Pass an AccessorBuilder instaed of an Accessor to a DMA handler

src/backplane.rs

@@ -1,5 +1,6 @@
 use std::fmt::Debug;
 use std::fmt::Display;
+use std::marker::PhantomData;
 
 use anyhow::anyhow;
 use itertools::Itertools;
@@ -14,23 +15,41 @@ use crate::{
 };
 
 pub trait DMAHandler: Debug {
-    fn handle<'a>(&mut self, backplane: &'a Backplane, card_accessor: DMACardAccessor<'a>);
+    fn handle<'a>(&mut self, backplane: &'a Backplane, card_accessor: DMACardAccessorBuilder<'a>);
 }
 
 #[derive(Copy, Clone, Debug)]
 #[allow(dead_code)]
-pub struct DMACardAccessor<'a> {
+pub struct DMACardAccessorBuilder<'a> {
     backplane: &'a Backplane,
     card_no: usize,
 }
 
-impl DMACardAccessor<'_> {
+impl<'a> DMACardAccessorBuilder<'a> {
     #[allow(dead_code)]
-    pub fn get<T: Card>(&self) -> Option<MappedMutexGuard<T>> {
+    pub fn build<T: Card>(self) -> DMACardAccessor<'a, T> {
-        MutexGuard::try_map(self.backplane.cards.lock(), |cards| {
+        DMACardAccessor {
-            cards[self.card_no].downcast_mut::<T>()
+            backplane: self.backplane,
+            card_no: self.card_no,
+            card_type: PhantomData,
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+#[allow(dead_code)]
+pub struct DMACardAccessor<'a, T> {
+    backplane: &'a Backplane,
+    card_no: usize,
+    card_type: PhantomData<T>,
+}
+
+impl<T: Card> DMACardAccessor<'_, T> {
+    #[allow(dead_code)]
+    pub fn get(&self) -> MappedMutexGuard<T> {
+        MutexGuard::map(self.backplane.cards.lock(), |cards| {
+            cards[self.card_no].downcast_mut::<T>().unwrap()
         })
-        .ok()
     }
 }
 
@@ -176,7 +195,7 @@ impl Backplane {
         for handler in self.dma_handlers.lock().iter_mut() {
             handler.1.handle(
                 self,
-                DMACardAccessor {
+                DMACardAccessorBuilder {
                     backplane: self,
                     card_no: handler.0,
                 },