ref(proguard): Redeclare mappings as immutable

To make it easier to reason about the code when working on #2328, we redeclare the `mappings` variable as immutable after we are done constructing it.

Author: szokeasaurusrex

src/commands/upload_proguard.rs

@@ -214,6 +214,9 @@ pub fn execute(matches: &ArgMatches) -> Result<()> {
         }
     }
 
+    // We are done constructing the mappings, redeclare as immutable.
+    let mappings = mappings;
+
     let api = Api::current();
     let config = Config::current();