driver: Extract handling of --explain to separate function

src/librustc_driver/lib.rs

@@ -329,15 +329,9 @@ pub trait CompilerCalls<'a> {
 #[derive(Copy, Clone)]
 pub struct RustcDefaultCalls;
 
-impl<'a> CompilerCalls<'a> for RustcDefaultCalls {
-    fn early_callback(&mut self,
-                      matches: &getopts::Matches,
-                      _sopts: &config::Options,
+fn handle_explain(code: &str,
                   descriptions: &diagnostics::registry::Registry,
-                      output: ErrorOutputType)
-                      -> Compilation {
-        match matches.opt_str("explain") {
-            Some(ref code) => {
+                  output: ErrorOutputType) {
     let normalised = if !code.starts_with("E") {
         format!("E{0:0>4}", code)
     } else {
@@ -352,12 +346,21 @@ impl<'a> CompilerCalls<'a> for RustcDefaultCalls {
             early_error(output, &format!("no extended information for {}", code));
         }
     }
+}
+
+impl<'a> CompilerCalls<'a> for RustcDefaultCalls {
+    fn early_callback(&mut self,
+                      matches: &getopts::Matches,
+                      _sopts: &config::Options,
+                      descriptions: &diagnostics::registry::Registry,
+                      output: ErrorOutputType)
+                      -> Compilation {
+        if let Some(ref code) = matches.opt_str("explain") {
+            handle_explain(code, descriptions, output);
             return Compilation::Stop;
         }
-            None => (),
-        }
 
-        return Compilation::Continue;
+        Compilation::Continue
     }
 
     fn no_input(&mut self,