Replace ParseSess::set_dcx with DiagCtxt::set_emitter

Replacing the error emitter doesn't accidentally clear the error count.

Author: bjorn3

compiler/rustc_errors/src/lib.rs

@@ -712,6 +712,10 @@ impl DiagCtxt {
         inner.emitter = new_emitter;
     }
 
+    pub fn set_emitter(&self, emitter: Box<dyn Emitter + DynSend>) {
+        self.inner.borrow_mut().emitter = emitter;
+    }
+
     /// Translate `message` eagerly with `args` to `SubdiagMessage::Eager`.
     pub fn eagerly_translate<'a>(
         &self,

compiler/rustc_session/src/parse.rs

@@ -337,8 +337,4 @@ impl ParseSess {
     pub fn dcx(&self) -> DiagCtxtHandle<'_> {
         self.dcx.handle()
     }
-
-    pub fn set_dcx(&mut self, dcx: DiagCtxt) {
-        self.dcx = dcx;
-    }
 }