diff --git a/sim/mcuboot-sys/src/api.rs b/sim/mcuboot-sys/src/api.rs
index 7bd4fc4..a6acd53 100644
--- a/sim/mcuboot-sys/src/api.rs
+++ b/sim/mcuboot-sys/src/api.rs
@@ -94,20 +94,27 @@
     pub static SIM_CTX: RefCell<CSimContextPtr> = RefCell::new(CSimContextPtr::new());
 }
 
-// Set the flash device to be used by the simulation.  The pointer is unsafely stashed away.
-pub unsafe fn set_flash(dev_id: u8, dev: &mut dyn Flash) {
+/// Set the flash device to be used by the simulation.  The pointer is unsafely stashed away.
+///
+/// # Safety
+///
+/// This uses mem::transmute to stash a Rust pointer into a C value to
+/// retrieve later.  It should be safe to use this.
+pub fn set_flash(dev_id: u8, dev: &mut dyn Flash) {
     THREAD_CTX.with(|ctx| {
         ctx.borrow_mut().flash_params.insert(dev_id, FlashParamsStruct {
             align: dev.align() as u16,
             erased_val: dev.erased_val(),
         });
-        let dev: &'static mut dyn Flash = mem::transmute(dev);
-        ctx.borrow_mut().flash_map.insert(
-            dev_id, FlashPtr{ptr: dev as *mut dyn Flash});
+        unsafe {
+            let dev: &'static mut dyn Flash = mem::transmute(dev);
+            ctx.borrow_mut().flash_map.insert(
+                dev_id, FlashPtr{ptr: dev as *mut dyn Flash});
+        }
     });
 }
 
-pub unsafe fn clear_flash(dev_id: u8) {
+pub fn clear_flash(dev_id: u8) {
     THREAD_CTX.with(|ctx| {
         ctx.borrow_mut().flash_map.remove(&dev_id);
     });