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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 | //! | |
32 | //! ```ignore (illustrative) | |
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; | |
69 | use crate::errors::SubstsOnOverriddenImpl; | |
70 | ||
71 | use rustc_data_structures::fx::FxHashSet; | |
72 | use rustc_hir as hir; | |
73 | use rustc_hir::def_id::{DefId, LocalDefId}; | |
74 | use rustc_infer::infer::outlives::env::OutlivesEnvironment; | |
75 | use rustc_infer::infer::TyCtxtInferExt; | |
76 | use rustc_infer::traits::specialization_graph::Node; | |
77 | use rustc_middle::ty::subst::{GenericArg, InternalSubsts, SubstsRef}; | |
78 | use rustc_middle::ty::trait_def::TraitSpecializationKind; | |
79 | use rustc_middle::ty::{self, TyCtxt, TypeVisitable}; | |
80 | use rustc_span::Span; | |
81 | use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt; | |
82 | use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _; | |
83 | use rustc_trait_selection::traits::{self, translate_substs, wf, ObligationCtxt}; | |
84 | ||
85 | pub(super) fn check_min_specialization(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) { | |
86 | if let Some(node) = parent_specialization_node(tcx, impl_def_id) { | |
87 | check_always_applicable(tcx, impl_def_id, node); | |
88 | } | |
89 | } | |
90 | ||
91 | fn parent_specialization_node(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId) -> Option<Node> { | |
92 | let trait_ref = tcx.impl_trait_ref(impl1_def_id)?; | |
93 | let trait_def = tcx.trait_def(trait_ref.skip_binder().def_id); | |
94 | ||
95 | let impl2_node = trait_def.ancestors(tcx, impl1_def_id.to_def_id()).ok()?.nth(1)?; | |
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 | |
107 | #[instrument(level = "debug", skip(tcx))] | |
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) { | |
110 | let impl2_def_id = impl2_node.def_id(); | |
111 | debug!(?impl2_def_id, ?impl2_substs); | |
112 | ||
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 | ||
119 | let span = tcx.def_span(impl1_def_id); | |
120 | check_constness(tcx, impl1_def_id, impl2_node, span); | |
121 | check_static_lifetimes(tcx, &parent_substs, span); | |
122 | check_duplicate_params(tcx, impl1_substs, &parent_substs, span); | |
123 | check_predicates(tcx, impl1_def_id, impl1_substs, impl2_node, impl2_substs, span); | |
124 | } | |
125 | } | |
126 | ||
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 | ||
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 | /// | |
153 | /// ```ignore (illustrative) | |
154 | /// impl<A, B> Foo<A> for B { /* impl2 */ } | |
155 | /// impl<C> Foo<Vec<C>> for C { /* impl1 */ } | |
156 | /// ``` | |
157 | /// | |
158 | /// Would return `S1 = [C]` and `S2 = [Vec<C>, C]`. | |
159 | fn get_impl_substs( | |
160 | tcx: TyCtxt<'_>, | |
161 | impl1_def_id: LocalDefId, | |
162 | impl2_node: Node, | |
163 | ) -> Option<(SubstsRef<'_>, SubstsRef<'_>)> { | |
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); | |
168 | ||
169 | let assumed_wf_types = | |
170 | ocx.assumed_wf_types(param_env, tcx.def_span(impl1_def_id), impl1_def_id); | |
171 | ||
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); | |
175 | ||
176 | let errors = ocx.select_all_or_error(); | |
177 | if !errors.is_empty() { | |
178 | ocx.infcx.err_ctxt().report_fulfillment_errors(&errors, None); | |
179 | return None; | |
180 | } | |
181 | ||
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); | |
184 | let _ = | |
185 | infcx.err_ctxt().check_region_obligations_and_report_errors(impl1_def_id, &outlives_env); | |
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)) | |
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(); | |
210 | let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).map(ty::EarlyBinder::subst_identity); | |
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() { | |
217 | if let ty::PredicateKind::Clause(ty::Clause::Projection(proj)) = | |
218 | predicate.kind().skip_binder() | |
219 | { | |
220 | let projection_ty = proj.projection_ty; | |
221 | let projected_ty = proj.term; | |
222 | ||
223 | let unbound_trait_ref = projection_ty.trait_ref(tcx); | |
224 | if Some(unbound_trait_ref) == impl_trait_ref { | |
225 | continue; | |
226 | } | |
227 | ||
228 | unconstrained_parameters.extend(cgp::parameters_for(&projection_ty, true)); | |
229 | ||
230 | for param in cgp::parameters_for(&projected_ty, false) { | |
231 | if !unconstrained_parameters.contains(¶m) { | |
232 | constrained_params.insert(param.0); | |
233 | } | |
234 | } | |
235 | ||
236 | unconstrained_parameters.extend(cgp::parameters_for(&projected_ty, true)); | |
237 | } | |
238 | } | |
239 | ||
240 | impl_substs | |
241 | .iter() | |
242 | .enumerate() | |
243 | .filter(|&(idx, _)| !constrained_params.contains(&(idx as u32))) | |
244 | .map(|(_, arg)| arg) | |
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 | /// | |
253 | /// ```ignore (illustrative) | |
254 | /// impl<A> Tr for A { } | |
255 | /// impl<B> Tr for (B, B) { } | |
256 | /// ``` | |
257 | /// | |
258 | /// Note that only consider the unconstrained parameters of the base impl: | |
259 | /// | |
260 | /// ```ignore (illustrative) | |
261 | /// impl<S, I: IntoIterator<Item = S>> Tr<S> for I { } | |
262 | /// impl<T> Tr<T> for Vec<T> { } | |
263 | /// ``` | |
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 | ) { | |
274 | let mut base_params = cgp::parameters_for(parent_substs, true); | |
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 | /// | |
288 | /// ```ignore (illustrative) | |
289 | /// impl<A> Tr for A { } | |
290 | /// impl Tr for &'static i32 { } | |
291 | /// ``` | |
292 | fn check_static_lifetimes<'tcx>( | |
293 | tcx: TyCtxt<'tcx>, | |
294 | parent_substs: &Vec<GenericArg<'tcx>>, | |
295 | span: Span, | |
296 | ) { | |
297 | if tcx.any_free_region_meets(parent_substs, |r| r.is_static()) { | |
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 | /// | |
304 | /// Each predicate `P` must be one of: | |
305 | /// | |
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, | |
311 | /// including the `Self`-type. | |
312 | #[instrument(level = "debug", skip(tcx))] | |
313 | fn check_predicates<'tcx>( | |
314 | tcx: TyCtxt<'tcx>, | |
315 | impl1_def_id: LocalDefId, | |
316 | impl1_substs: SubstsRef<'tcx>, | |
317 | impl2_node: Node, | |
318 | impl2_substs: SubstsRef<'tcx>, | |
319 | span: Span, | |
320 | ) { | |
321 | let instantiated = tcx.predicates_of(impl1_def_id).instantiate(tcx, impl1_substs); | |
322 | let impl1_predicates: Vec<_> = traits::elaborate_predicates_with_span( | |
323 | tcx, | |
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 | ), | |
329 | ) | |
330 | .map(|obligation| (obligation.predicate, obligation.cause.span)) | |
331 | .collect(); | |
332 | ||
333 | let mut impl2_predicates = if impl2_node.is_from_trait() { | |
334 | // Always applicable traits have to be always applicable without any | |
335 | // assumptions. | |
336 | Vec::new() | |
337 | } else { | |
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() | |
347 | }; | |
348 | debug!(?impl1_predicates, ?impl2_predicates); | |
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 { } | |
365 | let always_applicable_traits = impl1_predicates.iter().copied().filter(|&(predicate, _)| { | |
366 | matches!( | |
367 | trait_predicate_kind(tcx, predicate), | |
368 | Some(TraitSpecializationKind::AlwaysApplicable) | |
369 | ) | |
370 | }); | |
371 | ||
372 | // Include the well-formed predicates of the type parameters of the impl. | |
373 | for arg in tcx.impl_trait_ref(impl1_def_id).unwrap().subst_identity().substs { | |
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(); | |
384 | ||
385 | assert!(!obligations.needs_infer()); | |
386 | impl2_predicates.extend( | |
387 | traits::elaborate_obligations(tcx, obligations).map(|obligation| obligation.predicate), | |
388 | ) | |
389 | } | |
390 | impl2_predicates.extend( | |
391 | traits::elaborate_predicates_with_span(tcx, always_applicable_traits) | |
392 | .map(|obligation| obligation.predicate), | |
393 | ); | |
394 | ||
395 | for (predicate, span) in impl1_predicates { | |
396 | if !impl2_predicates.iter().any(|pred2| trait_predicates_eq(tcx, predicate, *pred2, span)) { | |
397 | check_specialization_on(tcx, predicate, span) | |
398 | } | |
399 | } | |
400 | } | |
401 | ||
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))] | |
468 | fn check_specialization_on<'tcx>(tcx: TyCtxt<'tcx>, predicate: ty::Predicate<'tcx>, span: Span) { | |
469 | match predicate.kind().skip_binder() { | |
470 | // Global predicates are either always true or always false, so we | |
471 | // are fine to specialize on. | |
472 | _ if predicate.is_global() => (), | |
473 | // We allow specializing on explicitly marked traits with no associated | |
474 | // items. | |
475 | ty::PredicateKind::Clause(ty::Clause::Trait(ty::TraitPredicate { | |
476 | trait_ref, | |
477 | constness: _, | |
478 | polarity: _, | |
479 | })) => { | |
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 `{}`", | |
489 | tcx.def_path_str(trait_ref.def_id), | |
490 | ), | |
491 | ) | |
492 | .emit(); | |
493 | } | |
494 | } | |
495 | ty::PredicateKind::Clause(ty::Clause::Projection(ty::ProjectionPredicate { | |
496 | projection_ty, | |
497 | term, | |
498 | })) => { | |
499 | tcx.sess | |
500 | .struct_span_err( | |
501 | span, | |
502 | &format!("cannot specialize on associated type `{projection_ty} == {term}`",), | |
503 | ) | |
504 | .emit(); | |
505 | } | |
506 | _ => { | |
507 | tcx.sess | |
508 | .struct_span_err(span, &format!("cannot specialize on predicate `{}`", predicate)) | |
509 | .emit(); | |
510 | } | |
511 | } | |
512 | } | |
513 | ||
514 | fn trait_predicate_kind<'tcx>( | |
515 | tcx: TyCtxt<'tcx>, | |
516 | predicate: ty::Predicate<'tcx>, | |
517 | ) -> Option<TraitSpecializationKind> { | |
518 | match predicate.kind().skip_binder() { | |
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(_)) | |
527 | | ty::PredicateKind::WellFormed(_) | |
528 | | ty::PredicateKind::Subtype(_) | |
529 | | ty::PredicateKind::Coerce(_) | |
530 | | ty::PredicateKind::ObjectSafe(_) | |
531 | | ty::PredicateKind::ClosureKind(..) | |
532 | | ty::PredicateKind::ConstEvaluatable(..) | |
533 | | ty::PredicateKind::ConstEquate(..) | |
534 | | ty::PredicateKind::Ambiguous | |
535 | | ty::PredicateKind::TypeWellFormedFromEnv(..) => None, | |
536 | } | |
537 | } |