feat: Implement Step trait for BigInt and BigUint

Cargo.toml

@@ -22,6 +22,7 @@ arbitrary = ["dep:arbitrary"]
 quickcheck = ["dep:quickcheck"]
 rand = ["dep:rand"]
 serde = ["dep:serde"]
+nightly-step = []
 
 [package.metadata.docs.rs]
 features = ["std", "serde", "rand", "quickcheck", "arbitrary"]
@@ -73,3 +74,6 @@ default-features = false
 optional = true
 version = "1"
 default-features = false
+
+[dev-dependencies]
+itertools = { version = "0.12.1", default-features = false }

README.md

@@ -36,6 +36,10 @@ num-bigint = { version = "0.4", features = ["rand"] }
 Note that you must use the version of `rand` that `num-bigint` is compatible
 with: `0.8`.
 
+### Range iterators
+
+On nightly you can enable `nightly-step` feature that allows you to iterate over ranges such as `Bigint::zero()..`; keep in mind that it relies on `step_trait` nightly features and is prone to break with a new Rust or `num-bigint` release.
+
 ## Releases
 
 Release notes are available in [RELEASES.md](RELEASES.md).

src/lib.rs

@@ -78,11 +78,19 @@
 //! Note that you must use the version of `rand` that `num-bigint` is compatible
 //! with: `0.8`.
 //!
+//! ### Range iterators
+//!
+//! On nightly you can enable `nightly-step` feature that allows you to iterate
+//! over ranges such as `Bigint::zero()..`;
+//! keep in mind that it relies on `step_trait` nightly features and is prone
+//! to break with a new Rust or `num-bigint` release.
+//!
 //!
 //! ## Compatibility
 //!
 //! The `num-bigint` crate is tested for rustc 1.60 and greater.
 
+#![cfg_attr(feature = "nightly-step", feature(step_trait))]
 #![doc(html_root_url = "https://docs.rs/num-bigint/0.4")]
 #![warn(rust_2018_idioms)]
 #![no_std]
@@ -126,6 +134,9 @@ mod biguint;
 #[cfg(feature = "rand")]
 mod bigrand;
 
+#[cfg(feature = "nightly-step")]
+mod step;
+
 #[cfg(target_pointer_width = "32")]
 type UsizePromotion = u32;
 #[cfg(target_pointer_width = "64")]

src/step.rs

@@ -0,0 +1,62 @@
+use core::iter::Step;
+
+use num_traits::CheckedSub;
+
+use crate::{BigInt, BigUint};
+
+impl Step for BigUint {
+    fn steps_between(start: &Self, end: &Self) -> Option<usize> {
+        end.checked_sub(start)?.try_into().ok()
+    }
+
+    fn forward_checked(start: Self, count: usize) -> Option<Self> {
+        Some(Self::forward(start, count))
+    }
+
+    fn backward_checked(start: Self, count: usize) -> Option<Self> {
+        start.checked_sub(&count.into())
+    }
+
+    fn forward(start: Self, count: usize) -> Self {
+        start + count
+    }
+}
+
+impl Step for BigInt {
+    fn steps_between(start: &Self, end: &Self) -> Option<usize> {
+        (end - start).try_into().ok()
+    }
+
+    fn forward_checked(start: Self, count: usize) -> Option<Self> {
+        Some(Self::forward(start, count))
+    }
+
+    fn backward_checked(start: Self, count: usize) -> Option<Self> {
+        Some(Self::backward(start, count))
+    }
+
+    fn forward(start: Self, count: usize) -> Self {
+        start + count
+    }
+
+    fn backward(start: Self, count: usize) -> Self {
+        start - count
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use itertools::assert_equal;
+    use num_traits::Zero;
+
+    use crate::{BigInt, BigUint};
+
+    #[test]
+    fn ranges() {
+        assert_equal(
+            BigUint::zero()..BigUint::from(6u8),
+            (0u8..6).map(BigUint::from),
+        );
+        assert_equal(BigInt::from(-6)..BigInt::from(6), (-6..6).map(BigInt::from));
+    }
+}