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 rustc::ty::query::Providers;
13 use rustc::ty::{self, TyCtxt, TypeFoldable};
14 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
15 use rustc_errors::struct_span_err;
17 use rustc_hir::def_id::DefId;
18 use rustc_hir::itemlikevisit::ItemLikeVisitor;
19 use std::collections::hash_map::Entry::{Occupied, Vacant};
23 use rustc_error_codes::*;
25 /// Checks that all the type/lifetime parameters on an impl also
26 /// appear in the trait ref or self type (or are constrained by a
27 /// where-clause). These rules are needed to ensure that, given a
28 /// trait ref like `<T as Trait<U>>`, we can derive the values of all
29 /// parameters on the impl (which is needed to make specialization
32 /// However, in the case of lifetimes, we only enforce these rules if
33 /// the lifetime parameter is used in an associated type. This is a
34 /// concession to backwards compatibility; see comment at the end of
35 /// the fn for details.
39 /// ```rust,ignore (pseudo-Rust)
40 /// impl<T> Trait<Foo> for Bar { ... }
41 /// // ^ T does not appear in `Foo` or `Bar`, error!
43 /// impl<T> Trait<Foo<T>> for Bar { ... }
44 /// // ^ T appears in `Foo<T>`, ok.
46 /// impl<T> Trait<Foo> for Bar where Bar: Iterator<Item = T> { ... }
47 /// // ^ T is bound to `<Bar as Iterator>::Item`, ok.
49 /// impl<'a> Trait<Foo> for Bar { }
50 /// // ^ 'a is unused, but for back-compat we allow it
52 /// impl<'a> Trait<Foo> for Bar { type X = &'a i32; }
53 /// // ^ 'a is unused and appears in assoc type, error
55 pub fn impl_wf_check(tcx: TyCtxt<'_>) {
56 // We will tag this as part of the WF check -- logically, it is,
57 // but it's one that we must perform earlier than the rest of
59 for &module in tcx.hir().krate().modules.keys() {
60 tcx.ensure().check_mod_impl_wf(tcx.hir().local_def_id(module));
64 fn check_mod_impl_wf(tcx: TyCtxt<'_>, module_def_id: DefId) {
65 tcx.hir().visit_item_likes_in_module(module_def_id, &mut ImplWfCheck { tcx });
68 pub fn provide(providers: &mut Providers<'_>) {
69 *providers = Providers { check_mod_impl_wf, ..*providers };
72 struct ImplWfCheck<'tcx> {
76 impl ItemLikeVisitor<'tcx> for ImplWfCheck<'tcx> {
77 fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
78 if let hir::ItemKind::Impl { ref items, .. } = item.kind {
79 let impl_def_id = self.tcx.hir().local_def_id(item.hir_id);
80 enforce_impl_params_are_constrained(self.tcx, impl_def_id, items);
81 enforce_impl_items_are_distinct(self.tcx, items);
85 fn visit_trait_item(&mut self, _trait_item: &'tcx hir::TraitItem<'tcx>) {}
87 fn visit_impl_item(&mut self, _impl_item: &'tcx hir::ImplItem<'tcx>) {}
90 fn enforce_impl_params_are_constrained(
93 impl_item_refs: &[hir::ImplItemRef<'_>],
95 // Every lifetime used in an associated type must be constrained.
96 let impl_self_ty = tcx.type_of(impl_def_id);
97 if impl_self_ty.references_error() {
98 // Don't complain about unconstrained type params when self ty isn't known due to errors.
100 tcx.sess.delay_span_bug(
101 tcx.def_span(impl_def_id),
103 "potentially unconstrained type parameters weren't evaluated: {:?}",
109 let impl_generics = tcx.generics_of(impl_def_id);
110 let impl_predicates = tcx.predicates_of(impl_def_id);
111 let impl_trait_ref = tcx.impl_trait_ref(impl_def_id);
113 let mut input_parameters = cgp::parameters_for_impl(impl_self_ty, impl_trait_ref);
114 cgp::identify_constrained_generic_params(
118 &mut input_parameters,
121 // Disallow unconstrained lifetimes, but only if they appear in assoc types.
122 let lifetimes_in_associated_types: FxHashSet<_> = impl_item_refs
124 .map(|item_ref| tcx.hir().local_def_id(item_ref.id.hir_id))
126 let item = tcx.associated_item(def_id);
127 item.kind == ty::AssocKind::Type && item.defaultness.has_value()
129 .flat_map(|def_id| cgp::parameters_for(&tcx.type_of(def_id), true))
132 for param in &impl_generics.params {
134 // Disallow ANY unconstrained type parameters.
135 ty::GenericParamDefKind::Type { .. } => {
136 let param_ty = ty::ParamTy::for_def(param);
137 if !input_parameters.contains(&cgp::Parameter::from(param_ty)) {
138 report_unused_parameter(
140 tcx.def_span(param.def_id),
142 ¶m_ty.to_string(),
146 ty::GenericParamDefKind::Lifetime => {
147 let param_lt = cgp::Parameter::from(param.to_early_bound_region_data());
148 if lifetimes_in_associated_types.contains(¶m_lt) && // (*)
149 !input_parameters.contains(¶m_lt)
151 report_unused_parameter(
153 tcx.def_span(param.def_id),
155 ¶m.name.to_string(),
159 ty::GenericParamDefKind::Const => {
160 let param_ct = ty::ParamConst::for_def(param);
161 if !input_parameters.contains(&cgp::Parameter::from(param_ct)) {
162 report_unused_parameter(
164 tcx.def_span(param.def_id),
166 ¶m_ct.to_string(),
173 // (*) This is a horrible concession to reality. I think it'd be
174 // better to just ban unconstrianed lifetimes outright, but in
175 // practice people do non-hygenic macros like:
178 // macro_rules! __impl_slice_eq1 {
179 // ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
180 // impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
187 // In a concession to backwards compatibility, we continue to
188 // permit those, so long as the lifetimes aren't used in
189 // associated types. I believe this is sound, because lifetimes
190 // used elsewhere are not projected back out.
193 fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: &str) {
198 "the {} parameter `{}` is not constrained by the \
199 impl trait, self type, or predicates",
203 .span_label(span, format!("unconstrained {} parameter", kind))
207 /// Enforce that we do not have two items in an impl with the same name.
208 fn enforce_impl_items_are_distinct(tcx: TyCtxt<'_>, impl_item_refs: &[hir::ImplItemRef<'_>]) {
209 let mut seen_type_items = FxHashMap::default();
210 let mut seen_value_items = FxHashMap::default();
211 for impl_item_ref in impl_item_refs {
212 let impl_item = tcx.hir().impl_item(impl_item_ref.id);
213 let seen_items = match impl_item.kind {
214 hir::ImplItemKind::TyAlias(_) => &mut seen_type_items,
215 _ => &mut seen_value_items,
217 match seen_items.entry(impl_item.ident.modern()) {
219 let mut err = struct_span_err!(
223 "duplicate definitions with name `{}`:",
228 format!("previous definition of `{}` here", impl_item.ident),
230 err.span_label(impl_item.span, "duplicate definition");
234 entry.insert(impl_item.span);