bootstrap: work around "unused attribute" errors in incremental stdlib rebuilds.

src/bootstrap/builder.rs

@@ -1099,6 +1099,13 @@ pub fn cargo(
 
             if self.config.deny_warnings {
                 rustflags.arg("-Dwarnings");
+
+                // FIXME(#58633) hide "unused attribute" errors in incremental
+                // builds of the standard library, as the underlying checks are
+                // not yet properly integrated with incremental recompilation.
+                if mode == Mode::Std && compiler.stage == 0 && self.config.incremental {
+                    rustflags.arg("-Aunused-attributes");
+                }
             }
         }