errors: implement fallback diagnostic translation

This commit updates the signatures of all diagnostic functions to accept
types that can be converted into a `DiagnosticMessage`. This enables
existing diagnostic calls to continue to work as before and Fluent
identifiers to be provided. The `SessionDiagnostic` derive just
generates normal diagnostic calls, so these APIs had to be modified to
accept Fluent identifiers.

In addition, loading of the "fallback" Fluent bundle, which contains the
built-in English messages, has been implemented.

Each diagnostic now has "arguments" which correspond to variables in the
Fluent messages (necessary to render a Fluent message) but no API for
adding arguments has been added yet. Therefore, diagnostics (that do not
require interpolation) can be converted to use Fluent identifiers and
will be output as before.

Author: davidtwco

src/parse/session.rs

@@ -33,6 +33,12 @@ impl Emitter for SilentEmitter {
         None
     }
     fn emit_diagnostic(&mut self, _db: &Diagnostic) {}
+    fn fluent_bundle(&self) -> Option<&Lrc<rustc_errors::FluentBundle>> {
+        None
+    }
+    fn fallback_fluent_bundle(&self) -> &Lrc<rustc_errors::FluentBundle> {
+        panic!("silent emitter attempted to translate a diagnostic");
+    }
 }
 
 fn silent_emitter() -> Box<dyn Emitter + Send> {
@@ -82,6 +88,14 @@ impl Emitter for SilentOnIgnoredFilesEmitter {
         }
         self.handle_non_ignoreable_error(db);
     }
+
+    fn fluent_bundle(&self) -> Option<&Lrc<rustc_errors::FluentBundle>> {
+        self.emitter.fluent_bundle()
+    }
+
+    fn fallback_fluent_bundle(&self) -> &Lrc<rustc_errors::FluentBundle> {
+        self.emitter.fallback_fluent_bundle()
+    }
 }
 
 fn default_handler(
@@ -100,9 +114,11 @@ fn default_handler(
     let emitter = if hide_parse_errors {
         silent_emitter()
     } else {
+        let fallback_bundle = rustc_errors::fallback_fluent_bundle();
         Box::new(EmitterWriter::stderr(
             color_cfg,
             Some(source_map.clone()),
+            fallback_bundle,
             false,
             false,
             None,
@@ -329,6 +345,12 @@ mod tests {
             fn emit_diagnostic(&mut self, _db: &Diagnostic) {
                 self.num_emitted_errors.fetch_add(1, Ordering::Release);
             }
+            fn fluent_bundle(&self) -> Option<&Lrc<rustc_errors::FluentBundle>> {
+                None
+            }
+            fn fallback_fluent_bundle(&self) -> &Lrc<rustc_errors::FluentBundle> {
+                panic!("test emitter attempted to translate a diagnostic");
+            }
         }
 
         fn build_diagnostic(level: DiagnosticLevel, span: Option<MultiSpan>) -> Diagnostic {