message: Add comment about arrow distillation
diff: @@ -369,6 +369,9 @@ impl<'interner, 'arena, 'env> Context<'interner, 'arena, 'env> {
  )
  }
  core::Term::Universe(_span) => Term::Universe(()),
  +
  + // Use arrow sugar if the the input binding is not referenced in the
  + // output type.
  core::Term::FunType(_span, _, input_type, output_type)
  if !output_type.binds_rigid_var(LocalVar::last()) =>
  {
  @@ -384,6 +387,7 @@ impl<'interner, 'arena, 'env> Context<'interner, 'arena, 'env> {
  self.scope.to_scope(output_type),
  )
  }
  + // Otherwise distill to a function type with an explicit parameter.
  core::Term::FunType(_span, input_name, input_type, output_type) => {
  let input_type = self.check(input_type);
  @@ -398,6 +402,7 @@ impl<'interner, 'arena, 'env> Context<'interner, 'arena, 'env> {
  self.scope.to_scope(output_type),
  )
  }
  +
  core::Term::FunLit(_span, input_name, output_expr) => {
  let input_name = self.push_rigid(*input_name);
  let output_expr = self.synth(output_expr);
  