Added support for Arbitrary

[LucaCappelletti94]

This PR adds support for the arbitrary crate by implementing the Arbitrary trait for BigDecimal. This enables using BigDecimal with fuzzing tools like cargo-fuzz and libfuzzer, and more importantly allows to use the Arbitrary derive in third party structs which include this struct as a field.

A new optional feature fuzz has been added to Cargo.toml:

[features]
fuzz = ["std", "arbitrary", "num-bigint/arbitrary"]

Note: The feature is named fuzz (not arbitrary) to avoid a name collision with the dependency, which is required for compatibility with older Cargo versions (pre-1.60). The feature requires std because the arbitrary crate itself does not support no_std

Implementation Details

To prevent memory allocation failures during fuzzing, the implementation limits the generated scale to ±1,000,000. Without this limit, extremely large scale values could cause the BigDecimal to attempt allocating massive amounts of memory when converted to strings or performing certain operations.

• New module: src/impl_arbitrary.rs implementing Arbitrary<'a> for BigDecimal
• Implements all three trait methods:
  • arbitrary() - Generate a random BigDecimal from unstructured data
  • arbitrary_take_rest() - Consume all remaining data to generate a BigDecimal
  • size_hint() - Provide size hints for the fuzzer

Usage

Enable the feature in your Cargo.toml:

[dependencies]
bigdecimal = { version = "0.4", features = ["fuzz"] }

Example fuzz target:

#![no_main]
use libfuzzer_sys::fuzz_target;
use bigdecimal::BigDecimal;

fuzz_target!(|data: BigDecimal| {
    // Your fuzzing logic here
    let _ = data.to_string();
    let _ = data.sqrt();
});

Closes #122

[LucaCappelletti94]

For added context, this PR is needed to allow for adding Arbitrary derives in sqlparser, as shown here: apache/datafusion-sqlparser-rs#2193