1 //! # Minimal Specialization
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*.
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.
14 //! To enforce this requirement on specializations we take the following
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
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
30 //! Suppose we have the following always applicable impl:
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 */ }
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.
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.
50 //! ### rustc_specialization_trait
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
58 //! ### rustc_unsafe_specialization_marker
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.
68 use crate::constrained_generic_params as cgp;
69 use crate::errors::SubstsOnOverriddenImpl;
71 use rustc_data_structures::fx::FxHashSet;
72 use rustc_hir::def_id::{DefId, LocalDefId};
73 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
74 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
75 use rustc_infer::traits::specialization_graph::Node;
76 use rustc_middle::ty::subst::{GenericArg, InternalSubsts, SubstsRef};
77 use rustc_middle::ty::trait_def::TraitSpecializationKind;
78 use rustc_middle::ty::{self, TyCtxt, TypeVisitable};
80 use rustc_trait_selection::traits::{self, translate_substs, wf};
82 pub(super) fn check_min_specialization(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) {
83 if let Some(node) = parent_specialization_node(tcx, impl_def_id) {
84 tcx.infer_ctxt().enter(|infcx| {
85 check_always_applicable(&infcx, impl_def_id, node);
90 fn parent_specialization_node(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId) -> Option<Node> {
91 let trait_ref = tcx.impl_trait_ref(impl1_def_id)?;
92 let trait_def = tcx.trait_def(trait_ref.def_id);
94 let impl2_node = trait_def.ancestors(tcx, impl1_def_id.to_def_id()).ok()?.nth(1)?;
96 let always_applicable_trait =
97 matches!(trait_def.specialization_kind, TraitSpecializationKind::AlwaysApplicable);
98 if impl2_node.is_from_trait() && !always_applicable_trait {
99 // Implementing a normal trait isn't a specialization.
105 /// Check that `impl1` is a sound specialization
106 fn check_always_applicable(infcx: &InferCtxt<'_, '_>, impl1_def_id: LocalDefId, impl2_node: Node) {
107 if let Some((impl1_substs, impl2_substs)) = get_impl_substs(infcx, impl1_def_id, impl2_node) {
108 let impl2_def_id = impl2_node.def_id();
110 "check_always_applicable(\nimpl1_def_id={:?},\nimpl2_def_id={:?},\nimpl2_substs={:?}\n)",
111 impl1_def_id, impl2_def_id, impl2_substs
116 let parent_substs = if impl2_node.is_from_trait() {
117 impl2_substs.to_vec()
119 unconstrained_parent_impl_substs(tcx, impl2_def_id, impl2_substs)
122 let span = tcx.def_span(impl1_def_id);
123 check_static_lifetimes(tcx, &parent_substs, span);
124 check_duplicate_params(tcx, impl1_substs, &parent_substs, span);
125 check_predicates(infcx, impl1_def_id, impl1_substs, impl2_node, impl2_substs, span);
129 /// Given a specializing impl `impl1`, and the base impl `impl2`, returns two
130 /// substitutions `(S1, S2)` that equate their trait references. The returned
131 /// types are expressed in terms of the generics of `impl1`.
135 /// impl<A, B> Foo<A> for B { /* impl2 */ }
136 /// impl<C> Foo<Vec<C>> for C { /* impl1 */ }
138 /// Would return `S1 = [C]` and `S2 = [Vec<C>, C]`.
139 fn get_impl_substs<'tcx>(
140 infcx: &InferCtxt<'_, 'tcx>,
141 impl1_def_id: LocalDefId,
143 ) -> Option<(SubstsRef<'tcx>, SubstsRef<'tcx>)> {
145 let param_env = tcx.param_env(impl1_def_id);
147 let impl1_substs = InternalSubsts::identity_for_item(tcx, impl1_def_id.to_def_id());
149 translate_substs(infcx, param_env, impl1_def_id.to_def_id(), impl1_substs, impl2_node);
151 // Conservatively use an empty `ParamEnv`.
152 let outlives_env = OutlivesEnvironment::new(ty::ParamEnv::empty());
153 infcx.resolve_regions_and_report_errors(&outlives_env);
154 let Ok(impl2_substs) = infcx.fully_resolve(impl2_substs) else {
155 let span = tcx.def_span(impl1_def_id);
156 tcx.sess.emit_err(SubstsOnOverriddenImpl { span });
159 Some((impl1_substs, impl2_substs))
162 /// Returns a list of all of the unconstrained subst of the given impl.
164 /// For example given the impl:
166 /// impl<'a, T, I> ... where &'a I: IntoIterator<Item=&'a T>
168 /// This would return the substs corresponding to `['a, I]`, because knowing
169 /// `'a` and `I` determines the value of `T`.
170 fn unconstrained_parent_impl_substs<'tcx>(
173 impl_substs: SubstsRef<'tcx>,
174 ) -> Vec<GenericArg<'tcx>> {
175 let impl_generic_predicates = tcx.predicates_of(impl_def_id);
176 let mut unconstrained_parameters = FxHashSet::default();
177 let mut constrained_params = FxHashSet::default();
178 let impl_trait_ref = tcx.impl_trait_ref(impl_def_id);
180 // Unfortunately the functions in `constrained_generic_parameters` don't do
181 // what we want here. We want only a list of constrained parameters while
182 // the functions in `cgp` add the constrained parameters to a list of
183 // unconstrained parameters.
184 for (predicate, _) in impl_generic_predicates.predicates.iter() {
185 if let ty::PredicateKind::Projection(proj) = predicate.kind().skip_binder() {
186 let projection_ty = proj.projection_ty;
187 let projected_ty = proj.term;
189 let unbound_trait_ref = projection_ty.trait_ref(tcx);
190 if Some(unbound_trait_ref) == impl_trait_ref {
194 unconstrained_parameters.extend(cgp::parameters_for(&projection_ty, true));
196 for param in cgp::parameters_for(&projected_ty, false) {
197 if !unconstrained_parameters.contains(¶m) {
198 constrained_params.insert(param.0);
202 unconstrained_parameters.extend(cgp::parameters_for(&projected_ty, true));
209 .filter(|&(idx, _)| !constrained_params.contains(&(idx as u32)))
214 /// Check that parameters of the derived impl don't occur more than once in the
215 /// equated substs of the base impl.
217 /// For example forbid the following:
219 /// impl<A> Tr for A { }
220 /// impl<B> Tr for (B, B) { }
222 /// Note that only consider the unconstrained parameters of the base impl:
224 /// impl<S, I: IntoIterator<Item = S>> Tr<S> for I { }
225 /// impl<T> Tr<T> for Vec<T> { }
227 /// The substs for the parent impl here are `[T, Vec<T>]`, which repeats `T`,
228 /// but `S` is constrained in the parent impl, so `parent_substs` is only
229 /// `[Vec<T>]`. This means we allow this impl.
230 fn check_duplicate_params<'tcx>(
232 impl1_substs: SubstsRef<'tcx>,
233 parent_substs: &Vec<GenericArg<'tcx>>,
236 let mut base_params = cgp::parameters_for(parent_substs, true);
237 base_params.sort_by_key(|param| param.0);
238 if let (_, [duplicate, ..]) = base_params.partition_dedup() {
239 let param = impl1_substs[duplicate.0 as usize];
241 .struct_span_err(span, &format!("specializing impl repeats parameter `{}`", param))
246 /// Check that `'static` lifetimes are not introduced by the specializing impl.
248 /// For example forbid the following:
250 /// impl<A> Tr for A { }
251 /// impl Tr for &'static i32 { }
252 fn check_static_lifetimes<'tcx>(
254 parent_substs: &Vec<GenericArg<'tcx>>,
257 if tcx.any_free_region_meets(parent_substs, |r| r.is_static()) {
258 tcx.sess.struct_span_err(span, "cannot specialize on `'static` lifetime").emit();
262 /// Check whether predicates on the specializing impl (`impl1`) are allowed.
264 /// Each predicate `P` must be:
266 /// * global (not reference any parameters)
267 /// * `T: Tr` predicate where `Tr` is an always-applicable trait
268 /// * on the base `impl impl2`
269 /// * Currently this check is done using syntactic equality, which is
270 /// conservative but generally sufficient.
271 /// * a well-formed predicate of a type argument of the trait being implemented,
272 /// including the `Self`-type.
273 fn check_predicates<'tcx>(
274 infcx: &InferCtxt<'_, 'tcx>,
275 impl1_def_id: LocalDefId,
276 impl1_substs: SubstsRef<'tcx>,
278 impl2_substs: SubstsRef<'tcx>,
282 let instantiated = tcx.predicates_of(impl1_def_id).instantiate(tcx, impl1_substs);
283 let impl1_predicates: Vec<_> = traits::elaborate_predicates_with_span(
286 instantiated.predicates,
287 // Don't drop predicates (unsound!) because `spans` is too short
288 instantiated.spans.into_iter().chain(std::iter::repeat(span)),
291 .map(|obligation| (obligation.predicate, obligation.cause.span))
294 let mut impl2_predicates = if impl2_node.is_from_trait() {
295 // Always applicable traits have to be always applicable without any
299 traits::elaborate_predicates(
301 tcx.predicates_of(impl2_node.def_id())
302 .instantiate(tcx, impl2_substs)
306 .map(|obligation| obligation.predicate)
310 "check_always_applicable(\nimpl1_predicates={:?},\nimpl2_predicates={:?}\n)",
311 impl1_predicates, impl2_predicates,
314 // Since impls of always applicable traits don't get to assume anything, we
315 // can also assume their supertraits apply.
317 // For example, we allow:
319 // #[rustc_specialization_trait]
320 // trait AlwaysApplicable: Debug { }
322 // impl<T> Tr for T { }
323 // impl<T: AlwaysApplicable> Tr for T { }
325 // Specializing on `AlwaysApplicable` allows also specializing on `Debug`
326 // which is sound because we forbid impls like the following
328 // impl<D: Debug> AlwaysApplicable for D { }
329 let always_applicable_traits = impl1_predicates.iter().copied().filter(|&(predicate, _)| {
331 trait_predicate_kind(tcx, predicate),
332 Some(TraitSpecializationKind::AlwaysApplicable)
336 // Include the well-formed predicates of the type parameters of the impl.
337 for arg in tcx.impl_trait_ref(impl1_def_id).unwrap().substs {
338 if let Some(obligations) = wf::obligations(
340 tcx.param_env(impl1_def_id),
341 tcx.hir().local_def_id_to_hir_id(impl1_def_id),
346 impl2_predicates.extend(
347 traits::elaborate_obligations(tcx, obligations)
348 .map(|obligation| obligation.predicate),
352 impl2_predicates.extend(
353 traits::elaborate_predicates_with_span(tcx, always_applicable_traits)
354 .map(|obligation| obligation.predicate),
357 for (predicate, span) in impl1_predicates {
358 if !impl2_predicates.contains(&predicate) {
359 check_specialization_on(tcx, predicate, span)
364 fn check_specialization_on<'tcx>(tcx: TyCtxt<'tcx>, predicate: ty::Predicate<'tcx>, span: Span) {
365 debug!("can_specialize_on(predicate = {:?})", predicate);
366 match predicate.kind().skip_binder() {
367 // Global predicates are either always true or always false, so we
368 // are fine to specialize on.
369 _ if predicate.is_global() => (),
370 // We allow specializing on explicitly marked traits with no associated
372 ty::PredicateKind::Trait(ty::TraitPredicate {
374 constness: ty::BoundConstness::NotConst,
378 trait_predicate_kind(tcx, predicate),
379 Some(TraitSpecializationKind::Marker)
385 "cannot specialize on trait `{}`",
386 tcx.def_path_str(trait_ref.def_id),
392 ty::PredicateKind::Projection(ty::ProjectionPredicate { projection_ty, term }) => {
396 &format!("cannot specialize on associated type `{projection_ty} == {term}`",),
402 .struct_span_err(span, &format!("cannot specialize on predicate `{}`", predicate))
408 fn trait_predicate_kind<'tcx>(
410 predicate: ty::Predicate<'tcx>,
411 ) -> Option<TraitSpecializationKind> {
412 match predicate.kind().skip_binder() {
413 ty::PredicateKind::Trait(ty::TraitPredicate {
415 constness: ty::BoundConstness::NotConst,
417 }) => Some(tcx.trait_def(trait_ref.def_id).specialization_kind),
418 ty::PredicateKind::Trait(_)
419 | ty::PredicateKind::RegionOutlives(_)
420 | ty::PredicateKind::TypeOutlives(_)
421 | ty::PredicateKind::Projection(_)
422 | ty::PredicateKind::WellFormed(_)
423 | ty::PredicateKind::Subtype(_)
424 | ty::PredicateKind::Coerce(_)
425 | ty::PredicateKind::ObjectSafe(_)
426 | ty::PredicateKind::ClosureKind(..)
427 | ty::PredicateKind::ConstEvaluatable(..)
428 | ty::PredicateKind::ConstEquate(..)
429 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,