Stop stripping serde attributes

To still prevent 'unknown attribute' errors, the attribute is registered
using a dummy derive macro where needed.

ruma-api-macros/src/api.rs

@@ -4,7 +4,7 @@ use proc_macro2::TokenStream;
 use quote::{quote, ToTokens};
 use syn::{
     parse::{Parse, ParseStream},
-    Field, Token, Type,
+    Token, Type,
 };
 
 pub(crate) mod attribute;
@@ -15,13 +15,6 @@ pub(crate) mod response;
 use self::{metadata::Metadata, request::Request, response::Response};
 use crate::util;
 
-/// Removes `serde` attributes from struct fields.
-pub fn strip_serde_attrs(field: &Field) -> Field {
-    let mut field = field.clone();
-    field.attrs.retain(|attr| !attr.path.is_ident("serde"));
-    field
-}
-
 /// The result of processing the `ruma_api` macro, ready for output back to source code.
 pub struct Api {
     /// The `metadata` section of the macro.

ruma-api-macros/src/api/request.rs

@@ -12,10 +12,7 @@ use syn::{
 };
 
 use crate::{
-    api::{
-        attribute::{Meta, MetaNameValue},
-        strip_serde_attrs,
-    },
+    api::attribute::{Meta, MetaNameValue},
     util,
 };
 
@@ -450,8 +447,7 @@ impl ToTokens for Request {
         let request_def = if self.fields.is_empty() {
             quote!(;)
         } else {
-            let fields =
-                self.fields.iter().map(|request_field| strip_serde_attrs(request_field.field()));
+            let fields = self.fields.iter().map(|request_field| request_field.field());
             quote! { { #(#fields),* } }
         };
 
@@ -540,7 +536,7 @@ impl ToTokens for Request {
         };
 
         let request = quote! {
-            #[derive(Debug, Clone, #ruma_serde::Outgoing)]
+            #[derive(Debug, Clone, #ruma_serde::Outgoing, #ruma_serde::_FakeDeriveSerde)]
             #[cfg_attr(not(feature = "unstable-exhaustive-types"), non_exhaustive)]
             #[incoming_derive(!Deserialize)]
             #( #struct_attributes )*

ruma-api-macros/src/api/response.rs

@@ -12,10 +12,7 @@ use syn::{
 };
 
 use crate::{
-    api::{
-        attribute::{Meta, MetaNameValue},
-        strip_serde_attrs,
-    },
+    api::attribute::{Meta, MetaNameValue},
     util,
 };
 
@@ -320,9 +317,7 @@ impl ToTokens for Response {
         let response_def = if self.fields.is_empty() {
             quote!(;)
         } else {
-            let fields =
-                self.fields.iter().map(|response_field| strip_serde_attrs(response_field.field()));
-
+            let fields = self.fields.iter().map(|response_field| response_field.field());
             quote! { { #(#fields),* } }
         };
 
@@ -347,7 +342,7 @@ impl ToTokens for Response {
         };
 
         let response = quote! {
-            #[derive(Debug, Clone, #ruma_serde::Outgoing)]
+            #[derive(Debug, Clone, #ruma_serde::Outgoing, #ruma_serde::_FakeDeriveSerde)]
             #[cfg_attr(not(feature = "unstable-exhaustive-types"), non_exhaustive)]
             #[incoming_derive(!Deserialize)]
             #( #struct_attributes )*

ruma-serde-macros/src/lib.rs

@@ -125,3 +125,12 @@ pub fn derive_string_enum(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as ItemEnum);
     expand_all(input).unwrap_or_else(|err| err.to_compile_error()).into()
 }
+
+/// A derive macro that generates no code, but registers the serde attribute so both `#[serde(...)]`
+/// and `#[cfg_attr(..., serde(...))]` are accepted on the type, its fields and (in case the input
+/// is an enum) variants fields.
+#[doc(hidden)]
+#[proc_macro_derive(_FakeDeriveSerde, attributes(serde))]
+pub fn fake_derive_serde(_input: TokenStream) -> TokenStream {
+    TokenStream::new()
+}

ruma-serde-macros/src/outgoing.rs

@@ -45,9 +45,12 @@ pub fn expand_derive_outgoing(input: DeriveInput) -> syn::Result<TokenStream> {
                 }
             }),
     );
-    if derive_deserialize {
-        derives.push(quote! { #ruma_serde::exports::serde::Deserialize });
-    }
+
+    derives.push(if derive_deserialize {
+        quote! { #ruma_serde::exports::serde::Deserialize }
+    } else {
+        quote! { #ruma_serde::_FakeDeriveSerde }
+    });
 
     let input_attrs =
         input.attrs.iter().filter(|attr| filter_input_attrs(attr)).collect::<Vec<_>>();