diff --git a/src/comment.rs b/src/comment.rs
index 05d7310122a..40e42b53ed1 100644
--- a/src/comment.rs
+++ b/src/comment.rs
@@ -1074,16 +1074,26 @@ fn light_rewrite_comment(
             // `*` in `/*`.
             let first_non_whitespace = l.find(|c| !char::is_whitespace(c));
             let left_trimmed = if let Some(fnw) = first_non_whitespace {
-                if l.as_bytes()[fnw] == b'*' && fnw > 0 {
-                    &l[fnw - 1..]
+                if l.as_bytes()[fnw] == b'*' {
+                    Cow::Owned(format!(" {}", &l[fnw..]))
                 } else {
-                    &l[fnw..]
+                    Cow::Borrowed(&l[fnw..])
                 }
             } else {
-                ""
+                Cow::Borrowed("")
             };
+
             // Preserve markdown's double-space line break syntax in doc comment.
-            trim_end_unless_two_whitespaces(left_trimmed, is_doc_comment)
+            match left_trimmed {
+                Cow::Borrowed(left_trimmed) => Cow::Borrowed(trim_end_unless_two_whitespaces(
+                    left_trimmed,
+                    is_doc_comment,
+                )),
+                Cow::Owned(left_trimmed) => {
+                    let trimmed = trim_end_unless_two_whitespaces(&left_trimmed, is_doc_comment);
+                    Cow::Owned(trimmed.to_string())
+                }
+            }
         })
         .join(&format!("\n{}", offset.to_string(config)))
 }
diff --git a/tests/source/6639-non-idempotent-block-doc-comments/inner-block-doc-comment.rs b/tests/source/6639-non-idempotent-block-doc-comments/inner-block-doc-comment.rs
new file mode 100644
index 00000000000..d46cf3688ec
--- /dev/null
+++ b/tests/source/6639-non-idempotent-block-doc-comments/inner-block-doc-comment.rs
@@ -0,0 +1,9 @@
+/*!First comment.
+*/
+pub mod outer {
+    /*!Second comment.
+*/
+    pub struct Inner {
+        pub octets: Vec<u8>,
+    }
+}
diff --git a/tests/source/6639-non-idempotent-block-doc-comments/naive-outer-block-comment.rs b/tests/source/6639-non-idempotent-block-doc-comments/naive-outer-block-comment.rs
new file mode 100644
index 00000000000..581dea9e3ab
--- /dev/null
+++ b/tests/source/6639-non-idempotent-block-doc-comments/naive-outer-block-comment.rs
@@ -0,0 +1,3 @@
+/**
+*/
+mod foo {}
diff --git a/tests/source/6639-non-idempotent-block-doc-comments/outer-block-doc-comment.rs b/tests/source/6639-non-idempotent-block-doc-comments/outer-block-doc-comment.rs
new file mode 100644
index 00000000000..b1c8e1c8518
--- /dev/null
+++ b/tests/source/6639-non-idempotent-block-doc-comments/outer-block-doc-comment.rs
@@ -0,0 +1,13 @@
+/**First comment.
+*
+*/
+pub mod outer {
+    /**Second comment.
+foo
+*/
+    pub struct Inner {
+        /**Third comment.
+*/
+        pub octets: Vec<u8>,
+    }
+}
diff --git a/tests/target/6639-non-idempotent-block-doc-comments/inner-block-doc-comment.rs b/tests/target/6639-non-idempotent-block-doc-comments/inner-block-doc-comment.rs
new file mode 100644
index 00000000000..660b855dadd
--- /dev/null
+++ b/tests/target/6639-non-idempotent-block-doc-comments/inner-block-doc-comment.rs
@@ -0,0 +1,9 @@
+/*!First comment.
+ */
+pub mod outer {
+    /*!Second comment.
+     */
+    pub struct Inner {
+        pub octets: Vec<u8>,
+    }
+}
diff --git a/tests/target/6639-non-idempotent-block-doc-comments/naive-outer-block-comment.rs b/tests/target/6639-non-idempotent-block-doc-comments/naive-outer-block-comment.rs
new file mode 100644
index 00000000000..42f39ee29ae
--- /dev/null
+++ b/tests/target/6639-non-idempotent-block-doc-comments/naive-outer-block-comment.rs
@@ -0,0 +1,3 @@
+/**
+ */
+mod foo {}
diff --git a/tests/target/6639-non-idempotent-block-doc-comments/outer-block-doc-comment.rs b/tests/target/6639-non-idempotent-block-doc-comments/outer-block-doc-comment.rs
new file mode 100644
index 00000000000..13bd081a846
--- /dev/null
+++ b/tests/target/6639-non-idempotent-block-doc-comments/outer-block-doc-comment.rs
@@ -0,0 +1,13 @@
+/**First comment.
+ *
+ */
+pub mod outer {
+    /**Second comment.
+    foo
+    */
+    pub struct Inner {
+        /**Third comment.
+         */
+        pub octets: Vec<u8>,
+    }
+}