2 files changed, 10 insertions, 5 deletions
diff --git a/src/libsyntax_pos/hygiene.rs b/src/libsyntax_pos/hygiene.rs
index 0c645fc678c..d5c0a2ca85f 100644
--- a/src/libsyntax_pos/hygiene.rs
+++ b/src/libsyntax_pos/hygiene.rs
@@ -12,6 +12,7 @@ use crate::symbol::{keywords, Symbol};
 
 use serialize::{Encodable, Decodable, Encoder, Decoder};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::sync::Lrc;
 use std::{fmt, mem};
 
 /// A SyntaxContext represents a chain of macro expansions (represented by marks).
@@ -550,10 +551,10 @@ pub struct ExpnInfo {
     pub def_site: Option<Span>,
     /// The format with which the macro was invoked.
     pub format: ExpnFormat,
-    /// Whether the macro is allowed to use #[unstable]/feature-gated
-    /// features internally without forcing the whole crate to opt-in
+    /// 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: bool,
+    pub allow_internal_unstable: Option<Lrc<[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 dbb4f8f8159..042005ea538 100644
--- a/src/libsyntax_pos/lib.rs
+++ b/src/libsyntax_pos/lib.rs
@@ -386,9 +386,13 @@ impl Span {
     /// Check if a span is "internal" to a macro in which `#[unstable]`
     /// items can be used (that is, a macro marked with
     /// `#[allow_internal_unstable]`).
-    pub fn allows_unstable(&self) -> bool {
+    pub fn allows_unstable(&self, feature: &str) -> bool {
         match self.ctxt().outer().expn_info() {
-            Some(info) => info.allow_internal_unstable,
+            Some(info) => info
+                .allow_internal_unstable
+                .map_or(false, |features| features.iter().any(|&f|
+                    f == feature || f == "allow_internal_unstable_backcompat_hack"
+                )),
             None => false,
         }
     }