Give precedence to local shared files over global ones

When serving 'essential files', we can either serve the global one,
created when building `empty_library`, or the local one, created when
building the local crate. Currently we default to the global one, but
this causes issues when the file should never have been global in the
first place (such as recently for `crates.js`: see
#1313).

This gives precedence to the local file so that the bug will be fixed
when rustdoc fixes it, even if we forget to update
`ESSENTIAL_FILES_UNVERSIONED`.

src/web/mod.rs

@@ -171,12 +171,23 @@ impl Handler for CratesfyiHandler {
             }
         }
 
-        // try serving shared rustdoc resources first, then db/static file handler and last router
-        // return 404 if none of them return Ok. It is important that the router comes last,
-        // because it gives the most specific errors, e.g. CrateNotFound or VersionNotFound
-        self.shared_resource_handler
+        // This is kind of a mess.
+        //
+        // Almost all files should be served through the `router_handler`; eventually
+        // `shared_resource_handler` should go through the router too, and `database_file_handler`
+        // could be removed altogether.
+        //
+        // Unfortunately, combining `shared_resource_handler` with the `router_handler` breaks
+        // things, because right now `shared_resource_handler` allows requesting files from *any*
+        // subdirectory and the router requires us to give a specific path. Changing them to a
+        // specific path means that buggy docs from 2018 will have missing CSS (#1181) so until
+        // that's fixed, we need to keep the current (buggy) behavior.
+        //
+        // It's important that `router_handler` comes first so that local rustdoc files take
+        // precedence over global ones (see #1324).
+        self.router_handler
             .handle(req)
-            .or_else(|e| if_404(e, || self.router_handler.handle(req)))
+            .or_else(|e| if_404(e, || self.shared_resource_handler.handle(req)))
             .or_else(|e| if_404(e, || self.database_file_handler.handle(req)))
             .or_else(|e| {
                 let err = if let Some(err) = e.error.downcast_ref::<error::Nope>() {