errors: lazily load fallback fluent bundle

Loading the fallback bundle in compilation sessions that won't go on to
emit any errors unnecessarily degrades compile time performance, so
lazily create the Fluent bundle when it is first required.

Signed-off-by: David Wood <[email protected]>

Author: davidtwco

src/parse/session.rs

@@ -36,7 +36,7 @@ impl Emitter for SilentEmitter {
     fn fluent_bundle(&self) -> Option<&Lrc<rustc_errors::FluentBundle>> {
         None
     }
-    fn fallback_fluent_bundle(&self) -> &Lrc<rustc_errors::FluentBundle> {
+    fn fallback_fluent_bundle(&self) -> &rustc_errors::FluentBundle {
         panic!("silent emitter attempted to translate a diagnostic");
     }
 }
@@ -93,7 +93,7 @@ impl Emitter for SilentOnIgnoredFilesEmitter {
         self.emitter.fluent_bundle()
     }
 
-    fn fallback_fluent_bundle(&self) -> &Lrc<rustc_errors::FluentBundle> {
+    fn fallback_fluent_bundle(&self) -> &rustc_errors::FluentBundle {
         self.emitter.fallback_fluent_bundle()
     }
 }
@@ -114,8 +114,8 @@ fn default_handler(
     let emitter = if hide_parse_errors {
         silent_emitter()
     } else {
-        let fallback_bundle = rustc_errors::fallback_fluent_bundle(false)
-            .expect("failed to load fallback fluent bundle");
+        let fallback_bundle =
+            rustc_errors::fallback_fluent_bundle(rustc_errors::DEFAULT_LOCALE_RESOURCES, false);
         Box::new(EmitterWriter::stderr(
             color_cfg,
             Some(source_map.clone()),
@@ -350,7 +350,7 @@ mod tests {
             fn fluent_bundle(&self) -> Option<&Lrc<rustc_errors::FluentBundle>> {
                 None
             }
-            fn fallback_fluent_bundle(&self) -> &Lrc<rustc_errors::FluentBundle> {
+            fn fallback_fluent_bundle(&self) -> &rustc_errors::FluentBundle {
                 panic!("test emitter attempted to translate a diagnostic");
             }
         }