From 4537025c7193b4f1412807ab15f723f51ff0b01e Mon Sep 17 00:00:00 2001
From: Michael Woerister <michaelwoerister@posteo>
Date: Wed, 9 May 2018 21:32:18 +0200
Subject: [PATCH] Add comment about first element in CStore::metas.

---
 src/librustc_metadata/cstore.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/librustc_metadata/cstore.rs b/src/librustc_metadata/cstore.rs
index 4872d560d27f0..0c54ec7c27ab2 100644
--- a/src/librustc_metadata/cstore.rs
+++ b/src/librustc_metadata/cstore.rs
@@ -96,6 +96,10 @@ pub struct CStore {
 impl CStore {
     pub fn new(metadata_loader: Box<MetadataLoader + Sync>) -> CStore {
         CStore {
+            // We add an empty entry for LOCAL_CRATE (which maps to zero) in
+            // order to make array indices in `metas` match with the
+            // corresponding `CrateNum`. This first entry will always remain
+            // `None`.
             metas: RwLock::new(IndexVec::from_elem_n(None, 1)),
             extern_mod_crate_map: Lock::new(FxHashMap()),
             metadata_loader,