macros: Ensure that crates using ruma_api macro have client and server features

crates/ruma-macros/Cargo.toml

@@ -18,8 +18,11 @@ proc-macro = true
 compat = []
 
 [dependencies]
+once_cell = "1.13.0"
 proc-macro-crate = "1.0.0"
 proc-macro2 = "1.0.24"
 quote = "1.0.8"
 ruma-identifiers-validation = { version = "0.8.1", path = "../ruma-identifiers-validation", default-features = false }
+serde = { version = "1.0.139", features = ["derive"] }
 syn = { version = "1.0.57", features = ["extra-traits", "full", "visit"] }
+toml = "0.5.9"

crates/ruma-macros/src/api.rs

@@ -1,7 +1,11 @@
 //! Methods and types for generating API endpoints.
 
-use proc_macro2::TokenStream;
+use std::{env, fs, path::Path};
+
+use once_cell::sync::Lazy;
+use proc_macro2::{Span, TokenStream};
 use quote::quote;
+use serde::{de::IgnoredAny, Deserialize};
 use syn::{
     braced,
     parse::{Parse, ParseStream},
@@ -46,6 +50,8 @@ pub struct Api {
 
 impl Api {
     pub fn expand_all(self) -> TokenStream {
+        let maybe_error = ensure_feature_presence().map(syn::Error::to_compile_error);
+
         let ruma_common = import_ruma_common();
         let http = quote! { #ruma_common::exports::http };
 
@@ -73,6 +79,8 @@ impl Api {
         let metadata_doc = format!("Metadata for the `{}` API endpoint.", name.value());
 
         quote! {
+            #maybe_error
+
             #[doc = #metadata_doc]
             pub const METADATA: #ruma_common::api::Metadata = #ruma_common::api::Metadata {
                 description: #description,
@@ -158,3 +166,52 @@ fn parse_response(input: ParseStream<'_>, attributes: Vec<Attribute>) -> syn::Re
 
     Ok(Response { attributes, fields, response_kw })
 }
+
+// Returns an error with a helpful error if the crate `ruma_api!` is used from doesn't declare both
+// a `client` and a `server` feature.
+fn ensure_feature_presence() -> Option<&'static syn::Error> {
+    #[derive(Deserialize)]
+    struct CargoToml {
+        features: Features,
+    }
+
+    #[derive(Deserialize)]
+    struct Features {
+        client: Option<IgnoredAny>,
+        server: Option<IgnoredAny>,
+    }
+
+    static RESULT: Lazy<Result<(), syn::Error>> = Lazy::new(|| {
+        let manifest_dir = env::var("CARGO_MANIFEST_DIR")
+            .map_err(|_| syn::Error::new(Span::call_site(), "Failed to read CARGO_MANIFEST_DIR"))?;
+
+        let manifest_file = Path::new(&manifest_dir).join("Cargo.toml");
+        let manifest_bytes = fs::read(manifest_file)
+            .map_err(|_| syn::Error::new(Span::call_site(), "Failed to read Cargo.toml"))?;
+
+        let manifest_parsed: CargoToml = toml::from_slice(&manifest_bytes)
+            .map_err(|_| syn::Error::new(Span::call_site(), "Failed to parse Cargo.toml"))?;
+
+        if manifest_parsed.features.client.is_none() {
+            return Err(syn::Error::new(
+                Span::call_site(),
+                "This crate doesn't define a `client` feature in its `Cargo.toml`.\n\
+                 Please add a `client` feature such that generated `OutgoingRequest` and \
+                 `IncomingResponse` implementations can be enabled.",
+            ));
+        }
+
+        if manifest_parsed.features.server.is_none() {
+            return Err(syn::Error::new(
+                Span::call_site(),
+                "This crate doesn't define a `server` feature in its `Cargo.toml`.\n\
+                 Please add a `server` feature such that generated `IncomingRequest` and \
+                 `OutgoingResponse` implementations can be enabled.",
+            ));
+        }
+
+        Ok(())
+    });
+
+    RESULT.as_ref().err()
+}