Import crossbeam-queue rust crate (attempt #2)

Bug: b/189133061
Test: cd external/rush/crates/crossbeam-queue && mm
Change-Id: Id6484239524470e0df585221a2e2255ef9f81aab

20 files changed, 2023 insertions, 0 deletions

diff --git a/Android.bp b/Android.bp
new file mode 100644
index 0000000..35b55ba
--- /dev/null
+++ b/Android.bp
@@ -0,0 +1,108 @@
+// This file is generated by cargo2android.py --config cargo2android.json.
+// Do not modify this file as changes will be overridden on upgrade.
+
+
+
+rust_defaults {
+    name: "crossbeam-queue_test_defaults",
+    crate_name: "crossbeam_queue",
+    srcs: ["src/lib.rs"],
+    cargo_env_compat: true,
+    cargo_pkg_version: "0.3.2",
+    test_suites: ["general-tests"],
+    auto_gen_config: true,
+    edition: "2018",
+    features: [
+        "alloc",
+        "default",
+        "std",
+    ],
+    rustlibs: [
+        "libcfg_if",
+        "libcrossbeam_utils",
+        "librand",
+    ],
+}
+
+rust_test_host {
+    name: "crossbeam-queue_host_test_src_lib",
+    defaults: ["crossbeam-queue_test_defaults"],
+    test_options: {
+        unit_test: true,
+    },
+}
+
+rust_test {
+    name: "crossbeam-queue_device_test_src_lib",
+    defaults: ["crossbeam-queue_test_defaults"],
+}
+
+rust_defaults {
+    name: "crossbeam-queue_test_defaults_crossbeam_queue",
+    crate_name: "crossbeam_queue",
+    cargo_env_compat: true,
+    cargo_pkg_version: "0.3.2",
+    test_suites: ["general-tests"],
+    auto_gen_config: true,
+    edition: "2018",
+    features: [
+        "alloc",
+        "default",
+        "std",
+    ],
+    rustlibs: [
+        "libcfg_if",
+        "libcrossbeam_queue",
+        "libcrossbeam_utils",
+        "librand",
+    ],
+}
+
+rust_test_host {
+    name: "crossbeam-queue_host_test_tests_array_queue",
+    defaults: ["crossbeam-queue_test_defaults_crossbeam_queue"],
+    srcs: ["tests/array_queue.rs"],
+    test_options: {
+        unit_test: true,
+    },
+}
+
+rust_test {
+    name: "crossbeam-queue_device_test_tests_array_queue",
+    defaults: ["crossbeam-queue_test_defaults_crossbeam_queue"],
+    srcs: ["tests/array_queue.rs"],
+}
+
+rust_test_host {
+    name: "crossbeam-queue_host_test_tests_seg_queue",
+    defaults: ["crossbeam-queue_test_defaults_crossbeam_queue"],
+    srcs: ["tests/seg_queue.rs"],
+    test_options: {
+        unit_test: true,
+    },
+}
+
+rust_test {
+    name: "crossbeam-queue_device_test_tests_seg_queue",
+    defaults: ["crossbeam-queue_test_defaults_crossbeam_queue"],
+    srcs: ["tests/seg_queue.rs"],
+}
+
+rust_library {
+    name: "libcrossbeam_queue",
+    host_supported: true,
+    crate_name: "crossbeam_queue",
+    cargo_env_compat: true,
+    cargo_pkg_version: "0.3.2",
+    srcs: ["src/lib.rs"],
+    edition: "2018",
+    features: [
+        "alloc",
+        "default",
+        "std",
+    ],
+    rustlibs: [
+        "libcfg_if",
+        "libcrossbeam_utils",
+    ],
+}
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..68306c8
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,42 @@
+# Version 0.3.2
+
+- Support targets that do not have atomic CAS on stable Rust (#698)
+
+# Version 0.3.1
+
+- Make `SegQueue::new` const fn. (#584)
+- Change license to "MIT OR Apache-2.0".
+
+# Version 0.3.0
+
+- Bump the minimum supported Rust version to 1.36.
+- Remove `PushError` and `PopError`.
+
+# Version 0.2.3
+
+- Fix bug in release (yanking 0.2.2)
+
+# Version 0.2.2
+
+- Fix unsoundness issues by adopting `MaybeUninit`. (#458)
+
+# Version 0.2.1
+
+- Add `no_std` support.
+
+# Version 0.2.0
+
+- Bump the minimum required version to 1.28.
+- Bump `crossbeam-utils` to `0.7`.
+
+# Version 0.1.2
+
+- Update `crossbeam-utils` to `0.6.5`.
+
+# Version 0.1.1
+
+- Update `crossbeam-utils` to `0.6.4`.
+
+# Version 0.1.0
+
+- Initial version with `ArrayQueue` and `SegQueue`.
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..dc16305
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,38 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies
+#
+# If you believe there's an error in this file please file an
+# issue against the rust-lang/cargo repository. If you're
+# editing this file be aware that the upstream Cargo.toml
+# will likely look very different (and much more reasonable)
+
+[package]
+edition = "2018"
+name = "crossbeam-queue"
+version = "0.3.2"
+authors = ["The Crossbeam Project Developers"]
+description = "Concurrent queues"
+homepage = "https://github.com/crossbeam-rs/crossbeam/tree/master/crossbeam-queue"
+documentation = "https://docs.rs/crossbeam-queue"
+keywords = ["queue", "mpmc", "lock-free", "producer", "consumer"]
+categories = ["concurrency", "data-structures", "no-std"]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/crossbeam-rs/crossbeam"
+[dependencies.cfg-if]
+version = "1"
+
+[dependencies.crossbeam-utils]
+version = "0.8.5"
+default-features = false
+[dev-dependencies.rand]
+version = "0.8"
+
+[features]
+alloc = []
+default = ["std"]
+nightly = ["crossbeam-utils/nightly"]
+std = ["alloc", "crossbeam-utils/std"]
diff --git a/Cargo.toml.orig b/Cargo.toml.orig
new file mode 100644
index 0000000..dbefdc5
--- /dev/null
+++ b/Cargo.toml.orig
@@ -0,0 +1,49 @@
+[package]
+name = "crossbeam-queue"
+# When publishing a new version:
+# - Update CHANGELOG.md
+# - Update README.md
+# - Create "crossbeam-queue-X.Y.Z" git tag
+version = "0.3.2"
+authors = ["The Crossbeam Project Developers"]
+edition = "2018"
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/crossbeam-rs/crossbeam"
+homepage = "https://github.com/crossbeam-rs/crossbeam/tree/master/crossbeam-queue"
+documentation = "https://docs.rs/crossbeam-queue"
+description = "Concurrent queues"
+keywords = ["queue", "mpmc", "lock-free", "producer", "consumer"]
+categories = ["concurrency", "data-structures", "no-std"]
+
+[features]
+default = ["std"]
+
+# Enable to use APIs that require `std`.
+# This is enabled by default.
+std = ["alloc", "crossbeam-utils/std"]
+
+# Enable to use APIs that require `alloc`.
+# This is enabled by default and also enabled if the `std` feature is enabled.
+#
+# NOTE: Disabling both `std` *and* `alloc` features is not supported yet.
+alloc = []
+
+# These features are no longer used.
+# TODO: remove in the next major version.
+# Enable to use of unstable functionality.
+# This is disabled by default and requires recent nightly compiler.
+#
+# NOTE: This feature is outside of the normal semver guarantees and minor or
+# patch versions of crossbeam may make breaking changes to them at any time.
+nightly = ["crossbeam-utils/nightly"]
+
+[dependencies]
+cfg-if = "1"
+
+[dependencies.crossbeam-utils]
+version = "0.8.5"
+path = "../crossbeam-utils"
+default-features = false
+
+[dev-dependencies]
+rand = "0.8"
diff --git a/LICENSE b/LICENSE
new file mode 120000
index 0000000..6b579aa
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1 @@
+LICENSE-APACHE
\ No newline at end of file
diff --git a/LICENSE-APACHE b/LICENSE-APACHE
new file mode 100644
index 0000000..16fe87b
--- /dev/null
+++ b/LICENSE-APACHE
@@ -0,0 +1,201 @@
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diff --git a/LICENSE-MIT b/LICENSE-MIT
new file mode 100644
index 0000000..068d491
--- /dev/null
+++ b/LICENSE-MIT
@@ -0,0 +1,27 @@
+The MIT License (MIT)
+
+Copyright (c) 2019 The Crossbeam Project Developers
+
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
diff --git a/METADATA b/METADATA
new file mode 100644
index 0000000..37b8868
--- /dev/null
+++ b/METADATA
@@ -0,0 +1,20 @@
+name: "crossbeam-queue"
+description: "Concurrent queues"
+third_party {
+  url {
+    type: HOMEPAGE
+    value: "https://crates.io/crates/crossbeam-queue"
+  }
+  url {
+    type: ARCHIVE
+    value: "https://static.crates.io/crates/crossbeam-queue/crossbeam-queue-0.3.2.crate"
+  }
+  version: "0.3.2"
+  # Dual-licensed, using the least restrictive per go/thirdpartylicenses#same.
+  license_type: NOTICE
+  last_upgrade_date {
+    year: 2021
+    month: 8
+    day: 30
+  }
+}
diff --git a/MODULE_LICENSE_APACHE2 b/MODULE_LICENSE_APACHE2
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/MODULE_LICENSE_APACHE2
diff --git a/OWNERS b/OWNERS
new file mode 100644
index 0000000..45dc4dd
--- /dev/null
+++ b/OWNERS
@@ -0,0 +1 @@
+include platform/prebuilts/rust:master:/OWNERS
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..2f30b39
--- /dev/null
+++ b/README.md
@@ -0,0 +1,54 @@
+# Crossbeam Queue
+
+[![Build Status](https://github.com/crossbeam-rs/crossbeam/workflows/CI/badge.svg)](
+https://github.com/crossbeam-rs/crossbeam/actions)
+[![License](https://img.shields.io/badge/license-MIT_OR_Apache--2.0-blue.svg)](
+https://github.com/crossbeam-rs/crossbeam/tree/master/crossbeam-queue#license)
+[![Cargo](https://img.shields.io/crates/v/crossbeam-queue.svg)](
+https://crates.io/crates/crossbeam-queue)
+[![Documentation](https://docs.rs/crossbeam-queue/badge.svg)](
+https://docs.rs/crossbeam-queue)
+[![Rust 1.36+](https://img.shields.io/badge/rust-1.36+-lightgray.svg)](
+https://www.rust-lang.org)
+[![chat](https://img.shields.io/discord/569610676205781012.svg?logo=discord)](https://discord.com/invite/JXYwgWZ)
+
+This crate provides concurrent queues that can be shared among threads:
+
+* [`ArrayQueue`], a bounded MPMC queue that allocates a fixed-capacity buffer on construction.
+* [`SegQueue`], an unbounded MPMC queue that allocates small buffers, segments, on demand.
+
+Everything in this crate can be used in `no_std` environments, provided that `alloc` feature is
+enabled.
+
+[`ArrayQueue`]: https://docs.rs/crossbeam-queue/*/crossbeam_queue/struct.ArrayQueue.html
+[`SegQueue`]: https://docs.rs/crossbeam-queue/*/crossbeam_queue/struct.SegQueue.html
+
+## Usage
+
+Add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+crossbeam-queue = "0.3"
+```
+
+## Compatibility
+
+Crossbeam Queue supports stable Rust releases going back at least six months,
+and every time the minimum supported Rust version is increased, a new minor
+version is released. Currently, the minimum supported Rust version is 1.36.
+
+## License
+
+Licensed under either of
+
+ * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+#### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
diff --git a/TEST_MAPPING b/TEST_MAPPING
new file mode 100644
index 0000000..53eefac
--- /dev/null
+++ b/TEST_MAPPING
@@ -0,0 +1,19 @@
+// Generated by update_crate_tests.py for tests that depend on this crate.
+{
+  "presubmit": [
+    {
+      "name": "crossbeam-queue_device_test_src_lib"
+    },
+    {
+      "name": "crossbeam-queue_device_test_tests_array_queue"
+    },
+    {
+      "name": "crossbeam-queue_device_test_tests_seg_queue"
+    }
+    // Manually disabled as these tests depend on specific graphics libraries
+    // being available on the machine running the tests.
+    //{
+    //  "name": "vulkano_device_test_src_lib"
+    //}
+  ]
+}
diff --git a/build.rs b/build.rs
new file mode 100644
index 0000000..4ef1279
--- /dev/null
+++ b/build.rs
@@ -0,0 +1,32 @@
+#![warn(rust_2018_idioms)]
+
+use std::env;
+
+include!("no_atomic.rs");
+
+// The rustc-cfg strings below are *not* public API. Please let us know by
+// opening a GitHub issue if your build environment requires some way to enable
+// these cfgs other than by executing our build script.
+fn main() {
+    let target = match env::var("TARGET") {
+        Ok(target) => target,
+        Err(e) => {
+            println!(
+                "cargo:warning={}: unable to get TARGET environment variable: {}",
+                env!("CARGO_PKG_NAME"),
+                e
+            );
+            return;
+        }
+    };
+
+    // Note that this is `no_*`, not `has_*`. This allows treating
+    // `cfg(target_has_atomic = "ptr")` as true when the build script doesn't
+    // run. This is needed for compatibility with non-cargo build systems that
+    // don't run the build script.
+    if NO_ATOMIC_CAS.contains(&&*target) {
+        println!("cargo:rustc-cfg=crossbeam_no_atomic_cas");
+    }
+
+    println!("cargo:rerun-if-changed=no_atomic.rs");
+}
diff --git a/cargo2android.json b/cargo2android.json
new file mode 100644
index 0000000..d36fb44
--- /dev/null
+++ b/cargo2android.json
@@ -0,0 +1,5 @@
+{
+  "device": true,
+  "run": true,
+  "tests": true
+}
\ No newline at end of file
diff --git a/no_atomic.rs b/no_atomic.rs
new file mode 100644
index 0000000..522b3b8
--- /dev/null
+++ b/no_atomic.rs
@@ -0,0 +1,59 @@
+// This file is @generated by no_atomic.sh.
+// It is not intended for manual editing.
+
+const NO_ATOMIC_CAS: &[&str] = &[
+    "avr-unknown-gnu-atmega328",
+    "msp430-none-elf",
+    "riscv32i-unknown-none-elf",
+    "riscv32imc-unknown-none-elf",
+    "thumbv4t-none-eabi",
+    "thumbv6m-none-eabi",
+];
+#[allow(dead_code)]
+const NO_ATOMIC_64: &[&str] = &[
+    "arm-linux-androideabi",
+    "armebv7r-none-eabi",
+    "armebv7r-none-eabihf",
+    "armv4t-unknown-linux-gnueabi",
+    "armv5te-unknown-linux-gnueabi",
+    "armv5te-unknown-linux-musleabi",
+    "armv5te-unknown-linux-uclibceabi",
+    "armv7r-none-eabi",
+    "armv7r-none-eabihf",
+    "hexagon-unknown-linux-musl",
+    "mips-unknown-linux-gnu",
+    "mips-unknown-linux-musl",
+    "mips-unknown-linux-uclibc",
+    "mipsel-unknown-linux-gnu",
+    "mipsel-unknown-linux-musl",
+    "mipsel-unknown-linux-uclibc",
+    "mipsel-unknown-none",
+    "mipsisa32r6-unknown-linux-gnu",
+    "mipsisa32r6el-unknown-linux-gnu",
+    "powerpc-unknown-linux-gnu",
+    "powerpc-unknown-linux-gnuspe",
+    "powerpc-unknown-linux-musl",
+    "powerpc-unknown-netbsd",
+    "powerpc-unknown-openbsd",
+    "powerpc-wrs-vxworks",
+    "powerpc-wrs-vxworks-spe",
+    "riscv32gc-unknown-linux-gnu",
+    "riscv32gc-unknown-linux-musl",
+    "riscv32imac-unknown-none-elf",
+    "thumbv7em-none-eabi",
+    "thumbv7em-none-eabihf",
+    "thumbv7m-none-eabi",
+    "thumbv8m.base-none-eabi",
+    "thumbv8m.main-none-eabi",
+    "thumbv8m.main-none-eabihf",
+    "mipsel-sony-psp",
+    "thumbv4t-none-eabi",
+    "thumbv6m-none-eabi",
+];
+#[allow(dead_code)]
+const NO_ATOMIC: &[&str] = &[
+    "avr-unknown-gnu-atmega328",
+    "msp430-none-elf",
+    "riscv32i-unknown-none-elf",
+    "riscv32imc-unknown-none-elf",
+];
diff --git a/src/array_queue.rs b/src/array_queue.rs
new file mode 100644
index 0000000..ff1efaa
--- /dev/null
+++ b/src/array_queue.rs
@@ -0,0 +1,434 @@
+//! The implementation is based on Dmitry Vyukov's bounded MPMC queue.
+//!
+//! Source:
+//!   - <http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue>
+
+use alloc::boxed::Box;
+use core::cell::UnsafeCell;
+use core::fmt;
+use core::marker::PhantomData;
+use core::mem::{self, MaybeUninit};
+use core::sync::atomic::{self, AtomicUsize, Ordering};
+
+use crossbeam_utils::{Backoff, CachePadded};
+
+/// A slot in a queue.
+struct Slot<T> {
+    /// The current stamp.
+    ///
+    /// If the stamp equals the tail, this node will be next written to. If it equals head + 1,
+    /// this node will be next read from.
+    stamp: AtomicUsize,
+
+    /// The value in this slot.
+    value: UnsafeCell<MaybeUninit<T>>,
+}
+
+/// A bounded multi-producer multi-consumer queue.
+///
+/// This queue allocates a fixed-capacity buffer on construction, which is used to store pushed
+/// elements. The queue cannot hold more elements than the buffer allows. Attempting to push an
+/// element into a full queue will fail. Having a buffer allocated upfront makes this queue a bit
+/// faster than [`SegQueue`].
+///
+/// [`SegQueue`]: super::SegQueue
+///
+/// # Examples
+///
+/// ```
+/// use crossbeam_queue::ArrayQueue;
+///
+/// let q = ArrayQueue::new(2);
+///
+/// assert_eq!(q.push('a'), Ok(()));
+/// assert_eq!(q.push('b'), Ok(()));
+/// assert_eq!(q.push('c'), Err('c'));
+/// assert_eq!(q.pop(), Some('a'));
+/// ```
+pub struct ArrayQueue<T> {
+    /// The head of the queue.
+    ///
+    /// This value is a "stamp" consisting of an index into the buffer and a lap, but packed into a
+    /// single `usize`. The lower bits represent the index, while the upper bits represent the lap.
+    ///
+    /// Elements are popped from the head of the queue.
+    head: CachePadded<AtomicUsize>,
+
+    /// The tail of the queue.
+    ///
+    /// This value is a "stamp" consisting of an index into the buffer and a lap, but packed into a
+    /// single `usize`. The lower bits represent the index, while the upper bits represent the lap.
+    ///
+    /// Elements are pushed into the tail of the queue.
+    tail: CachePadded<AtomicUsize>,
+
+    /// The buffer holding slots.
+    buffer: *mut Slot<T>,
+
+    /// The queue capacity.
+    cap: usize,
+
+    /// A stamp with the value of `{ lap: 1, index: 0 }`.
+    one_lap: usize,
+
+    /// Indicates that dropping an `ArrayQueue<T>` may drop elements of type `T`.
+    _marker: PhantomData<T>,
+}
+
+unsafe impl<T: Send> Sync for ArrayQueue<T> {}
+unsafe impl<T: Send> Send for ArrayQueue<T> {}
+
+impl<T> ArrayQueue<T> {
+    /// Creates a new bounded queue with the given capacity.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the capacity is zero.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::ArrayQueue;
+    ///
+    /// let q = ArrayQueue::<i32>::new(100);
+    /// ```
+    pub fn new(cap: usize) -> ArrayQueue<T> {
+        assert!(cap > 0, "capacity must be non-zero");
+
+        // Head is initialized to `{ lap: 0, index: 0 }`.
+        // Tail is initialized to `{ lap: 0, index: 0 }`.
+        let head = 0;
+        let tail = 0;
+
+        // Allocate a buffer of `cap` slots initialized
+        // with stamps.
+        let buffer = {
+            let mut boxed: Box<[Slot<T>]> = (0..cap)
+                .map(|i| {
+                    // Set the stamp to `{ lap: 0, index: i }`.
+                    Slot {
+                        stamp: AtomicUsize::new(i),
+                        value: UnsafeCell::new(MaybeUninit::uninit()),
+                    }
+                })
+                .collect();
+            let ptr = boxed.as_mut_ptr();
+            mem::forget(boxed);
+            ptr
+        };
+
+        // One lap is the smallest power of two greater than `cap`.
+        let one_lap = (cap + 1).next_power_of_two();
+
+        ArrayQueue {
+            buffer,
+            cap,
+            one_lap,
+            head: CachePadded::new(AtomicUsize::new(head)),
+            tail: CachePadded::new(AtomicUsize::new(tail)),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Attempts to push an element into the queue.
+    ///
+    /// If the queue is full, the element is returned back as an error.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::ArrayQueue;
+    ///
+    /// let q = ArrayQueue::new(1);
+    ///
+    /// assert_eq!(q.push(10), Ok(()));
+    /// assert_eq!(q.push(20), Err(20));
+    /// ```
+    pub fn push(&self, value: T) -> Result<(), T> {
+        let backoff = Backoff::new();
+        let mut tail = self.tail.load(Ordering::Relaxed);
+
+        loop {
+            // Deconstruct the tail.
+            let index = tail & (self.one_lap - 1);
+            let lap = tail & !(self.one_lap - 1);
+
+            // Inspect the corresponding slot.
+            let slot = unsafe { &*self.buffer.add(index) };
+            let stamp = slot.stamp.load(Ordering::Acquire);
+
+            // If the tail and the stamp match, we may attempt to push.
+            if tail == stamp {
+                let new_tail = if index + 1 < self.cap {
+                    // Same lap, incremented index.
+                    // Set to `{ lap: lap, index: index + 1 }`.
+                    tail + 1
+                } else {
+                    // One lap forward, index wraps around to zero.
+                    // Set to `{ lap: lap.wrapping_add(1), index: 0 }`.
+                    lap.wrapping_add(self.one_lap)
+                };
+
+                // Try moving the tail.
+                match self.tail.compare_exchange_weak(
+                    tail,
+                    new_tail,
+                    Ordering::SeqCst,
+                    Ordering::Relaxed,
+                ) {
+                    Ok(_) => {
+                        // Write the value into the slot and update the stamp.
+                        unsafe {
+                            slot.value.get().write(MaybeUninit::new(value));
+                        }
+                        slot.stamp.store(tail + 1, Ordering::Release);
+                        return Ok(());
+                    }
+                    Err(t) => {
+                        tail = t;
+                        backoff.spin();
+                    }
+                }
+            } else if stamp.wrapping_add(self.one_lap) == tail + 1 {
+                atomic::fence(Ordering::SeqCst);
+                let head = self.head.load(Ordering::Relaxed);
+
+                // If the head lags one lap behind the tail as well...
+                if head.wrapping_add(self.one_lap) == tail {
+                    // ...then the queue is full.
+                    return Err(value);
+                }
+
+                backoff.spin();
+                tail = self.tail.load(Ordering::Relaxed);
+            } else {
+                // Snooze because we need to wait for the stamp to get updated.
+                backoff.snooze();
+                tail = self.tail.load(Ordering::Relaxed);
+            }
+        }
+    }
+
+    /// Attempts to pop an element from the queue.
+    ///
+    /// If the queue is empty, `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::ArrayQueue;
+    ///
+    /// let q = ArrayQueue::new(1);
+    /// assert_eq!(q.push(10), Ok(()));
+    ///
+    /// assert_eq!(q.pop(), Some(10));
+    /// assert!(q.pop().is_none());
+    /// ```
+    pub fn pop(&self) -> Option<T> {
+        let backoff = Backoff::new();
+        let mut head = self.head.load(Ordering::Relaxed);
+
+        loop {
+            // Deconstruct the head.
+            let index = head & (self.one_lap - 1);
+            let lap = head & !(self.one_lap - 1);
+
+            // Inspect the corresponding slot.
+            let slot = unsafe { &*self.buffer.add(index) };
+            let stamp = slot.stamp.load(Ordering::Acquire);
+
+            // If the the stamp is ahead of the head by 1, we may attempt to pop.
+            if head + 1 == stamp {
+                let new = if index + 1 < self.cap {
+                    // Same lap, incremented index.
+                    // Set to `{ lap: lap, index: index + 1 }`.
+                    head + 1
+                } else {
+                    // One lap forward, index wraps around to zero.
+                    // Set to `{ lap: lap.wrapping_add(1), index: 0 }`.
+                    lap.wrapping_add(self.one_lap)
+                };
+
+                // Try moving the head.
+                match self.head.compare_exchange_weak(
+                    head,
+                    new,
+                    Ordering::SeqCst,
+                    Ordering::Relaxed,
+                ) {
+                    Ok(_) => {
+                        // Read the value from the slot and update the stamp.
+                        let msg = unsafe { slot.value.get().read().assume_init() };
+                        slot.stamp
+                            .store(head.wrapping_add(self.one_lap), Ordering::Release);
+                        return Some(msg);
+                    }
+                    Err(h) => {
+                        head = h;
+                        backoff.spin();
+                    }
+                }
+            } else if stamp == head {
+                atomic::fence(Ordering::SeqCst);
+                let tail = self.tail.load(Ordering::Relaxed);
+
+                // If the tail equals the head, that means the channel is empty.
+                if tail == head {
+                    return None;
+                }
+
+                backoff.spin();
+                head = self.head.load(Ordering::Relaxed);
+            } else {
+                // Snooze because we need to wait for the stamp to get updated.
+                backoff.snooze();
+                head = self.head.load(Ordering::Relaxed);
+            }
+        }
+    }
+
+    /// Returns the capacity of the queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::ArrayQueue;
+    ///
+    /// let q = ArrayQueue::<i32>::new(100);
+    ///
+    /// assert_eq!(q.capacity(), 100);
+    /// ```
+    pub fn capacity(&self) -> usize {
+        self.cap
+    }
+
+    /// Returns `true` if the queue is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::ArrayQueue;
+    ///
+    /// let q = ArrayQueue::new(100);
+    ///
+    /// assert!(q.is_empty());
+    /// q.push(1).unwrap();
+    /// assert!(!q.is_empty());
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        let head = self.head.load(Ordering::SeqCst);
+        let tail = self.tail.load(Ordering::SeqCst);
+
+        // Is the tail lagging one lap behind head?
+        // Is the tail equal to the head?
+        //
+        // Note: If the head changes just before we load the tail, that means there was a moment
+        // when the channel was not empty, so it is safe to just return `false`.
+        tail == head
+    }
+
+    /// Returns `true` if the queue is full.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::ArrayQueue;
+    ///
+    /// let q = ArrayQueue::new(1);
+    ///
+    /// assert!(!q.is_full());
+    /// q.push(1).unwrap();
+    /// assert!(q.is_full());
+    /// ```
+    pub fn is_full(&self) -> bool {
+        let tail = self.tail.load(Ordering::SeqCst);
+        let head = self.head.load(Ordering::SeqCst);
+
+        // Is the head lagging one lap behind tail?
+        //
+        // Note: If the tail changes just before we load the head, that means there was a moment
+        // when the queue was not full, so it is safe to just return `false`.
+        head.wrapping_add(self.one_lap) == tail
+    }
+
+    /// Returns the number of elements in the queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::ArrayQueue;
+    ///
+    /// let q = ArrayQueue::new(100);
+    /// assert_eq!(q.len(), 0);
+    ///
+    /// q.push(10).unwrap();
+    /// assert_eq!(q.len(), 1);
+    ///
+    /// q.push(20).unwrap();
+    /// assert_eq!(q.len(), 2);
+    /// ```
+    pub fn len(&self) -> usize {
+        loop {
+            // Load the tail, then load the head.
+            let tail = self.tail.load(Ordering::SeqCst);
+            let head = self.head.load(Ordering::SeqCst);
+
+            // If the tail didn't change, we've got consistent values to work with.
+            if self.tail.load(Ordering::SeqCst) == tail {
+                let hix = head & (self.one_lap - 1);
+                let tix = tail & (self.one_lap - 1);
+
+                return if hix < tix {
+                    tix - hix
+                } else if hix > tix {
+                    self.cap - hix + tix
+                } else if tail == head {
+                    0
+                } else {
+                    self.cap
+                };
+            }
+        }
+    }
+}
+
+impl<T> Drop for ArrayQueue<T> {
+    fn drop(&mut self) {
+        // Get the index of the head.
+        let hix = self.head.load(Ordering::Relaxed) & (self.one_lap - 1);
+
+        // Loop over all slots that hold a message and drop them.
+        for i in 0..self.len() {
+            // Compute the index of the next slot holding a message.
+            let index = if hix + i < self.cap {
+                hix + i
+            } else {
+                hix + i - self.cap
+            };
+
+            unsafe {
+                let p = {
+                    let slot = &mut *self.buffer.add(index);
+                    let value = &mut *slot.value.get();
+                    value.as_mut_ptr()
+                };
+                p.drop_in_place();
+            }
+        }
+
+        // Finally, deallocate the buffer, but don't run any destructors.
+        unsafe {
+            // Create a slice from the buffer to make
+            // a fat pointer. Then, use Box::from_raw
+            // to deallocate it.
+            let ptr = core::slice::from_raw_parts_mut(self.buffer, self.cap) as *mut [Slot<T>];
+            Box::from_raw(ptr);
+        }
+    }
+}
+
+impl<T> fmt::Debug for ArrayQueue<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("ArrayQueue { .. }")
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..846d7c2
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,34 @@
+//! Concurrent queues.
+//!
+//! This crate provides concurrent queues that can be shared among threads:
+//!
+//! * [`ArrayQueue`], a bounded MPMC queue that allocates a fixed-capacity buffer on construction.
+//! * [`SegQueue`], an unbounded MPMC queue that allocates small buffers, segments, on demand.
+
+#![doc(test(
+    no_crate_inject,
+    attr(
+        deny(warnings, rust_2018_idioms),
+        allow(dead_code, unused_assignments, unused_variables)
+    )
+))]
+#![warn(
+    missing_docs,
+    missing_debug_implementations,
+    rust_2018_idioms,
+    unreachable_pub
+)]
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(not(crossbeam_no_atomic_cas))]
+cfg_if::cfg_if! {
+    if #[cfg(feature = "alloc")] {
+        extern crate alloc;
+
+        mod array_queue;
+        mod seg_queue;
+
+        pub use self::array_queue::ArrayQueue;
+        pub use self::seg_queue::SegQueue;
+    }
+}
diff --git a/src/seg_queue.rs b/src/seg_queue.rs
new file mode 100644
index 0000000..8545541
--- /dev/null
+++ b/src/seg_queue.rs
@@ -0,0 +1,486 @@
+use alloc::boxed::Box;
+use core::cell::UnsafeCell;
+use core::fmt;
+use core::marker::PhantomData;
+use core::mem::MaybeUninit;
+use core::ptr;
+use core::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
+
+use crossbeam_utils::{Backoff, CachePadded};
+
+// Bits indicating the state of a slot:
+// * If a value has been written into the slot, `WRITE` is set.
+// * If a value has been read from the slot, `READ` is set.
+// * If the block is being destroyed, `DESTROY` is set.
+const WRITE: usize = 1;
+const READ: usize = 2;
+const DESTROY: usize = 4;
+
+// Each block covers one "lap" of indices.
+const LAP: usize = 32;
+// The maximum number of values a block can hold.
+const BLOCK_CAP: usize = LAP - 1;
+// How many lower bits are reserved for metadata.
+const SHIFT: usize = 1;
+// Indicates that the block is not the last one.
+const HAS_NEXT: usize = 1;
+
+/// A slot in a block.
+struct Slot<T> {
+    /// The value.
+    value: UnsafeCell<MaybeUninit<T>>,
+
+    /// The state of the slot.
+    state: AtomicUsize,
+}
+
+impl<T> Slot<T> {
+    /// Waits until a value is written into the slot.
+    fn wait_write(&self) {
+        let backoff = Backoff::new();
+        while self.state.load(Ordering::Acquire) & WRITE == 0 {
+            backoff.snooze();
+        }
+    }
+}
+
+/// A block in a linked list.
+///
+/// Each block in the list can hold up to `BLOCK_CAP` values.
+struct Block<T> {
+    /// The next block in the linked list.
+    next: AtomicPtr<Block<T>>,
+
+    /// Slots for values.
+    slots: [Slot<T>; BLOCK_CAP],
+}
+
+impl<T> Block<T> {
+    /// Creates an empty block that starts at `start_index`.
+    fn new() -> Block<T> {
+        // SAFETY: This is safe because:
+        //  [1] `Block::next` (AtomicPtr) may be safely zero initialized.
+        //  [2] `Block::slots` (Array) may be safely zero initialized because of [3, 4].
+        //  [3] `Slot::value` (UnsafeCell) may be safely zero initialized because it
+        //       holds a MaybeUninit.
+        //  [4] `Slot::state` (AtomicUsize) may be safely zero initialized.
+        unsafe { MaybeUninit::zeroed().assume_init() }
+    }
+
+    /// Waits until the next pointer is set.
+    fn wait_next(&self) -> *mut Block<T> {
+        let backoff = Backoff::new();
+        loop {
+            let next = self.next.load(Ordering::Acquire);
+            if !next.is_null() {
+                return next;
+            }
+            backoff.snooze();
+        }
+    }
+
+    /// Sets the `DESTROY` bit in slots starting from `start` and destroys the block.
+    unsafe fn destroy(this: *mut Block<T>, start: usize) {
+        // It is not necessary to set the `DESTROY` bit in the last slot because that slot has
+        // begun destruction of the block.
+        for i in start..BLOCK_CAP - 1 {
+            let slot = (*this).slots.get_unchecked(i);
+
+            // Mark the `DESTROY` bit if a thread is still using the slot.
+            if slot.state.load(Ordering::Acquire) & READ == 0
+                && slot.state.fetch_or(DESTROY, Ordering::AcqRel) & READ == 0
+            {
+                // If a thread is still using the slot, it will continue destruction of the block.
+                return;
+            }
+        }
+
+        // No thread is using the block, now it is safe to destroy it.
+        drop(Box::from_raw(this));
+    }
+}
+
+/// A position in a queue.
+struct Position<T> {
+    /// The index in the queue.
+    index: AtomicUsize,
+
+    /// The block in the linked list.
+    block: AtomicPtr<Block<T>>,
+}
+
+/// An unbounded multi-producer multi-consumer queue.
+///
+/// This queue is implemented as a linked list of segments, where each segment is a small buffer
+/// that can hold a handful of elements. There is no limit to how many elements can be in the queue
+/// at a time. However, since segments need to be dynamically allocated as elements get pushed,
+/// this queue is somewhat slower than [`ArrayQueue`].
+///
+/// [`ArrayQueue`]: super::ArrayQueue
+///
+/// # Examples
+///
+/// ```
+/// use crossbeam_queue::SegQueue;
+///
+/// let q = SegQueue::new();
+///
+/// q.push('a');
+/// q.push('b');
+///
+/// assert_eq!(q.pop(), Some('a'));
+/// assert_eq!(q.pop(), Some('b'));
+/// assert!(q.pop().is_none());
+/// ```
+pub struct SegQueue<T> {
+    /// The head of the queue.
+    head: CachePadded<Position<T>>,
+
+    /// The tail of the queue.
+    tail: CachePadded<Position<T>>,
+
+    /// Indicates that dropping a `SegQueue<T>` may drop values of type `T`.
+    _marker: PhantomData<T>,
+}
+
+unsafe impl<T: Send> Send for SegQueue<T> {}
+unsafe impl<T: Send> Sync for SegQueue<T> {}
+
+impl<T> SegQueue<T> {
+    /// Creates a new unbounded queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::SegQueue;
+    ///
+    /// let q = SegQueue::<i32>::new();
+    /// ```
+    pub const fn new() -> SegQueue<T> {
+        SegQueue {
+            head: CachePadded::new(Position {
+                block: AtomicPtr::new(ptr::null_mut()),
+                index: AtomicUsize::new(0),
+            }),
+            tail: CachePadded::new(Position {
+                block: AtomicPtr::new(ptr::null_mut()),
+                index: AtomicUsize::new(0),
+            }),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Pushes an element into the queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::SegQueue;
+    ///
+    /// let q = SegQueue::new();
+    ///
+    /// q.push(10);
+    /// q.push(20);
+    /// ```
+    pub fn push(&self, value: T) {
+        let backoff = Backoff::new();
+        let mut tail = self.tail.index.load(Ordering::Acquire);
+        let mut block = self.tail.block.load(Ordering::Acquire);
+        let mut next_block = None;
+
+        loop {
+            // Calculate the offset of the index into the block.
+            let offset = (tail >> SHIFT) % LAP;
+
+            // If we reached the end of the block, wait until the next one is installed.
+            if offset == BLOCK_CAP {
+                backoff.snooze();
+                tail = self.tail.index.load(Ordering::Acquire);
+                block = self.tail.block.load(Ordering::Acquire);
+                continue;
+            }
+
+            // If we're going to have to install the next block, allocate it in advance in order to
+            // make the wait for other threads as short as possible.
+            if offset + 1 == BLOCK_CAP && next_block.is_none() {
+                next_block = Some(Box::new(Block::<T>::new()));
+            }
+
+            // If this is the first push operation, we need to allocate the first block.
+            if block.is_null() {
+                let new = Box::into_raw(Box::new(Block::<T>::new()));
+
+                if self
+                    .tail
+                    .block
+                    .compare_exchange(block, new, Ordering::Release, Ordering::Relaxed)
+                    .is_ok()
+                {
+                    self.head.block.store(new, Ordering::Release);
+                    block = new;
+                } else {
+                    next_block = unsafe { Some(Box::from_raw(new)) };
+                    tail = self.tail.index.load(Ordering::Acquire);
+                    block = self.tail.block.load(Ordering::Acquire);
+                    continue;
+                }
+            }
+
+            let new_tail = tail + (1 << SHIFT);
+
+            // Try advancing the tail forward.
+            match self.tail.index.compare_exchange_weak(
+                tail,
+                new_tail,
+                Ordering::SeqCst,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => unsafe {
+                    // If we've reached the end of the block, install the next one.
+                    if offset + 1 == BLOCK_CAP {
+                        let next_block = Box::into_raw(next_block.unwrap());
+                        let next_index = new_tail.wrapping_add(1 << SHIFT);
+
+                        self.tail.block.store(next_block, Ordering::Release);
+                        self.tail.index.store(next_index, Ordering::Release);
+                        (*block).next.store(next_block, Ordering::Release);
+                    }
+
+                    // Write the value into the slot.
+                    let slot = (*block).slots.get_unchecked(offset);
+                    slot.value.get().write(MaybeUninit::new(value));
+                    slot.state.fetch_or(WRITE, Ordering::Release);
+
+                    return;
+                },
+                Err(t) => {
+                    tail = t;
+                    block = self.tail.block.load(Ordering::Acquire);
+                    backoff.spin();
+                }
+            }
+        }
+    }
+
+    /// Pops an element from the queue.
+    ///
+    /// If the queue is empty, `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::SegQueue;
+    ///
+    /// let q = SegQueue::new();
+    ///
+    /// q.push(10);
+    /// assert_eq!(q.pop(), Some(10));
+    /// assert!(q.pop().is_none());
+    /// ```
+    pub fn pop(&self) -> Option<T> {
+        let backoff = Backoff::new();
+        let mut head = self.head.index.load(Ordering::Acquire);
+        let mut block = self.head.block.load(Ordering::Acquire);
+
+        loop {
+            // Calculate the offset of the index into the block.
+            let offset = (head >> SHIFT) % LAP;
+
+            // If we reached the end of the block, wait until the next one is installed.
+            if offset == BLOCK_CAP {
+                backoff.snooze();
+                head = self.head.index.load(Ordering::Acquire);
+                block = self.head.block.load(Ordering::Acquire);
+                continue;
+            }
+
+            let mut new_head = head + (1 << SHIFT);
+
+            if new_head & HAS_NEXT == 0 {
+                atomic::fence(Ordering::SeqCst);
+                let tail = self.tail.index.load(Ordering::Relaxed);
+
+                // If the tail equals the head, that means the queue is empty.
+                if head >> SHIFT == tail >> SHIFT {
+                    return None;
+                }
+
+                // If head and tail are not in the same block, set `HAS_NEXT` in head.
+                if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP {
+                    new_head |= HAS_NEXT;
+                }
+            }
+
+            // The block can be null here only if the first push operation is in progress. In that
+            // case, just wait until it gets initialized.
+            if block.is_null() {
+                backoff.snooze();
+                head = self.head.index.load(Ordering::Acquire);
+                block = self.head.block.load(Ordering::Acquire);
+                continue;
+            }
+
+            // Try moving the head index forward.
+            match self.head.index.compare_exchange_weak(
+                head,
+                new_head,
+                Ordering::SeqCst,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => unsafe {
+                    // If we've reached the end of the block, move to the next one.
+                    if offset + 1 == BLOCK_CAP {
+                        let next = (*block).wait_next();
+                        let mut next_index = (new_head & !HAS_NEXT).wrapping_add(1 << SHIFT);
+                        if !(*next).next.load(Ordering::Relaxed).is_null() {
+                            next_index |= HAS_NEXT;
+                        }
+
+                        self.head.block.store(next, Ordering::Release);
+                        self.head.index.store(next_index, Ordering::Release);
+                    }
+
+                    // Read the value.
+                    let slot = (*block).slots.get_unchecked(offset);
+                    slot.wait_write();
+                    let value = slot.value.get().read().assume_init();
+
+                    // Destroy the block if we've reached the end, or if another thread wanted to
+                    // destroy but couldn't because we were busy reading from the slot.
+                    if offset + 1 == BLOCK_CAP {
+                        Block::destroy(block, 0);
+                    } else if slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0 {
+                        Block::destroy(block, offset + 1);
+                    }
+
+                    return Some(value);
+                },
+                Err(h) => {
+                    head = h;
+                    block = self.head.block.load(Ordering::Acquire);
+                    backoff.spin();
+                }
+            }
+        }
+    }
+
+    /// Returns `true` if the queue is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::SegQueue;
+    ///
+    /// let q = SegQueue::new();
+    ///
+    /// assert!(q.is_empty());
+    /// q.push(1);
+    /// assert!(!q.is_empty());
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        let head = self.head.index.load(Ordering::SeqCst);
+        let tail = self.tail.index.load(Ordering::SeqCst);
+        head >> SHIFT == tail >> SHIFT
+    }
+
+    /// Returns the number of elements in the queue.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_queue::SegQueue;
+    ///
+    /// let q = SegQueue::new();
+    /// assert_eq!(q.len(), 0);
+    ///
+    /// q.push(10);
+    /// assert_eq!(q.len(), 1);
+    ///
+    /// q.push(20);
+    /// assert_eq!(q.len(), 2);
+    /// ```
+    pub fn len(&self) -> usize {
+        loop {
+            // Load the tail index, then load the head index.
+            let mut tail = self.tail.index.load(Ordering::SeqCst);
+            let mut head = self.head.index.load(Ordering::SeqCst);
+
+            // If the tail index didn't change, we've got consistent indices to work with.
+            if self.tail.index.load(Ordering::SeqCst) == tail {
+                // Erase the lower bits.
+                tail &= !((1 << SHIFT) - 1);
+                head &= !((1 << SHIFT) - 1);
+
+                // Fix up indices if they fall onto block ends.
+                if (tail >> SHIFT) & (LAP - 1) == LAP - 1 {
+                    tail = tail.wrapping_add(1 << SHIFT);
+                }
+                if (head >> SHIFT) & (LAP - 1) == LAP - 1 {
+                    head = head.wrapping_add(1 << SHIFT);
+                }
+
+                // Rotate indices so that head falls into the first block.
+                let lap = (head >> SHIFT) / LAP;
+                tail = tail.wrapping_sub((lap * LAP) << SHIFT);
+                head = head.wrapping_sub((lap * LAP) << SHIFT);
+
+                // Remove the lower bits.
+                tail >>= SHIFT;
+                head >>= SHIFT;
+
+                // Return the difference minus the number of blocks between tail and head.
+                return tail - head - tail / LAP;
+            }
+        }
+    }
+}
+
+impl<T> Drop for SegQueue<T> {
+    fn drop(&mut self) {
+        let mut head = self.head.index.load(Ordering::Relaxed);
+        let mut tail = self.tail.index.load(Ordering::Relaxed);
+        let mut block = self.head.block.load(Ordering::Relaxed);
+
+        // Erase the lower bits.
+        head &= !((1 << SHIFT) - 1);
+        tail &= !((1 << SHIFT) - 1);
+
+        unsafe {
+            // Drop all values between `head` and `tail` and deallocate the heap-allocated blocks.
+            while head != tail {
+                let offset = (head >> SHIFT) % LAP;
+
+                if offset < BLOCK_CAP {
+                    // Drop the value in the slot.
+                    let slot = (*block).slots.get_unchecked(offset);
+                    let p = &mut *slot.value.get();
+                    p.as_mut_ptr().drop_in_place();
+                } else {
+                    // Deallocate the block and move to the next one.
+                    let next = (*block).next.load(Ordering::Relaxed);
+                    drop(Box::from_raw(block));
+                    block = next;
+                }
+
+                head = head.wrapping_add(1 << SHIFT);
+            }
+
+            // Deallocate the last remaining block.
+            if !block.is_null() {
+                drop(Box::from_raw(block));
+            }
+        }
+    }
+}
+
+impl<T> fmt::Debug for SegQueue<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("SegQueue { .. }")
+    }
+}
+
+impl<T> Default for SegQueue<T> {
+    fn default() -> SegQueue<T> {
+        SegQueue::new()
+    }
+}
diff --git a/tests/array_queue.rs b/tests/array_queue.rs
new file mode 100644
index 0000000..63007eb
--- /dev/null
+++ b/tests/array_queue.rs
@@ -0,0 +1,250 @@
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+use crossbeam_queue::ArrayQueue;
+use crossbeam_utils::thread::scope;
+use rand::{thread_rng, Rng};
+
+#[test]
+fn smoke() {
+    let q = ArrayQueue::new(1);
+
+    q.push(7).unwrap();
+    assert_eq!(q.pop(), Some(7));
+
+    q.push(8).unwrap();
+    assert_eq!(q.pop(), Some(8));
+    assert!(q.pop().is_none());
+}
+
+#[test]
+fn capacity() {
+    for i in 1..10 {
+        let q = ArrayQueue::<i32>::new(i);
+        assert_eq!(q.capacity(), i);
+    }
+}
+
+#[test]
+#[should_panic(expected = "capacity must be non-zero")]
+fn zero_capacity() {
+    let _ = ArrayQueue::<i32>::new(0);
+}
+
+#[test]
+fn len_empty_full() {
+    let q = ArrayQueue::new(2);
+
+    assert_eq!(q.len(), 0);
+    assert_eq!(q.is_empty(), true);
+    assert_eq!(q.is_full(), false);
+
+    q.push(()).unwrap();
+
+    assert_eq!(q.len(), 1);
+    assert_eq!(q.is_empty(), false);
+    assert_eq!(q.is_full(), false);
+
+    q.push(()).unwrap();
+
+    assert_eq!(q.len(), 2);
+    assert_eq!(q.is_empty(), false);
+    assert_eq!(q.is_full(), true);
+
+    q.pop().unwrap();
+
+    assert_eq!(q.len(), 1);
+    assert_eq!(q.is_empty(), false);
+    assert_eq!(q.is_full(), false);
+}
+
+#[test]
+fn len() {
+    const COUNT: usize = 25_000;
+    const CAP: usize = 1000;
+
+    let q = ArrayQueue::new(CAP);
+    assert_eq!(q.len(), 0);
+
+    for _ in 0..CAP / 10 {
+        for i in 0..50 {
+            q.push(i).unwrap();
+            assert_eq!(q.len(), i + 1);
+        }
+
+        for i in 0..50 {
+            q.pop().unwrap();
+            assert_eq!(q.len(), 50 - i - 1);
+        }
+    }
+    assert_eq!(q.len(), 0);
+
+    for i in 0..CAP {
+        q.push(i).unwrap();
+        assert_eq!(q.len(), i + 1);
+    }
+
+    for _ in 0..CAP {
+        q.pop().unwrap();
+    }
+    assert_eq!(q.len(), 0);
+
+    scope(|scope| {
+        scope.spawn(|_| {
+            for i in 0..COUNT {
+                loop {
+                    if let Some(x) = q.pop() {
+                        assert_eq!(x, i);
+                        break;
+                    }
+                }
+                let len = q.len();
+                assert!(len <= CAP);
+            }
+        });
+
+        scope.spawn(|_| {
+            for i in 0..COUNT {
+                while q.push(i).is_err() {}
+                let len = q.len();
+                assert!(len <= CAP);
+            }
+        });
+    })
+    .unwrap();
+    assert_eq!(q.len(), 0);
+}
+
+#[test]
+fn spsc() {
+    const COUNT: usize = 100_000;
+
+    let q = ArrayQueue::new(3);
+
+    scope(|scope| {
+        scope.spawn(|_| {
+            for i in 0..COUNT {
+                loop {
+                    if let Some(x) = q.pop() {
+                        assert_eq!(x, i);
+                        break;
+                    }
+                }
+            }
+            assert!(q.pop().is_none());
+        });
+
+        scope.spawn(|_| {
+            for i in 0..COUNT {
+                while q.push(i).is_err() {}
+            }
+        });
+    })
+    .unwrap();
+}
+
+#[test]
+fn mpmc() {
+    const COUNT: usize = 25_000;
+    const THREADS: usize = 4;
+
+    let q = ArrayQueue::<usize>::new(3);
+    let v = (0..COUNT).map(|_| AtomicUsize::new(0)).collect::<Vec<_>>();
+
+    scope(|scope| {
+        for _ in 0..THREADS {
+            scope.spawn(|_| {
+                for _ in 0..COUNT {
+                    let n = loop {
+                        if let Some(x) = q.pop() {
+                            break x;
+                        }
+                    };
+                    v[n].fetch_add(1, Ordering::SeqCst);
+                }
+            });
+        }
+        for _ in 0..THREADS {
+            scope.spawn(|_| {
+                for i in 0..COUNT {
+                    while q.push(i).is_err() {}
+                }
+            });
+        }
+    })
+    .unwrap();
+
+    for c in v {
+        assert_eq!(c.load(Ordering::SeqCst), THREADS);
+    }
+}
+
+#[test]
+fn drops() {
+    const RUNS: usize = 100;
+
+    static DROPS: AtomicUsize = AtomicUsize::new(0);
+
+    #[derive(Debug, PartialEq)]
+    struct DropCounter;
+
+    impl Drop for DropCounter {
+        fn drop(&mut self) {
+            DROPS.fetch_add(1, Ordering::SeqCst);
+        }
+    }
+
+    let mut rng = thread_rng();
+
+    for _ in 0..RUNS {
+        let steps = rng.gen_range(0..10_000);
+        let additional = rng.gen_range(0..50);
+
+        DROPS.store(0, Ordering::SeqCst);
+        let q = ArrayQueue::new(50);
+
+        scope(|scope| {
+            scope.spawn(|_| {
+                for _ in 0..steps {
+                    while q.pop().is_none() {}
+                }
+            });
+
+            scope.spawn(|_| {
+                for _ in 0..steps {
+                    while q.push(DropCounter).is_err() {
+                        DROPS.fetch_sub(1, Ordering::SeqCst);
+                    }
+                }
+            });
+        })
+        .unwrap();
+
+        for _ in 0..additional {
+            q.push(DropCounter).unwrap();
+        }
+
+        assert_eq!(DROPS.load(Ordering::SeqCst), steps);
+        drop(q);
+        assert_eq!(DROPS.load(Ordering::SeqCst), steps + additional);
+    }
+}
+
+#[test]
+fn linearizable() {
+    const COUNT: usize = 25_000;
+    const THREADS: usize = 4;
+
+    let q = ArrayQueue::new(THREADS);
+
+    scope(|scope| {
+        for _ in 0..THREADS {
+            scope.spawn(|_| {
+                for _ in 0..COUNT {
+                    while q.push(0).is_err() {}
+                    q.pop().unwrap();
+                }
+            });
+        }
+    })
+    .unwrap();
+}
diff --git a/tests/seg_queue.rs b/tests/seg_queue.rs
new file mode 100644
index 0000000..63df9a0
--- /dev/null
+++ b/tests/seg_queue.rs
@@ -0,0 +1,163 @@
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+use crossbeam_queue::SegQueue;
+use crossbeam_utils::thread::scope;
+use rand::{thread_rng, Rng};
+
+#[test]
+fn smoke() {
+    let q = SegQueue::new();
+    q.push(7);
+    assert_eq!(q.pop(), Some(7));
+
+    q.push(8);
+    assert_eq!(q.pop(), Some(8));
+    assert!(q.pop().is_none());
+}
+
+#[test]
+fn len_empty_full() {
+    let q = SegQueue::new();
+
+    assert_eq!(q.len(), 0);
+    assert_eq!(q.is_empty(), true);
+
+    q.push(());
+
+    assert_eq!(q.len(), 1);
+    assert_eq!(q.is_empty(), false);
+
+    q.pop().unwrap();
+
+    assert_eq!(q.len(), 0);
+    assert_eq!(q.is_empty(), true);
+}
+
+#[test]
+fn len() {
+    let q = SegQueue::new();
+
+    assert_eq!(q.len(), 0);
+
+    for i in 0..50 {
+        q.push(i);
+        assert_eq!(q.len(), i + 1);
+    }
+
+    for i in 0..50 {
+        q.pop().unwrap();
+        assert_eq!(q.len(), 50 - i - 1);
+    }
+
+    assert_eq!(q.len(), 0);
+}
+
+#[test]
+fn spsc() {
+    const COUNT: usize = 100_000;
+
+    let q = SegQueue::new();
+
+    scope(|scope| {
+        scope.spawn(|_| {
+            for i in 0..COUNT {
+                loop {
+                    if let Some(x) = q.pop() {
+                        assert_eq!(x, i);
+                        break;
+                    }
+                }
+            }
+            assert!(q.pop().is_none());
+        });
+        scope.spawn(|_| {
+            for i in 0..COUNT {
+                q.push(i);
+            }
+        });
+    })
+    .unwrap();
+}
+
+#[test]
+fn mpmc() {
+    const COUNT: usize = 25_000;
+    const THREADS: usize = 4;
+
+    let q = SegQueue::<usize>::new();
+    let v = (0..COUNT).map(|_| AtomicUsize::new(0)).collect::<Vec<_>>();
+
+    scope(|scope| {
+        for _ in 0..THREADS {
+            scope.spawn(|_| {
+                for _ in 0..COUNT {
+                    let n = loop {
+                        if let Some(x) = q.pop() {
+                            break x;
+                        }
+                    };
+                    v[n].fetch_add(1, Ordering::SeqCst);
+                }
+            });
+        }
+        for _ in 0..THREADS {
+            scope.spawn(|_| {
+                for i in 0..COUNT {
+                    q.push(i);
+                }
+            });
+        }
+    })
+    .unwrap();
+
+    for c in v {
+        assert_eq!(c.load(Ordering::SeqCst), THREADS);
+    }
+}
+
+#[test]
+fn drops() {
+    static DROPS: AtomicUsize = AtomicUsize::new(0);
+
+    #[derive(Debug, PartialEq)]
+    struct DropCounter;
+
+    impl Drop for DropCounter {
+        fn drop(&mut self) {
+            DROPS.fetch_add(1, Ordering::SeqCst);
+        }
+    }
+
+    let mut rng = thread_rng();
+
+    for _ in 0..100 {
+        let steps = rng.gen_range(0..10_000);
+        let additional = rng.gen_range(0..1000);
+
+        DROPS.store(0, Ordering::SeqCst);
+        let q = SegQueue::new();
+
+        scope(|scope| {
+            scope.spawn(|_| {
+                for _ in 0..steps {
+                    while q.pop().is_none() {}
+                }
+            });
+
+            scope.spawn(|_| {
+                for _ in 0..steps {
+                    q.push(DropCounter);
+                }
+            });
+        })
+        .unwrap();
+
+        for _ in 0..additional {
+            q.push(DropCounter);
+        }
+
+        assert_eq!(DROPS.load(Ordering::SeqCst), steps);
+        drop(q);
+        assert_eq!(DROPS.load(Ordering::SeqCst), steps + additional);
+    }
+}