Use `Rc<[Symbol]>` instead of `Vec<Symbol>` to reduce # of allocs

1 files changed, 4 insertions, 3 deletions

diff --git a/src/libsyntax_ext/deriving/mod.rs b/src/libsyntax_ext/deriving/mod.rs
index bc86ffa852b..fff54814a38 100644
--- a/src/libsyntax_ext/deriving/mod.rs
+++ b/src/libsyntax_ext/deriving/mod.rs
@@ -138,13 +138,14 @@ fn call_intrinsic(cx: &ExtCtxt<'_>,
                   -> P<ast::Expr> {
     let intrinsic_allowed_via_allow_internal_unstable = cx
         .current_expansion.mark.expn_info().unwrap()
-        .allow_internal_unstable.iter()
-        .any(|&s| s == "core_intrinsics");
+        .allow_internal_unstable.map_or(false, |features| features.iter().any(|&s|
+            s == "core_intrinsics"
+        ));
     if intrinsic_allowed_via_allow_internal_unstable {
         span = span.with_ctxt(cx.backtrace());
     } else { // Avoid instability errors with user defined curstom derives, cc #36316
         let mut info = cx.current_expansion.mark.expn_info().unwrap();
-        info.allow_internal_unstable = vec![Symbol::intern("core_intrinsics")];
+        info.allow_internal_unstable = Some(vec![Symbol::intern("core_intrinsics")].into());
         let mark = Mark::fresh(Mark::root());
         mark.set_expn_info(info);
         span = span.with_ctxt(SyntaxContext::empty().apply_mark(mark));