crypto-common: vendor getrandom::SysRng (#2135)

It's now been a month since #2096 was merged and there still has not
been a corresponding `getrandom` release.

This commit temporarily internally vendors `SysRng` (which has a very
small implementation) until there is a `getrandom` crate release which
is usable. This will unblock work on rolling out the `Generate` trait.

Once there is, this commit can be reverted.

Author: tarcieri

Cargo.lock

@@ -16,5 +16,3 @@ members = [
 [patch.crates-io]
 digest = { path = "digest" }
 signature = { path = "signature" }
-
-getrandom = { git = "https://github.com/rust-random/getrandom" }

Cargo.toml

@@ -16,7 +16,7 @@ description = "Common cryptographic traits"
 hybrid-array = "0.4"
 
 # optional dependencies
-getrandom = { version = "0.3", optional = true, features = ["sys_rng"] }
+getrandom = { version = "0.3", optional = true }
 rand_core = { version = "0.10.0-rc-3", optional = true }
 
 [features]

crypto-common/Cargo.toml

@@ -22,7 +22,7 @@ pub trait Generate: Sized {
     /// Returns [`RngError`] in the event the system's ambient RNG experiences an internal failure.
     #[cfg(feature = "getrandom")]
     fn try_generate() -> Result<Self, RngError> {
-        Self::try_from_rng(&mut getrandom::SysRng)
+        Self::try_from_rng(&mut sys_rng::SysRng)
     }
 
     /// Randomly generate a value of this type using the system's ambient cryptographically secure
@@ -84,3 +84,35 @@ impl<U: ArraySize> Generate for Array<u64, U> {
         Self::try_from_fn(|_| rng.try_next_u64())
     }
 }
+
+#[cfg(feature = "getrandom")]
+mod sys_rng {
+    use getrandom::Error;
+    use rand_core::{TryCryptoRng, TryRngCore};
+
+    /// A [`TryRngCore`] interface over the system's preferred random number source
+    // TODO(tarcieri): replace this with `getrandom::SysRng` when `sys_rng` feature is available
+    #[derive(Clone, Copy, Debug, Default)]
+    pub struct SysRng;
+
+    impl TryRngCore for SysRng {
+        type Error = Error;
+
+        #[inline]
+        fn try_next_u32(&mut self) -> Result<u32, Error> {
+            getrandom::u32()
+        }
+
+        #[inline]
+        fn try_next_u64(&mut self) -> Result<u64, Error> {
+            getrandom::u64()
+        }
+
+        #[inline]
+        fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
+            getrandom::fill(dest)
+        }
+    }
+
+    impl TryCryptoRng for SysRng {}
+}