Micro example of proptest for micro AST | as reading https://proptest-rs.github.io/proptest/proptest/tutorial/index.html "Proptest from the Bottom Up"

proptest_addition_AST_micro_example.rs

Maybe some inspiration (proptesting , not fuzzing, per say, still maybe helpful/inspiring):

Micro example of proptest for micro AST | as reading https://proptest-rs.github.io/proptest/proptest/tutorial/index.html "Proptest from the Bottom Up"

```rust
use std::fmt;
use proptest::prelude::*;

// Definition of Micro Language AST Grammar

#[derive(Debug)]
enum Expr {
    Add(Box<Expr>, Box<Expr>),
    Int(i32),
}

// Definiotions as `proptest-derive` failed on stable...

impl Arbitrary for Expr {
    type Parameters = ();
    type Strategy = BoxedStrategy<Self>;

    fn arbitrary_with(_args: Self::Parameters) -> Self::Strategy {
        let leaf = any::<i32>().prop_map(Expr::Int).boxed();

        leaf.prop_recursive(8, 256, 10, |inner| {
            (inner.clone(), inner)
                .prop_map(|(lhs, rhs)| Expr::Add(Box::new(lhs), Box::new(rhs)))
                .boxed()
        }).boxed()
    }
}

// probably could be for bigger grammar solved with some nice "derive".

impl fmt::Display for Expr {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            Expr::Add(lhs, rhs) => write!(f, "({} + {})", lhs, rhs),
            Expr::Int(i) => write!(f, "{}", i),
        }
    }
}

// Let's generate example bunch of ASTs and print them:

fn main() {
    proptest!(|(expr in any::<Expr>())| {
        println!("{}", expr);
    });
}
```