Add NeverMarker (#4674)

Fixes #4186

provider/core/src/lib.rs

@@ -186,6 +186,7 @@ pub use crate::buf::BufferMarker;
 pub use crate::buf::BufferProvider;
 pub use crate::marker::DataMarker;
 pub use crate::marker::KeyedDataMarker;
+pub use crate::marker::NeverMarker;
 #[cfg(feature = "serde")]
 pub use crate::serde::AsDeserializingBufferProvider;
 

provider/core/src/marker.rs

@@ -4,7 +4,9 @@
 
 //! Marker types and traits for DataProvider.
 
-use crate::key::DataKey;
+use core::marker::PhantomData;
+
+use crate::{data_key, key::DataKey};
 use yoke::Yokeable;
 
 /// Trait marker for data structs. All types delivered by the data provider must be associated with
@@ -83,3 +85,107 @@ pub trait KeyedDataMarker: DataMarker {
  /// The single [`DataKey`] associated with this marker.
  const KEY: DataKey;
 }
+
+/// A [`DataMarker`] that never returns data.
+///
+/// All types that have non-blanket impls of `DataProvider<M>` are expected to explicitly
+/// implement `DataProvider<NeverMarker<Y>>`, returning [`DataErrorKind::MissingDataKey`].
+/// See [`impl_data_provider_never_marker!`].
+///
+/// [`DataErrorKind::MissingDataKey`]: crate::DataErrorKind::MissingDataKey
+/// [`impl_data_provider_never_marker!`]: crate::impl_data_provider_never_marker
+///
+/// # Examples
+///
+/// ```
+/// use icu_locid::locale;
+/// use icu_provider::hello_world::*;
+/// use icu_provider::prelude::*;
+/// use icu_provider::NeverMarker;
+///
+/// let buffer_provider = HelloWorldProvider.into_json_provider();
+///
+/// let result = DataProvider::<NeverMarker<HelloWorldV1<'static>>>::load(
+/// &buffer_provider.as_deserializing(),
+/// DataRequest {
+/// locale: &locale!("en").into(),
+/// metadata: Default::default(),
+/// },
+/// );
+///
+/// assert!(matches!(
+/// result,
+/// Err(DataError {
+/// kind: DataErrorKind::MissingDataKey,
+/// ..
+/// })
+/// ));
+/// ```
+#[derive(Debug, Copy, Clone)]
+pub struct NeverMarker<Y>(PhantomData<Y>);
+
+impl<Y> DataMarker for NeverMarker<Y>
+where
+ for<'a> Y: Yokeable<'a>,
+{
+ type Yokeable = Y;
+}
+
+impl<Y> KeyedDataMarker for NeverMarker<Y>
+where
+ for<'a> Y: Yokeable<'a>,
+{
+ const KEY: DataKey = data_key!("_never@1");
+}
+
+/// Implements `DataProvider<NeverMarker<Y>>` on a struct.
+///
+/// For more information, see [`NeverMarker`].
+///
+/// # Examples
+///
+/// ```
+/// use icu_locid::locale;
+/// use icu_provider::hello_world::*;
+/// use icu_provider::prelude::*;
+/// use icu_provider::NeverMarker;
+///
+/// struct MyProvider;
+///
+/// icu_provider::impl_data_provider_never_marker!(MyProvider);
+///
+/// let result = DataProvider::<NeverMarker<HelloWorldV1<'static>>>::load(
+/// &MyProvider,
+/// DataRequest {
+/// locale: &locale!("und").into(),
+/// metadata: Default::default(),
+/// },
+/// );
+///
+/// assert!(matches!(
+/// result,
+/// Err(DataError {
+/// kind: DataErrorKind::MissingDataKey,
+/// ..
+/// })
+/// ));
+/// ```
+#[macro_export]
+macro_rules! impl_data_provider_never_marker {
+ ($ty:path) => {
+ impl<Y> $crate::DataProvider<$crate::NeverMarker<Y>> for $ty
+ where
+ for<'a> Y: $crate::yoke::Yokeable<'a>,
+ {
+ fn load(
+ &self,
+ req: $crate::DataRequest,
+ ) -> Result<$crate::DataResponse<$crate::NeverMarker<Y>>, $crate::DataError> {
+ Err($crate::DataErrorKind::MissingDataKey.with_req(
+ <$crate::NeverMarker<Y> as $crate::KeyedDataMarker>::KEY,
+ req,
+ ))
+ }
+ }
+ };
+}

provider/datagen/src/baked_exporter.rs

@@ -657,6 +657,7 @@ impl BakedExporter {
  #[allow(dead_code)]
  pub const MUST_USE_MAKE_PROVIDER_MACRO: () = ();
  }
+ icu_provider::impl_data_provider_never_marker!($name);
  };
  }
  #[doc(inline)]

provider/datagen/src/provider.rs

@@ -33,6 +33,7 @@ pub struct DatagenProvider {
 }
 
 crate::registry::make_exportable_provider!(DatagenProvider);
+icu_provider::impl_data_provider_never_marker!(DatagenProvider);
 
 impl DatagenProvider {
  /// The latest CLDR JSON tag that has been verified to work with this version of `icu_datagen`.