Move OsRng down to rand (#1674)

dhardy · web-flow · commit 5d07c2dc7f5f · 2025-10-30T08:54:50.000Z
Moves `OsRng` from `rand_core` to `rand`.
Removes fns `SeedableRng::from_os_rng` and `try_from_os_rng`.
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
@@ -117,9 +117,8 @@ jobs:
           cargo test --target ${{ matrix.target }} --features=serde,log,small_rng
       - name: Test rand_core
         run: |
-          cargo test --target ${{ matrix.target }} --manifest-path rand_core/Cargo.toml
           cargo test --target ${{ matrix.target }} --manifest-path rand_core/Cargo.toml --no-default-features
-          cargo test --target ${{ matrix.target }} --manifest-path rand_core/Cargo.toml --no-default-features --features=os_rng
+          cargo test --target ${{ matrix.target }} --manifest-path rand_core/Cargo.toml --features serde
       - name: Test rand_pcg
         run: cargo test --target ${{ matrix.target }} --manifest-path rand_pcg/Cargo.toml --features=serde
       - name: Test rand_chacha
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -14,7 +14,9 @@ You may also find the [Upgrade Guide](https://rust-random.github.io/book/update.
 - 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)
 - Let `Fill` be implemented for element types, not sliceable types (#1652)
+- Fix `OsError::raw_os_error` on UEFI targets by returning `Option<usize>` (#1665)
 - Replace fn `TryRngCore::read_adapter(..) -> RngReadAdapter` with simpler struct `RngReader` (#1669)
+- Remove fns `SeedableRng::from_os_rng`, `try_from_os_rng` (#1674)
 
 ### Additions
 - Add fns `IndexedRandom::choose_iter`, `choose_weighted_iter` (#1632)
diff --git a/Cargo.toml b/Cargo.toml
@@ -34,13 +34,13 @@ serde = ["dep:serde", "rand_core/serde"]
 
 # Option (enabled by default): without "std" rand uses libcore; this option
 # enables functionality expected to be available on a standard platform.
-std = ["rand_core/std", "alloc"]
+std = ["alloc", "getrandom?/std"]
 
 # Option: "alloc" enables support for Vec and Box when not using "std"
 alloc = []
 
 # Option: enable OsRng
-os_rng = ["rand_core/os_rng"]
+os_rng = ["dep:getrandom"]
 
 # Option (requires nightly Rust): experimental SIMD support
 simd_support = []
@@ -83,6 +83,7 @@ rand_core = { path = "rand_core", version = "0.9.0", default-features = false }
 log = { version = "0.4.4", optional = true }
 serde = { version = "1.0.103", features = ["derive"], optional = true }
 chacha20 = { version = "=0.10.0-rc.2", default-features = false, features = ["rng"], optional = true }
+getrandom = { version = "0.3.0", optional = true }
 
 [dev-dependencies]
 rand_pcg = { path = "rand_pcg", version = "0.9.0" }
diff --git a/README.md b/README.md
@@ -91,7 +91,7 @@ compiler versions will be compatible. This is especially true of Rand's
 experimental `simd_support` feature.
 
 Rand supports limited functionality in `no_std` mode (enabled via
-`default-features = false`). In this case, `OsRng` and `from_os_rng` are
+`default-features = false`). In this case, `OsRng` is
 unavailable (unless `os_rng` is enabled), large parts of `seq` are
 unavailable (unless `alloc` is enabled), and `ThreadRng` is unavailable.
 
diff --git a/SECURITY.md b/SECURITY.md
@@ -24,8 +24,7 @@ used according to these additional constraints:
 
     -   Via `SeedableRng::from_seed` using a cryptographically secure seed value
     -   Via `SeedableRng::from_rng` or `try_from_rng` using a cryptographically
-        secure source `rng`
-    -   Via `SeedableRng::from_os_rng` or `try_from_os_rng`
+        secure source `rng` such as `OsRng` or `ThreadRng`.
 -   The state (memory) of the generator and its seed value (or source `rng`) are
     not exposed
 
diff --git a/rand_chacha/CHANGELOG.md b/rand_chacha/CHANGELOG.md
@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [0.10.0] - UNRELEASED
 ### Changed
 - Bump MSRV to 1.85 and edition to 2024 (#1671)
+- Remove feature `os_rng` (#1674)
 
 ## [0.9.0] - 2025-01-27
 ### Dependencies and features
diff --git a/rand_chacha/Cargo.toml b/rand_chacha/Cargo.toml
@@ -27,10 +27,10 @@ serde = { version = "1.0", features = ["derive"], optional = true }
 [dev-dependencies]
 # Only to test serde
 serde_json = "1.0.120"
-rand_core = { path = "../rand_core", version = "0.9.0", features = ["os_rng"] }
+rand_core = { path = "../rand_core", version = "0.9.0" }
+rand = { path = "..", version = "0.10.0-rc.0" }
 
 [features]
 default = ["std"]
-os_rng = ["rand_core/os_rng"]
-std = ["ppv-lite86/std", "rand_core/std"]
+std = ["ppv-lite86/std"]
 serde = ["dep:serde"]
diff --git a/rand_chacha/README.md b/rand_chacha/README.md
@@ -35,8 +35,7 @@ Links:
 
 `rand_chacha` is `no_std` compatible when disabling default features; the `std`
 feature can be explicitly required to re-enable `std` support. Using `std`
-allows detection of CPU features and thus better optimisation. Using `std`
-also enables `os_rng` functionality, such as `ChaCha20Rng::from_os_rng()`.
+allows detection of CPU features and thus better optimisation.
 
 
 # License
diff --git a/rand_chacha/src/lib.rs b/rand_chacha/src/lib.rs
@@ -24,7 +24,7 @@
 //!
 //! Where secure unpredictable generators are required, it is suggested to use
 //! [`ChaCha12Rng`] or [`ChaCha20Rng`] and to seed via
-//! [`SeedableRng::from_os_rng`].
+//! [`OsRng`].
 //!
 //! See also the [Security] chapter in the rand book. The crate is provided
 //! "as is", without any form of guarantee, and without a security audit.
@@ -37,8 +37,8 @@
 //!
 //! 1.  With a fresh seed, **direct from the OS** (implies a syscall):
 //!     ```
-//!     # use {rand_core::SeedableRng, rand_chacha::ChaCha12Rng};
-//!     let rng = ChaCha12Rng::from_os_rng();
+//!     # use {rand_core::SeedableRng, rand_chacha::ChaCha12Rng, rand::rngs::OsRng};
+//!     let rng = ChaCha12Rng::try_from_rng(&mut OsRng).unwrap();
 //!     # let _: ChaCha12Rng = rng;
 //!     ```
 //! 2.  **From a master generator.** This could be [`rand::rng`]
@@ -76,7 +76,7 @@
 //! [`BlockRngCore`]: rand_core::block::BlockRngCore
 //! [`RngCore`]: rand_core::RngCore
 //! [`SeedableRng`]: rand_core::SeedableRng
-//! [`SeedableRng::from_os_rng`]: rand_core::SeedableRng::from_os_rng
+//! [`OsRng`]: https://docs.rs/rand/latest/rand/rngs/struct.OsRng.html
 //! [`rand::rng`]: https://docs.rs/rand/latest/rand/fn.rng.html
 //! [`rand::Rng`]: https://docs.rs/rand/latest/rand/trait.Rng.html
 
diff --git a/rand_core/CHANGELOG.md b/rand_core/CHANGELOG.md
@@ -7,10 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## Unreleased
 ### API changes
 - Relax `Sized` bound on impls of `SeedableRng` (#1641)
-- Fix `OsError::raw_os_error` on UEFI targets by returning `Option<usize>` (#1665)
 - Move `rand_core::impls::*` to `rand_core::le` module (#1667)
 - Use Edition 2024 and MSRV 1.85 (#1668)
 - Remove fn `TryRngCore::read_adapter(..) -> RngReadAdapter` (replaced with `rand::RngReader`) (#1669)
+- Remove feature `os_rng`, structs `OsRng` and `OsError` and fns `from_os_rng`, `try_from_os_rng` (#1674)
+- Remove feature `std` (#1674)
+- Removed dependency `getrandom` (#1674)
 
 ## [0.9.3] — 2025-02-29
 ### Other
diff --git a/rand_core/Cargo.toml b/rand_core/Cargo.toml
@@ -25,10 +25,7 @@ rustdoc-args = ["--generate-link-to-definition"]
 all-features = true
 
 [features]
-std = ["getrandom?/std"]
-os_rng = ["dep:getrandom"]
 serde = ["dep:serde"] # enables serde for BlockRng wrapper
 
 [dependencies]
 serde = { version = "1", features = ["derive"], optional = true }
-getrandom = { version = "0.3.0", optional = true }
diff --git a/rand_core/src/lib.rs b/rand_core/src/lib.rs
@@ -36,18 +36,10 @@
 #![cfg_attr(docsrs, feature(doc_cfg))]
 #![no_std]
 
-#[cfg(feature = "std")]
-extern crate std;
-
 use core::{fmt, ops::DerefMut};
 
 pub mod block;
 pub mod le;
-#[cfg(feature = "os_rng")]
-mod os;
-
-#[cfg(feature = "os_rng")]
-pub use os::{OsError, OsRng};
 
 /// Implementation-level interface for RNGs
 ///
@@ -168,6 +160,8 @@ where
 /// An optional property of CSPRNGs is backtracking resistance: if the CSPRNG's
 /// state is revealed, it will not be computationally-feasible to reconstruct
 /// prior output values. This property is not required by `CryptoRng`.
+///
+/// [`OsRng`]: https://docs.rs/rand/latest/rand/rngs/struct.OsRng.html
 pub trait CryptoRng: RngCore {}
 
 impl<T: DerefMut> CryptoRng for T where T::Target: CryptoRng {}
@@ -184,6 +178,8 @@ impl<T: DerefMut> CryptoRng for T where T::Target: CryptoRng {}
 /// An implementation of this trait may be made compatible with code requiring
 /// an [`RngCore`] through [`TryRngCore::unwrap_err`]. The resulting RNG will
 /// panic in case the underlying fallible RNG yields an error.
+///
+/// [`OsRng`]: https://docs.rs/rand/latest/rand/rngs/struct.OsRng.html
 pub trait TryRngCore {
     /// The type returned in the event of a RNG error.
     type Error: fmt::Debug + fmt::Display;
@@ -246,6 +242,8 @@ impl<R: RngCore + ?Sized> TryRngCore for R {
 /// `default()` instances are themselves secure generators: for example if the
 /// implementing type is a stateless interface over a secure external generator
 /// (like [`OsRng`]) or if the `default()` instance uses a strong, fresh seed.
+///
+/// [`OsRng`]: https://docs.rs/rand/latest/rand/rngs/struct.OsRng.html
 pub trait TryCryptoRng: TryRngCore {}
 
 impl<R: CryptoRng + ?Sized> TryCryptoRng for R {}
@@ -490,48 +488,6 @@ pub trait SeedableRng: Sized {
         rng.try_fill_bytes(seed.as_mut())?;
         Ok(Self::from_seed(seed))
     }
-
-    /// Creates a new instance of the RNG seeded via [`getrandom`].
-    ///
-    /// This method is the recommended way to construct non-deterministic PRNGs
-    /// since it is convenient and secure.
-    ///
-    /// Note that this method may panic on (extremely unlikely) [`getrandom`] errors.
-    /// If it's not desirable, use the [`try_from_os_rng`] method instead.
-    ///
-    /// In case the overhead of using [`getrandom`] to seed *many* PRNGs is an
-    /// issue, one may prefer to seed from a local PRNG, e.g.
-    /// `from_rng(rand::rng()).unwrap()`.
-    ///
-    /// # Panics
-    ///
-    /// If [`getrandom`] is unable to provide secure entropy this method will panic.
-    ///
-    /// [`getrandom`]: https://docs.rs/getrandom
-    /// [`try_from_os_rng`]: SeedableRng::try_from_os_rng
-    #[cfg(feature = "os_rng")]
-    fn from_os_rng() -> Self {
-        match Self::try_from_os_rng() {
-            Ok(res) => res,
-            Err(err) => panic!("from_os_rng failed: {}", err),
-        }
-    }
-
-    /// Creates a new instance of the RNG seeded via [`getrandom`] without unwrapping
-    /// potential [`getrandom`] errors.
-    ///
-    /// In case the overhead of using [`getrandom`] to seed *many* PRNGs is an
-    /// issue, one may prefer to seed from a local PRNG, e.g.
-    /// `from_rng(&mut rand::rng()).unwrap()`.
-    ///
-    /// [`getrandom`]: https://docs.rs/getrandom
-    #[cfg(feature = "os_rng")]
-    fn try_from_os_rng() -> Result<Self, getrandom::Error> {
-        let mut seed = Self::Seed::default();
-        getrandom::fill(seed.as_mut())?;
-        let res = Self::from_seed(seed);
-        Ok(res)
-    }
 }
 
 #[cfg(test)]
diff --git a/rand_pcg/CHANGELOG.md b/rand_pcg/CHANGELOG.md
@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [0.10.0 — Unreleased]
 ### Changes
 - Use Edition 2024 and MSRV 1.85 (#1653)
+- Remove feature `os_rng` (#1674)
 
 ## [0.9.0] - 2025-01-27
 ### Dependencies and features
diff --git a/rand_pcg/Cargo.toml b/rand_pcg/Cargo.toml
@@ -21,7 +21,6 @@ rustdoc-args = ["--generate-link-to-definition"]
 
 [features]
 serde = ["dep:serde"]
-os_rng = ["rand_core/os_rng"]
 
 [dependencies]
 rand_core = { path = "../rand_core", version = "0.9.0" }
@@ -32,4 +31,4 @@ serde = { version = "1", features = ["derive"], optional = true }
 # deps yet, see: https://github.com/rust-lang/cargo/issues/1596
 # Versions prior to 1.1.4 had incorrect minimal dependencies.
 bincode = { version = "1.1.4" }
-rand_core = { path = "../rand_core", version = "0.9.0", features = ["os_rng"] }
+rand_core = { path = "../rand_core", version = "0.9.0" }
diff --git a/src/rngs/mod.rs b/src/rngs/mod.rs
@@ -110,6 +110,9 @@ mod std;
 #[cfg(feature = "thread_rng")]
 pub(crate) mod thread;
 
+#[cfg(feature = "os_rng")]
+mod os;
+
 #[cfg(feature = "small_rng")]
 pub use self::small::SmallRng;
 #[cfg(feature = "small_rng")]
@@ -126,4 +129,4 @@ pub use self::thread::ThreadRng;
 pub use chacha20::{ChaCha8Rng, ChaCha12Rng, ChaCha20Rng};
 
 #[cfg(feature = "os_rng")]
-pub use rand_core::OsRng;
+pub use os::{OsError, OsRng};
diff --git a/src/rngs/os.rs b/src/rngs/os.rs
@@ -35,7 +35,7 @@ use crate::{TryCryptoRng, TryRngCore};
 ///
 /// # Usage example
 /// ```
-/// use rand_core::{TryRngCore, OsRng};
+/// use rand::{TryRngCore, rngs::OsRng};
 ///
 /// let mut key = [0u8; 16];
 /// OsRng.try_fill_bytes(&mut key).unwrap();
diff --git a/src/rngs/small.rs b/src/rngs/small.rs
@@ -47,13 +47,6 @@ type Rng = super::xoshiro256plusplus::Xoshiro256PlusPlus;
 ///     let rng = SmallRng::from_rng(&mut rand::rng());
 ///     # let _: SmallRng = rng;
 ///     ```
-///     or [`SeedableRng::from_os_rng`]:
-///     ```
-///     # use rand::SeedableRng;
-///     # use rand::rngs::SmallRng;
-///     let rng = SmallRng::from_os_rng();
-///     # let _: SmallRng = rng;
-///     ```
 /// 2.  To use a deterministic integral seed, use `seed_from_u64`. This uses a
 ///     hash function internally to yield a (typically) good seed from any
 ///     input.
diff --git a/src/rngs/std.rs b/src/rngs/std.rs
@@ -39,8 +39,8 @@ use chacha20::ChaCha12Rng as Rng;
 ///
 /// Using a fresh seed **direct from the OS** is the most secure option:
 /// ```
-/// # use rand::{SeedableRng, rngs::StdRng};
-/// let rng = StdRng::from_os_rng();
+/// # use rand::{SeedableRng, rngs::{StdRng, OsRng}};
+/// let rng = StdRng::try_from_rng(&mut OsRng).unwrap();
 /// # let _: StdRng = rng;
 /// ```
 ///
diff --git a/src/rngs/thread.rs b/src/rngs/thread.rs
@@ -13,12 +13,9 @@ use std::fmt;
 use std::rc::Rc;
 use std::thread_local;
 
+use super::{OsError, OsRng, ReseedingRng, std::Core};
 use rand_core::{CryptoRng, RngCore};
 
-use super::std::Core;
-use crate::rngs::OsRng;
-use crate::rngs::ReseedingRng;
-
 // Rationale for using `UnsafeCell` in `ThreadRng`:
 //
 // Previously we used a `RefCell`, with an overhead of ~15%. There will only
@@ -100,7 +97,7 @@ impl ThreadRng {
     /// Immediately reseed the generator
     ///
     /// This discards any remaining random data in the cache.
-    pub fn reseed(&mut self) -> Result<(), rand_core::OsError> {
+    pub fn reseed(&mut self) -> Result<(), OsError> {
         // SAFETY: We must make sure to stop using `rng` before anyone else
         // creates another mutable reference
         let rng = unsafe { &mut *self.rng.get() };
