auto merge of #10509 : cmr/rust/trait_name, r=alexcrichton

Rebase + fix of #10118

Author: bors

src/librustc/middle/typeck/check/mod.rs

@@ -863,11 +863,13 @@ pub fn compare_impl_method(tcx: ty::ctxt,
     if impl_m.fty.sig.inputs.len() != trait_m.fty.sig.inputs.len() {
         tcx.sess.span_err(
             impl_m_span,
-            format!("method `{}` has {} parameter(s) \
-                    but the trait has {} parameter(s)",
-                    tcx.sess.str_of(trait_m.ident),
-                    impl_m.fty.sig.inputs.len(),
-                    trait_m.fty.sig.inputs.len()));
+            format!("method `{}` has {} parameter{} \
+                  but the declaration in trait `{}` has {}",
+                 tcx.sess.str_of(trait_m.ident),
+                 impl_m.fty.sig.inputs.len(),
+                 if impl_m.fty.sig.inputs.len() == 1 { "" } else { "s" },
+                 ty::item_path_str(tcx, trait_m.def_id),
+                 trait_m.fty.sig.inputs.len()));
         return;
     }
 

src/test/compile-fail/trait-impl-different-num-params.rs

@@ -13,7 +13,7 @@ trait foo {
 }
 impl foo for int {
     fn bar(&self) -> int {
-        //~^ ERROR method `bar` has 0 parameter(s) but the trait has 1
+        //~^ ERROR method `bar` has 0 parameters but the declaration in trait `foo::bar` has 1
         *self
     }
 }