ARROW-9534: [Rust] [DataFusion] Add functions to create literal expressions

[andygrove]

No description provided.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-9534

[andygrove]

@jorgecarleitao fyi

[jorgecarleitao]

Nice! I just needed this in my ballista's PR!

following this SO answer, what do you think of

trait Literal {
    fn lit(&self) -> ScalarValue;
}

impl Literal for i32 {
    fn lit(&self) -> ScalarValue {
        ScalarValue::Int(self.clone())
    }
}
impl Literal for bool {
    fn lit(&self) -> ScalarValue {
        ScalarValue::Boolean(self.clone())
    }
}

fn lit<T: Literal>(n: T) -> ScalarValue {
    n.lit()
}

# usage:
# lit(2);
# lit(true);

this way, the user does not have to import nor remember the value's type; they can just call lit. It seems to be a common pattern in rust.

[andygrove]

@jorgecarleitao Oh, that is much nicer. Do you want to PR that and I'll close this one?

[jorgecarleitao]

I can take it.