require reimplementations of all items when a defaulted associated type is overriden

This is a
[breaking-change]
but it follows the RFC (not sure whether it will be accepted).

src/librustc_typeck/check/mod.rs

@@ -846,6 +846,7 @@ fn check_impl_items_against_trait<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
     // Locate trait methods
     let tcx = ccx.tcx;
     let trait_items = tcx.trait_items(impl_trait_ref.def_id);
+    let mut overridden_associated_type = None;
 
     // Check existing impl methods to see if they are both present in trait
     // and compatible with trait signature
@@ -911,8 +912,10 @@ fn check_impl_items_against_trait<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                     _ => tcx.sess.span_bug(impl_item.span, "non-type impl-item for type")
                 };
 
-                if let &ty::TypeTraitItem(..) = ty_trait_item {
-                    // ...
+                if let &ty::TypeTraitItem(ref at) = ty_trait_item {
+                    if let Some(_) = at.ty {
+                        overridden_associated_type = Some(impl_item);
+                    }
                 } else {
                     span_err!(tcx.sess, impl_item.span, E0325,
                               "item `{}` is an associated type, \
@@ -930,6 +933,8 @@ fn check_impl_items_against_trait<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
     let provided_methods = tcx.provided_trait_methods(impl_trait_ref.def_id);
     let associated_consts = tcx.associated_consts(impl_trait_ref.def_id);
     let mut missing_items = Vec::new();
+    let mut invalidated_items = Vec::new();
+    let associated_type_overridden = overridden_associated_type.is_some();
     for trait_item in trait_items.iter() {
         match *trait_item {
             ty::ConstTraitItem(ref associated_const) => {
@@ -944,9 +949,12 @@ fn check_impl_items_against_trait<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                 let is_provided =
                     associated_consts.iter().any(|ac| ac.default.is_some() &&
                                                  ac.name == associated_const.name);
-                if !is_implemented && !is_provided {
-                    missing_items.push(format!("`{}`",
-                                               token::get_name(associated_const.name)));
+                if !is_implemented {
+                    if !is_provided {
+                        missing_items.push(associated_const.name);
+                    } else if associated_type_overridden {
+                        invalidated_items.push(associated_const.name);
+                    }
                 }
             }
             ty::MethodTraitItem(ref trait_method) => {
@@ -961,8 +969,12 @@ fn check_impl_items_against_trait<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                     });
                 let is_provided =
                     provided_methods.iter().any(|m| m.name == trait_method.name);
-                if !is_implemented && !is_provided {
-                    missing_items.push(format!("`{}`", token::get_name(trait_method.name)));
+                if !is_implemented {
+                    if !is_provided {
+                        missing_items.push(trait_method.name);
+                    } else if associated_type_overridden {
+                        invalidated_items.push(trait_method.name);
+                    }
                 }
             }
             ty::TypeTraitItem(ref associated_type) => {
@@ -975,8 +987,12 @@ fn check_impl_items_against_trait<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                     }
                 });
                 let is_provided = associated_type.ty.is_some();
-                if !is_implemented && !is_provided {
-                    missing_items.push(format!("`{}`", token::get_name(associated_type.name)));
+                if !is_implemented {
+                    if !is_provided {
+                        missing_items.push(associated_type.name);
+                    } else if associated_type_overridden {
+                        invalidated_items.push(associated_type.name);
+                    }
                 }
             }
         }
@@ -984,8 +1000,21 @@ fn check_impl_items_against_trait<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
 
     if !missing_items.is_empty() {
         span_err!(tcx.sess, impl_span, E0046,
-            "not all trait items implemented, missing: {}",
-            missing_items.connect(", "));
+            "not all trait items implemented, missing: `{}`",
+            missing_items.iter()
+                  .map(<ast::Name>::as_str)
+                  .collect::<Vec<_>>().connect("`, `"))
+    }
+
+    if !invalidated_items.is_empty() {
+        let invalidator = overridden_associated_type.unwrap();
+        span_err!(tcx.sess, invalidator.span, E0399,
+                  "the following trait items need to be reimplemented \
+                   as `{}` was overridden: `{}`",
+                  invalidator.ident.as_str(),
+                  invalidated_items.iter()
+                                   .map(<ast::Name>::as_str)
+                                   .collect::<Vec<_>>().connect("`, `"))
     }
 }
 

src/librustc_typeck/diagnostics.rs

@@ -2066,6 +2066,8 @@ register_diagnostics! {
            // `#[lang = \"{}\"]` is allowed for the `{}` primitive
     E0391, // unsupported cyclic reference between types/traits detected
     E0392, // parameter `{}` is never used
-    E0393  // the type parameter `{}` must be explicitly specified in an object
+    E0393, // the type parameter `{}` must be explicitly specified in an object
            // type because its default value `{}` references the type `Self`"
+    E0399  // trait items need to be implemented because the associated
+           // type `{}` was overridden
 }

src/test/compile-fail/associated-types-overridden-default.rs

@@ -0,0 +1,25 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(associated_consts)]
+
+pub trait Tr {
+    type Assoc = u8;
+    type Assoc2 = Self::Assoc;
+    const C: u8 = 11;
+    fn foo(&self) {}
+}
+
+impl Tr for () {
+    type Assoc = ();
+    //~^ ERROR need to be reimplemented as `Assoc` was overridden: `Assoc2`, `C`, `foo`
+}
+
+fn main() {}