Remove rand dependency by using our own PRNG

.travis.yml

@@ -112,7 +112,11 @@ matrix:
         - cargo build --manifest-path futures/Cargo.toml
             --target thumbv6m-none-eabi
             --no-default-features
-            --features unstable,alloc,cfg-target-has-atomic
+            --features unstable,cfg-target-has-atomic,alloc
+        - cargo build --manifest-path futures/Cargo.toml
+            --target thumbv6m-none-eabi
+            --no-default-features
+            --features unstable,cfg-target-has-atomic,async-await
 
     - name: cargo build --target=thumbv7m-none-eabi
       rust: nightly
@@ -127,6 +131,10 @@ matrix:
             --target thumbv7m-none-eabi
             --no-default-features
             --features alloc
+        - cargo build --manifest-path futures/Cargo.toml
+            --target thumbv7m-none-eabi
+            --no-default-features
+            --features async-await
 
     - name: cargo check (futures-util)
       rust: nightly

futures-select-macro/src/lib.rs

@@ -149,7 +149,6 @@ pub fn select(input: TokenStream) -> TokenStream {
     let parsed = syn::parse_macro_input!(input as Select);
 
     let futures_crate: syn::Path = parsed.futures_crate_path.unwrap_or_else(|| parse_quote!(::futures_util));
-    let rand_crate: syn::Path = parse_quote!(#futures_crate::rand_reexport);
 
     // should be def_site, but that's unstable
     let span = Span::call_site();
@@ -265,10 +264,7 @@ pub fn select(input: TokenStream) -> TokenStream {
             #( #poll_functions )*
 
             let mut __select_arr = [#( #variant_names ),*];
-            <[_] as #rand_crate::SliceRandom>::shuffle(
-                &mut __select_arr,
-                &mut #rand_crate::thread_rng(),
-            );
+            #futures_crate::async_await::shuffle(&mut __select_arr);
             for poller in &mut __select_arr {
                 let poller: &mut &mut dyn FnMut(
                     &mut #futures_crate::task::Context<'_>

futures-util/Cargo.toml

@@ -25,7 +25,7 @@ sink = ["futures-sink-preview"]
 io = ["std", "futures-io-preview", "memchr"]
 channel = ["std", "futures-channel-preview"]
 join-macro = ["async-await", "futures-join-macro-preview", "proc-macro-hack", "proc-macro-nested"]
-select-macro = ["async-await", "futures-select-macro-preview", "proc-macro-hack", "proc-macro-nested", "rand"]
+select-macro = ["async-await", "futures-select-macro-preview", "proc-macro-hack", "proc-macro-nested"]
 
 # Unstable features
 # These features are outside of the normal semver guarantees and require the
@@ -43,7 +43,6 @@ futures-join-macro-preview = { path = "../futures-join-macro", version = "=0.3.0
 futures-select-macro-preview = { path = "../futures-select-macro", version = "=0.3.0-alpha.18", default-features = false, optional = true }
 proc-macro-hack = { version = "0.5.9", optional = true }
 proc-macro-nested = { version = "0.1.2", optional = true }
-rand = { version = "0.7.0", optional = true }
 slab = { version = "0.4", optional = true }
 memchr = { version = "2.2", optional = true }
 futures_01 = { version = "0.1.25", optional = true, package = "futures" }

futures-util/src/async_await/mod.rs

@@ -31,6 +31,11 @@ mod select_mod;
 #[cfg(feature = "select-macro")]
 pub use self::select_mod::*;
 
+#[cfg(feature = "select-macro")]
+mod random;
+#[cfg(feature = "select-macro")]
+pub use self::random::*;
+
 #[doc(hidden)]
 #[inline(always)]
 pub fn assert_unpin<T: Unpin>(_: &T) {}

futures-util/src/async_await/random.rs

@@ -0,0 +1,51 @@
+use std::{
+    cell::Cell,
+    collections::hash_map::DefaultHasher,
+    hash::Hasher,
+    num::Wrapping,
+    sync::atomic::{AtomicUsize, Ordering},
+};
+
+#[doc(hidden)]
+pub fn shuffle<T>(slice: &mut [T]) {
+    for i in (1..slice.len()).rev() {
+        slice.swap(i, gen_index(i + 1));
+    }
+}
+
+/// Return a value from `0..n`.
+fn gen_index(n: usize) -> usize {
+    (random() % n as u64) as usize
+}
+
+/// Pseudorandom number generator based on [xorshift*].
+///
+/// [xorshift*]: https://en.wikipedia.org/wiki/Xorshift#xorshift*
+fn random() -> u64 {
+    thread_local! {
+        static RNG: Cell<Wrapping<u64>> = Cell::new(Wrapping(prng_seed()));
+    }
+
+    fn prng_seed() -> u64 {
+        static COUNTER: AtomicUsize = AtomicUsize::new(0);
+
+        // Any non-zero seed will do
+        let mut seed = 0;
+        while seed == 0 {
+            let mut hasher = DefaultHasher::new();
+            hasher.write_usize(COUNTER.fetch_add(1, Ordering::Relaxed));
+            seed = hasher.finish();
+        }
+        seed
+    }
+
+    RNG.with(|rng| {
+        let mut x = rng.get();
+        debug_assert_ne!(x.0, 0);
+        x ^= x >> 12;
+        x ^= x << 25;
+        x ^= x >> 27;
+        rng.set(x);
+        x.0.wrapping_mul(0x2545_f491_4f6c_dd1d)
+    })
+}

futures-util/src/lib.rs

@@ -39,14 +39,6 @@ pub mod async_await;
 #[doc(hidden)]
 pub use self::async_await::*;
 
-#[cfg(feature = "std")]
-#[cfg(feature = "select-macro")]
-#[doc(hidden)]
-pub mod rand_reexport { // used by select!
-    #[doc(hidden)]
-    pub use rand::{prelude::SliceRandom, thread_rng};
-}
-
 #[doc(hidden)]
 pub use futures_core::core_reexport;
 

futures/src/lib.rs

@@ -527,7 +527,7 @@ pub mod never {
 #[cfg(feature = "std")]
 #[cfg(feature = "async-await")]
 #[doc(hidden)]
-pub use futures_util::rand_reexport;
+pub use futures_util::async_await;
 
 #[cfg(feature = "std")]
 #[cfg(feature = "async-await")]