rustc_expand: make diagnostic translatable

Author: Xiretza

compiler/rustc_expand/messages.ftl

@@ -10,6 +10,11 @@ expand_attribute_meta_item =
 expand_attribute_single_word =
     attribute must only be a single word
 
+expand_attributes_on_expressions_experimental =
+    attributes on expressions are experimental
+    .help_outer_doc = `///` is used for outer documentation comments; for a plain comment, use `//`.
+    .help_inner_doc = `//!` is used for inner documentation comments; for a plain comment, use `//` by removing the `!` or inserting a space in between them: `// !`
+
 expand_attributes_wrong_form =
     attribute must be of form: `attributes(foo, bar)`
 

compiler/rustc_expand/src/config.rs

@@ -383,14 +383,14 @@ impl<'a> StripUnconfigured<'a> {
                 &self.sess,
                 sym::stmt_expr_attributes,
                 attr.span,
-                "attributes on expressions are experimental",
+                crate::fluent_generated::expand_attributes_on_expressions_experimental,
             );
 
             if attr.is_doc_comment() {
                 err.help(if attr.style == AttrStyle::Outer {
-                    "`///` is used for outer documentation comments; for a plain comment, use `//`."
+                    crate::fluent_generated::expand_help_outer_doc
                 } else {
-                    "`//!` is used for inner documentation comments; for a plain comment, use `//` by removing the `!` or inserting a space in between them: `// !`"
+                    crate::fluent_generated::expand_help_inner_doc
                 });
             }