src/librustc_typeck/impl_wf_check.rs

   1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! This pass enforces various "well-formedness constraints" on impls.
  12 //! Logically, it is part of wfcheck -- but we do it early so that we
  13 //! can stop compilation afterwards, since part of the trait matching
  14 //! infrastructure gets very grumpy if these conditions don't hold. In
  15 //! particular, if there are type parameters that are not part of the
  16 //! impl, then coherence will report strange inference ambiguity
  17 //! errors; if impls have duplicate items, we get misleading
  18 //! specialization errors. These things can (and probably should) be
  19 //! fixed, but for the moment it's easier to do these checks early.
  20
  21 use constrained_type_params as ctp;
  22 use rustc::hir;
  23 use rustc::hir::itemlikevisit::ItemLikeVisitor;
  24 use rustc::hir::def_id::DefId;
  25 use rustc::ty::{self, TyCtxt};
  26 use rustc::util::nodemap::{FxHashMap, FxHashSet};
  27 use std::collections::hash_map::Entry::{Occupied, Vacant};
  28
  29 use syntax_pos::Span;
  30
  31 /// Checks that all the type/lifetime parameters on an impl also
  32 /// appear in the trait ref or self-type (or are constrained by a
  33 /// where-clause). These rules are needed to ensure that, given a
  34 /// trait ref like `<T as Trait<U>>`, we can derive the values of all
  35 /// parameters on the impl (which is needed to make specialization
  36 /// possible).
  37 ///
  38 /// However, in the case of lifetimes, we only enforce these rules if
  39 /// the lifetime parameter is used in an associated type.  This is a
  40 /// concession to backwards compatibility; see comment at the end of
  41 /// the fn for details.
  42 ///
  43 /// Example:
  44 ///
  45 /// ```rust,ignore (pseudo-Rust)
  46 /// impl<T> Trait<Foo> for Bar { ... }
  47 /// //   ^ T does not appear in `Foo` or `Bar`, error!
  48 ///
  49 /// impl<T> Trait<Foo<T>> for Bar { ... }
  50 /// //   ^ T appears in `Foo<T>`, ok.
  51 ///
  52 /// impl<T> Trait<Foo> for Bar where Bar: Iterator<Item=T> { ... }
  53 /// //   ^ T is bound to `<Bar as Iterator>::Item`, ok.
  54 ///
  55 /// impl<'a> Trait<Foo> for Bar { }
  56 /// //   ^ 'a is unused, but for back-compat we allow it
  57 ///
  58 /// impl<'a> Trait<Foo> for Bar { type X = &'a i32; }
  59 /// //   ^ 'a is unused and appears in assoc type, error
  60 /// ```
  61 pub fn impl_wf_check<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
  62     // We will tag this as part of the WF check -- logically, it is,
  63     // but it's one that we must perform earlier than the rest of
  64     // WfCheck.
  65     tcx.hir.krate().visit_all_item_likes(&mut ImplWfCheck { tcx: tcx });
  66 }
  67
  68 struct ImplWfCheck<'a, 'tcx: 'a> {
  69     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  70 }
  71
  72 impl<'a, 'tcx> ItemLikeVisitor<'tcx> for ImplWfCheck<'a, 'tcx> {
  73     fn visit_item(&mut self, item: &'tcx hir::Item) {
  74         match item.node {
  75             hir::ItemImpl(.., ref impl_item_refs) => {
  76                 let impl_def_id = self.tcx.hir.local_def_id(item.id);
  77                 enforce_impl_params_are_constrained(self.tcx,
  78                                                     impl_def_id,
  79                                                     impl_item_refs);
  80                 enforce_impl_items_are_distinct(self.tcx, impl_item_refs);
  81             }
  82             _ => { }
  83         }
  84     }
  85
  86     fn visit_trait_item(&mut self, _trait_item: &'tcx hir::TraitItem) { }
  87
  88     fn visit_impl_item(&mut self, _impl_item: &'tcx hir::ImplItem) { }
  89 }
  90
  91 fn enforce_impl_params_are_constrained<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  92                                                  impl_def_id: DefId,
  93                                                  impl_item_refs: &[hir::ImplItemRef])
  94 {
  95     // Every lifetime used in an associated type must be constrained.
  96     let impl_self_ty = tcx.type_of(impl_def_id);
  97     let impl_generics = tcx.generics_of(impl_def_id);
  98     let impl_predicates = tcx.predicates_of(impl_def_id);
  99     let impl_trait_ref = tcx.impl_trait_ref(impl_def_id);
 100
 101     let mut input_parameters = ctp::parameters_for_impl(impl_self_ty, impl_trait_ref);
 102     ctp::identify_constrained_type_params(
 103         tcx, &impl_predicates.predicates.as_slice(), impl_trait_ref, &mut input_parameters);
 104
 105     // Disallow unconstrained lifetimes, but only if they appear in assoc types.
 106     let lifetimes_in_associated_types: FxHashSet<_> = impl_item_refs.iter()
 107         .map(|item_ref| tcx.hir.local_def_id(item_ref.id.node_id))
 108         .filter(|&def_id| {
 109             let item = tcx.associated_item(def_id);
 110             item.kind == ty::AssociatedKind::Type && item.defaultness.has_value()
 111         })
 112         .flat_map(|def_id| {
 113             ctp::parameters_for(&tcx.type_of(def_id), true)
 114         }).collect();
 115
 116     for param in &impl_generics.params {
 117         match param.kind {
 118             // Disallow ANY unconstrained type parameters.
 119             ty::GenericParamDefKind::Type {..} => {
 120                 let param_ty = ty::ParamTy::for_def(param);
 121                 if !input_parameters.contains(&ctp::Parameter::from(param_ty)) {
 122                     report_unused_parameter(tcx,
 123                                             tcx.def_span(param.def_id),
 124                                             "type",
 125                                             &param_ty.to_string());
 126                 }
 127             }
 128             ty::GenericParamDefKind::Lifetime => {
 129                 let param_lt = ctp::Parameter::from(param.to_early_bound_region_data());
 130                 if lifetimes_in_associated_types.contains(&param_lt) && // (*)
 131                     !input_parameters.contains(&param_lt) {
 132                     report_unused_parameter(tcx,
 133                                             tcx.def_span(param.def_id),
 134                                             "lifetime",
 135                                             &param.name.to_string());
 136                 }
 137             }
 138         }
 139     }
 140
 141     // (*) This is a horrible concession to reality. I think it'd be
 142     // better to just ban unconstrianed lifetimes outright, but in
 143     // practice people do non-hygenic macros like:
 144     //
 145     // ```
 146     // macro_rules! __impl_slice_eq1 {
 147     //     ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
 148     //         impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
 149     //            ....
 150     //         }
 151     //     }
 152     // }
 153     // ```
 154     //
 155     // In a concession to backwards compatbility, we continue to
 156     // permit those, so long as the lifetimes aren't used in
 157     // associated types. I believe this is sound, because lifetimes
 158     // used elsewhere are not projected back out.
 159 }
 160
 161 fn report_unused_parameter(tcx: TyCtxt,
 162                            span: Span,
 163                            kind: &str,
 164                            name: &str)
 165 {
 166     struct_span_err!(
 167         tcx.sess, span, E0207,
 168         "the {} parameter `{}` is not constrained by the \
 169         impl trait, self type, or predicates",
 170         kind, name)
 171         .span_label(span, format!("unconstrained {} parameter", kind))
 172         .emit();
 173 }
 174
 175 /// Enforce that we do not have two items in an impl with the same name.
 176 fn enforce_impl_items_are_distinct<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 177                                              impl_item_refs: &[hir::ImplItemRef])
 178 {
 179     let mut seen_type_items = FxHashMap();
 180     let mut seen_value_items = FxHashMap();
 181     for impl_item_ref in impl_item_refs {
 182         let impl_item = tcx.hir.impl_item(impl_item_ref.id);
 183         let seen_items = match impl_item.node {
 184             hir::ImplItemKind::Type(_) => &mut seen_type_items,
 185             _                    => &mut seen_value_items,
 186         };
 187         match seen_items.entry(impl_item.name) {
 188             Occupied(entry) => {
 189                 let mut err = struct_span_err!(tcx.sess, impl_item.span, E0201,
 190                                                "duplicate definitions with name `{}`:",
 191                                                impl_item.name);
 192                 err.span_label(*entry.get(),
 193                                format!("previous definition of `{}` here",
 194                                         impl_item.name));
 195                 err.span_label(impl_item.span, "duplicate definition");
 196                 err.emit();
 197             }
 198             Vacant(entry) => {
 199                 entry.insert(impl_item.span);
 200             }
 201         }
 202     }
 203 }