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// 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 ).
//! Collection of traits for providers that support type erasure of data structs.
use crate::error::Error;
use crate::prelude::*;
use crate::yoke::*;
use alloc::boxed::Box;
use alloc::rc::Rc;
use core::any::Any;
use core::any::TypeId;
/// Auto-implemented trait allowing for type erasure of data provider structs.
///
/// Requires the static lifetime in order to be convertible to [`Any`].
pub trait ErasedDataStruct: 'static {
/// Return this boxed trait object as [`Box`]`<dyn `[`Any`]`>`.
///
/// # Examples
///
/// ```
/// use icu_provider::erased::ErasedDataStruct;
/// use icu_provider::hello_world::HelloWorldV1;
///
/// // Create type-erased box
/// let erased: Box<dyn ErasedDataStruct> = Box::new(HelloWorldV1::default());
///
/// // Convert to typed box
/// let boxed: Box<HelloWorldV1> = erased.into_any().downcast().expect("Types should match");
/// ```
fn into_any(self: Box<Self>) -> Box<dyn Any>;
fn into_any_rc(self: Rc<Self>) -> Rc<dyn Any>;
/// Return this trait object reference as `&dyn `[`Any`].
///
/// Also see associated method [`downcast_ref()`](trait.ErasedDataStruct.html#method.downcast_ref).
///
/// # Examples
///
/// ```
/// use icu_provider::erased::ErasedDataStruct;
/// use icu_provider::hello_world::HelloWorldV1;
///
/// // Create type-erased reference
/// let data = HelloWorldV1::default();
/// let erased: &dyn ErasedDataStruct = &data;
///
/// // Borrow as typed reference
/// let borrowed: &HelloWorldV1 = erased.as_any().downcast_ref().expect("Types should match");
/// ```
fn as_any(&self) -> &dyn Any;
}
impl ZeroCopyFrom<dyn ErasedDataStruct> for &'static dyn ErasedDataStruct {
#[allow(clippy::needless_lifetimes)]
fn zero_copy_from<'b>(this: &'b (dyn ErasedDataStruct)) -> &'b dyn ErasedDataStruct {
this
}
}
/// Marker type for [`ErasedDataStruct`].
pub struct ErasedDataStructMarker {}
impl DataMarker<'static> for ErasedDataStructMarker {
type Yokeable = &'static dyn ErasedDataStruct;
type Cart = dyn ErasedDataStruct;
}
impl<'data, M> crate::dynutil::UpcastDataPayload<'static, M> for ErasedDataStructMarker
where
M: DataMarker<'static>,
M::Cart: Sized,
{
/// Upcast for ErasedDataStruct creates an `Rc<dyn ErasedDataStruct>` from the current inner
/// `Yoke` (i.e., `Rc::from(yoke)`).
fn upcast(other: DataPayload<'static, M>) -> DataPayload<'static, ErasedDataStructMarker> {
use crate::data_provider::DataPayloadInner::*;
let cart: Rc<dyn ErasedDataStruct> = match other.inner {
RcStruct(yoke) => Rc::from(yoke),
Owned(yoke) => Rc::from(yoke),
RcBuf(yoke) => Rc::from(yoke),
};
DataPayload::from_partial_owned(cart)
}
}
impl<'data> DataPayload<'static, ErasedDataStructMarker> {
/// Convert this [`DataPayload`] of an [`ErasedDataStruct`] into a [`DataPayload`] of a
/// concrete type.
///
/// Returns an error if the type is not compatible.
///
/// This is the main way to consume data returned from an [`ErasedDataProvider`].
///
/// Internally, this method reverses the transformation performed by
/// [`UpcastDataPayload::upcast`](crate::dynutil::UpcastDataPayload::upcast) as implemented
/// for [`ErasedDataStructMarker`].
///
/// # Examples
///
/// ```
/// use icu_provider::prelude::*;
/// use icu_provider::erased::*;
/// use icu_provider::hello_world::*;
/// use icu_locid_macros::langid;
///
/// let provider = HelloWorldProvider::new_with_placeholder_data();
///
/// let erased_payload: DataPayload<ErasedDataStructMarker> = provider
/// .load_payload(&DataRequest {
/// resource_path: ResourcePath {
/// key: key::HELLO_WORLD_V1,
/// options: ResourceOptions {
/// variant: None,
/// langid: Some(langid!("de")),
/// }
/// }
/// })
/// .expect("Loading should succeed")
/// .take_payload()
/// .expect("Data should be present");
///
/// let downcast_payload: DataPayload<HelloWorldV1Marker> = erased_payload
/// .downcast()
/// .expect("Types should match");
///
/// assert_eq!("Hallo Welt", downcast_payload.get().message);
/// ```
pub fn downcast<M>(self) -> Result<DataPayload<'static, M>, Error>
where
M: DataMarker<'static>,
M::Cart: Sized,
M::Yokeable: ZeroCopyFrom<M::Cart>,
{
use crate::data_provider::DataPayloadInner::*;
match self.inner {
RcStruct(yoke) => {
let any_rc: Rc<dyn Any> = yoke.into_backing_cart().into_any_rc();
// `any_rc` is the Yoke that was converted into the `dyn ErasedDataStruct`. It
// could have been either the RcStruct or the Owned variant of Yoke.
// Check first for Case 2: an RcStruct Yoke.
let y1 = any_rc.downcast::<Yoke<M::Yokeable, Rc<M::Cart>>>();
let any_rc = match y1 {
Ok(rc_yoke) => match Rc::try_unwrap(rc_yoke) {
Ok(yoke) => {
return Ok(DataPayload {
inner: RcStruct(yoke),
})
}
// Note: We could consider cloning the Yoke instead of erroring out.
Err(_) => return Err(Error::MultipleReferences),
},
Err(any_rc) => any_rc,
};
// Check for Case 3: an Owned Yoke.
let y2 = any_rc.downcast::<Yoke<M::Yokeable, ()>>();
let any_rc = match y2 {
Ok(rc_yoke) => match Rc::try_unwrap(rc_yoke) {
Ok(yoke) => return Ok(DataPayload { inner: Owned(yoke) }),
// Note: We could consider cloning the Yoke instead of erroring out.
Err(_) => return Err(Error::MultipleReferences),
},
Err(any_rc) => any_rc,
};
// Check for Case 4: an RcBuf Yoke.
let y2 = any_rc.downcast::<Yoke<M::Yokeable, Rc<[u8]>>>();
let any_rc = match y2 {
Ok(rc_yoke) => match Rc::try_unwrap(rc_yoke) {
Ok(yoke) => return Ok(DataPayload { inner: RcBuf(yoke) }),
// Note: We could consider cloning the Yoke instead of erroring out.
Err(_) => return Err(Error::MultipleReferences),
},
Err(any_rc) => any_rc,
};
// None of the downcasts succeeded; return an error.
Err(Error::MismatchedType {
actual: Some(any_rc.type_id()),
generic: Some(TypeId::of::<M::Cart>()),
})
}
// This is unreachable because ErasedDataStruct cannot be fully owned, since it
// contains a reference.
Owned(_) => unreachable!(),
// This is unreachable because ErasedDataStruct needs to reference an object.
RcBuf(_) => unreachable!(),
}
}
}
impl<T> ErasedDataStruct for T
where
T: Any,
for<'a> &'a T: Clone,
{
fn into_any(self: Box<Self>) -> Box<dyn Any> {
self
}
fn into_any_rc(self: Rc<Self>) -> Rc<dyn Any> {
self
}
fn as_any(&self) -> &dyn Any {
self
}
}
/// A type-erased data provider that loads a payload of types implementing [`Any`].
///
/// Note: This trait is redundant with [`DataProvider`]`<dyn `[`ErasedDataStruct`]`>` and auto-implemented
/// for all types implementing that trait. This trait may eventually be removed when the following
/// Rust issues are resolved:
///
/// - [#41517](https://github.com/rust-lang/rust/issues/41517) (trait aliases are not supported)
/// - [#68636](https://github.com/rust-lang/rust/issues/68636) (identical traits can't be auto-implemented)
pub trait ErasedDataProvider<'data> {
/// Query the provider for data, returning the result as an [`ErasedDataStruct`] trait object.
///
/// Returns [`Ok`] if the request successfully loaded data. If data failed to load, returns an
/// Error with more information.
fn load_erased(
&self,
req: &DataRequest,
) -> Result<DataResponse<'static, ErasedDataStructMarker>, Error>;
}
// Auto-implement `ErasedDataProvider` on types implementing `DataProvider<dyn ErasedDataStruct>`
impl<'data, T> ErasedDataProvider<'data> for T
where
T: DataProvider<'static, ErasedDataStructMarker>,
{
fn load_erased(
&self,
req: &DataRequest,
) -> Result<DataResponse<'static, ErasedDataStructMarker>, Error> {
DataProvider::<ErasedDataStructMarker>::load_payload(self, req)
}
}
impl<'data, M> DataProvider<'static, M> for dyn ErasedDataProvider<'data> + 'data
where
M: DataMarker<'static>,
<M::Yokeable as Yokeable<'static>>::Output: Clone + Any,
M::Yokeable: ZeroCopyFrom<M::Cart>,
M::Cart: Sized,
{
/// Serve [`Sized`] objects from an [`ErasedDataProvider`] via downcasting.
fn load_payload(&self, req: &DataRequest) -> Result<DataResponse<'static, M>, Error> {
let result = ErasedDataProvider::load_erased(self, req)?;
Ok(DataResponse {
metadata: result.metadata,
payload: result.payload.map(|p| p.downcast()).transpose()?,
})
}
}
#[cfg(test)]
mod test {
use super::*;
use crate::dynutil::UpcastDataPayload;
use crate::marker::CowStringMarker;
use alloc::borrow::Cow;
#[test]
fn test_erased_case_2() {
let data = Rc::new("foo".to_string());
let original = DataPayload::<CowStringMarker>::from_partial_owned(data);
let upcasted = ErasedDataStructMarker::upcast(original);
let downcasted = upcasted
.downcast::<CowStringMarker>()
.expect("Type conversion");
assert_eq!(downcasted.get(), "foo");
}
#[test]
fn test_erased_case_3() {
let data = "foo".to_string();
let original = DataPayload::<CowStringMarker>::from_owned(Cow::Owned(data));
let upcasted = ErasedDataStructMarker::upcast(original);
let downcasted = upcasted
.downcast::<CowStringMarker>()
.expect("Type conversion");
assert_eq!(downcasted.get(), "foo");
}
#[test]
fn test_erased_case_4() {
let data: Rc<[u8]> = "foo".as_bytes().into();
let original = DataPayload::<CowStringMarker>::try_from_rc_buffer_badly(data, |bytes| {
core::str::from_utf8(bytes).map(|s| Cow::Borrowed(s))
})
.expect("String is valid UTF-8");
let upcasted = ErasedDataStructMarker::upcast(original);
let downcasted = upcasted
.downcast::<CowStringMarker>()
.expect("Type conversion");
assert_eq!(downcasted.get(), "foo");
}
}