Replace ParseSess::set_dcx with DiagCtxt::set_emitter

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

compiler/rustc_errors/src/lib.rs

@@ -712,6 +712,10 @@ fn fallback_fluent_bundle(&self) -> &FluentBundle {
         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 @@ pub fn proc_macro_quoted_spans(&self) -> impl Iterator<Item = (usize, Span)> + '
     pub fn dcx(&self) -> DiagCtxtHandle<'_> {
         self.dcx.handle()
     }
-
-    pub fn set_dcx(&mut self, dcx: DiagCtxt) {
-        self.dcx = dcx;
-    }
 }

src/doc/rustc-dev-guide/examples/rustc-interface-getting-diagnostics.rs

@@ -10,6 +10,8 @@
 extern crate rustc_session;
 extern crate rustc_span;
 
+use std::sync::{Arc, Mutex};
+
 use rustc_errors::emitter::Emitter;
 use rustc_errors::registry::{self, Registry};
 use rustc_errors::translation::Translate;
@@ -17,8 +19,6 @@
 use rustc_session::config;
 use rustc_span::source_map::SourceMap;
 
-use std::sync::{Arc, Mutex};
-
 struct DebugEmitter {
     source_map: Arc<SourceMap>,
     diagnostics: Arc<Mutex<Vec<DiagInner>>>,
@@ -67,10 +67,10 @@ fn main() {
         locale_resources: rustc_driver::DEFAULT_LOCALE_RESOURCES.to_owned(),
         lint_caps: rustc_hash::FxHashMap::default(),
         psess_created: Some(Box::new(|parse_sess| {
-            parse_sess.set_dcx(DiagCtxt::new(Box::new(DebugEmitter {
+            parse_sess.dcx().set_emitter(Box::new(DebugEmitter {
                 source_map: parse_sess.clone_source_map(),
                 diagnostics,
-            })));
+            }));
         })),
         register_lints: None,
         override_queries: None,