From ca0b89aa040acf5df95d39579cbd7ff03c73baf1 Mon Sep 17 00:00:00 2001
From: Josh Stone <jistone@redhat.com>
Date: Sun, 9 Aug 2020 12:41:09 -0700
Subject: [PATCH] rustc_span: note the perf loss seen from IndexSet in
 symbol::Interner

---
 src/librustc_span/symbol.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/librustc_span/symbol.rs b/src/librustc_span/symbol.rs
index 5203bfdb3b7ce..ca9702784a268 100644
--- a/src/librustc_span/symbol.rs
+++ b/src/librustc_span/symbol.rs
@@ -1481,6 +1481,10 @@ impl<CTX> ToStableHashKey<CTX> for Symbol {
 }
 
 // The `&'static str`s in this type actually point into the arena.
+//
+// The `FxHashMap`+`Vec` pair could be replaced by `FxIndexSet`, but #75278
+// found that to regress performance up to 2% in some cases. This might be
+// revisited after further improvements to `indexmap`.
 #[derive(Default)]
 pub struct Interner {
     arena: DroplessArena,