Replace run_compiler with RunCompiler builder pattern.

RunCompiler::new takes non-optional params, and optional
params can be set using set_*field_name* method.
finally `run` will forward all fields to `run_compiler`.

compiler/rustc_driver/src/lib.rs

@@ -134,9 +134,52 @@ pub fn diagnostics_registry() -> Registry {
     Registry::new(&rustc_error_codes::DIAGNOSTICS)
 }
 
+pub struct RunCompiler<'a, 'b> {
+    at_args: &'a [String],
+    callbacks: &'b mut (dyn Callbacks + Send),
+    file_loader: Option<Box<dyn FileLoader + Send + Sync>>,
+    emitter: Option<Box<dyn Write + Send>>,
+    make_codegen_backend:
+        Option<Box<dyn FnOnce(&config::Options) -> Box<dyn CodegenBackend> + Send>>,
+}
+
+impl<'a, 'b> RunCompiler<'a, 'b> {
+    pub fn new(at_args: &'a [String], callbacks: &'b mut (dyn Callbacks + Send)) -> Self {
+        Self { at_args, callbacks, file_loader: None, emitter: None, make_codegen_backend: None }
+    }
+    pub fn set_make_codegen_backend(
+        &mut self,
+        make_codegen_backend: Option<
+            Box<dyn FnOnce(&config::Options) -> Box<dyn CodegenBackend> + Send>,
+        >,
+    ) -> &mut Self {
+        self.make_codegen_backend = make_codegen_backend;
+        self
+    }
+    pub fn set_emitter(&mut self, emitter: Option<Box<dyn Write + Send>>) -> &mut Self {
+        self.emitter = emitter;
+        self
+    }
+    pub fn set_file_loader(
+        &mut self,
+        file_loader: Option<Box<dyn FileLoader + Send + Sync>>,
+    ) -> &mut Self {
+        self.file_loader = file_loader;
+        self
+    }
+    pub fn run(self) -> interface::Result<()> {
+        run_compiler(
+            self.at_args,
+            self.callbacks,
+            self.file_loader,
+            self.emitter,
+            self.make_codegen_backend,
+        )
+    }
+}
 // Parse args and run the compiler. This is the primary entry point for rustc.
 // The FileLoader provides a way to load files from sources other than the file system.
-pub fn run_compiler(
+fn run_compiler(
     at_args: &[String],
     callbacks: &mut (dyn Callbacks + Send),
     file_loader: Option<Box<dyn FileLoader + Send + Sync>>,
@@ -1285,7 +1328,7 @@ pub fn main() -> ! {
                 })
             })
             .collect::<Vec<_>>();
-        run_compiler(&args, &mut callbacks, None, None, None)
+        RunCompiler::new(&args, &mut callbacks).run()
     });
     // The extra `\t` is necessary to align this label with the others.
     print_time_passes_entry(callbacks.time_passes, "\ttotal", start.elapsed());