Make non-found module name optional

No longer uses a magic string for missing or root module.

src/librustc_resolve/lib.rs

@@ -4062,7 +4062,7 @@ fn show_candidates(err: &mut DiagnosticBuilder,
 }
 
 /// A somewhat inefficient routine to obtain the name of a module.
-fn module_to_string(module: Module) -> String {
+fn module_to_string(module: Module) -> Option<String> {
     let mut names = Vec::new();
 
     fn collect_mod(names: &mut Vec<Ident>, module: Module) {
@@ -4080,12 +4080,12 @@ fn collect_mod(names: &mut Vec<Ident>, module: Module) {
     collect_mod(&mut names, module);
 
     if names.is_empty() {
-        return "???".to_string();
+        return None;
     }
-    names_to_string(&names.into_iter()
+    Some(names_to_string(&names.into_iter()
                         .rev()
                         .map(|n| dummy_spanned(n))
-                        .collect::<Vec<_>>())
+                        .collect::<Vec<_>>()))
 }
 
 fn err_path_resolution() -> PathResolution {

src/librustc_resolve/resolve_imports.rs

@@ -524,7 +524,7 @@ pub fn finalize_imports(&mut self) {
     fn resolve_import(&mut self, directive: &'b ImportDirective<'b>) -> bool {
         debug!("(resolving import for module) resolving import `{}::...` in `{}`",
                names_to_string(&directive.module_path[..]),
-               module_to_string(self.current_module));
+               module_to_string(self.current_module).unwrap_or("???".to_string()));
 
         self.current_module = directive.parent;
 
@@ -773,10 +773,10 @@ fn finalize_import(&mut self, directive: &'b ImportDirective<'b>) -> Option<(Spa
                         None => "".to_owned(),
                     };
                 let module_str = module_to_string(module);
-                let msg = if &module_str == "???" {
-                    format!("no `{}` in the root{}", ident, lev_suggestion)
-                } else {
+                let msg = if let Some(module_str) = module_str {
                     format!("no `{}` in `{}`{}", ident, module_str, lev_suggestion)
+                } else {
+                    format!("no `{}` in the root{}", ident, lev_suggestion)
                 };
                 Some((span, msg))
             } else {