compiler/rustc_typeck/src/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 min_specialization::check_min_specialization;
  13
  14 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  15 use rustc_errors::struct_span_err;
  16 use rustc_hir as hir;
  17 use rustc_hir::def_id::LocalDefId;
  18 use rustc_hir::itemlikevisit::ItemLikeVisitor;
  19 use rustc_middle::ty::query::Providers;
  20 use rustc_middle::ty::{self, TyCtxt, TypeFoldable};
  21 use rustc_span::Span;
  22
  23 use std::collections::hash_map::Entry::{Occupied, Vacant};
  24
  25 mod min_specialization;
  26
  27 /// Checks that all the type/lifetime parameters on an impl also
  28 /// appear in the trait ref or self type (or are constrained by a
  29 /// where-clause). These rules are needed to ensure that, given a
  30 /// trait ref like `<T as Trait<U>>`, we can derive the values of all
  31 /// parameters on the impl (which is needed to make specialization
  32 /// possible).
  33 ///
  34 /// However, in the case of lifetimes, we only enforce these rules if
  35 /// the lifetime parameter is used in an associated type. This is a
  36 /// concession to backwards compatibility; see comment at the end of
  37 /// the fn for details.
  38 ///
  39 /// Example:
  40 ///
  41 /// ```rust,ignore (pseudo-Rust)
  42 /// impl<T> Trait<Foo> for Bar { ... }
  43 /// //   ^ T does not appear in `Foo` or `Bar`, error!
  44 ///
  45 /// impl<T> Trait<Foo<T>> for Bar { ... }
  46 /// //   ^ T appears in `Foo<T>`, ok.
  47 ///
  48 /// impl<T> Trait<Foo> for Bar where Bar: Iterator<Item = T> { ... }
  49 /// //   ^ T is bound to `<Bar as Iterator>::Item`, ok.
  50 ///
  51 /// impl<'a> Trait<Foo> for Bar { }
  52 /// //   ^ 'a is unused, but for back-compat we allow it
  53 ///
  54 /// impl<'a> Trait<Foo> for Bar { type X = &'a i32; }
  55 /// //   ^ 'a is unused and appears in assoc type, error
  56 /// ```
  57 pub fn impl_wf_check(tcx: TyCtxt<'_>) {
  58     // We will tag this as part of the WF check -- logically, it is,
  59     // but it's one that we must perform earlier than the rest of
  60     // WfCheck.
  61     for &module in tcx.hir().krate().modules.keys() {
  62         tcx.ensure().check_mod_impl_wf(tcx.hir().local_def_id(module));
  63     }
  64 }
  65
  66 fn check_mod_impl_wf(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
  67     let min_specialization = tcx.features().min_specialization;
  68     tcx.hir()
  69         .visit_item_likes_in_module(module_def_id, &mut ImplWfCheck { tcx, min_specialization });
  70 }
  71
  72 pub fn provide(providers: &mut Providers) {
  73     *providers = Providers { check_mod_impl_wf, ..*providers };
  74 }
  75
  76 struct ImplWfCheck<'tcx> {
  77     tcx: TyCtxt<'tcx>,
  78     min_specialization: bool,
  79 }
  80
  81 impl ItemLikeVisitor<'tcx> for ImplWfCheck<'tcx> {
  82     fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
  83         if let hir::ItemKind::Impl { ref items, .. } = item.kind {
  84             let impl_def_id = self.tcx.hir().local_def_id(item.hir_id);
  85             enforce_impl_params_are_constrained(self.tcx, impl_def_id, items);
  86             enforce_impl_items_are_distinct(self.tcx, items);
  87             if self.min_specialization {
  88                 check_min_specialization(self.tcx, impl_def_id.to_def_id(), item.span);
  89             }
  90         }
  91     }
  92
  93     fn visit_trait_item(&mut self, _trait_item: &'tcx hir::TraitItem<'tcx>) {}
  94
  95     fn visit_impl_item(&mut self, _impl_item: &'tcx hir::ImplItem<'tcx>) {}
  96 }
  97
  98 fn enforce_impl_params_are_constrained(
  99     tcx: TyCtxt<'_>,
 100     impl_def_id: LocalDefId,
 101     impl_item_refs: &[hir::ImplItemRef<'_>],
 102 ) {
 103     // Every lifetime used in an associated type must be constrained.
 104     let impl_self_ty = tcx.type_of(impl_def_id);
 105     if impl_self_ty.references_error() {
 106         // Don't complain about unconstrained type params when self ty isn't known due to errors.
 107         // (#36836)
 108         tcx.sess.delay_span_bug(
 109             tcx.def_span(impl_def_id),
 110             &format!(
 111                 "potentially unconstrained type parameters weren't evaluated: {:?}",
 112                 impl_self_ty,
 113             ),
 114         );
 115         return;
 116     }
 117     let impl_generics = tcx.generics_of(impl_def_id);
 118     let impl_predicates = tcx.predicates_of(impl_def_id);
 119     let impl_trait_ref = tcx.impl_trait_ref(impl_def_id);
 120
 121     let mut input_parameters = cgp::parameters_for_impl(impl_self_ty, impl_trait_ref);
 122     cgp::identify_constrained_generic_params(
 123         tcx,
 124         impl_predicates,
 125         impl_trait_ref,
 126         &mut input_parameters,
 127     );
 128
 129     // Disallow unconstrained lifetimes, but only if they appear in assoc types.
 130     let lifetimes_in_associated_types: FxHashSet<_> = impl_item_refs
 131         .iter()
 132         .map(|item_ref| tcx.hir().local_def_id(item_ref.id.hir_id))
 133         .flat_map(|def_id| {
 134             let item = tcx.associated_item(def_id);
 135             match item.kind {
 136                 ty::AssocKind::Type => {
 137                     if item.defaultness.has_value() {
 138                         cgp::parameters_for(&tcx.type_of(def_id), true)
 139                     } else {
 140                         Vec::new()
 141                     }
 142                 }
 143                 ty::AssocKind::Fn | ty::AssocKind::Const => Vec::new(),
 144             }
 145         })
 146         .collect();
 147
 148     for param in &impl_generics.params {
 149         match param.kind {
 150             // Disallow ANY unconstrained type parameters.
 151             ty::GenericParamDefKind::Type { .. } => {
 152                 let param_ty = ty::ParamTy::for_def(param);
 153                 if !input_parameters.contains(&cgp::Parameter::from(param_ty)) {
 154                     report_unused_parameter(
 155                         tcx,
 156                         tcx.def_span(param.def_id),
 157                         "type",
 158                         &param_ty.to_string(),
 159                     );
 160                 }
 161             }
 162             ty::GenericParamDefKind::Lifetime => {
 163                 let param_lt = cgp::Parameter::from(param.to_early_bound_region_data());
 164                 if lifetimes_in_associated_types.contains(&param_lt) && // (*)
 165                     !input_parameters.contains(&param_lt)
 166                 {
 167                     report_unused_parameter(
 168                         tcx,
 169                         tcx.def_span(param.def_id),
 170                         "lifetime",
 171                         &param.name.to_string(),
 172                     );
 173                 }
 174             }
 175             ty::GenericParamDefKind::Const => {
 176                 let param_ct = ty::ParamConst::for_def(param);
 177                 if !input_parameters.contains(&cgp::Parameter::from(param_ct)) {
 178                     report_unused_parameter(
 179                         tcx,
 180                         tcx.def_span(param.def_id),
 181                         "const",
 182                         &param_ct.to_string(),
 183                     );
 184                 }
 185             }
 186         }
 187     }
 188
 189     // (*) This is a horrible concession to reality. I think it'd be
 190     // better to just ban unconstrained lifetimes outright, but in
 191     // practice people do non-hygenic macros like:
 192     //
 193     // ```
 194     // macro_rules! __impl_slice_eq1 {
 195     //     ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
 196     //         impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
 197     //            ....
 198     //         }
 199     //     }
 200     // }
 201     // ```
 202     //
 203     // In a concession to backwards compatibility, we continue to
 204     // permit those, so long as the lifetimes aren't used in
 205     // associated types. I believe this is sound, because lifetimes
 206     // used elsewhere are not projected back out.
 207 }
 208
 209 fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: &str) {
 210     let mut err = struct_span_err!(
 211         tcx.sess,
 212         span,
 213         E0207,
 214         "the {} parameter `{}` is not constrained by the \
 215         impl trait, self type, or predicates",
 216         kind,
 217         name
 218     );
 219     err.span_label(span, format!("unconstrained {} parameter", kind));
 220     if kind == "const" {
 221         err.note(
 222             "expressions using a const parameter must map each value to a distinct output value",
 223         );
 224         err.note(
 225             "proving the result of expressions other than the parameter are unique is not supported",
 226         );
 227     }
 228     err.emit();
 229 }
 230
 231 /// Enforce that we do not have two items in an impl with the same name.
 232 fn enforce_impl_items_are_distinct(tcx: TyCtxt<'_>, impl_item_refs: &[hir::ImplItemRef<'_>]) {
 233     let mut seen_type_items = FxHashMap::default();
 234     let mut seen_value_items = FxHashMap::default();
 235     for impl_item_ref in impl_item_refs {
 236         let impl_item = tcx.hir().impl_item(impl_item_ref.id);
 237         let seen_items = match impl_item.kind {
 238             hir::ImplItemKind::TyAlias(_) => &mut seen_type_items,
 239             _ => &mut seen_value_items,
 240         };
 241         match seen_items.entry(impl_item.ident.normalize_to_macros_2_0()) {
 242             Occupied(entry) => {
 243                 let mut err = struct_span_err!(
 244                     tcx.sess,
 245                     impl_item.span,
 246                     E0201,
 247                     "duplicate definitions with name `{}`:",
 248                     impl_item.ident
 249                 );
 250                 err.span_label(
 251                     *entry.get(),
 252                     format!("previous definition of `{}` here", impl_item.ident),
 253                 );
 254                 err.span_label(impl_item.span, "duplicate definition");
 255                 err.emit();
 256             }
 257             Vacant(entry) => {
 258                 entry.insert(impl_item.span);
 259             }
 260         }
 261     }
 262 }