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 /// Checks that all the type/lifetime parameters on an impl also
24 /// appear in the trait ref or self type (or are constrained by a
25 /// where-clause). These rules are needed to ensure that, given a
26 /// trait ref like `<T as Trait<U>>`, we can derive the values of all
27 /// parameters on the impl (which is needed to make specialization
30 /// However, in the case of lifetimes, we only enforce these rules if
31 /// the lifetime parameter is used in an associated type. This is a
32 /// concession to backwards compatibility; see comment at the end of
33 /// the fn for details.
37 /// ```rust,ignore (pseudo-Rust)
38 /// impl<T> Trait<Foo> for Bar { ... }
39 /// // ^ T does not appear in `Foo` or `Bar`, error!
41 /// impl<T> Trait<Foo<T>> for Bar { ... }
42 /// // ^ T appears in `Foo<T>`, ok.
44 /// impl<T> Trait<Foo> for Bar where Bar: Iterator<Item = T> { ... }
45 /// // ^ T is bound to `<Bar as Iterator>::Item`, ok.
47 /// impl<'a> Trait<Foo> for Bar { }
48 /// // ^ 'a is unused, but for back-compat we allow it
50 /// impl<'a> Trait<Foo> for Bar { type X = &'a i32; }
51 /// // ^ 'a is unused and appears in assoc type, error
53 pub fn impl_wf_check(tcx: TyCtxt<'_>) {
54 // We will tag this as part of the WF check -- logically, it is,
55 // but it's one that we must perform earlier than the rest of
57 for &module in tcx.hir().krate().modules.keys() {
58 tcx.ensure().check_mod_impl_wf(tcx.hir().local_def_id(module));
62 fn check_mod_impl_wf(tcx: TyCtxt<'_>, module_def_id: DefId) {
63 tcx.hir().visit_item_likes_in_module(module_def_id, &mut ImplWfCheck { tcx });
66 pub fn provide(providers: &mut Providers<'_>) {
67 *providers = Providers { check_mod_impl_wf, ..*providers };
70 struct ImplWfCheck<'tcx> {
74 impl ItemLikeVisitor<'tcx> for ImplWfCheck<'tcx> {
75 fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
76 if let hir::ItemKind::Impl { ref items, .. } = item.kind {
77 let impl_def_id = self.tcx.hir().local_def_id(item.hir_id);
78 enforce_impl_params_are_constrained(self.tcx, impl_def_id, items);
79 enforce_impl_items_are_distinct(self.tcx, items);
83 fn visit_trait_item(&mut self, _trait_item: &'tcx hir::TraitItem<'tcx>) {}
85 fn visit_impl_item(&mut self, _impl_item: &'tcx hir::ImplItem<'tcx>) {}
88 fn enforce_impl_params_are_constrained(
91 impl_item_refs: &[hir::ImplItemRef<'_>],
93 // Every lifetime used in an associated type must be constrained.
94 let impl_self_ty = tcx.type_of(impl_def_id);
95 if impl_self_ty.references_error() {
96 // Don't complain about unconstrained type params when self ty isn't known due to errors.
98 tcx.sess.delay_span_bug(
99 tcx.def_span(impl_def_id),
101 "potentially unconstrained type parameters weren't evaluated: {:?}",
107 let impl_generics = tcx.generics_of(impl_def_id);
108 let impl_predicates = tcx.predicates_of(impl_def_id);
109 let impl_trait_ref = tcx.impl_trait_ref(impl_def_id);
111 let mut input_parameters = cgp::parameters_for_impl(impl_self_ty, impl_trait_ref);
112 cgp::identify_constrained_generic_params(
116 &mut input_parameters,
119 // Disallow unconstrained lifetimes, but only if they appear in assoc types.
120 let lifetimes_in_associated_types: FxHashSet<_> = impl_item_refs
122 .map(|item_ref| tcx.hir().local_def_id(item_ref.id.hir_id))
124 let item = tcx.associated_item(def_id);
125 item.kind == ty::AssocKind::Type && item.defaultness.has_value()
127 .flat_map(|def_id| cgp::parameters_for(&tcx.type_of(def_id), true))
130 for param in &impl_generics.params {
132 // Disallow ANY unconstrained type parameters.
133 ty::GenericParamDefKind::Type { .. } => {
134 let param_ty = ty::ParamTy::for_def(param);
135 if !input_parameters.contains(&cgp::Parameter::from(param_ty)) {
136 report_unused_parameter(
138 tcx.def_span(param.def_id),
140 ¶m_ty.to_string(),
144 ty::GenericParamDefKind::Lifetime => {
145 let param_lt = cgp::Parameter::from(param.to_early_bound_region_data());
146 if lifetimes_in_associated_types.contains(¶m_lt) && // (*)
147 !input_parameters.contains(¶m_lt)
149 report_unused_parameter(
151 tcx.def_span(param.def_id),
153 ¶m.name.to_string(),
157 ty::GenericParamDefKind::Const => {
158 let param_ct = ty::ParamConst::for_def(param);
159 if !input_parameters.contains(&cgp::Parameter::from(param_ct)) {
160 report_unused_parameter(
162 tcx.def_span(param.def_id),
164 ¶m_ct.to_string(),
171 // (*) This is a horrible concession to reality. I think it'd be
172 // better to just ban unconstrianed lifetimes outright, but in
173 // practice people do non-hygenic macros like:
176 // macro_rules! __impl_slice_eq1 {
177 // ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
178 // impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
185 // In a concession to backwards compatibility, we continue to
186 // permit those, so long as the lifetimes aren't used in
187 // associated types. I believe this is sound, because lifetimes
188 // used elsewhere are not projected back out.
191 fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: &str) {
196 "the {} parameter `{}` is not constrained by the \
197 impl trait, self type, or predicates",
201 .span_label(span, format!("unconstrained {} parameter", kind))
205 /// Enforce that we do not have two items in an impl with the same name.
206 fn enforce_impl_items_are_distinct(tcx: TyCtxt<'_>, impl_item_refs: &[hir::ImplItemRef<'_>]) {
207 let mut seen_type_items = FxHashMap::default();
208 let mut seen_value_items = FxHashMap::default();
209 for impl_item_ref in impl_item_refs {
210 let impl_item = tcx.hir().impl_item(impl_item_ref.id);
211 let seen_items = match impl_item.kind {
212 hir::ImplItemKind::TyAlias(_) => &mut seen_type_items,
213 _ => &mut seen_value_items,
215 match seen_items.entry(impl_item.ident.modern()) {
217 let mut err = struct_span_err!(
221 "duplicate definitions with name `{}`:",
226 format!("previous definition of `{}` here", impl_item.ident),
228 err.span_label(impl_item.span, "duplicate definition");
232 entry.insert(impl_item.span);