Rollup merge of #123694 - Xiretza:expand-diagnostics, r=compiler-errors

expand: fix minor diagnostics bug

The error mentions `///`, when it's actually `//!`:

```
error[E0658]: attributes on expressions are experimental
 --> test.rs:4:9
  |
4 |         //! wah
  |         ^^^^^^^
  |
  = note: see issue https://github.com/rust-lang/rust/issues/15701 <https://github.com/rust-lang/rust/issues/15701> for more information
  = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable
  = help: `///` is for documentation comments. For a plain comment, use `//`.
```

2 files changed, 11 insertions, 3 deletions

diff --git a/compiler/rustc_expand/messages.ftl b/compiler/rustc_expand/messages.ftl
index 7ae988a5be6..530b37aadb1 100644
--- a/compiler/rustc_expand/messages.ftl
+++ b/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)`
 
diff --git a/compiler/rustc_expand/src/config.rs b/compiler/rustc_expand/src/config.rs
index 67a5a9128cc..35f0d8abffc 100644
--- a/compiler/rustc_expand/src/config.rs
+++ b/compiler/rustc_expand/src/config.rs
@@ -382,7 +382,6 @@ impl<'a> StripUnconfigured<'a> {
     }
 
     /// If attributes are not allowed on expressions, emit an error for `attr`
-    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     #[instrument(level = "trace", skip(self))]
     pub(crate) fn maybe_emit_expr_attr_err(&self, attr: &Attribute) {
         if self.features.is_some_and(|features| !features.stmt_expr_attributes)
@@ -392,11 +391,15 @@ 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("`///` is for documentation comments. For a plain comment, use `//`.");
+                err.help(if attr.style == AttrStyle::Outer {
+                    crate::fluent_generated::expand_help_outer_doc
+                } else {
+                    crate::fluent_generated::expand_help_inner_doc
+                });
             }
 
             err.emit();