Add FIXME-s that some types should be transparent

src/libstd/sys_common/os_str_bytes.rs

@@ -18,6 +18,12 @@ pub(crate) struct Buf {
     pub inner: Vec<u8>
 }
 
+// FIXME:
+// `Buf::as_slice` current implementation relies
+// on `Slice` being layout-compatible with `[u8]`.
+// When attribute privacy is implemented, `Slice` should be annotated as `#[repr(transparent)]`.
+// Anyway, `Slice` representation and layout are considered implementation detail, are
+// not documented and must not be relied upon.
 pub(crate) struct Slice {
     pub inner: [u8]
 }