Better error reporting with renaming imports

src/librustc/middle/resolve.rs

@@ -2593,7 +2593,12 @@ pub impl Resolver {
         let start_index;
         match module_prefix_result {
             Failed => {
-                self.session.span_err(span, ~"unresolved name");
+                let mpath = self.idents_to_str(module_path);
+                let idx = str::rfind(self.idents_to_str(module_path), |c| { c == ':' }).unwrap();
+                self.session.span_err(span, fmt!("unresolved import: could not find `%s` in `%s`",
+                                                 str::substr(mpath, idx, mpath.len() - idx),
+                                                 // idx - 1 to account for the extra semicolon
+                                                 str::substr(mpath, 0, idx - 1)));
                 return Failed;
             }
             Indeterminate => {

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

@@ -8,5 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use foo::bar; //~ ERROR unresolved import. maybe a missing
+use foo::bar; //~ ERROR unresolved import. maybe a missing `extern mod foo`?
               //~^ ERROR failed to resolve import
+use x = bar::baz; //~ ERROR unresolved import: could not find `baz` in `bar`
+                  //~^ ERROR failed to resolve import
+
+mod bar {
+    struct bar;
+}