fix: Do not try to connect via postcard to proc-macro-srv

src/tools/rust-analyzer/crates/proc-macro-api/src/lib.rs

@@ -129,7 +129,7 @@ pub fn spawn<'a>(
         let process = ProcMacroServerProcess::run(
             process_path,
             env,
-            process::Protocol::Postcard { mode: SpanMode::Id },
+            process::Protocol::LegacyJson { mode: SpanMode::Id },
         )?;
         Ok(ProcMacroClient { process: Arc::new(process), path: process_path.to_owned() })
     }

src/tools/rust-analyzer/crates/proc-macro-api/src/process.rs

@@ -30,8 +30,13 @@ pub(crate) struct ProcMacroServerProcess {
 
 #[derive(Debug, Clone)]
 pub(crate) enum Protocol {
-    LegacyJson { mode: SpanMode },
-    Postcard { mode: SpanMode },
+    LegacyJson {
+        mode: SpanMode,
+    },
+    #[expected(dead_code)]
+    Postcard {
+        mode: SpanMode,
+    },
 }
 
 /// Maintains the state of the proc-macro server process.