derive: Add delegate_to option for ExtensionDispatch

This patch adds a delegate_to option to the ExtensionDispatch derive
macro that makes it possible to create an alias for a backend using a
different set of extensions.  To avoid misunderstandings, delegating
backends are forced to use the unit type ().

derive/examples/extension-dispatch.rs

@@ -1,7 +1,9 @@
 use trussed::Error;
 
 mod backends {
-    use super::extensions::{TestExtension, TestReply, TestRequest};
+    use super::extensions::{
+        SampleExtension, SampleReply, SampleRequest, TestExtension, TestReply, TestRequest,
+    };
     use trussed::{
         backend::Backend, platform::Platform, serde_extensions::ExtensionImpl,
         service::ServiceResources, types::CoreContext, Error,
@@ -26,6 +28,18 @@ mod backends {
         }
     }
 
+    impl ExtensionImpl<SampleExtension> for ABackend {
+        fn extension_request<P: Platform>(
+            &mut self,
+            _core_ctx: &mut CoreContext,
+            _backend_ctx: &mut Self::Context,
+            _request: &SampleRequest,
+            _resources: &mut ServiceResources<P>,
+        ) -> Result<SampleReply, Error> {
+            Ok(SampleReply)
+        }
+    }
+
     #[derive(Default)]
     pub struct BBackend;
 
@@ -88,6 +102,7 @@ mod extensions {
 
 enum Backend {
     A,
+    ASample,
     B,
 }
 
@@ -106,6 +121,11 @@ enum Extension {
 struct Dispatch {
     #[extensions("Test")]
     a: backends::ABackend,
+
+    #[dispatch(delegate_to = "a")]
+    #[extensions("Sample")]
+    a_sample: (),
+
     b: backends::BBackend,
 }
 
@@ -135,5 +155,9 @@ fn main() {
         &[BackendId::Custom(Backend::B)],
         Some(Error::RequestNotAvailable),
     );
+    run(
+        &[BackendId::Custom(Backend::ASample)],
+        Some(Error::RequestNotAvailable),
+    );
     run(&[BackendId::Custom(Backend::A)], None);
 }

derive/src/extension_dispatch.rs

@@ -15,6 +15,7 @@ pub struct ExtensionDispatch {
     dispatch_attrs: DispatchAttrs,
     extension_attrs: ExtensionAttrs,
     backends: Vec<Backend>,
+    delegated_backends: Vec<DelegatedBackend>,
 }
 
 impl ExtensionDispatch {
@@ -27,18 +28,37 @@ impl ExtensionDispatch {
         };
         let dispatch_attrs = DispatchAttrs::new(&input)?;
         let extension_attrs = ExtensionAttrs::new(&input)?;
-        let backends = data_struct
+        let raw_backends: Vec<_> = data_struct
             .fields
             .iter()
-            .enumerate()
-            .map(|(i, field)| Backend::new(i, field, &extension_attrs.extensions))
+            .map(RawBackend::new)
+            .collect::<Result<_>>()?;
+        let mut backends = Vec::new();
+        let mut delegated_backends = Vec::new();
+        for raw_backend in raw_backends {
+            if let Some(delegate_to) = raw_backend.delegate_to.clone() {
+                delegated_backends.push((raw_backend, delegate_to));
+            } else {
+                backends.push(Backend::new(
+                    backends.len(),
+                    raw_backend,
+                    &extension_attrs.extensions,
+                )?);
+            }
+        }
+        let delegated_backends = delegated_backends
+            .into_iter()
+            .map(|(raw, delegate_to)| {
+                DelegatedBackend::new(raw, delegate_to, &backends, &extension_attrs.extensions)
+            })
             .collect::<Result<_>>()?;
         Ok(Self {
             name: input.ident,
             generics: input.generics,
             dispatch_attrs,
             extension_attrs,
             backends,
+            delegated_backends,
         })
     }
 
@@ -49,7 +69,15 @@ impl ExtensionDispatch {
         let (impl_generics, ty_generics, where_clause) = self.generics.split_for_impl();
         let context = self.backends.iter().map(Backend::context);
         let requests = self.backends.iter().map(Backend::request);
+        let delegated_requests = self
+            .delegated_backends
+            .iter()
+            .map(DelegatedBackend::request);
         let extension_requests = self.backends.iter().map(Backend::extension_request);
+        let delegated_extension_requests = self
+            .delegated_backends
+            .iter()
+            .map(DelegatedBackend::extension_request);
         let extension_impls = self
             .extension_attrs
             .extensions
@@ -71,6 +99,7 @@ impl ExtensionDispatch {
                 ) -> ::core::result::Result<::trussed::api::Reply, ::trussed::error::Error> {
                     match backend {
                         #(#requests)*
+                        #(#delegated_requests)*
                     }
                 }
 
@@ -84,6 +113,7 @@ impl ExtensionDispatch {
                 ) -> ::core::result::Result<::trussed::api::reply::SerdeExtension, ::trussed::error::Error> {
                     match backend {
                         #(#extension_requests)*
+                        #(#delegated_extension_requests)*
                     }
                 }
             }
@@ -165,32 +195,70 @@ impl ExtensionAttrs {
     }
 }
 
-struct Backend {
+struct RawBackend {
     id: Ident,
     field: Ident,
     ty: Type,
-    index: Index,
-    extensions: Vec<Extension>,
+    delegate_to: Option<Ident>,
+    extensions: Vec<Ident>,
 }
 
-impl Backend {
-    fn new(i: usize, field: &Field, extension_types: &HashMap<Ident, Path>) -> Result<Self> {
+impl RawBackend {
+    fn new(field: &Field) -> Result<Self> {
         let ident = field.ident.clone().ok_or_else(|| {
             Error::new_spanned(
                 field,
                 "ExtensionDispatch can only be derived for a struct with named fields",
             )
         })?;
+        let mut delegate_to = None;
+        for attr in util::get_attrs(&field.attrs, "dispatch") {
+            attr.parse_nested_meta(|meta| {
+                if meta.path.is_ident("delegate_to") {
+                    let s: LitStr = meta.value()?.parse()?;
+                    delegate_to = Some(s.parse()?);
+                    Ok(())
+                } else {
+                    Err(meta.error("unsupported dispatch attribute"))
+                }
+            })?;
+        }
         let mut extensions = Vec::new();
         for attr in util::get_attrs(&field.attrs, "extensions") {
             for s in attr.parse_args_with(Punctuated::<LitStr, Token![,]>::parse_terminated)? {
-                extensions.push(Extension::new(&s, extension_types)?);
+                extensions.push(s.parse()?);
             }
         }
         Ok(Self {
             id: util::to_camelcase(&ident),
             field: ident,
             ty: field.ty.clone(),
+            delegate_to,
+            extensions,
+        })
+    }
+}
+
+#[derive(Clone)]
+struct Backend {
+    id: Ident,
+    field: Ident,
+    ty: Type,
+    index: Index,
+    extensions: Vec<Extension>,
+}
+
+impl Backend {
+    fn new(i: usize, raw: RawBackend, extensions: &HashMap<Ident, Path>) -> Result<Self> {
+        let extensions = raw
+            .extensions
+            .into_iter()
+            .map(|i| Extension::new(i, extensions))
+            .collect::<Result<_>>()?;
+        Ok(Self {
+            id: raw.id,
+            field: raw.field,
+            ty: raw.ty,
             index: Index::from(i),
             extensions,
         })
@@ -224,17 +292,98 @@ impl Backend {
     }
 }
 
+struct DelegatedBackend {
+    id: Ident,
+    field: Ident,
+    backend: Backend,
+    extensions: Vec<Extension>,
+}
+
+impl DelegatedBackend {
+    fn new(
+        raw: RawBackend,
+        delegate_to: Ident,
+        backends: &[Backend],
+        extensions: &HashMap<Ident, Path>,
+    ) -> Result<Self> {
+        match raw.ty {
+            Type::Tuple(tuple) if tuple.elems.is_empty() => (),
+            _ => {
+                return Err(Error::new_spanned(
+                    &raw.ty,
+                    "delegated backends must use the unit type ()",
+                ));
+            }
+        }
+
+        let extensions = raw
+            .extensions
+            .into_iter()
+            .map(|i| Extension::new(i, extensions))
+            .collect::<Result<_>>()?;
+        let backend = backends
+            .iter()
+            .find(|backend| backend.field == delegate_to)
+            .ok_or_else(|| Error::new_spanned(delegate_to, "unknown backend"))?
+            .clone();
+        Ok(Self {
+            id: raw.id,
+            field: raw.field,
+            backend,
+            extensions,
+        })
+    }
+
+    fn request(&self) -> TokenStream {
+        let Self {
+            id, backend, field, ..
+        } = self;
+        let Backend {
+            field: delegated_field,
+            index: delegated_index,
+            ..
+        } = backend;
+        quote! {
+            Self::BackendId::#id => {
+                let _ = self.#field;
+                ::trussed::backend::Backend::request(
+                    &mut self.#delegated_field, &mut ctx.core, &mut ctx.backends.#delegated_index, request, resources,
+                )
+            }
+        }
+    }
+
+    fn extension_request(&self) -> TokenStream {
+        let Self {
+            id,
+            extensions,
+            backend,
+            field,
+        } = self;
+        let extension_requests = extensions.iter().map(|e| e.extension_request(backend));
+        quote! {
+            Self::BackendId::#id => {
+                let _ = self.#field;
+                match extension {
+                    #(#extension_requests)*
+                    _ => Err(::trussed::error::Error::RequestNotAvailable),
+                }
+            }
+        }
+    }
+}
+
+#[derive(Clone)]
 struct Extension {
     id: Ident,
     ty: Path,
 }
 
 impl Extension {
-    fn new(s: &LitStr, extensions: &HashMap<Ident, Path>) -> Result<Self> {
-        let id = s.parse()?;
+    fn new(id: Ident, extensions: &HashMap<Ident, Path>) -> Result<Self> {
         let ty = extensions
             .get(&id)
-            .ok_or_else(|| Error::new_spanned(s, "unknown extension ID"))?
+            .ok_or_else(|| Error::new_spanned(&id, "unknown extension ID"))?
             .clone();
         Ok(Self { id, ty })
     }