errors: only eagerly translate subdiagnostics

Subdiagnostics don't need to be lazily translated, they can always be
eagerly translated. Eager translation is slightly more complex as we need
to have a `DiagCtxt` available to perform the translation, which involves
slightly more threading of that context.

This slight increase in complexity should enable later simplifications -
like passing `DiagCtxt` into `AddToDiagnostic` and moving Fluent messages
into the diagnostic structs rather than having them in separate files
(working on that was what led to this change).

Signed-off-by: David Wood <david@davidtw.co>

src/parse/session.rs

@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::path::Path;
 use std::sync::atomic::{AtomicBool, Ordering};
 
@@ -40,6 +41,16 @@ impl Translate for SilentEmitter {
     fn fallback_fluent_bundle(&self) -> &rustc_errors::FluentBundle {
         panic!("silent emitter attempted to translate a diagnostic");
     }
+
+    // Override `translate_message` for the silent emitter because eager translation of
+    // subdiagnostics result in a call to this.
+    fn translate_message<'a>(
+        &'a self,
+        message: &'a rustc_errors::DiagnosticMessage,
+        _: &'a rustc_errors::translation::FluentArgs<'_>,
+    ) -> Result<Cow<'_, str>, rustc_errors::error::TranslateError<'_>> {
+        rustc_errors::emitter::silent_translate(message)
+    }
 }
 
 impl Emitter for SilentEmitter {