How to get query colums as String - rust

Question

I'm porting a PHP-app to Rust (as a noob). Since my query results will go straight into html, I don't really need any type-safety. Just NULL -> "" and everything else as String.
Also time::PrimitiveDateTime doesn't fully implement format, such that {mytime:.19} doesn't cut off the useless ".0" milliseconds. So it'd be nice to have that as String too. Unless there is an easier conversion to get that?
Not having found (or understood) any appropriate converter, I tried
fn as_string(row: mysql::Row, index: usize) -> String {
if let Some(&value) = row.as_ref(index) {
value.to_string()
} else {
String::from("")
}
}
but
error[E0599]: `mysql::Value` doesn't implement `std::fmt::Display`

Answer 1

Your value variable is of type mysql::Value, which is an Enum type.
As it doesn't implement Display, you probably want to write a conversion to String manually.
For enums, this usually involves a match:
use mysql::Value;
fn as_string(mut row: mysql::Row, index: usize) -> String {
if let Some(value) = row.as_ref(index) {
match value {
Value::NULL => String::from(""),
Value::Bytes(v) => String::from_utf8_lossy(v.as_slice()).into_owned(),
Value::Int(v) => format!("{v}"),
Value::UInt(v) => format!("{v}"),
Value::Float(v) => format!("{v}"),
Value::Double(v) => format!("{v}"),
Value::Date(year, month, day, hour, minutes, seconds, micro) => todo!(),
Value::Time(negative, days, hours, minutes, seconds, micro) => todo!(),
}
} else {
String::from("")
}
}
You should of course replace the todo!() statements with implementations for those specific types.
Note that you shouldn't actually handle the None case like this, because it indicates a programming error. It happens if you already moved the value out of Row before, not if your cell is empty.
So I would argue the cleanest way is to return an Option<String>, which simplifies this function even further:
use mysql::Value;
fn as_string(mut row: mysql::Row, index: usize) -> Option<String> {
row.as_ref(index).map(|value| match value {
Value::NULL => String::from(""),
Value::Bytes(v) => String::from_utf8_lossy(v.as_slice()).into_owned(),
Value::Int(v) => format!("{v}"),
Value::UInt(v) => format!("{v}"),
Value::Float(v) => format!("{v}"),
Value::Double(v) => format!("{v}"),
Value::Date(year, month, day, hour, minutes, seconds, micro) => todo!(),
Value::Time(negative, days, hours, minutes, seconds, micro) => todo!(),
})
}