Add missing error code for private method

Author: GuillaumeGomez

src/librustc_typeck/check/method/suggest.rs

@@ -312,8 +312,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             }
 
             MethodError::PrivateMatch(def) => {
-                let msg = format!("{} `{}` is private", def.kind_name(), item_name);
-                self.tcx.sess.span_err(span, &msg);
+                struct_span_err!(self.tcx.sess, span, E0624,
+                                 "{} `{}` is private", def.kind_name(), item_name).emit();
             }
 
             MethodError::IllegalSizedBound(candidates) => {

src/librustc_typeck/diagnostics.rs

@@ -4644,6 +4644,62 @@ whose implementation is handled specially by the compiler. In order to fix this
 error, just declare a function.
 "##,
 
+E0624: r##"
+A private item was used outside of its scope.
+
+Erroneous code example:
+
+```compile_fail,E0624
+mod inner {
+    pub struct Foo;
+
+    impl Foo {
+        fn method(&self) {}
+    }
+}
+
+let foo = inner::Foo;
+foo.method(); // error: method `method` is private
+```
+
+Two possibilities are available to solve this issue:
+
+1. Only use the item in the scope it has been defined:
+
+```
+mod inner {
+    pub struct Foo;
+
+    impl Foo {
+        fn method(&self) {}
+    }
+
+    pub fn call_method(foo: &Foo) { // We create a public function.
+        foo.method(); // Which calls the item.
+    }
+}
+
+let foo = inner::Foo;
+inner::call_method(&foo); // And since the function is public, we can call the
+                          // method through it.
+```
+
+2. Make the item public:
+
+```
+mod inner {
+    pub struct Foo;
+
+    impl Foo {
+        pub fn method(&self) {} // It's now public.
+    }
+}
+
+let foo = inner::Foo;
+foo.method(); // Ok!
+```
+"##,
+
 }
 
 register_diagnostics! {

src/test/compile-fail/E0624.rs

@@ -0,0 +1,22 @@
+// Copyright 2017 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.
+
+mod inner {
+    pub struct Foo;
+
+    impl Foo {
+        fn method(&self) {}
+    }
+}
+
+fn main() {
+    let foo = inner::Foo;
+    foo.method(); //~ ERROR method `method` is private [E0624]
+}