Prohibit unused type parameters in impls.

src/libcollections/btree/node.rs

@@ -1417,7 +1417,7 @@ pub type MutTraversal<'a, K, V> = AbsTraversal<ElemsAndEdges<Zip<slice::Iter<'a,
 /// An owning traversal over a node's entries and edges
 pub type MoveTraversal<K, V> = AbsTraversal<MoveTraversalImpl<K, V>>;
 
-
+#[old_impl_check]
 impl<K, V, E, Impl: TraversalImpl<K, V, E>> Iterator for AbsTraversal<Impl> {
     type Item = TraversalItem<K, V, E>;
 
@@ -1433,6 +1433,7 @@ impl<K, V, E, Impl: TraversalImpl<K, V, E>> Iterator for AbsTraversal<Impl> {
     }
 }
 
+#[old_impl_check]
 impl<K, V, E, Impl: TraversalImpl<K, V, E>> DoubleEndedIterator for AbsTraversal<Impl> {
     fn next_back(&mut self) -> Option<TraversalItem<K, V, E>> {
         let tail_is_edge = self.tail_is_edge;

src/libcollections/lib.rs

@@ -26,6 +26,7 @@
 #![feature(unsafe_destructor, slicing_syntax)]
 #![feature(unboxed_closures)]
 #![feature(old_orphan_check)]
+#![feature(old_impl_check)]
 #![feature(associated_types)]
 #![no_std]
 

src/librustc/lib.rs

@@ -28,6 +28,7 @@
 #![feature(rustc_diagnostic_macros)]
 #![feature(unboxed_closures)]
 #![feature(old_orphan_check)]
+#![feature(old_impl_check)]
 #![feature(associated_types)]
 
 extern crate arena;

src/librustc/lint/builtin.rs

@@ -669,6 +669,7 @@ impl LintPass for UnusedAttributes {
 
             // FIXME: #19470 this shouldn't be needed forever
             "old_orphan_check",
+            "old_impl_check",
         ];
 
         static CRATE_ATTRS: &'static [&'static str] = &[

src/librustc/util/ppaux.rs

@@ -1349,6 +1349,7 @@ impl<'tcx, T:Repr<'tcx>> Repr<'tcx> for ty::Binder<T> {
     }
 }
 
+#[old_impl_check]
 impl<'tcx, S, H, K, V> Repr<'tcx> for HashMap<K,V,H>
     where K : Hash<S> + Eq + Repr<'tcx>,
           V : Repr<'tcx>,

src/librustc_typeck/collect.rs

@@ -35,7 +35,7 @@ use middle::lang_items::SizedTraitLangItem;
 use middle::region;
 use middle::resolve_lifetime;
 use middle::subst;
-use middle::subst::{Substs};
+use middle::subst::{Substs, TypeSpace};
 use middle::ty::{AsPredicate, ImplContainer, ImplOrTraitItemContainer, TraitContainer};
 use middle::ty::{self, RegionEscape, Ty, TypeScheme};
 use middle::ty_fold::{self, TypeFolder, TypeFoldable};
@@ -47,6 +47,7 @@ use util::ppaux;
 use util::ppaux::{Repr,UserString};
 use write_ty_to_tcx;
 
+use std::collections::HashSet;
 use std::rc::Rc;
 
 use syntax::abi;
@@ -644,6 +645,10 @@ fn convert(ccx: &CollectCtxt, it: &ast::Item) {
                                                Some(selfty),
                                                None);
             }
+
+            enforce_impl_ty_params_are_constrained(ccx.tcx,
+                                                   generics,
+                                                   local_def(it.id));
         },
         ast::ItemTrait(_, _, _, ref trait_methods) => {
             let trait_def = trait_def_of_item(ccx, it);
@@ -1605,3 +1610,96 @@ fn check_method_self_type<'a, 'tcx, RS:RegionScope>(
         })
     }
 }
+
+/// Checks that all the type parameters on an impl
+fn enforce_impl_ty_params_are_constrained<'tcx>(tcx: &ty::ctxt<'tcx>,
+                                                ast_generics: &ast::Generics,
+                                                impl_def_id: ast::DefId)
+{
+    let impl_scheme = ty::lookup_item_type(tcx, impl_def_id);
+    let impl_trait_ref = ty::impl_trait_ref(tcx, impl_def_id);
+
+    // The trait reference is an input, so find all type parameters
+    // reachable from there, to start (if this is an inherent impl,
+    // then just examine the self type).
+    let mut input_parameters: HashSet<_> =
+        impl_trait_ref.iter()
+                      .flat_map(|t| t.input_types().iter()) // Types in trait ref, if any
+                      .chain(Some(impl_scheme.ty).iter())  // Self type, always
+                      .flat_map(|t| t.walk())
+                      .filter_map(to_opt_param_ty)
+                      .collect();
+
+    loop {
+        let num_inputs = input_parameters.len();
+
+        let mut projection_predicates =
+            impl_scheme.generics.predicates
+            .iter()
+            .filter_map(|predicate| {
+                match *predicate {
+                    // Ignore higher-ranked binders. For the purposes
+                    // of this check, they don't matter because they
+                    // only affect named regions, and we're just
+                    // concerned about type parameters here.
+                    ty::Predicate::Projection(ref data) => Some(data.0.clone()),
+                    _ => None,
+                }
+            });
+
+        for projection in projection_predicates {
+            // Special case: watch out for some kind of sneaky attempt
+            // to project out an associated type defined by this very trait.
+            if Some(projection.projection_ty.trait_ref.clone()) == impl_trait_ref {
+                continue;
+            }
+
+            let relies_only_on_inputs =
+                projection.projection_ty.trait_ref.input_types().iter()
+                .flat_map(|t| t.walk())
+                .filter_map(to_opt_param_ty)
+                .all(|t| input_parameters.contains(&t));
+
+            if relies_only_on_inputs {
+                input_parameters.extend(
+                    projection.ty.walk().filter_map(to_opt_param_ty));
+            }
+        }
+
+        if input_parameters.len() == num_inputs {
+            break;
+        }
+    }
+
+    for (index, ty_param) in ast_generics.ty_params.iter().enumerate() {
+        let param_ty = ty::ParamTy { space: TypeSpace,
+                                     idx: index as u32,
+                                     name: ty_param.ident.name };
+        if !input_parameters.contains(&param_ty) {
+            if ty::has_attr(tcx, impl_def_id, "old_impl_check") {
+                tcx.sess.span_warn(
+                    ty_param.span,
+                    format!("the type parameter `{}` is not constrained by the \
+                             impl trait, self type, or predicates",
+                            param_ty.user_string(tcx)).as_slice());
+            } else {
+                tcx.sess.span_err(
+                    ty_param.span,
+                    format!("the type parameter `{}` is not constrained by the \
+                             impl trait, self type, or predicates",
+                            param_ty.user_string(tcx)).as_slice());
+                tcx.sess.span_help(
+                    ty_param.span,
+                    format!("you can temporarily opt out of this rule by placing \
+                             the `#[old_impl_check]` attribute on the impl").as_slice());
+            }
+        }
+    }
+
+    fn to_opt_param_ty<'tcx>(ty: Ty<'tcx>) -> Option<ty::ParamTy> {
+        match ty.sty {
+            ty::ty_param(ref d) => Some(d.clone()),
+            _ => None,
+        }
+    }
+}

src/libserialize/collection_impls.rs

@@ -156,6 +156,7 @@ impl<
     }
 }
 
+#[old_impl_check]
 impl<
     K: Encodable + Hash<X> + Eq,
     V: Encodable,
@@ -175,6 +176,7 @@ impl<
     }
 }
 
+#[old_impl_check]
 impl<
     K: Decodable + Hash<S> + Eq,
     V: Decodable,
@@ -195,6 +197,7 @@ impl<
     }
 }
 
+#[old_impl_check]
 impl<
     T: Encodable + Hash<X> + Eq,
     X,
@@ -212,6 +215,7 @@ impl<
     }
 }
 
+#[old_impl_check]
 impl<
     T: Decodable + Hash<S> + Eq,
     S,

src/libserialize/lib.rs

@@ -26,6 +26,7 @@ Core encoding and decoding interfaces.
 #![feature(macro_rules, default_type_params, phase, slicing_syntax, globs)]
 #![feature(unboxed_closures)]
 #![feature(associated_types)]
+#![feature(old_impl_check)]
 
 // test harness access
 #[cfg(test)]

src/libstd/collections/hash/map.rs

@@ -439,6 +439,7 @@ impl<K, V, M> SearchResult<K, V, M> {
     }
 }
 
+#[old_impl_check]
 impl<K: Eq + Hash<S>, V, S, H: Hasher<S>> HashMap<K, V, H> {
     fn make_hash<X: ?Sized + Hash<S>>(&self, x: &X) -> SafeHash {
         table::make_hash(&self.hasher, x)
@@ -517,6 +518,7 @@ impl<K: Hash + Eq, V> HashMap<K, V, RandomSipHasher> {
     }
 }
 
+#[old_impl_check]
 impl<K: Eq + Hash<S>, V, S, H: Hasher<S>> HashMap<K, V, H> {
     /// Creates an empty hashmap which will use the given hasher to hash keys.
     ///
@@ -1191,6 +1193,7 @@ fn search_entry_hashed<'a, K, V, Q: ?Sized>(table: &'a mut RawTable<K,V>, hash:
 }
 
 #[stable]
+#[old_impl_check]
 impl<K: Eq + Hash<S>, V: PartialEq, S, H: Hasher<S>> PartialEq for HashMap<K, V, H> {
     fn eq(&self, other: &HashMap<K, V, H>) -> bool {
         if self.len() != other.len() { return false; }
@@ -1202,9 +1205,11 @@ impl<K: Eq + Hash<S>, V: PartialEq, S, H: Hasher<S>> PartialEq for HashMap<K, V,
 }
 
 #[stable]
+#[old_impl_check]
 impl<K: Eq + Hash<S>, V: Eq, S, H: Hasher<S>> Eq for HashMap<K, V, H> {}
 
 #[stable]
+#[old_impl_check]
 impl<K: Eq + Hash<S> + Show, V: Show, S, H: Hasher<S>> Show for HashMap<K, V, H> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         try!(write!(f, "{{"));
@@ -1219,6 +1224,7 @@ impl<K: Eq + Hash<S> + Show, V: Show, S, H: Hasher<S>> Show for HashMap<K, V, H>
 }
 
 #[stable]
+#[old_impl_check]
 impl<K: Eq + Hash<S>, V, S, H: Hasher<S> + Default> Default for HashMap<K, V, H> {
     #[stable]
     fn default() -> HashMap<K, V, H> {
@@ -1227,6 +1233,7 @@ impl<K: Eq + Hash<S>, V, S, H: Hasher<S> + Default> Default for HashMap<K, V, H>
 }
 
 #[stable]
+#[old_impl_check]
 impl<K: Hash<S> + Eq, Q: ?Sized, V, S, H: Hasher<S>> Index<Q> for HashMap<K, V, H>
     where Q: BorrowFrom<K> + Hash<S> + Eq
 {
@@ -1239,6 +1246,7 @@ impl<K: Hash<S> + Eq, Q: ?Sized, V, S, H: Hasher<S>> Index<Q> for HashMap<K, V,
 }
 
 #[stable]
+#[old_impl_check]
 impl<K: Hash<S> + Eq, Q: ?Sized, V, S, H: Hasher<S>> IndexMut<Q> for HashMap<K, V, H>
     where Q: BorrowFrom<K> + Hash<S> + Eq
 {
@@ -1472,6 +1480,7 @@ impl<'a, Q: ?Sized + 'a + ToOwned<K>, K: 'a, V: 'a> VacantEntry<'a, Q, K, V> {
 }
 
 #[stable]
+#[old_impl_check]
 impl<K: Eq + Hash<S>, V, S, H: Hasher<S> + Default> FromIterator<(K, V)> for HashMap<K, V, H> {
     fn from_iter<T: Iterator<Item=(K, V)>>(iter: T) -> HashMap<K, V, H> {
         let lower = iter.size_hint().0;
@@ -1482,6 +1491,7 @@ impl<K: Eq + Hash<S>, V, S, H: Hasher<S> + Default> FromIterator<(K, V)> for Has
 }
 
 #[stable]
+#[old_impl_check]
 impl<K: Eq + Hash<S>, V, S, H: Hasher<S>> Extend<(K, V)> for HashMap<K, V, H> {
     fn extend<T: Iterator<Item=(K, V)>>(&mut self, mut iter: T) {
         for (k, v) in iter {