Use horizontal layout for a function with a single argument

foo(long_arg)

instead of

foo(
    long_arg,
)

src/expr.rs

@@ -2215,12 +2215,23 @@ where
         _ if args.len() >= 1 => {
             item_vec[args.len() - 1].item = args.last()
                 .and_then(|last_arg| last_arg.rewrite(context, shape));
-            tactic = definitive_tactic(
+            // Use horizontal layout for a function with a single argument as long as
-                &*item_vec,
+            // everything fits in a single line.
-                ListTactic::LimitedHorizontalVertical(args_max_width),
+            if args.len() == 1
-                Separator::Comma,
+                && args_max_width != 0 // Vertical layout is forced.
-                one_line_width,
+                && !item_vec[0].has_comment()
-            );
+                && !item_vec[0].inner_as_ref().contains('\n')
+                && ::lists::total_item_width(&item_vec[0]) <= one_line_width
+            {
+                tactic = DefinitiveListTactic::Horizontal;
+            } else {
+                tactic = definitive_tactic(
+                    &*item_vec,
+                    ListTactic::LimitedHorizontalVertical(args_max_width),
+                    Separator::Comma,
+                    one_line_width,
+                );
+            }
         }
         _ => (),
     }

src/lists.rs

@@ -713,7 +713,7 @@ where
         .fold((0, 0), |acc, l| (acc.0 + 1, acc.1 + l))
 }
 
-fn total_item_width(item: &ListItem) -> usize {
+pub fn total_item_width(item: &ListItem) -> usize {
     comment_len(item.pre_comment.as_ref().map(|x| &(*x)[..]))
         + comment_len(item.post_comment.as_ref().map(|x| &(*x)[..]))
         + item.item.as_ref().map_or(0, |str| str.len())