src/librustc_typeck/impl_wf_check.rs

   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.
  10
  11 use crate::constrained_generic_params as cgp;
  12 use rustc::hir;
  13 use rustc::hir::itemlikevisit::ItemLikeVisitor;
  14 use rustc::hir::def_id::DefId;
  15 use rustc::ty::{self, TyCtxt, TypeFoldable};
  16 use rustc::ty::query::Providers;
  17 use rustc::util::nodemap::{FxHashMap, FxHashSet};
  18 use std::collections::hash_map::Entry::{Occupied, Vacant};
  19
  20 use syntax_pos::Span;
  21
  22 use rustc_error_codes::*;
  23
  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
  29 /// possible).
  30 ///
  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.
  35 ///
  36 /// Example:
  37 ///
  38 /// ```rust,ignore (pseudo-Rust)
  39 /// impl<T> Trait<Foo> for Bar { ... }
  40 /// //   ^ T does not appear in `Foo` or `Bar`, error!
  41 ///
  42 /// impl<T> Trait<Foo<T>> for Bar { ... }
  43 /// //   ^ T appears in `Foo<T>`, ok.
  44 ///
  45 /// impl<T> Trait<Foo> for Bar where Bar: Iterator<Item = T> { ... }
  46 /// //   ^ T is bound to `<Bar as Iterator>::Item`, ok.
  47 ///
  48 /// impl<'a> Trait<Foo> for Bar { }
  49 /// //   ^ 'a is unused, but for back-compat we allow it
  50 ///
  51 /// impl<'a> Trait<Foo> for Bar { type X = &'a i32; }
  52 /// //   ^ 'a is unused and appears in assoc type, error
  53 /// ```
  54 pub fn impl_wf_check(tcx: TyCtxt<'_>) {
  55     // We will tag this as part of the WF check -- logically, it is,
  56     // but it's one that we must perform earlier than the rest of
  57     // WfCheck.
  58     for &module in tcx.hir().krate().modules.keys() {
  59         tcx.ensure().check_mod_impl_wf(tcx.hir().local_def_id(module));
  60     }
  61 }
  62
  63 fn check_mod_impl_wf(tcx: TyCtxt<'_>, module_def_id: DefId) {
  64     tcx.hir().visit_item_likes_in_module(
  65         module_def_id,
  66         &mut ImplWfCheck { tcx }
  67     );
  68 }
  69
  70 pub fn provide(providers: &mut Providers<'_>) {
  71     *providers = Providers {
  72         check_mod_impl_wf,
  73         ..*providers
  74     };
  75 }
  76
  77 struct ImplWfCheck<'tcx> {
  78     tcx: TyCtxt<'tcx>,
  79 }
  80
  81 impl ItemLikeVisitor<'tcx> for ImplWfCheck<'tcx> {
  82     fn visit_item(&mut self, item: &'tcx hir::Item) {
  83         if let hir::ItemKind::Impl(.., ref impl_item_refs) = item.kind {
  84             let impl_def_id = self.tcx.hir().local_def_id(item.hir_id);
  85             enforce_impl_params_are_constrained(self.tcx,
  86                                                 impl_def_id,
  87                                                 impl_item_refs);
  88             enforce_impl_items_are_distinct(self.tcx, impl_item_refs);
  89         }
  90     }
  91
  92     fn visit_trait_item(&mut self, _trait_item: &'tcx hir::TraitItem) { }
  93
  94     fn visit_impl_item(&mut self, _impl_item: &'tcx hir::ImplItem) { }
  95 }
  96
  97 fn enforce_impl_params_are_constrained(
  98     tcx: TyCtxt<'_>,
  99     impl_def_id: DefId,
 100     impl_item_refs: &[hir::ImplItemRef],
 101 ) {
 102     // Every lifetime used in an associated type must be constrained.
 103     let impl_self_ty = tcx.type_of(impl_def_id);
 104     if impl_self_ty.references_error() {
 105         // Don't complain about unconstrained type params when self ty isn't known due to errors.
 106         // (#36836)
 107         tcx.sess.delay_span_bug(
 108             tcx.def_span(impl_def_id),
 109             "potentially unconstrained type parameters weren't evaluated",
 110         );
 111         return;
 112     }
 113     let impl_generics = tcx.generics_of(impl_def_id);
 114     let impl_predicates = tcx.predicates_of(impl_def_id);
 115     let impl_trait_ref = tcx.impl_trait_ref(impl_def_id);
 116
 117     let mut input_parameters = cgp::parameters_for_impl(impl_self_ty, impl_trait_ref);
 118     cgp::identify_constrained_generic_params(
 119         tcx, impl_predicates, impl_trait_ref, &mut input_parameters);
 120
 121     // Disallow unconstrained lifetimes, but only if they appear in assoc types.
 122     let lifetimes_in_associated_types: FxHashSet<_> = impl_item_refs.iter()
 123         .map(|item_ref| tcx.hir().local_def_id(item_ref.id.hir_id))
 124         .filter(|&def_id| {
 125             let item = tcx.associated_item(def_id);
 126             item.kind == ty::AssocKind::Type && item.defaultness.has_value()
 127         })
 128         .flat_map(|def_id| {
 129             cgp::parameters_for(&tcx.type_of(def_id), true)
 130         }).collect();
 131
 132     for param in &impl_generics.params {
 133         match param.kind {
 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(tcx,
 139                                             tcx.def_span(param.def_id),
 140                                             "type",
 141                                             &param_ty.to_string());
 142                 }
 143             }
 144             ty::GenericParamDefKind::Lifetime => {
 145                 let param_lt = cgp::Parameter::from(param.to_early_bound_region_data());
 146                 if lifetimes_in_associated_types.contains(&param_lt) && // (*)
 147                     !input_parameters.contains(&param_lt) {
 148                     report_unused_parameter(tcx,
 149                                             tcx.def_span(param.def_id),
 150                                             "lifetime",
 151                                             &param.name.to_string());
 152                 }
 153             }
 154             ty::GenericParamDefKind::Const => {
 155                 let param_ct = ty::ParamConst::for_def(param);
 156                 if !input_parameters.contains(&cgp::Parameter::from(param_ct)) {
 157                     report_unused_parameter(tcx,
 158                                            tcx.def_span(param.def_id),
 159                                            "const",
 160                                            &param_ct.to_string());
 161                 }
 162             }
 163         }
 164     }
 165
 166     // (*) This is a horrible concession to reality. I think it'd be
 167     // better to just ban unconstrianed lifetimes outright, but in
 168     // practice people do non-hygenic macros like:
 169     //
 170     // ```
 171     // macro_rules! __impl_slice_eq1 {
 172     //     ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
 173     //         impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
 174     //            ....
 175     //         }
 176     //     }
 177     // }
 178     // ```
 179     //
 180     // In a concession to backwards compatibility, we continue to
 181     // permit those, so long as the lifetimes aren't used in
 182     // associated types. I believe this is sound, because lifetimes
 183     // used elsewhere are not projected back out.
 184 }
 185
 186 fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: &str) {
 187     struct_span_err!(
 188         tcx.sess, span, E0207,
 189         "the {} parameter `{}` is not constrained by the \
 190         impl trait, self type, or predicates",
 191         kind, name)
 192         .span_label(span, format!("unconstrained {} parameter", kind))
 193         .emit();
 194 }
 195
 196 /// Enforce that we do not have two items in an impl with the same name.
 197 fn enforce_impl_items_are_distinct(tcx: TyCtxt<'_>, impl_item_refs: &[hir::ImplItemRef]) {
 198     let mut seen_type_items = FxHashMap::default();
 199     let mut seen_value_items = FxHashMap::default();
 200     for impl_item_ref in impl_item_refs {
 201         let impl_item = tcx.hir().impl_item(impl_item_ref.id);
 202         let seen_items = match impl_item.kind {
 203             hir::ImplItemKind::TyAlias(_) => &mut seen_type_items,
 204             _                          => &mut seen_value_items,
 205         };
 206         match seen_items.entry(impl_item.ident.modern()) {
 207             Occupied(entry) => {
 208                 let mut err = struct_span_err!(tcx.sess, impl_item.span, E0201,
 209                                                "duplicate definitions with name `{}`:",
 210                                                impl_item.ident);
 211                 err.span_label(*entry.get(),
 212                                format!("previous definition of `{}` here",
 213                                        impl_item.ident));
 214                 err.span_label(impl_item.span, "duplicate definition");
 215                 err.emit();
 216             }
 217             Vacant(entry) => {
 218                 entry.insert(impl_item.span);
 219             }
 220         }
 221     }
 222 }