Didn't find any other examples. At the moment rust code is recognized
as ASCII C program text.
Index: usr.bin/file/magdir/rust
===================================================================
RCS file: usr.bin/file/magdir/rust
diff -N usr.bin/file/magdir/rust
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ usr.bin/file/magdir/rust 14 Jan 2019 12:20:22 -0000
@@ -0,0 +1,5 @@
+#------------------------------------------------------------------------------
+# rust: file(1) magic for Rust language
+#
+0 search fn\ main()\ { Rust program text
+
