Overhaul Diagnostic args.

First, introduce a typedef `DiagnosticArgMap`.

Second, make the `args` field public, and remove the `args` getter and
`replace_args` setter. These were necessary previously because the getter
had a `#[allow(rustc::potential_query_instability)]` attribute, but that
was removed in #120931 when the args were changed from `FxHashMap` to
`FxIndexMap`. (All the other `Diagnostic` fields are public.)

compiler/rustc_errors/src/lib.rs

@@ -37,9 +37,10 @@ extern crate self as rustc_errors;
 
 pub use codes::*;
 pub use diagnostic::{
-    AddToDiagnostic, BugAbort, DecorateLint, Diagnostic, DiagnosticArg, DiagnosticArgName,
-    DiagnosticArgValue, DiagnosticBuilder, DiagnosticStyledString, EmissionGuarantee, FatalAbort,
-    IntoDiagnostic, IntoDiagnosticArg, StringPart, SubDiagnostic, SubdiagnosticMessageOp,
+    AddToDiagnostic, BugAbort, DecorateLint, Diagnostic, DiagnosticArg, DiagnosticArgMap,
+    DiagnosticArgName, DiagnosticArgValue, DiagnosticBuilder, DiagnosticStyledString,
+    EmissionGuarantee, FatalAbort, IntoDiagnostic, IntoDiagnosticArg, StringPart, SubDiagnostic,
+    SubdiagnosticMessageOp,
 };
 pub use diagnostic_impls::{
     DiagnosticArgFromDisplay, DiagnosticSymbolList, ExpectedLifetimeParameter,
@@ -1482,9 +1483,8 @@ impl DiagCtxtInner {
         diag: &Diagnostic,
         msg: impl Into<SubdiagnosticMessage>,
     ) -> SubdiagnosticMessage {
-        let args = diag.args();
         let msg = diag.subdiagnostic_message_to_diagnostic_message(msg);
-        self.eagerly_translate(msg, args)
+        self.eagerly_translate(msg, diag.args.iter())
     }
 
     fn flush_delayed(&mut self) {