Add some necessary tracing commands.

src/librustc_resolve/resolve_imports.rs

@@ -499,6 +499,11 @@ impl<'a, 'b:'a> ImportResolver<'a, 'b> {
                 // resolution for it so that later resolve stages won't complain.
                 self.import_dummy_binding(import);
                 if !seen_spans.contains(&span) {
+                    info!(
+                        "preparing to import_path_to_string(import={:?}, span={:?})",
+                        import,
+                        span
+                    );
                     let path = import_path_to_string(&import.module_path[..],
                                                      &import.subclass,
                                                      span);
@@ -1015,6 +1020,14 @@ impl<'a, 'b:'a> ImportResolver<'a, 'b> {
 fn import_path_to_string(names: &[SpannedIdent],
                          subclass: &ImportDirectiveSubclass,
                          span: Span) -> String {
+    info!(
+        "import_path_to_string(names={:?} ({:p}/{}), subclass={:?}, span={:?})",
+        names,
+        names.as_ptr(),
+        names.len(),
+        subclass,
+        span,
+    );
     let pos = names.iter()
         .position(|p| span == p.span && p.node.name != keywords::CrateRoot.name());
     let global = !names.is_empty() && names[0].node.name == keywords::CrateRoot.name();
@@ -1039,11 +1052,12 @@ fn import_path_to_string(names: &[SpannedIdent],
             if names.is_empty() || x.starts_with("::") {
                 span_bug!(
                     span,
-                    "invalid name `{}` at {:?}; global = {}, names = {:?}, subclass = {:?}",
+                    "invalid name `{}` at {:?}; global = {}, names = {:p}/{}, subclass = {:?}",
                     x,
                     span,
                     global,
-                    names,
+                    names.as_ptr(),
+                    names.len(),
                     subclass
                 );
             }