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 <david.wood@huawei.com>

compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs

@@ -8,8 +8,8 @@
 use crate::emitter::FileWithAnnotatedLines;
 use crate::snippet::Line;
 use crate::{
-    CodeSuggestion, Diagnostic, DiagnosticId, DiagnosticMessage, Emitter, FluentBundle, Level,
-    MultiSpan, Style, SubDiagnostic,
+    CodeSuggestion, Diagnostic, DiagnosticId, DiagnosticMessage, Emitter, FluentBundle,
+    LazyFallbackBundle, Level, MultiSpan, Style, SubDiagnostic,
 };
 use annotate_snippets::display_list::{DisplayList, FormatOptions};
 use annotate_snippets::snippet::*;
@@ -22,7 +22,7 @@ use rustc_span::SourceFile;
 pub struct AnnotateSnippetEmitterWriter {
     source_map: Option<Lrc<SourceMap>>,
     fluent_bundle: Option<Lrc<FluentBundle>>,
-    fallback_bundle: Lrc<FluentBundle>,
+    fallback_bundle: LazyFallbackBundle,
 
     /// If true, hides the longer explanation text
     short_message: bool,
@@ -67,8 +67,8 @@ impl Emitter for AnnotateSnippetEmitterWriter {
         self.fluent_bundle.as_ref()
     }
 
-    fn fallback_fluent_bundle(&self) -> &Lrc<FluentBundle> {
-        &self.fallback_bundle
+    fn fallback_fluent_bundle(&self) -> &FluentBundle {
+        &**self.fallback_bundle
     }
 
     fn should_show_explain(&self) -> bool {
@@ -101,7 +101,7 @@ impl AnnotateSnippetEmitterWriter {
     pub fn new(
         source_map: Option<Lrc<SourceMap>>,
         fluent_bundle: Option<Lrc<FluentBundle>>,
-        fallback_bundle: Lrc<FluentBundle>,
+        fallback_bundle: LazyFallbackBundle,
         short_message: bool,
         macro_backtrace: bool,
     ) -> Self {