Keep blank lines in chained block-style comments (#4012)

src/formatting/chains.rs

@@ -859,9 +859,18 @@ fn trim_tries(s: &str) -> String {
     for (kind, rich_char) in CharClasses::new(s.chars()) {
         match rich_char.get_char() {
             '\n' => {
-                if result.is_empty() || !line_buffer.trim().is_empty() {
-                    result.push_str(&line_buffer);
-                    result.push('\n')
+                let last_new_line = result.rfind('\n').unwrap_or(0);
+                match result[..last_new_line].rfind('\n') {
+                    // Remove the current line if it's blank and follows a line-style comment
+                    Some(second_last_newline)
+                        if result[second_last_newline + 1..]
+                            .trim_start()
+                            .starts_with("//")
+                            && line_buffer.trim().is_empty() => {}
+                    _ => {
+                        result.push_str(&line_buffer);
+                        result.push('\n');
+                    }
                 }
                 line_buffer.clear();
             }

tests/source/chains_with_comment.rs

@@ -119,3 +119,14 @@ fn foo() {
         ? ? ?
         .baz;
 }
+
+// #4012
+pub fn foo2() {
+    let a = chain
+        .get()
+        /*
+
+
+        */
+        .set();
+}

tests/target/chains_with_comment.rs

@@ -135,3 +135,14 @@ fn foo() {
         // comment
         .baz;
 }
+
+// #4012
+pub fn foo2() {
+    let a = chain
+        .get()
+        /*
+
+
+        */
+        .set();
+}