Ensure FlashRegion can only be created within this crate

embassy-stm32/build.rs

@@ -206,6 +206,7 @@ fn main() {
                 erase_size: #erase_size,
                 write_size: #write_size,
                 erase_value: #erase_value,
+                _ensure_internal: (),
             };
         });
 

embassy-stm32/src/flash/mod.rs

@@ -19,6 +19,7 @@ pub struct FlashRegion {
     pub erase_size: u32,
     pub write_size: u32,
     pub erase_value: u8,
+    pub(crate) _ensure_internal: (),
 }
 
 #[derive(Debug, PartialEq)]