Function retarget_assignment_cast 

fn retarget_assignment_cast(
    expr: &mut Expr,
    schema: &DFSchemaRef,
    source_plan: Option<&LogicalPlan>,
    timezone: &SessionTimezone,
) -> Result<bool>

Reinterprets one assignment cast, returning whether it was rewritten.

source_plan is the projection’s input, used to resolve a literal behind a column reference; Values rows carry their expression inline and pass None.