Remove rustc_symbol_mangling/messages.ftl.

It contains a single message that (a) doesn't contain any natural
language, and (b) is only used in tests.

compiler/rustc_symbol_mangling/src/errors.rs

@@ -1,18 +1,32 @@
 //! Errors emitted by symbol_mangling.
 
-use rustc_errors::{DiagnosticArgValue, IntoDiagnosticArg};
-use rustc_macros::Diagnostic;
+use rustc_errors::{ErrorGuaranteed, IntoDiagnostic};
 use rustc_span::Span;
+use std::fmt;
 
-#[derive(Diagnostic)]
-#[diag(symbol_mangling_test_output)]
 pub struct TestOutput {
-    #[primary_span]
     pub span: Span,
     pub kind: Kind,
     pub content: String,
 }
 
+// This diagnostic doesn't need translation because (a) it doesn't contain any
+// natural language, and (b) it's only used in tests. So we construct it
+// manually and avoid the fluent machinery.
+impl IntoDiagnostic<'_> for TestOutput {
+    fn into_diagnostic(
+        self,
+        handler: &'_ rustc_errors::Handler,
+    ) -> rustc_errors::DiagnosticBuilder<'_, ErrorGuaranteed> {
+        let TestOutput { span, kind, content } = self;
+
+        #[allow(rustc::untranslatable_diagnostic)]
+        let mut diag = handler.struct_err(format!("{kind}({content})"));
+        diag.set_span(span);
+        diag
+    }
+}
+
 pub enum Kind {
     SymbolName,
     Demangling,
@@ -20,15 +34,13 @@ pub enum Kind {
     DefPath,
 }
 
-impl IntoDiagnosticArg for Kind {
-    fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
-        let kind = match self {
-            Kind::SymbolName => "symbol-name",
-            Kind::Demangling => "demangling",
-            Kind::DemanglingAlt => "demangling-alt",
-            Kind::DefPath => "def-path",
+impl fmt::Display for Kind {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Kind::SymbolName => write!(f, "symbol-name"),
+            Kind::Demangling => write!(f, "demangling"),
+            Kind::DemanglingAlt => write!(f, "demangling-alt"),
+            Kind::DefPath => write!(f, "def-path"),
         }
-        .into();
-        DiagnosticArgValue::Str(kind)
     }
 }