This is needed for the forms and validation enhancements in Components for Preview 3. It extends the pattern around binding to component properties with a new optional "*Expression" parameter, e.g.:

    [Parameter] DateTime StartDate { get; set; }
    [Parameter] Action<DateTime> StartDateChanged { get; set; }
    [Parameter] Expression<Func<DateTime>> StartDateExpression { get; set; } // This is new

It's optional, so if the component doesn't declare *Expression, then there's no change to the generated code versus what we had before.

If the component does declare *Expression (where * is something our existing rules match for binding based on the presence of *Changed), then when a consumer uses the binding, e.g.:

    <MyEditor bind-StartDate="@model.StartDate" />

... then it expands to be equivalent to:

    <MyEditor ... StartDateExpression="@(() => model.StartDate)" />

This makes it possible for MyEditor to read any metadata it wants about the thing you're binding to (e.g., validation annotations). What we actually use it for in the new built-in forms/validation system is uniquely identifying the bound field within the EditContext, so for example MyEditor could highlight different states based on whether that field is currently tracked as modified/invalid/etc.

Note: This is only for binding to components. There's no equivalent when binding to plain elements, since plain elements have no way of declaring that they want *Expression (and no reason why they should, since they couldn't do anything with it).