From 0098506094adb842135a4b3bdcb8a69036155a8f Mon Sep 17 00:00:00 2001
From: "Shane F. Carr" <shane@unicode.org>
Date: Wed, 13 Aug 2025 23:56:15 -0700
Subject: [PATCH] Add icu_provider_adapter::delegate module with helper macro

---
 provider/adapters/src/delegate.rs | 97 +++++++++++++++++++++++++++++++
 provider/adapters/src/lib.rs      |  4 ++
 2 files changed, 101 insertions(+)
 create mode 100644 provider/adapters/src/delegate.rs

diff --git a/provider/adapters/src/delegate.rs b/provider/adapters/src/delegate.rs
new file mode 100644
index 00000000000..936c795c09a
--- /dev/null
+++ b/provider/adapters/src/delegate.rs
@@ -0,0 +1,97 @@
+// This file is part of ICU4X. For terms of use, please see the file
+// called LICENSE at the top level of the ICU4X source tree
+// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
+
+//! Macros for delegating impls to inner data providers.
+
+/// Delegate an impl of `DataProvider<M>` to a field of a struct.
+///
+/// # Examples
+///
+/// ```
+/// use icu_provider::hello_world::*;
+/// use icu_locale::locale;
+///
+/// struct Wrap(HelloWorldProvider);
+///
+/// // Delegate `DataProvider<HelloWorldV1>` to the field `self.0`
+/// icu_provider_adapters::delegate::data_provider_to_field!(Wrap, HelloWorldV1, &self.0);
+///
+/// // Test that it works
+/// let wrap = Wrap(HelloWorldProvider::default());
+/// HelloWorldFormatter::try_new_unstable(&wrap, locale!("de").into()).unwrap();
+/// ```
+///
+/// Also works if the field is a [`BufferProvider`]:
+///
+/// ```
+/// use icu_provider::hello_world::*;
+/// use icu_locale::locale;
+///
+/// struct Wrap(HelloWorldJsonProvider);
+///
+/// // Delegate `DataProvider<HelloWorldV1>` to the field `self.0`, calling `as_deserializing()` on the field
+/// icu_provider_adapters::delegate::data_provider_to_field!(Wrap, HelloWorldV1, &self.0.as_deserializing());
+///
+/// // Test that it works
+/// let wrap = Wrap(HelloWorldProvider::default().into_json_provider());
+/// HelloWorldFormatter::try_new_unstable(&wrap, locale!("de").into()).unwrap();
+/// ```
+///
+/// [`BufferProvider`]: icu_provider::prelude::BufferProvider
+#[doc(hidden)] // macro
+#[macro_export]
+macro_rules! __data_provider_to_field {
+    ($provider:path, $marker:path, &self.$field:tt.as_deserializing()) => {
+        impl $crate::icu_provider::DataProvider<$marker> for $provider {
+            fn load(&self, req: $crate::icu_provider::DataRequest) -> Result<$crate::icu_provider::DataResponse<$marker>, $crate::icu_provider::DataError> {
+                let provider = $crate::icu_provider::prelude::AsDeserializingBufferProvider::as_deserializing(&self.$field);
+                $crate::icu_provider::DataProvider::<$marker>::load(&provider, req)
+            }
+        }
+    };
+    ($provider:path, $marker:path, &self.$field:tt) => {
+        impl $crate::icu_provider::DataProvider<$marker> for $provider {
+            fn load(&self, req: $crate::icu_provider::DataRequest) -> Result<$crate::icu_provider::DataResponse<$marker>, $crate::icu_provider::DataError> {
+                $crate::icu_provider::DataProvider::<$marker>::load(&self.$field, req)
+            }
+        }
+    };
+}
+
+#[doc(inline)]
+pub use __data_provider_to_field as data_provider_to_field;
+
+#[cfg(test)]
+mod tests {
+    use icu_locale::locale;
+    use icu_provider::hello_world::{
+        HelloWorldFormatter, HelloWorldJsonProvider, HelloWorldProvider, HelloWorldV1,
+    };
+
+    use super::*;
+
+    #[test]
+    fn test_delegate() {
+        struct Wrap(HelloWorldProvider);
+        data_provider_to_field!(Wrap, HelloWorldV1, &self.0);
+
+        let hello1 = HelloWorldProvider::default();
+        let wrap = Wrap(hello1);
+
+        let formatter = HelloWorldFormatter::try_new_unstable(&wrap, locale!("de").into()).unwrap();
+        assert_eq!(formatter.format_to_string(), "Hallo Welt");
+    }
+
+    #[test]
+    fn test_delegate_to_buffer() {
+        struct Wrap(HelloWorldJsonProvider);
+        data_provider_to_field!(Wrap, HelloWorldV1, &self.0.as_deserializing());
+
+        let hello1 = HelloWorldProvider::default().into_json_provider();
+        let wrap = Wrap(hello1);
+
+        let formatter = HelloWorldFormatter::try_new_unstable(&wrap, locale!("de").into()).unwrap();
+        assert_eq!(formatter.format_to_string(), "Hallo Welt");
+    }
+}
diff --git a/provider/adapters/src/lib.rs b/provider/adapters/src/lib.rs
index 2b172a5d733..d238009a3a9 100644
--- a/provider/adapters/src/lib.rs
+++ b/provider/adapters/src/lib.rs
@@ -28,9 +28,13 @@
 
 extern crate alloc;
 
+pub mod delegate;
 pub mod either;
 pub mod empty;
 pub mod fallback;
 pub mod filter;
 pub mod fixed;
 pub mod fork;
+
+#[doc(hidden)] // internal for macros
+pub use icu_provider;