Reuse views when moving between diagnostic view and editors

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/gpui/src/app.rs

@@ -3097,14 +3097,39 @@ impl Drop for AnyViewHandle {
 
 pub struct AnyModelHandle {
     model_id: usize,
+    model_type: TypeId,
     ref_counts: Arc<Mutex<RefCounts>>,
 }
 
+impl AnyModelHandle {
+    pub fn downcast<T: Entity>(self) -> Option<ModelHandle<T>> {
+        if self.is::<T>() {
+            let result = Some(ModelHandle {
+                model_id: self.model_id,
+                model_type: PhantomData,
+                ref_counts: self.ref_counts.clone(),
+            });
+            unsafe {
+                Arc::decrement_strong_count(&self.ref_counts);
+            }
+            std::mem::forget(self);
+            result
+        } else {
+            None
+        }
+    }
+
+    pub fn is<T: Entity>(&self) -> bool {
+        self.model_type == TypeId::of::<T>()
+    }
+}
+
 impl<T: Entity> From<ModelHandle<T>> for AnyModelHandle {
     fn from(handle: ModelHandle<T>) -> Self {
         handle.ref_counts.lock().inc_model(handle.model_id);
         Self {
             model_id: handle.model_id,
+            model_type: TypeId::of::<T>(),
             ref_counts: handle.ref_counts.clone(),
         }
     }