Add fixme

Author: bjorn3

build_system/prepare.rs

@@ -15,6 +15,7 @@ pub(crate) fn prepare(dirs: &Dirs, rustc: &Path) {
     super::tests::REGEX_REPO.fetch(dirs);
     super::tests::PORTABLE_SIMD_REPO.fetch(dirs);
 
+    // FIXME do this on the fly?
     prepare_stdlib(dirs, rustc);
     prepare_coretests(dirs, rustc);