Change inherent overlap error to a warning for now, to ease the breakage.

src/librustc/lint/builtin.rs

@@ -154,6 +154,12 @@
     "transmute from function item type to pointer-sized type erroneously allowed"
 }
 
+declare_lint! {
+    pub OVERLAPPING_INHERENT_IMPLS,
+    Warn,
+    "two overlapping inherent impls define an item with the same name were erroneously allowed"
+}
+
 /// Does nothing as a lint pass, but registers some `Lint`s
 /// which are used by other parts of the compiler.
 #[derive(Copy, Clone)]
@@ -184,7 +190,8 @@ fn get_lints(&self) -> LintArray {
             MATCH_OF_UNIT_VARIANT_VIA_PAREN_DOTDOT,
             CONST_ERR,
             RAW_POINTER_DERIVE,
-            TRANSMUTE_FROM_FN_ITEM_TYPES
+            TRANSMUTE_FROM_FN_ITEM_TYPES,
+            OVERLAPPING_INHERENT_IMPLS
         )
     }
 }

src/librustc_lint/lib.rs

@@ -175,6 +175,10 @@ macro_rules! add_lint_group {
             id: LintId::of(TRANSMUTE_FROM_FN_ITEM_TYPES),
             reference: "issue #19925 <https://github.com/rust-lang/rust/issues/19925>",
         },
+        FutureIncompatibleInfo {
+            id: LintId::of(OVERLAPPING_INHERENT_IMPLS),
+            reference: "issue #22889 <https://github.com/rust-lang/rust/issues/22889>",
+        },
         ]);
 
     // We have one lint pass defined specially

src/librustc_typeck/coherence/overlap.rs

@@ -18,11 +18,11 @@
 use middle::infer;
 use middle::ty::{self, TyCtxt};
 use syntax::ast;
-use syntax::codemap::Span;
 use rustc::dep_graph::DepNode;
 use rustc_front::hir;
 use rustc_front::intravisit;
 use util::nodemap::DefIdMap;
+use lint;
 
 pub fn check(tcx: &TyCtxt) {
     let mut overlap = OverlapChecker { tcx: tcx,
@@ -41,11 +41,6 @@ struct OverlapChecker<'cx, 'tcx:'cx> {
 }
 
 impl<'cx, 'tcx> OverlapChecker<'cx, 'tcx> {
-    fn span_of_def_id(&self, did: DefId) -> Span {
-        let node_id = self.tcx.map.as_local_node_id(did).unwrap();
-        self.tcx.map.span(node_id)
-    }
-
     fn check_for_common_items_in_impls(&self, impl1: DefId, impl2: DefId) {
         #[derive(Copy, Clone, PartialEq)]
         enum Namespace { Type, Value }
@@ -68,11 +63,12 @@ fn name_and_namespace(tcx: &TyCtxt, item: &ty::ImplOrTraitItemId)
 
             for item2 in &impl_items[&impl2] {
                 if (name, namespace) == name_and_namespace(&self.tcx, item2) {
-                    let mut err = super::report_duplicate_item(
-                        &self.tcx, self.span_of_def_id(item1.def_id()), name);
-                    span_note!(&mut err, self.span_of_def_id(item2.def_id()),
-                               "conflicting definition is here:");
-                    err.emit();
+                    let msg = format!("duplicate definitions with name `{}`", name);
+                    let node_id = self.tcx.map.as_local_node_id(item1.def_id()).unwrap();
+                    self.tcx.sess.add_lint(lint::builtin::OVERLAPPING_INHERENT_IMPLS,
+                                           node_id,
+                                           self.tcx.span_of_impl(item1.def_id()).unwrap(),
+                                           msg);
                 }
             }
         }

src/test/compile-fail/inherent-overlap.rs

@@ -11,10 +11,14 @@
 // Test that you cannot define items with the same name in overlapping inherent
 // impl blocks.
 
+#![feature(rustc_attrs)]
+#![allow(dead_code)]
+
 struct Foo;
 
 impl Foo {
-    fn id() {} //~ ERROR E0201
+    fn id() {} //~ WARN duplicate definitions
+    //~^ WARN previously accepted
 }
 
 impl Foo {
@@ -24,7 +28,8 @@ fn id() {}
 struct Bar<T>(T);
 
 impl<T> Bar<T> {
-    fn bar(&self) {} //~ ERROR E0201
+    fn bar(&self) {} //~ WARN duplicate definitions
+    //~^ WARN previously accepted
 }
 
 impl Bar<u32> {
@@ -34,11 +39,13 @@ fn bar(&self) {}
 struct Baz<T>(T);
 
 impl<T: Copy> Baz<T> {
-    fn baz(&self) {} //~ ERROR E0201
+    fn baz(&self) {} //~ WARN duplicate definitions
+    //~^ WARN previously accepted
 }
 
 impl<T> Baz<Vec<T>> {
     fn baz(&self) {}
 }
 
-fn main() {}
+#[rustc_error]
+fn main() {} //~ ERROR compilation successful