: Span) { | |
130 | if impl2_node.is_from_trait() { | |
131 | // This isn't a specialization | |
132 | return; | |
133 | } | |
134 | ||
135 | let impl1_constness = tcx.constness(impl1_def_id.to_def_id()); | |
136 | let impl2_constness = tcx.constness(impl2_node.def_id()); | |
137 | ||
138 | if let hir::Constness::Const = impl2_constness { | |
139 | if let hir::Constness::NotConst = impl1_constness { | |
140 | tcx.sess | |
141 | .struct_span_err(span, "cannot specialize on const impl with non-const impl") | |
142 | .emit(); | |
143 | } | |
144 | } | |
145 | } | |
146 | ||
ba9703b0 XL |
147 | /// Given a specializing impl `impl1`, and the base impl `impl2`, returns two |
148 | /// substitutions `(S1, S2)` that equate their trait references. The returned | |
149 | /// types are expressed in terms of the generics of `impl1`. | |
150 | /// | |
151 | /// Example | |
152 | /// | |
2b03887a | 153 | /// ```ignore (illustrative) |
ba9703b0 XL |
154 | /// impl<A, B> Foo<A> for B { /* impl2 */ } |
155 | /// impl<C> Foo<Vec<C>> for C { /* impl1 */ } | |
2b03887a | 156 | /// ``` |
ba9703b0 XL |
157 | /// |
158 | /// Would return `S1 = [C]` and `S2 = [Vec<C>, C]`. | |
f25598a0 FG |
159 | fn get_impl_substs( |
160 | tcx: TyCtxt<'_>, | |
064997fb | 161 | impl1_def_id: LocalDefId, |
ba9703b0 | 162 | impl2_node: Node, |
f25598a0 | 163 | ) -> Option<(SubstsRef<'_>, SubstsRef<'_>)> { |
2b03887a FG |
164 | let infcx = &tcx.infer_ctxt().build(); |
165 | let ocx = ObligationCtxt::new(infcx); | |
166 | let param_env = tcx.param_env(impl1_def_id); | |
167 | let impl1_hir_id = tcx.hir().local_def_id_to_hir_id(impl1_def_id); | |
ba9703b0 | 168 | |
2b03887a FG |
169 | let assumed_wf_types = |
170 | ocx.assumed_wf_types(param_env, tcx.def_span(impl1_def_id), impl1_def_id); | |
ba9703b0 | 171 | |
2b03887a FG |
172 | let impl1_substs = InternalSubsts::identity_for_item(tcx, impl1_def_id.to_def_id()); |
173 | let impl2_substs = | |
174 | translate_substs(infcx, param_env, impl1_def_id.to_def_id(), impl1_substs, impl2_node); | |
f2b60f7d | 175 | |
2b03887a FG |
176 | let errors = ocx.select_all_or_error(); |
177 | if !errors.is_empty() { | |
487cf647 | 178 | ocx.infcx.err_ctxt().report_fulfillment_errors(&errors, None); |
2b03887a FG |
179 | return None; |
180 | } | |
f2b60f7d | 181 | |
2b03887a FG |
182 | let implied_bounds = infcx.implied_bounds_tys(param_env, impl1_hir_id, assumed_wf_types); |
183 | let outlives_env = OutlivesEnvironment::with_bounds(param_env, Some(infcx), implied_bounds); | |
f25598a0 FG |
184 | let _ = |
185 | infcx.err_ctxt().check_region_obligations_and_report_errors(impl1_def_id, &outlives_env); | |
2b03887a FG |
186 | let Ok(impl2_substs) = infcx.fully_resolve(impl2_substs) else { |
187 | let span = tcx.def_span(impl1_def_id); | |
188 | tcx.sess.emit_err(SubstsOnOverriddenImpl { span }); | |
189 | return None; | |
190 | }; | |
191 | Some((impl1_substs, impl2_substs)) | |
ba9703b0 XL |
192 | } |
193 | ||
194 | /// Returns a list of all of the unconstrained subst of the given impl. | |
195 | /// | |
196 | /// For example given the impl: | |
197 | /// | |
198 | /// impl<'a, T, I> ... where &'a I: IntoIterator<Item=&'a T> | |
199 | /// | |
200 | /// This would return the substs corresponding to `['a, I]`, because knowing | |
201 | /// `'a` and `I` determines the value of `T`. | |
202 | fn unconstrained_parent_impl_substs<'tcx>( | |
203 | tcx: TyCtxt<'tcx>, | |
204 | impl_def_id: DefId, | |
205 | impl_substs: SubstsRef<'tcx>, | |
206 | ) -> Vec<GenericArg<'tcx>> { | |
207 | let impl_generic_predicates = tcx.predicates_of(impl_def_id); | |
208 | let mut unconstrained_parameters = FxHashSet::default(); | |
209 | let mut constrained_params = FxHashSet::default(); | |
f25598a0 | 210 | let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).map(ty::EarlyBinder::subst_identity); |
ba9703b0 XL |
211 | |
212 | // Unfortunately the functions in `constrained_generic_parameters` don't do | |
213 | // what we want here. We want only a list of constrained parameters while | |
214 | // the functions in `cgp` add the constrained parameters to a list of | |
215 | // unconstrained parameters. | |
216 | for (predicate, _) in impl_generic_predicates.predicates.iter() { | |
487cf647 FG |
217 | if let ty::PredicateKind::Clause(ty::Clause::Projection(proj)) = |
218 | predicate.kind().skip_binder() | |
219 | { | |
3dfed10e | 220 | let projection_ty = proj.projection_ty; |
5099ac24 | 221 | let projected_ty = proj.term; |
ba9703b0 XL |
222 | |
223 | let unbound_trait_ref = projection_ty.trait_ref(tcx); | |
224 | if Some(unbound_trait_ref) == impl_trait_ref { | |
225 | continue; | |
226 | } | |
227 | ||
5099ac24 | 228 | unconstrained_parameters.extend(cgp::parameters_for(&projection_ty, true)); |
ba9703b0 | 229 | |
5099ac24 | 230 | for param in cgp::parameters_for(&projected_ty, false) { |
ba9703b0 XL |
231 | if !unconstrained_parameters.contains(¶m) { |
232 | constrained_params.insert(param.0); | |
233 | } | |
234 | } | |
235 | ||
5099ac24 | 236 | unconstrained_parameters.extend(cgp::parameters_for(&projected_ty, true)); |
ba9703b0 XL |
237 | } |
238 | } | |
239 | ||
240 | impl_substs | |
241 | .iter() | |
242 | .enumerate() | |
243 | .filter(|&(idx, _)| !constrained_params.contains(&(idx as u32))) | |
f9f354fc | 244 | .map(|(_, arg)| arg) |
ba9703b0 XL |
245 | .collect() |
246 | } | |
247 | ||
248 | /// Check that parameters of the derived impl don't occur more than once in the | |
249 | /// equated substs of the base impl. | |
250 | /// | |
251 | /// For example forbid the following: | |
252 | /// | |
2b03887a | 253 | /// ```ignore (illustrative) |
ba9703b0 XL |
254 | /// impl<A> Tr for A { } |
255 | /// impl<B> Tr for (B, B) { } | |
2b03887a | 256 | /// ``` |
ba9703b0 XL |
257 | /// |
258 | /// Note that only consider the unconstrained parameters of the base impl: | |
259 | /// | |
2b03887a | 260 | /// ```ignore (illustrative) |
ba9703b0 XL |
261 | /// impl<S, I: IntoIterator<Item = S>> Tr<S> for I { } |
262 | /// impl<T> Tr<T> for Vec<T> { } | |
2b03887a | 263 | /// ``` |
ba9703b0 XL |
264 | /// |
265 | /// The substs for the parent impl here are `[T, Vec<T>]`, which repeats `T`, | |
266 | /// but `S` is constrained in the parent impl, so `parent_substs` is only | |
267 | /// `[Vec<T>]`. This means we allow this impl. | |
268 | fn check_duplicate_params<'tcx>( | |
269 | tcx: TyCtxt<'tcx>, | |
270 | impl1_substs: SubstsRef<'tcx>, | |
271 | parent_substs: &Vec<GenericArg<'tcx>>, | |
272 | span: Span, | |
273 | ) { | |
5099ac24 | 274 | let mut base_params = cgp::parameters_for(parent_substs, true); |
ba9703b0 XL |
275 | base_params.sort_by_key(|param| param.0); |
276 | if let (_, [duplicate, ..]) = base_params.partition_dedup() { | |
277 | let param = impl1_substs[duplicate.0 as usize]; | |
278 | tcx.sess | |
279 | .struct_span_err(span, &format!("specializing impl repeats parameter `{}`", param)) | |
280 | .emit(); | |
281 | } | |
282 | } | |
283 | ||
284 | /// Check that `'static` lifetimes are not introduced by the specializing impl. | |
285 | /// | |
286 | /// For example forbid the following: | |
287 | /// | |
2b03887a | 288 | /// ```ignore (illustrative) |
ba9703b0 XL |
289 | /// impl<A> Tr for A { } |
290 | /// impl Tr for &'static i32 { } | |
2b03887a | 291 | /// ``` |
ba9703b0 XL |
292 | fn check_static_lifetimes<'tcx>( |
293 | tcx: TyCtxt<'tcx>, | |
294 | parent_substs: &Vec<GenericArg<'tcx>>, | |
295 | span: Span, | |
296 | ) { | |
5099ac24 | 297 | if tcx.any_free_region_meets(parent_substs, |r| r.is_static()) { |
ba9703b0 XL |
298 | tcx.sess.struct_span_err(span, "cannot specialize on `'static` lifetime").emit(); |
299 | } | |
300 | } | |
301 | ||
302 | /// Check whether predicates on the specializing impl (`impl1`) are allowed. | |
303 | /// | |
487cf647 | 304 | /// Each predicate `P` must be one of: |
ba9703b0 | 305 | /// |
487cf647 FG |
306 | /// * Global (not reference any parameters). |
307 | /// * A `T: Tr` predicate where `Tr` is an always-applicable trait. | |
308 | /// * Present on the base impl `impl2`. | |
309 | /// * This check is done using the `trait_predicates_eq` function below. | |
310 | /// * A well-formed predicate of a type argument of the trait being implemented, | |
ba9703b0 | 311 | /// including the `Self`-type. |
487cf647 | 312 | #[instrument(level = "debug", skip(tcx))] |
ba9703b0 | 313 | fn check_predicates<'tcx>( |
f2b60f7d | 314 | tcx: TyCtxt<'tcx>, |
f9f354fc | 315 | impl1_def_id: LocalDefId, |
ba9703b0 XL |
316 | impl1_substs: SubstsRef<'tcx>, |
317 | impl2_node: Node, | |
318 | impl2_substs: SubstsRef<'tcx>, | |
319 | span: Span, | |
320 | ) { | |
064997fb FG |
321 | let instantiated = tcx.predicates_of(impl1_def_id).instantiate(tcx, impl1_substs); |
322 | let impl1_predicates: Vec<_> = traits::elaborate_predicates_with_span( | |
c295e0f8 | 323 | tcx, |
064997fb FG |
324 | std::iter::zip( |
325 | instantiated.predicates, | |
326 | // Don't drop predicates (unsound!) because `spans` is too short | |
327 | instantiated.spans.into_iter().chain(std::iter::repeat(span)), | |
328 | ), | |
c295e0f8 | 329 | ) |
064997fb | 330 | .map(|obligation| (obligation.predicate, obligation.cause.span)) |
c295e0f8 XL |
331 | .collect(); |
332 | ||
ba9703b0 XL |
333 | let mut impl2_predicates = if impl2_node.is_from_trait() { |
334 | // Always applicable traits have to be always applicable without any | |
335 | // assumptions. | |
c295e0f8 | 336 | Vec::new() |
ba9703b0 | 337 | } else { |
c295e0f8 XL |
338 | traits::elaborate_predicates( |
339 | tcx, | |
340 | tcx.predicates_of(impl2_node.def_id()) | |
341 | .instantiate(tcx, impl2_substs) | |
342 | .predicates | |
343 | .into_iter(), | |
344 | ) | |
345 | .map(|obligation| obligation.predicate) | |
346 | .collect() | |
ba9703b0 | 347 | }; |
487cf647 | 348 | debug!(?impl1_predicates, ?impl2_predicates); |
ba9703b0 XL |
349 | |
350 | // Since impls of always applicable traits don't get to assume anything, we | |
351 | // can also assume their supertraits apply. | |
352 | // | |
353 | // For example, we allow: | |
354 | // | |
355 | // #[rustc_specialization_trait] | |
356 | // trait AlwaysApplicable: Debug { } | |
357 | // | |
358 | // impl<T> Tr for T { } | |
359 | // impl<T: AlwaysApplicable> Tr for T { } | |
360 | // | |
361 | // Specializing on `AlwaysApplicable` allows also specializing on `Debug` | |
362 | // which is sound because we forbid impls like the following | |
363 | // | |
364 | // impl<D: Debug> AlwaysApplicable for D { } | |
064997fb | 365 | let always_applicable_traits = impl1_predicates.iter().copied().filter(|&(predicate, _)| { |
c295e0f8 XL |
366 | matches!( |
367 | trait_predicate_kind(tcx, predicate), | |
368 | Some(TraitSpecializationKind::AlwaysApplicable) | |
369 | ) | |
370 | }); | |
ba9703b0 XL |
371 | |
372 | // Include the well-formed predicates of the type parameters of the impl. | |
f25598a0 | 373 | for arg in tcx.impl_trait_ref(impl1_def_id).unwrap().subst_identity().substs { |
2b03887a FG |
374 | let infcx = &tcx.infer_ctxt().build(); |
375 | let obligations = wf::obligations( | |
376 | infcx, | |
377 | tcx.param_env(impl1_def_id), | |
378 | tcx.hir().local_def_id_to_hir_id(impl1_def_id), | |
379 | 0, | |
380 | arg, | |
381 | span, | |
382 | ) | |
383 | .unwrap(); | |
f2b60f7d | 384 | |
2b03887a FG |
385 | assert!(!obligations.needs_infer()); |
386 | impl2_predicates.extend( | |
387 | traits::elaborate_obligations(tcx, obligations).map(|obligation| obligation.predicate), | |
388 | ) | |
ba9703b0 | 389 | } |
c295e0f8 | 390 | impl2_predicates.extend( |
064997fb | 391 | traits::elaborate_predicates_with_span(tcx, always_applicable_traits) |
ba9703b0 XL |
392 | .map(|obligation| obligation.predicate), |
393 | ); | |
394 | ||
064997fb | 395 | for (predicate, span) in impl1_predicates { |
487cf647 | 396 | if !impl2_predicates.iter().any(|pred2| trait_predicates_eq(tcx, predicate, *pred2, span)) { |
f9f354fc | 397 | check_specialization_on(tcx, predicate, span) |
ba9703b0 XL |
398 | } |
399 | } | |
400 | } | |
401 | ||
487cf647 FG |
402 | /// Checks if some predicate on the specializing impl (`predicate1`) is the same |
403 | /// as some predicate on the base impl (`predicate2`). | |
404 | /// | |
405 | /// This basically just checks syntactic equivalence, but is a little more | |
406 | /// forgiving since we want to equate `T: Tr` with `T: ~const Tr` so this can work: | |
407 | /// | |
408 | /// ```ignore (illustrative) | |
409 | /// #[rustc_specialization_trait] | |
410 | /// trait Specialize { } | |
411 | /// | |
412 | /// impl<T: Bound> Tr for T { } | |
413 | /// impl<T: ~const Bound + Specialize> const Tr for T { } | |
414 | /// ``` | |
415 | /// | |
416 | /// However, we *don't* want to allow the reverse, i.e., when the bound on the | |
417 | /// specializing impl is not as const as the bound on the base impl: | |
418 | /// | |
419 | /// ```ignore (illustrative) | |
420 | /// impl<T: ~const Bound> const Tr for T { } | |
421 | /// impl<T: Bound + Specialize> const Tr for T { } // should be T: ~const Bound | |
422 | /// ``` | |
423 | /// | |
424 | /// So we make that check in this function and try to raise a helpful error message. | |
425 | fn trait_predicates_eq<'tcx>( | |
426 | tcx: TyCtxt<'tcx>, | |
427 | predicate1: ty::Predicate<'tcx>, | |
428 | predicate2: ty::Predicate<'tcx>, | |
429 | span: Span, | |
430 | ) -> bool { | |
431 | let pred1_kind = predicate1.kind().skip_binder(); | |
432 | let pred2_kind = predicate2.kind().skip_binder(); | |
433 | let (trait_pred1, trait_pred2) = match (pred1_kind, pred2_kind) { | |
434 | ( | |
435 | ty::PredicateKind::Clause(ty::Clause::Trait(pred1)), | |
436 | ty::PredicateKind::Clause(ty::Clause::Trait(pred2)), | |
437 | ) => (pred1, pred2), | |
438 | // Just use plain syntactic equivalence if either of the predicates aren't | |
439 | // trait predicates or have bound vars. | |
440 | _ => return predicate1 == predicate2, | |
441 | }; | |
442 | ||
443 | let predicates_equal_modulo_constness = { | |
444 | let pred1_unconsted = | |
445 | ty::TraitPredicate { constness: ty::BoundConstness::NotConst, ..trait_pred1 }; | |
446 | let pred2_unconsted = | |
447 | ty::TraitPredicate { constness: ty::BoundConstness::NotConst, ..trait_pred2 }; | |
448 | pred1_unconsted == pred2_unconsted | |
449 | }; | |
450 | ||
451 | if !predicates_equal_modulo_constness { | |
452 | return false; | |
453 | } | |
454 | ||
455 | // Check that the predicate on the specializing impl is at least as const as | |
456 | // the one on the base. | |
457 | match (trait_pred2.constness, trait_pred1.constness) { | |
458 | (ty::BoundConstness::ConstIfConst, ty::BoundConstness::NotConst) => { | |
459 | tcx.sess.struct_span_err(span, "missing `~const` qualifier for specialization").emit(); | |
460 | } | |
461 | _ => {} | |
462 | } | |
463 | ||
464 | true | |
465 | } | |
466 | ||
467 | #[instrument(level = "debug", skip(tcx))] | |
f9f354fc | 468 | fn check_specialization_on<'tcx>(tcx: TyCtxt<'tcx>, predicate: ty::Predicate<'tcx>, span: Span) { |
5869c6ff | 469 | match predicate.kind().skip_binder() { |
ba9703b0 XL |
470 | // Global predicates are either always true or always false, so we |
471 | // are fine to specialize on. | |
5099ac24 | 472 | _ if predicate.is_global() => (), |
ba9703b0 XL |
473 | // We allow specializing on explicitly marked traits with no associated |
474 | // items. | |
487cf647 | 475 | ty::PredicateKind::Clause(ty::Clause::Trait(ty::TraitPredicate { |
94222f64 | 476 | trait_ref, |
487cf647 | 477 | constness: _, |
3c0e092e | 478 | polarity: _, |
487cf647 | 479 | })) => { |
ba9703b0 XL |
480 | if !matches!( |
481 | trait_predicate_kind(tcx, predicate), | |
482 | Some(TraitSpecializationKind::Marker) | |
483 | ) { | |
484 | tcx.sess | |
485 | .struct_span_err( | |
486 | span, | |
487 | &format!( | |
488 | "cannot specialize on trait `{}`", | |
94222f64 | 489 | tcx.def_path_str(trait_ref.def_id), |
ba9703b0 XL |
490 | ), |
491 | ) | |
5e7ed085 | 492 | .emit(); |
ba9703b0 XL |
493 | } |
494 | } | |
487cf647 FG |
495 | ty::PredicateKind::Clause(ty::Clause::Projection(ty::ProjectionPredicate { |
496 | projection_ty, | |
497 | term, | |
498 | })) => { | |
064997fb FG |
499 | tcx.sess |
500 | .struct_span_err( | |
501 | span, | |
502 | &format!("cannot specialize on associated type `{projection_ty} == {term}`",), | |
503 | ) | |
504 | .emit(); | |
505 | } | |
5e7ed085 FG |
506 | _ => { |
507 | tcx.sess | |
064997fb | 508 | .struct_span_err(span, &format!("cannot specialize on predicate `{}`", predicate)) |
5e7ed085 FG |
509 | .emit(); |
510 | } | |
ba9703b0 XL |
511 | } |
512 | } | |
513 | ||
514 | fn trait_predicate_kind<'tcx>( | |
515 | tcx: TyCtxt<'tcx>, | |
f9f354fc | 516 | predicate: ty::Predicate<'tcx>, |
ba9703b0 | 517 | ) -> Option<TraitSpecializationKind> { |
5869c6ff | 518 | match predicate.kind().skip_binder() { |
487cf647 FG |
519 | ty::PredicateKind::Clause(ty::Clause::Trait(ty::TraitPredicate { |
520 | trait_ref, | |
521 | constness: _, | |
522 | polarity: _, | |
523 | })) => Some(tcx.trait_def(trait_ref.def_id).specialization_kind), | |
524 | ty::PredicateKind::Clause(ty::Clause::RegionOutlives(_)) | |
525 | | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(_)) | |
526 | | ty::PredicateKind::Clause(ty::Clause::Projection(_)) | |
5869c6ff XL |
527 | | ty::PredicateKind::WellFormed(_) |
528 | | ty::PredicateKind::Subtype(_) | |
94222f64 | 529 | | ty::PredicateKind::Coerce(_) |
5869c6ff XL |
530 | | ty::PredicateKind::ObjectSafe(_) |
531 | | ty::PredicateKind::ClosureKind(..) | |
532 | | ty::PredicateKind::ConstEvaluatable(..) | |
533 | | ty::PredicateKind::ConstEquate(..) | |
487cf647 | 534 | | ty::PredicateKind::Ambiguous |
5869c6ff | 535 | | ty::PredicateKind::TypeWellFormedFromEnv(..) => None, |
ba9703b0 XL |
536 | } |
537 | } |