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

2 files changed, 5 insertions, 3 deletions

diff --git a/src/libsyntax_pos/hygiene.rs b/src/libsyntax_pos/hygiene.rs
index f9394826c9b..fb9b4c9d70e 100644
--- a/src/libsyntax_pos/hygiene.rs
+++ b/src/libsyntax_pos/hygiene.rs
@@ -13,6 +13,7 @@ use crate::symbol::{keywords, Symbol};
 use serialize::{Encodable, Decodable, Encoder, Decoder};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use std::{fmt, mem};
+use std::rc::Rc;
 
 /// A SyntaxContext represents a chain of macro expansions (represented by marks).
 #[derive(Clone, Copy, PartialEq, Eq, Default, PartialOrd, Ord, Hash)]
@@ -553,7 +554,7 @@ pub struct ExpnInfo {
     /// List of #[unstable]/feature-gated features that the macro is allowed to use
     /// internally without forcing the whole crate to opt-in
     /// to them.
-    pub allow_internal_unstable: Vec<Symbol>,
+    pub allow_internal_unstable: Option<Rc<[Symbol]>>,
     /// Whether the macro is allowed to use `unsafe` internally
     /// even if the user crate has `#![forbid(unsafe_code)]`.
     pub allow_internal_unsafe: bool,
diff --git a/src/libsyntax_pos/lib.rs b/src/libsyntax_pos/lib.rs
index 87c4d02fa4f..f0dc1b00fd9 100644
--- a/src/libsyntax_pos/lib.rs
+++ b/src/libsyntax_pos/lib.rs
@@ -389,8 +389,9 @@ impl Span {
         match self.ctxt().outer().expn_info() {
             Some(info) => info
                 .allow_internal_unstable
-                .iter()
-                .any(|&f| f == feature || f == "allow_internal_unstable_backcompat_hack"),
+                .map_or(false, |features| features.iter().any(|&f|
+                    f == feature || f == "allow_internal_unstable_backcompat_hack"
+                )),
             None => false,
         }
     }