Deprecate rand::rngs::mock module and StepRng

Author: dhardy

CHANGELOG.md

@@ -8,6 +8,10 @@ A [separate changelog is kept for rand_core](rand_core/CHANGELOG.md).
 
 You may also find the [Upgrade Guide](https://rust-random.github.io/book/update.html) useful.
 
+## [Unreleased]
+### Deprecated
+- Deprecate `rand::rngs::mock` module and `StepRng` generator (#1634)
+
 ## [0.9.1] - 2025-04-17
 ### Security and unsafe
 - Revise "not a crypto library" policy again (#1565)

src/rng.rs

@@ -110,11 +110,11 @@ pub trait Rng: RngCore {
     /// # Example
     ///
     /// ```
-    /// use rand::{rngs::mock::StepRng, Rng};
+    /// use rand::{rngs::SmallRng, Rng, SeedableRng};
     ///
-    /// let rng = StepRng::new(1, 1);
+    /// let rng = SmallRng::seed_from_u64(0);
     /// let v: Vec<i32> = rng.random_iter().take(5).collect();
-    /// assert_eq!(&v, &[1, 2, 3, 4, 5]);
+    /// assert_eq!(v.len(), 5);
     /// ```
     #[inline]
     fn random_iter<T>(self) -> distr::Iter<StandardUniform, Self, T>

src/rngs/mock.rs

@@ -8,6 +8,8 @@
 
 //! Mock random number generator
 
+#![allow(deprecated)]
+
 use rand_core::{impls, RngCore};
 
 #[cfg(feature = "serde")]
@@ -31,6 +33,7 @@ use serde::{Deserialize, Serialize};
 /// # Example
 ///
 /// ```
+/// # #![allow(deprecated)]
 /// use rand::Rng;
 /// use rand::rngs::mock::StepRng;
 ///
@@ -40,6 +43,7 @@ use serde::{Deserialize, Serialize};
 /// ```
 #[derive(Debug, Clone, PartialEq, Eq)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[deprecated(since = "0.9.2", note = "Deprecated without replacement")]
 pub struct StepRng {
     v: u64,
     a: u64,

src/rngs/mod.rs

@@ -80,6 +80,7 @@
 mod reseeding;
 pub use reseeding::ReseedingRng;
 
+#[deprecated(since = "0.9.2")]
 pub mod mock; // Public so we don't export `StepRng` directly, making it a bit
               // more clear it is intended for testing.