Merge interface::run_compiler calls.

`rustc_driver_impl::run_compiler` currently has two
`interface::run_compiler` calls: one for the "no input" case, and one
for the normal case.

This commit merges the former into the latter, which makes the control
flow easier to read and avoids some duplication.

It also makes it clearer that the "no input" case will describe lints
before printing crate info, while the normal case does it in the reverse
order. Possibly a bug?

Author: nnethercote

compiler/rustc_driver_impl/src/lib.rs

@@ -338,41 +338,14 @@ fn run_compiler(
         expanded_args: args,
     };
 
-    match make_input(&default_handler, &matches.free) {
+    let has_input = match make_input(&default_handler, &matches.free) {
         Err(reported) => return Err(reported),
         Ok(Some(input)) => {
             config.input = input;
-
-            callbacks.config(&mut config);
+            true // has input: normal compilation
         }
         Ok(None) => match matches.free.len() {
-            0 => {
-                callbacks.config(&mut config);
-
-                default_handler.abort_if_errors();
-
-                interface::run_compiler(config, |compiler| {
-                    let sopts = &compiler.session().opts;
-                    let handler = EarlyErrorHandler::new(sopts.error_format);
-
-                    if sopts.describe_lints {
-                        describe_lints(compiler.session());
-                        return;
-                    }
-                    let should_stop = print_crate_info(
-                        &handler,
-                        &**compiler.codegen_backend(),
-                        compiler.session(),
-                        false,
-                    );
-
-                    if should_stop == Compilation::Stop {
-                        return;
-                    }
-                    handler.early_error("no input filename given")
-                });
-                return Ok(());
-            }
+            0 => false, // no input: we will exit early
             1 => panic!("make_input should have provided valid inputs"),
             _ => default_handler.early_error(format!(
                 "multiple input filenames provided (first two filenames are `{}` and `{}`)",
@@ -381,12 +354,32 @@ fn run_compiler(
         },
     };
 
+    callbacks.config(&mut config);
+
     default_handler.abort_if_errors();
+    drop(default_handler);
 
     interface::run_compiler(config, |compiler| {
         let sess = compiler.session();
         let handler = EarlyErrorHandler::new(sess.opts.error_format);
 
+        if !has_input {
+            if sess.opts.describe_lints {
+                describe_lints(compiler.session());
+                return sess.compile_status();
+            }
+            let should_stop = print_crate_info(
+                &handler,
+                &**compiler.codegen_backend(),
+                compiler.session(),
+                false,
+            );
+            if should_stop == Compilation::Continue {
+                handler.early_error("no input filename given")
+            }
+            return sess.compile_status();
+        }
+
         let should_stop = print_crate_info(&handler, &**compiler.codegen_backend(), sess, true)
             .and_then(|| {
                 list_metadata(&handler, sess, &*compiler.codegen_backend().metadata_loader())