Rand: add std_rng feature flag

Cargo.toml

@@ -22,18 +22,28 @@ appveyor = { repository = "rust-random/rand" }
 
 [features]
 # Meta-features:
-default = ["std"] # without "std" rand uses libcore
+default = ["std", "std_rng"]
 nightly = ["simd_support"] # enables all features requiring nightly rust
 serde1 = [] # does nothing, deprecated
 
-# Optional dependencies:
+# Option: without "std" rand uses libcore; this option enables functionality
+# expected to be available on a standard platform.
 std = ["rand_core/std", "rand_chacha/std", "alloc", "getrandom", "libc"]
-alloc = ["rand_core/alloc"] # enables Vec and Box support (without std)
+
+# Option: "alloc" enables support for Vec and Box when not using "std"
+alloc = ["rand_core/alloc"]
+
+# Option: use getrandom package for seeding
 getrandom = ["rand_core/getrandom"]
 
-# Configuration:
-simd_support = ["packed_simd"] # enables SIMD support
-small_rng = ["rand_pcg"] # enables SmallRng
+# Option: experimental SIMD support
+simd_support = ["packed_simd"]
+
+# Option: enable StdRng (enabled by default)
+std_rng = ["rand_chacha", "rand_hc"]
+
+# Option: enable SmallRng
+small_rng = ["rand_pcg"]
 
 [workspace]
 members = [
@@ -63,9 +73,9 @@ libc = { version = "0.2.22", optional = true, default-features = false }
 # Emscripten does not support 128-bit integers, which are used by ChaCha code.
 # We work around this by using a different RNG.
 [target.'cfg(not(target_os = "emscripten"))'.dependencies]
-rand_chacha = { path = "rand_chacha", version = "0.2.1", default-features = false }
+rand_chacha = { path = "rand_chacha", version = "0.2.1", default-features = false, optional = true }
 [target.'cfg(target_os = "emscripten")'.dependencies]
-rand_hc = { path = "rand_hc", version = "0.2" }
+rand_hc = { path = "rand_hc", version = "0.2", optional = true }
 
 [dev-dependencies]
 rand_pcg = { path = "rand_pcg", version = "0.2" }

README.md

@@ -90,19 +90,17 @@ Rand release if required, but the change must be noted in the changelog.
 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 (when using this feature in `no_std`, Rand requires Rustc version 1.36 or greater)
+- `alloc` (implied by `std`) enables functionality requiring an allocator
+ (when using this feature in `no_std`, Rand requires Rustc version 1.36 or
+ greater)
 - `getrandom` (implied by `std`) is an optional dependency providing the code
  behind `rngs::OsRng`
+- `std_rng` enables inclusion of `StdRng`, `thread_rng` and `random`
+ (the latter two *also* require that `std` be enabled)
 
 Optionally, the following dependencies can be enabled:
 
-- `log` enables logging via the `log` crate
-- `stdweb` implies `getrandom/stdweb` to enable
- `getrandom` support on `wasm32-unknown-unknown`
- (will be removed in rand 0.8; activate via `getrandom` crate instead)
-- `wasm-bindgen` implies `getrandom/wasm-bindgen` to enable
- `getrandom` support on `wasm32-unknown-unknown`
- (will be removed in rand 0.8; activate via `getrandom` crate instead)
+- `log` enables logging via the `log` crate` crate
 
 Additionally, these features configure Rand:
 

src/lib.rs

@@ -100,10 +100,12 @@ pub mod rngs;
 pub mod seq;
 
 // Public exports
-#[cfg(feature = "std")] pub use crate::rngs::thread::thread_rng;
+#[cfg(all(feature = "std", feature = "std_rng"))]
+pub use crate::rngs::thread::thread_rng;
 pub use rng::{Fill, Rng};
 
-#[cfg(feature = "std")] use crate::distributions::{Distribution, Standard};
+#[cfg(all(feature = "std", feature = "std_rng"))]
+use crate::distributions::{Distribution, Standard};
 
 /// Generates a random value using the thread-local random number generator.
 ///
@@ -147,7 +149,7 @@ pub use rng::{Fill, Rng};
 /// ```
 ///
 /// [`Standard`]: distributions::Standard
-#[cfg(feature = "std")]
+#[cfg(all(feature = "std", feature = "std_rng"))]
 #[inline]
 pub fn random<T>() -> T
 where Standard: Distribution<T> {
@@ -167,7 +169,7 @@ mod test {
  }
 
  #[test]
- #[cfg(feature = "std")]
+ #[cfg(all(feature = "std", feature = "std_rng"))]
  fn test_random() {
  // not sure how to test this aside from just getting some values
  let _n: usize = random();

src/prelude.rs

@@ -22,12 +22,13 @@
 #[cfg(feature = "small_rng")]
 #[doc(no_inline)]
 pub use crate::rngs::SmallRng;
+#[cfg(feature = "std_rng")]
 #[doc(no_inline)] pub use crate::rngs::StdRng;
 #[doc(no_inline)]
-#[cfg(feature = "std")]
+#[cfg(all(feature = "std", feature = "std_rng"))]
 pub use crate::rngs::ThreadRng;
 #[doc(no_inline)] pub use crate::seq::{IteratorRandom, SliceRandom};
 #[doc(no_inline)]
-#[cfg(feature = "std")]
+#[cfg(all(feature = "std", feature = "std_rng"))]
 pub use crate::{random, thread_rng};
 #[doc(no_inline)] pub use crate::{CryptoRng, Rng, RngCore, SeedableRng};

src/rngs/adapter/reseeding.rs

@@ -325,6 +325,7 @@ mod fork {
 }
 
 
+#[cfg(feature = "std_rng")]
 #[cfg(test)]
 mod test {
  use super::ReseedingRng;

src/rngs/mod.rs

@@ -102,15 +102,15 @@ pub mod adapter;
 pub mod mock; // Public so we don't export `StepRng` directly, making it a bit
  // more clear it is intended for testing.
 #[cfg(feature = "small_rng")] mod small;
-mod std;
-#[cfg(feature = "std")] pub(crate) mod thread;
+#[cfg(feature = "std_rng")] mod std;
+#[cfg(all(feature = "std", feature = "std_rng"))] pub(crate) mod thread;
 
 #[allow(deprecated)]
 #[cfg(feature = "std")]
 pub use self::entropy::EntropyRng;
 
 #[cfg(feature = "small_rng")] pub use self::small::SmallRng;
-pub use self::std::StdRng;
-#[cfg(feature = "std")] pub use self::thread::ThreadRng;
+#[cfg(feature = "std_rng")] pub use self::std::StdRng;
+#[cfg(all(feature = "std", feature = "std_rng"))] pub use self::thread::ThreadRng;
 
 #[cfg(feature = "getrandom")] pub use rand_core::OsRng;

src/rngs/thread.rs

@@ -8,8 +8,8 @@
 
 //! Thread-local random number generator
 
-use std::cell::UnsafeCell;
-use std::ptr::NonNull;
+use core::cell::UnsafeCell;
+use core::ptr::NonNull;
 
 use super::std::Core;
 use crate::rngs::adapter::ReseedingRng;