rustc_expand: remember module #[path]s during expansion

this fixes cycle detection for modules that need a second invocation collection pass after parsing

compiler/rustc_expand/src/expand.rs

@@ -41,7 +41,9 @@
 };
 use crate::fluent_generated;
 use crate::mbe::diagnostics::annotate_err_with_kind;
-use crate::module::{mod_dir_path, parse_external_mod, DirOwnership, ParsedExternalMod};
+use crate::module::{
+    mod_dir_path, mod_file_path_from_attr, parse_external_mod, DirOwnership, ParsedExternalMod,
+};
 use crate::placeholders::{placeholder, PlaceholderExpander};
 
 macro_rules! ast_fragments {
@@ -1202,8 +1204,14 @@ fn wrap_flat_map_node_walk_flat_map(
                     ecx.current_expansion.dir_ownership,
                     *inline,
                 );
+                // If the module was parsed from an external file, recover its path.
+                // This lets `parse_external_mod` catch cycles if it's self-referential.
+                let file_path = match inline {
+                    Inline::Yes => None,
+                    Inline::No => mod_file_path_from_attr(ecx.sess, &attrs, &dir_path),
+                };
                 node.attrs = attrs;
-                (None, dir_path, dir_ownership)
+                (file_path, dir_path, dir_ownership)
             }
             ModKind::Unloaded => {
                 // We have an outline `mod foo;` so we need to parse the file.

compiler/rustc_expand/src/module.rs

@@ -171,7 +171,7 @@ fn mod_file_path<'a>(
 
 /// Derive a submodule path from the first found `#[path = "path_string"]`.
 /// The provided `dir_path` is joined with the `path_string`.
-fn mod_file_path_from_attr(
+pub(crate) fn mod_file_path_from_attr(
     sess: &Session,
     attrs: &[Attribute],
     dir_path: &Path,