Disable std_rng, sys_rng and thread_rng by default

CHANGELOG.md

@@ -10,6 +10,7 @@ You may also find the [Upgrade Guide](https://rust-random.github.io/book/update.
 
 ## [Unreleased]
 ### Changes
+- The features `std_rng`, `sys_rng` and `thread_rng` are now disabled by default ([#1733]).
 - The dependency on `rand_chacha` has been replaced with a dependency on `chacha20`. This changes the implementation behind `StdRng`, but the output remains the same. There may be some API breakage when using the ChaCha-types directly as these are now the ones in `chacha20` instead of `rand_chacha` (#1642).
 - Rename fns `IndexedRandom::choose_multiple` -> `sample`, `choose_multiple_array` -> `sample_array`, `choose_multiple_weighted` -> `sample_weighted`, struct `SliceChooseIter` -> `IndexedSamples` and fns `IteratorRandom::choose_multiple` -> `sample`, `choose_multiple_fill` -> `sample_fill` (#1632)
 - Use Edition 2024 and MSRV 1.85 (#1653)
@@ -38,6 +39,7 @@ You may also find the [Upgrade Guide](https://rust-random.github.io/book/update.
 [#1717]: https://github.com/rust-random/rand/pull/1717
 [#1722]: https://github.com/rust-random/rand/pull/1722
 [#1732]: https://github.com/rust-random/rand/pull/1732
+[#1733]: https://github.com/rust-random/rand/pull/1733
 
 ## [0.9.2] - 2025-07-20
 ### Deprecated

Cargo.toml

@@ -24,11 +24,11 @@ all-features = true
 rustdoc-args = ["--generate-link-to-definition"]
 
 [package.metadata.playground]
-features = ["serde"]
+features = ["serde", "std_rng", "sys_rng", "thread_rng"]
 
 [features]
 # Meta-features:
-default = ["std", "std_rng", "sys_rng", "thread_rng"]
+default = ["std"]
 serde = ["dep:serde"]
 
 # Option (enabled by default): without "std" rand uses libcore; this option
@@ -75,6 +75,7 @@ chacha20 = { version = "0.10.0-rc.10", default-features = false, features = ["rn
 getrandom = { version = "0.4.0-rc.1", optional = true }
 
 [dev-dependencies]
+rand = { path = ".", features = ["chacha", "thread_rng"] }
 rand_pcg = { path = "rand_pcg", version = "0.10.0-rc.9" }
 # Only to test serde
 postcard = {version = "1.1.3", default-features = false, features = ["alloc"]}

README.md

@@ -9,7 +9,7 @@ Rand is a set of crates supporting (pseudo-)random generators:
 
 -   Built over a standard RNG trait: [`rand_core::RngCore`](https://docs.rs/rand_core/latest/rand_core/trait.RngCore.html)
 -   With fast implementations of both [strong](https://rust-random.github.io/book/guide-rngs.html#cryptographically-secure-pseudo-random-number-generators-csprngs) and
-    [small](https://rust-random.github.io/book/guide-rngs.html#basic-pseudo-random-number-generators-prngs) generators: [`rand::rngs`](https://docs.rs/rand/latest/rand/rngs/index.html), and more RNGs: [`rand_chacha`](https://docs.rs/rand_chacha), [`rand_xoshiro`](https://docs.rs/rand_xoshiro/), [`rand_pcg`](https://docs.rs/rand_pcg/), [rngs repo](https://github.com/rust-random/rngs/)
+    [small](https://rust-random.github.io/book/guide-rngs.html#basic-pseudo-random-number-generators-prngs) generators: [`rand::rngs`](https://docs.rs/rand/latest/rand/rngs/index.html), and more RNGs: [`chacha20`](https://docs.rs/chacha20), [`rand_xoshiro`](https://docs.rs/rand_xoshiro/), [`rand_pcg`](https://docs.rs/rand_pcg/), [`rand_sfc`](https://docs.rs/rand_sfc/), [`rand_seeder`](https://docs.rs/rand_seeder/), [rngs repo](https://github.com/rust-random/rngs/)
 -   [`rand::rng`](https://docs.rs/rand/latest/rand/fn.rng.html) is an asymptotically-fast, automatically-seeded and reasonably strong generator available on all `std` targets
 -   Direct support for seeding generators from the [getrandom] crate
 
@@ -64,13 +64,16 @@ See the [CHANGELOG](https://github.com/rust-random/rand/blob/master/CHANGELOG.md
 Rand is built with these features enabled by default:
 
 -   `std` enables functionality dependent on the `std` lib
--   `alloc` (implied by `std`) enables functionality requiring an allocator
--   `sys_rng` (implied by `std`) enables `rngs::SysRng`, using the [getrandom] crate
--   `std_rng` enables inclusion of `StdRng`, `ThreadRng`
+-   `alloc` (implied by `std`) enables functionality requiring an allocator; a
+    significant portion of sequence and distribution functionality requires this
 
 Optionally, the following dependencies can be enabled:
 
--   `log` enables logging via [log](https://crates.io/crates/log)
+-   `sys_rng` enables `rand::rngs::SysRng` (uses the [getrandom] crate)
+-   `std_rng` enables `rand::rngs::StdRng` (uses the [chacha20] crate)
+-   `chacha` enables `rand::rngs::{ChaCha8Rng, ChaCha12Rng, ChaCha20Rng}` (uses the [chacha20] crate)
+-   `thread_rng` (implies `std`, `std_rng`, `sys_rng`) enables `rand::rngs::ThreadRng` and `rand::rng()`
+-   `log` enables logging (uses the [log] crate)
 
 Additionally, these features configure Rand:
 
@@ -83,11 +86,6 @@ Additionally, these features configure Rand:
 
     Note: enabling this option is expected to affect reproducibility of results.
 
-Rand supports limited functionality in `no_std` mode (enabled via
-`default-features = false`). In this case, `SysRng` is
-unavailable (unless `sys_rng` is enabled), large parts of `seq` are
-unavailable (unless `alloc` is enabled), and `ThreadRng` is unavailable.
-
 ## Portability
 
 ### Reproducibility
@@ -111,3 +109,5 @@ See [LICENSE-APACHE](https://github.com/rust-random/rand/blob/master/LICENSE-APA
 [COPYRIGHT](https://github.com/rust-random/rand/blob/master/COPYRIGHT) for details.
 
 [getrandom]: https://crates.io/crates/getrandom
+[chacha20]: https://crates.io/crates/chacha20
+[log]: https://crates.io/crates/log

examples/rayon-monte-carlo.rs

@@ -38,8 +38,8 @@
 //! over BATCH_SIZE trials. Manually batching also turns out to be faster
 //! for the nondeterministic version of this program as well.
 
-use chacha20::ChaCha8Rng;
 use rand::distr::{Distribution, Uniform};
+use rand::rngs::ChaCha8Rng;
 use rand_core::SeedableRng;
 use rayon::prelude::*;