make "expected paren or brace" error translatable

compiler/rustc_expand/messages.ftl

@@ -33,6 +33,9 @@ expand_duplicate_matcher_binding = duplicate matcher binding
 expand_expected_comma_in_list =
     expected token: `,`
 
+expand_expected_paren_or_brace =
+    expected `(` or `{"{"}`, found `{$token}`
+
 expand_explain_doc_comment_inner =
     inner doc comments expand to `#![doc = "..."]`, which is what this macro attempted to match
 

compiler/rustc_expand/src/errors.rs

@@ -448,3 +448,11 @@ pub struct InvalidFragmentSpecifier {
     pub fragment: Ident,
     pub help: String,
 }
+
+#[derive(Diagnostic)]
+#[diag(expand_expected_paren_or_brace)]
+pub struct ExpectedParenOrBrace<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub token: Cow<'a, str>,
+}

compiler/rustc_expand/src/mbe/quoted.rs

@@ -194,9 +194,11 @@ fn parse_tree<'a>(
                             }
                             Delimiter::Parenthesis => {}
                             _ => {
-                                let tok = pprust::token_kind_to_string(&token::OpenDelim(delim));
-                                let msg = format!("expected `(` or `{{`, found `{tok}`");
-                                sess.dcx().span_err(delim_span.entire(), msg);
+                                let token = pprust::token_kind_to_string(&token::OpenDelim(delim));
+                                sess.dcx().emit_err(errors::ExpectedParenOrBrace {
+                                    span: delim_span.entire(),
+                                    token,
+                                });
                             }
                         }
                     }