1 //! This pass enforces various "well-formedness constraints" on impls.
2 //! Logically, it is part of wfcheck -- but we do it early so that we
3 //! can stop compilation afterwards, since part of the trait matching
4 //! infrastructure gets very grumpy if these conditions don't hold. In
5 //! particular, if there are type parameters that are not part of the
6 //! impl, then coherence will report strange inference ambiguity
7 //! errors; if impls have duplicate items, we get misleading
8 //! specialization errors. These things can (and probably should) be
9 //! fixed, but for the moment it's easier to do these checks early.
11 use crate::constrained_generic_params as cgp;
12 use min_specialization::check_min_specialization;
14 use rustc_data_structures::fx::FxHashSet;
15 use rustc_errors::struct_span_err;
16 use rustc_hir::def::DefKind;
17 use rustc_hir::def_id::LocalDefId;
18 use rustc_middle::ty::query::Providers;
19 use rustc_middle::ty::{self, TyCtxt, TypeVisitable};
20 use rustc_span::{Span, Symbol};
22 mod min_specialization;
24 /// Checks that all the type/lifetime parameters on an impl also
25 /// appear in the trait ref or self type (or are constrained by a
26 /// where-clause). These rules are needed to ensure that, given a
27 /// trait ref like `<T as Trait<U>>`, we can derive the values of all
28 /// parameters on the impl (which is needed to make specialization
31 /// However, in the case of lifetimes, we only enforce these rules if
32 /// the lifetime parameter is used in an associated type. This is a
33 /// concession to backwards compatibility; see comment at the end of
34 /// the fn for details.
38 /// ```rust,ignore (pseudo-Rust)
39 /// impl<T> Trait<Foo> for Bar { ... }
40 /// // ^ T does not appear in `Foo` or `Bar`, error!
42 /// impl<T> Trait<Foo<T>> for Bar { ... }
43 /// // ^ T appears in `Foo<T>`, ok.
45 /// impl<T> Trait<Foo> for Bar where Bar: Iterator<Item = T> { ... }
46 /// // ^ T is bound to `<Bar as Iterator>::Item`, ok.
48 /// impl<'a> Trait<Foo> for Bar { }
49 /// // ^ 'a is unused, but for back-compat we allow it
51 /// impl<'a> Trait<Foo> for Bar { type X = &'a i32; }
52 /// // ^ 'a is unused and appears in assoc type, error
54 fn check_mod_impl_wf(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
55 let min_specialization = tcx.features().min_specialization;
56 let module = tcx.hir_module_items(module_def_id);
57 for id in module.items() {
58 if matches!(tcx.def_kind(id.owner_id), DefKind::Impl) {
59 enforce_impl_params_are_constrained(tcx, id.owner_id.def_id);
60 if min_specialization {
61 check_min_specialization(tcx, id.owner_id.def_id);
67 pub fn provide(providers: &mut Providers) {
68 *providers = Providers { check_mod_impl_wf, ..*providers };
71 fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) {
72 // Every lifetime used in an associated type must be constrained.
73 let impl_self_ty = tcx.type_of(impl_def_id);
74 if impl_self_ty.references_error() {
75 // Don't complain about unconstrained type params when self ty isn't known due to errors.
77 tcx.sess.delay_span_bug(
78 tcx.def_span(impl_def_id),
80 "potentially unconstrained type parameters weren't evaluated: {:?}",
86 let impl_generics = tcx.generics_of(impl_def_id);
87 let impl_predicates = tcx.predicates_of(impl_def_id);
88 let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).map(ty::EarlyBinder::subst_identity);
90 let mut input_parameters = cgp::parameters_for_impl(impl_self_ty, impl_trait_ref);
91 cgp::identify_constrained_generic_params(
95 &mut input_parameters,
98 // Disallow unconstrained lifetimes, but only if they appear in assoc types.
99 let lifetimes_in_associated_types: FxHashSet<_> = tcx
100 .associated_item_def_ids(impl_def_id)
103 let item = tcx.associated_item(def_id);
105 ty::AssocKind::Type => {
106 if item.defaultness(tcx).has_value() {
107 cgp::parameters_for(&tcx.type_of(def_id), true)
112 ty::AssocKind::Fn | ty::AssocKind::Const => Vec::new(),
117 for param in &impl_generics.params {
119 // Disallow ANY unconstrained type parameters.
120 ty::GenericParamDefKind::Type { .. } => {
121 let param_ty = ty::ParamTy::for_def(param);
122 if !input_parameters.contains(&cgp::Parameter::from(param_ty)) {
123 report_unused_parameter(tcx, tcx.def_span(param.def_id), "type", param_ty.name);
126 ty::GenericParamDefKind::Lifetime => {
127 let param_lt = cgp::Parameter::from(param.to_early_bound_region_data());
128 if lifetimes_in_associated_types.contains(¶m_lt) && // (*)
129 !input_parameters.contains(¶m_lt)
131 report_unused_parameter(
133 tcx.def_span(param.def_id),
139 ty::GenericParamDefKind::Const { .. } => {
140 let param_ct = ty::ParamConst::for_def(param);
141 if !input_parameters.contains(&cgp::Parameter::from(param_ct)) {
142 report_unused_parameter(
144 tcx.def_span(param.def_id),
153 // (*) This is a horrible concession to reality. I think it'd be
154 // better to just ban unconstrained lifetimes outright, but in
155 // practice people do non-hygienic macros like:
158 // macro_rules! __impl_slice_eq1 {
159 // ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
160 // impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
167 // In a concession to backwards compatibility, we continue to
168 // permit those, so long as the lifetimes aren't used in
169 // associated types. I believe this is sound, because lifetimes
170 // used elsewhere are not projected back out.
173 fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: Symbol) {
174 let mut err = struct_span_err!(
178 "the {} parameter `{}` is not constrained by the \
179 impl trait, self type, or predicates",
183 err.span_label(span, format!("unconstrained {} parameter", kind));
186 "expressions using a const parameter must map each value to a distinct output value",
189 "proving the result of expressions other than the parameter are unique is not supported",