3 files changed, 28 insertions, 9 deletions
diff --git a/compiler/rustc_index/Cargo.toml b/compiler/rustc_index/Cargo.toml
index d8ea5aa80..e1cda5a9e 100644
--- a/compiler/rustc_index/Cargo.toml
+++ b/compiler/rustc_index/Cargo.toml
@@ -7,6 +7,10 @@ edition = "2021"
 
 [dependencies]
 arrayvec = { version = "0.7", default-features = false }
-rustc_serialize = { path = "../rustc_serialize" }
-rustc_macros = { path = "../rustc_macros" }
+rustc_serialize = { path = "../rustc_serialize", optional = true }
+rustc_macros = { path = "../rustc_macros", optional = true }
 smallvec = "1.8.1"
+
+[features]
+default = ["nightly"]
+nightly = ["rustc_serialize", "rustc_macros"]
diff --git a/compiler/rustc_index/src/lib.rs b/compiler/rustc_index/src/lib.rs
index 23a4c1f06..db2c79152 100644
--- a/compiler/rustc_index/src/lib.rs
+++ b/compiler/rustc_index/src/lib.rs
@@ -1,17 +1,25 @@
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
-#![feature(allow_internal_unstable)]
-#![feature(extend_one)]
-#![feature(min_specialization)]
-#![feature(new_uninit)]
-#![feature(step_trait)]
-#![feature(stmt_expr_attributes)]
-#![feature(test)]
+#![cfg_attr(
+    feature = "nightly",
+    feature(
+        allow_internal_unstable,
+        extend_one,
+        min_specialization,
+        new_uninit,
+        step_trait,
+        stmt_expr_attributes,
+        test
+    )
+)]
 
+#[cfg(feature = "nightly")]
 pub mod bit_set;
+#[cfg(feature = "nightly")]
 pub mod interval;
 pub mod vec;
 
+#[cfg(feature = "rustc_macros")]
 pub use rustc_macros::newtype_index;
 
 /// Type size assertion. The first argument is a type and the second argument is its expected size.
diff --git a/compiler/rustc_index/src/vec.rs b/compiler/rustc_index/src/vec.rs
index 4172ce3bb..39aa27a23 100644
--- a/compiler/rustc_index/src/vec.rs
+++ b/compiler/rustc_index/src/vec.rs
@@ -1,3 +1,4 @@
+#[cfg(feature = "rustc_serialize")]
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 
 use std::fmt;
@@ -17,10 +18,12 @@ pub trait Idx: Copy + 'static + Eq + PartialEq + Debug + Hash {
 
     fn index(self) -> usize;
 
+    #[inline]
     fn increment_by(&mut self, amount: usize) {
         *self = self.plus(amount);
     }
 
+    #[inline]
     fn plus(self, amount: usize) -> Self {
         Self::new(self.index() + amount)
     }
@@ -59,12 +62,14 @@ pub struct IndexVec<I: Idx, T> {
 // not the phantom data.
 unsafe impl<I: Idx, T> Send for IndexVec<I, T> where T: Send {}
 
+#[cfg(feature = "rustc_serialize")]
 impl<S: Encoder, I: Idx, T: Encodable<S>> Encodable<S> for IndexVec<I, T> {
     fn encode(&self, s: &mut S) {
         Encodable::encode(&self.raw, s);
     }
 }
 
+#[cfg(feature = "rustc_serialize")]
 impl<D: Decoder, I: Idx, T: Decodable<D>> Decodable<D> for IndexVec<I, T> {
     fn decode(d: &mut D) -> Self {
         IndexVec { raw: Decodable::decode(d), _marker: PhantomData }
@@ -357,11 +362,13 @@ impl<I: Idx, T> Extend<T> for IndexVec<I, T> {
     }
 
     #[inline]
+    #[cfg(feature = "nightly")]
     fn extend_one(&mut self, item: T) {
         self.raw.push(item);
     }
 
     #[inline]
+    #[cfg(feature = "nightly")]
     fn extend_reserve(&mut self, additional: usize) {
         self.raw.reserve(additional);
     }