Keep the comment after where clause that has no predicate (rust-lang#4649)

Author: ChinYing-Li

src/formatting/items.rs

@@ -2528,7 +2528,7 @@ fn rewrite_fn_base(
         } else {
             result.push_str(&ret_str);
         }
-
+        /*
         if where_clause.predicates.is_empty() {
             // Comment between return type and the end of the decl.
             // Even if there are no predicates in the where clause, the "where" kw may be present,
@@ -2555,7 +2555,7 @@ fn rewrite_fn_base(
                     force_new_line_for_brace = true;
                 }
             }
-        }
+        }*/
     }
 
     let pos_before_where = match fd.output {
@@ -2585,27 +2585,42 @@ fn rewrite_fn_base(
         pos_before_where,
         option,
     )?;
-    // If there are neither where-clause nor return type, we may be missing comments between
-    // params and `{`.
-    if where_clause_str.is_empty() {
-        if let ast::FnRetTy::Default(ret_span) = fd.output {
-            match recover_missing_comment_in_span(
-                mk_sp(params_span.hi(), ret_span.hi()),
-                shape,
-                context,
-                last_line_width(&result),
-            ) {
-                Some(ref missing_comment) if !missing_comment.is_empty() => {
-                    result.push_str(missing_comment);
-                    force_new_line_for_brace = true;
-                }
-                _ => {}
+
+    result.push_str(&where_clause_str);
+    if where_clause.predicates.is_empty() {
+        // Comment between return type and the end of the decl.
+        // Even if there are no predicates in the where clause, the "where" kw may be present,
+        // so start the snippet after it.
+        let snippet_lo = where_clause.span.hi();
+        let snippet_hi = span.hi();
+        let extra_indent = match where_clause.has_where_token {
+            true => Indent::new(context.config.tab_spaces(), 0),
+            false => Indent::new(0, 0),
+        };
+        let missing_comment = recover_missing_comment_in_span(
+            mk_sp(snippet_lo, snippet_hi),
+            shape,
+            context,
+            shape.indent.width() + extra_indent.width(),
+        )
+        .unwrap_or(String::new());
+        if !missing_comment.is_empty() {
+            // If the comment is more than one line, start it at the line after where
+            if where_clause.has_where_token {
+                result.push_str(&*format!(
+                    "\n{}where\n{}",
+                    (shape.indent + extra_indent).to_string(context.config),
+                    missing_comment.trim_start_matches('\n'),
+                ));
+            } else {
+                result.push_str(&*missing_comment);
             }
+            force_new_line_for_brace = context
+                .snippet(mk_sp(snippet_lo, snippet_hi))
+                .trim_end_matches(' ')
+                .ends_with('\n');
         }
     }
-
-    result.push_str(&where_clause_str);
-
     let ends_with_comment = last_line_contains_single_line_comment(&result);
     force_new_line_for_brace |= ends_with_comment;
     force_new_line_for_brace |=

tests/source/issue-4649.rs

@@ -0,0 +1,9 @@
+trait Bar {
+    fn bar(&self, a: T)
+        where
+    //     Self: Bar
+    // Some comment
+    ;
+
+    fn bar2(&self, a: T) where /* Self: Bar */ ;
+}

tests/source/where-clause.rs

@@ -65,3 +65,13 @@ pub trait AsUnindented {
 fn f<T: AsUnindented<
 Output<'static> = U>, U>() where U: AsUnindented<
 Output<'static> = i32> {}
+
+fn foo2<T>()
+    where T: FnOnce()
+// Comments
+{}
+
+fn foo3<T, U>()
+    where T: FnOnce(), // Comments
+          U: FnOnce()
+{}

tests/target/issue-1113.rs

@@ -25,9 +25,9 @@ pub fn foo() -> fmt::Result
 }
 
 pub fn foo() -> fmt::Result /*
-                             *
-                             *
-                             */
+ *
+ *
+ */
 {
     panic!()
 }

tests/target/issue-4001.rs

@@ -1,3 +1,5 @@
-fn unit() -> () /* comment */ {
+fn unit() -> ()
+    where
+/* comment */ {
     ()
 }

tests/target/issue-4649.rs

@@ -0,0 +1,11 @@
+trait Bar {
+    fn bar(&self, a: T)
+        where
+    //     Self: Bar
+    // Some comment
+    ;
+
+    fn bar2(&self, a: T)
+        where
+/* Self: Bar */;
+}

tests/target/where-clause.rs

@@ -115,3 +115,14 @@ fn f<T: AsUnindented<Output<'static> = U>, U>()
     where U: AsUnindented<Output<'static> = i32>
 {
 }
+
+fn foo2<T>()
+    where T: FnOnce() // Comments
+{
+}
+
+fn foo3<T, U>()
+    where T: FnOnce(), // Comments
+          U: FnOnce()
+{
+}