diff -r ea35afdb1366 -r 99a6bcd1e8e4 src/Pure/Tools/build.scala
--- a/src/Pure/Tools/build.scala	Wed Nov 18 21:34:13 2020 +0100
+++ b/src/Pure/Tools/build.scala	Wed Nov 18 21:39:55 2020 +0100
@@ -533,7 +533,7 @@
         full_sessions(session_name).meta_digest ::
         deps.sources(session_name) :::
         deps.imported_sources(session_name)
-      SHA1.digest(cat_lines(digests.map(_.toString).sorted)).toString
+      SHA1.digest_set(digests).toString
     }
 
     val deps =