[metalos/netos] clean rust-check

Summary:
When making large refactors (like the following diff in this stack),
`arc rust-check` is a super valuable tool.
But the noisier existing code is, the less useful it is.
I asked Devmate to clean it up and it actually worked pretty well.

Test Plan:
```
❯ arc rust-check fbcode//metalos/... fbcode//netos/... fbcode//edge_cloud/echos/...
Buck UI: https://www.internalfb.com/buck2/3a186d72-051e-4299-a882-ac9845e80e6b
Network: Up: 0B  Down: 0B
Loading targets.   Remaining     0/1                                                                                                                                                                                                   30 targets declared
Analyzing targets. Remaining     0/483
Executing actions. Remaining     0/1798
Command: bxl.
Time elapsed: 2.9s
BXL SUCCEEDED
```

Reviewed By: vjt

Differential Revision: D85584087

fbshipit-source-id: 7df23970315d7c0f5cea15a90cad7c0a4ddbf434

Author: vmagro

antlir/bzl/shape2/templates/struct.rs.handlebars

@@ -89,6 +89,7 @@ where
         ];
         let _ = p.read_struct_begin(|_| ())?;
         loop {
+            #[allow(unused_variables)]
             let (_, fty, fid) = p.read_field_begin(|_| (), FIELDS)?;
             if fty == ::fbthrift::TType::Stop {
                 break;