Indentation of secont and following lines in multiline comment

src/formatting/comment.rs

@@ -948,24 +948,29 @@ fn light_rewrite_comment(
     config: &Config,
     is_doc_comment: bool,
 ) -> String {
-    let lines: Vec<&str> = orig
+    let lines: Vec<String> = orig
         .lines()
         .map(|l| {
             // This is basically just l.trim(), but in the case that a line starts
             // with `*` we want to leave one space before it, so it aligns with the
             // `*` 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..]
+            let (blank, left_trimmed) = if let Some(fnw) = first_non_whitespace {
+                if l.as_bytes()[fnw] == b'*' {
+                    // Ensure '*' is preceeded by blank and not by a tab.
+                    (" ", &l[fnw..])
                 } else {
-                    &l[fnw..]
+                    ("", &l[fnw..])
                 }
             } else {
-                ""
+                ("", "")
             };
             // Preserve markdown's double-space line break syntax in doc comment.
-            trim_end_unless_two_whitespaces(left_trimmed, is_doc_comment)
+            format!(
+                "{}{}",
+                blank,
+                trim_end_unless_two_whitespaces(left_trimmed, is_doc_comment),
+            )
         })
         .collect();
     lines.join(&format!("\n{}", offset.to_string(config)))

tests/source/comment-multiline-indentation-hard-tabs.rs

@@ -0,0 +1,47 @@
+// rustfmt-hard_tabs: true
+// Ensure multiline comments are indented properly,
+// including when second line is prefixed by tab or at the beginning of the line
+
+/* First comment line
+* second comment line - no prefix
+* last comment line */
+
+/* First comment line
+ * second comment line - blank prefix
+ * last comment line */
+
+	/* First comment line
+	* second comment line - tab prefix
+	* last comment line */
+
+/* First comment line
+                    * second comment line - blank prefix
+* last comment line - no prefix */
+
+/* First comment line
+                 * second comment line - blank prefix
+          * last comment line */
+
+type T1 = TT<
+u32, /* First comment line
+* second comment line - no prefix
+* last comment line */
+>;
+
+type T2 = TT<
+u32, /* First comment line
+      * second comment line - blank prefix
+      * last comment line */
+>;
+
+type T2 = TT<
+u32, /* First comment line
+	* second comment line - tab prefix
+	* last comment line */
+>;
+
+type T3 = TT<
+u32,	/* First comment line
+		* second comment line - tab prefix
+		* last comment line */
+>;

tests/source/comment-multiline-indentation.rs

@@ -0,0 +1,47 @@
+// rustfmt-hard_tabs: false
+// Ensure multiline comments are indented properly,
+// including when second line is prefixed by tab or at the beginning of the line
+
+/* First comment line
+* second comment line - no prefix
+* last comment line */
+
+/* First comment line
+ * second comment line - blank prefix
+ * last comment line */
+
+	/* First comment line
+	* second comment line - tab prefix
+	* last comment line */
+
+/* First comment line
+                    * second comment line - blank prefix
+* last comment line - no prefix */
+
+/* First comment line
+                 * second comment line - blank prefix
+          * last comment line */
+
+type T1 = TT<
+u32, /* First comment line
+* second comment line - no prefix
+* last comment line */
+>;
+
+type T2 = TT<
+u32, /* First comment line
+      * second comment line - blank prefix
+      * last comment line */
+>;
+
+type T2 = TT<
+u32, /* First comment line
+	* second comment line - tab prefix
+	* last comment line */
+>;
+
+type T3 = TT<
+u32,	/* First comment line
+		* second comment line - tab prefix
+		* last comment line */
+>;