Fix #2640: Handle hidden crate attributes in fn main wrapping

crates/mdbook-html/src/html/hide_lines.rs

@@ -132,7 +132,7 @@ fn partition_rust_source(s: &str) -> (&str, &str) {
         r"^(?mx)
         (
             (?:
-                ^[ \t]*\#!\[.* (?:\r?\n)?
+                ^[ \t]*(?:\#\x20)?[ \t]*\#!\[.* (?:\r?\n)?
                 |
                 ^\s* (?:\r?\n)?
             )*
@@ -155,6 +155,36 @@ fn partition_rust_source(s: &str) -> (&str, &str) {
     s.split_at(split_idx)
 }
 
+#[test]
+fn wrap_rust_main_basic() {
+    // Code without fn main gets wrapped
+    assert_eq!(
+        wrap_rust_main("let x = 1;"),
+        Some("# #![allow(unused)]\n# fn main() {\nlet x = 1;\n# }".to_string())
+    );
+}
+
+#[test]
+fn wrap_rust_main_with_feature_attr() {
+    // Crate-level attributes should be placed before fn main (issue #2640)
+    // Without hidden prefix:
+    assert_eq!(
+        wrap_rust_main("#![feature(rustc_attrs)]\n#[rustc_on_unimplemented = \"oh no\"]\npub trait Foo {}"),
+        Some("# #![allow(unused)]\n#![feature(rustc_attrs)]\n# fn main() {\n#[rustc_on_unimplemented = \"oh no\"]\npub trait Foo {}\n# }".to_string())
+    );
+    // With hidden line prefix `# ` (the actual input from markdown):
+    assert_eq!(
+        wrap_rust_main("# #![feature(rustc_attrs)]\n#[rustc_on_unimplemented = \"oh no\"]\npub trait Foo {}"),
+        Some("# #![allow(unused)]\n# #![feature(rustc_attrs)]\n# fn main() {\n#[rustc_on_unimplemented = \"oh no\"]\npub trait Foo {}\n# }".to_string())
+    );
+}
+
+#[test]
+fn wrap_rust_main_already_has_main() {
+    // Code with fn main should not be wrapped
+    assert_eq!(wrap_rust_main("fn main() {}"), None);
+}
+
 #[test]
 fn it_partitions_rust_source() {
     assert_eq!(partition_rust_source(""), ("", ""));
@@ -190,4 +220,9 @@ fn it_partitions_rust_source() {
         partition_rust_source("    // Example"),
         ("", "    // Example")
     );
+    // Hidden line prefix with crate attribute (issue #2640)
+    assert_eq!(
+        partition_rust_source("# #![feature(rustc_attrs)]\n#[foo]\npub trait Foo {}"),
+        ("# #![feature(rustc_attrs)]\n", "#[foo]\npub trait Foo {}")
+    );
 }

tests/testsuite/playground.rs

@@ -17,6 +17,25 @@ fn playground_on_rust_code() {
     );
 }
 
+// Verifies that hidden crate-level attributes (like #![feature(...)]) are
+// properly wrapped when fn main is generated. The attributes should appear
+// before fn main, not inside it. (issue #2640)
+#[test]
+fn playground_hidden_feature_attr() {
+    BookTest::from_dir("playground/playground_hidden_feature_attr").check_main_file(
+        "book/feature-attr.html",
+        str![[r##"
+<h1 id="feature-attr"><a class="header" href="#feature-attr">Feature Attr</a></h1>
+<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
+</span><span class="boring">#![feature(rustc_attrs)]
+</span><span class="boring">fn main() {
+</span>#[rustc_on_unimplemented = "oh no"]
+pub trait Foo {}
+<span class="boring">}</span></code></pre>
+"##]],
+    );
+}
+
 // When the playground is disabled, there should be no playground class.
 #[test]
 fn disabled_playground() {

tests/testsuite/playground/playground_hidden_feature_attr/book.toml

@@ -0,0 +1,2 @@
+[book]
+title = "playground_on_rust_code"

tests/testsuite/playground/playground_hidden_feature_attr/src/SUMMARY.md

@@ -0,0 +1,3 @@
+# Summary
+
+[Hidden Feature Attr](feature-attr.md)

tests/testsuite/playground/playground_hidden_feature_attr/src/feature-attr.md

@@ -0,0 +1,7 @@
+# Feature Attr
+
+```rust
+# #![feature(rustc_attrs)]
+#[rustc_on_unimplemented = "oh no"]
+pub trait Foo {}
+```