Desugar separated elements

src/desugar.rs

@@ -1 +1,2 @@
+mod basic;
 mod program;

src/desugar/basic.rs

@@ -0,0 +1,36 @@
+use crate::ast::Separated;
+
+impl<E, S1, S2> Separated<E, S1, S2> {
+    pub fn desugar_elem(self, desugar_elem: impl Fn(E) -> (E, bool)) -> (Self, bool) {
+        match self {
+            Self::Empty(span) => (Self::Empty(span), false),
+
+            Self::NonEmpty {
+                first_elem,
+                last_elems,
+                trailing,
+                span,
+            } => {
+                let (new_first_elem, mut desugared) = desugar_elem(first_elem);
+                let mut new_last_elems = vec![];
+                for (separator, elem) in last_elems {
+                    if desugared {
+                        new_last_elems.push((separator, elem));
+                    } else {
+                        let (elem, elem_desugared) = desugar_elem(elem);
+                        desugared = desugared || elem_desugared;
+                        new_last_elems.push((separator, elem));
+                    }
+                }
+
+                let new = Self::NonEmpty {
+                    first_elem: new_first_elem,
+                    last_elems: new_last_elems,
+                    trailing,
+                    span,
+                };
+                (new, desugared)
+            }
+        }
+    }
+}

src/desugar/program.rs

@@ -16,7 +16,7 @@ impl Program {
                 s2,
                 span,
             } => {
-                let (elems, desugared) = (elems, false); // TODO Implement
+                let (elems, desugared) = elems.desugar_elem(|e| (e, false)); // TODO Implement
                 if desugared {
                     let new = Self::Module {
                         s0,