Fix rust fenced code blocks with an indent

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

@@ -141,7 +141,14 @@ fn partition_rust_source(s: &str) -> (&str, &str) {
     let split_idx = match HEADER_RE.captures(s) {
         Some(caps) => {
             let attributes = &caps[1];
-            attributes.len()
+            if attributes.trim().is_empty() {
+                // Don't include pure whitespace as an attribute. The
+                // whitespace in the regex is intended to handle multiple
+                // attributes *separated* by potential whitespace.
+                0
+            } else {
+                attributes.len()
+            }
         }
         None => 0,
     };
@@ -179,4 +186,8 @@ fn it_partitions_rust_source() {
         ),
         ("\n#![allow(foo)]\n\n#![allow(bar)]\n\n", "let x = 1;")
     );
+    assert_eq!(
+        partition_rust_source("    // Example"),
+        ("", "    // Example")
+    );
 }

tests/testsuite/rendering.rs

@@ -223,3 +223,9 @@ Html text was:
 fn html_blocks() {
     BookTest::from_dir("rendering/html_blocks").check_all_main_files();
 }
+
+// Test for a fenced code block that is also indented.
+#[test]
+fn code_block_fenced_with_indent() {
+    BookTest::from_dir("rendering/code_blocks_fenced_with_indent").check_all_main_files();
+}

tests/testsuite/rendering/code_blocks_fenced_with_indent/book.toml

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

tests/testsuite/rendering/code_blocks_fenced_with_indent/expected/code-blocks-fenced-with-indent.html

@@ -0,0 +1,6 @@
+<h1 id="code-blocks-fenced-with-indent"><a class="header" href="#code-blocks-fenced-with-indent">Code blocks fenced with indent</a></h1>
+<pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
+</span><span class="boring">fn main() {
+</span>    // This has a first line that is indented.
+    println!("hello");
+<span class="boring">}</span></code></pre>

tests/testsuite/rendering/code_blocks_fenced_with_indent/src/SUMMARY.md

@@ -0,0 +1,3 @@
+# Summary
+
+- [Code blocks fenced with indent](./code-blocks-fenced-with-indent.md)

tests/testsuite/rendering/code_blocks_fenced_with_indent/src/code-blocks-fenced-with-indent.md

@@ -0,0 +1,6 @@
+# Code blocks fenced with indent
+
+```rust
+    // This has a first line that is indented.
+    println!("hello");
+```