StoreBackend::Data: don't require Sized

src/lib.rs

@@ -1,6 +1,6 @@
 #![no_std]
 
-use core::{mem::size_of, str};
+use core::str;
 use byteorder::{ByteOrder, BigEndian};
 #[cfg(feature = "postcard-values")]
 use serde::{Deserialize, Serialize};
@@ -14,12 +14,12 @@ mod test;
 
 /// Backend interface for `Store`
 pub trait StoreBackend {
-    type Data: Sized + AsRef<[u8]>;
+    type Data: ?Sized + AsRef<[u8]>;
 
     /// Memory-mapped
     fn data(&self) -> &Self::Data;
     fn len(&self) -> usize {
-        size_of::<Self::Data>()
+        self.data().as_ref().len()
     }
 
     type Error;