13 files changed, 989 insertions, 0 deletions
diff --git a/xpcom/rust/gtest/bench-collections/Bench.cpp b/xpcom/rust/gtest/bench-collections/Bench.cpp
new file mode 100644
index 0000000000..5ed4629938
--- /dev/null
+++ b/xpcom/rust/gtest/bench-collections/Bench.cpp
@@ -0,0 +1,297 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Overview
+// --------
+// This file measures the speed of various implementations of C++ and Rust
+// collections (hash tables, etc.) used within the codebase. There are a small
+// number of benchmarks for each collection type; each benchmark tests certain
+// operations (insertion, lookup, iteration, etc.) More benchmarks could easily
+// be envisioned, but this small number is enough to characterize the major
+// differences between implementations, while keeping the file size and
+// complexity low.
+//
+// Details
+// -------
+// The file uses `MOZ_GTEST_BENCH_F` so that results are integrated into
+// PerfHerder. It is also designed so that individual test benchmarks can be
+// run under a profiler.
+//
+// The C++ code uses `MOZ_RELEASE_ASSERT` extensively to check values and
+// ensure operations aren't optimized away by the compiler. The Rust code uses
+// `assert!()`. These should be roughly equivalent, but aren't guaranteed to be
+// the same. As a result, the intra-C++ comparisons should be reliable, and the
+// intra-Rust comparisons should be reliable, but the C++ vs. Rust comparisons
+// may be less reliable.
+//
+// Note that the Rust implementations run very slowly without --enable-release.
+//
+// Profiling
+// ---------
+// If you want to measure a particular implementation under a profiler such as
+// Callgrind, do something like this:
+//
+//   MOZ_RUN_GTEST=1 GTEST_FILTER='*BenchCollections*$IMPL*'
+//       valgrind --tool=callgrind --callgrind-out-file=clgout
+//       $OBJDIR/dist/bin/firefox -unittest
+//   callgrind_annotate --auto=yes clgout > clgann
+//
+// where $IMPL is part of an implementation name in a test (e.g. "PLDHash",
+// "MozHash") and $OBJDIR is an objdir containing a --enable-release build.
+//
+// Note that multiple processes are spawned, so `clgout` gets overwritten
+// multiple times, but the last process to write its profiling data to file is
+// the one of interest. (Alternatively, use --callgrind-out-file=clgout.%p to
+// get separate output files for each process, with a PID suffix.)
+
+#include "gtest/gtest.h"
+#include "gtest/MozGTestBench.h"  // For MOZ_GTEST_BENCH
+#include "mozilla/AllocPolicy.h"
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/HashFunctions.h"
+#include "mozilla/HashTable.h"
+#include "mozilla/StaticMutex.h"
+#include "mozilla/TimeStamp.h"
+#include "PLDHashTable.h"
+#include <unordered_set>
+
+using namespace mozilla;
+
+// This function gives a pseudo-random sequence with the following properties:
+// - Deterministic and platform-independent.
+// - No duplicates in the first VALS_LEN results, which is useful for ensuring
+//   the tables get to a particular size, and also for guaranteeing lookups
+//   that fail.
+static uintptr_t MyRand() {
+  static uintptr_t s = 0;
+  s = s * 1103515245 + 12345;
+  return s;
+}
+
+// Keep this in sync with Params in bench.rs.
+struct Params {
+  const char* mConfigName;
+  size_t mNumInserts;            // Insert this many unique keys
+  size_t mNumSuccessfulLookups;  // Does mNumInserts lookups each time
+  size_t mNumFailingLookups;     // Does mNumInserts lookups each time
+  size_t mNumIterations;         // Iterates the full table each time
+  bool mRemoveInserts;           // Remove all entries at end?
+};
+
+// We don't use std::unordered_{set,map}, but it's an interesting thing to
+// benchmark against.
+//
+// Keep this in sync with all the other Bench_*() functions.
+static void Bench_Cpp_unordered_set(const Params* aParams, void** aVals,
+                                    size_t aLen) {
+  std::unordered_set<void*> hs;
+
+  for (size_t j = 0; j < aParams->mNumInserts; j++) {
+    hs.insert(aVals[j]);
+  }
+
+  for (size_t i = 0; i < aParams->mNumSuccessfulLookups; i++) {
+    for (size_t j = 0; j < aParams->mNumInserts; j++) {
+      MOZ_RELEASE_ASSERT(hs.find(aVals[j]) != hs.end());
+    }
+  }
+
+  for (size_t i = 0; i < aParams->mNumFailingLookups; i++) {
+    for (size_t j = aParams->mNumInserts; j < aParams->mNumInserts * 2; j++) {
+      MOZ_RELEASE_ASSERT(hs.find(aVals[j]) == hs.end());
+    }
+  }
+
+  for (size_t i = 0; i < aParams->mNumIterations; i++) {
+    size_t n = 0;
+    for (const auto& elem : hs) {
+      (void)elem;
+      n++;
+    }
+    MOZ_RELEASE_ASSERT(aParams->mNumInserts == n);
+    MOZ_RELEASE_ASSERT(hs.size() == n);
+  }
+
+  if (aParams->mRemoveInserts) {
+    for (size_t j = 0; j < aParams->mNumInserts; j++) {
+      MOZ_RELEASE_ASSERT(hs.erase(aVals[j]) == 1);
+    }
+    MOZ_RELEASE_ASSERT(hs.size() == 0);
+  } else {
+    MOZ_RELEASE_ASSERT(hs.size() == aParams->mNumInserts);
+  }
+}
+
+// Keep this in sync with all the other Bench_*() functions.
+static void Bench_Cpp_PLDHashTable(const Params* aParams, void** aVals,
+                                   size_t aLen) {
+  PLDHashTable hs(PLDHashTable::StubOps(), sizeof(PLDHashEntryStub));
+
+  for (size_t j = 0; j < aParams->mNumInserts; j++) {
+    auto entry = static_cast<PLDHashEntryStub*>(hs.Add(aVals[j]));
+    MOZ_RELEASE_ASSERT(!entry->key);
+    entry->key = aVals[j];
+  }
+
+  for (size_t i = 0; i < aParams->mNumSuccessfulLookups; i++) {
+    for (size_t j = 0; j < aParams->mNumInserts; j++) {
+      MOZ_RELEASE_ASSERT(hs.Search(aVals[j]));
+    }
+  }
+
+  for (size_t i = 0; i < aParams->mNumFailingLookups; i++) {
+    for (size_t j = aParams->mNumInserts; j < aParams->mNumInserts * 2; j++) {
+      MOZ_RELEASE_ASSERT(!hs.Search(aVals[j]));
+    }
+  }
+
+  for (size_t i = 0; i < aParams->mNumIterations; i++) {
+    size_t n = 0;
+    for (auto iter = hs.Iter(); !iter.Done(); iter.Next()) {
+      n++;
+    }
+    MOZ_RELEASE_ASSERT(aParams->mNumInserts == n);
+    MOZ_RELEASE_ASSERT(hs.EntryCount() == n);
+  }
+
+  if (aParams->mRemoveInserts) {
+    for (size_t j = 0; j < aParams->mNumInserts; j++) {
+      hs.Remove(aVals[j]);
+    }
+    MOZ_RELEASE_ASSERT(hs.EntryCount() == 0);
+  } else {
+    MOZ_RELEASE_ASSERT(hs.EntryCount() == aParams->mNumInserts);
+  }
+}
+
+// Keep this in sync with all the other Bench_*() functions.
+static void Bench_Cpp_MozHashSet(const Params* aParams, void** aVals,
+                                 size_t aLen) {
+  mozilla::HashSet<void*, mozilla::DefaultHasher<void*>, MallocAllocPolicy> hs;
+
+  for (size_t j = 0; j < aParams->mNumInserts; j++) {
+    MOZ_RELEASE_ASSERT(hs.put(aVals[j]));
+  }
+
+  for (size_t i = 0; i < aParams->mNumSuccessfulLookups; i++) {
+    for (size_t j = 0; j < aParams->mNumInserts; j++) {
+      MOZ_RELEASE_ASSERT(hs.has(aVals[j]));
+    }
+  }
+
+  for (size_t i = 0; i < aParams->mNumFailingLookups; i++) {
+    for (size_t j = aParams->mNumInserts; j < aParams->mNumInserts * 2; j++) {
+      MOZ_RELEASE_ASSERT(!hs.has(aVals[j]));
+    }
+  }
+
+  for (size_t i = 0; i < aParams->mNumIterations; i++) {
+    size_t n = 0;
+    for (auto iter = hs.iter(); !iter.done(); iter.next()) {
+      n++;
+    }
+    MOZ_RELEASE_ASSERT(aParams->mNumInserts == n);
+    MOZ_RELEASE_ASSERT(hs.count() == n);
+  }
+
+  if (aParams->mRemoveInserts) {
+    for (size_t j = 0; j < aParams->mNumInserts; j++) {
+      hs.remove(aVals[j]);
+    }
+    MOZ_RELEASE_ASSERT(hs.count() == 0);
+  } else {
+    MOZ_RELEASE_ASSERT(hs.count() == aParams->mNumInserts);
+  }
+}
+
+extern "C" {
+void Bench_Rust_HashSet(const Params* params, void** aVals, size_t aLen);
+void Bench_Rust_FnvHashSet(const Params* params, void** aVals, size_t aLen);
+void Bench_Rust_FxHashSet(const Params* params, void** aVals, size_t aLen);
+}
+
+static const size_t VALS_LEN = 131072;
+
+// Each benchmark measures a different aspect of performance.
+// Note that no "Inserts" value can exceed VALS_LEN.
+// Also, if any failing lookups are done, Inserts must be <= VALS_LEN/2.
+const Params gParamsList[] = {
+    // clang-format off
+  //                           Successful Failing              Remove
+  //                 Inserts   lookups    lookups  Iterations  inserts
+  { "succ_lookups",  1024,     5000,      0,       0,          false },
+  { "fail_lookups",  1024,     0,         5000,    0,          false },
+  { "insert_remove", VALS_LEN, 0,         0,       0,          true  },
+  { "iterate",       1024,     0,         0,       5000,       false },
+    // clang-format on
+};
+
+class BenchCollections : public ::testing::Test {
+ protected:
+  void SetUp() override {
+    StaticMutexAutoLock lock(sValsMutex);
+
+    if (!sVals) {
+      sVals = (void**)malloc(VALS_LEN * sizeof(void*));
+      for (size_t i = 0; i < VALS_LEN; i++) {
+        // This leaves the high 32 bits zero on 64-bit platforms, but that
+        // should still be enough randomness to get typical behaviour.
+        sVals[i] = reinterpret_cast<void*>(uintptr_t(MyRand()));
+      }
+    }
+
+    printf("\n");
+    for (size_t i = 0; i < ArrayLength(gParamsList); i++) {
+      const Params* params = &gParamsList[i];
+      printf("%14s", params->mConfigName);
+    }
+    printf("%14s\n", "total");
+  }
+
+ public:
+  void BenchImpl(void (*aBench)(const Params*, void**, size_t)) {
+    StaticMutexAutoLock lock(sValsMutex);
+
+    double total = 0;
+    for (size_t i = 0; i < ArrayLength(gParamsList); i++) {
+      const Params* params = &gParamsList[i];
+      TimeStamp t1 = TimeStamp::Now();
+      aBench(params, sVals, VALS_LEN);
+      TimeStamp t2 = TimeStamp::Now();
+      double t = (t2 - t1).ToMilliseconds();
+      printf("%11.1f ms", t);
+      total += t;
+    }
+    printf("%11.1f ms\n", total);
+  }
+
+ private:
+  // Random values used in the benchmarks.
+  static void** sVals;
+
+  // A mutex that protects all benchmark operations, ensuring that two
+  // benchmarks never run concurrently.
+  static StaticMutex sValsMutex;
+};
+
+void** BenchCollections::sVals;
+StaticMutex BenchCollections::sValsMutex;
+
+MOZ_GTEST_BENCH_F(BenchCollections, unordered_set,
+                  [this] { BenchImpl(Bench_Cpp_unordered_set); });
+
+MOZ_GTEST_BENCH_F(BenchCollections, PLDHash,
+                  [this] { BenchImpl(Bench_Cpp_PLDHashTable); });
+
+MOZ_GTEST_BENCH_F(BenchCollections, MozHash,
+                  [this] { BenchImpl(Bench_Cpp_MozHashSet); });
+
+MOZ_GTEST_BENCH_F(BenchCollections, RustHash,
+                  [this] { BenchImpl(Bench_Rust_HashSet); });
+
+MOZ_GTEST_BENCH_F(BenchCollections, RustFnvHash,
+                  [this] { BenchImpl(Bench_Rust_FnvHashSet); });
+
+MOZ_GTEST_BENCH_F(BenchCollections, RustFxHash,
+                  [this] { BenchImpl(Bench_Rust_FxHashSet); });
diff --git a/xpcom/rust/gtest/bench-collections/Cargo.toml b/xpcom/rust/gtest/bench-collections/Cargo.toml
new file mode 100644
index 0000000000..857d87cffd
--- /dev/null
+++ b/xpcom/rust/gtest/bench-collections/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "bench-collections-gtest"
+version = "0.1.0"
+license = "MPL-2.0"
+description = "Benchmarks for various collections"
+
+[dependencies]
+fnv = "1.0"
+fxhash = "0.2.1"
+
+[lib]
+path = "bench.rs"
diff --git a/xpcom/rust/gtest/bench-collections/bench.rs b/xpcom/rust/gtest/bench-collections/bench.rs
new file mode 100644
index 0000000000..632cbd63e3
--- /dev/null
+++ b/xpcom/rust/gtest/bench-collections/bench.rs
@@ -0,0 +1,101 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#![allow(non_snake_case)]
+
+extern crate fnv;
+extern crate fxhash;
+
+use fnv::FnvHashSet;
+use fxhash::FxHashSet;
+use std::collections::HashSet;
+use std::os::raw::{c_char, c_void};
+use std::slice;
+
+/// Keep this in sync with Params in Bench.cpp.
+#[derive(Debug)]
+#[repr(C)]
+pub struct Params {
+    config_name: *const c_char,
+    num_inserts: usize,
+    num_successful_lookups: usize,
+    num_failing_lookups: usize,
+    num_iterations: usize,
+    remove_inserts: bool,
+}
+
+#[no_mangle]
+pub extern "C" fn Bench_Rust_HashSet(
+    params: *const Params,
+    vals: *const *const c_void,
+    len: usize,
+) {
+    let hs: HashSet<_> = std::collections::HashSet::default();
+    Bench_Rust(hs, params, vals, len);
+}
+
+#[no_mangle]
+pub extern "C" fn Bench_Rust_FnvHashSet(
+    params: *const Params,
+    vals: *const *const c_void,
+    len: usize,
+) {
+    let hs = FnvHashSet::default();
+    Bench_Rust(hs, params, vals, len);
+}
+
+#[no_mangle]
+pub extern "C" fn Bench_Rust_FxHashSet(
+    params: *const Params,
+    vals: *const *const c_void,
+    len: usize,
+) {
+    let hs = FxHashSet::default();
+    Bench_Rust(hs, params, vals, len);
+}
+
+// Keep this in sync with all the other Bench_*() functions.
+fn Bench_Rust<H: std::hash::BuildHasher>(
+    mut hs: HashSet<*const c_void, H>,
+    params: *const Params,
+    vals: *const *const c_void,
+    len: usize,
+) {
+    let params = unsafe { &*params };
+    let vals = unsafe { slice::from_raw_parts(vals, len) };
+
+    for j in 0..params.num_inserts {
+        hs.insert(vals[j]);
+    }
+
+    for _i in 0..params.num_successful_lookups {
+        for j in 0..params.num_inserts {
+            assert!(hs.contains(&vals[j]));
+        }
+    }
+
+    for _i in 0..params.num_failing_lookups {
+        for j in params.num_inserts..params.num_inserts * 2 {
+            assert!(!hs.contains(&vals[j]));
+        }
+    }
+
+    for _i in 0..params.num_iterations {
+        let mut n = 0;
+        for _ in hs.iter() {
+            n += 1;
+        }
+        assert!(params.num_inserts == n);
+        assert!(hs.len() == n);
+    }
+
+    if params.remove_inserts {
+        for j in 0..params.num_inserts {
+            assert!(hs.remove(&vals[j]));
+        }
+        assert!(hs.len() == 0);
+    } else {
+        assert!(hs.len() == params.num_inserts);
+    }
+}
diff --git a/xpcom/rust/gtest/moz.build b/xpcom/rust/gtest/moz.build
new file mode 100644
index 0000000000..0e64cdc9ca
--- /dev/null
+++ b/xpcom/rust/gtest/moz.build
@@ -0,0 +1,14 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+UNIFIED_SOURCES += [
+    "bench-collections/Bench.cpp",
+    "moz_task/Test.cpp",
+    "nsstring/Test.cpp",
+    "xpcom/Test.cpp",
+]
+
+FINAL_LIBRARY = "xul-gtest"
diff --git a/xpcom/rust/gtest/moz_task/Cargo.toml b/xpcom/rust/gtest/moz_task/Cargo.toml
new file mode 100644
index 0000000000..09d240c309
--- /dev/null
+++ b/xpcom/rust/gtest/moz_task/Cargo.toml
@@ -0,0 +1,13 @@
+[package]
+name = "moz_task-gtest"
+version = "0.1.0"
+authors = ["nobody@mozilla.com"]
+license = "MPL-2.0"
+description = "Tests for rust bindings to xpcom event target types"
+edition = "2018"
+
+[dependencies]
+moz_task = { path = "../../moz_task" }
+
+[lib]
+path = "test.rs"
diff --git a/xpcom/rust/gtest/moz_task/Test.cpp b/xpcom/rust/gtest/moz_task/Test.cpp
new file mode 100644
index 0000000000..6c40f1bc97
--- /dev/null
+++ b/xpcom/rust/gtest/moz_task/Test.cpp
@@ -0,0 +1,14 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "gtest/gtest.h"
+
+extern "C" void Rust_Future(bool* aItWorked);
+
+TEST(RustMozTask, Future)
+{
+  bool itWorked = false;
+  Rust_Future(&itWorked);
+  EXPECT_TRUE(itWorked);
+}
diff --git a/xpcom/rust/gtest/moz_task/test.rs b/xpcom/rust/gtest/moz_task/test.rs
new file mode 100644
index 0000000000..4261030f47
--- /dev/null
+++ b/xpcom/rust/gtest/moz_task/test.rs
@@ -0,0 +1,72 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use moz_task;
+use std::{
+    future::Future,
+    pin::Pin,
+    sync::atomic::{AtomicBool, Ordering::Relaxed},
+    sync::Arc,
+    task::{Context, Poll, Waker},
+};
+
+/// Demo `Future` to demonstrate executing futures to completion via `nsIEventTarget`.
+struct MyFuture {
+    poll_count: u32,
+    waker: Option<Waker>,
+}
+
+impl Default for MyFuture {
+    fn default() -> Self {
+        Self {
+            poll_count: 0,
+            waker: None,
+        }
+    }
+}
+
+impl Future for MyFuture {
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
+        self.poll_count += 1;
+
+        if let Some(waker) = &mut self.waker {
+            if !waker.will_wake(cx.waker()) {
+                *waker = cx.waker().clone();
+            }
+        } else {
+            let waker = cx.waker().clone();
+            self.waker = Some(waker);
+        }
+
+        println!("Poll count = {}", self.poll_count);
+        if self.poll_count > 5 {
+                Poll::Ready(())
+        } else {
+            // Just notify the task that we need to re-polled.
+            if let Some(waker) = &self.waker {
+                waker.wake_by_ref();
+            }
+            Poll::Pending
+        }
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn Rust_Future(it_worked: *mut bool) {
+    let done = Arc::new(AtomicBool::new(false));
+    let done2 = done.clone();
+
+    moz_task::spawn_current_thread(async move {
+        MyFuture::default().await;
+        done.store(true, Relaxed);
+    })
+    .unwrap();
+
+    unsafe {
+        moz_task::gtest_only::spin_event_loop_until(move || done2.load(Relaxed)).unwrap();
+        *it_worked = true;
+    };
+}
diff --git a/xpcom/rust/gtest/nsstring/Cargo.toml b/xpcom/rust/gtest/nsstring/Cargo.toml
new file mode 100644
index 0000000000..fd997926a7
--- /dev/null
+++ b/xpcom/rust/gtest/nsstring/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "nsstring-gtest"
+version = "0.1.0"
+authors = ["nobody@mozilla.com"]
+license = "MPL-2.0"
+description = "Tests for rust bindings to xpcom string types"
+
+[dependencies]
+nsstring = { path = "../../nsstring" }
+
+[lib]
+path = "test.rs"
diff --git a/xpcom/rust/gtest/nsstring/Test.cpp b/xpcom/rust/gtest/nsstring/Test.cpp
new file mode 100644
index 0000000000..4e14c7bca5
--- /dev/null
+++ b/xpcom/rust/gtest/nsstring/Test.cpp
@@ -0,0 +1,177 @@
+#include "gtest/gtest.h"
+#include <stdint.h>
+#include <utility>
+#include "nsString.h"
+
+extern "C" {
+// This function is called by the rust code in test.rs if a non-fatal test
+// failure occurs.
+void GTest_ExpectFailure(const char* aMessage) { EXPECT_STREQ(aMessage, ""); }
+}
+
+#define SIZE_ALIGN_CHECK(Clazz)                                   \
+  extern "C" void Rust_Test_ReprSizeAlign_##Clazz(size_t* size,   \
+                                                  size_t* align); \
+  TEST(RustNsString, ReprSizeAlign_##Clazz)                       \
+  {                                                               \
+    size_t size, align;                                           \
+    Rust_Test_ReprSizeAlign_##Clazz(&size, &align);               \
+    EXPECT_EQ(size, sizeof(Clazz));                               \
+    EXPECT_EQ(align, alignof(Clazz));                             \
+  }
+
+SIZE_ALIGN_CHECK(nsString)
+SIZE_ALIGN_CHECK(nsCString)
+
+#define MEMBER_CHECK(Clazz, Member)                                       \
+  extern "C" void Rust_Test_Member_##Clazz##_##Member(                    \
+      size_t* size, size_t* align, size_t* offset);                       \
+  TEST(RustNsString, ReprMember_##Clazz##_##Member)                       \
+  {                                                                       \
+    class Hack : public Clazz {                                           \
+     public:                                                              \
+      static void RunTest() {                                             \
+        size_t size, align, offset;                                       \
+        Rust_Test_Member_##Clazz##_##Member(&size, &align, &offset);      \
+        EXPECT_EQ(size, sizeof(std::declval<Hack>().Member));             \
+        EXPECT_EQ(align, alignof(decltype(std::declval<Hack>().Member))); \
+        EXPECT_EQ(offset, offsetof(Hack, Member));                        \
+      }                                                                   \
+    };                                                                    \
+    static_assert(sizeof(Clazz) == sizeof(Hack), "Hack matches class");   \
+    Hack::RunTest();                                                      \
+  }
+
+MEMBER_CHECK(nsString, mData)
+MEMBER_CHECK(nsString, mLength)
+MEMBER_CHECK(nsString, mDataFlags)
+MEMBER_CHECK(nsString, mClassFlags)
+MEMBER_CHECK(nsCString, mData)
+MEMBER_CHECK(nsCString, mLength)
+MEMBER_CHECK(nsCString, mDataFlags)
+MEMBER_CHECK(nsCString, mClassFlags)
+
+extern "C" void Rust_Test_NsStringFlags(
+    uint16_t* f_terminated, uint16_t* f_voided, uint16_t* f_refcounted,
+    uint16_t* f_owned, uint16_t* f_inline, uint16_t* f_literal,
+    uint16_t* f_class_inline, uint16_t* f_class_null_terminated);
+TEST(RustNsString, NsStringFlags)
+{
+  uint16_t f_terminated, f_voided, f_refcounted, f_owned, f_inline, f_literal,
+      f_class_inline, f_class_null_terminated;
+  Rust_Test_NsStringFlags(&f_terminated, &f_voided, &f_refcounted, &f_owned,
+                          &f_inline, &f_literal, &f_class_inline,
+                          &f_class_null_terminated);
+  EXPECT_EQ(f_terminated, uint16_t(nsAString::DataFlags::TERMINATED));
+  EXPECT_EQ(f_terminated, uint16_t(nsACString::DataFlags::TERMINATED));
+  EXPECT_EQ(f_voided, uint16_t(nsAString::DataFlags::VOIDED));
+  EXPECT_EQ(f_voided, uint16_t(nsACString::DataFlags::VOIDED));
+  EXPECT_EQ(f_refcounted, uint16_t(nsAString::DataFlags::REFCOUNTED));
+  EXPECT_EQ(f_refcounted, uint16_t(nsACString::DataFlags::REFCOUNTED));
+  EXPECT_EQ(f_owned, uint16_t(nsAString::DataFlags::OWNED));
+  EXPECT_EQ(f_owned, uint16_t(nsACString::DataFlags::OWNED));
+  EXPECT_EQ(f_inline, uint16_t(nsAString::DataFlags::INLINE));
+  EXPECT_EQ(f_inline, uint16_t(nsACString::DataFlags::INLINE));
+  EXPECT_EQ(f_literal, uint16_t(nsAString::DataFlags::LITERAL));
+  EXPECT_EQ(f_literal, uint16_t(nsACString::DataFlags::LITERAL));
+  EXPECT_EQ(f_class_inline, uint16_t(nsAString::ClassFlags::INLINE));
+  EXPECT_EQ(f_class_inline, uint16_t(nsACString::ClassFlags::INLINE));
+  EXPECT_EQ(f_class_null_terminated,
+            uint16_t(nsAString::ClassFlags::NULL_TERMINATED));
+  EXPECT_EQ(f_class_null_terminated,
+            uint16_t(nsACString::ClassFlags::NULL_TERMINATED));
+}
+
+extern "C" void Rust_StringFromCpp(const nsACString* aCStr,
+                                   const nsAString* aStr);
+TEST(RustNsString, StringFromCpp)
+{
+  nsAutoCString foo;
+  foo.AssignASCII("Hello, World!");
+
+  nsAutoString bar;
+  bar.AssignASCII("Hello, World!");
+
+  Rust_StringFromCpp(&foo, &bar);
+}
+
+extern "C" void Rust_AssignFromRust(nsACString* aCStr, nsAString* aStr);
+TEST(RustNsString, AssignFromRust)
+{
+  nsAutoCString cs;
+  nsAutoString s;
+  Rust_AssignFromRust(&cs, &s);
+  EXPECT_TRUE(cs.EqualsASCII("Hello, World!"));
+  EXPECT_TRUE(s.EqualsASCII("Hello, World!"));
+}
+
+extern "C" {
+void Cpp_AssignFromCpp(nsACString* aCStr, nsAString* aStr) {
+  aCStr->AssignASCII("Hello, World!");
+  aStr->AssignASCII("Hello, World!");
+}
+}
+extern "C" void Rust_AssignFromCpp();
+TEST(RustNsString, AssignFromCpp)
+{ Rust_AssignFromCpp(); }
+
+extern "C" void Rust_StringWrite();
+TEST(RustNsString, StringWrite)
+{ Rust_StringWrite(); }
+
+extern "C" void Rust_FromEmptyRustString();
+TEST(RustNsString, FromEmptyRustString)
+{ Rust_FromEmptyRustString(); }
+
+extern "C" void Rust_WriteToBufferFromRust(nsACString* aCStr, nsAString* aStr,
+                                           nsACString* aFallibleCStr,
+                                           nsAString* aFallibleStr);
+TEST(RustNsString, WriteToBufferFromRust)
+{
+  nsAutoCString cStr;
+  nsAutoString str;
+  nsAutoCString fallibleCStr;
+  nsAutoString fallibleStr;
+
+  cStr.AssignLiteral("abc");
+  str.AssignLiteral("abc");
+  fallibleCStr.AssignLiteral("abc");
+  fallibleStr.AssignLiteral("abc");
+
+  Rust_WriteToBufferFromRust(&cStr, &str, &fallibleCStr, &fallibleStr);
+
+  EXPECT_TRUE(cStr.EqualsASCII("ABC"));
+  EXPECT_TRUE(str.EqualsASCII("ABC"));
+  EXPECT_TRUE(fallibleCStr.EqualsASCII("ABC"));
+  EXPECT_TRUE(fallibleStr.EqualsASCII("ABC"));
+}
+
+extern "C" void Rust_InlineCapacityFromRust(const nsACString* aCStr,
+                                            const nsAString* aStr,
+                                            size_t* aCStrCapacity,
+                                            size_t* aStrCapacity);
+TEST(RustNsString, InlineCapacityFromRust)
+{
+  size_t cStrCapacity;
+  size_t strCapacity;
+  nsAutoCStringN<93> cs;
+  nsAutoStringN<93> s;
+  Rust_InlineCapacityFromRust(&cs, &s, &cStrCapacity, &strCapacity);
+  EXPECT_EQ(cStrCapacity, 92U);
+  EXPECT_EQ(strCapacity, 92U);
+}
+
+extern "C" void Rust_VoidStringFromRust(nsACString* aCStr, nsAString* aStr);
+TEST(RustNsString, VoidStringFromRust)
+{
+  nsAutoCString cs;
+  nsAutoString s;
+
+  EXPECT_FALSE(cs.IsVoid());
+  EXPECT_FALSE(s.IsVoid());
+
+  Rust_VoidStringFromRust(&cs, &s);
+
+  EXPECT_TRUE(cs.IsVoid());
+  EXPECT_TRUE(s.IsVoid());
+}
diff --git a/xpcom/rust/gtest/nsstring/test.rs b/xpcom/rust/gtest/nsstring/test.rs
new file mode 100644
index 0000000000..a5d142f2b2
--- /dev/null
+++ b/xpcom/rust/gtest/nsstring/test.rs
@@ -0,0 +1,131 @@
+#![allow(non_snake_case)]
+
+extern crate nsstring;
+
+use nsstring::*;
+use std::ffi::CString;
+use std::fmt::Write;
+use std::os::raw::c_char;
+
+fn nonfatal_fail(msg: String) {
+    extern "C" {
+        fn GTest_ExpectFailure(message: *const c_char);
+    }
+    unsafe {
+        let msg = CString::new(msg).unwrap();
+        GTest_ExpectFailure(msg.as_ptr());
+    }
+}
+
+/// This macro checks if the two arguments are equal, and causes a non-fatal
+/// GTest test failure if they are not.
+macro_rules! expect_eq {
+    ($x:expr, $y:expr) => {
+        match (&$x, &$y) {
+            (x, y) => {
+                if *x != *y {
+                    nonfatal_fail(format!(
+                        "check failed: (`{:?}` == `{:?}`) at {}:{}",
+                        x,
+                        y,
+                        file!(),
+                        line!()
+                    ))
+                }
+            }
+        }
+    };
+}
+
+#[no_mangle]
+pub extern "C" fn Rust_StringFromCpp(cs: *const nsACString, s: *const nsAString) {
+    unsafe {
+        expect_eq!(&*cs, "Hello, World!");
+        expect_eq!(&*s, "Hello, World!");
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn Rust_AssignFromRust(cs: *mut nsACString, s: *mut nsAString) {
+    unsafe {
+        (*cs).assign(&nsCString::from("Hello, World!"));
+        expect_eq!(&*cs, "Hello, World!");
+        (*s).assign(&nsString::from("Hello, World!"));
+        expect_eq!(&*s, "Hello, World!");
+    }
+}
+
+extern "C" {
+    fn Cpp_AssignFromCpp(cs: *mut nsACString, s: *mut nsAString);
+}
+
+#[no_mangle]
+pub extern "C" fn Rust_AssignFromCpp() {
+    let mut cs = nsCString::new();
+    let mut s = nsString::new();
+    unsafe {
+        Cpp_AssignFromCpp(&mut *cs, &mut *s);
+    }
+    expect_eq!(cs, "Hello, World!");
+    expect_eq!(s, "Hello, World!");
+}
+
+#[no_mangle]
+pub extern "C" fn Rust_StringWrite() {
+    let mut cs = nsCString::new();
+    let mut s = nsString::new();
+
+    write!(s, "a").unwrap();
+    write!(cs, "a").unwrap();
+    expect_eq!(s, "a");
+    expect_eq!(cs, "a");
+    write!(s, "bc").unwrap();
+    write!(cs, "bc").unwrap();
+    expect_eq!(s, "abc");
+    expect_eq!(cs, "abc");
+    write!(s, "{}", 123).unwrap();
+    write!(cs, "{}", 123).unwrap();
+    expect_eq!(s, "abc123");
+    expect_eq!(cs, "abc123");
+}
+
+#[no_mangle]
+pub extern "C" fn Rust_FromEmptyRustString() {
+    let mut test = nsString::from("Blah");
+    test.assign_utf8(&nsCString::from(String::new()));
+    assert!(test.is_empty());
+}
+
+#[no_mangle]
+pub extern "C" fn Rust_WriteToBufferFromRust(
+    cs: *mut nsACString,
+    s: *mut nsAString,
+    fallible_cs: *mut nsACString,
+    fallible_s: *mut nsAString,
+) {
+    unsafe {
+        let cs_buf = (*cs).to_mut();
+        let s_buf = (*s).to_mut();
+        let fallible_cs_buf = (*fallible_cs).fallible_to_mut().unwrap();
+        let fallible_s_buf = (*fallible_s).fallible_to_mut().unwrap();
+
+        cs_buf[0] = b'A';
+        cs_buf[1] = b'B';
+        cs_buf[2] = b'C';
+        s_buf[0] = b'A' as u16;
+        s_buf[1] = b'B' as u16;
+        s_buf[2] = b'C' as u16;
+        fallible_cs_buf[0] = b'A';
+        fallible_cs_buf[1] = b'B';
+        fallible_cs_buf[2] = b'C';
+        fallible_s_buf[0] = b'A' as u16;
+        fallible_s_buf[1] = b'B' as u16;
+        fallible_s_buf[2] = b'C' as u16;
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn Rust_VoidStringFromRust(cs: &mut nsACString, s: &mut nsAString) {
+    cs.set_is_void(true);
+    s.set_is_void(true);
+}
diff --git a/xpcom/rust/gtest/xpcom/Cargo.toml b/xpcom/rust/gtest/xpcom/Cargo.toml
new file mode 100644
index 0000000000..777080b33b
--- /dev/null
+++ b/xpcom/rust/gtest/xpcom/Cargo.toml
@@ -0,0 +1,14 @@
+[package]
+name = "xpcom-gtest"
+version = "0.1.0"
+authors = ["michael@thelayzells.com"]
+license = "MPL-2.0"
+description = "Tests for rust bindings to xpcom interfaces"
+
+[dependencies]
+xpcom = { path = "../../xpcom" }
+nserror = { path = "../../nserror" }
+nsstring = { path = "../../nsstring" }
+
+[lib]
+path = "test.rs"
diff --git a/xpcom/rust/gtest/xpcom/Test.cpp b/xpcom/rust/gtest/xpcom/Test.cpp
new file mode 100644
index 0000000000..4c2a6fb117
--- /dev/null
+++ b/xpcom/rust/gtest/xpcom/Test.cpp
@@ -0,0 +1,33 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "gtest/gtest.h"
+#include "nsCOMPtr.h"
+#include "nsIRunnable.h"
+#include "mozilla/Services.h"
+#include "nsIObserverService.h"
+
+extern "C" nsIObserverService* Rust_ObserveFromRust();
+
+TEST(RustXpcom, ObserverFromRust)
+{
+  nsCOMPtr<nsIObserverService> rust = Rust_ObserveFromRust();
+  nsCOMPtr<nsIObserverService> cpp = mozilla::services::GetObserverService();
+  EXPECT_EQ(rust, cpp);
+}
+
+extern "C" void Rust_ImplementRunnableInRust(bool* aItWorked,
+                                             nsIRunnable** aRunnable);
+
+TEST(RustXpcom, ImplementRunnableInRust)
+{
+  bool itWorked = false;
+  nsCOMPtr<nsIRunnable> runnable;
+  Rust_ImplementRunnableInRust(&itWorked, getter_AddRefs(runnable));
+
+  EXPECT_TRUE(runnable);
+  EXPECT_FALSE(itWorked);
+  runnable->Run();
+  EXPECT_TRUE(itWorked);
+}
diff --git a/xpcom/rust/gtest/xpcom/test.rs b/xpcom/rust/gtest/xpcom/test.rs
new file mode 100644
index 0000000000..8e3e08701c
--- /dev/null
+++ b/xpcom/rust/gtest/xpcom/test.rs
@@ -0,0 +1,99 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#![allow(non_snake_case)]
+
+#[macro_use]
+extern crate xpcom;
+
+extern crate nserror;
+
+use nserror::{nsresult, NS_OK};
+use std::ffi::{CStr, CString};
+use std::os::raw::c_char;
+use std::ptr;
+use xpcom::interfaces;
+
+#[no_mangle]
+pub unsafe extern "C" fn Rust_ObserveFromRust() -> *const interfaces::nsIObserverService {
+    let obssvc = xpcom::services::get_ObserverService().unwrap();
+
+    // Define an observer
+    #[derive(xpcom)]
+    #[xpimplements(nsIObserver)]
+    #[refcnt = "nonatomic"]
+    struct InitObserver {
+        run: *mut bool,
+    }
+    impl Observer {
+        unsafe fn Observe(
+            &self,
+            _subject: *const interfaces::nsISupports,
+            topic: *const c_char,
+            _data: *const i16,
+        ) -> nsresult {
+            *self.run = true;
+            assert!(CStr::from_ptr(topic).to_str() == Ok("test-rust-observe"));
+            NS_OK
+        }
+    }
+
+    let topic = CString::new("test-rust-observe").unwrap();
+
+    let mut run = false;
+    let observer = Observer::allocate(InitObserver { run: &mut run });
+    let rv = obssvc.AddObserver(
+        observer.coerce::<interfaces::nsIObserver>(),
+        topic.as_ptr(),
+        false,
+    );
+    assert!(rv.succeeded());
+
+    let rv = obssvc.NotifyObservers(ptr::null(), topic.as_ptr(), ptr::null());
+    assert!(rv.succeeded());
+    assert!(run, "The observer should have been run!");
+
+    let rv = obssvc.RemoveObserver(observer.coerce::<interfaces::nsIObserver>(), topic.as_ptr());
+    assert!(rv.succeeded());
+
+    assert!(
+        observer.coerce::<interfaces::nsISupports>() as *const _
+            == &*observer
+                .query_interface::<interfaces::nsISupports>()
+                .unwrap() as *const _
+    );
+
+    &*obssvc
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn Rust_ImplementRunnableInRust(
+    it_worked: *mut bool,
+    runnable: *mut *const interfaces::nsIRunnable,
+) {
+    // Define a type which implements nsIRunnable in rust.
+    #[derive(xpcom)]
+    #[xpimplements(nsIRunnable)]
+    #[refcnt = "atomic"]
+    struct InitRunnableFn<F: Fn() + 'static> {
+        run: F,
+    }
+
+    impl<F: Fn() + 'static> RunnableFn<F> {
+        unsafe fn Run(&self) -> nsresult {
+            (self.run)();
+            NS_OK
+        }
+    }
+
+    let my_runnable = RunnableFn::allocate(InitRunnableFn {
+        run: move || {
+            *it_worked = true;
+        },
+    });
+    my_runnable
+        .query_interface::<interfaces::nsIRunnable>()
+        .unwrap()
+        .forget(&mut *runnable);
+}