From 6daae3e9536c82aad78d612a307189fbb76ad9c2 Mon Sep 17 00:00:00 2001
From: Diggory Hardy <git@dhardy.name>
Date: Thu, 16 May 2019 13:12:20 +0100
Subject: [PATCH 1/9] rand_core: remove ErrorKind enum

This is virtually no usage, thus it was decided to remove
---
 rand_core/src/error.rs        | 105 ++++++----------------------------
 rand_core/src/lib.rs          |   2 +-
 rand_jitter/src/error.rs      |   7 ++-
 rand_os/src/lib.rs            |   7 +--
 src/lib.rs                    |   9 +--
 src/rngs/adapter/read.rs      |  17 ++----
 src/rngs/adapter/reseeding.rs |  22 +++----
 src/rngs/os.rs                |   7 +--
 8 files changed, 43 insertions(+), 133 deletions(-)

diff --git a/rand_core/src/error.rs b/rand_core/src/error.rs
index 5a8459ea8be..6d343be14f4 100644
--- a/rand_core/src/error.rs
+++ b/rand_core/src/error.rs
@@ -15,80 +15,18 @@ use std::error::Error as stdError;
 #[cfg(feature="std")]
 use std::io;
 
-/// Error kind which can be matched over.
-#[derive(PartialEq, Eq, Debug, Copy, Clone)]
-pub enum ErrorKind {
-    /// Feature is not available; not recoverable.
-    /// 
-    /// This is the most permanent failure type and implies the error cannot be
-    /// resolved simply by retrying (e.g. the feature may not exist in this
-    /// build of the application or on the current platform).
-    Unavailable,
-    /// General failure; there may be a chance of recovery on retry.
-    /// 
-    /// This is the catch-all kind for errors from known and unknown sources
-    /// which do not have a more specific kind / handling method.
-    /// 
-    /// It is suggested to retry a couple of times or retry later when
-    /// handling; some error sources may be able to resolve themselves,
-    /// although this is not likely.
-    Unexpected,
-    /// A transient failure which likely can be resolved or worked around.
-    /// 
-    /// This error kind exists for a few specific cases where it is known that
-    /// the error likely can be resolved internally, but is reported anyway.
-    Transient,
-    /// Not ready yet: recommended to try again a little later.
-    /// 
-    /// This error kind implies the generator needs more time or needs some
-    /// other part of the application to do something else first before it is
-    /// ready for use; for example this may be used by external generators
-    /// which require time for initialization.
-    NotReady,
-    #[doc(hidden)]
-    __Nonexhaustive,
-}
-
-impl ErrorKind {
-    /// True if this kind of error may resolve itself on retry.
-    /// 
-    /// See also `should_wait()`.
-    pub fn should_retry(self) -> bool {
-        self != ErrorKind::Unavailable
-    }
-    
-    /// True if we should retry but wait before retrying
-    /// 
-    /// This implies `should_retry()` is true.
-    pub fn should_wait(self) -> bool {
-        self == ErrorKind::NotReady
-    }
-    
-    /// A description of this error kind
-    pub fn description(self) -> &'static str {
-        match self {
-            ErrorKind::Unavailable => "permanently unavailable",
-            ErrorKind::Unexpected => "unexpected failure",
-            ErrorKind::Transient => "transient failure",
-            ErrorKind::NotReady => "not ready yet",
-            ErrorKind::__Nonexhaustive => unreachable!(),
-        }
-    }
-}
 
 
 /// Error type of random number generators
 /// 
 /// This is a relatively simple error type, designed for compatibility with and
-/// without the Rust `std` library. It embeds a "kind" code, a message (static
-/// string only), and an optional chained cause (`std` only). The `kind` and
-/// `msg` fields can be accessed directly; cause can be accessed via
+/// without the Rust `std` library. It embeds a message (static
+/// string only), and an optional chained cause (`std` only). The
+/// `msg` field can be accessed directly; cause can be accessed via
 /// `std::error::Error::cause` or `Error::take_cause`. Construction can only be
 /// done via `Error::new` or `Error::with_cause`.
 #[derive(Debug)]
 pub struct Error {
-    /// The error kind
-    pub kind: ErrorKind,
     /// The error message
     pub msg: &'static str,
     #[cfg(feature="std")]
@@ -96,18 +34,17 @@ pub struct Error {
 }
 
 impl Error {
-    /// Create a new instance, with specified kind and a message.
-    pub fn new(kind: ErrorKind, msg: &'static str) -> Self {
+    /// Create a new instance, with a message.
+    pub fn new(msg: &'static str) -> Self {
         #[cfg(feature="std")] {
-            Error { kind, msg, cause: None }
+            Error { msg, cause: None }
         }
         #[cfg(not(feature="std"))] {
-            Error { kind, msg }
+            Error {  msg }
         }
     }
     
-    /// Create a new instance, with specified kind, message, and a
-    /// chained cause.
+    /// Create a new instance, with a message and a chained cause.
     /// 
     /// Note: `stdError` is an alias for `std::error::Error`.
     /// 
@@ -116,19 +53,18 @@ impl Error {
     /// type `E` (because both `Box` and `stdError` are unavailable), and the
     /// `cause` is ignored.
     #[cfg(feature="std")]
-    pub fn with_cause<E>(kind: ErrorKind, msg: &'static str, cause: E) -> Self
+    pub fn with_cause<E>(msg: &'static str, cause: E) -> Self
         where E: Into<Box<stdError + Send + Sync>>
     {
-        Error { kind, msg, cause: Some(cause.into()) }
+        Error { msg, cause: Some(cause.into()) }
     }
     
-    /// Create a new instance, with specified kind, message, and a
-    /// chained cause.
+    /// Create a new instance, with a message and a chained cause.
     /// 
     /// In `no_std` mode the *cause* is ignored.
     #[cfg(not(feature="std"))]
-    pub fn with_cause<E>(kind: ErrorKind, msg: &'static str, _cause: E) -> Self {
-        Error { kind, msg }
+    pub fn with_cause<E>(msg: &'static str, _cause: E) -> Self {
+        Error { msg }
     }
     
     /// Take the cause, if any. This allows the embedded cause to be extracted.
@@ -143,11 +79,11 @@ impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         #[cfg(feature="std")] {
             if let Some(ref cause) = self.cause {
-                return write!(f, "{} ({}); cause: {}",
-                        self.msg, self.kind.description(), cause);
+                return write!(f, "{}; cause: {}",
+                        self.msg, cause);
             }
         }
-        write!(f, "{} ({})", self.msg, self.kind.description())
+        write!(f, "{}", self.msg)
     }
 }
 
@@ -165,13 +101,6 @@ impl stdError for Error {
 #[cfg(feature="std")]
 impl From<Error> for io::Error {
     fn from(error: Error) -> Self {
-        use std::io::ErrorKind::*;
-        match error.kind {
-            ErrorKind::Unavailable => io::Error::new(NotFound, error),
-            ErrorKind::Unexpected |
-            ErrorKind::Transient => io::Error::new(Other, error),
-            ErrorKind::NotReady => io::Error::new(WouldBlock, error),
-            ErrorKind::__Nonexhaustive => unreachable!(),
-        }
+        io::Error::new(io::ErrorKind::Other, error)
     }
 }
diff --git a/rand_core/src/lib.rs b/rand_core/src/lib.rs
index 4b0e6e48b01..6d48e892592 100644
--- a/rand_core/src/lib.rs
+++ b/rand_core/src/lib.rs
@@ -50,7 +50,7 @@ use core::ptr::copy_nonoverlapping;
 
 #[cfg(all(feature="alloc", not(feature="std")))] use alloc::boxed::Box;
 
-pub use error::{ErrorKind, Error};
+pub use error::Error;
 
 
 mod error;
diff --git a/rand_jitter/src/error.rs b/rand_jitter/src/error.rs
index a3483e84d0f..b643aebc9db 100644
--- a/rand_jitter/src/error.rs
+++ b/rand_jitter/src/error.rs
@@ -7,7 +7,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use rand_core::{Error, ErrorKind};
+use rand_core::Error;
 use core::fmt;
 
 /// An error that can occur when [`JitterRng::test_timer`] fails.
@@ -60,10 +60,11 @@ impl From<TimerError> for Error {
         // Timer check is already quite permissive of failures so we don't
         // expect false-positive failures, i.e. any error is irrecoverable.
         #[cfg(feature = "std")] {
-            Error::with_cause(ErrorKind::Unavailable, "timer jitter failed basic quality tests", err)
+            Error::with_cause("timer jitter failed basic quality tests", err)
         }
         #[cfg(not(feature = "std"))] {
-            Error::new(ErrorKind::Unavailable, "timer jitter failed basic quality tests")
+            let _ = err;
+            Error::new("timer jitter failed basic quality tests")
         }
     }
 }
diff --git a/rand_os/src/lib.rs b/rand_os/src/lib.rs
index 2849f4fa2d8..34ec811039b 100644
--- a/rand_os/src/lib.rs
+++ b/rand_os/src/lib.rs
@@ -35,13 +35,13 @@
 pub use rand_core;  // re-export
 
 use getrandom::getrandom;
-use rand_core::{CryptoRng, RngCore, Error, ErrorKind, impls};
+use rand_core::{CryptoRng, RngCore, Error, impls};
 
 /// A random number generator that retrieves randomness from from the
 /// operating system.
 ///
 /// This is a zero-sized struct. It can be freely constructed with `OsRng`.
-/// 
+///
 /// The implementation is provided by the [getrandom] crate. Refer to
 /// [getrandom] documentation for details.
 ///
@@ -88,8 +88,7 @@ impl RngCore for OsRng {
         // (And why waste a system call?)
         if dest.len() == 0 { return Ok(()); }
         
-        getrandom(dest).map_err(|e|
-            Error::with_cause(ErrorKind::Unavailable, "OsRng failed", e))
+        getrandom(dest).map_err(|e| Error::with_cause("OsRng failed", e))
     }
 }
 
diff --git a/src/lib.rs b/src/lib.rs
index 35ac0b94d48..dbaf668eae4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -79,8 +79,7 @@ extern crate rand_pcg;
 
 
 // Re-exports from rand_core
-pub use rand_core::{RngCore, CryptoRng, SeedableRng};
-pub use rand_core::{ErrorKind, Error};
+pub use rand_core::{RngCore, CryptoRng, SeedableRng, Error};
 
 // Public exports
 #[cfg(feature="std")] pub use rngs::thread::thread_rng;
@@ -276,10 +275,8 @@ pub trait Rng: RngCore {
     /// On big-endian platforms this performs byte-swapping to ensure
     /// portability of results from reproducible generators.
     ///
-    /// This uses [`try_fill_bytes`] internally and forwards all RNG errors. In
-    /// some cases errors may be resolvable; see [`ErrorKind`] and
-    /// documentation for the RNG in use. If you do not plan to handle these
-    /// errors you may prefer to use [`fill`].
+    /// This is identical to [`fill`] except that it uses [`try_fill_bytes`]
+    /// internally and forwards RNG errors.
     ///
     /// # Example
     ///
diff --git a/src/rngs/adapter/read.rs b/src/rngs/adapter/read.rs
index b7787702d6a..4c08f97362b 100644
--- a/src/rngs/adapter/read.rs
+++ b/src/rngs/adapter/read.rs
@@ -11,7 +11,7 @@
 
 use std::io::Read;
 
-use rand_core::{RngCore, Error, ErrorKind, impls};
+use rand_core::{RngCore, Error, impls};
 
 
 /// An RNG that reads random bytes straight from any type supporting
@@ -73,22 +73,15 @@ impl<R: Read> RngCore for ReadRng<R> {
     fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
         if dest.len() == 0 { return Ok(()); }
         // Use `std::io::read_exact`, which retries on `ErrorKind::Interrupted`.
-        self.reader.read_exact(dest).map_err(|err| {
-            match err.kind() {
-                ::std::io::ErrorKind::UnexpectedEof => Error::with_cause(
-                    ErrorKind::Unavailable,
-                    "not enough bytes available, reached end of source", err),
-                _ => Error::with_cause(ErrorKind::Unavailable,
-                    "error reading from Read source", err)
-            }
-        })
+        self.reader.read_exact(dest).map_err(|err|
+                Error::with_cause("error reading from Read source", err))
     }
 }
 
 #[cfg(test)]
 mod test {
     use super::ReadRng;
-    use {RngCore, ErrorKind};
+    use RngCore;
 
     #[test]
     fn test_reader_rng_u64() {
@@ -131,6 +124,6 @@ mod test {
 
         let mut rng = ReadRng::new(&v[..]);
 
-        assert!(rng.try_fill_bytes(&mut w).err().unwrap().kind == ErrorKind::Unavailable);
+        assert!(rng.try_fill_bytes(&mut w).is_err());
     }
 }
diff --git a/src/rngs/adapter/reseeding.rs b/src/rngs/adapter/reseeding.rs
index d58ce569f0e..30e8288ff0e 100644
--- a/src/rngs/adapter/reseeding.rs
+++ b/src/rngs/adapter/reseeding.rs
@@ -12,7 +12,7 @@
 
 use core::mem::size_of;
 
-use rand_core::{RngCore, CryptoRng, SeedableRng, Error, ErrorKind};
+use rand_core::{RngCore, CryptoRng, SeedableRng, Error};
 use rand_core::block::{BlockRngCore, BlockRng};
 
 /// A wrapper around any PRNG that implements [`BlockRngCore`], that adds the
@@ -240,21 +240,13 @@ where R: BlockRngCore + SeedableRng,
         let num_bytes =
             results.as_ref().len() * size_of::<<R as BlockRngCore>::Item>();
 
-        let threshold = if let Err(e) = self.reseed() {
-            let delay = match e.kind {
-                ErrorKind::Transient => num_bytes as i64,
-                kind @ _ if kind.should_retry() => self.threshold >> 8,
-                _ => self.threshold,
-            };
-            warn!("Reseeding RNG delayed reseeding by {} bytes due to \
-                   error from source: {}", delay, e);
-            delay
-        } else {
-            self.fork_counter = global_fork_counter;
-            self.threshold
-        };
+        if let Err(e) = self.reseed() {
+            warn!("Reseeding RNG failed: {}", e);
+            let _ = e;
+        }
+        self.fork_counter = global_fork_counter;
 
-        self.bytes_until_reseed = threshold - num_bytes as i64;
+        self.bytes_until_reseed = self.threshold - num_bytes as i64;
         self.inner.generate(results);
     }
 }
diff --git a/src/rngs/os.rs b/src/rngs/os.rs
index e71ea80498c..02333ac68c1 100644
--- a/src/rngs/os.rs
+++ b/src/rngs/os.rs
@@ -9,13 +9,13 @@
 //! Interface to the random number generator of the operating system.
 
 use getrandom::getrandom;
-use rand_core::{CryptoRng, RngCore, Error, ErrorKind, impls};
+use rand_core::{CryptoRng, RngCore, Error, impls};
 
 /// A random number generator that retrieves randomness from from the
 /// operating system.
 ///
 /// This is a zero-sized struct. It can be freely constructed with `OsRng`.
-/// 
+///
 /// The implementation is provided by the [getrandom] crate. Refer to
 /// [getrandom] documentation for details.
 ///
@@ -60,8 +60,7 @@ impl RngCore for OsRng {
     }
 
     fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
-        getrandom(dest).map_err(|e|
-            Error::with_cause(ErrorKind::Unavailable, "OsRng failed", e))
+        getrandom(dest).map_err(|e| Error::with_cause("OsRng failed", e))
     }
 }
 

From a4e5f019f76175dfc42e9cdd4e8f3feb6ae2df71 Mon Sep 17 00:00:00 2001
From: Diggory Hardy <git@dhardy.name>
Date: Thu, 16 May 2019 15:38:44 +0100
Subject: [PATCH 2/9] Support From<getrandom::Error> for rand_core::Error

This gives us an alternative to feature-specific behaviour
---
 rand_core/Cargo.toml   |  3 +-
 rand_core/src/error.rs | 73 +++++++++++++++++++++++++++---------------
 rand_os/src/lib.rs     |  3 +-
 src/rngs/os.rs         |  3 +-
 4 files changed, 53 insertions(+), 29 deletions(-)

diff --git a/rand_core/Cargo.toml b/rand_core/Cargo.toml
index 98e47ddf899..8c39865486a 100644
--- a/rand_core/Cargo.toml
+++ b/rand_core/Cargo.toml
@@ -18,10 +18,11 @@ travis-ci = { repository = "rust-random/rand" }
 appveyor = { repository = "rust-random/rand" }
 
 [features]
-std = ["alloc"]    # use std library; should be default but for above bug
+std = ["alloc", "getrandom", "getrandom/std"]    # use std library; should be default but for above bug
 alloc = []  # enables Vec and Box support without std
 serde1 = ["serde", "serde_derive"] # enables serde for BlockRng wrapper
 
 [dependencies]
 serde = { version = "1", optional = true }
 serde_derive = { version = "^1.0.38", optional = true }
+getrandom = { version = "0.1", optional = true }
diff --git a/rand_core/src/error.rs b/rand_core/src/error.rs
index 6d343be14f4..f803399e8d8 100644
--- a/rand_core/src/error.rs
+++ b/rand_core/src/error.rs
@@ -9,13 +9,13 @@
 //! Error types
 
 use core::fmt;
-
-#[cfg(feature="std")]
-use std::error::Error as stdError;
-#[cfg(feature="std")]
-use std::io;
+#[cfg(not(feature="std"))]
+use core::num::NonZeroU32;
 
 
+// A randomly-chosen 24-bit prefix for our codes.
+#[cfg(not(feature="std"))]
+pub(crate) const CODE_PREFIX: u32 = 0x517e8100;
 
 /// Error type of random number generators
 /// 
@@ -30,7 +30,9 @@ pub struct Error {
     /// The error message
     pub msg: &'static str,
     #[cfg(feature="std")]
-    cause: Option<Box<stdError + Send + Sync>>,
+    cause: Option<Box<std::error::Error + Send + Sync>>,
+    #[cfg(not(feature="std"))]
+    code: NonZeroU32,
 }
 
 impl Error {
@@ -40,39 +42,45 @@ impl Error {
             Error { msg, cause: None }
         }
         #[cfg(not(feature="std"))] {
-            Error {  msg }
+            Error {  msg, code: NonZeroU32::new(CODE_PREFIX).unwrap() }
         }
     }
     
     /// Create a new instance, with a message and a chained cause.
-    /// 
-    /// Note: `stdError` is an alias for `std::error::Error`.
-    /// 
-    /// If not targetting `std` (i.e. `no_std`), this function is replaced by
-    /// another with the same prototype, except that there are no bounds on the
-    /// type `E` (because both `Box` and `stdError` are unavailable), and the
-    /// `cause` is ignored.
+    ///
+    /// This function is only available with the `std` feature.
+    // NOTE: with specialisation we could support both.
     #[cfg(feature="std")]
     pub fn with_cause<E>(msg: &'static str, cause: E) -> Self
-        where E: Into<Box<stdError + Send + Sync>>
+        where E: Into<Box<std::error::Error + Send + Sync>>
     {
         Error { msg, cause: Some(cause.into()) }
     }
     
-    /// Create a new instance, with a message and a chained cause.
-    /// 
-    /// In `no_std` mode the *cause* is ignored.
+    /// Create a new instance, with a message and an error code.
+    ///
+    /// This function is only available without the `std` feature.
     #[cfg(not(feature="std"))]
-    pub fn with_cause<E>(msg: &'static str, _cause: E) -> Self {
-        Error { msg }
+    pub fn with_code(msg: &'static str, code: NonZeroU32) -> Self {
+        Error { msg, code }
     }
     
     /// Take the cause, if any. This allows the embedded cause to be extracted.
     /// This uses `Option::take`, leaving `self` with no cause.
+    ///
+    /// This function is only available with the `std` feature.
     #[cfg(feature="std")]
-    pub fn take_cause(&mut self) -> Option<Box<stdError + Send + Sync>> {
+    pub fn take_cause(&mut self) -> Option<Box<std::error::Error + Send + Sync>> {
         self.cause.take()
     }
+    
+    /// Retrieve the error code.
+    ///
+    /// This function is only available without the `std` feature.
+    #[cfg(not(feature="std"))]
+    pub fn code(&self) -> NonZeroU32 {
+        self.code
+    }
 }
 
 impl fmt::Display for Error {
@@ -87,20 +95,33 @@ impl fmt::Display for Error {
     }
 }
 
+#[cfg(feature="getrandom")]
+impl From<getrandom::Error> for Error {
+    fn from(error: getrandom::Error) -> Self {
+        let msg = "getrandom error";
+        #[cfg(feature="std")] {
+            Error { msg, cause: Some(Box::new(error)) }
+        }
+        #[cfg(not(feature="std"))] {
+            Error { msg, code: error.code() }
+        }
+    }
+}
+
 #[cfg(feature="std")]
-impl stdError for Error {
+impl std::error::Error for Error {
     fn description(&self) -> &str {
         self.msg
     }
 
-    fn cause(&self) -> Option<&stdError> {
-        self.cause.as_ref().map(|e| e.as_ref() as &stdError)
+    fn cause(&self) -> Option<&std::error::Error> {
+        self.cause.as_ref().map(|e| e.as_ref() as &std::error::Error)
     }
 }
 
 #[cfg(feature="std")]
-impl From<Error> for io::Error {
+impl From<Error> for std::io::Error {
     fn from(error: Error) -> Self {
-        io::Error::new(io::ErrorKind::Other, error)
+        std::io::Error::new(std::io::ErrorKind::Other, error)
     }
 }
diff --git a/rand_os/src/lib.rs b/rand_os/src/lib.rs
index 34ec811039b..39530d812a7 100644
--- a/rand_os/src/lib.rs
+++ b/rand_os/src/lib.rs
@@ -88,7 +88,8 @@ impl RngCore for OsRng {
         // (And why waste a system call?)
         if dest.len() == 0 { return Ok(()); }
         
-        getrandom(dest).map_err(|e| Error::with_cause("OsRng failed", e))
+        getrandom(dest)?;
+        Ok(())
     }
 }
 
diff --git a/src/rngs/os.rs b/src/rngs/os.rs
index 02333ac68c1..bdba2f6eabb 100644
--- a/src/rngs/os.rs
+++ b/src/rngs/os.rs
@@ -60,7 +60,8 @@ impl RngCore for OsRng {
     }
 
     fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
-        getrandom(dest).map_err(|e| Error::with_cause("OsRng failed", e))
+        getrandom(dest)?;
+        Ok(())
     }
 }
 

From 25650c4a347dca72090fb71c3a178a3f4acf979f Mon Sep 17 00:00:00 2001
From: Diggory Hardy <git@dhardy.name>
Date: Thu, 16 May 2019 15:51:11 +0100
Subject: [PATCH 3/9] Update and synchronise OsRng modules

---
 rand_os/src/lib.rs | 35 +++++++++++++++--------------------
 src/rngs/os.rs     | 27 ++++++++++++++++++++++++---
 2 files changed, 39 insertions(+), 23 deletions(-)

diff --git a/rand_os/src/lib.rs b/rand_os/src/lib.rs
index 39530d812a7..9090ab8250a 100644
--- a/rand_os/src/lib.rs
+++ b/rand_os/src/lib.rs
@@ -8,19 +8,8 @@
 // except according to those terms.
 
 //! Interface to the random number generator of the operating system.
-//!
-//! # Blocking and error handling
-//!
-//! It is possible that when used during early boot the first call to `OsRng`
-//! will block until the system's RNG is initialised. It is also possible
-//! (though highly unlikely) for `OsRng` to fail on some platforms, most
-//! likely due to system mis-configuration.
-//!
-//! After the first successful call, it is highly unlikely that failures or
-//! significant delays will occur (although performance should be expected to
-//! be much slower than a user-space PRNG).
-//!
-//! [getrandom]: https://crates.io/crates/getrandom
+// Note: keep this code in sync with the rand::rngs::os module!
+
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",
        html_favicon_url = "https://www.rust-lang.org/favicon.ico",
        html_root_url = "https://rust-random.github.io/rand/")]
@@ -28,8 +17,6 @@
 #![deny(missing_debug_implementations)]
 #![doc(test(attr(allow(unused_variables), deny(warnings))))]
 
-#![cfg_attr(feature = "stdweb", recursion_limit="128")]
-
 #![no_std]  // but see getrandom crate
 
 pub use rand_core;  // re-export
@@ -45,9 +32,21 @@ use rand_core::{CryptoRng, RngCore, Error, impls};
 /// The implementation is provided by the [getrandom] crate. Refer to
 /// [getrandom] documentation for details.
 ///
+/// # Blocking and error handling
+///
+/// It is possible that when used during early boot the first call to `OsRng`
+/// will block until the system's RNG is initialised. It is also possible
+/// (though highly unlikely) for `OsRng` to fail on some platforms, most
+/// likely due to system mis-configuration.
+///
+/// After the first successful call, it is highly unlikely that failures or
+/// significant delays will occur (although performance should be expected to
+/// be much slower than a user-space PRNG).
+///
 /// # Usage example
 /// ```
-/// use rand_os::{OsRng, rand_core::RngCore};
+/// use rand_os::rand_core::RngCore;
+/// use rand_os::OsRng;
 ///
 /// let mut key = [0u8; 16];
 /// OsRng.fill_bytes(&mut key);
@@ -84,10 +83,6 @@ impl RngCore for OsRng {
     }
 
     fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
-        // Some systems do not support reading 0 random bytes.
-        // (And why waste a system call?)
-        if dest.len() == 0 { return Ok(()); }
-        
         getrandom(dest)?;
         Ok(())
     }
diff --git a/src/rngs/os.rs b/src/rngs/os.rs
index bdba2f6eabb..c7fc75f6941 100644
--- a/src/rngs/os.rs
+++ b/src/rngs/os.rs
@@ -7,6 +7,7 @@
 // except according to those terms.
 
 //! Interface to the random number generator of the operating system.
+// Note: keep this code in sync with the rand_os crate!
 
 use getrandom::getrandom;
 use rand_core::{CryptoRng, RngCore, Error, impls};
@@ -19,15 +20,29 @@ use rand_core::{CryptoRng, RngCore, Error, impls};
 /// The implementation is provided by the [getrandom] crate. Refer to
 /// [getrandom] documentation for details.
 ///
-/// ## Example
+/// # Blocking and error handling
 ///
+/// It is possible that when used during early boot the first call to `OsRng`
+/// will block until the system's RNG is initialised. It is also possible
+/// (though highly unlikely) for `OsRng` to fail on some platforms, most
+/// likely due to system mis-configuration.
+///
+/// After the first successful call, it is highly unlikely that failures or
+/// significant delays will occur (although performance should be expected to
+/// be much slower than a user-space PRNG).
+///
+/// # Usage example
 /// ```
 /// use rand::rngs::{StdRng, OsRng};
 /// use rand::{RngCore, SeedableRng};
 ///
-/// println!("Random number, straight from the OS: {}", OsRng.next_u32());
+/// let mut key = [0u8; 16];
+/// OsRng.fill_bytes(&mut key);
+/// let random_u64 = OsRng.next_u64();
+/// 
+/// // OsRng is especially useful for seeding other RNGs (see also from_entropy)
 /// let mut rng = StdRng::from_rng(OsRng).unwrap();
-/// println!("Another random number: {}", rng.next_u32());
+/// let _ = rng.next_u32();
 /// ```
 ///
 /// [getrandom]: https://crates.io/crates/getrandom
@@ -72,3 +87,9 @@ fn test_os_rng() {
     assert!(x != 0);
     assert!(x != y);
 }
+
+#[test]
+fn test_construction() {
+    let mut rng = OsRng::default();
+    assert!(rng.next_u64() != 0);
+}

From b61e61b3a7c8e9b7b1df1653f6e333ed3efd69cb Mon Sep 17 00:00:00 2001
From: Diggory Hardy <git@dhardy.name>
Date: Thu, 16 May 2019 17:12:04 +0100
Subject: [PATCH 4/9] Move from_entropy to SeedableRng and update RngCore doc

---
 rand_core/src/lib.rs           | 84 ++++++++++++++++++++--------------
 rand_distr/tests/uniformity.rs |  3 +-
 src/lib.rs                     | 50 --------------------
 src/prelude.rs                 |  2 +-
 src/rngs/mod.rs                |  4 +-
 src/rngs/small.rs              |  6 +--
 tests/wasm_bindgen/src/lib.rs  |  1 -
 7 files changed, 55 insertions(+), 95 deletions(-)

diff --git a/rand_core/src/lib.rs b/rand_core/src/lib.rs
index 6d48e892592..7472a666038 100644
--- a/rand_core/src/lib.rs
+++ b/rand_core/src/lib.rs
@@ -213,10 +213,6 @@ pub trait CryptoRng {}
 /// This trait encapsulates the low-level functionality common to all
 /// pseudo-random number generators (PRNGs, or algorithmic generators).
 ///
-/// The `FromEntropy` trait from the [`rand`] crate is automatically
-/// implemented for every type implementing `SeedableRng`, providing
-/// a convenient `from_entropy()` constructor.
-///
 /// [`rand`]: https://docs.rs/rand
 pub trait SeedableRng: Sized {
     /// Seed type, which is restricted to types mutably-dereferencable as `u8`
@@ -270,14 +266,18 @@ pub trait SeedableRng: Sized {
     ///
     /// PRNG implementations are allowed to assume that bits in the seed are
     /// well distributed. That means usually that the number of one and zero
-    /// bits are about equal, and values like 0, 1 and (size - 1) are unlikely.
-    ///
-    /// PRNG implementations are recommended to be reproducible. A PRNG seeded
-    /// using this function with a fixed seed should produce the same sequence
-    /// of output in the future and on different architectures (with for example
-    /// different endianness).
-    ///
-    /// It is however not required that this function yield the same state as a
+    /// bits are roughly equal, and values like 0, 1 and (size - 1) are unlikely.
+    /// Note that many non-cryptographic PRNGs will show poor quality output
+    /// if this is not adhered to. If you wish to seed from simple numbers, use
+    /// `seed_from_u64` instead.
+    ///
+    /// All PRNG implementations should be reproducible unless otherwise noted:
+    /// given a fixed `seed`, the same sequence of output should be produced
+    /// on all runs, library versions and architectures (e.g. check endianness).
+    /// Any "value-breaking" changes to the generator should require bumping at
+    /// least the minor version and documentation of the change.
+    ///
+    /// It is not required that this function yield the same state as a
     /// reference implementation of the PRNG given equivalent seed; if necessary
     /// another constructor replicating behaviour from a reference
     /// implementation can be added.
@@ -330,33 +330,26 @@ pub trait SeedableRng: Sized {
 
     /// Create a new PRNG seeded from another `Rng`.
     ///
-    /// This is the recommended way to initialize PRNGs with fresh entropy. The
-    /// `FromEntropy` trait from the [`rand`] crate provides a convenient
-    /// `from_entropy` method based on `from_rng`.
-    ///
-    /// Usage of this method is not recommended when reproducibility is required
-    /// since implementing PRNGs are not required to fix Endianness and are
-    /// allowed to modify implementations in new releases.
+    /// This may be useful when needing to rapidly seed many PRNGs from a master
+    /// PRNG, and to allow forking of PRNGs. It may be considered deterministic.
     ///
-    /// It is important to use a good source of randomness to initialize the
-    /// PRNG. Cryptographic PRNG may be rendered insecure when seeded from a
-    /// non-cryptographic PRNG or with insufficient entropy.
-    /// Many non-cryptographic PRNGs will show statistical bias in their first
-    /// results if their seed numbers are small or if there is a simple pattern
-    /// between them.
+    /// The master PRNG should be at least as high quality as the child PRNGs.
+    /// When seeding non-cryptographic child PRNGs, we recommend using a
+    /// different algorithm for the master PRNG (ideally a CSPRNG) to avoid
+    /// correlations between the child PRNGs. If this is not possible (e.g.
+    /// forking using small non-crypto PRNGs) ensure that your PRNG has a good
+    /// mixing function on the output or consider use of a hash function with
+    /// `from_seed`.
     ///
-    /// Prefer to seed from a strong external entropy source like `OsRng` from
-    /// the [`rand_os`] crate or from a cryptographic PRNG; if creating a new
-    /// generator for cryptographic uses you *must* seed from a strong source.
-    ///
-    /// Seeding a small PRNG from another small PRNG is possible, but
-    /// something to be careful with. An extreme example of how this can go
-    /// wrong is seeding an Xorshift RNG from another Xorshift RNG, which
-    /// will effectively clone the generator. In general seeding from a
-    /// generator which is hard to predict is probably okay.
+    /// Note that seeding `XorShiftRng` from another `XorShiftRng` provides an
+    /// extreme example of what can go wrong: the new PRNG will be a clone
+    /// of the parent.
     ///
     /// PRNG implementations are allowed to assume that a good RNG is provided
     /// for seeding, and that it is cryptographically secure when appropriate.
+    /// As of `rand` 0.7 / `rand_core` 0.5, implementations overriding this
+    /// method should ensure the implementation satisfies reproducibility
+    /// (in prior versions this was not required).
     ///
     /// [`rand`]: https://docs.rs/rand
     /// [`rand_os`]: https://docs.rs/rand_os
@@ -365,6 +358,29 @@ 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.
+    ///
+    /// 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(thread_rng()).unwrap()`.
+    ///
+    /// # Panics
+    ///
+    /// If [`getrandom`] is unable to provide secure entropy this method will panic.
+    ///
+    /// [`getrandom`]: https://docs.rs/getrandom
+    #[cfg(feature="getrandom")]
+    fn from_entropy() -> Self {
+        let mut seed = Self::Seed::default();
+        if let Err(err) = getrandom::getrandom(seed.as_mut()) {
+            panic!("from_entropy failed: {}", err);
+        }
+        Self::from_seed(seed)
+    }
 }
 
 // Implement `RngCore` for references to an `RngCore`.
diff --git a/rand_distr/tests/uniformity.rs b/rand_distr/tests/uniformity.rs
index 3b9c7493e14..6ecea032be1 100644
--- a/rand_distr/tests/uniformity.rs
+++ b/rand_distr/tests/uniformity.rs
@@ -13,8 +13,7 @@ extern crate average;
 extern crate rand;
 
 use average::Histogram;
-use rand::distributions::Distribution;
-use rand::FromEntropy;
+use rand::prelude::*;
 use std as core;
 
 const N_BINS: usize = 100;
diff --git a/src/lib.rs b/src/lib.rs
index dbaf668eae4..8fbdc80df21 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -451,56 +451,6 @@ impl_as_byte_slice_arrays!(32, N,N,N,N,N,N,N,N,N,N,N,N,N,N,N,N,N,N,N,N,N,N,N,N,N
 impl_as_byte_slice_arrays!(!div 4096, N,N,N,N,N,N,N,);
 
 
-/// A convenience extension to [`SeedableRng`] allowing construction from fresh
-/// entropy. This trait is automatically implemented for any PRNG implementing
-/// [`SeedableRng`] and is not intended to be implemented by users.
-///
-/// This is equivalent to using `SeedableRng::from_rng(OsRng)` then
-/// unwrapping the result.
-///
-/// Since this is convenient and secure, it is the recommended way to create
-/// PRNGs, though two alternatives may be considered:
-///
-/// *   Deterministic creation using [`SeedableRng::from_seed`] with a fixed seed
-/// *   Seeding from `thread_rng`: `SeedableRng::from_rng(thread_rng())?`;
-///     this will usually be faster and should also be secure, but requires
-///     trusting one extra component.
-///
-/// ## Example
-///
-/// ```
-/// use rand::{Rng, FromEntropy};
-/// use rand::rngs::StdRng;
-///
-/// let mut rng = StdRng::from_entropy();
-/// println!("Random die roll: {}", rng.gen_range(1, 7));
-/// ```
-///
-/// [`OsRng`]: rngs::OsRng
-#[cfg(feature="std")]
-pub trait FromEntropy: SeedableRng {
-    /// Creates a new instance of the RNG seeded from [`OsRng`].
-    ///
-    /// This method is equivalent to `SeedableRng::from_rng(OsRng).unwrap()`.
-    ///
-    /// # Panics
-    ///
-    /// If [`OsRng`] is unable to obtain secure entropy this method will panic.
-    /// It is also possible for an RNG overriding the [`SeedableRng::from_rng`]
-    /// method to cause a panic. Both causes are extremely unlikely to occur;
-    /// most users need not worry about this.
-    fn from_entropy() -> Self;
-}
-
-#[cfg(feature="std")]
-impl<R: SeedableRng> FromEntropy for R {
-    fn from_entropy() -> R {
-        R::from_rng(rngs::OsRng).unwrap_or_else(|err|
-            panic!("FromEntropy::from_entropy() failed: {}", err))
-    }
-}
-
-
 /// Generates a random value using the thread-local random number generator.
 ///
 /// This is simply a shortcut for `thread_rng().gen()`. See [`thread_rng`] for
diff --git a/src/prelude.rs b/src/prelude.rs
index 5d8a0e9b2aa..5218c91c975 100644
--- a/src/prelude.rs
+++ b/src/prelude.rs
@@ -23,5 +23,5 @@
 #[doc(no_inline)] pub use rngs::{SmallRng, StdRng};
 #[doc(no_inline)] #[cfg(feature="std")] pub use rngs::ThreadRng;
 #[doc(no_inline)] pub use {Rng, RngCore, CryptoRng, SeedableRng};
-#[doc(no_inline)] #[cfg(feature="std")] pub use {FromEntropy, random, thread_rng};
+#[doc(no_inline)] #[cfg(feature="std")] pub use {random, thread_rng};
 #[doc(no_inline)] pub use seq::{SliceRandom, IteratorRandom};
diff --git a/src/rngs/mod.rs b/src/rngs/mod.rs
index 08dde4496f2..940dfab20af 100644
--- a/src/rngs/mod.rs
+++ b/src/rngs/mod.rs
@@ -42,9 +42,7 @@
 //! -   `from_seed` accepts a type specific to the PRNG
 //! -   `from_rng` allows a PRNG to be seeded from any other RNG
 //! -   `seed_from_u64` allows any PRNG to be seeded from a `u64` insecurely
-//!
-//! Additionally, [`FromEntropy::from_entropy`] is a shortcut for seeding from
-//! [`OsRng`].
+//! -   `from_entropy` securely seeds a PRNG from fresh entropy
 //!
 //! Use the [`rand_core`] crate when implementing your own RNGs.
 //!
diff --git a/src/rngs/small.rs b/src/rngs/small.rs
index 9874d2469b8..0e39adba9c1 100644
--- a/src/rngs/small.rs
+++ b/src/rngs/small.rs
@@ -34,11 +34,10 @@ type Rng = ::rand_pcg::Pcg32;
 ///
 /// # Examples
 ///
-/// Initializing `SmallRng` with a random seed can be done using [`FromEntropy`]:
+/// Initializing `SmallRng` with a random seed can be done using [`SeedableRng::from_entropy`]:
 ///
 /// ```
-/// # use rand::Rng;
-/// use rand::FromEntropy;
+/// use rand::{Rng, SeedableRng};
 /// use rand::rngs::SmallRng;
 ///
 /// // Create small, cheap to initialize and fast RNG with a random seed.
@@ -66,7 +65,6 @@ type Rng = ::rand_pcg::Pcg32;
 ///     .collect();
 /// ```
 ///
-/// [`FromEntropy`]: crate::FromEntropy
 /// [`StdRng`]: crate::rngs::StdRng
 /// [`thread_rng`]: crate::thread_rng
 /// [rand_pcg]: https://crates.io/crates/rand_pcg
diff --git a/tests/wasm_bindgen/src/lib.rs b/tests/wasm_bindgen/src/lib.rs
index 6610b8cbdbc..9de50958a2c 100644
--- a/tests/wasm_bindgen/src/lib.rs
+++ b/tests/wasm_bindgen/src/lib.rs
@@ -15,7 +15,6 @@ extern crate wasm_bindgen;
 extern crate wasm_bindgen_test;
 
 use rand::rngs::{OsRng, StdRng};
-use rand::FromEntropy;
 use rand::{Rng, SeedableRng};
 use wasm_bindgen::prelude::*;
 

From f42abf8ff197b8c7b15122c1bbfafe12f4b66a45 Mon Sep 17 00:00:00 2001
From: Diggory Hardy <git@dhardy.name>
Date: Thu, 16 May 2019 17:14:33 +0100
Subject: [PATCH 5/9] Make Error::msg a private field

---
 rand_core/src/error.rs | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/rand_core/src/error.rs b/rand_core/src/error.rs
index f803399e8d8..95b62e4eb9e 100644
--- a/rand_core/src/error.rs
+++ b/rand_core/src/error.rs
@@ -27,8 +27,7 @@ pub(crate) const CODE_PREFIX: u32 = 0x517e8100;
 /// done via `Error::new` or `Error::with_cause`.
 #[derive(Debug)]
 pub struct Error {
-    /// The error message
-    pub msg: &'static str,
+    msg: &'static str,
     #[cfg(feature="std")]
     cause: Option<Box<std::error::Error + Send + Sync>>,
     #[cfg(not(feature="std"))]
@@ -64,6 +63,11 @@ impl Error {
     pub fn with_code(msg: &'static str, code: NonZeroU32) -> Self {
         Error { msg, code }
     }
+
+    /// Retrieve the error message.
+    pub fn msg(&self) -> &str {
+        self.msg
+    }
     
     /// Take the cause, if any. This allows the embedded cause to be extracted.
     /// This uses `Option::take`, leaving `self` with no cause.

From 959043d7d6f727ad085c97a21e2547c4c34b1f4f Mon Sep 17 00:00:00 2001
From: Diggory Hardy <git@dhardy.name>
Date: Fri, 17 May 2019 17:52:22 +0100
Subject: [PATCH 6/9] rand_core::Error: support using a code also with std

---
 rand_core/src/error.rs | 46 ++++++++++++++++++++++++++----------------
 1 file changed, 29 insertions(+), 17 deletions(-)

diff --git a/rand_core/src/error.rs b/rand_core/src/error.rs
index 95b62e4eb9e..cad05ab78a7 100644
--- a/rand_core/src/error.rs
+++ b/rand_core/src/error.rs
@@ -9,7 +9,6 @@
 //! Error types
 
 use core::fmt;
-#[cfg(not(feature="std"))]
 use core::num::NonZeroU32;
 
 
@@ -57,11 +56,13 @@ impl Error {
     }
     
     /// Create a new instance, with a message and an error code.
-    ///
-    /// This function is only available without the `std` feature.
-    #[cfg(not(feature="std"))]
     pub fn with_code(msg: &'static str, code: NonZeroU32) -> Self {
-        Error { msg, code }
+        #[cfg(feature="std")] {
+            Error { msg, cause: Some(Box::new(ErrorCode(code))) }
+        }
+        #[cfg(not(feature="std"))] {
+            Error { msg, code }
+        }
     }
 
     /// Retrieve the error message.
@@ -78,23 +79,20 @@ impl Error {
         self.cause.take()
     }
     
-    /// Retrieve the error code.
-    ///
-    /// This function is only available without the `std` feature.
+    /// Retrieve the error code, if any.
     #[cfg(not(feature="std"))]
-    pub fn code(&self) -> NonZeroU32 {
-        self.code
+    pub fn code(&self) -> Option<NonZeroU32> {
+        #[cfg(feature="std")] {
+            self.cause.as_ref().and_then(|b| b.downcast_ref::<ErrorCode>()).map(|c| c.0)
+        }
+        #[cfg(not(feature="std"))] {
+            Some(self.code)
+        }
     }
 }
 
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        #[cfg(feature="std")] {
-            if let Some(ref cause) = self.cause {
-                return write!(f, "{}; cause: {}",
-                        self.msg, cause);
-            }
-        }
         write!(f, "{}", self.msg)
     }
 }
@@ -118,7 +116,7 @@ impl std::error::Error for Error {
         self.msg
     }
 
-    fn cause(&self) -> Option<&std::error::Error> {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
         self.cause.as_ref().map(|e| e.as_ref() as &std::error::Error)
     }
 }
@@ -129,3 +127,17 @@ impl From<Error> for std::io::Error {
         std::io::Error::new(std::io::ErrorKind::Other, error)
     }
 }
+
+#[cfg(feature="std")]
+#[derive(Debug, Copy, Clone)]
+struct ErrorCode(NonZeroU32);
+
+#[cfg(feature="std")]
+impl fmt::Display for ErrorCode {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "ErrorCode({})", self.0)
+    }
+}
+
+#[cfg(feature="std")]
+impl std::error::Error for ErrorCode {}

From ca1b7c429af3155f9bd2aecfe7f9014d9b4afff3 Mon Sep 17 00:00:00 2001
From: Diggory Hardy <git@dhardy.name>
Date: Tue, 28 May 2019 10:27:26 +0100
Subject: [PATCH 7/9] rand_core::Error: replace internal representation and
 adjust API

This is a significant breaking change, discussed in #800.
Also rand::rngs::adapter::read is no longer publically exported.
---
 rand_core/src/error.rs   | 113 ++++++++++++++++++---------------------
 src/rngs/adapter/mod.rs  |   4 +-
 src/rngs/adapter/read.rs |  25 +++++++--
 3 files changed, 76 insertions(+), 66 deletions(-)

diff --git a/rand_core/src/error.rs b/rand_core/src/error.rs
index cad05ab78a7..7e8f0c9ba32 100644
--- a/rand_core/src/error.rs
+++ b/rand_core/src/error.rs
@@ -12,78 +12,58 @@ use core::fmt;
 use core::num::NonZeroU32;
 
 
-// A randomly-chosen 24-bit prefix for our codes.
-#[cfg(not(feature="std"))]
-pub(crate) const CODE_PREFIX: u32 = 0x517e8100;
-
 /// Error type of random number generators
-/// 
-/// This is a relatively simple error type, designed for compatibility with and
-/// without the Rust `std` library. It embeds a message (static
-/// string only), and an optional chained cause (`std` only). The
-/// `msg` field can be accessed directly; cause can be accessed via
-/// `std::error::Error::cause` or `Error::take_cause`. Construction can only be
-/// done via `Error::new` or `Error::with_cause`.
+///
+/// In order to be compatible with `std` and `no_std`, this type has two
+/// possible implementations: with `std` a boxed `Error` trait object is stored,
+/// while with `no_std` we merely store an error code.
 #[derive(Debug)]
 pub struct Error {
-    msg: &'static str,
     #[cfg(feature="std")]
-    cause: Option<Box<std::error::Error + Send + Sync>>,
+    inner: Box<dyn std::error::Error + Send + Sync + 'static>,
     #[cfg(not(feature="std"))]
     code: NonZeroU32,
 }
 
 impl Error {
-    /// Create a new instance, with a message.
-    pub fn new(msg: &'static str) -> Self {
-        #[cfg(feature="std")] {
-            Error { msg, cause: None }
-        }
-        #[cfg(not(feature="std"))] {
-            Error {  msg, code: NonZeroU32::new(CODE_PREFIX).unwrap() }
-        }
-    }
-    
-    /// Create a new instance, with a message and a chained cause.
-    ///
-    /// This function is only available with the `std` feature.
-    // NOTE: with specialisation we could support both.
+    /// Construct from any type supporting `std::error::Error`
+    /// 
+    /// Available only when configured with `std`.
+    /// 
+    /// See also `From<NonZeroU32>`, which is available with and without `std`.
     #[cfg(feature="std")]
-    pub fn with_cause<E>(msg: &'static str, cause: E) -> Self
-        where E: Into<Box<std::error::Error + Send + Sync>>
+    pub fn new<E>(err: E) -> Self
+    where E: Into<Box<dyn std::error::Error + Send + Sync + 'static>>
     {
-        Error { msg, cause: Some(cause.into()) }
+        Error { inner: err.into() }
     }
     
-    /// Create a new instance, with a message and an error code.
-    pub fn with_code(msg: &'static str, code: NonZeroU32) -> Self {
-        #[cfg(feature="std")] {
-            Error { msg, cause: Some(Box::new(ErrorCode(code))) }
-        }
-        #[cfg(not(feature="std"))] {
-            Error { msg, code }
-        }
-    }
-
-    /// Retrieve the error message.
-    pub fn msg(&self) -> &str {
-        self.msg
+    /// Reference the inner error (`std` only)
+    /// 
+    /// When configured with `std`, this is a trivial operation and never
+    /// panics. Without `std`, this method is simply unavailable.
+    #[cfg(feature="std")]
+    pub fn inner(&self) -> &(dyn std::error::Error + Send + Sync + 'static) {
+        &*self.inner
     }
     
-    /// Take the cause, if any. This allows the embedded cause to be extracted.
-    /// This uses `Option::take`, leaving `self` with no cause.
-    ///
-    /// This function is only available with the `std` feature.
+    /// Unwrap the inner error (`std` only)
+    /// 
+    /// When configured with `std`, this is a trivial operation and never
+    /// panics. Without `std`, this method is simply unavailable.
     #[cfg(feature="std")]
-    pub fn take_cause(&mut self) -> Option<Box<std::error::Error + Send + Sync>> {
-        self.cause.take()
+    pub fn take_inner(self) -> Box<dyn std::error::Error + Send + Sync + 'static> {
+        self.inner
     }
     
     /// Retrieve the error code, if any.
-    #[cfg(not(feature="std"))]
+    /// 
+    /// If this `Error` was constructed via `From<NonZeroU32>`, then this method
+    /// will return this `NonZeroU32` code (for `no_std` this is always the
+    /// case). Otherwise, this method will return `None`.
     pub fn code(&self) -> Option<NonZeroU32> {
         #[cfg(feature="std")] {
-            self.cause.as_ref().and_then(|b| b.downcast_ref::<ErrorCode>()).map(|c| c.0)
+            self.inner.downcast_ref::<ErrorCode>().map(|c| c.0)
         }
         #[cfg(not(feature="std"))] {
             Some(self.code)
@@ -93,31 +73,42 @@ impl Error {
 
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{}", self.msg)
+        #[cfg(feature="std")] {
+            write!(f, "{}", self.inner)
+        }
+        #[cfg(not(feature="std"))] {
+            write!(f, "error code {}", self.code)
+        }
+    }
+}
+
+impl From<NonZeroU32> for Error {
+    fn from(code: NonZeroU32) -> Self {
+        #[cfg(feature="std")] {
+            Error { inner: Box::new(ErrorCode(code)) }
+        }
+        #[cfg(not(feature="std"))] {
+            Error { code }
+        }
     }
 }
 
 #[cfg(feature="getrandom")]
 impl From<getrandom::Error> for Error {
     fn from(error: getrandom::Error) -> Self {
-        let msg = "getrandom error";
         #[cfg(feature="std")] {
-            Error { msg, cause: Some(Box::new(error)) }
+            Error { inner: Box::new(error) }
         }
         #[cfg(not(feature="std"))] {
-            Error { msg, code: error.code() }
+            Error { code: error.code() }
         }
     }
 }
 
 #[cfg(feature="std")]
 impl std::error::Error for Error {
-    fn description(&self) -> &str {
-        self.msg
-    }
-
     fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
-        self.cause.as_ref().map(|e| e.as_ref() as &std::error::Error)
+        self.inner.source()
     }
 }
 
@@ -135,7 +126,7 @@ struct ErrorCode(NonZeroU32);
 #[cfg(feature="std")]
 impl fmt::Display for ErrorCode {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "ErrorCode({})", self.0)
+        write!(f, "error code {}", self.0)
     }
 }
 
diff --git a/src/rngs/adapter/mod.rs b/src/rngs/adapter/mod.rs
index 60b832e9a4d..659ff26218d 100644
--- a/src/rngs/adapter/mod.rs
+++ b/src/rngs/adapter/mod.rs
@@ -8,8 +8,8 @@
 
 //! Wrappers / adapters forming RNGs
 
-#[cfg(feature="std")] #[doc(hidden)] pub mod read;
+#[cfg(feature="std")] mod read;
 mod reseeding;
 
-#[cfg(feature="std")] pub use self::read::ReadRng;
+#[cfg(feature="std")] pub use self::read::{ReadRng, ReadError};
 pub use self::reseeding::ReseedingRng;
diff --git a/src/rngs/adapter/read.rs b/src/rngs/adapter/read.rs
index 4c08f97362b..53b741491ab 100644
--- a/src/rngs/adapter/read.rs
+++ b/src/rngs/adapter/read.rs
@@ -10,6 +10,7 @@
 //! A wrapper around any Read to treat it as an RNG.
 
 use std::io::Read;
+use std::fmt;
 
 use rand_core::{RngCore, Error, impls};
 
@@ -73,11 +74,27 @@ impl<R: Read> RngCore for ReadRng<R> {
     fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
         if dest.len() == 0 { return Ok(()); }
         // Use `std::io::read_exact`, which retries on `ErrorKind::Interrupted`.
-        self.reader.read_exact(dest).map_err(|err|
-                Error::with_cause("error reading from Read source", err))
+        self.reader.read_exact(dest).map_err(|e| Error::new(ReadError(e)))
     }
 }
 
+/// `ReadRng` error type
+#[derive(Debug)]
+pub struct ReadError(std::io::Error);
+
+impl fmt::Display for ReadError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "ReadError: {}", self.0)
+    }
+}
+
+impl std::error::Error for ReadError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        Some(&self.0)
+    }
+}
+
+
 #[cfg(test)]
 mod test {
     use super::ReadRng;
@@ -124,6 +141,8 @@ mod test {
 
         let mut rng = ReadRng::new(&v[..]);
 
-        assert!(rng.try_fill_bytes(&mut w).is_err());
+        let result = rng.try_fill_bytes(&mut w);
+        assert!(result.is_err());
+        println!("Error: {}", result.unwrap_err());
     }
 }

From c5a5ab17937a3c96ca8f08fe711714d1e14061a4 Mon Sep 17 00:00:00 2001
From: Diggory Hardy <git@dhardy.name>
Date: Tue, 28 May 2019 10:54:17 +0100
Subject: [PATCH 8/9] Fix support for getrandom without std

---
 .travis.yml | 1 +
 Cargo.toml  | 8 +++++---
 src/lib.rs  | 2 +-
 3 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/.travis.yml b/.travis.yml
index 4e754799448..4baf810c8bd 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -268,6 +268,7 @@ before_install:
 
 script:
   - cargo test --tests --no-default-features
+  - cargo test --no-default-features --features getrandom
   # TODO: add simd_support feature:
   - cargo test --features=serde1,log
   - cargo test --examples
diff --git a/Cargo.toml b/Cargo.toml
index 7d80b25f934..14e4cd9bf80 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -28,8 +28,9 @@ i128_support = [] # enables i128 and u128 support
 simd_support = ["packed_simd"] # enables SIMD support
 serde1 = ["rand_core/serde1", "rand_isaac/serde1", "rand_xorshift/serde1"] # enables serialization for PRNGs
 # re-export optional WASM dependencies to avoid breakage:
-wasm-bindgen = ["getrandom/wasm-bindgen"]
-stdweb = ["getrandom/stdweb"]
+wasm-bindgen = ["getrandom_package/wasm-bindgen"]
+stdweb = ["getrandom_package/stdweb"]
+getrandom = ["getrandom_package", "rand_core/getrandom"]
 
 [workspace]
 members = [
@@ -50,7 +51,8 @@ members = [
 rand_core = { path = "rand_core", version = "0.4" }
 rand_pcg = { path = "rand_pcg", version = "0.1" }
 rand_hc = { path = "rand_hc", version = "0.1" }
-getrandom = { version = "0.1.1", optional = true }
+# Do not depend on 'getrandom_package' directly; use the 'getrandom' feature!
+getrandom_package = { version = "0.1.1", package = "getrandom", optional = true }
 log = { version = "0.4", optional = true }
 
 [dependencies.packed_simd]
diff --git a/src/lib.rs b/src/lib.rs
index 1ea2fb2e8a9..dcfdb8bbf03 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -59,7 +59,7 @@
 #[cfg(feature="simd_support")] extern crate packed_simd;
 
 #[cfg(feature = "getrandom")]
-extern crate getrandom;
+extern crate getrandom_package as getrandom;
 
 extern crate rand_core;
 extern crate rand_hc;

From 2d5594760ee5cfd5099e8e66bd676f7373b5fce2 Mon Sep 17 00:00:00 2001
From: Diggory Hardy <git@dhardy.name>
Date: Tue, 28 May 2019 14:22:46 +0100
Subject: [PATCH 9/9] Fix rand_jitter error conversion

---
 rand_jitter/src/error.rs | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/rand_jitter/src/error.rs b/rand_jitter/src/error.rs
index b643aebc9db..3ea72c38424 100644
--- a/rand_jitter/src/error.rs
+++ b/rand_jitter/src/error.rs
@@ -11,20 +11,23 @@ use rand_core::Error;
 use core::fmt;
 
 /// An error that can occur when [`JitterRng::test_timer`] fails.
+/// 
+/// All variants have a value of 0x6e530400 = 1850934272 plus a small
+/// increment (1 through 5).
 ///
 /// [`JitterRng::test_timer`]: crate::JitterRng::test_timer
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum TimerError {
     /// No timer available.
-    NoTimer,
+    NoTimer = 0x6e530401,
     /// Timer too coarse to use as an entropy source.
-    CoarseTimer,
+    CoarseTimer = 0x6e530402,
     /// Timer is not monotonically increasing.
-    NotMonotonic,
+    NotMonotonic = 0x6e530403,
     /// Variations of deltas of time too small.
-    TinyVariantions,
+    TinyVariantions = 0x6e530404,
     /// Too many stuck results (indicating no added entropy).
-    TooManyStuck,
+    TooManyStuck = 0x6e530405,
     #[doc(hidden)]
     __Nonexhaustive,
 }
@@ -60,11 +63,10 @@ impl From<TimerError> for Error {
         // Timer check is already quite permissive of failures so we don't
         // expect false-positive failures, i.e. any error is irrecoverable.
         #[cfg(feature = "std")] {
-            Error::with_cause("timer jitter failed basic quality tests", err)
+            Error::new(err)
         }
         #[cfg(not(feature = "std"))] {
-            let _ = err;
-            Error::new("timer jitter failed basic quality tests")
+            Error::from(core::num::NonZeroU32::new(err as u32).unwrap())
         }
     }
 }