Maybe it works

Still compiling, but I think I have it!

Author: tbelaire

src/librustc_resolve/resolve_imports.rs

@@ -896,6 +896,7 @@ impl<'a, 'b:'a, 'tcx:'b> ImportResolver<'a, 'b, 'tcx> {
 
         match target {
             Some(ref target) if target.shadowable != Shadowable::Always => {
+                use syntax::ast_map::NodeItem;
                 let ns_word = match namespace {
                     TypeNS => "type",
                     ValueNS => "value",
@@ -904,12 +905,21 @@ impl<'a, 'b:'a, 'tcx:'b> ImportResolver<'a, 'b, 'tcx> {
                           "a {} named `{}` has already been imported \
                            in this module", ns_word,
                                   &token::get_name(name));
+                let use_id = import_resolution.id(namespace);
+                if let NodeItem(item) = self.resolver.ast_map.get(use_id) {
+                    // Assert item.node is ItemUse
+                    // I feel like this should maybe mention the type,
+                    // as it's otherwise a bit of work to look up...
+                    // use syntax::ast::Item;
+                    span_note!(self.resolver.session, item.span,
+                               "Previously import of {} `{}` here",
+                               ns_word, token::get_name(name));
+                }
+                // Also showing the definition is reasonable?
                 if let Some(sp) = target.bindings.span_for_namespace(namespace) {
                     span_note!(self.resolver.session, sp,
-                               "first import of {} `{}` here",
+                               "definition of {} `{}` here",
                                ns_word, token::get_name(name));
-                } else {
-                    span_note!(self.resolver.session, import_span, "I can't find where it was previously imported");
                 }
             }
             Some(_) | None => {}