add design notes

capnproto · Sep 23, 2024 · d1d3b01 · d1d3b01
1 parent 436de1b
commit d1d3b01
Showing 1 changed file with 10 additions and 0 deletions.
diff --git a/capnp/src/dynamic_value.rs b/capnp/src/dynamic_value.rs
@@ -71,6 +71,11 @@ impl<'a> Reader<'a> {
 
     /// Downcasts the `Reader` into a specific struct type. Panics if the
     /// expected type does not match the value.
+    ///
+    /// Design note: instead of this method, it would be better to add a blanket impl
+    /// of the `DowncastBuilder` trait that covered every struct type. Unfortunately,
+    /// the current way the `Introspect` and `OwnedStruct` traits are set up does not
+    /// seem to allow this.
     pub fn downcast_struct<T: crate::traits::OwnedStruct>(self) -> T::Reader<'a> {
         let sr: dynamic_struct::Reader = self.downcast();
         let TypeVariant::Struct(rs) = T::introspect().which() else {
@@ -231,6 +236,11 @@ impl<'a> Builder<'a> {
 
     /// Downcasts the `Builder` into a specific struct type. Panics if the
     /// expected type does not match the value.
+    ///
+    /// Design note: instead of this method, it would be better to add a blanket impl
+    /// of the `DowncastBuilder` trait that covered every struct type. Unfortunately,
+    /// the current way the `Introspect` and `OwnedStruct` traits are set up does not
+    /// seem to allow this.
     pub fn downcast_struct<T: crate::traits::OwnedStruct>(self) -> T::Builder<'a> {
         let sb: dynamic_struct::Builder = self.downcast();
         let TypeVariant::Struct(rs) = T::introspect().which() else {