Convenience funcs for some_option.unwrap_or(...)

This ensures consistent handling of default values for options that are
None if not specified on the command line.

Author: Rich Kadel

src/driver.rs

@@ -85,7 +85,7 @@ impl rustc_driver::Callbacks for ClippyCallbacks {
         // run on the unoptimized MIR. On the other hand this results in some false negatives. If
         // MIR passes can be enabled / disabled separately, we should figure out, what passes to
         // use for Clippy.
-        config.opts.debugging_opts.mir_opt_level = Some(0);
+        config.opts.debugging_opts.mir_opt_level = 0;
     }
 }