From 0b7e94626b6409fcaa4024088246a32327468248 Mon Sep 17 00:00:00 2001
From: David Renshaw <dwrenshaw@gmail.com>
Date: Mon, 23 Sep 2024 14:10:21 -0400
Subject: [PATCH] add dynamic_struct::{Builder, Reader}::downcast()

---
 capnp/src/dynamic_struct.rs | 24 ++++++++++++++++++++++++
 capnp/src/dynamic_value.rs  | 18 +++---------------
 2 files changed, 27 insertions(+), 15 deletions(-)

diff --git a/capnp/src/dynamic_struct.rs b/capnp/src/dynamic_struct.rs
index a8a4c866b..aecafc010 100644
--- a/capnp/src/dynamic_struct.rs
+++ b/capnp/src/dynamic_struct.rs
@@ -232,6 +232,18 @@ impl<'a> Reader<'a> {
         let field = self.schema.get_field_by_name(field_name)?;
         self.has(field)
     }
+
+    /// Downcasts the `Reader` into a specific struct type. Panics if the
+    /// expected type does not match the value.
+    pub fn downcast<T: crate::traits::OwnedStruct>(self) -> T::Reader<'a> {
+        assert!(
+            Into::<crate::introspect::Type>::into(crate::introspect::TypeVariant::Struct(
+                self.schema.raw
+            ))
+            .may_downcast_to(T::introspect())
+        );
+        self.reader.into()
+    }
 }
 
 /// A mutable dynamically-typed struct.
@@ -773,6 +785,18 @@ impl<'a> Builder<'a> {
         }
         Ok(())
     }
+
+    /// Downcasts the `Builder` into a specific struct type. Panics if the
+    /// expected type does not match the value.
+    pub fn downcast<T: crate::traits::OwnedStruct>(self) -> T::Builder<'a> {
+        assert!(
+            Into::<crate::introspect::Type>::into(crate::introspect::TypeVariant::Struct(
+                self.schema.raw
+            ))
+            .may_downcast_to(T::introspect())
+        );
+        self.builder.into()
+    }
 }
 
 impl<'a> crate::traits::SetterInput<crate::any_pointer::Owned> for Reader<'a> {
diff --git a/capnp/src/dynamic_value.rs b/capnp/src/dynamic_value.rs
index 8ef956327..3a049edf6 100644
--- a/capnp/src/dynamic_value.rs
+++ b/capnp/src/dynamic_value.rs
@@ -77,14 +77,8 @@ impl<'a> Reader<'a> {
     /// 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 sb: dynamic_struct::Reader = self.downcast();
-        assert!(
-            Into::<crate::introspect::Type>::into(crate::introspect::TypeVariant::Struct(
-                sb.schema.raw
-            ))
-            .may_downcast_to(T::introspect())
-        );
-        sb.reader.into()
+        let sr: dynamic_struct::Reader = self.downcast();
+        sr.downcast::<T>()
     }
 }
 
@@ -243,13 +237,7 @@ impl<'a> Builder<'a> {
     /// seem to allow this.
     pub fn downcast_struct<T: crate::traits::OwnedStruct>(self) -> T::Builder<'a> {
         let sb: dynamic_struct::Builder = self.downcast();
-        assert!(
-            Into::<crate::introspect::Type>::into(crate::introspect::TypeVariant::Struct(
-                sb.schema.raw
-            ))
-            .may_downcast_to(T::introspect())
-        );
-        sb.builder.into()
+        sb.downcast::<T>()
     }
 }