Fix rust fenced code blocks with an indent

This fixes a bug in the Rust code block partitioning that was
incorrectly removing the whitespace from the beginning of a code block.

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,
     };
@@ -181,6 +188,6 @@ fn it_partitions_rust_source() {
     );
     assert_eq!(
         partition_rust_source("    // Example"),
-        ("    ", "// Example")
+        ("", "    // Example")
     );
 }

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

@@ -1,6 +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.
+</span><span class="boring">fn main() {
+</span>    // This has a first line that is indented.
     println!("hello");
 <span class="boring">}</span></code></pre>