First attempt at fixing #20591

This isn't quite right, but it's interesting.

src/librustc_resolve/resolve_imports.rs

@@ -903,6 +903,17 @@ impl<'a, 'b:'a, 'tcx:'b> ImportResolver<'a, 'b, 'tcx> {
                                   },
                                   &token::get_name(name));
                 span_err!(self.resolver.session, import_span, E0252, "{}", &msg[..]);
+                if let Some(sp) = target.bindings.span_for_namespace(namespace) {
+                    span_note!(self.resolver.session, sp,
+                               "first import of {} `{}` here",
+                               match namespace {
+                                   TypeNS => "type",
+                                   ValueNS => "value",
+                               },
+                               token::get_name(name));
+                } else {
+                    span_note!(self.resolver.session, import_span, "I can't find where it was previously imported");
+                }
             }
             Some(_) | None => {}
         }

src/test/compile-fail/double-import.rs

@@ -0,0 +1,25 @@
+// Copyright 2013 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(no_std)]
+#![no_std]
+
+
+mod sub1 {
+    fn foo() {} // Implementation 1
+}
+
+mod sub2 {
+    fn foo() {} // Implementation 2
+}
+
+use sub1::foo; //~ NOTE first imported here
+use sub2::foo; //~ ERROR a value named `foo` has already been imported in this module [E0252]
+
+fn main() {}