Crate proptest_arbitrary_interop

proptest-arbitrary-interop

This crate provides the necessary glue to reuse an implementation of arbitrary::Arbitrary as a proptest::strategy::Strategy.

Usage

in Cargo.toml:

[dependencies]
arbitrary = "1.1.3"
proptest  = "1.0.0"

In your code:

// Part 1: suppose you implement Arbitrary for one of your types
// because you want to fuzz it.

use arbitrary::{Arbitrary, Result, Unstructured};
#[derive(Copy, Clone, Debug)]
pub struct Rgb {
    pub r: u8,
    pub g: u8,
    pub b: u8,
}
impl<'a> Arbitrary<'a> for Rgb {
    fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self> {
        let r = u8::arbitrary(u)?;
        let g = u8::arbitrary(u)?;
        let b = u8::arbitrary(u)?;
        Ok(Rgb { r, g, b })
    }
}

// Part 2: suppose you later decide that in addition to fuzzing
// you want to use that Arbitrary impl, but with proptest.

use proptest::prelude::*;
use proptest_arbitrary_interop::arb;

proptest! {
    #[test]
    #[should_panic]
    fn always_red(color in arb::<Rgb>()) {
        prop_assert!(color.g == 0 || color.r > color.g);
    }
}

Caveats

It only works with types that implement arbitrary::Arbitrary in a particular fashion: those conforming to the requirements of ArbInterop. These are roughly “types that, when randomly-generated, don’t retain pointers into the random-data buffer wrapped by the arbitrary::Unstructured they are generated from”. Many implementations of arbitrary::Arbitrary will fit the bill, but certain kinds of “zero-copy” implementations of arbitrary::Arbitrary will not work. This requirement appears to be a necessary part of the semantic model of proptest – generated values have to own their pointer graph, no borrows. Patches welcome if you can figure out a way to not require it.

This crate is based on proptest-quickcheck-interop by Mazdak Farrokhzad, without whose work I wouldn’t have had a clue how to approach this. The exact type signatures for the ArbInterop type are courtesy of Jim Blandy, who I hereby officially designate for-all-time as the Rust Puzzle King. Any errors I’ve introduced along the way are, of course, my own.

Structs

ArbStrategy

ArbValueTree

Constants

DEFAULT_SIZE

Default size (256) passed to arb_sized by arb.

Traits

ArbInterop

The subset of possible arbitrary::Arbitrary implementations that this crate works with. The main concern here is the for<'a> Arbitrary<'a> business, which (in practice) decouples the generated Arbitrary value from the lifetime of the random buffer it’s fed; I can’t actually explain how, because Rust’s type system is way over my head.

Functions

arb

Calls arb_sized with DEFAULT_SIZE which is 256.

arb_sized

Constructs a proptest::strategy::Strategy for a given arbitrary::Arbitrary type, generating size bytes of random data as input to the arbitrary::Arbitrary type.