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::{FxHashMap, 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, TypeFoldable};
22 use std::collections::hash_map::Entry::{Occupied, Vacant};
24 mod min_specialization;
26 /// Checks that all the type/lifetime parameters on an impl also
27 /// appear in the trait ref or self type (or are constrained by a
28 /// where-clause). These rules are needed to ensure that, given a
29 /// trait ref like `<T as Trait<U>>`, we can derive the values of all
30 /// parameters on the impl (which is needed to make specialization
33 /// However, in the case of lifetimes, we only enforce these rules if
34 /// the lifetime parameter is used in an associated type. This is a
35 /// concession to backwards compatibility; see comment at the end of
36 /// the fn for details.
40 /// ```rust,ignore (pseudo-Rust)
41 /// impl<T> Trait<Foo> for Bar { ... }
42 /// // ^ T does not appear in `Foo` or `Bar`, error!
44 /// impl<T> Trait<Foo<T>> for Bar { ... }
45 /// // ^ T appears in `Foo<T>`, ok.
47 /// impl<T> Trait<Foo> for Bar where Bar: Iterator<Item = T> { ... }
48 /// // ^ T is bound to `<Bar as Iterator>::Item`, ok.
50 /// impl<'a> Trait<Foo> for Bar { }
51 /// // ^ 'a is unused, but for back-compat we allow it
53 /// impl<'a> Trait<Foo> for Bar { type X = &'a i32; }
54 /// // ^ 'a is unused and appears in assoc type, error
56 fn check_mod_impl_wf(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
57 let min_specialization = tcx.features().min_specialization;
58 let module = tcx.hir_module_items(module_def_id);
59 for id in module.items() {
60 if matches!(tcx.def_kind(id.def_id), DefKind::Impl) {
61 enforce_impl_params_are_constrained(tcx, id.def_id);
62 enforce_impl_items_are_distinct(tcx, id.def_id);
63 if min_specialization {
64 check_min_specialization(tcx, id.def_id);
70 pub fn provide(providers: &mut Providers) {
71 *providers = Providers { check_mod_impl_wf, ..*providers };
74 fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) {
75 // Every lifetime used in an associated type must be constrained.
76 let impl_self_ty = tcx.type_of(impl_def_id);
77 if impl_self_ty.references_error() {
78 // Don't complain about unconstrained type params when self ty isn't known due to errors.
80 tcx.sess.delay_span_bug(
81 tcx.def_span(impl_def_id),
83 "potentially unconstrained type parameters weren't evaluated: {:?}",
89 let impl_generics = tcx.generics_of(impl_def_id);
90 let impl_predicates = tcx.predicates_of(impl_def_id);
91 let impl_trait_ref = tcx.impl_trait_ref(impl_def_id);
93 let mut input_parameters = cgp::parameters_for_impl(impl_self_ty, impl_trait_ref);
94 cgp::identify_constrained_generic_params(
98 &mut input_parameters,
101 // Disallow unconstrained lifetimes, but only if they appear in assoc types.
102 let lifetimes_in_associated_types: FxHashSet<_> = tcx
103 .associated_item_def_ids(impl_def_id)
106 let item = tcx.associated_item(def_id);
108 ty::AssocKind::Type => {
109 if item.defaultness.has_value() {
110 cgp::parameters_for(&tcx.type_of(def_id), true)
115 ty::AssocKind::Fn | ty::AssocKind::Const => Vec::new(),
120 for param in &impl_generics.params {
122 // Disallow ANY unconstrained type parameters.
123 ty::GenericParamDefKind::Type { .. } => {
124 let param_ty = ty::ParamTy::for_def(param);
125 if !input_parameters.contains(&cgp::Parameter::from(param_ty)) {
126 report_unused_parameter(
128 tcx.def_span(param.def_id),
130 ¶m_ty.to_string(),
134 ty::GenericParamDefKind::Lifetime => {
135 let param_lt = cgp::Parameter::from(param.to_early_bound_region_data());
136 if lifetimes_in_associated_types.contains(¶m_lt) && // (*)
137 !input_parameters.contains(¶m_lt)
139 report_unused_parameter(
141 tcx.def_span(param.def_id),
143 ¶m.name.to_string(),
147 ty::GenericParamDefKind::Const { .. } => {
148 let param_ct = ty::ParamConst::for_def(param);
149 if !input_parameters.contains(&cgp::Parameter::from(param_ct)) {
150 report_unused_parameter(
152 tcx.def_span(param.def_id),
154 ¶m_ct.to_string(),
161 // (*) This is a horrible concession to reality. I think it'd be
162 // better to just ban unconstrained lifetimes outright, but in
163 // practice people do non-hygienic macros like:
166 // macro_rules! __impl_slice_eq1 {
167 // ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
168 // impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
175 // In a concession to backwards compatibility, we continue to
176 // permit those, so long as the lifetimes aren't used in
177 // associated types. I believe this is sound, because lifetimes
178 // used elsewhere are not projected back out.
181 fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: &str) {
182 let mut err = struct_span_err!(
186 "the {} parameter `{}` is not constrained by the \
187 impl trait, self type, or predicates",
191 err.span_label(span, format!("unconstrained {} parameter", kind));
194 "expressions using a const parameter must map each value to a distinct output value",
197 "proving the result of expressions other than the parameter are unique is not supported",
203 /// Enforce that we do not have two items in an impl with the same name.
204 fn enforce_impl_items_are_distinct(tcx: TyCtxt<'_>, impl_def_id: LocalDefId) {
205 let mut seen_type_items = FxHashMap::default();
206 let mut seen_value_items = FxHashMap::default();
207 for &impl_item_ref in tcx.associated_item_def_ids(impl_def_id) {
208 let impl_item = tcx.associated_item(impl_item_ref);
209 let seen_items = match impl_item.kind {
210 ty::AssocKind::Type => &mut seen_type_items,
211 _ => &mut seen_value_items,
213 let span = tcx.def_span(impl_item_ref);
214 let ident = impl_item.ident(tcx);
215 match seen_items.entry(ident.normalize_to_macros_2_0()) {
217 let mut err = struct_span_err!(
221 "duplicate definitions with name `{}`:",
224 err.span_label(*entry.get(), format!("previous definition of `{}` here", ident));
225 err.span_label(span, "duplicate definition");