Add icu_provider_adapter::delegate module with helper macro

[sffc]

See it in use in #6852

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[sffc]

/gemini review

[gemini-code-assist]

Code Review

This pull request adds a new delegate module containing a helper macro data_provider_to_field!. The overall approach is good, with clear documentation and tests. I've identified a significant limitation in the macro's implementation that restricts its usability to only direct fields of a struct. I've provided a suggestion to make the macro more flexible by allowing delegation to nested fields, which would greatly improve its utility.

[gemini-code-assist]

The current macro implementation is very restrictive as it only allows delegating to a direct field of self. The use of $field:tt matches only a single token tree, so it won't work for nested fields like self.foo.bar.

To make the macro more flexible and support nested field access, you can change $field:tt to $($field:tt)+. This will match one or more token trees, allowing for expressions like self.a.b.c.

Suggested change

	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.$field, req)
	}
	}
	};
	}
	macro_rules! __data_provider_to_field {
	($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)
	}
	}
	};
	}

[robertbastian]

this is a very small implementation which I'm not sure is worth a macro that has to be special cased for different providers

[Manishearth]

I'm ambivalent on this. I think it's fine, but I'm not strongly in favor of adding such a macro.

[sffc]

I see it less about the macro and more about blessing this pattern as one we want to encourage, with a docs page, etc.

[robertbastian]

so just write a docs page? the macro seems brittle and learning how to use the macro is more effort than writing the redirecting impl

[CLAassistant]

All committers have signed the CLA.