buck2/rust: turn unused-dependencies errors into actual errors

Summary:
For Reasons, `rustc` does not exit with an error status for unused
dependency errors (rust-lang/rust#96068).

We can promote them to proper errors in the rustc_action script.
Also improve how the diagnostics are rendered to make them a bit more
readable.

Reviewed By: dtolnay

Differential Revision: D35690034

fbshipit-source-id: a2571f9d6682a83527bfca9ce73dc474162a6f9c

prelude/rust/tools/rustc_action.py

@@ -102,13 +102,26 @@ async def handle_output(proc, args, crate_map):
 
         # Add more information to unused crate warnings
         if diag.get("unused_extern_names", []) != []:
+            # Treat error-level unused dep warnings as errors
+            if diag.get("lint_level") in ("deny", "forbid"):
+                got_error_diag = True
+
             unused_names = diag["unused_extern_names"]
 
             if args.buck_target:
+                rendered_unused = []
+                for name in unused_names:
+                    if name in crate_map:
+                        rendered_unused.append("{}: {}".format(crate_map[name], name))
+                    else:
+                        rendered_unused.append("{}".format(name))
+                rendered_unused.sort()
+                rendered_unused = "\n    ".join(rendered_unused)
+
                 diag["buck_target"] = args.buck_target
                 diag[
                     "rendered"
-                ] = f"Target `{args.buck_target}` has unused dependencies: {','.join(unused_names)}"
+                ] = f"Target `{args.buck_target}` has unused dependencies:\n    {rendered_unused}"
             diag["unused_deps"] = {
                 name: crate_map[name] for name in unused_names if name in crate_map
             }
@@ -160,6 +173,10 @@ async def main():
             f"res={repr(res)} got_error_diag={got_error_diag} args.failure_filter {args.failure_filter}"
         )
 
+    # If rustc is reporting a silent error, make it loud
+    if res == 0 and got_error_diag:
+        res = 1
+
     # Check for death by signal - this is always considered a failure
     if res < 0:
         cmdline = " ".join(shlex.quote(arg) for arg in args.rustc)