make "proc-macro derive panicked" translatable

compiler/rustc_expand/messages.ftl

@@ -114,6 +114,10 @@ expand_only_one_argument =
 expand_only_one_word =
     must only be one word
 
+expand_proc_macro_derive_panicked =
+    proc-macro derive panicked
+    .help = message: {$message}
+
 expand_proc_macro_derive_tokens =
     proc-macro derive produced unparsable tokens
 

compiler/rustc_expand/src/errors.rs

@@ -392,6 +392,21 @@ pub(crate) struct ProcMacroPanickedHelp {
     pub message: String,
 }
 
+#[derive(Diagnostic)]
+#[diag(expand_proc_macro_derive_panicked)]
+pub(crate) struct ProcMacroDerivePanicked {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub message: Option<ProcMacroDerivePanickedHelp>,
+}
+
+#[derive(Subdiagnostic)]
+#[help(expand_help)]
+pub(crate) struct ProcMacroDerivePanickedHelp {
+    pub message: String,
+}
+
 #[derive(Diagnostic)]
 #[diag(expand_custom_attribute_panicked)]
 pub(crate) struct CustomAttributePanicked {

compiler/rustc_expand/src/proc_macro.rs

@@ -147,11 +147,14 @@ impl MultiItemModifier for DeriveProcMacro {
             match self.client.run(&strategy, server, input, proc_macro_backtrace) {
                 Ok(stream) => stream,
                 Err(e) => {
-                    let mut err = ecx.dcx().struct_span_err(span, "proc-macro derive panicked");
+                    ecx.dcx().emit_err({
-                    if let Some(s) = e.as_str() {
+                        errors::ProcMacroDerivePanicked {
-                        err.help(format!("message: {s}"));
+                            span,
-                    }
+                            message: e.as_str().map(|message| {
-                    err.emit();
+                                errors::ProcMacroDerivePanickedHelp { message: message.into() }
+                            }),
+                        }
+                    });
                     return ExpandResult::Ready(vec![]);
                 }
             }