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::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::error_reporting::TypeErrCtxtExt;
81 use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
82 use rustc_trait_selection::traits::{self, translate_substs, wf, ObligationCtxt};
84 pub(super) fn check_min_specialization(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) {
85 if let Some(node) = parent_specialization_node(tcx, impl_def_id) {
86 check_always_applicable(tcx, 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(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node: Node) {
107 if let Some((impl1_substs, impl2_substs)) = get_impl_substs(tcx, 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
114 let parent_substs = if impl2_node.is_from_trait() {
115 impl2_substs.to_vec()
117 unconstrained_parent_impl_substs(tcx, impl2_def_id, impl2_substs)
120 let span = tcx.def_span(impl1_def_id);
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);
127 /// Given a specializing impl `impl1`, and the base impl `impl2`, returns two
128 /// substitutions `(S1, S2)` that equate their trait references. The returned
129 /// types are expressed in terms of the generics of `impl1`.
133 /// impl<A, B> Foo<A> for B { /* impl2 */ }
134 /// impl<C> Foo<Vec<C>> for C { /* impl1 */ }
136 /// Would return `S1 = [C]` and `S2 = [Vec<C>, C]`.
137 fn get_impl_substs<'tcx>(
139 impl1_def_id: LocalDefId,
141 ) -> Option<(SubstsRef<'tcx>, SubstsRef<'tcx>)> {
142 let infcx = &tcx.infer_ctxt().build();
143 let ocx = ObligationCtxt::new(infcx);
144 let param_env = tcx.param_env(impl1_def_id);
145 let impl1_hir_id = tcx.hir().local_def_id_to_hir_id(impl1_def_id);
147 let assumed_wf_types =
148 ocx.assumed_wf_types(param_env, tcx.def_span(impl1_def_id), impl1_def_id);
150 let impl1_substs = InternalSubsts::identity_for_item(tcx, impl1_def_id.to_def_id());
152 translate_substs(infcx, param_env, impl1_def_id.to_def_id(), impl1_substs, impl2_node);
154 let errors = ocx.select_all_or_error();
155 if !errors.is_empty() {
156 ocx.infcx.err_ctxt().report_fulfillment_errors(&errors, None, false);
160 let implied_bounds = infcx.implied_bounds_tys(param_env, impl1_hir_id, assumed_wf_types);
161 let outlives_env = OutlivesEnvironment::with_bounds(param_env, Some(infcx), implied_bounds);
162 infcx.check_region_obligations_and_report_errors(impl1_def_id, &outlives_env);
163 let Ok(impl2_substs) = infcx.fully_resolve(impl2_substs) else {
164 let span = tcx.def_span(impl1_def_id);
165 tcx.sess.emit_err(SubstsOnOverriddenImpl { span });
168 Some((impl1_substs, impl2_substs))
171 /// Returns a list of all of the unconstrained subst of the given impl.
173 /// For example given the impl:
175 /// impl<'a, T, I> ... where &'a I: IntoIterator<Item=&'a T>
177 /// This would return the substs corresponding to `['a, I]`, because knowing
178 /// `'a` and `I` determines the value of `T`.
179 fn unconstrained_parent_impl_substs<'tcx>(
182 impl_substs: SubstsRef<'tcx>,
183 ) -> Vec<GenericArg<'tcx>> {
184 let impl_generic_predicates = tcx.predicates_of(impl_def_id);
185 let mut unconstrained_parameters = FxHashSet::default();
186 let mut constrained_params = FxHashSet::default();
187 let impl_trait_ref = tcx.impl_trait_ref(impl_def_id);
189 // Unfortunately the functions in `constrained_generic_parameters` don't do
190 // what we want here. We want only a list of constrained parameters while
191 // the functions in `cgp` add the constrained parameters to a list of
192 // unconstrained parameters.
193 for (predicate, _) in impl_generic_predicates.predicates.iter() {
194 if let ty::PredicateKind::Projection(proj) = predicate.kind().skip_binder() {
195 let projection_ty = proj.projection_ty;
196 let projected_ty = proj.term;
198 let unbound_trait_ref = projection_ty.trait_ref(tcx);
199 if Some(unbound_trait_ref) == impl_trait_ref {
203 unconstrained_parameters.extend(cgp::parameters_for(&projection_ty, true));
205 for param in cgp::parameters_for(&projected_ty, false) {
206 if !unconstrained_parameters.contains(¶m) {
207 constrained_params.insert(param.0);
211 unconstrained_parameters.extend(cgp::parameters_for(&projected_ty, true));
218 .filter(|&(idx, _)| !constrained_params.contains(&(idx as u32)))
223 /// Check that parameters of the derived impl don't occur more than once in the
224 /// equated substs of the base impl.
226 /// For example forbid the following:
228 /// impl<A> Tr for A { }
229 /// impl<B> Tr for (B, B) { }
231 /// Note that only consider the unconstrained parameters of the base impl:
233 /// impl<S, I: IntoIterator<Item = S>> Tr<S> for I { }
234 /// impl<T> Tr<T> for Vec<T> { }
236 /// The substs for the parent impl here are `[T, Vec<T>]`, which repeats `T`,
237 /// but `S` is constrained in the parent impl, so `parent_substs` is only
238 /// `[Vec<T>]`. This means we allow this impl.
239 fn check_duplicate_params<'tcx>(
241 impl1_substs: SubstsRef<'tcx>,
242 parent_substs: &Vec<GenericArg<'tcx>>,
245 let mut base_params = cgp::parameters_for(parent_substs, true);
246 base_params.sort_by_key(|param| param.0);
247 if let (_, [duplicate, ..]) = base_params.partition_dedup() {
248 let param = impl1_substs[duplicate.0 as usize];
250 .struct_span_err(span, &format!("specializing impl repeats parameter `{}`", param))
255 /// Check that `'static` lifetimes are not introduced by the specializing impl.
257 /// For example forbid the following:
259 /// impl<A> Tr for A { }
260 /// impl Tr for &'static i32 { }
261 fn check_static_lifetimes<'tcx>(
263 parent_substs: &Vec<GenericArg<'tcx>>,
266 if tcx.any_free_region_meets(parent_substs, |r| r.is_static()) {
267 tcx.sess.struct_span_err(span, "cannot specialize on `'static` lifetime").emit();
271 /// Check whether predicates on the specializing impl (`impl1`) are allowed.
273 /// Each predicate `P` must be:
275 /// * global (not reference any parameters)
276 /// * `T: Tr` predicate where `Tr` is an always-applicable trait
277 /// * on the base `impl impl2`
278 /// * Currently this check is done using syntactic equality, which is
279 /// conservative but generally sufficient.
280 /// * a well-formed predicate of a type argument of the trait being implemented,
281 /// including the `Self`-type.
282 fn check_predicates<'tcx>(
284 impl1_def_id: LocalDefId,
285 impl1_substs: SubstsRef<'tcx>,
287 impl2_substs: SubstsRef<'tcx>,
290 let instantiated = tcx.predicates_of(impl1_def_id).instantiate(tcx, impl1_substs);
291 let impl1_predicates: Vec<_> = traits::elaborate_predicates_with_span(
294 instantiated.predicates,
295 // Don't drop predicates (unsound!) because `spans` is too short
296 instantiated.spans.into_iter().chain(std::iter::repeat(span)),
299 .map(|obligation| (obligation.predicate, obligation.cause.span))
302 let mut impl2_predicates = if impl2_node.is_from_trait() {
303 // Always applicable traits have to be always applicable without any
307 traits::elaborate_predicates(
309 tcx.predicates_of(impl2_node.def_id())
310 .instantiate(tcx, impl2_substs)
314 .map(|obligation| obligation.predicate)
318 "check_always_applicable(\nimpl1_predicates={:?},\nimpl2_predicates={:?}\n)",
319 impl1_predicates, impl2_predicates,
322 // Since impls of always applicable traits don't get to assume anything, we
323 // can also assume their supertraits apply.
325 // For example, we allow:
327 // #[rustc_specialization_trait]
328 // trait AlwaysApplicable: Debug { }
330 // impl<T> Tr for T { }
331 // impl<T: AlwaysApplicable> Tr for T { }
333 // Specializing on `AlwaysApplicable` allows also specializing on `Debug`
334 // which is sound because we forbid impls like the following
336 // impl<D: Debug> AlwaysApplicable for D { }
337 let always_applicable_traits = impl1_predicates.iter().copied().filter(|&(predicate, _)| {
339 trait_predicate_kind(tcx, predicate),
340 Some(TraitSpecializationKind::AlwaysApplicable)
344 // Include the well-formed predicates of the type parameters of the impl.
345 for arg in tcx.impl_trait_ref(impl1_def_id).unwrap().substs {
346 let infcx = &tcx.infer_ctxt().build();
347 let obligations = wf::obligations(
349 tcx.param_env(impl1_def_id),
350 tcx.hir().local_def_id_to_hir_id(impl1_def_id),
357 assert!(!obligations.needs_infer());
358 impl2_predicates.extend(
359 traits::elaborate_obligations(tcx, obligations).map(|obligation| obligation.predicate),
362 impl2_predicates.extend(
363 traits::elaborate_predicates_with_span(tcx, always_applicable_traits)
364 .map(|obligation| obligation.predicate),
367 for (predicate, span) in impl1_predicates {
368 if !impl2_predicates.contains(&predicate) {
369 check_specialization_on(tcx, predicate, span)
374 fn check_specialization_on<'tcx>(tcx: TyCtxt<'tcx>, predicate: ty::Predicate<'tcx>, span: Span) {
375 debug!("can_specialize_on(predicate = {:?})", predicate);
376 match predicate.kind().skip_binder() {
377 // Global predicates are either always true or always false, so we
378 // are fine to specialize on.
379 _ if predicate.is_global() => (),
380 // We allow specializing on explicitly marked traits with no associated
382 ty::PredicateKind::Trait(ty::TraitPredicate {
384 constness: ty::BoundConstness::NotConst,
388 trait_predicate_kind(tcx, predicate),
389 Some(TraitSpecializationKind::Marker)
395 "cannot specialize on trait `{}`",
396 tcx.def_path_str(trait_ref.def_id),
402 ty::PredicateKind::Projection(ty::ProjectionPredicate { projection_ty, term }) => {
406 &format!("cannot specialize on associated type `{projection_ty} == {term}`",),
412 .struct_span_err(span, &format!("cannot specialize on predicate `{}`", predicate))
418 fn trait_predicate_kind<'tcx>(
420 predicate: ty::Predicate<'tcx>,
421 ) -> Option<TraitSpecializationKind> {
422 match predicate.kind().skip_binder() {
423 ty::PredicateKind::Trait(ty::TraitPredicate { trait_ref, constness: _, polarity: _ }) => {
424 Some(tcx.trait_def(trait_ref.def_id).specialization_kind)
426 ty::PredicateKind::RegionOutlives(_)
427 | ty::PredicateKind::TypeOutlives(_)
428 | ty::PredicateKind::Projection(_)
429 | ty::PredicateKind::WellFormed(_)
430 | ty::PredicateKind::Subtype(_)
431 | ty::PredicateKind::Coerce(_)
432 | ty::PredicateKind::ObjectSafe(_)
433 | ty::PredicateKind::ClosureKind(..)
434 | ty::PredicateKind::ConstEvaluatable(..)
435 | ty::PredicateKind::ConstEquate(..)
436 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,