hygiene: Introduce a helper method for creating new expansions

Creating a fresh expansion and immediately generating a span from it is the most common scenario.

Also avoid allocating `allow_internal_unstable` lists for derive markers repeatedly.
And rename `ExpnInfo::with_unstable` to `ExpnInfo::allow_unstable`, seems to be a better fitting name.

1 files changed, 2 insertions, 4 deletions

diff --git a/src/libsyntax_ext/proc_macro_decls.rs b/src/libsyntax_ext/proc_macro_decls.rs
index 0733a8ec95c..2f78644dff2 100644
--- a/src/libsyntax_ext/proc_macro_decls.rs
+++ b/src/libsyntax_ext/proc_macro_decls.rs
@@ -346,12 +346,10 @@ fn mk_decls(
     custom_attrs: &[ProcMacroDef],
     custom_macros: &[ProcMacroDef],
 ) -> P<ast::Item> {
-    let mark = Mark::fresh(Mark::root());
-    mark.set_expn_info(ExpnInfo::with_unstable(
+    let span = DUMMY_SP.fresh_expansion(Mark::root(), ExpnInfo::allow_unstable(
         ExpnKind::Macro(MacroKind::Attr, sym::proc_macro), DUMMY_SP, cx.parse_sess.edition,
-        &[sym::rustc_attrs, Symbol::intern("proc_macro_internals")],
+        [sym::rustc_attrs, sym::proc_macro_internals][..].into(),
     ));
-    let span = DUMMY_SP.apply_mark(mark);
 
     let hidden = cx.meta_list_item_word(span, sym::hidden);
     let doc = cx.meta_list(span, sym::doc, vec![hidden]);