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>>,
@@ -1275,7 +1318,7 @@ 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());

src/test/ui-fulldeps/compiler-calls.rs

@@ -26,13 +26,8 @@ fn main() {
     let mut count = 1;
     let args = vec!["compiler-calls".to_string(), "foo.rs".to_string()];
     rustc_driver::catch_fatal_errors(|| {
-        rustc_driver::run_compiler(
-            &args,
-            &mut TestCalls { count: &mut count },
-            None,
-            None,
-            None,
-        ).ok();
-    }).ok();
+        rustc_driver::RunCompiler::new(&args, &mut TestCalls { count: &mut count }).run().ok();
+    })
+    .ok();
     assert_eq!(count, 2);
 }

src/tools/clippy/src/driver.rs

@@ -357,7 +357,7 @@ pub fn main() {
                 args.extend(vec!["--sysroot".into(), sys_root]);
             };
 
-            return rustc_driver::run_compiler(&args, &mut DefaultCallbacks, None, None, None);
+            return rustc_driver::RunCompiler::new(&args, &mut DefaultCallbacks).run();
         }
 
         if orig_args.iter().any(|a| a == "--version" || a == "-V") {
@@ -420,6 +420,6 @@ pub fn main() {
         let mut default = DefaultCallbacks;
         let callbacks: &mut (dyn rustc_driver::Callbacks + Send) =
             if clippy_enabled { &mut clippy } else { &mut default };
-        rustc_driver::run_compiler(&args, callbacks, None, None, None)
+        rustc_driver::RunCompiler::new(&args, callbacks).run()
     }))
 }