Add an --output option for specifying an error emitter

src/libsyntax/errors/emitter.rs

@@ -43,7 +43,7 @@ pub trait Emitter {
 /// maximum number of lines we will print for each error; arbitrary.
 const MAX_LINES: usize = 6;
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum ColorConfig {
     Auto,
     Always,

src/libsyntax/errors/mod.rs

@@ -276,6 +276,7 @@ pub struct Handler {
 }
 
 impl Handler {
+    // TODO remove
     pub fn new(color_config: ColorConfig,
                registry: Option<diagnostics::registry::Registry>,
                can_emit_warnings: bool,