[GitHub] [arrow-rs] stuartcarnie commented on a diff in pull request #3887: feat: Add Commands enum to decode prost messages to strong type

["stuartcarnie (via GitHub)" <gi...@apache.org>]

stuartcarnie commented on code in PR #3887:
URL: https://github.com/apache/arrow-rs/pull/3887#discussion_r1142749508


##########
arrow-flight/src/sql/mod.rs:
##########
@@ -71,22 +72,60 @@ pub trait ProstMessageExt: prost::Message + Default {
     fn as_any(&self) -> Any;
 }
 
+/// Macro to coerce a token to an item, specifically
+/// to build the `Commands` enum.
+///
+/// See: <https://danielkeep.github.io/tlborm/book/blk-ast-coercion.html>
+macro_rules! as_item {
+    ($i:item) => {
+        $i
+    };
+}
+
 macro_rules! prost_message_ext {
-    ($($name:ty,)*) => {
-        $(
-            impl ProstMessageExt for $name {
-                fn type_url() -> &'static str {
-                    concat!("type.googleapis.com/arrow.flight.protocol.sql.", stringify!($name))
+    ($($name:tt,)*) => {
+        paste! {
+            $(
+            const [<$name:snake:upper _TYPE_URL>]: &'static str = concat!("type.googleapis.com/arrow.flight.protocol.sql.", stringify!($name));
+            )*
+
+            as_item! {
+                pub enum Commands {

Review Comment:
   Good call – do you think adding some docs to the enum and then describing using the `TryFrom<Any>` trait to decoding them?



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