Make Collection formattable

compiler/fmt/src/annotation.rs

@@ -3,7 +3,7 @@ use crate::{
     spaces::{fmt_comments_only, fmt_spaces, NewlineAt, INDENT},
     Buf,
 };
-use roc_parse::ast::{AliasHeader, AssignedField, Expr, Tag, TypeAnnotation};
+use roc_parse::ast::{AliasHeader, AssignedField, Collection, Expr, Tag, TypeAnnotation};
 use roc_parse::ident::UppercaseIdent;
 use roc_region::all::Loc;
 
@@ -83,6 +83,15 @@ where
     }
 }
 
+impl<'a, T> Formattable for Collection<'a, T>
+where
+    T: Formattable,
+{
+    fn is_multiline(&self) -> bool {
+        self.items.iter().any(|item| item.is_multiline()) || !self.final_comments().is_empty()
+    }
+}
+
 /// A Located formattable value is also formattable
 impl<T> Formattable for Loc<T>
 where

compiler/fmt/src/collection.rs

@@ -17,10 +17,8 @@ pub fn fmt_collection<'a, 'buf, T: ExtractSpaces<'a> + Formattable>(
     <T as ExtractSpaces<'a>>::Item: Formattable,
 {
     buf.indent(indent);
-    let is_multiline =
-        items.iter().any(|item| item.is_multiline()) || !items.final_comments().is_empty();
 
-    if is_multiline {
+    if items.is_multiline() {
         let braces_indent = indent;
         let item_indent = braces_indent + INDENT;
         if newline == Newlines::Yes {