Add checks for self: _ and self: &_

src/librustc/hir/lowering.rs

@@ -775,6 +775,16 @@ impl<'a> LoweringContext<'a> {
     }
 
     fn lower_method_sig(&mut self, sig: &MethodSig) -> hir::MethodSig {
+        // Check for `self: _` and `self: &_`
+        if let SelfKind::Explicit(ref ty, _) = sig.explicit_self.node {
+            match sig.decl.inputs.get(0).and_then(Arg::to_self).map(|eself| eself.node) {
+                Some(SelfKind::Value(..)) | Some(SelfKind::Region(..)) => {
+                    self.id_assigner.diagnostic().span_err(ty.span,
+                        "the type placeholder `_` is not allowed within types on item signatures");
+                }
+                _ => {}
+            }
+        }
         hir::MethodSig {
             generics: self.lower_generics(&sig.generics),
             abi: sig.abi,

src/test/compile-fail/self-infer.rs

@@ -0,0 +1,18 @@
+// Copyright 2016 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.
+
+struct S;
+
+impl S {
+    fn f(self: _) {} //~ERROR the type placeholder `_` is not allowed within types on item sig
+    fn g(self: &_) {} //~ERROR the type placeholder `_` is not allowed within types on item sig
+}
+
+fn main() {}