Add test case for issue 5997

Author: ytmimi

tests/source/issue_5997.rs

@@ -0,0 +1,15 @@
+// rustfmt-version: Two
+
+// This formats with two spaces:
+pub struct Newtype(
+    /// Doc
+    #[doc()] //
+    pub  Vec<u8>,
+);
+
+// This formats with one:
+pub struct Newtype(
+    /// Doc
+    #[doc()]
+    pub Vec<u8>,
+);

tests/target/issue_5997.rs

@@ -0,0 +1,15 @@
+// rustfmt-version: Two
+
+// This formats with two spaces:
+pub struct Newtype(
+    /// Doc
+    #[doc()] //
+    pub Vec<u8>,
+);
+
+// This formats with one:
+pub struct Newtype(
+    /// Doc
+    #[doc()]
+    pub Vec<u8>,
+);