fix rust code in comment with a line containing only a hash sign (rust-lang#3818)

Author: scampi

src/comment.rs

@@ -823,7 +823,8 @@ fn rewrite_comment_inner(
 const RUSTFMT_CUSTOM_COMMENT_PREFIX: &str = "//#### ";
 
 fn hide_sharp_behind_comment(s: &str) -> Cow<'_, str> {
-    if s.trim_start().starts_with("# ") {
+    let s_trimmed = s.trim();
+    if s_trimmed.starts_with("# ") || s_trimmed == "#" {
         Cow::from(format!("{}{}", RUSTFMT_CUSTOM_COMMENT_PREFIX, s))
     } else {
         Cow::from(s)

tests/source/issue-3751.rs

@@ -0,0 +1,10 @@
+// rustfmt-format_code_in_doc_comments: true
+
+//! Empty pound line
+//!
+//! ```rust
+//! #
+//! # fn main() {
+//!  foo ( )  ;
+//! # }
+//! ```

tests/target/issue-3751.rs

@@ -0,0 +1,10 @@
+// rustfmt-format_code_in_doc_comments: true
+
+//! Empty pound line
+//!
+//! ```rust
+//! #
+//! # fn main() {
+//! foo();
+//! # }
+//! ```