diff --git a/rustfmt-core/rustfmt-lib/src/missed_spans.rs b/rustfmt-core/rustfmt-lib/src/missed_spans.rs
index 92fcb000ff9..6c326bffa48 100644
--- a/rustfmt-core/rustfmt-lib/src/missed_spans.rs
+++ b/rustfmt-core/rustfmt-lib/src/missed_spans.rs
@@ -48,6 +48,12 @@ impl<'a> FmtVisitor<'a> {
             self.last_pos = end;
             return;
         }
+
+        let last_line_str = self.buffer.lines().last().unwrap_or("");
+        if !missing_snippet.starts_with("\n") && last_line_str.find("//").is_some() {
+            self.push_str("\n");
+        }
+
         self.format_missing_inner(end, |this, last_snippet, _| this.push_str(last_snippet))
     }
 
diff --git a/rustfmt-core/rustfmt-lib/tests/source/issue-3988.rs b/rustfmt-core/rustfmt-lib/tests/source/issue-3988.rs
new file mode 100644
index 00000000000..f56290a1522
--- /dev/null
+++ b/rustfmt-core/rustfmt-lib/tests/source/issue-3988.rs
@@ -0,0 +1,9 @@
+// rustfmt-normalize_doc_attributes: true
+// rustfmt-max_width: 10
+
+mod a {
+  mod b {
+      #[doc = "test"]fn foo() -> () { 
+      }
+  }
+}
diff --git a/rustfmt-core/rustfmt-lib/tests/target/issue-3988.rs b/rustfmt-core/rustfmt-lib/tests/target/issue-3988.rs
new file mode 100644
index 00000000000..8a1e830298e
--- /dev/null
+++ b/rustfmt-core/rustfmt-lib/tests/target/issue-3988.rs
@@ -0,0 +1,10 @@
+// rustfmt-normalize_doc_attributes: true
+// rustfmt-max_width: 10
+
+mod a {
+    mod b {
+        ///test
+fn foo() -> (){
+        }
+    }
+}