Introduce Collection as a general abstraction in the ast

compiler/parse/src/ast.rs

@@ -5,6 +5,18 @@ use bumpalo::Bump;
 use roc_module::operator::{BinOp, CalledVia, UnaryOp};
 use roc_region::all::{Loc, Position, Region};
 
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Collection<'a, T> {
+    pub items: &'a [T],
+    pub final_comments: &'a [CommentOrNewline<'a>],
+}
+
+impl<'a, T> Collection<'a, T> {
+    pub fn empty() -> Collection<'a, T> {
+        Collection { items: &[], final_comments: &[] }
+    }
+}
+
 #[derive(Clone, Debug, PartialEq)]
 pub enum Module<'a> {
     Interface { header: InterfaceHeader<'a> },
@@ -231,11 +243,11 @@ pub enum TypeAnnotation<'a> {
     ),
 
     Record {
-        fields: &'a [Loc<AssignedField<'a, TypeAnnotation<'a>>>],
+        fields: Collection<'a, Loc<AssignedField<'a, TypeAnnotation<'a>>>>,
         /// The row type variable in an open record, e.g. the `r` in `{ name: Str }r`.
         /// This is None if it's a closed record annotation like `{ name: Str }`.
         ext: Option<&'a Loc<TypeAnnotation<'a>>>,
-        final_comments: &'a [CommentOrNewline<'a>],
+        // final_comments: &'a [CommentOrNewline<'a>],
     },
 
     /// A tag union, e.g. `[