expand: Move module file path stack from global session to expansion data

Also don't push the paths on the stack directly in `fn parse_external_mod`, return them instead.

compiler/rustc_session/src/parse.rs

@@ -13,7 +13,6 @@ use rustc_span::hygiene::ExpnId;
 use rustc_span::source_map::{FilePathMapping, SourceMap};
 use rustc_span::{MultiSpan, Span, Symbol};
 
-use std::path::PathBuf;
 use std::str;
 
 /// The set of keys (and, optionally, values) that define the compilation
@@ -122,8 +121,6 @@ pub struct ParseSess {
     pub missing_fragment_specifiers: Lock<FxHashMap<Span, NodeId>>,
     /// Places where raw identifiers were used. This is used for feature-gating raw identifiers.
     pub raw_identifier_spans: Lock<Vec<Span>>,
-    /// Used to determine and report recursive module inclusions.
-    pub included_mod_stack: Lock<Vec<PathBuf>>,
     source_map: Lrc<SourceMap>,
     pub buffered_lints: Lock<Vec<BufferedEarlyLint>>,
     /// Contains the spans of block expressions that could have been incomplete based on the
@@ -157,7 +154,6 @@ impl ParseSess {
             edition: ExpnId::root().expn_data().edition,
             missing_fragment_specifiers: Default::default(),
             raw_identifier_spans: Lock::new(Vec::new()),
-            included_mod_stack: Lock::new(vec![]),
             source_map,
             buffered_lints: Lock::new(vec![]),
             ambiguous_block_expr_parse: Lock::new(FxHashMap::default()),