Add NonAuthEndpoint trait

This is a way to mark any type implementing `Endpoint` as not
requiring authentication information through the type system.
This is useful for clients that build on top of the ruma crates
to check this condition at compile time, avoiding a runtime error.

ruma-api-macros/src/api.rs

@@ -90,6 +90,14 @@ impl ToTokens for Api {
         let rate_limited = &self.metadata.rate_limited;
         let requires_authentication = &self.metadata.requires_authentication;
 
+        let non_auth_endpoint_impl = if requires_authentication.value {
+            quote! {
+                impl ruma_api::NonAuthEndpoint for Request {}
+            }
+        } else {
+            TokenStream::new()
+        };
+
         let request_type = &self.request;
         let response_type = &self.response;
 
@@ -507,6 +515,8 @@ impl ToTokens for Api {
                     requires_authentication: #requires_authentication,
                 };
             }
+
+            #non_auth_endpoint_impl
         };
 
         api.to_tokens(tokens);

src/lib.rs

@@ -236,6 +236,11 @@ pub trait Endpoint:
     const METADATA: Metadata;
 }
 
+/// A Matrix API endpoint that doesn't require authentication.
+///
+/// This marker trait is to indicate that a type implementing `Endpoint` doesn't require any authentication.
+pub trait NonAuthEndpoint: Endpoint {}
+
 /// Metadata about an API endpoint.
 #[derive(Clone, Debug)]
 pub struct Metadata {