Fix up comment about OnDiskCache::foreign_expn_data.

diff --git a/compiler/rustc_query_impl/src/on_disk_cache.rs b/compiler/rustc_query_impl/src/on_disk_cache.rs
index f9456a6f2cdb8575a78f1c2c839635e9a9cc0ed3..cb50c851e9a455daac7426e6dfc3b9e979597e69 100644 (file)
--- a/compiler/rustc_query_impl/src/on_disk_cache.rs
+++ b/compiler/rustc_query_impl/src/on_disk_cache.rs
@@ -86,8 +86,10 @@ pub struct OnDiskCache<'sess> {
     expn_data: UnhashMap<ExpnHash, AbsoluteBytePos>,
     // Additional information used when decoding hygiene data.
     hygiene_context: HygieneDecodeContext,
-    // FIXME(mw): Update this comment:
-    // Likewise for ExpnId.
+    // Maps `ExpnHash`es to their raw value from the *previous*
+    // compilation session. This is used as an initial 'guess' when
+    // we try to map an `ExpnHash` to its value in the current
+    // compilation session.
     foreign_expn_data: UnhashMap<ExpnHash, u32>,
 }