Add missing DiagnosticBuilder::eager_diagnostic method.

This lets us avoid the use of `DiagnosticBuilder::into_diagnostic` in
miri, when then means that `DiagnosticBuilder::into_diagnostic` can
become private, being now only used by `stash` and `buffer`.

Author: nnethercote

Diff for: compiler/rustc_errors/src/diagnostic_builder.rs

@@ -265,7 +265,7 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
 
     /// Converts the builder to a `Diagnostic` for later emission,
     /// unless dcx has disabled such buffering.
-    pub fn into_diagnostic(mut self) -> Option<(Diagnostic, &'a DiagCtxt)> {
+    fn into_diagnostic(mut self) -> Option<(Diagnostic, &'a DiagCtxt)> {
         let flags = self.dcx.inner.lock().flags;
         if flags.dont_buffer_diagnostics || flags.treat_err_as_bug.is_some() {
             self.emit();
@@ -427,6 +427,10 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
     forward!((subdiagnostic, with_subdiagnostic)(
         subdiagnostic: impl crate::AddToDiagnostic,
     ));
+    forward!((eager_subdiagnostic, with_eager_subdiagnostic)(
+        dcx: &DiagCtxt,
+        subdiagnostic: impl crate::AddToDiagnostic,
+    ));
 }
 
 impl<G: EmissionGuarantee> Debug for DiagnosticBuilder<'_, G> {

Diff for: src/tools/miri/src/diagnostics.rs

@@ -499,22 +499,20 @@ pub fn report_msg<'tcx>(
         err.note(if extra_span { "BACKTRACE (of the first span):" } else { "BACKTRACE:" });
     }
 
-    let (mut err, handler) = err.into_diagnostic().unwrap();
-
     // Add backtrace
     for (idx, frame_info) in stacktrace.iter().enumerate() {
         let is_local = machine.is_local(frame_info);
         // No span for non-local frames and the first frame (which is the error site).
         if is_local && idx > 0 {
-            err.eager_subdiagnostic(handler, frame_info.as_note(machine.tcx));
+            err.eager_subdiagnostic(err.dcx, frame_info.as_note(machine.tcx));
         } else {
             let sm = sess.source_map();
             let span = sm.span_to_embeddable_string(frame_info.span);
             err.note(format!("{frame_info} at {span}"));
         }
     }
 
-    handler.emit_diagnostic(err);
+    err.emit();
 }
 
 impl<'mir, 'tcx> MiriMachine<'mir, 'tcx> {