| //! This pass enforces various "well-formedness constraints" on impls. |
| //! Logically, it is part of wfcheck -- but we do it early so that we |
| //! can stop compilation afterwards, since part of the trait matching |
| //! infrastructure gets very grumpy if these conditions don't hold. In |
| //! particular, if there are type parameters that are not part of the |
| //! impl, then coherence will report strange inference ambiguity |
| //! errors; if impls have duplicate items, we get misleading |
| //! specialization errors. These things can (and probably should) be |
| //! fixed, but for the moment it's easier to do these checks early. |
| |
| use crate::constrained_generic_params as cgp; |
| use rustc::hir; |
| use rustc::hir::itemlikevisit::ItemLikeVisitor; |
| use rustc::hir::def_id::DefId; |
| use rustc::ty::{self, TyCtxt, TypeFoldable}; |
| use rustc::ty::query::Providers; |
| use rustc::util::nodemap::{FxHashMap, FxHashSet}; |
| use std::collections::hash_map::Entry::{Occupied, Vacant}; |
| |
| use syntax_pos::Span; |
| |
| /// Checks that all the type/lifetime parameters on an impl also |
| /// appear in the trait ref or self type (or are constrained by a |
| /// where-clause). These rules are needed to ensure that, given a |
| /// trait ref like `<T as Trait<U>>`, we can derive the values of all |
| /// parameters on the impl (which is needed to make specialization |
| /// possible). |
| /// |
| /// However, in the case of lifetimes, we only enforce these rules if |
| /// the lifetime parameter is used in an associated type. This is a |
| /// concession to backwards compatibility; see comment at the end of |
| /// the fn for details. |
| /// |
| /// Example: |
| /// |
| /// ```rust,ignore (pseudo-Rust) |
| /// impl<T> Trait<Foo> for Bar { ... } |
| /// // ^ T does not appear in `Foo` or `Bar`, error! |
| /// |
| /// impl<T> Trait<Foo<T>> for Bar { ... } |
| /// // ^ T appears in `Foo<T>`, ok. |
| /// |
| /// impl<T> Trait<Foo> for Bar where Bar: Iterator<Item = T> { ... } |
| /// // ^ T is bound to `<Bar as Iterator>::Item`, ok. |
| /// |
| /// impl<'a> Trait<Foo> for Bar { } |
| /// // ^ 'a is unused, but for back-compat we allow it |
| /// |
| /// impl<'a> Trait<Foo> for Bar { type X = &'a i32; } |
| /// // ^ 'a is unused and appears in assoc type, error |
| /// ``` |
| pub fn impl_wf_check(tcx: TyCtxt<'_>) { |
| // We will tag this as part of the WF check -- logically, it is, |
| // but it's one that we must perform earlier than the rest of |
| // WfCheck. |
| for &module in tcx.hir().krate().modules.keys() { |
| tcx.ensure().check_mod_impl_wf(tcx.hir().local_def_id_from_node_id(module)); |
| } |
| } |
| |
| fn check_mod_impl_wf(tcx: TyCtxt<'_>, module_def_id: DefId) { |
| tcx.hir().visit_item_likes_in_module( |
| module_def_id, |
| &mut ImplWfCheck { tcx } |
| ); |
| } |
| |
| pub fn provide(providers: &mut Providers<'_>) { |
| *providers = Providers { |
| check_mod_impl_wf, |
| ..*providers |
| }; |
| } |
| |
| struct ImplWfCheck<'tcx> { |
| tcx: TyCtxt<'tcx>, |
| } |
| |
| impl ItemLikeVisitor<'tcx> for ImplWfCheck<'tcx> { |
| fn visit_item(&mut self, item: &'tcx hir::Item) { |
| if let hir::ItemKind::Impl(.., ref impl_item_refs) = item.node { |
| let impl_def_id = self.tcx.hir().local_def_id(item.hir_id); |
| enforce_impl_params_are_constrained(self.tcx, |
| impl_def_id, |
| impl_item_refs); |
| enforce_impl_items_are_distinct(self.tcx, impl_item_refs); |
| } |
| } |
| |
| fn visit_trait_item(&mut self, _trait_item: &'tcx hir::TraitItem) { } |
| |
| fn visit_impl_item(&mut self, _impl_item: &'tcx hir::ImplItem) { } |
| } |
| |
| fn enforce_impl_params_are_constrained( |
| tcx: TyCtxt<'_>, |
| impl_def_id: DefId, |
| impl_item_refs: &[hir::ImplItemRef], |
| ) { |
| // Every lifetime used in an associated type must be constrained. |
| let impl_self_ty = tcx.type_of(impl_def_id); |
| if impl_self_ty.references_error() { |
| // Don't complain about unconstrained type params when self ty isn't known due to errors. |
| // (#36836) |
| tcx.sess.delay_span_bug( |
| tcx.def_span(impl_def_id), |
| "potentially unconstrained type parameters weren't evaluated", |
| ); |
| return; |
| } |
| let impl_generics = tcx.generics_of(impl_def_id); |
| let impl_predicates = tcx.predicates_of(impl_def_id); |
| let impl_trait_ref = tcx.impl_trait_ref(impl_def_id); |
| |
| let mut input_parameters = cgp::parameters_for_impl(impl_self_ty, impl_trait_ref); |
| cgp::identify_constrained_generic_params( |
| tcx, &impl_predicates, impl_trait_ref, &mut input_parameters); |
| |
| // Disallow unconstrained lifetimes, but only if they appear in assoc types. |
| let lifetimes_in_associated_types: FxHashSet<_> = impl_item_refs.iter() |
| .map(|item_ref| tcx.hir().local_def_id(item_ref.id.hir_id)) |
| .filter(|&def_id| { |
| let item = tcx.associated_item(def_id); |
| item.kind == ty::AssocKind::Type && item.defaultness.has_value() |
| }) |
| .flat_map(|def_id| { |
| cgp::parameters_for(&tcx.type_of(def_id), true) |
| }).collect(); |
| |
| for param in &impl_generics.params { |
| match param.kind { |
| // Disallow ANY unconstrained type parameters. |
| ty::GenericParamDefKind::Type { .. } => { |
| let param_ty = ty::ParamTy::for_def(param); |
| if !input_parameters.contains(&cgp::Parameter::from(param_ty)) { |
| report_unused_parameter(tcx, |
| tcx.def_span(param.def_id), |
| "type", |
| ¶m_ty.to_string()); |
| } |
| } |
| ty::GenericParamDefKind::Lifetime => { |
| let param_lt = cgp::Parameter::from(param.to_early_bound_region_data()); |
| if lifetimes_in_associated_types.contains(¶m_lt) && // (*) |
| !input_parameters.contains(¶m_lt) { |
| report_unused_parameter(tcx, |
| tcx.def_span(param.def_id), |
| "lifetime", |
| ¶m.name.to_string()); |
| } |
| } |
| ty::GenericParamDefKind::Const => { |
| let param_ct = ty::ParamConst::for_def(param); |
| if !input_parameters.contains(&cgp::Parameter::from(param_ct)) { |
| report_unused_parameter(tcx, |
| tcx.def_span(param.def_id), |
| "const", |
| ¶m_ct.to_string()); |
| } |
| } |
| } |
| } |
| |
| // (*) This is a horrible concession to reality. I think it'd be |
| // better to just ban unconstrianed lifetimes outright, but in |
| // practice people do non-hygenic macros like: |
| // |
| // ``` |
| // macro_rules! __impl_slice_eq1 { |
| // ($Lhs: ty, $Rhs: ty, $Bound: ident) => { |
| // impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> { |
| // .... |
| // } |
| // } |
| // } |
| // ``` |
| // |
| // In a concession to backwards compatibility, we continue to |
| // permit those, so long as the lifetimes aren't used in |
| // associated types. I believe this is sound, because lifetimes |
| // used elsewhere are not projected back out. |
| } |
| |
| fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: &str) { |
| struct_span_err!( |
| tcx.sess, span, E0207, |
| "the {} parameter `{}` is not constrained by the \ |
| impl trait, self type, or predicates", |
| kind, name) |
| .span_label(span, format!("unconstrained {} parameter", kind)) |
| .emit(); |
| } |
| |
| /// Enforce that we do not have two items in an impl with the same name. |
| fn enforce_impl_items_are_distinct(tcx: TyCtxt<'_>, impl_item_refs: &[hir::ImplItemRef]) { |
| let mut seen_type_items = FxHashMap::default(); |
| let mut seen_value_items = FxHashMap::default(); |
| for impl_item_ref in impl_item_refs { |
| let impl_item = tcx.hir().impl_item(impl_item_ref.id); |
| let seen_items = match impl_item.node { |
| hir::ImplItemKind::TyAlias(_) => &mut seen_type_items, |
| _ => &mut seen_value_items, |
| }; |
| match seen_items.entry(impl_item.ident.modern()) { |
| Occupied(entry) => { |
| let mut err = struct_span_err!(tcx.sess, impl_item.span, E0201, |
| "duplicate definitions with name `{}`:", |
| impl_item.ident); |
| err.span_label(*entry.get(), |
| format!("previous definition of `{}` here", |
| impl_item.ident)); |
| err.span_label(impl_item.span, "duplicate definition"); |
| err.emit(); |
| } |
| Vacant(entry) => { |
| entry.insert(impl_item.span); |
| } |
| } |
| } |
| } |
