Function rewrite_placeholder

datafusion/expr/src/expr.rs:2883–2902

Modifies expr to match the DataType, metadata, and nullability of other if it is a placeholder with previously unspecified type information (i.e., most placeholders)

(expr: &mut Expr, other: &Expr, schema: &DFSchema)

// Modifies expr to match the DataType, metadata, and nullability of other if it is
// a placeholder with previously unspecified type information (i.e., most placeholders)
fn rewrite_placeholder(expr: &mut Expr, other: &Expr, schema: &DFSchema) -> Result<()> {
    if let Expr::Placeholder(Placeholder { id: _, field }) = expr
        && field.is_none()
    {
        let other_field = other.to_field(schema);
        match other_field {
            Err(e) => {
                Err(e.context(format!(
                    "Can not find type of {other} needed to infer type of {expr}"
                )))?;
            }
            Ok((_, other_field)) => {
                // We can't infer the nullability of the future parameter that might
                // be bound, so ensure this is set to true
                *field = Some(other_field.as_ref().clone().with_nullable(true).into());
            }
        }
    };
    Ok(())
}

#[macro_export]
macro_rules! expr_vec_fmt {