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>

compiler/rustc_errors/src/diagnostic.rs

@@ -851,18 +851,11 @@ impl Diagnostic {
         self
     }
 
-    /// Add a subdiagnostic from a type that implements `Subdiagnostic` (see
-    /// [rustc_macros::Subdiagnostic]).
-    pub fn subdiagnostic(&mut self, subdiagnostic: impl AddToDiagnostic) -> &mut Self {
-        subdiagnostic.add_to_diagnostic(self);
-        self
-    }
-
     /// Add a subdiagnostic from a type that implements `Subdiagnostic` (see
     /// [rustc_macros::Subdiagnostic]). Performs eager translation of any translatable messages
     /// used in the subdiagnostic, so suitable for use with repeated messages (i.e. re-use of
     /// interpolated variables).
-    pub fn eager_subdiagnostic(
+    pub fn subdiagnostic(
         &mut self,
         dcx: &crate::DiagCtxt,
         subdiagnostic: impl AddToDiagnostic,
@@ -921,7 +914,7 @@ impl Diagnostic {
     /// Helper function that takes a `SubdiagnosticMessage` and returns a `DiagnosticMessage` by
     /// combining it with the primary message of the diagnostic (if translatable, otherwise it just
     /// passes the user's string along).
-    fn subdiagnostic_message_to_diagnostic_message(
+    pub(crate) fn subdiagnostic_message_to_diagnostic_message(
         &self,
         attr: impl Into<SubdiagnosticMessage>,
     ) -> DiagnosticMessage {