Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

42 files changed, 6714 insertions, 0 deletions

diff --git a/xpcom/rust/gecko_logger/Cargo.toml b/xpcom/rust/gecko_logger/Cargo.toml
new file mode 100644
index 0000000000..2f8ca84a97
--- /dev/null
+++ b/xpcom/rust/gecko_logger/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "gecko_logger"
+version = "0.1.0"
+authors = ["nobody@mozilla.com"]
+edition = "2018"
+license = "MPL-2.0"
+
+[dependencies]
+lazy_static = "1"
+log = {version = "0.4", features = ["release_max_level_info"]}
+env_logger = {version = "0.10", default-features = false, features = ["color"]} # disable `regex` to reduce code size
+app_services_logger = { path = "../../../services/common/app_services_logger" }
diff --git a/xpcom/rust/gecko_logger/src/lib.rs b/xpcom/rust/gecko_logger/src/lib.rs
new file mode 100644
index 0000000000..5da85b9d89
--- /dev/null
+++ b/xpcom/rust/gecko_logger/src/lib.rs
@@ -0,0 +1,256 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+//! This provides a way to direct rust logging into the gecko logger.
+
+#[macro_use]
+extern crate lazy_static;
+
+use app_services_logger::{AppServicesLogger, LOGGERS_BY_TARGET};
+use log::Log;
+use log::{Level, LevelFilter};
+use std::boxed::Box;
+use std::collections::HashMap;
+use std::ffi::{CStr, CString};
+use std::os::raw::c_char;
+use std::os::raw::c_int;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::RwLock;
+use std::{cmp, env};
+
+extern "C" {
+    fn ExternMozLog(tag: *const c_char, prio: c_int, text: *const c_char);
+    fn gfx_critical_note(msg: *const c_char);
+    #[cfg(target_os = "android")]
+    fn __android_log_write(prio: c_int, tag: *const c_char, text: *const c_char) -> c_int;
+}
+
+lazy_static! {
+    // This could be a proper static once [1] is fixed or parking_lot's const fn
+    // support is not nightly-only.
+    //
+    // [1]: https://github.com/rust-lang/rust/issues/73714
+    static ref LOG_MODULE_MAP: RwLock<HashMap<String, (LevelFilter, bool)>> = RwLock::new(HashMap::new());
+}
+
+/// This tells us whether LOG_MODULE_MAP is possibly non-empty.
+static LOGGING_ACTIVE: AtomicBool = AtomicBool::new(false);
+
+/// This function searches for the module's name in the hashmap. If that is not
+/// found, it proceeds to search for the parent modules.
+/// It returns a tuple containing the matched string, if the matched module
+/// was a pattern match, and the level we found in the hashmap.
+/// If none is found, it will return the module's name and LevelFilter::Off
+fn get_level_for_module<'a>(
+    map: &HashMap<String, (LevelFilter, bool)>,
+    key: &'a str,
+) -> (&'a str, bool, LevelFilter) {
+    if let Some((level, is_pattern_match)) = map.get(key) {
+        return (key, *is_pattern_match, level.clone());
+    }
+
+    let mut mod_name = &key[..];
+    while let Some(pos) = mod_name.rfind("::") {
+        mod_name = &mod_name[..pos];
+        if let Some((level, is_pattern_match)) = map.get(mod_name) {
+            return (mod_name, *is_pattern_match, level.clone());
+        }
+    }
+
+    return (key, false, LevelFilter::Off);
+}
+
+/// This function takes a record to maybe log to Gecko.
+/// It returns true if the record was handled by Gecko's logging, and false
+/// otherwise.
+pub fn log_to_gecko(record: &log::Record) -> bool {
+    if !LOGGING_ACTIVE.load(Ordering::Relaxed) {
+        return false;
+    }
+
+    let key = match record.module_path() {
+        Some(key) => key,
+        None => return false,
+    };
+
+    let (mod_name, is_pattern_match, level) = {
+        let map = LOG_MODULE_MAP.read().unwrap();
+        get_level_for_module(&map, &key)
+    };
+
+    if level == LevelFilter::Off {
+        return false;
+    }
+
+    if level < record.metadata().level() {
+        return false;
+    }
+
+    // Map the log::Level to mozilla::LogLevel.
+    let moz_log_level = match record.metadata().level() {
+        Level::Error => 1, // Error
+        Level::Warn => 2,  // Warning
+        Level::Info => 3,  // Info
+        Level::Debug => 4, // Debug
+        Level::Trace => 5, // Verbose
+    };
+
+    // If it was a pattern match, we need to append ::* to the matched string.
+    let (tag, msg) = if is_pattern_match {
+        (
+            CString::new(format!("{}::*", mod_name)).unwrap(),
+            CString::new(format!("[{}] {}", key, record.args())).unwrap(),
+        )
+    } else {
+        (
+            CString::new(key).unwrap(),
+            CString::new(format!("{}", record.args())).unwrap(),
+        )
+    };
+
+    unsafe {
+        ExternMozLog(tag.as_ptr(), moz_log_level, msg.as_ptr());
+    }
+
+    return true;
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn set_rust_log_level(module: *const c_char, level: u8) {
+    // Convert the Gecko level to a rust LevelFilter.
+    let rust_level = match level {
+        1 => LevelFilter::Error,
+        2 => LevelFilter::Warn,
+        3 => LevelFilter::Info,
+        4 => LevelFilter::Debug,
+        5 => LevelFilter::Trace,
+        _ => LevelFilter::Off,
+    };
+
+    // This is the name of the rust module that we're trying to log in Gecko.
+    let mut mod_name = CStr::from_ptr(module).to_string_lossy().into_owned();
+
+    let is_pattern_match = mod_name.ends_with("::*");
+
+    // If this is a pattern, remove the last "::*" from it so we can search it
+    // in the map.
+    if is_pattern_match {
+        let len = mod_name.len() - 3;
+        mod_name.truncate(len);
+    }
+
+    LOGGING_ACTIVE.store(true, Ordering::Relaxed);
+    let mut map = LOG_MODULE_MAP.write().unwrap();
+    map.insert(mod_name, (rust_level, is_pattern_match));
+
+    // Figure out the max level of all the modules.
+    let max = map
+        .values()
+        .map(|(lvl, _)| lvl)
+        .max()
+        .unwrap_or(&LevelFilter::Off);
+    log::set_max_level(*max);
+}
+
+pub struct GeckoLogger {
+    logger: env_logger::Logger,
+}
+
+impl GeckoLogger {
+    pub fn new() -> GeckoLogger {
+        let mut builder = env_logger::Builder::new();
+        let default_level = if cfg!(debug_assertions) {
+            "warn"
+        } else {
+            "error"
+        };
+        let logger = match env::var("RUST_LOG") {
+            Ok(v) => builder.parse_filters(&v).build(),
+            _ => builder.parse_filters(default_level).build(),
+        };
+
+        GeckoLogger { logger }
+    }
+
+    pub fn init() -> Result<(), log::SetLoggerError> {
+        let gecko_logger = Self::new();
+
+        // The max level may have already been set by gecko_logger. Don't
+        // set it to a lower level.
+        let level = cmp::max(log::max_level(), gecko_logger.logger.filter());
+        log::set_max_level(level);
+        log::set_boxed_logger(Box::new(gecko_logger))
+    }
+
+    fn should_log_to_app_services(target: &str) -> bool {
+        return AppServicesLogger::is_app_services_logger_registered(target.into());
+    }
+
+    fn maybe_log_to_app_services(&self, record: &log::Record) {
+        if Self::should_log_to_app_services(record.target()) {
+            if let Some(l) = LOGGERS_BY_TARGET.read().unwrap().get(record.target()) {
+                l.log(record);
+            }
+        }
+    }
+
+    fn should_log_to_gfx_critical_note(record: &log::Record) -> bool {
+        record.level() == log::Level::Error && record.target().contains("webrender")
+    }
+
+    fn maybe_log_to_gfx_critical_note(&self, record: &log::Record) {
+        if Self::should_log_to_gfx_critical_note(record) {
+            let msg = CString::new(format!("{}", record.args())).unwrap();
+            unsafe {
+                gfx_critical_note(msg.as_ptr());
+            }
+        }
+    }
+
+    #[cfg(not(target_os = "android"))]
+    fn log_out(&self, record: &log::Record) {
+        // If the log wasn't handled by the gecko platform logger, just pass it
+        // to the env_logger.
+        if !log_to_gecko(record) {
+            self.logger.log(record);
+        }
+    }
+
+    #[cfg(target_os = "android")]
+    fn log_out(&self, record: &log::Record) {
+        if !self.logger.matches(record) {
+            return;
+        }
+
+        let msg = CString::new(format!("{}", record.args())).unwrap();
+        let tag = CString::new(record.module_path().unwrap()).unwrap();
+        let prio = match record.metadata().level() {
+            Level::Error => 6, /* ERROR */
+            Level::Warn => 5,  /* WARN */
+            Level::Info => 4,  /* INFO */
+            Level::Debug => 3, /* DEBUG */
+            Level::Trace => 2, /* VERBOSE */
+        };
+        // Output log directly to android log, since env_logger can output log
+        // only to stderr or stdout.
+        unsafe {
+            __android_log_write(prio, tag.as_ptr(), msg.as_ptr());
+        }
+    }
+}
+
+impl log::Log for GeckoLogger {
+    fn enabled(&self, metadata: &log::Metadata) -> bool {
+        self.logger.enabled(metadata) || GeckoLogger::should_log_to_app_services(metadata.target())
+    }
+
+    fn log(&self, record: &log::Record) {
+        // Forward log to gfxCriticalNote, if the log should be in gfx crash log.
+        self.maybe_log_to_gfx_critical_note(record);
+        self.maybe_log_to_app_services(record);
+        self.log_out(record);
+    }
+
+    fn flush(&self) {}
+}
diff --git a/xpcom/rust/gkrust_utils/Cargo.toml b/xpcom/rust/gkrust_utils/Cargo.toml
new file mode 100644
index 0000000000..e398725260
--- /dev/null
+++ b/xpcom/rust/gkrust_utils/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "gkrust_utils"
+version = "0.1.0"
+authors = ["Jonathan Kingston <jkt@mozilla.com>"]
+license = "MPL-2.0"
+
+[dependencies]
+semver = "1.0"
+nsstring = { path = "../nsstring" }
diff --git a/xpcom/rust/gkrust_utils/cbindgen.toml b/xpcom/rust/gkrust_utils/cbindgen.toml
new file mode 100644
index 0000000000..967fbefd47
--- /dev/null
+++ b/xpcom/rust/gkrust_utils/cbindgen.toml
@@ -0,0 +1,31 @@
+header = """/* 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/. */"""
+autogen_warning = """/* DO NOT MODIFY THIS MANUALLY! This file was generated using cbindgen.
+ * To generate this file:
+ *   1. Get the latest cbindgen using `cargo install --force cbindgen`
+ *      a. Alternatively, you can clone `https://github.com/eqrion/cbindgen` and use a tagged release
+ *   2. Run `rustup run nightly cbindgen xpcom/rust/gkrust_utils --lockfile Cargo.lock --crate gkrust_utils -o xpcom/base/gk_rust_utils_ffi_generated.h`
+ */
+#include "nsError.h"
+#include "nsString.h"
+"""
+include_version = true
+braces = "SameLine"
+line_length = 100
+tab_width = 2
+language = "C++"
+namespaces = ["mozilla"]
+
+[export]
+# Skip constants because we don't have any
+item_types = ["globals", "enums", "structs", "unions", "typedefs", "opaque", "functions"]
+
+[enum]
+add_sentinel = true
+derive_helper_methods = true
+
+[defines]
+"target_os = windows" = "XP_WIN"
+"target_os = macos" = "XP_MACOSX"
+"target_os = android" = "ANDROID"
diff --git a/xpcom/rust/gkrust_utils/src/lib.rs b/xpcom/rust/gkrust_utils/src/lib.rs
new file mode 100644
index 0000000000..c519d7e002
--- /dev/null
+++ b/xpcom/rust/gkrust_utils/src/lib.rs
@@ -0,0 +1,24 @@
+/* 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/. */
+
+extern crate nsstring;
+extern crate semver;
+use nsstring::nsACString;
+
+#[no_mangle]
+pub unsafe extern "C" fn GkRustUtils_ParseSemVer(
+    ver: &nsACString,
+    out_major: *mut u64,
+    out_minor: *mut u64,
+    out_patch: *mut u64,
+) -> bool {
+    let version = match semver::Version::parse(&ver.to_utf8()) {
+        Ok(ver) => ver,
+        Err(_) => return false,
+    };
+    *out_major = version.major;
+    *out_minor = version.minor;
+    *out_patch = version.patch;
+    true
+}
diff --git a/xpcom/rust/gtest/bench-collections/Bench.cpp b/xpcom/rust/gtest/bench-collections/Bench.cpp
new file mode 100644
index 0000000000..41152470c7
--- /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 MOZ_UNANNOTATED;
+};
+
+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..1aba69f01f
--- /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.is_empty());
+    } 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..d3457930c8
--- /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/TestMozTask.cpp",
+    "nsstring/TestnsString.cpp",
+    "xpcom/TestXpcom.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/TestMozTask.cpp b/xpcom/rust/gtest/moz_task/TestMozTask.cpp
new file mode 100644
index 0000000000..6c40f1bc97
--- /dev/null
+++ b/xpcom/rust/gtest/moz_task/TestMozTask.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..fa4c0af852
--- /dev/null
+++ b/xpcom/rust/gtest/moz_task/test.rs
@@ -0,0 +1,78 @@
+/* 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,
+    task::{Context, Poll, Waker},
+};
+
+/// Demo `Future` to demonstrate executing futures to completion via `nsIEventTarget`.
+struct MyFuture {
+    poll_count: u32,
+    waker: Option<Waker>,
+    expect_main_thread: bool,
+}
+
+impl MyFuture {
+    fn new(expect_main_thread: bool) -> Self {
+        Self {
+            poll_count: 0,
+            waker: None,
+            expect_main_thread,
+        }
+    }
+}
+
+impl Future for MyFuture {
+    type Output = u32;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<u32> {
+        assert_eq!(moz_task::is_main_thread(), self.expect_main_thread);
+
+        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(self.poll_count)
+        } 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 future = async move {
+        assert_eq!(MyFuture::new(true).await, 6);
+        assert_eq!(
+            moz_task::spawn_local("Rust_Future inner spawn_local", MyFuture::new(true)).await,
+            6
+        );
+        assert_eq!(
+            moz_task::spawn("Rust_Future inner spawn", MyFuture::new(false)).await,
+            6
+        );
+        unsafe {
+            *it_worked = true;
+        }
+    };
+    unsafe {
+        moz_task::gtest_only::spin_event_loop_until("Rust_Future", future).unwrap();
+    };
+}
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/TestnsString.cpp b/xpcom/rust/gtest/nsstring/TestnsString.cpp
new file mode 100644
index 0000000000..4e14c7bca5
--- /dev/null
+++ b/xpcom/rust/gtest/nsstring/TestnsString.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/TestXpcom.cpp b/xpcom/rust/gtest/xpcom/TestXpcom.cpp
new file mode 100644
index 0000000000..69425274f6
--- /dev/null
+++ b/xpcom/rust/gtest/xpcom/TestXpcom.cpp
@@ -0,0 +1,66 @@
+/* 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 "nsIObserver.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);
+}
+
+extern "C" void Rust_GetMultipleInterfaces(nsIRunnable** aRunnable,
+                                           nsIObserver** aObserver);
+
+TEST(RustXpcom, DynamicCastVoid)
+{
+  nsCOMPtr<nsIRunnable> runnable;
+  nsCOMPtr<nsIObserver> observer;
+  Rust_GetMultipleInterfaces(getter_AddRefs(runnable),
+                             getter_AddRefs(observer));
+
+  // They should have different addresses when `static_cast` to void*
+  EXPECT_NE(static_cast<void*>(runnable.get()),
+            static_cast<void*>(observer.get()));
+
+  // These should be the same object
+  nsCOMPtr<nsISupports> runnableSupports = do_QueryInterface(runnable);
+  nsCOMPtr<nsISupports> observerSupports = do_QueryInterface(observer);
+  EXPECT_EQ(runnableSupports.get(), observerSupports.get());
+
+#ifndef XP_WIN
+  // They should have the same address when dynamic_cast to void*
+  // dynamic_cast<void*> is not supported without rtti on windows.
+  EXPECT_EQ(dynamic_cast<void*>(runnable.get()),
+            dynamic_cast<void*>(observer.get()));
+
+  // The nsISupports pointer from `do_QueryInterface` should match
+  // `dynamic_cast<void*>`
+  EXPECT_EQ(dynamic_cast<void*>(observer.get()),
+            static_cast<void*>(observerSupports.get()));
+#endif
+}
diff --git a/xpcom/rust/gtest/xpcom/test.rs b/xpcom/rust/gtest/xpcom/test.rs
new file mode 100644
index 0000000000..f26a0140f3
--- /dev/null
+++ b/xpcom/rust/gtest/xpcom/test.rs
@@ -0,0 +1,131 @@
+/* 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, RefPtr};
+
+#[no_mangle]
+pub unsafe extern "C" fn Rust_ObserveFromRust() -> *const interfaces::nsIObserverService {
+    let obssvc: RefPtr<interfaces::nsIObserverService> =
+        xpcom::components::Observer::service().unwrap();
+
+    // Define an observer
+    #[xpcom(implement(nsIObserver), nonatomic)]
+    struct Observer {
+        run: *mut bool,
+    }
+    impl Observer {
+        unsafe fn Observe(
+            &self,
+            _subject: *const interfaces::nsISupports,
+            topic: *const c_char,
+            _data: *const u16,
+        ) -> 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.
+    #[xpcom(implement(nsIRunnable), atomic)]
+    struct RunnableFn<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);
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn Rust_GetMultipleInterfaces(
+    runnable: *mut *const interfaces::nsIRunnable,
+    observer: *mut *const interfaces::nsIObserver,
+) {
+    // Define a type which implements nsIRunnable and nsIObserver in rust, and
+    // hand both references back to c++
+    #[xpcom(implement(nsIRunnable, nsIObserver), atomic)]
+    struct MultipleInterfaces {}
+
+    impl MultipleInterfaces {
+        unsafe fn Run(&self) -> nsresult {
+            NS_OK
+        }
+        unsafe fn Observe(
+            &self,
+            _subject: *const interfaces::nsISupports,
+            _topic: *const c_char,
+            _data: *const u16,
+        ) -> nsresult {
+            NS_OK
+        }
+    }
+
+    let instance = MultipleInterfaces::allocate(InitMultipleInterfaces {});
+    instance
+        .query_interface::<interfaces::nsIRunnable>()
+        .unwrap()
+        .forget(&mut *runnable);
+    instance
+        .query_interface::<interfaces::nsIObserver>()
+        .unwrap()
+        .forget(&mut *observer);
+}
diff --git a/xpcom/rust/moz_task/Cargo.toml b/xpcom/rust/moz_task/Cargo.toml
new file mode 100644
index 0000000000..2e3e43ef7a
--- /dev/null
+++ b/xpcom/rust/moz_task/Cargo.toml
@@ -0,0 +1,16 @@
+[package]
+name = "moz_task"
+version = "0.1.0"
+authors = ["Myk Melez <myk@mykzilla.org>"]
+license = "MPL-2.0"
+description = "Rust wrappers around XPCOM threading functions"
+edition = "2018"
+
+[dependencies]
+log = "0.4"
+cstr = "0.2"
+libc = "0.2"
+async-task = { version = "4.3" }
+nserror = { path = "../nserror" }
+nsstring = { path = "../nsstring" }
+xpcom = { path = "../xpcom" }
diff --git a/xpcom/rust/moz_task/src/dispatcher.rs b/xpcom/rust/moz_task/src/dispatcher.rs
new file mode 100644
index 0000000000..17ad9ceb81
--- /dev/null
+++ b/xpcom/rust/moz_task/src/dispatcher.rs
@@ -0,0 +1,153 @@
+/* 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 crate::{
+    dispatch_background_task_runnable, dispatch_runnable, get_current_thread, DispatchOptions,
+};
+use nserror::{nsresult, NS_OK};
+use nsstring::nsACString;
+use std::sync::Mutex;
+use xpcom::interfaces::{nsIEventTarget, nsIRunnablePriority};
+use xpcom::xpcom;
+
+/// Basic wrapper to convert a FnOnce callback into a `nsIRunnable` to be
+/// dispatched using XPCOM.
+#[xpcom(implement(nsIRunnable, nsINamed, nsIRunnablePriority), atomic)]
+struct RunnableFunction<F: FnOnce() + 'static> {
+    name: &'static str,
+    priority: u32,
+    function: Mutex<Option<F>>,
+}
+
+impl<F: FnOnce() + 'static> RunnableFunction<F> {
+    #[allow(non_snake_case)]
+    fn Run(&self) -> nsresult {
+        let function = self.function.lock().unwrap().take();
+        debug_assert!(function.is_some(), "runnable invoked twice?");
+        if let Some(function) = function {
+            function();
+        }
+        NS_OK
+    }
+
+    #[allow(non_snake_case)]
+    unsafe fn GetName(&self, result: *mut nsACString) -> nsresult {
+        (*result).assign(self.name);
+        NS_OK
+    }
+
+    #[allow(non_snake_case)]
+    unsafe fn GetPriority(&self, result: *mut u32) -> nsresult {
+        *result = self.priority;
+        NS_OK
+    }
+}
+
+pub struct RunnableBuilder<F> {
+    name: &'static str,
+    function: F,
+    priority: u32,
+    options: DispatchOptions,
+}
+
+impl<F> RunnableBuilder<F> {
+    pub fn new(name: &'static str, function: F) -> Self {
+        RunnableBuilder {
+            name,
+            function,
+            priority: nsIRunnablePriority::PRIORITY_NORMAL,
+            options: DispatchOptions::default(),
+        }
+    }
+
+    pub fn priority(mut self, priority: u32) -> Self {
+        self.priority = priority;
+        self
+    }
+
+    pub fn options(mut self, options: DispatchOptions) -> Self {
+        self.options = options;
+        self
+    }
+
+    pub fn may_block(mut self, may_block: bool) -> Self {
+        self.options = self.options.may_block(may_block);
+        self
+    }
+
+    pub unsafe fn at_end(mut self, at_end: bool) -> Self {
+        self.options = self.options.at_end(at_end);
+        self
+    }
+}
+
+impl<F> RunnableBuilder<F>
+where
+    F: FnOnce() + Send + 'static,
+{
+    /// Dispatch this Runnable to the specified EventTarget. The runnable function must be `Send`.
+    pub fn dispatch(self, target: &nsIEventTarget) -> Result<(), nsresult> {
+        let runnable = RunnableFunction::allocate(InitRunnableFunction {
+            name: self.name,
+            priority: self.priority,
+            function: Mutex::new(Some(self.function)),
+        });
+        unsafe { dispatch_runnable(runnable.coerce(), target, self.options) }
+    }
+
+    /// Dispatch this Runnable to the specified EventTarget as a background
+    /// task. The runnable function must be `Send`.
+    pub fn dispatch_background_task(self) -> Result<(), nsresult> {
+        let runnable = RunnableFunction::allocate(InitRunnableFunction {
+            name: self.name,
+            priority: self.priority,
+            function: Mutex::new(Some(self.function)),
+        });
+        unsafe { dispatch_background_task_runnable(runnable.coerce(), self.options) }
+    }
+}
+
+impl<F> RunnableBuilder<F>
+where
+    F: FnOnce() + 'static,
+{
+    /// Dispatch this Runnable to the current thread.
+    ///
+    /// Unlike `dispatch` and `dispatch_background_task`, the runnable does not
+    /// need to be `Send` to dispatch to the current thread.
+    pub fn dispatch_local(self) -> Result<(), nsresult> {
+        let target = get_current_thread()?;
+        let runnable = RunnableFunction::allocate(InitRunnableFunction {
+            name: self.name,
+            priority: self.priority,
+            function: Mutex::new(Some(self.function)),
+        });
+        unsafe { dispatch_runnable(runnable.coerce(), target.coerce(), self.options) }
+    }
+}
+
+pub fn dispatch_onto<F>(
+    name: &'static str,
+    target: &nsIEventTarget,
+    function: F,
+) -> Result<(), nsresult>
+where
+    F: FnOnce() + Send + 'static,
+{
+    RunnableBuilder::new(name, function).dispatch(target)
+}
+
+pub fn dispatch_background_task<F>(name: &'static str, function: F) -> Result<(), nsresult>
+where
+    F: FnOnce() + Send + 'static,
+{
+    RunnableBuilder::new(name, function).dispatch_background_task()
+}
+
+pub fn dispatch_local<F>(name: &'static str, function: F) -> Result<(), nsresult>
+where
+    F: FnOnce() + 'static,
+{
+    RunnableBuilder::new(name, function).dispatch_local()
+}
diff --git a/xpcom/rust/moz_task/src/event_loop.rs b/xpcom/rust/moz_task/src/event_loop.rs
new file mode 100644
index 0000000000..f8d113ed57
--- /dev/null
+++ b/xpcom/rust/moz_task/src/event_loop.rs
@@ -0,0 +1,66 @@
+/* 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/. */
+
+extern crate nsstring;
+
+use cstr::cstr;
+use nserror::{nsresult, NS_ERROR_SERVICE_NOT_AVAILABLE, NS_ERROR_UNEXPECTED, NS_OK};
+use nsstring::*;
+use std::cell::RefCell;
+use std::future::Future;
+use xpcom::{interfaces::nsIThreadManager, xpcom, xpcom_method};
+
+#[xpcom(implement(nsINestedEventLoopCondition), nonatomic)]
+struct FutureCompleteCondition<T: 'static> {
+    value: RefCell<Option<T>>,
+}
+
+impl<T: 'static> FutureCompleteCondition<T> {
+    xpcom_method!(is_done => IsDone() -> bool);
+    fn is_done(&self) -> Result<bool, nsresult> {
+        Ok(self.value.borrow().is_some())
+    }
+}
+
+/// Spin the event loop on the current thread until `future` is resolved.
+///
+/// # Safety
+///
+/// Spinning a nested event loop should always be avoided when possible, as it
+/// can cause hangs, break JS run-to-completion guarantees, and break other C++
+/// code currently on the stack relying on heap invariants. While in a pure-rust
+/// codebase this method would only be ill-advised and not technically "unsafe",
+/// it is marked as unsafe due to the potential for triggering unsafety in
+/// unrelated C++ code.
+pub unsafe fn spin_event_loop_until<F>(
+    reason: &'static str,
+    future: F,
+) -> Result<F::Output, nsresult>
+where
+    F: Future + 'static,
+    F::Output: 'static,
+{
+    let thread_manager =
+        xpcom::get_service::<nsIThreadManager>(cstr!("@mozilla.org/thread-manager;1"))
+            .ok_or(NS_ERROR_SERVICE_NOT_AVAILABLE)?;
+
+    let cond = FutureCompleteCondition::<F::Output>::allocate(InitFutureCompleteCondition {
+        value: RefCell::new(None),
+    });
+
+    // Spawn our future onto the current thread event loop, and record the
+    // completed value as it completes.
+    let cond2 = cond.clone();
+    crate::spawn_local(reason, async move {
+        let rv = future.await;
+        *cond2.value.borrow_mut() = Some(rv);
+    })
+    .detach();
+
+    thread_manager
+        .SpinEventLoopUntil(&*nsCStr::from(reason), cond.coerce())
+        .to_result()?;
+    let rv = cond.value.borrow_mut().take();
+    rv.ok_or(NS_ERROR_UNEXPECTED)
+}
diff --git a/xpcom/rust/moz_task/src/executor.rs b/xpcom/rust/moz_task/src/executor.rs
new file mode 100644
index 0000000000..0016839373
--- /dev/null
+++ b/xpcom/rust/moz_task/src/executor.rs
@@ -0,0 +1,291 @@
+/* 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 crate::{get_current_thread, DispatchOptions, RunnableBuilder};
+use std::{
+    cell::Cell,
+    fmt::Debug,
+    future::Future,
+    pin::Pin,
+    ptr,
+    sync::Arc,
+    task::{Context, Poll},
+};
+use xpcom::interfaces::{nsIEventTarget, nsIRunnablePriority};
+use xpcom::RefPtr;
+
+/// A spawned task.
+///
+/// A [`AsyncTask`] can be awaited to retrieve the output of its future.
+///
+/// Dropping an [`AsyncTask`] cancels it, which means its future won't be polled
+/// again. To drop the [`AsyncTask`] handle without canceling it, use
+/// [`detach()`][`AsyncTask::detach()`] instead. To cancel a task gracefully and
+/// wait until it is fully destroyed, use the [`cancel()`][AsyncTask::cancel()]
+/// method.
+///
+/// A task which is cancelled due to the nsIEventTarget it was dispatched to no
+/// longer accepting events will never be resolved.
+#[derive(Debug)]
+#[must_use = "tasks get canceled when dropped, use `.detach()` to run them in the background"]
+pub struct AsyncTask<T> {
+    task: async_task::FallibleTask<T>,
+}
+
+impl<T> AsyncTask<T> {
+    fn new(task: async_task::Task<T>) -> Self {
+        AsyncTask {
+            task: task.fallible(),
+        }
+    }
+
+    /// Detaches the task to let it keep running in the background.
+    pub fn detach(self) {
+        self.task.detach()
+    }
+
+    /// Cancels the task and waits for it to stop running.
+    ///
+    /// Returns the task's output if it was completed just before it got canceled, or [`None`] if
+    /// it didn't complete.
+    ///
+    /// While it's possible to simply drop the [`Task`] to cancel it, this is a cleaner way of
+    /// canceling because it also waits for the task to stop running.
+    pub async fn cancel(self) -> Option<T> {
+        self.task.cancel().await
+    }
+}
+
+impl<T> Future for AsyncTask<T> {
+    type Output = T;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        // Wrap the future produced by `AsyncTask` to never resolve if the
+        // Runnable was dropped, and the task was cancelled.
+        match Pin::new(&mut self.task).poll(cx) {
+            Poll::Ready(Some(t)) => Poll::Ready(t),
+            Poll::Ready(None) | Poll::Pending => Poll::Pending,
+        }
+    }
+}
+
+enum SpawnTarget {
+    BackgroundTask,
+    EventTarget(RefPtr<nsIEventTarget>),
+}
+
+// SAFETY: All XPCOM interfaces are considered !Send + !Sync, however all
+// well-behaved nsIEventTarget instances must be threadsafe.
+unsafe impl Send for SpawnTarget {}
+unsafe impl Sync for SpawnTarget {}
+
+/// Information used by tasks as they are spawned. Stored in an Arc such that
+/// their identity can be used for `POLLING_TASK`.
+struct TaskSpawnConfig {
+    name: &'static str,
+    priority: u32,
+    options: DispatchOptions,
+    target: SpawnTarget,
+}
+
+thread_local! {
+    /// Raw pointer to the TaskSpawnConfig for the currently polling task. Used
+    /// to detect scheduling callbacks for a runnable while it is polled, to set
+    /// `DISPATCH_AT_END` on the notification.
+    static POLLING_TASK: Cell<*const TaskSpawnConfig> = Cell::new(ptr::null());
+}
+
+fn schedule(config: Arc<TaskSpawnConfig>, runnable: async_task::Runnable) {
+    // If we're dispatching this task while it is currently running on the same
+    // thread, set the `DISPATCH_AT_END` flag in the dispatch options to tell
+    // our threadpool target to not bother to spin up another thread.
+    let currently_polling = POLLING_TASK.with(|t| t.get() == Arc::as_ptr(&config));
+
+    // SAFETY: We use the POLLING_TASK thread local to check if we meet the
+    // requirements for `at_end`.
+    let options = unsafe { config.options.at_end(currently_polling) };
+
+    // Build the RunnableBuilder for our task to be dispatched.
+    let config2 = config.clone();
+    let builder = RunnableBuilder::new(config.name, move || {
+        // Record the pointer for the currently executing task in the
+        // POLLING_TASK thread-local so that nested dispatches can detect it.
+        POLLING_TASK.with(|t| {
+            let prev = t.get();
+            t.set(Arc::as_ptr(&config2));
+            runnable.run();
+            t.set(prev);
+        });
+    })
+    .priority(config.priority)
+    .options(options);
+
+    let rv = match &config.target {
+        SpawnTarget::BackgroundTask => builder.dispatch_background_task(),
+        SpawnTarget::EventTarget(target) => builder.dispatch(&*target),
+    };
+    if let Err(err) = rv {
+        log::warn!(
+            "dispatch for spawned task '{}' failed: {:?}",
+            config.name,
+            err
+        );
+    }
+}
+
+/// Helper for starting an async task which will run a future to completion.
+#[derive(Debug)]
+pub struct TaskBuilder<F> {
+    name: &'static str,
+    future: F,
+    priority: u32,
+    options: DispatchOptions,
+}
+
+impl<F> TaskBuilder<F> {
+    pub fn new(name: &'static str, future: F) -> TaskBuilder<F> {
+        TaskBuilder {
+            name,
+            future,
+            priority: nsIRunnablePriority::PRIORITY_NORMAL,
+            options: DispatchOptions::default(),
+        }
+    }
+
+    /// Specify the priority of the task's runnables.
+    pub fn priority(mut self, priority: u32) -> Self {
+        self.priority = priority;
+        self
+    }
+
+    /// Specify options to use when dispatching the task.
+    pub fn options(mut self, options: DispatchOptions) -> Self {
+        self.options = options;
+        self
+    }
+
+    /// Set whether or not the event may block, and should be run on the IO
+    /// thread pool.
+    pub fn may_block(mut self, may_block: bool) -> Self {
+        self.options = self.options.may_block(may_block);
+        self
+    }
+}
+
+impl<F> TaskBuilder<F>
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    /// Run the future on the background task pool.
+    pub fn spawn(self) -> AsyncTask<F::Output> {
+        let config = Arc::new(TaskSpawnConfig {
+            name: self.name,
+            priority: self.priority,
+            options: self.options,
+            target: SpawnTarget::BackgroundTask,
+        });
+        let (runnable, task) = async_task::spawn(self.future, move |runnable| {
+            schedule(config.clone(), runnable)
+        });
+        runnable.schedule();
+        AsyncTask::new(task)
+    }
+
+    /// Run the future on the specified nsIEventTarget.
+    pub fn spawn_onto(self, target: &nsIEventTarget) -> AsyncTask<F::Output> {
+        let config = Arc::new(TaskSpawnConfig {
+            name: self.name,
+            priority: self.priority,
+            options: self.options,
+            target: SpawnTarget::EventTarget(RefPtr::new(target)),
+        });
+        let (runnable, task) = async_task::spawn(self.future, move |runnable| {
+            schedule(config.clone(), runnable)
+        });
+        runnable.schedule();
+        AsyncTask::new(task)
+    }
+}
+
+impl<F> TaskBuilder<F>
+where
+    F: Future + 'static,
+    F::Output: 'static,
+{
+    /// Run the future on the current thread.
+    ///
+    /// Unlike the other `spawn` methods, this method supports non-Send futures.
+    ///
+    /// # Panics
+    ///
+    /// This method may panic if run on a thread which cannot run local futures
+    /// (e.g. due to it is not being an XPCOM thread, or if we are very late
+    /// during shutdown).
+    pub fn spawn_local(self) -> AsyncTask<F::Output> {
+        let current_thread = get_current_thread().expect("cannot get current thread");
+        let config = Arc::new(TaskSpawnConfig {
+            name: self.name,
+            priority: self.priority,
+            options: self.options,
+            target: SpawnTarget::EventTarget(RefPtr::new(current_thread.coerce())),
+        });
+        let (runnable, task) = async_task::spawn_local(self.future, move |runnable| {
+            schedule(config.clone(), runnable)
+        });
+        runnable.schedule();
+        AsyncTask::new(task)
+    }
+}
+
+/// Spawn a future onto the background task pool. The future will not be run on
+/// the main thread.
+pub fn spawn<F>(name: &'static str, future: F) -> AsyncTask<F::Output>
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    TaskBuilder::new(name, future).spawn()
+}
+
+/// Spawn a potentially-blocking future onto the background task pool. The
+/// future will not be run on the main thread.
+pub fn spawn_blocking<F>(name: &'static str, future: F) -> AsyncTask<F::Output>
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    TaskBuilder::new(name, future).may_block(true).spawn()
+}
+
+/// Spawn a local future onto the current thread.
+pub fn spawn_local<F>(name: &'static str, future: F) -> AsyncTask<F::Output>
+where
+    F: Future + 'static,
+    F::Output: 'static,
+{
+    TaskBuilder::new(name, future).spawn_local()
+}
+
+pub fn spawn_onto<F>(name: &'static str, target: &nsIEventTarget, future: F) -> AsyncTask<F::Output>
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    TaskBuilder::new(name, future).spawn_onto(target)
+}
+
+pub fn spawn_onto_blocking<F>(
+    name: &'static str,
+    target: &nsIEventTarget,
+    future: F,
+) -> AsyncTask<F::Output>
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    TaskBuilder::new(name, future)
+        .may_block(true)
+        .spawn_onto(target)
+}
diff --git a/xpcom/rust/moz_task/src/lib.rs b/xpcom/rust/moz_task/src/lib.rs
new file mode 100644
index 0000000000..2f0c0cfd0a
--- /dev/null
+++ b/xpcom/rust/moz_task/src/lib.rs
@@ -0,0 +1,378 @@
+/* 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/. */
+
+//! This module wraps XPCOM threading functions with Rust functions
+//! to make it safer and more convenient to call the XPCOM functions.
+//! It also provides the Task trait and TaskRunnable struct,
+//! which make it easier to dispatch tasks to threads.
+
+mod dispatcher;
+pub use dispatcher::{dispatch_background_task, dispatch_local, dispatch_onto, RunnableBuilder};
+mod event_loop;
+mod executor;
+pub use executor::{
+    spawn, spawn_blocking, spawn_local, spawn_onto, spawn_onto_blocking, AsyncTask, TaskBuilder,
+};
+
+// Expose functions intended to be used only in gtest via this module.
+// We don't use a feature gate here to stop the need to compile all crates that
+// depend upon `moz_task` twice.
+pub mod gtest_only {
+    pub use crate::event_loop::spin_event_loop_until;
+}
+
+use nserror::nsresult;
+use nsstring::{nsACString, nsCString};
+use std::{ffi::CStr, marker::PhantomData, mem, ptr};
+use xpcom::{
+    getter_addrefs,
+    interfaces::{nsIEventTarget, nsIRunnable, nsISerialEventTarget, nsISupports, nsIThread},
+    AtomicRefcnt, RefCounted, RefPtr, XpCom,
+};
+
+extern "C" {
+    fn NS_GetCurrentThreadRust(result: *mut *const nsIThread) -> nsresult;
+    fn NS_GetMainThreadRust(result: *mut *const nsIThread) -> nsresult;
+    fn NS_IsMainThread() -> bool;
+    fn NS_NewNamedThreadWithDefaultStackSize(
+        name: *const nsACString,
+        result: *mut *const nsIThread,
+        event: *const nsIRunnable,
+    ) -> nsresult;
+    fn NS_IsOnCurrentThread(target: *const nsIEventTarget) -> bool;
+    fn NS_ProxyReleaseISupports(
+        name: *const libc::c_char,
+        target: *const nsIEventTarget,
+        doomed: *const nsISupports,
+        always_proxy: bool,
+    );
+    fn NS_CreateBackgroundTaskQueue(
+        name: *const libc::c_char,
+        target: *mut *const nsISerialEventTarget,
+    ) -> nsresult;
+    fn NS_DispatchBackgroundTask(event: *const nsIRunnable, flags: u32) -> nsresult;
+}
+
+pub fn get_current_thread() -> Result<RefPtr<nsIThread>, nsresult> {
+    getter_addrefs(|p| unsafe { NS_GetCurrentThreadRust(p) })
+}
+
+pub fn get_main_thread() -> Result<RefPtr<nsIThread>, nsresult> {
+    getter_addrefs(|p| unsafe { NS_GetMainThreadRust(p) })
+}
+
+pub fn is_main_thread() -> bool {
+    unsafe { NS_IsMainThread() }
+}
+
+// There's no OS requirement that thread names be static, but dynamic thread
+// names tend to conceal more than they reveal when processing large numbers of
+// crash reports.
+pub fn create_thread(name: &'static str) -> Result<RefPtr<nsIThread>, nsresult> {
+    getter_addrefs(|p| unsafe {
+        NS_NewNamedThreadWithDefaultStackSize(&*nsCString::from(name), p, ptr::null())
+    })
+}
+
+pub fn is_on_current_thread(target: &nsIEventTarget) -> bool {
+    unsafe { NS_IsOnCurrentThread(target) }
+}
+
+/// Creates a queue that runs tasks on the background thread pool. The tasks
+/// will run in the order they're dispatched, one after the other.
+pub fn create_background_task_queue(
+    name: &'static CStr,
+) -> Result<RefPtr<nsISerialEventTarget>, nsresult> {
+    getter_addrefs(|p| unsafe { NS_CreateBackgroundTaskQueue(name.as_ptr(), p) })
+}
+
+/// Dispatches a one-shot runnable to an event target, like a thread or a
+/// task queue, with the given options.
+///
+/// This function leaks the runnable if dispatch fails.
+///
+/// # Safety
+///
+/// As there is no guarantee that the runnable is actually `Send + Sync`, we
+/// can't know that it's safe to dispatch an `nsIRunnable` to any
+/// `nsIEventTarget`.
+pub unsafe fn dispatch_runnable(
+    runnable: &nsIRunnable,
+    target: &nsIEventTarget,
+    options: DispatchOptions,
+) -> Result<(), nsresult> {
+    // NOTE: DispatchFromScript performs an AddRef on `runnable` which is
+    // why this function leaks on failure.
+    target
+        .DispatchFromScript(runnable, options.flags())
+        .to_result()
+}
+
+/// Dispatches a one-shot task runnable to the background thread pool with the
+/// given options. The task may run concurrently with other background tasks.
+/// If you need tasks to run in a specific order, please create a background
+/// task queue using `create_background_task_queue`, and dispatch tasks to it
+/// instead.
+///
+/// This function leaks the runnable if dispatch fails. This avoids a race where
+/// a runnable can be destroyed on either the original or target thread, which
+/// is important if the runnable holds thread-unsafe members.
+///
+/// ### Safety
+///
+/// As there is no guarantee that the runnable is actually `Send + Sync`, we
+/// can't know that it's safe to dispatch an `nsIRunnable` to any
+/// `nsIEventTarget`.
+pub unsafe fn dispatch_background_task_runnable(
+    runnable: &nsIRunnable,
+    options: DispatchOptions,
+) -> Result<(), nsresult> {
+    // This eventually calls the non-`already_AddRefed<nsIRunnable>` overload of
+    // `nsIEventTarget::Dispatch` (see xpcom/threads/nsIEventTarget.idl#20-25),
+    // which adds an owning reference and leaks if dispatch fails.
+    NS_DispatchBackgroundTask(runnable, options.flags()).to_result()
+}
+
+/// Options to control how task runnables are dispatched.
+#[derive(Copy, Clone, Debug, Eq, Hash, PartialEq)]
+pub struct DispatchOptions(u32);
+
+impl Default for DispatchOptions {
+    #[inline]
+    fn default() -> Self {
+        DispatchOptions(nsIEventTarget::DISPATCH_NORMAL)
+    }
+}
+
+impl DispatchOptions {
+    /// Creates a blank set of options. The runnable will be dispatched using
+    /// the default mode.
+    #[inline]
+    pub fn new() -> Self {
+        DispatchOptions::default()
+    }
+
+    /// Indicates whether or not the dispatched runnable may block its target
+    /// thread by waiting on I/O. If `true`, the runnable may be dispatched to a
+    /// dedicated thread pool, leaving the main pool free for CPU-bound tasks.
+    #[inline]
+    pub fn may_block(self, may_block: bool) -> DispatchOptions {
+        const FLAG: u32 = nsIEventTarget::DISPATCH_EVENT_MAY_BLOCK;
+        if may_block {
+            DispatchOptions(self.flags() | FLAG)
+        } else {
+            DispatchOptions(self.flags() & !FLAG)
+        }
+    }
+
+    /// Specifies that the dispatch is occurring from a running event that was
+    /// dispatched to the same event target, and that event is about to finish.
+    ///
+    /// A thread pool can use this as an optimization hint to not spin up
+    /// another thread, since the current thread is about to become idle.
+    ///
+    /// Setting this flag is unsafe, as it may only be used from the target
+    /// event target when the event is about to finish.
+    #[inline]
+    pub unsafe fn at_end(self, may_block: bool) -> DispatchOptions {
+        const FLAG: u32 = nsIEventTarget::DISPATCH_AT_END;
+        if may_block {
+            DispatchOptions(self.flags() | FLAG)
+        } else {
+            DispatchOptions(self.flags() & !FLAG)
+        }
+    }
+
+    /// Returns the set of bitflags to pass to `DispatchFromScript`.
+    #[inline]
+    fn flags(self) -> u32 {
+        self.0
+    }
+}
+
+/// A task represents an operation that asynchronously executes on a target
+/// thread, and returns its result to the original thread.
+///
+/// # Alternatives
+///
+/// This trait is no longer necessary for basic tasks to be dispatched to
+/// another thread with a callback on the originating thread. `moz_task` now has
+/// a series of more rust-like primitives which can be used instead. For
+/// example, it may be preferable to use the async executor over `Task`:
+///
+/// ```ignore
+/// // Spawn a task onto the background task pool, and capture the result of its
+/// // execution.
+/// let bg_task = moz_task::spawn("Example", async move {
+///     do_background_work(captured_state)
+/// });
+///
+/// // Spawn another task on the calling thread which will await on the result
+/// // of the async operation, and invoke a non-Send callback. This task won't
+/// // be awaited on, so needs to be `detach`-ed.
+/// moz_task::spawn_local("Example", async move {
+///     callback.completed(bg_task.await);
+/// })
+/// .detach();
+/// ```
+///
+/// If no result is needed, the task returned from `spawn` may be also detached
+/// directly.
+pub trait Task {
+    // FIXME: These could accept `&mut`.
+    fn run(&self);
+    fn done(&self) -> Result<(), nsresult>;
+}
+
+pub struct TaskRunnable {
+    name: &'static str,
+    task: Box<dyn Task + Send + Sync>,
+}
+
+impl TaskRunnable {
+    // XXX: Fixme: clean up this old API. (bug 1744312)
+    pub fn new(
+        name: &'static str,
+        task: Box<dyn Task + Send + Sync>,
+    ) -> Result<TaskRunnable, nsresult> {
+        Ok(TaskRunnable { name, task })
+    }
+
+    pub fn dispatch(self, target: &nsIEventTarget) -> Result<(), nsresult> {
+        self.dispatch_with_options(target, DispatchOptions::default())
+    }
+
+    pub fn dispatch_with_options(
+        self,
+        target: &nsIEventTarget,
+        options: DispatchOptions,
+    ) -> Result<(), nsresult> {
+        // Perform `task.run()` on a background thread.
+        let task = self.task;
+        let handle = TaskBuilder::new(self.name, async move {
+            task.run();
+            task
+        })
+        .options(options)
+        .spawn_onto(target);
+
+        // Run `task.done()` on the starting thread once the background thread
+        // is done with the task.
+        spawn_local(self.name, async move {
+            let task = handle.await;
+            let _ = task.done();
+        })
+        .detach();
+        Ok(())
+    }
+
+    pub fn dispatch_background_task_with_options(
+        self,
+        options: DispatchOptions,
+    ) -> Result<(), nsresult> {
+        // Perform `task.run()` on a background thread.
+        let task = self.task;
+        let handle = TaskBuilder::new(self.name, async move {
+            task.run();
+            task
+        })
+        .options(options)
+        .spawn();
+
+        // Run `task.done()` on the starting thread once the background thread
+        // is done with the task.
+        spawn_local(self.name, async move {
+            let task = handle.await;
+            let _ = task.done();
+        })
+        .detach();
+        Ok(())
+    }
+}
+
+pub type ThreadPtrHandle<T> = RefPtr<ThreadPtrHolder<T>>;
+
+/// A Rust analog to `nsMainThreadPtrHolder` that wraps an `nsISupports` object
+/// with thread-safe refcounting. The holder keeps one reference to the wrapped
+/// object that's released when the holder's refcount reaches zero.
+pub struct ThreadPtrHolder<T: XpCom + 'static> {
+    ptr: *const T,
+    marker: PhantomData<T>,
+    name: &'static CStr,
+    owning_thread: RefPtr<nsIThread>,
+    refcnt: AtomicRefcnt,
+}
+
+unsafe impl<T: XpCom + 'static> Send for ThreadPtrHolder<T> {}
+unsafe impl<T: XpCom + 'static> Sync for ThreadPtrHolder<T> {}
+
+unsafe impl<T: XpCom + 'static> RefCounted for ThreadPtrHolder<T> {
+    unsafe fn addref(&self) {
+        self.refcnt.inc();
+    }
+
+    unsafe fn release(&self) {
+        let rc = self.refcnt.dec();
+        if rc == 0 {
+            // Once the holder's count reaches zero, release the wrapped
+            // object...
+            if !self.ptr.is_null() {
+                // The holder can be released on any thread. If we're on the
+                // owning thread, we can release the object directly. Otherwise,
+                // we need to post a proxy release event to release the object
+                // on the owning thread.
+                if is_on_current_thread(&self.owning_thread) {
+                    (*self.ptr).release()
+                } else {
+                    NS_ProxyReleaseISupports(
+                        self.name.as_ptr(),
+                        self.owning_thread.coerce(),
+                        self.ptr as *const T as *const nsISupports,
+                        false,
+                    );
+                }
+            }
+            // ...And deallocate the holder.
+            mem::drop(Box::from_raw(self as *const Self as *mut Self));
+        }
+    }
+}
+
+impl<T: XpCom + 'static> ThreadPtrHolder<T> {
+    /// Creates a new owning thread pointer holder. Returns an error if the
+    /// thread manager has shut down. Panics if `name` isn't a valid C string.
+    pub fn new(name: &'static CStr, ptr: RefPtr<T>) -> Result<RefPtr<Self>, nsresult> {
+        let owning_thread = get_current_thread()?;
+        // Take ownership of the `RefPtr`. This does _not_ decrement its
+        // refcount, which is what we want. Once we've released all references
+        // to the holder, we'll release the wrapped `RefPtr`.
+        let raw: *const T = &*ptr;
+        mem::forget(ptr);
+        unsafe {
+            let boxed = Box::new(ThreadPtrHolder {
+                name,
+                ptr: raw,
+                marker: PhantomData,
+                owning_thread,
+                refcnt: AtomicRefcnt::new(),
+            });
+            Ok(RefPtr::from_raw(Box::into_raw(boxed)).unwrap())
+        }
+    }
+
+    /// Returns the wrapped object's owning thread.
+    pub fn owning_thread(&self) -> &nsIThread {
+        &self.owning_thread
+    }
+
+    /// Returns the wrapped object if called from the owning thread, or
+    /// `None` if called from any other thread.
+    pub fn get(&self) -> Option<&T> {
+        if is_on_current_thread(&self.owning_thread) && !self.ptr.is_null() {
+            unsafe { Some(&*self.ptr) }
+        } else {
+            None
+        }
+    }
+}
diff --git a/xpcom/rust/nserror/Cargo.toml b/xpcom/rust/nserror/Cargo.toml
new file mode 100644
index 0000000000..4fca5a4c2c
--- /dev/null
+++ b/xpcom/rust/nserror/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "nserror"
+version = "0.1.0"
+authors = ["Nika Layzell <nika@thelayzells.com>"]
+license = "MPL-2.0"
+description = "Rust bindings to xpcom nsresult and NS_ERROR_ values"
+edition = "2018"
+
+[dependencies]
+nsstring = { path = "../nsstring" }
+mozbuild = "0.1"
diff --git a/xpcom/rust/nserror/src/lib.rs b/xpcom/rust/nserror/src/lib.rs
new file mode 100644
index 0000000000..e4107d1b57
--- /dev/null
+++ b/xpcom/rust/nserror/src/lib.rs
@@ -0,0 +1,79 @@
+/* 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 nsstring::{nsACString, nsCString};
+use std::error::Error;
+use std::fmt;
+
+/// The type of errors in gecko.  Uses a newtype to provide additional type
+/// safety in Rust and #[repr(transparent)] to ensure the same representation
+/// as the C++ equivalent.
+#[repr(transparent)]
+#[allow(non_camel_case_types)]
+#[derive(Clone, Copy, Eq, PartialEq, Ord, PartialOrd, Hash)]
+pub struct nsresult(pub u32);
+
+impl nsresult {
+    pub fn failed(self) -> bool {
+        (self.0 >> 31) != 0
+    }
+
+    pub fn succeeded(self) -> bool {
+        !self.failed()
+    }
+
+    pub fn to_result(self) -> Result<(), nsresult> {
+        if self.failed() {
+            Err(self)
+        } else {
+            Ok(())
+        }
+    }
+
+    /// Get a printable name for the nsresult error code. This function returns
+    /// a nsCString<'static>, which implements `Display`.
+    pub fn error_name(self) -> nsCString {
+        let mut cstr = nsCString::new();
+        unsafe {
+            Gecko_GetErrorName(self, &mut *cstr);
+        }
+        cstr
+    }
+}
+
+impl fmt::Display for nsresult {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.error_name())
+    }
+}
+
+impl fmt::Debug for nsresult {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.error_name())
+    }
+}
+
+impl<T, E> From<Result<T, E>> for nsresult
+where
+    E: Into<nsresult>,
+{
+    fn from(result: Result<T, E>) -> nsresult {
+        match result {
+            Ok(_) => NS_OK,
+            Err(e) => e.into(),
+        }
+    }
+}
+
+impl Error for nsresult {}
+
+extern "C" {
+    fn Gecko_GetErrorName(rv: nsresult, cstr: *mut nsACString);
+}
+
+mod error_list {
+    include!(mozbuild::objdir_path!("xpcom/base/error_list.rs"));
+}
+
+pub use error_list::*;
diff --git a/xpcom/rust/nsstring/Cargo.toml b/xpcom/rust/nsstring/Cargo.toml
new file mode 100644
index 0000000000..c2f8e34b45
--- /dev/null
+++ b/xpcom/rust/nsstring/Cargo.toml
@@ -0,0 +1,14 @@
+[package]
+name = "nsstring"
+version = "0.1.0"
+authors = ["nobody@mozilla.com"]
+license = "MPL-2.0"
+description = "Rust bindings to xpcom string types"
+edition = "2018"
+
+[features]
+gecko_debug = []
+
+[dependencies]
+bitflags = "1.0"
+encoding_rs = "0.8.0"
diff --git a/xpcom/rust/nsstring/src/conversions.rs b/xpcom/rust/nsstring/src/conversions.rs
new file mode 100644
index 0000000000..c72c195c08
--- /dev/null
+++ b/xpcom/rust/nsstring/src/conversions.rs
@@ -0,0 +1,751 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use crate::{
+    nsACString, nsAString, nsCStringLike, BulkWriteOk, Gecko_FallibleAssignCString,
+    Latin1StringLike,
+};
+use encoding_rs::mem::*;
+use encoding_rs::Encoding;
+use std::slice;
+
+/// Required math stated in the docs of
+/// `convert_utf16_to_utf8()`.
+#[inline(always)]
+fn times_three(a: usize) -> Option<usize> {
+    a.checked_mul(3)
+}
+
+#[inline(always)]
+fn identity(a: usize) -> Option<usize> {
+    Some(a)
+}
+
+#[inline(always)]
+fn plus_one(a: usize) -> Option<usize> {
+    a.checked_add(1)
+}
+
+/// Typical cache line size per
+/// https://stackoverflow.com/questions/14707803/line-size-of-l1-and-l2-caches
+///
+/// For consistent behavior, not trying to use 128 on aarch64
+/// or other fanciness like that.
+const CACHE_LINE: usize = 64;
+
+const CACHE_LINE_MASK: usize = CACHE_LINE - 1;
+
+/// Returns true if the string is both longer than a cache line
+/// and the first cache line is ASCII.
+#[inline(always)]
+fn long_string_starts_with_ascii(buffer: &[u8]) -> bool {
+    // We examine data only up to the end of the cache line
+    // to make this check minimally disruptive.
+    if buffer.len() <= CACHE_LINE {
+        return false;
+    }
+    let bound = CACHE_LINE - ((buffer.as_ptr() as usize) & CACHE_LINE_MASK);
+    is_ascii(&buffer[..bound])
+}
+
+/// Returns true if the string is both longer than two cache lines
+/// and the first two cache lines are Basic Latin.
+#[inline(always)]
+fn long_string_stars_with_basic_latin(buffer: &[u16]) -> bool {
+    // We look at two cache lines with code unit size of two. There is need
+    // to look at more than one cache line in the UTF-16 case, because looking
+    // at just one cache line wouldn't catch non-ASCII Latin with high enough
+    // probability with Latin-script languages that have relatively infrequent
+    // non-ASCII characters.
+    if buffer.len() <= CACHE_LINE {
+        return false;
+    }
+    let bound = (CACHE_LINE * 2 - ((buffer.as_ptr() as usize) & CACHE_LINE_MASK)) / 2;
+    is_basic_latin(&buffer[..bound])
+}
+
+// Ignoring the copy avoidance complications of conversions between Latin1 and
+// UTF-8, a conversion function has the outward form of
+// `fn F(&mut self, other: &[T], old_len: usize) -> Result<BulkWriteOk, ()>`,
+// where `T` is either `u8` or `u16`. `other` is the slice whose converted
+// content are to be appended to `self` and `old_len` indicates how many
+// code unit of `self` are to be preserved (0 for the assignment case and
+// `self.len()` for the appending case).
+//
+// As implementation parameters a conversion function needs to know the
+// math for computing the worst case conversion length in code units given
+// the input length in code units. For a _constant conversion_ the number
+// of code units the conversion produces equals the number of code units
+// in the input. For a _shinking conversion_ the maximum number of code
+// units the conversion can produce equals the number of code units in
+// the input, but the conversion can produce fewer code units. Still, due
+// to implementation details, the function might want _one_ unit more of
+// output space. For an _expanding conversion_ (no need for macro), the
+// minimum number of code units produced by the conversion is the number
+// of code units in the input, but the conversion can produce more.
+//
+// Copy avoidance conversions avoid copying a refcounted buffer when it's
+// ASCII-only.
+//
+// Internally, a conversion function needs to know the underlying
+// encoding_rs conversion function, the math for computing the required
+// output buffer size and, depending on the case, the underlying
+// encoding_rs ASCII prefix handling function.
+
+/// A conversion where the number of code units in the output is potentially
+/// smaller than the number of code units in the input.
+///
+/// Takes the name of the method to be generated, the name of the conversion
+/// function and the type of the input slice.
+///
+/// `$name` is the name of the function to generate
+/// `$convert` is the underlying `encoding_rs::mem` function to use
+/// `$other_ty` is the type of the input slice
+/// `$math` is the worst-case length math that `$convert` expects
+macro_rules! shrinking_conversion {
+    (name = $name:ident,
+     convert = $convert:ident,
+     other_ty = $other_ty:ty,
+     math = $math:ident) => {
+        fn $name(&mut self, other: $other_ty, old_len: usize) -> Result<BulkWriteOk, ()> {
+            let needed = $math(other.len()).ok_or(())?;
+            let mut handle =
+                unsafe { self.bulk_write(old_len.checked_add(needed).ok_or(())?, old_len, false)? };
+            let written = $convert(other, &mut handle.as_mut_slice()[old_len..]);
+            let new_len = old_len + written;
+            Ok(handle.finish(new_len, new_len > CACHE_LINE))
+        }
+    };
+}
+
+/// A conversion where the number of code units in the output is always equal
+/// to the number of code units in the input.
+///
+/// Takes the name of the method to be generated, the name of the conversion
+/// function and the type of the input slice.
+///
+/// `$name` is the name of the function to generate
+/// `$convert` is the underlying `encoding_rs::mem` function to use
+/// `$other_ty` is the type of the input slice
+macro_rules! constant_conversion {
+    (name = $name:ident,
+     convert = $convert:ident,
+     other_ty = $other_ty:ty) => {
+        fn $name(
+            &mut self,
+            other: $other_ty,
+            old_len: usize,
+            allow_shrinking: bool,
+        ) -> Result<BulkWriteOk, ()> {
+            let new_len = old_len.checked_add(other.len()).ok_or(())?;
+            let mut handle = unsafe { self.bulk_write(new_len, old_len, allow_shrinking)? };
+            $convert(other, &mut handle.as_mut_slice()[old_len..]);
+            Ok(handle.finish(new_len, false))
+        }
+    };
+}
+
+/// An intermediate check for avoiding a copy and having an `nsStringBuffer`
+/// refcount increment instead when both `self` and `other` are `nsACString`s,
+/// `other` is entirely ASCII and all old data in `self` is discarded.
+///
+/// `$name` is the name of the function to generate
+/// `$impl` is the underlying conversion that takes a slice and that is used
+///         when we can't just adopt the incoming buffer as-is
+/// `$string_like` is the kind of input taken
+macro_rules! ascii_copy_avoidance {
+    (name = $name:ident,
+     implementation = $implementation:ident,
+     string_like = $string_like:ident) => {
+        fn $name<T: $string_like + ?Sized>(
+            &mut self,
+            other: &T,
+            old_len: usize,
+        ) -> Result<BulkWriteOk, ()> {
+            let adapter = other.adapt();
+            let other_slice = adapter.as_ref();
+            let num_ascii = if adapter.is_abstract() && old_len == 0 {
+                let up_to = Encoding::ascii_valid_up_to(other_slice);
+                if up_to == other_slice.len() {
+                    // Calling something whose argument can be obtained from
+                    // the adapter rather than an nsStringLike avoids a huge
+                    // lifetime mess by keeping nsStringLike and
+                    // Latin1StringLike free of lifetime interdependencies.
+                    if unsafe { Gecko_FallibleAssignCString(self, other.adapt().as_ptr()) } {
+                        return Ok(BulkWriteOk {});
+                    } else {
+                        return Err(());
+                    }
+                }
+                Some(up_to)
+            } else {
+                None
+            };
+            self.$implementation(other_slice, old_len, num_ascii)
+        }
+    };
+}
+
+impl nsAString {
+    // Valid UTF-8 to UTF-16
+
+    // Documentation says the destination buffer needs to have
+    // as many code units as the input.
+    shrinking_conversion!(
+        name = fallible_append_str_impl,
+        convert = convert_str_to_utf16,
+        other_ty = &str,
+        math = identity
+    );
+
+    /// Convert a valid UTF-8 string into valid UTF-16 and replace the content
+    /// of this string with the conversion result.
+    pub fn assign_str(&mut self, other: &str) {
+        self.fallible_append_str_impl(other, 0)
+            .expect("Out of memory");
+    }
+
+    /// Convert a valid UTF-8 string into valid UTF-16 and fallibly replace the
+    /// content of this string with the conversion result.
+    pub fn fallible_assign_str(&mut self, other: &str) -> Result<(), ()> {
+        self.fallible_append_str_impl(other, 0).map(|_| ())
+    }
+
+    /// Convert a valid UTF-8 string into valid UTF-16 and append the conversion
+    /// to this string.
+    pub fn append_str(&mut self, other: &str) {
+        let len = self.len();
+        self.fallible_append_str_impl(other, len)
+            .expect("Out of memory");
+    }
+
+    /// Convert a valid UTF-8 string into valid UTF-16 and fallibly append the
+    /// conversion to this string.
+    pub fn fallible_append_str(&mut self, other: &str) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_str_impl(other, len).map(|_| ())
+    }
+
+    // Potentially-invalid UTF-8 to UTF-16
+
+    // Documentation says the destination buffer needs to have
+    // one more code unit than the input.
+    shrinking_conversion!(
+        name = fallible_append_utf8_impl,
+        convert = convert_utf8_to_utf16,
+        other_ty = &[u8],
+        math = plus_one
+    );
+
+    /// Convert a potentially-invalid UTF-8 string into valid UTF-16
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// replace the content of this string with the conversion result.
+    pub fn assign_utf8(&mut self, other: &[u8]) {
+        self.fallible_append_utf8_impl(other, 0)
+            .expect("Out of memory");
+    }
+
+    /// Convert a potentially-invalid UTF-8 string into valid UTF-16
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// fallibly replace the content of this string with the conversion result.
+    pub fn fallible_assign_utf8(&mut self, other: &[u8]) -> Result<(), ()> {
+        self.fallible_append_utf8_impl(other, 0).map(|_| ())
+    }
+
+    /// Convert a potentially-invalid UTF-8 string into valid UTF-16
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// append the conversion result to this string.
+    pub fn append_utf8(&mut self, other: &[u8]) {
+        let len = self.len();
+        self.fallible_append_utf8_impl(other, len)
+            .expect("Out of memory");
+    }
+
+    /// Convert a potentially-invalid UTF-8 string into valid UTF-16
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// fallibly append the conversion result to this string.
+    pub fn fallible_append_utf8(&mut self, other: &[u8]) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_utf8_impl(other, len).map(|_| ())
+    }
+
+    // Latin1 to UTF-16
+
+    constant_conversion!(
+        name = fallible_append_latin1_impl,
+        convert = convert_latin1_to_utf16,
+        other_ty = &[u8]
+    );
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-16 and replace the content of this string with the conversion result.
+    pub fn assign_latin1(&mut self, other: &[u8]) {
+        self.fallible_append_latin1_impl(other, 0, true)
+            .expect("Out of memory");
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-16 and fallibly replace the content of this string with the
+    /// conversion result.
+    pub fn fallible_assign_latin1(&mut self, other: &[u8]) -> Result<(), ()> {
+        self.fallible_append_latin1_impl(other, 0, true).map(|_| ())
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-16 and append the conversion result to this string.
+    pub fn append_latin1(&mut self, other: &[u8]) {
+        let len = self.len();
+        self.fallible_append_latin1_impl(other, len, false)
+            .expect("Out of memory");
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-16 and fallibly append the conversion result to this string.
+    pub fn fallible_append_latin1(&mut self, other: &[u8]) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_latin1_impl(other, len, false)
+            .map(|_| ())
+    }
+}
+
+impl nsACString {
+    // UTF-16 to UTF-8
+
+    fn fallible_append_utf16_to_utf8_impl(
+        &mut self,
+        other: &[u16],
+        old_len: usize,
+    ) -> Result<BulkWriteOk, ()> {
+        // We first size the buffer for ASCII if the first two cache lines are ASCII. If that turns out
+        // not to be enough, we size for the worst case given the length of the remaining input at that
+        // point. BUT if the worst case fits inside the inline capacity of an autostring, we skip
+        // the ASCII stuff.
+        let worst_case_needed = if let Some(inline_capacity) = self.inline_capacity() {
+            let worst_case = times_three(other.len()).ok_or(())?;
+            if worst_case <= inline_capacity {
+                Some(worst_case)
+            } else {
+                None
+            }
+        } else {
+            None
+        };
+        let (filled, read, mut handle) =
+            if worst_case_needed.is_none() && long_string_stars_with_basic_latin(other) {
+                let new_len_with_ascii = old_len.checked_add(other.len()).ok_or(())?;
+                let mut handle = unsafe { self.bulk_write(new_len_with_ascii, old_len, false)? };
+                let (read, written) =
+                    convert_utf16_to_utf8_partial(other, &mut handle.as_mut_slice()[old_len..]);
+                let left = other.len() - read;
+                if left == 0 {
+                    return Ok(handle.finish(old_len + written, true));
+                }
+                let filled = old_len + written;
+                let needed = times_three(left).ok_or(())?;
+                let new_len = filled.checked_add(needed).ok_or(())?;
+                unsafe {
+                    handle.restart_bulk_write(new_len, filled, false)?;
+                }
+                (filled, read, handle)
+            } else {
+                // Started with non-ASCII. Compute worst case
+                let needed = if let Some(n) = worst_case_needed {
+                    n
+                } else {
+                    times_three(other.len()).ok_or(())?
+                };
+                let new_len = old_len.checked_add(needed).ok_or(())?;
+                let handle = unsafe { self.bulk_write(new_len, old_len, false)? };
+                (old_len, 0, handle)
+            };
+        let written = convert_utf16_to_utf8(&other[read..], &mut handle.as_mut_slice()[filled..]);
+        Ok(handle.finish(filled + written, true))
+    }
+
+    /// Convert a potentially-invalid UTF-16 string into valid UTF-8
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// replace the content of this string with the conversion result.
+    pub fn assign_utf16_to_utf8(&mut self, other: &[u16]) {
+        self.fallible_append_utf16_to_utf8_impl(other, 0)
+            .expect("Out of memory");
+    }
+
+    /// Convert a potentially-invalid UTF-16 string into valid UTF-8
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// fallibly replace the content of this string with the conversion result.
+    pub fn fallible_assign_utf16_to_utf8(&mut self, other: &[u16]) -> Result<(), ()> {
+        self.fallible_append_utf16_to_utf8_impl(other, 0)
+            .map(|_| ())
+    }
+
+    /// Convert a potentially-invalid UTF-16 string into valid UTF-8
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// append the conversion result to this string.
+    pub fn append_utf16_to_utf8(&mut self, other: &[u16]) {
+        let len = self.len();
+        self.fallible_append_utf16_to_utf8_impl(other, len)
+            .expect("Out of memory");
+    }
+
+    /// Convert a potentially-invalid UTF-16 string into valid UTF-8
+    /// (replacing invalid sequences with the REPLACEMENT CHARACTER) and
+    /// fallibly append the conversion result to this string.
+    pub fn fallible_append_utf16_to_utf8(&mut self, other: &[u16]) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_utf16_to_utf8_impl(other, len)
+            .map(|_| ())
+    }
+
+    // UTF-16 to Latin1
+
+    constant_conversion!(
+        name = fallible_append_utf16_to_latin1_lossy_impl,
+        convert = convert_utf16_to_latin1_lossy,
+        other_ty = &[u16]
+    );
+
+    /// Convert a UTF-16 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// replace the content of this string with the conversion result.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-16,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn assign_utf16_to_latin1_lossy(&mut self, other: &[u16]) {
+        self.fallible_append_utf16_to_latin1_lossy_impl(other, 0, true)
+            .expect("Out of memory");
+    }
+
+    /// Convert a UTF-16 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// fallibly replace the content of this string with the conversion result.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-16,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn fallible_assign_utf16_to_latin1_lossy(&mut self, other: &[u16]) -> Result<(), ()> {
+        self.fallible_append_utf16_to_latin1_lossy_impl(other, 0, true)
+            .map(|_| ())
+    }
+
+    /// Convert a UTF-16 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// append the conversion result to this string.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-16,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn append_utf16_to_latin1_lossy(&mut self, other: &[u16]) {
+        let len = self.len();
+        self.fallible_append_utf16_to_latin1_lossy_impl(other, len, false)
+            .expect("Out of memory");
+    }
+
+    /// Convert a UTF-16 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// fallibly append the conversion result to this string.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-16,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn fallible_append_utf16_to_latin1_lossy(&mut self, other: &[u16]) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_utf16_to_latin1_lossy_impl(other, len, false)
+            .map(|_| ())
+    }
+
+    // UTF-8 to Latin1
+
+    ascii_copy_avoidance!(
+        name = fallible_append_utf8_to_latin1_lossy_check,
+        implementation = fallible_append_utf8_to_latin1_lossy_impl,
+        string_like = nsCStringLike
+    );
+
+    fn fallible_append_utf8_to_latin1_lossy_impl(
+        &mut self,
+        other: &[u8],
+        old_len: usize,
+        maybe_num_ascii: Option<usize>,
+    ) -> Result<BulkWriteOk, ()> {
+        let new_len = old_len.checked_add(other.len()).ok_or(())?;
+        let num_ascii = maybe_num_ascii.unwrap_or(0);
+        // Already checked for overflow above, so this can't overflow.
+        let old_len_plus_num_ascii = old_len + num_ascii;
+        let mut handle = unsafe { self.bulk_write(new_len, old_len, false)? };
+        let written = {
+            let buffer = handle.as_mut_slice();
+            if num_ascii != 0 {
+                (&mut buffer[old_len..old_len_plus_num_ascii]).copy_from_slice(&other[..num_ascii]);
+            }
+            convert_utf8_to_latin1_lossy(&other[num_ascii..], &mut buffer[old_len_plus_num_ascii..])
+        };
+        Ok(handle.finish(old_len_plus_num_ascii + written, true))
+    }
+
+    /// Convert a UTF-8 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// replace the content of this string with the conversion result.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-8,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn assign_utf8_to_latin1_lossy<T: nsCStringLike + ?Sized>(&mut self, other: &T) {
+        self.fallible_append_utf8_to_latin1_lossy_check(other, 0)
+            .expect("Out of memory");
+    }
+
+    /// Convert a UTF-8 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// fallibly replace the content of this string with the conversion result.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-8,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn fallible_assign_utf8_to_latin1_lossy<T: nsCStringLike + ?Sized>(
+        &mut self,
+        other: &T,
+    ) -> Result<(), ()> {
+        self.fallible_append_utf8_to_latin1_lossy_check(other, 0)
+            .map(|_| ())
+    }
+
+    /// Convert a UTF-8 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// append the conversion result to this string.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-8,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn append_utf8_to_latin1_lossy<T: nsCStringLike + ?Sized>(&mut self, other: &T) {
+        let len = self.len();
+        self.fallible_append_utf8_to_latin1_lossy_check(other, len)
+            .expect("Out of memory");
+    }
+
+    /// Convert a UTF-8 string whose all code points are below U+0100 into
+    /// a Latin1 (scalar value is byte value; not windows-1252!) string and
+    /// fallibly append the conversion result to this string.
+    ///
+    /// # Panics
+    ///
+    /// If the input contains code points above U+00FF or is not valid UTF-8,
+    /// panics in debug mode and produces garbage in a memory-safe way in
+    /// release builds. The nature of the garbage may differ based on CPU
+    /// architecture and must not be relied upon.
+    pub fn fallible_append_utf8_to_latin1_lossy<T: nsCStringLike + ?Sized>(
+        &mut self,
+        other: &T,
+    ) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_utf8_to_latin1_lossy_check(other, len)
+            .map(|_| ())
+    }
+
+    // Latin1 to UTF-8 CString
+
+    ascii_copy_avoidance!(
+        name = fallible_append_latin1_to_utf8_check,
+        implementation = fallible_append_latin1_to_utf8_impl,
+        string_like = Latin1StringLike
+    );
+
+    fn fallible_append_latin1_to_utf8_impl(
+        &mut self,
+        other: &[u8],
+        old_len: usize,
+        maybe_num_ascii: Option<usize>,
+    ) -> Result<BulkWriteOk, ()> {
+        let (filled, read, mut handle) = if let Some(num_ascii) = maybe_num_ascii {
+            // Wrapper checked for ASCII
+            let left = other.len() - num_ascii;
+            let filled = old_len + num_ascii;
+            let needed = left.checked_mul(2).ok_or(())?;
+            let new_len = filled.checked_add(needed).ok_or(())?;
+            let mut handle = unsafe { self.bulk_write(new_len, old_len, false)? };
+            if num_ascii != 0 {
+                (&mut handle.as_mut_slice()[old_len..filled]).copy_from_slice(&other[..num_ascii]);
+            }
+            (filled, num_ascii, handle)
+        } else {
+            let worst_case_needed = if let Some(inline_capacity) = self.inline_capacity() {
+                let worst_case = other.len().checked_mul(2).ok_or(())?;
+                if worst_case <= inline_capacity {
+                    Some(worst_case)
+                } else {
+                    None
+                }
+            } else {
+                None
+            };
+            if worst_case_needed.is_none() && long_string_starts_with_ascii(other) {
+                // Wrapper didn't check for ASCII, so let's see if `other` starts with ASCII
+                // `other` starts with ASCII, so let's first size the buffer
+                // with optimism that it's ASCII-only.
+                let new_len_with_ascii = old_len.checked_add(other.len()).ok_or(())?;
+                let mut handle = unsafe { self.bulk_write(new_len_with_ascii, old_len, false)? };
+                let (read, written) =
+                    convert_latin1_to_utf8_partial(other, &mut handle.as_mut_slice()[old_len..]);
+                let left = other.len() - read;
+                let filled = old_len + written;
+                if left == 0 {
+                    // `other` fit in the initial allocation
+                    return Ok(handle.finish(filled, true));
+                }
+                let needed = left.checked_mul(2).ok_or(())?;
+                let new_len = filled.checked_add(needed).ok_or(())?;
+                unsafe {
+                    handle.restart_bulk_write(new_len, filled, false)?;
+                }
+                (filled, read, handle)
+            } else {
+                // Started with non-ASCII. Assume worst case.
+                let needed = if let Some(n) = worst_case_needed {
+                    n
+                } else {
+                    other.len().checked_mul(2).ok_or(())?
+                };
+                let new_len = old_len.checked_add(needed).ok_or(())?;
+                let handle = unsafe { self.bulk_write(new_len, old_len, false)? };
+                (old_len, 0, handle)
+            }
+        };
+        let written = convert_latin1_to_utf8(&other[read..], &mut handle.as_mut_slice()[filled..]);
+        Ok(handle.finish(filled + written, true))
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-8 and replace the content of this string with the conversion result.
+    pub fn assign_latin1_to_utf8<T: Latin1StringLike + ?Sized>(&mut self, other: &T) {
+        self.fallible_append_latin1_to_utf8_check(other, 0)
+            .expect("Out of memory");
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-8 and fallibly replace the content of this string with the
+    /// conversion result.
+    pub fn fallible_assign_latin1_to_utf8<T: Latin1StringLike + ?Sized>(
+        &mut self,
+        other: &T,
+    ) -> Result<(), ()> {
+        self.fallible_append_latin1_to_utf8_check(other, 0)
+            .map(|_| ())
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-8 and append the conversion result to this string.
+    pub fn append_latin1_to_utf8<T: Latin1StringLike + ?Sized>(&mut self, other: &T) {
+        let len = self.len();
+        self.fallible_append_latin1_to_utf8_check(other, len)
+            .expect("Out of memory");
+    }
+
+    /// Convert a Latin1 (i.e. byte value equals scalar value; not windows-1252!)
+    /// into UTF-8 and fallibly append the conversion result to this string.
+    pub fn fallible_append_latin1_to_utf8<T: Latin1StringLike + ?Sized>(
+        &mut self,
+        other: &T,
+    ) -> Result<(), ()> {
+        let len = self.len();
+        self.fallible_append_latin1_to_utf8_check(other, len)
+            .map(|_| ())
+    }
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nsstring_fallible_append_utf8_impl(
+    this: *mut nsAString,
+    other: *const u8,
+    other_len: usize,
+    old_len: usize,
+) -> bool {
+    let other_slice = slice::from_raw_parts(other, other_len);
+    (*this)
+        .fallible_append_utf8_impl(other_slice, old_len)
+        .is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nsstring_fallible_append_latin1_impl(
+    this: *mut nsAString,
+    other: *const u8,
+    other_len: usize,
+    old_len: usize,
+    allow_shrinking: bool,
+) -> bool {
+    let other_slice = slice::from_raw_parts(other, other_len);
+    (*this)
+        .fallible_append_latin1_impl(other_slice, old_len, allow_shrinking)
+        .is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nscstring_fallible_append_utf16_to_utf8_impl(
+    this: *mut nsACString,
+    other: *const u16,
+    other_len: usize,
+    old_len: usize,
+) -> bool {
+    let other_slice = slice::from_raw_parts(other, other_len);
+    (*this)
+        .fallible_append_utf16_to_utf8_impl(other_slice, old_len)
+        .is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nscstring_fallible_append_utf16_to_latin1_lossy_impl(
+    this: *mut nsACString,
+    other: *const u16,
+    other_len: usize,
+    old_len: usize,
+    allow_shrinking: bool,
+) -> bool {
+    let other_slice = slice::from_raw_parts(other, other_len);
+    (*this)
+        .fallible_append_utf16_to_latin1_lossy_impl(other_slice, old_len, allow_shrinking)
+        .is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nscstring_fallible_append_utf8_to_latin1_lossy_check(
+    this: *mut nsACString,
+    other: *const nsACString,
+    old_len: usize,
+) -> bool {
+    (*this)
+        .fallible_append_utf8_to_latin1_lossy_check(&*other, old_len)
+        .is_ok()
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn nscstring_fallible_append_latin1_to_utf8_check(
+    this: *mut nsACString,
+    other: *const nsACString,
+    old_len: usize,
+) -> bool {
+    (*this)
+        .fallible_append_latin1_to_utf8_check(&*other, old_len)
+        .is_ok()
+}
diff --git a/xpcom/rust/nsstring/src/lib.rs b/xpcom/rust/nsstring/src/lib.rs
new file mode 100644
index 0000000000..521c2c8c04
--- /dev/null
+++ b/xpcom/rust/nsstring/src/lib.rs
@@ -0,0 +1,1543 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+//! This module provides rust bindings for the XPCOM string types.
+//!
+//! # TL;DR (what types should I use)
+//!
+//! Use `&{mut,} nsA[C]String` for functions in rust which wish to take or
+//! mutate XPCOM strings. The other string types `Deref` to this type.
+//!
+//! Use `ns[C]String` (`ns[C]String` in C++) for string struct members, and as
+//! an intermediate between rust string data structures (such as `String` or
+//! `Vec<u16>`) and `&{mut,} nsA[C]String` (using `ns[C]String::from(value)`).
+//! These conversions will attempt to re-use the passed-in buffer, appending a
+//! null.
+//!
+//! Use `ns[C]Str` (`nsDependent[C]String` in C++) as an intermediate between
+//! borrowed rust data structures (such as `&str` and `&[u16]`) and `&{mut,}
+//! nsA[C]String` (using `ns[C]Str::from(value)`). These conversions should not
+//! perform any allocations. This type is not safe to share with `C++` as a
+//! struct field, but passing the borrowed `&{mut,} nsA[C]String` over FFI is
+//! safe.
+//!
+//! Use `*{const,mut} nsA[C]String` (`{const,} nsA[C]String*` in C++) for
+//! function arguments passed across the rust/C++ language boundary.
+//!
+//! There is currently no Rust equivalent to `nsAuto[C]String`. Implementing a
+//! type that contains a pointer to an inline buffer is difficult in Rust due
+//! to its move semantics, which require that it be safe to move a value by
+//! copying its bits. If such a type is genuinely needed at some point,
+//! <https://bugzilla.mozilla.org/show_bug.cgi?id=1403506#c6> has a sketch of
+//! how to emulate it via macros.
+//!
+//! # String Types
+//!
+//! ## `nsA[C]String`
+//!
+//! The core types in this module are `nsAString` and `nsACString`. These types
+//! are zero-sized as far as rust is concerned, and are safe to pass around
+//! behind both references (in rust code), and pointers (in C++ code). They
+//! represent a handle to a XPCOM string which holds either `u16` or `u8`
+//! characters respectively. The backing character buffer is guaranteed to live
+//! as long as the reference to the `nsAString` or `nsACString`.
+//!
+//! These types in rust are simply used as dummy types. References to them
+//! represent a pointer to the beginning of a variable-sized `#[repr(C)]` struct
+//! which is common between both C++ and Rust implementations. In C++, their
+//! corresponding types are also named `nsAString` or `nsACString`, and they are
+//! defined within the `nsTSubstring.{cpp,h}` file.
+//!
+//! ### Valid Operations
+//!
+//! An `&nsA[C]String` acts like rust's `&str`, in that it is a borrowed
+//! reference to the backing data. When used as an argument to other functions
+//! on `&mut nsA[C]String`, optimizations can be performed to avoid copying
+//! buffers, as information about the backing storage is preserved.
+//!
+//! An `&mut nsA[C]String` acts like rust's `&mut Cow<str>`, in that it is a
+//! mutable reference to a potentially borrowed string, which when modified will
+//! ensure that it owns its own backing storage. This type can be appended to
+//! with the methods `.append`, `.append_utf{8,16}`, and with the `write!`
+//! macro, and can be assigned to with `.assign`.
+//!
+//! ## `ns[C]Str<'a>`
+//!
+//! This type is an maybe-owned string type. It acts similarially to a
+//! `Cow<[{u8,u16}]>`. This type provides `Deref` and `DerefMut` implementations
+//! to `nsA[C]String`, which provides the methods for manipulating this type.
+//! This type's lifetime parameter, `'a`, represents the lifetime of the backing
+//! storage. When modified this type may re-allocate in order to ensure that it
+//! does not mutate its backing storage.
+//!
+//! `ns[C]Str`s can be constructed either with `ns[C]Str::new()`, which creates
+//! an empty `ns[C]Str<'static>`, or through one of the provided `From`
+//! implementations. Only `nsCStr` can be constructed `From<'a str>`, as
+//! constructing a `nsStr` would require transcoding. Use `ns[C]String` instead.
+//!
+//! When passing this type by reference, prefer passing a `&nsA[C]String` or
+//! `&mut nsA[C]String`. to passing this type.
+//!
+//! When passing this type across the language boundary, pass it as `*const
+//! nsA[C]String` for an immutable reference, or `*mut nsA[C]String` for a
+//! mutable reference.
+//!
+//! ## `ns[C]String`
+//!
+//! This type is an owned, null-terminated string type. This type provides
+//! `Deref` and `DerefMut` implementations to `nsA[C]String`, which provides the
+//! methods for manipulating this type.
+//!
+//! `ns[C]String`s can be constructed either with `ns[C]String::new()`, which
+//! creates an empty `ns[C]String`, or through one of the provided `From`
+//! implementations, which will try to avoid reallocating when possible,
+//! although a terminating `null` will be added.
+//!
+//! When passing this type by reference, prefer passing a `&nsA[C]String` or
+//! `&mut nsA[C]String`. to passing this type.
+//!
+//! When passing this type across the language boundary, pass it as `*const
+//! nsA[C]String` for an immutable reference, or `*mut nsA[C]String` for a
+//! mutable reference. This struct may also be included in `#[repr(C)]` structs
+//! shared with C++.
+//!
+//! ## `ns[C]StringRepr`
+//!
+//! This crate also provides the type `ns[C]StringRepr` which acts conceptually
+//! similar to an `ns[C]String`, however, it does not have a `Drop`
+//! implementation.
+//!
+//! If this type is dropped in rust, it will not free its backing storage. This
+//! can be useful when implementing FFI types which contain `ns[C]String` members
+//! which invoke their member's destructors through C++ code.
+
+#![allow(non_camel_case_types)]
+#![allow(clippy::missing_safety_doc)]
+#![allow(clippy::new_without_default)]
+#![allow(clippy::result_unit_err)]
+
+use bitflags::bitflags;
+use std::borrow;
+use std::cmp;
+use std::fmt;
+use std::marker::PhantomData;
+use std::mem;
+use std::ops::{Deref, DerefMut};
+use std::os::raw::c_void;
+use std::ptr;
+use std::slice;
+use std::str;
+
+mod conversions;
+
+pub use self::conversions::nscstring_fallible_append_latin1_to_utf8_check;
+pub use self::conversions::nscstring_fallible_append_utf16_to_latin1_lossy_impl;
+pub use self::conversions::nscstring_fallible_append_utf16_to_utf8_impl;
+pub use self::conversions::nscstring_fallible_append_utf8_to_latin1_lossy_check;
+pub use self::conversions::nsstring_fallible_append_latin1_impl;
+pub use self::conversions::nsstring_fallible_append_utf8_impl;
+
+/// A type for showing that `finish()` was called on a `BulkWriteHandle`.
+/// Instantiating this type from elsewhere is basically an assertion that
+/// there is no `BulkWriteHandle` around, so be very careful with instantiating
+/// this type!
+pub struct BulkWriteOk;
+
+/// Semi-arbitrary threshold below which we don't care about shrinking
+/// buffers to size. Currently matches `CACHE_LINE` in the `conversions`
+/// module.
+const SHRINKING_THRESHOLD: usize = 64;
+
+///////////////////////////////////
+// Internal Implementation Flags //
+///////////////////////////////////
+
+bitflags! {
+    // While this has the same layout as u16, it cannot be passed
+    // over FFI safely as a u16.
+    #[repr(C)]
+    struct DataFlags: u16 {
+        const TERMINATED = 1 << 0; // IsTerminated returns true
+        const VOIDED = 1 << 1; // IsVoid returns true
+        const REFCOUNTED = 1 << 2; // mData points to a heap-allocated, shareable, refcounted
+                                    // buffer
+        const OWNED = 1 << 3; // mData points to a heap-allocated, raw buffer
+        const INLINE = 1 << 4; // mData points to a writable, inline buffer
+        const LITERAL = 1 << 5; // mData points to a string literal; TERMINATED will also be set
+    }
+}
+
+bitflags! {
+    // While this has the same layout as u16, it cannot be passed
+    // over FFI safely as a u16.
+    #[repr(C)]
+    struct ClassFlags: u16 {
+        const INLINE = 1 << 0; // |this|'s buffer is inline
+        const NULL_TERMINATED = 1 << 1; // |this| requires its buffer is null-terminated
+    }
+}
+
+////////////////////////////////////
+// Generic String Bindings Macros //
+////////////////////////////////////
+
+macro_rules! string_like {
+    {
+        char_t = $char_t: ty;
+
+        AString = $AString: ident;
+        String = $String: ident;
+        Str = $Str: ident;
+
+        StringLike = $StringLike: ident;
+        StringAdapter = $StringAdapter: ident;
+    } => {
+        /// This trait is implemented on types which are `ns[C]String`-like, in
+        /// that they can at very low cost be converted to a borrowed
+        /// `&nsA[C]String`. Unfortunately, the intermediate type
+        /// `ns[C]StringAdapter` is required as well due to types like `&[u8]`
+        /// needing to be (cheaply) wrapped in a `nsCString` on the stack to
+        /// create the `&nsACString`.
+        ///
+        /// This trait is used to DWIM when calling the methods on
+        /// `nsA[C]String`.
+        pub trait $StringLike {
+            fn adapt(&self) -> $StringAdapter;
+        }
+
+        impl<'a, T: $StringLike + ?Sized> $StringLike for &'a T {
+            fn adapt(&self) -> $StringAdapter {
+                <T as $StringLike>::adapt(*self)
+            }
+        }
+
+        impl<'a, T> $StringLike for borrow::Cow<'a, T>
+            where T: $StringLike + borrow::ToOwned + ?Sized {
+            fn adapt(&self) -> $StringAdapter {
+                <T as $StringLike>::adapt(self.as_ref())
+            }
+        }
+
+        impl $StringLike for $AString {
+            fn adapt(&self) -> $StringAdapter {
+                $StringAdapter::Abstract(self)
+            }
+        }
+
+        impl<'a> $StringLike for $Str<'a> {
+            fn adapt(&self) -> $StringAdapter {
+                $StringAdapter::Abstract(self)
+            }
+        }
+
+        impl $StringLike for $String {
+            fn adapt(&self) -> $StringAdapter {
+                $StringAdapter::Abstract(self)
+            }
+        }
+
+        impl $StringLike for [$char_t] {
+            fn adapt(&self) -> $StringAdapter {
+                $StringAdapter::Borrowed($Str::from(self))
+            }
+        }
+
+        impl $StringLike for Vec<$char_t> {
+            fn adapt(&self) -> $StringAdapter {
+                $StringAdapter::Borrowed($Str::from(&self[..]))
+            }
+        }
+
+        impl $StringLike for Box<[$char_t]> {
+            fn adapt(&self) -> $StringAdapter {
+                $StringAdapter::Borrowed($Str::from(&self[..]))
+            }
+        }
+    }
+}
+
+impl<'a> Drop for nsAStringBulkWriteHandle<'a> {
+    /// This only runs in error cases. In success cases, `finish()`
+    /// calls `forget(self)`.
+    fn drop(&mut self) {
+        if self.capacity == 0 {
+            // If capacity is 0, the string is a zero-length
+            // string, so we have nothing to do.
+            return;
+        }
+        // The old zero terminator may be gone by now, so we need
+        // to write a new one somewhere and make length match.
+        // We can use a length between 1 and self.capacity.
+        // Seems prudent to overwrite the uninitialized memory.
+        // Using the length 1 leaves the shortest memory to overwrite.
+        // U+FFFD is the safest placeholder. Merely truncating the
+        // string to a zero-length string might be dangerous in some
+        // scenarios. See
+        // https://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences
+        // for closely related scenario.
+        unsafe {
+            let mut this = self.string.as_repr_mut();
+            this.as_mut().length = 1u32;
+            *(this.as_mut().data.as_mut()) = 0xFFFDu16;
+            *(this.as_mut().data.as_ptr().add(1)) = 0;
+        }
+    }
+}
+
+impl<'a> Drop for nsACStringBulkWriteHandle<'a> {
+    /// This only runs in error cases. In success cases, `finish()`
+    /// calls `forget(self)`.
+    fn drop(&mut self) {
+        if self.capacity == 0 {
+            // If capacity is 0, the string is a zero-length
+            // string, so we have nothing to do.
+            return;
+        }
+        // The old zero terminator may be gone by now, so we need
+        // to write a new one somewhere and make length match.
+        // We can use a length between 1 and self.capacity.
+        // Seems prudent to overwrite the uninitialized memory.
+        // Using the length 1 leaves the shortest memory to overwrite.
+        // U+FFFD is the safest placeholder, but when it doesn't fit,
+        // let's use ASCII substitute. Merely truncating the
+        // string to a zero-length string might be dangerous in some
+        // scenarios. See
+        // https://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences
+        // for closely related scenario.
+        unsafe {
+            let mut this = self.string.as_repr_mut();
+            if self.capacity >= 3 {
+                this.as_mut().length = 3u32;
+                *(this.as_mut().data.as_mut()) = 0xEFu8;
+                *(this.as_mut().data.as_ptr().add(1)) = 0xBFu8;
+                *(this.as_mut().data.as_ptr().add(2)) = 0xBDu8;
+                *(this.as_mut().data.as_ptr().add(3)) = 0;
+            } else {
+                this.as_mut().length = 1u32;
+                *(this.as_mut().data.as_mut()) = 0x1Au8; // U+FFFD doesn't fit
+                *(this.as_mut().data.as_ptr().add(1)) = 0;
+            }
+        }
+    }
+}
+
+macro_rules! define_string_types {
+    {
+        char_t = $char_t: ty;
+
+        AString = $AString: ident;
+        String = $String: ident;
+        Str = $Str: ident;
+
+        StringLike = $StringLike: ident;
+        StringAdapter = $StringAdapter: ident;
+
+        StringRepr = $StringRepr: ident;
+        AutoStringRepr = $AutoStringRepr: ident;
+
+        BulkWriteHandle = $BulkWriteHandle: ident;
+
+        drop = $drop: ident;
+        assign = $assign: ident, $fallible_assign: ident;
+        take_from = $take_from: ident, $fallible_take_from: ident;
+        append = $append: ident, $fallible_append: ident;
+        set_length = $set_length: ident, $fallible_set_length: ident;
+        begin_writing = $begin_writing: ident, $fallible_begin_writing: ident;
+        start_bulk_write = $start_bulk_write: ident;
+    } => {
+        /// The representation of a `ns[C]String` type in C++. This type is
+        /// used internally by our definition of `ns[C]String` to ensure layout
+        /// compatibility with the C++ `ns[C]String` type.
+        ///
+        /// This type may also be used in place of a C++ `ns[C]String` inside of
+        /// struct definitions which are shared with C++, as it has identical
+        /// layout to our `ns[C]String` type.
+        ///
+        /// This struct will leak its data if dropped from rust. See the module
+        /// documentation for more information on this type.
+        #[repr(C)]
+        #[derive(Debug)]
+        pub struct $StringRepr {
+            data: ptr::NonNull<$char_t>,
+            length: u32,
+            dataflags: DataFlags,
+            classflags: ClassFlags,
+        }
+
+        impl $StringRepr {
+            fn new(classflags: ClassFlags) -> $StringRepr {
+                static NUL: $char_t = 0;
+                $StringRepr {
+                    data: unsafe { ptr::NonNull::new_unchecked(&NUL as *const _ as *mut _) },
+                    length: 0,
+                    dataflags: DataFlags::TERMINATED | DataFlags::LITERAL,
+                    classflags,
+                }
+            }
+        }
+
+        impl Deref for $StringRepr {
+            type Target = $AString;
+            fn deref(&self) -> &$AString {
+                unsafe {
+                    &*(self as *const _ as *const $AString)
+                }
+            }
+        }
+
+        impl DerefMut for $StringRepr {
+            fn deref_mut(&mut self) -> &mut $AString {
+                unsafe {
+                    &mut *(self as *mut _ as *mut $AString)
+                }
+            }
+        }
+
+        #[repr(C)]
+        #[derive(Debug)]
+        pub struct $AutoStringRepr {
+            super_repr: $StringRepr,
+            inline_capacity: u32,
+        }
+
+        pub struct $BulkWriteHandle<'a> {
+            string: &'a mut $AString,
+            capacity: usize,
+        }
+
+        impl<'a> $BulkWriteHandle<'a> {
+            fn new(string: &'a mut $AString, capacity: usize) -> Self {
+                $BulkWriteHandle{ string, capacity }
+            }
+
+            pub unsafe fn restart_bulk_write(&mut self,
+                                             capacity: usize,
+                                             units_to_preserve: usize,
+                                             allow_shrinking: bool) -> Result<(), ()> {
+                self.capacity =
+                    self.string.start_bulk_write_impl(capacity,
+                                                      units_to_preserve,
+                                                      allow_shrinking)?;
+                Ok(())
+            }
+
+            pub fn finish(mut self, length: usize, allow_shrinking: bool) -> BulkWriteOk {
+                // NOTE: Drop is implemented outside the macro earlier in this file,
+                // because it needs to deal with different code unit representations
+                // for the REPLACEMENT CHARACTER in the UTF-16 and UTF-8 cases and
+                // needs to deal with a REPLACEMENT CHARACTER not fitting in the
+                // buffer in the UTF-8 case.
+                assert!(length <= self.capacity);
+                if length == 0 {
+                    // `truncate()` is OK even when the string
+                    // is in invalid state.
+                    self.string.truncate();
+                    mem::forget(self); // Don't run the failure path in drop()
+                    return BulkWriteOk{};
+                }
+                if allow_shrinking && length > SHRINKING_THRESHOLD {
+                    unsafe {
+                        let _ = self.restart_bulk_write(length, length, true);
+                    }
+                }
+                unsafe {
+                    let mut this = self.string.as_repr_mut();
+                    this.as_mut().length = length as u32;
+                    *(this.as_mut().data.as_ptr().add(length)) = 0;
+                    if cfg!(debug_assertions) {
+                        // Overwrite the unused part in debug builds. Note
+                        // that capacity doesn't include space for the zero
+                        // terminator, so starting after the zero-terminator
+                        // we wrote ends up overwriting the terminator space
+                        // not reflected in the capacity number.
+                        // write_bytes() takes care of multiplying the length
+                        // by the size of T.
+                        ptr::write_bytes(this.as_mut().data.as_ptr().add(length + 1),
+                                         0xE4u8,
+                                         self.capacity - length);
+                    }
+                    // We don't have a Rust interface for mozilla/MemoryChecking.h,
+                    // so let's just not communicate with MSan/Valgrind here.
+                }
+                mem::forget(self); // Don't run the failure path in drop()
+                BulkWriteOk{}
+            }
+
+            pub fn as_mut_slice(&mut self) -> &mut [$char_t] {
+                unsafe {
+                    let mut this = self.string.as_repr_mut();
+                    slice::from_raw_parts_mut(this.as_mut().data.as_ptr(), self.capacity)
+                }
+            }
+        }
+
+        /// This type is the abstract type which is used for interacting with
+        /// strings in rust. Each string type can derefence to an instance of
+        /// this type, which provides the useful operations on strings.
+        ///
+        /// NOTE: Rust thinks this type has a size of 0, because the data
+        /// associated with it is not necessarially safe to move. It is not safe
+        /// to construct a nsAString yourself, unless it is received by
+        /// dereferencing one of these types.
+        ///
+        /// NOTE: The `[u8; 0]` member is zero sized, and only exists to prevent
+        /// the construction by code outside of this module. It is used instead
+        /// of a private `()` member because the `improper_ctypes` lint complains
+        /// about some ZST members in `extern "C"` function declarations.
+        #[repr(C)]
+        pub struct $AString {
+            _prohibit_constructor: [u8; 0],
+        }
+
+        impl $AString {
+            /// Assign the value of `other` into self, overwriting any value
+            /// currently stored. Performs an optimized assignment when possible
+            /// if `other` is a `nsA[C]String`.
+            pub fn assign<T: $StringLike + ?Sized>(&mut self, other: &T) {
+                unsafe { $assign(self, other.adapt().as_ptr()) };
+            }
+
+            /// Assign the value of `other` into self, overwriting any value
+            /// currently stored. Performs an optimized assignment when possible
+            /// if `other` is a `nsA[C]String`.
+            ///
+            /// Returns Ok(()) on success, and Err(()) if the allocation failed.
+            pub fn fallible_assign<T: $StringLike + ?Sized>(&mut self, other: &T) -> Result<(), ()> {
+                if unsafe { $fallible_assign(self, other.adapt().as_ptr()) } {
+                    Ok(())
+                } else {
+                    Err(())
+                }
+            }
+
+            /// Take the value of `other` and set `self`, overwriting any value
+            /// currently stored. The passed-in string will be truncated.
+            pub fn take_from(&mut self, other: &mut $AString) {
+                unsafe { $take_from(self, other) };
+            }
+
+            /// Take the value of `other` and set `self`, overwriting any value
+            /// currently stored. If this function fails, the source string will
+            /// be left untouched, otherwise it will be truncated.
+            ///
+            /// Returns Ok(()) on success, and Err(()) if the allocation failed.
+            pub fn fallible_take_from(&mut self, other: &mut $AString) -> Result<(), ()> {
+                if unsafe { $fallible_take_from(self, other) } {
+                    Ok(())
+                } else {
+                    Err(())
+                }
+            }
+
+            /// Append the value of `other` into self.
+            pub fn append<T: $StringLike + ?Sized>(&mut self, other: &T) {
+                unsafe { $append(self, other.adapt().as_ptr()) };
+            }
+
+            /// Append the value of `other` into self.
+            ///
+            /// Returns Ok(()) on success, and Err(()) if the allocation failed.
+            pub fn fallible_append<T: $StringLike + ?Sized>(&mut self, other: &T) -> Result<(), ()> {
+                if unsafe { $fallible_append(self, other.adapt().as_ptr()) } {
+                    Ok(())
+                } else {
+                    Err(())
+                }
+            }
+
+            /// Mark the string's data as void. If `true`, the string will be truncated.
+            ///
+            /// A void string is generally converted to a `null` JS value by bindings code.
+            pub fn set_is_void(&mut self, is_void: bool) {
+                if is_void {
+                    self.truncate();
+                }
+                unsafe {
+                    self.as_repr_mut().as_mut().dataflags.set(DataFlags::VOIDED, is_void);
+                }
+            }
+
+            /// Returns whether the string's data is voided.
+            pub fn is_void(&self) -> bool {
+                self.as_repr().dataflags.contains(DataFlags::VOIDED)
+            }
+
+            /// Set the length of the string to the passed-in length, and expand
+            /// the backing capacity to match. This method is unsafe as it can
+            /// expose uninitialized memory when len is greater than the current
+            /// length of the string.
+            pub unsafe fn set_length(&mut self, len: u32) {
+                $set_length(self, len);
+            }
+
+            /// Set the length of the string to the passed-in length, and expand
+            /// the backing capacity to match. This method is unsafe as it can
+            /// expose uninitialized memory when len is greater than the current
+            /// length of the string.
+            ///
+            /// Returns Ok(()) on success, and Err(()) if the allocation failed.
+            pub unsafe fn fallible_set_length(&mut self, len: u32) -> Result<(), ()> {
+                if $fallible_set_length(self, len) {
+                    Ok(())
+                } else {
+                    Err(())
+                }
+            }
+
+            pub fn truncate(&mut self) {
+                unsafe {
+                    self.set_length(0);
+                }
+            }
+
+            /// Get a `&mut` reference to the backing data for this string.
+            /// This method will allocate and copy if the current backing buffer
+            /// is immutable or shared.
+            pub fn to_mut(&mut self) -> &mut [$char_t] {
+                unsafe {
+                    let len = self.len();
+                    if len == 0 {
+                        // Use an arbitrary but aligned non-null value as the pointer
+                        slice::from_raw_parts_mut(ptr::NonNull::<$char_t>::dangling().as_ptr(), 0)
+                    } else {
+                        slice::from_raw_parts_mut($begin_writing(self), len)
+                    }
+                }
+            }
+
+            /// Get a `&mut` reference to the backing data for this string.
+            /// This method will allocate and copy if the current backing buffer
+            /// is immutable or shared.
+            ///
+            /// Returns `Ok(&mut [T])` on success, and `Err(())` if the
+            /// allocation failed.
+            pub fn fallible_to_mut(&mut self) -> Result<&mut [$char_t], ()> {
+                unsafe {
+                    let len = self.len();
+                    if len == 0 {
+                        // Use an arbitrary but aligned non-null value as the pointer
+                        Ok(slice::from_raw_parts_mut(
+                            ptr::NonNull::<$char_t>::dangling().as_ptr() as *mut $char_t, 0))
+                    } else {
+                        let ptr = $fallible_begin_writing(self);
+                        if ptr.is_null() {
+                            Err(())
+                        } else {
+                            Ok(slice::from_raw_parts_mut(ptr, len))
+                        }
+                    }
+                }
+            }
+
+            /// Unshares the buffer of the string and returns a handle
+            /// from which a writable slice whose length is the rounded-up
+            /// capacity can be obtained.
+            ///
+            /// Fails also if the new length doesn't fit in 32 bits.
+            ///
+            /// # Safety
+            ///
+            /// Unsafe because of exposure of uninitialized memory.
+            pub unsafe fn bulk_write(&mut self,
+                                     capacity: usize,
+                                     units_to_preserve: usize,
+                                     allow_shrinking: bool) -> Result<$BulkWriteHandle, ()> {
+                let capacity =
+                    self.start_bulk_write_impl(capacity, units_to_preserve, allow_shrinking)?;
+                Ok($BulkWriteHandle::new(self, capacity))
+            }
+
+            unsafe fn start_bulk_write_impl(&mut self,
+                                            capacity: usize,
+                                            units_to_preserve: usize,
+                                            allow_shrinking: bool) -> Result<usize, ()> {
+                if capacity > u32::MAX as usize {
+                    Err(())
+                } else {
+                    let capacity32 = capacity as u32;
+                    let rounded = $start_bulk_write(self,
+                                                    capacity32,
+                                                    units_to_preserve as u32,
+                                                    allow_shrinking && capacity > SHRINKING_THRESHOLD);
+                    if rounded == u32::MAX {
+                        return Err(())
+                    }
+                    Ok(rounded as usize)
+                }
+            }
+
+            fn as_repr(&self) -> &$StringRepr {
+                // All $AString values point to a struct prefix which is
+                // identical to $StringRepr, thus we can cast `self`
+                // into *const $StringRepr to get the reference to the
+                // underlying data.
+                unsafe {
+                    &*(self as *const _ as *const $StringRepr)
+                }
+            }
+
+            fn as_repr_mut(&mut self) -> ptr::NonNull<$StringRepr> {
+                unsafe { ptr::NonNull::new_unchecked(self as *mut _ as *mut $StringRepr)}
+            }
+
+            fn as_auto_string_repr(&self) -> Option<&$AutoStringRepr> {
+                if !self.as_repr().classflags.contains(ClassFlags::INLINE) {
+                    return None;
+                }
+
+                unsafe {
+                    Some(&*(self as *const _ as *const $AutoStringRepr))
+                }
+            }
+
+            /// If this is an autostring, returns the capacity (excluding the
+            /// zero terminator) of the inline buffer within `Some()`. Otherwise
+            /// returns `None`.
+            pub fn inline_capacity(&self) -> Option<usize> {
+                Some(self.as_auto_string_repr()?.inline_capacity as usize)
+            }
+        }
+
+        impl Deref for $AString {
+            type Target = [$char_t];
+            fn deref(&self) -> &[$char_t] {
+                unsafe {
+                    // All $AString values point to a struct prefix which is
+                    // identical to $StringRepr, thus we can cast `self`
+                    // into *const $StringRepr to get the reference to the
+                    // underlying data.
+                    let this = &*(self as *const _ as *const $StringRepr);
+                    slice::from_raw_parts(this.data.as_ptr(), this.length as usize)
+                }
+            }
+        }
+
+        impl AsRef<[$char_t]> for $AString {
+            fn as_ref(&self) -> &[$char_t] {
+                self
+            }
+        }
+
+        impl cmp::PartialEq for $AString {
+            fn eq(&self, other: &$AString) -> bool {
+                &self[..] == &other[..]
+            }
+        }
+
+        impl cmp::PartialEq<[$char_t]> for $AString {
+            fn eq(&self, other: &[$char_t]) -> bool {
+                &self[..] == other
+            }
+        }
+
+        impl cmp::PartialEq<$String> for $AString {
+            fn eq(&self, other: &$String) -> bool {
+                self.eq(&**other)
+            }
+        }
+
+        impl<'a> cmp::PartialEq<$Str<'a>> for $AString {
+            fn eq(&self, other: &$Str<'a>) -> bool {
+                self.eq(&**other)
+            }
+        }
+
+        #[repr(C)]
+        pub struct $Str<'a> {
+            hdr: $StringRepr,
+            _marker: PhantomData<&'a [$char_t]>,
+        }
+
+        impl $Str<'static> {
+            pub fn new() -> $Str<'static> {
+                $Str {
+                    hdr: $StringRepr::new(ClassFlags::empty()),
+                    _marker: PhantomData,
+                }
+            }
+        }
+
+        impl<'a> Drop for $Str<'a> {
+            fn drop(&mut self) {
+                unsafe {
+                    $drop(&mut **self);
+                }
+            }
+        }
+
+        impl<'a> Deref for $Str<'a> {
+            type Target = $AString;
+            fn deref(&self) -> &$AString {
+                &self.hdr
+            }
+        }
+
+        impl<'a> DerefMut for $Str<'a> {
+            fn deref_mut(&mut self) -> &mut $AString {
+                &mut self.hdr
+            }
+        }
+
+        impl<'a> AsRef<[$char_t]> for $Str<'a> {
+            fn as_ref(&self) -> &[$char_t] {
+                &self
+            }
+        }
+
+        impl<'a> From<&'a [$char_t]> for $Str<'a> {
+            fn from(s: &'a [$char_t]) -> $Str<'a> {
+                assert!(s.len() < (u32::MAX as usize));
+                if s.is_empty() {
+                    return $Str::new();
+                }
+                $Str {
+                    hdr: $StringRepr {
+                        data: unsafe { ptr::NonNull::new_unchecked(s.as_ptr() as *mut _) },
+                        length: s.len() as u32,
+                        dataflags: DataFlags::empty(),
+                        classflags: ClassFlags::empty(),
+                    },
+                    _marker: PhantomData,
+                }
+            }
+        }
+
+        impl<'a> From<&'a Vec<$char_t>> for $Str<'a> {
+            fn from(s: &'a Vec<$char_t>) -> $Str<'a> {
+                $Str::from(&s[..])
+            }
+        }
+
+        impl<'a> From<&'a $AString> for $Str<'a> {
+            fn from(s: &'a $AString) -> $Str<'a> {
+                $Str::from(&s[..])
+            }
+        }
+
+        impl<'a> fmt::Write for $Str<'a> {
+            fn write_str(&mut self, s: &str) -> Result<(), fmt::Error> {
+                $AString::write_str(self, s)
+            }
+        }
+
+        impl<'a> fmt::Display for $Str<'a> {
+            fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+                <$AString as fmt::Display>::fmt(self, f)
+            }
+        }
+
+        impl<'a> fmt::Debug for $Str<'a> {
+            fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+                <$AString as fmt::Debug>::fmt(self, f)
+            }
+        }
+
+        impl<'a> cmp::PartialEq for $Str<'a> {
+            fn eq(&self, other: &$Str<'a>) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl<'a> cmp::PartialEq<[$char_t]> for $Str<'a> {
+            fn eq(&self, other: &[$char_t]) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl<'a, 'b> cmp::PartialEq<&'b [$char_t]> for $Str<'a> {
+            fn eq(&self, other: &&'b [$char_t]) -> bool {
+                $AString::eq(self, *other)
+            }
+        }
+
+        impl<'a> cmp::PartialEq<str> for $Str<'a> {
+            fn eq(&self, other: &str) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl<'a, 'b> cmp::PartialEq<&'b str> for $Str<'a> {
+            fn eq(&self, other: &&'b str) -> bool {
+                $AString::eq(self, *other)
+            }
+        }
+
+        #[repr(C)]
+        pub struct $String {
+            hdr: $StringRepr,
+        }
+
+        unsafe impl Send for $String {}
+        unsafe impl Sync for $String {}
+
+        impl $String {
+            pub fn new() -> $String {
+                $String {
+                    hdr: $StringRepr::new(ClassFlags::NULL_TERMINATED),
+                }
+            }
+
+            /// Converts this String into a StringRepr, which will leak if the
+            /// repr is not passed to something that knows how to free it.
+            pub fn into_repr(mut self) -> $StringRepr {
+                mem::replace(&mut self.hdr, $StringRepr::new(ClassFlags::NULL_TERMINATED))
+            }
+        }
+
+        impl Drop for $String {
+            fn drop(&mut self) {
+                unsafe {
+                    $drop(&mut **self);
+                }
+            }
+        }
+
+        impl Deref for $String {
+            type Target = $AString;
+            fn deref(&self) -> &$AString {
+                &self.hdr
+            }
+        }
+
+        impl DerefMut for $String {
+            fn deref_mut(&mut self) -> &mut $AString {
+                &mut self.hdr
+            }
+        }
+
+        impl Clone for $String {
+            fn clone(&self) -> Self {
+                let mut copy = $String::new();
+                copy.assign(self);
+                copy
+            }
+        }
+
+        impl AsRef<[$char_t]> for $String {
+            fn as_ref(&self) -> &[$char_t] {
+                &self
+            }
+        }
+
+        impl<'a> From<&'a [$char_t]> for $String {
+            fn from(s: &'a [$char_t]) -> $String {
+                let mut res = $String::new();
+                res.assign(&$Str::from(&s[..]));
+                res
+            }
+        }
+
+        impl<'a> From<&'a Vec<$char_t>> for $String {
+            fn from(s: &'a Vec<$char_t>) -> $String {
+                $String::from(&s[..])
+            }
+        }
+
+        impl<'a> From<&'a $AString> for $String {
+            fn from(s: &'a $AString) -> $String {
+                $String::from(&s[..])
+            }
+        }
+
+        impl From<Box<[$char_t]>> for $String {
+            fn from(s: Box<[$char_t]>) -> $String {
+                s.into_vec().into()
+            }
+        }
+
+        impl From<Vec<$char_t>> for $String {
+            fn from(mut s: Vec<$char_t>) -> $String {
+                assert!(s.len() < (u32::MAX as usize));
+                if s.is_empty() {
+                    return $String::new();
+                }
+
+                let length = s.len() as u32;
+                s.push(0); // null terminator
+
+                // SAFETY NOTE: This method produces an data_flags::OWNED
+                // ns[C]String from a Box<[$char_t]>. this is only safe
+                // because in the Gecko tree, we use the same allocator for
+                // Rust code as for C++ code, meaning that our box can be
+                // legally freed with libc::free().
+                let ptr = s.as_mut_ptr();
+                mem::forget(s);
+                unsafe {
+                    Gecko_IncrementStringAdoptCount(ptr as *mut _);
+                }
+                $String {
+                    hdr: $StringRepr {
+                        data: unsafe { ptr::NonNull::new_unchecked(ptr) },
+                        length,
+                        dataflags: DataFlags::OWNED | DataFlags::TERMINATED,
+                        classflags: ClassFlags::NULL_TERMINATED,
+                    }
+                }
+            }
+        }
+
+        impl fmt::Write for $String {
+            fn write_str(&mut self, s: &str) -> Result<(), fmt::Error> {
+                $AString::write_str(self, s)
+            }
+        }
+
+        impl fmt::Display for $String {
+            fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+                <$AString as fmt::Display>::fmt(self, f)
+            }
+        }
+
+        impl fmt::Debug for $String {
+            fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+                <$AString as fmt::Debug>::fmt(self, f)
+            }
+        }
+
+        impl cmp::PartialEq for $String {
+            fn eq(&self, other: &$String) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl cmp::PartialEq<[$char_t]> for $String {
+            fn eq(&self, other: &[$char_t]) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl<'a> cmp::PartialEq<&'a [$char_t]> for $String {
+            fn eq(&self, other: &&'a [$char_t]) -> bool {
+                $AString::eq(self, *other)
+            }
+        }
+
+        impl cmp::PartialEq<str> for $String {
+            fn eq(&self, other: &str) -> bool {
+                $AString::eq(self, other)
+            }
+        }
+
+        impl<'a> cmp::PartialEq<&'a str> for $String {
+            fn eq(&self, other: &&'a str) -> bool {
+                $AString::eq(self, *other)
+            }
+        }
+
+        /// An adapter type to allow for passing both types which coerce to
+        /// &[$char_type], and &$AString to a function, while still performing
+        /// optimized operations when passed the $AString.
+        pub enum $StringAdapter<'a> {
+            Borrowed($Str<'a>),
+            Abstract(&'a $AString),
+        }
+
+        impl<'a> $StringAdapter<'a> {
+            fn as_ptr(&self) -> *const $AString {
+                &**self
+            }
+        }
+
+        impl<'a> Deref for $StringAdapter<'a> {
+            type Target = $AString;
+
+            fn deref(&self) -> &$AString {
+                match *self {
+                    $StringAdapter::Borrowed(ref s) => s,
+                    $StringAdapter::Abstract(ref s) => s,
+                }
+            }
+        }
+
+        impl<'a> $StringAdapter<'a> {
+            #[allow(dead_code)]
+            fn is_abstract(&self) -> bool {
+                match *self {
+                    $StringAdapter::Borrowed(_) => false,
+                    $StringAdapter::Abstract(_) => true,
+                }
+            }
+        }
+
+        string_like! {
+            char_t = $char_t;
+
+            AString = $AString;
+            String = $String;
+            Str = $Str;
+
+            StringLike = $StringLike;
+            StringAdapter = $StringAdapter;
+        }
+    }
+}
+
+///////////////////////////////////////////
+// Bindings for nsCString (u8 char type) //
+///////////////////////////////////////////
+
+define_string_types! {
+    char_t = u8;
+
+    AString = nsACString;
+    String = nsCString;
+    Str = nsCStr;
+
+    StringLike = nsCStringLike;
+    StringAdapter = nsCStringAdapter;
+
+    StringRepr = nsCStringRepr;
+    AutoStringRepr = nsAutoCStringRepr;
+
+    BulkWriteHandle = nsACStringBulkWriteHandle;
+
+    drop = Gecko_FinalizeCString;
+    assign = Gecko_AssignCString, Gecko_FallibleAssignCString;
+    take_from = Gecko_TakeFromCString, Gecko_FallibleTakeFromCString;
+    append = Gecko_AppendCString, Gecko_FallibleAppendCString;
+    set_length = Gecko_SetLengthCString, Gecko_FallibleSetLengthCString;
+    begin_writing = Gecko_BeginWritingCString, Gecko_FallibleBeginWritingCString;
+    start_bulk_write = Gecko_StartBulkWriteCString;
+}
+
+impl nsACString {
+    /// Gets a CString as an utf-8 str or a String, trying to avoid copies, and
+    /// replacing invalid unicode sequences with replacement characters.
+    #[inline]
+    pub fn to_utf8(&self) -> borrow::Cow<str> {
+        String::from_utf8_lossy(&self[..])
+    }
+
+    #[inline]
+    pub unsafe fn as_str_unchecked(&self) -> &str {
+        if cfg!(debug_assertions) {
+            str::from_utf8(self).expect("Should be utf-8")
+        } else {
+            str::from_utf8_unchecked(self)
+        }
+    }
+}
+
+impl<'a> From<&'a str> for nsCStr<'a> {
+    fn from(s: &'a str) -> nsCStr<'a> {
+        s.as_bytes().into()
+    }
+}
+
+impl<'a> From<&'a String> for nsCStr<'a> {
+    fn from(s: &'a String) -> nsCStr<'a> {
+        nsCStr::from(&s[..])
+    }
+}
+
+impl<'a> From<&'a str> for nsCString {
+    fn from(s: &'a str) -> nsCString {
+        s.as_bytes().into()
+    }
+}
+
+impl<'a> From<&'a String> for nsCString {
+    fn from(s: &'a String) -> nsCString {
+        nsCString::from(&s[..])
+    }
+}
+
+impl From<Box<str>> for nsCString {
+    fn from(s: Box<str>) -> nsCString {
+        s.into_string().into()
+    }
+}
+
+impl From<String> for nsCString {
+    fn from(s: String) -> nsCString {
+        s.into_bytes().into()
+    }
+}
+
+// Support for the write!() macro for appending to nsACStrings
+impl fmt::Write for nsACString {
+    fn write_str(&mut self, s: &str) -> Result<(), fmt::Error> {
+        self.append(s);
+        Ok(())
+    }
+}
+
+impl fmt::Display for nsACString {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        fmt::Display::fmt(&self.to_utf8(), f)
+    }
+}
+
+impl fmt::Debug for nsACString {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        fmt::Debug::fmt(&self.to_utf8(), f)
+    }
+}
+
+impl cmp::PartialEq<str> for nsACString {
+    fn eq(&self, other: &str) -> bool {
+        &self[..] == other.as_bytes()
+    }
+}
+
+impl nsCStringLike for str {
+    fn adapt(&self) -> nsCStringAdapter {
+        nsCStringAdapter::Borrowed(nsCStr::from(self))
+    }
+}
+
+impl nsCStringLike for String {
+    fn adapt(&self) -> nsCStringAdapter {
+        nsCStringAdapter::Borrowed(nsCStr::from(&self[..]))
+    }
+}
+
+impl nsCStringLike for Box<str> {
+    fn adapt(&self) -> nsCStringAdapter {
+        nsCStringAdapter::Borrowed(nsCStr::from(&self[..]))
+    }
+}
+
+// This trait is implemented on types which are Latin1 `nsCString`-like,
+// in that they can at very low cost be converted to a borrowed
+// `&nsACString` and do not denote UTF-8ness in the Rust type system.
+//
+// This trait is used to DWIM when calling the methods on
+// `nsACString`.
+string_like! {
+    char_t = u8;
+
+    AString = nsACString;
+    String = nsCString;
+    Str = nsCStr;
+
+    StringLike = Latin1StringLike;
+    StringAdapter = nsCStringAdapter;
+}
+
+///////////////////////////////////////////
+// Bindings for nsString (u16 char type) //
+///////////////////////////////////////////
+
+define_string_types! {
+    char_t = u16;
+
+    AString = nsAString;
+    String = nsString;
+    Str = nsStr;
+
+    StringLike = nsStringLike;
+    StringAdapter = nsStringAdapter;
+
+    StringRepr = nsStringRepr;
+    AutoStringRepr = nsAutoStringRepr;
+
+    BulkWriteHandle = nsAStringBulkWriteHandle;
+
+    drop = Gecko_FinalizeString;
+    assign = Gecko_AssignString, Gecko_FallibleAssignString;
+    take_from = Gecko_TakeFromString, Gecko_FallibleTakeFromString;
+    append = Gecko_AppendString, Gecko_FallibleAppendString;
+    set_length = Gecko_SetLengthString, Gecko_FallibleSetLengthString;
+    begin_writing = Gecko_BeginWritingString, Gecko_FallibleBeginWritingString;
+    start_bulk_write = Gecko_StartBulkWriteString;
+}
+
+// NOTE: The From impl for a string slice for nsString produces a <'static>
+// lifetime, as it allocates.
+impl<'a> From<&'a str> for nsString {
+    fn from(s: &'a str) -> nsString {
+        s.encode_utf16().collect::<Vec<u16>>().into()
+    }
+}
+
+impl<'a> From<&'a String> for nsString {
+    fn from(s: &'a String) -> nsString {
+        nsString::from(&s[..])
+    }
+}
+
+// Support for the write!() macro for writing to nsStrings
+impl fmt::Write for nsAString {
+    fn write_str(&mut self, s: &str) -> Result<(), fmt::Error> {
+        // Directly invoke gecko's routines for appending utf8 strings to
+        // nsAString values, to avoid as much overhead as possible
+        self.append_str(s);
+        Ok(())
+    }
+}
+
+impl nsAString {
+    /// Turns this utf-16 string into a string, replacing invalid unicode
+    /// sequences with replacement characters.
+    ///
+    /// This is needed because the default ToString implementation goes through
+    /// fmt::Display, and thus allocates the string twice.
+    #[allow(clippy::inherent_to_string_shadow_display)]
+    pub fn to_string(&self) -> String {
+        String::from_utf16_lossy(&self[..])
+    }
+}
+
+impl fmt::Display for nsAString {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        fmt::Display::fmt(&self.to_string(), f)
+    }
+}
+
+impl fmt::Debug for nsAString {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        fmt::Debug::fmt(&self.to_string(), f)
+    }
+}
+
+impl cmp::PartialEq<str> for nsAString {
+    fn eq(&self, other: &str) -> bool {
+        other.encode_utf16().eq(self.iter().cloned())
+    }
+}
+
+#[cfg(not(feature = "gecko_debug"))]
+#[allow(non_snake_case)]
+unsafe fn Gecko_IncrementStringAdoptCount(_: *mut c_void) {}
+
+extern "C" {
+    #[cfg(feature = "gecko_debug")]
+    fn Gecko_IncrementStringAdoptCount(data: *mut c_void);
+
+    // Gecko implementation in nsSubstring.cpp
+    fn Gecko_FinalizeCString(this: *mut nsACString);
+
+    fn Gecko_AssignCString(this: *mut nsACString, other: *const nsACString);
+    fn Gecko_TakeFromCString(this: *mut nsACString, other: *mut nsACString);
+    fn Gecko_AppendCString(this: *mut nsACString, other: *const nsACString);
+    fn Gecko_SetLengthCString(this: *mut nsACString, length: u32);
+    fn Gecko_BeginWritingCString(this: *mut nsACString) -> *mut u8;
+    fn Gecko_FallibleAssignCString(this: *mut nsACString, other: *const nsACString) -> bool;
+    fn Gecko_FallibleTakeFromCString(this: *mut nsACString, other: *mut nsACString) -> bool;
+    fn Gecko_FallibleAppendCString(this: *mut nsACString, other: *const nsACString) -> bool;
+    fn Gecko_FallibleSetLengthCString(this: *mut nsACString, length: u32) -> bool;
+    fn Gecko_FallibleBeginWritingCString(this: *mut nsACString) -> *mut u8;
+    fn Gecko_StartBulkWriteCString(
+        this: *mut nsACString,
+        capacity: u32,
+        units_to_preserve: u32,
+        allow_shrinking: bool,
+    ) -> u32;
+
+    fn Gecko_FinalizeString(this: *mut nsAString);
+
+    fn Gecko_AssignString(this: *mut nsAString, other: *const nsAString);
+    fn Gecko_TakeFromString(this: *mut nsAString, other: *mut nsAString);
+    fn Gecko_AppendString(this: *mut nsAString, other: *const nsAString);
+    fn Gecko_SetLengthString(this: *mut nsAString, length: u32);
+    fn Gecko_BeginWritingString(this: *mut nsAString) -> *mut u16;
+    fn Gecko_FallibleAssignString(this: *mut nsAString, other: *const nsAString) -> bool;
+    fn Gecko_FallibleTakeFromString(this: *mut nsAString, other: *mut nsAString) -> bool;
+    fn Gecko_FallibleAppendString(this: *mut nsAString, other: *const nsAString) -> bool;
+    fn Gecko_FallibleSetLengthString(this: *mut nsAString, length: u32) -> bool;
+    fn Gecko_FallibleBeginWritingString(this: *mut nsAString) -> *mut u16;
+    fn Gecko_StartBulkWriteString(
+        this: *mut nsAString,
+        capacity: u32,
+        units_to_preserve: u32,
+        allow_shrinking: bool,
+    ) -> u32;
+}
+
+//////////////////////////////////////
+// Repr Validation Helper Functions //
+//////////////////////////////////////
+
+pub mod test_helpers {
+    //! This module only exists to help with ensuring that the layout of the
+    //! structs inside of rust and C++ are identical.
+    //!
+    //! It is public to ensure that these testing functions are avaliable to
+    //! gtest code.
+
+    use super::{nsACString, nsAString};
+    use super::{nsCStr, nsCString, nsCStringRepr};
+    use super::{nsStr, nsString, nsStringRepr};
+    use super::{ClassFlags, DataFlags};
+    use std::mem;
+
+    /// Generates an #[no_mangle] extern "C" function which returns the size and
+    /// alignment of the given type with the given name.
+    macro_rules! size_align_check {
+        ($T:ty, $fname:ident) => {
+            #[no_mangle]
+            #[allow(non_snake_case)]
+            pub unsafe extern "C" fn $fname(size: *mut usize, align: *mut usize) {
+                *size = mem::size_of::<$T>();
+                *align = mem::align_of::<$T>();
+            }
+        };
+        ($T:ty, $U:ty, $V:ty, $fname:ident) => {
+            #[no_mangle]
+            #[allow(non_snake_case)]
+            pub unsafe extern "C" fn $fname(size: *mut usize, align: *mut usize) {
+                *size = mem::size_of::<$T>();
+                *align = mem::align_of::<$T>();
+
+                assert_eq!(*size, mem::size_of::<$U>());
+                assert_eq!(*align, mem::align_of::<$U>());
+                assert_eq!(*size, mem::size_of::<$V>());
+                assert_eq!(*align, mem::align_of::<$V>());
+            }
+        };
+    }
+
+    size_align_check!(
+        nsStringRepr,
+        nsString,
+        nsStr<'static>,
+        Rust_Test_ReprSizeAlign_nsString
+    );
+    size_align_check!(
+        nsCStringRepr,
+        nsCString,
+        nsCStr<'static>,
+        Rust_Test_ReprSizeAlign_nsCString
+    );
+
+    /// Generates a $[no_mangle] extern "C" function which returns the size,
+    /// alignment and offset in the parent struct of a given member, with the
+    /// given name.
+    ///
+    /// This method can trigger Undefined Behavior if the accessing the member
+    /// $member on a given type would use that type's `Deref` implementation.
+    macro_rules! member_check {
+        ($T:ty, $U:ty, $V:ty, $member:ident, $method:ident) => {
+            #[no_mangle]
+            #[allow(non_snake_case)]
+            pub unsafe extern "C" fn $method(
+                size: *mut usize,
+                align: *mut usize,
+                offset: *mut usize,
+            ) {
+                // Create a temporary value of type T to get offsets, sizes
+                // and alignments from.
+                let tmp: mem::MaybeUninit<$T> = mem::MaybeUninit::uninit();
+                // FIXME: This should use &raw references when available,
+                // this is technically UB as it creates a reference to
+                // uninitialized memory, but there's no better way to do
+                // this right now.
+                let tmp = &*tmp.as_ptr();
+                *size = mem::size_of_val(&tmp.$member);
+                *align = mem::align_of_val(&tmp.$member);
+                *offset = (&tmp.$member as *const _ as usize) - (tmp as *const $T as usize);
+
+                let tmp: mem::MaybeUninit<$U> = mem::MaybeUninit::uninit();
+                let tmp = &*tmp.as_ptr();
+                assert_eq!(*size, mem::size_of_val(&tmp.hdr.$member));
+                assert_eq!(*align, mem::align_of_val(&tmp.hdr.$member));
+                assert_eq!(
+                    *offset,
+                    (&tmp.hdr.$member as *const _ as usize) - (tmp as *const $U as usize)
+                );
+
+                let tmp: mem::MaybeUninit<$V> = mem::MaybeUninit::uninit();
+                let tmp = &*tmp.as_ptr();
+                assert_eq!(*size, mem::size_of_val(&tmp.hdr.$member));
+                assert_eq!(*align, mem::align_of_val(&tmp.hdr.$member));
+                assert_eq!(
+                    *offset,
+                    (&tmp.hdr.$member as *const _ as usize) - (tmp as *const $V as usize)
+                );
+            }
+        };
+    }
+
+    member_check!(
+        nsStringRepr,
+        nsString,
+        nsStr<'static>,
+        data,
+        Rust_Test_Member_nsString_mData
+    );
+    member_check!(
+        nsStringRepr,
+        nsString,
+        nsStr<'static>,
+        length,
+        Rust_Test_Member_nsString_mLength
+    );
+    member_check!(
+        nsStringRepr,
+        nsString,
+        nsStr<'static>,
+        dataflags,
+        Rust_Test_Member_nsString_mDataFlags
+    );
+    member_check!(
+        nsStringRepr,
+        nsString,
+        nsStr<'static>,
+        classflags,
+        Rust_Test_Member_nsString_mClassFlags
+    );
+    member_check!(
+        nsCStringRepr,
+        nsCString,
+        nsCStr<'static>,
+        data,
+        Rust_Test_Member_nsCString_mData
+    );
+    member_check!(
+        nsCStringRepr,
+        nsCString,
+        nsCStr<'static>,
+        length,
+        Rust_Test_Member_nsCString_mLength
+    );
+    member_check!(
+        nsCStringRepr,
+        nsCString,
+        nsCStr<'static>,
+        dataflags,
+        Rust_Test_Member_nsCString_mDataFlags
+    );
+    member_check!(
+        nsCStringRepr,
+        nsCString,
+        nsCStr<'static>,
+        classflags,
+        Rust_Test_Member_nsCString_mClassFlags
+    );
+
+    #[no_mangle]
+    #[allow(non_snake_case)]
+    pub unsafe extern "C" fn Rust_Test_NsStringFlags(
+        f_terminated: *mut u16,
+        f_voided: *mut u16,
+        f_refcounted: *mut u16,
+        f_owned: *mut u16,
+        f_inline: *mut u16,
+        f_literal: *mut u16,
+        f_class_inline: *mut u16,
+        f_class_null_terminated: *mut u16,
+    ) {
+        *f_terminated = DataFlags::TERMINATED.bits();
+        *f_voided = DataFlags::VOIDED.bits();
+        *f_refcounted = DataFlags::REFCOUNTED.bits();
+        *f_owned = DataFlags::OWNED.bits();
+        *f_inline = DataFlags::INLINE.bits();
+        *f_literal = DataFlags::LITERAL.bits();
+        *f_class_inline = ClassFlags::INLINE.bits();
+        *f_class_null_terminated = ClassFlags::NULL_TERMINATED.bits();
+    }
+
+    #[no_mangle]
+    #[allow(non_snake_case)]
+    pub unsafe extern "C" fn Rust_InlineCapacityFromRust(
+        cstring: *const nsACString,
+        string: *const nsAString,
+        cstring_capacity: *mut usize,
+        string_capacity: *mut usize,
+    ) {
+        *cstring_capacity = (*cstring).inline_capacity().unwrap();
+        *string_capacity = (*string).inline_capacity().unwrap();
+    }
+}
diff --git a/xpcom/rust/xpcom/Cargo.toml b/xpcom/rust/xpcom/Cargo.toml
new file mode 100644
index 0000000000..9c8a1ec83b
--- /dev/null
+++ b/xpcom/rust/xpcom/Cargo.toml
@@ -0,0 +1,20 @@
+[package]
+name = "xpcom"
+version = "0.1.0"
+authors = ["Nika Layzell <nika@thelayzells.com>"]
+edition = "2018"
+license = "MPL-2.0"
+
+[dependencies]
+cstr = "0.2"
+libc = "0.2"
+nsstring = { path = "../nsstring" }
+nserror = { path = "../nserror" }
+threadbound = "0.1"
+xpcom_macros = { path = "xpcom_macros" }
+thin-vec = { version = "0.2.1", features = ["gecko-ffi"] }
+mozbuild = "0.1"
+
+[features]
+thread_sanitizer = []
+gecko_refcount_logging = []
diff --git a/xpcom/rust/xpcom/src/base.rs b/xpcom/rust/xpcom/src/base.rs
new file mode 100644
index 0000000000..556768a179
--- /dev/null
+++ b/xpcom/rust/xpcom/src/base.rs
@@ -0,0 +1,59 @@
+/* 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 crate::interfaces::{nsIInterfaceRequestor, nsISupports};
+use crate::{GetterAddrefs, RefCounted, RefPtr};
+
+#[repr(C)]
+#[derive(Copy, Clone, Eq, PartialEq)]
+/// A "unique identifier". This is modeled after OSF DCE UUIDs.
+pub struct nsID(pub u32, pub u16, pub u16, pub [u8; 8]);
+
+/// Interface IDs
+pub type nsIID = nsID;
+/// Class IDs
+pub type nsCID = nsID;
+
+/// A type which implements XpCom must follow the following rules:
+///
+/// * It must be a legal XPCOM interface.
+/// * The result of a QueryInterface or similar call, passing IID, must return a
+///   valid reference to an object of the given type.
+/// * It must be valid to cast a &self reference to a &nsISupports reference.
+pub unsafe trait XpCom: RefCounted {
+    const IID: nsIID;
+
+    /// Perform a QueryInterface call on this object, attempting to dynamically
+    /// cast it to the requested interface type. Returns Some(RefPtr<T>) if the
+    /// cast succeeded, and None otherwise.
+    fn query_interface<T: XpCom>(&self) -> Option<RefPtr<T>> {
+        let mut ga = GetterAddrefs::<T>::new();
+        unsafe {
+            if (*(self as *const Self as *const nsISupports))
+                .QueryInterface(&T::IID, ga.void_ptr())
+                .succeeded()
+            {
+                ga.refptr()
+            } else {
+                None
+            }
+        }
+    }
+
+    /// Perform a `GetInterface` call on this object, returning `None` if the
+    /// object doesn't implement `nsIInterfaceRequestor`, or can't access the
+    /// interface `T`.
+    fn get_interface<T: XpCom>(&self) -> Option<RefPtr<T>> {
+        let ireq = self.query_interface::<nsIInterfaceRequestor>()?;
+
+        let mut ga = GetterAddrefs::<T>::new();
+        unsafe {
+            if ireq.GetInterface(&T::IID, ga.void_ptr()).succeeded() {
+                ga.refptr()
+            } else {
+                None
+            }
+        }
+    }
+}
diff --git a/xpcom/rust/xpcom/src/components.rs b/xpcom/rust/xpcom/src/components.rs
new file mode 100644
index 0000000000..c83e2df705
--- /dev/null
+++ b/xpcom/rust/xpcom/src/components.rs
@@ -0,0 +1,23 @@
+/* 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/. */
+
+//! This module contains convenient accessors for static XPCOM components.
+//!
+//! The contents of this file are generated from
+//! `xpcom/components/gen_static_components.py`.
+
+extern "C" {
+    fn Gecko_GetServiceByModuleID(
+        id: ModuleID,
+        iid: &crate::nsIID,
+        result: *mut *mut libc::c_void,
+    ) -> nserror::nsresult;
+    fn Gecko_CreateInstanceByModuleID(
+        id: ModuleID,
+        iid: &crate::nsIID,
+        result: *mut *mut libc::c_void,
+    ) -> nserror::nsresult;
+}
+
+include!(mozbuild::objdir_path!("xpcom/components/components.rs"));
diff --git a/xpcom/rust/xpcom/src/interfaces/idl.rs b/xpcom/rust/xpcom/src/interfaces/idl.rs
new file mode 100644
index 0000000000..c8d62ce716
--- /dev/null
+++ b/xpcom/rust/xpcom/src/interfaces/idl.rs
@@ -0,0 +1,12 @@
+/* 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(bad_style)]
+
+use crate::interfaces::*;
+use crate::*;
+
+// NOTE: This file contains a series of `include!()` invocations, defining all
+// idl interfaces directly within this module.
+include!(mozbuild::objdir_path!("dist/xpcrs/rt/all.rs"));
diff --git a/xpcom/rust/xpcom/src/interfaces/mod.rs b/xpcom/rust/xpcom/src/interfaces/mod.rs
new file mode 100644
index 0000000000..5b9de5cef5
--- /dev/null
+++ b/xpcom/rust/xpcom/src/interfaces/mod.rs
@@ -0,0 +1,31 @@
+/* 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/. */
+
+//! This module contains the xpcom interfaces exposed to rust code.
+//!
+//! The items in this module come in a few flavours:
+//!
+//! 1. `nsI*`: These are the types for XPCOM interfaces. They should always be
+//!    passed behind a reference, pointer, or `RefPtr`. They may be coerced to
+//!    their base interfaces using the `coerce` method.
+//!
+//! 2. `nsI*Coerce`: These traits provide the implementation mechanics for the
+//!    `coerce` method, and can usually be ignored. *These traits are hidden in
+//!    rustdoc*
+//!
+//! 3. `nsI*VTable`: These structs are the vtable definitions for each type.
+//!    They contain the base interface's vtable, followed by pointers for each
+//!    of the vtable's methods. If direct access is needed, a `*const nsI*` can
+//!    be safely transmuted to a `*const nsI*VTable`. *These structs are hidden
+//!    in rustdoc*
+//!
+//! 4. Typedefs used in idl file definitions.
+
+// Interfaces defined in .idl files
+mod idl;
+pub use self::idl::*;
+
+// Other interfaces which are needed to compile
+mod nonidl;
+pub use self::nonidl::*;
diff --git a/xpcom/rust/xpcom/src/interfaces/nonidl.rs b/xpcom/rust/xpcom/src/interfaces/nonidl.rs
new file mode 100644
index 0000000000..b9e3f1abe2
--- /dev/null
+++ b/xpcom/rust/xpcom/src/interfaces/nonidl.rs
@@ -0,0 +1,180 @@
+/* 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/. */
+
+//! This module contains definitions of interfaces which are used in idl files
+//! as forward declarations, but are not actually defined in an idl file.
+//!
+//! NOTE: The IIDs in these files must be kept in sync with the IDL definitions
+//! in the corresponding C++ files.
+
+use crate::nsID;
+
+// XXX: This macro should have an option for a custom base interface instead of
+// nsISupports, such that Document can have nsINode as a base, etc. For now,
+// query_interface should be sufficient.
+macro_rules! nonidl {
+    ($name:ident, $iid:expr) => {
+        /// This interface is referenced from idl files, but not defined in
+        /// them. It exports no methods to rust code.
+        #[repr(C)]
+        pub struct $name {
+            _vtable: *const $crate::interfaces::nsISupportsVTable,
+        }
+
+        unsafe impl $crate::XpCom for $name {
+            const IID: $crate::nsIID = $iid;
+        }
+
+        unsafe impl $crate::RefCounted for $name {
+            #[inline]
+            unsafe fn addref(&self) {
+                self.AddRef();
+            }
+            #[inline]
+            unsafe fn release(&self) {
+                self.Release();
+            }
+        }
+
+        impl ::std::ops::Deref for $name {
+            type Target = $crate::interfaces::nsISupports;
+            #[inline]
+            fn deref(&self) -> &$crate::interfaces::nsISupports {
+                unsafe { ::std::mem::transmute(self) }
+            }
+        }
+    };
+}
+
+// Must be kept in sync with Document.h
+nonidl!(
+    Document,
+    nsID(
+        0xce1f7627,
+        0x7109,
+        0x4977,
+        [0xba, 0x77, 0x49, 0x0f, 0xfd, 0xe0, 0x7a, 0xaa]
+    )
+);
+
+// Must be kept in sync with nsINode.h
+nonidl!(
+    nsINode,
+    nsID(
+        0x70ba4547,
+        0x7699,
+        0x44fc,
+        [0xb3, 0x20, 0x52, 0xdb, 0xe3, 0xd1, 0xf9, 0x0a]
+    )
+);
+
+// Must be kept in sync with nsIContent.h
+nonidl!(
+    nsIContent,
+    nsID(
+        0x8e1bab9d,
+        0x8815,
+        0x4d2c,
+        [0xa2, 0x4d, 0x7a, 0xba, 0x52, 0x39, 0xdc, 0x22]
+    )
+);
+
+// Must be kept in sync with nsIConsoleReportCollector.h
+nonidl!(
+    nsIConsoleReportCollector,
+    nsID(
+        0xdd98a481,
+        0xd2c4,
+        0x4203,
+        [0x8d, 0xfa, 0x85, 0xbf, 0xd7, 0xdc, 0xd7, 0x05]
+    )
+);
+
+// Must be kept in sync with nsIGlobalObject.h
+nonidl!(
+    nsIGlobalObject,
+    nsID(
+        0x11afa8be,
+        0xd997,
+        0x4e07,
+        [0xa6, 0xa3, 0x6f, 0x87, 0x2e, 0xc3, 0xee, 0x7f]
+    )
+);
+
+// Must be kept in sync with nsIScriptElement.h
+nonidl!(
+    nsIScriptElement,
+    nsID(
+        0xe60fca9b,
+        0x1b96,
+        0x4e4e,
+        [0xa9, 0xb4, 0xdc, 0x98, 0x4f, 0x88, 0x3f, 0x9c]
+    )
+);
+
+// Must be kept in sync with nsPIDOMWindow.h
+nonidl!(
+    nsPIDOMWindowOuter,
+    nsID(
+        0x769693d4,
+        0xb009,
+        0x4fe2,
+        [0xaf, 0x18, 0x7d, 0xc8, 0xdf, 0x74, 0x96, 0xdf]
+    )
+);
+
+// Must be kept in sync with nsPIDOMWindow.h
+nonidl!(
+    nsPIDOMWindowInner,
+    nsID(
+        0x775dabc9,
+        0x8f43,
+        0x4277,
+        [0x9a, 0xdb, 0xf1, 0x99, 0x0d, 0x77, 0xcf, 0xfb]
+    )
+);
+
+// Must be kept in sync with nsIScriptContext.h
+nonidl!(
+    nsIScriptContext,
+    nsID(
+        0x54cbe9cf,
+        0x7282,
+        0x421a,
+        [0x91, 0x6f, 0xd0, 0x70, 0x73, 0xde, 0xb8, 0xc0]
+    )
+);
+
+// Must be kept in sync with nsIScriptGlobalObject.h
+nonidl!(
+    nsIScriptGlobalObject,
+    nsID(
+        0x876f83bd,
+        0x6314,
+        0x460a,
+        [0xa0, 0x45, 0x1c, 0x8f, 0x46, 0x2f, 0xb8, 0xe1]
+    )
+);
+
+// Must be kept in sync with nsIScrollObserver.h
+nonidl!(
+    nsIScrollObserver,
+    nsID(
+        0xaa5026eb,
+        0x2f88,
+        0x4026,
+        [0xa4, 0x6b, 0xf4, 0x59, 0x6b, 0x4e, 0xdf, 0x00]
+    )
+);
+
+// Must be kept in sync with nsIWidget.h
+nonidl!(
+    nsIWidget,
+    nsID(
+        0x06396bf6,
+        0x2dd8,
+        0x45e5,
+        [0xac, 0x45, 0x75, 0x26, 0x53, 0xb1, 0xc9, 0x80]
+    )
+);
diff --git a/xpcom/rust/xpcom/src/lib.rs b/xpcom/rust/xpcom/src/lib.rs
new file mode 100644
index 0000000000..ac039ebe76
--- /dev/null
+++ b/xpcom/rust/xpcom/src/lib.rs
@@ -0,0 +1,43 @@
+/* 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/. */
+
+//! This crate contains the functionality required in order to both implement
+//! and call XPCOM methods from rust code.
+//!
+//! For documentation on how to implement XPCOM methods, see the documentation
+//! for the [`xpcom_macros`](../xpcom_macros/index.html) crate.
+
+#![allow(non_snake_case)]
+#![allow(non_camel_case_types)]
+
+// re-export the xpcom_macros macro
+pub use xpcom_macros::xpcom;
+
+// Helper functions and data structures are exported in the root of the crate.
+mod base;
+pub use base::*;
+
+// Declarative macro to generate XPCOM method stubs.
+mod method;
+pub use method::*;
+
+// dom::Promise resolving.
+mod promise;
+pub use promise::*;
+
+mod refptr;
+pub use refptr::*;
+
+mod statics;
+pub use statics::*;
+
+// XPCOM interface definitions.
+pub mod interfaces;
+
+// XPCOM component getters.
+pub mod components;
+
+// Implementation details of the xpcom_macros crate.
+#[doc(hidden)]
+pub mod reexports;
diff --git a/xpcom/rust/xpcom/src/method.rs b/xpcom/rust/xpcom/src/method.rs
new file mode 100644
index 0000000000..66c0510bd9
--- /dev/null
+++ b/xpcom/rust/xpcom/src/method.rs
@@ -0,0 +1,241 @@
+/* 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 nserror::{nsresult, NS_ERROR_NULL_POINTER};
+
+/// The xpcom_method macro generates a Rust XPCOM method stub that converts
+/// raw pointer arguments to references, calls a Rustic implementation
+/// of the method, writes its return value into the XPCOM method's outparameter,
+/// and returns an nsresult.
+///
+/// In other words, given an XPCOM method like:
+///
+/// ```ignore
+/// interface nsIFooBarBaz : nsISupports {
+///   nsIVariant foo(in AUTF8String bar, [optional] in bool baz);
+/// }
+/// ```
+///
+/// And a Rust implementation that uses #[xpcom] to implement it:
+///
+/// ```ignore
+/// #[xpcom(implement(nsIFooBarBaz), atomic)]
+/// struct FooBarBaz {
+///     // …
+/// }
+/// ```
+///
+/// With the appropriate extern crate and use declarations
+///
+/// ```ignore
+/// extern crate xpcom;
+/// use xpcom::xpcom_method;
+/// ```
+///
+/// Invoking the macro with the name of the XPCOM method, the name of its
+/// Rustic implementation, the set of its arguments, and its return value:
+///
+/// ```ignore
+/// impl FooBarBaz {
+///   xpcom_method!(
+///       foo => Foo(bar: *const nsACString, baz: bool) -> *const nsIVariant
+///   );
+/// }
+/// ```
+///
+/// Results in the macro generating an XPCOM stub like the following:
+///
+/// ```ignore
+/// unsafe fn Foo(&self, bar: *const nsACString, baz: bool, retval: *mut *const nsIVariant) -> nsresult {
+///     let bar = match Ensure::ensure(bar) {
+///         Ok(val) => val,
+///         Err(result) => return result,
+///     };
+///     let baz = match Ensure::ensure(baz) {
+///         Ok(val) => val,
+///         Err(result) => return result,
+///     };
+///
+///     match self.foo(bar, baz) {
+///         Ok(val) => {
+///             val.forget(&mut *retval);
+///             NS_OK
+///         }
+///         Err(error) => {
+///             error!("{}", error);
+///             error.into()
+///         }
+///     }
+/// }
+/// ```
+///
+/// Which calls a Rustic implementation (that you implement) like the following:
+///
+/// ```ignore
+/// impl FooBarBaz {
+///     fn foo(&self, bar: &nsACString, baz: bool) -> Result<RefPtr<nsIVariant>, nsresult> {
+///         // …
+///     }
+/// }
+/// ```
+///
+/// Notes:
+///
+/// On error, the Rustic implementation can return an Err(nsresult) or any
+/// other type that implements Into<nsresult>.  So you can define and return
+/// a custom error type, which the XPCOM stub will convert to nsresult.
+///
+/// This macro assumes that all non-null pointer arguments are valid!
+/// It does ensure that they aren't null, using the `ensure_param` macro.
+/// But it doesn't otherwise check their validity. That makes the function
+/// unsafe, so callers must ensure that they only call it with valid pointer
+/// arguments.
+#[macro_export]
+macro_rules! xpcom_method {
+    // This rule is provided to ensure external modules don't need to import
+    // internal implementation details of xpcom_method.
+    // The @ensure_param rule converts raw pointer arguments to references,
+    // returning NS_ERROR_NULL_POINTER if the argument is_null().
+    //
+    // Notes:
+    //
+    // This rule can be called on a non-pointer copy parameter, but there's no
+    // benefit to doing so.  The macro will just set the value of the parameter
+    // to itself. (This macro does this anyway due to limitations in declarative
+    // macros; it isn't currently possible to distinguish between pointer and
+    // copy types when processing a set of parameters.)
+    //
+    // The macro currently supports only in-parameters (*const nsIFoo); It
+    // doesn't (yet?) support out-parameters (*mut nsIFoo).  The xpcom_method
+    // macro itself does, however, support the return value out-parameter.
+    (@ensure_param $name:ident) => {
+        let $name = match $crate::Ensure::ensure($name) {
+            Ok(val) => val,
+            Err(result) => return result,
+        };
+    };
+
+    // `#[allow(non_snake_case)]` is used for each method because `$xpcom_name`
+    // is almost always UpperCamelCase, and Rust gives a warning that it should
+    // be snake_case. It isn't reasonable to rename the XPCOM methods, so
+    // silence the warning.
+
+    // A method whose return value is a *mut *const nsISomething type.
+    // Example: foo => Foo(bar: *const nsACString, baz: bool) -> *const nsIVariant
+    ($rust_name:ident => $xpcom_name:ident($($param_name:ident: $param_type:ty),*) -> *const $retval:ty) => {
+        #[allow(non_snake_case)]
+        unsafe fn $xpcom_name(&self, $($param_name: $param_type,)* retval: *mut *const $retval) -> nsresult {
+            $(xpcom_method!(@ensure_param $param_name);)*
+            match self.$rust_name($($param_name, )*) {
+                Ok(val) => {
+                    val.forget(&mut *retval);
+                    NS_OK
+                }
+                Err(error) => {
+                    error.into()
+                }
+            }
+        }
+    };
+
+    // A method whose return value is a *mut nsAString type.
+    // Example: foo => Foo(bar: *const nsACString, baz: bool) -> nsAString
+    ($rust_name:ident => $xpcom_name:ident($($param_name:ident: $param_type:ty),*) -> nsAString) => {
+        #[allow(non_snake_case)]
+        unsafe fn $xpcom_name(&self, $($param_name: $param_type,)* retval: *mut nsAString) -> nsresult {
+            $(xpcom_method!(@ensure_param $param_name);)*
+            match self.$rust_name($($param_name, )*) {
+                Ok(val) => {
+                    (*retval).assign(&val);
+                    NS_OK
+                }
+                Err(error) => {
+                    error.into()
+                }
+            }
+        }
+    };
+
+    // A method whose return value is a *mut nsACString type.
+    // Example: foo => Foo(bar: *const nsACString, baz: bool) -> nsACString
+    ($rust_name:ident => $xpcom_name:ident($($param_name:ident: $param_type:ty),*) -> nsACString) => {
+        #[allow(non_snake_case)]
+        unsafe fn $xpcom_name(&self, $($param_name: $param_type,)* retval: *mut nsACString) -> nsresult {
+            $(xpcom_method!(@ensure_param $param_name);)*
+            match self.$rust_name($($param_name, )*) {
+                Ok(val) => {
+                    (*retval).assign(&val);
+                    NS_OK
+                }
+                Err(error) => {
+                    error.into()
+                }
+            }
+        }
+    };
+
+    // A method whose return value is a non-nsA[C]String *mut type.
+    // Example: foo => Foo(bar: *const nsACString, baz: bool) -> bool
+    ($rust_name:ident => $xpcom_name:ident($($param_name:ident: $param_type:ty),*) -> $retval:ty) => {
+        #[allow(non_snake_case)]
+        unsafe fn $xpcom_name(&self, $($param_name: $param_type,)* retval: *mut $retval) -> nsresult {
+            $(xpcom_method!(@ensure_param $param_name);)*
+            match self.$rust_name($($param_name, )*) {
+                Ok(val) => {
+                    *retval = val;
+                    NS_OK
+                }
+                Err(error) => {
+                    error.into()
+                }
+            }
+        }
+    };
+
+    // A method that doesn't have a return value.
+    // Example: foo => Foo(bar: *const nsACString, baz: bool)
+    ($rust_name:ident => $xpcom_name:ident($($param_name:ident: $param_type:ty),*)) => {
+        #[allow(non_snake_case)]
+        unsafe fn $xpcom_name(&self, $($param_name: $param_type,)*) -> nsresult {
+            $(xpcom_method!(@ensure_param $param_name);)*
+            match self.$rust_name($($param_name, )*) {
+                Ok(_) => NS_OK,
+                Err(error) => {
+                    error.into()
+                }
+            }
+        }
+    };
+}
+
+/// A trait that ensures that a raw pointer isn't null and converts it to
+/// a reference.  Because of limitations in declarative macros, this includes an
+/// implementation for types that are Copy, which simply returns the value
+/// itself.
+#[doc(hidden)]
+pub trait Ensure<T> {
+    unsafe fn ensure(value: T) -> Self;
+}
+
+impl<'a, T: 'a> Ensure<*const T> for Result<&'a T, nsresult> {
+    unsafe fn ensure(ptr: *const T) -> Result<&'a T, nsresult> {
+        if ptr.is_null() {
+            Err(NS_ERROR_NULL_POINTER)
+        } else {
+            Ok(&*ptr)
+        }
+    }
+}
+
+impl<'a, T: 'a> Ensure<*const T> for Result<Option<&'a T>, nsresult> {
+    unsafe fn ensure(ptr: *const T) -> Result<Option<&'a T>, nsresult> {
+        Ok(if ptr.is_null() { None } else { Some(&*ptr) })
+    }
+}
+
+impl<T: Copy> Ensure<T> for Result<T, nsresult> {
+    unsafe fn ensure(copyable: T) -> Result<T, nsresult> {
+        Ok(copyable)
+    }
+}
diff --git a/xpcom/rust/xpcom/src/promise.rs b/xpcom/rust/xpcom/src/promise.rs
new file mode 100644
index 0000000000..0fdab9b6aa
--- /dev/null
+++ b/xpcom/rust/xpcom/src/promise.rs
@@ -0,0 +1,62 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use crate::{
+    create_instance,
+    interfaces::{nsIVariant, nsIWritableVariant},
+    RefCounted,
+};
+
+use cstr::*;
+
+mod ffi {
+    use super::*;
+
+    extern "C" {
+        // These are implemented in dom/promise/Promise.cpp
+        pub fn DomPromise_AddRef(promise: *const Promise);
+        pub fn DomPromise_Release(promise: *const Promise);
+        pub fn DomPromise_RejectWithVariant(promise: *const Promise, variant: *const nsIVariant);
+        pub fn DomPromise_ResolveWithVariant(promise: *const Promise, variant: *const nsIVariant);
+    }
+}
+
+#[repr(C)]
+pub struct Promise {
+    private: [u8; 0],
+
+    /// This field is a phantomdata to ensure that the Promise type and any
+    /// struct containing it is not safe to send across threads, as DOM is
+    /// generally not threadsafe.
+    __nosync: ::std::marker::PhantomData<::std::rc::Rc<u8>>,
+}
+
+impl Promise {
+    pub fn reject_with_undefined(&self) {
+        let variant = create_instance::<nsIWritableVariant>(cstr!("@mozilla.org/variant;1"))
+            .expect("Failed to create writable variant");
+        unsafe {
+            variant.SetAsVoid();
+        }
+        self.reject_with_variant(&variant);
+    }
+
+    pub fn reject_with_variant(&self, variant: &nsIVariant) {
+        unsafe { ffi::DomPromise_RejectWithVariant(self, variant) }
+    }
+
+    pub fn resolve_with_variant(&self, variant: &nsIVariant) {
+        unsafe { ffi::DomPromise_ResolveWithVariant(self, variant) }
+    }
+}
+
+unsafe impl RefCounted for Promise {
+    unsafe fn addref(&self) {
+        ffi::DomPromise_AddRef(self)
+    }
+
+    unsafe fn release(&self) {
+        ffi::DomPromise_Release(self)
+    }
+}
diff --git a/xpcom/rust/xpcom/src/reexports.rs b/xpcom/rust/xpcom/src/reexports.rs
new file mode 100644
index 0000000000..d198899497
--- /dev/null
+++ b/xpcom/rust/xpcom/src/reexports.rs
@@ -0,0 +1,52 @@
+/* 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/. */
+
+//! The automatically generated code from `xpcom_macros` depends on some types
+//! which are defined in other libraries which `xpcom` depends on, but which may
+//! not be `extern crate`-ed into the crate the macros are expanded into. This
+//! module re-exports those types from `xpcom` so that they can be used from the
+//! macro.
+// re-export libc so it can be used by the procedural macro.
+pub extern crate libc;
+
+pub use nsstring::{nsACString, nsAString, nsCString, nsString};
+
+pub use nserror::{nsresult, NS_ERROR_NO_INTERFACE, NS_OK};
+
+pub use std::ops::Deref;
+
+/// Helper method used by the xpcom codegen, it is not public API or meant for
+/// calling outside of that context.
+///
+/// Takes a reference to the `this` pointer received from XPIDL, and offsets and
+/// casts it to a reference to the concrete rust `struct` type, `U`.
+///
+/// `vtable_index` is the index, and therefore the offset in pointers, of the
+/// vtable for `T` in `U`.
+///
+/// A reference to `this` is taken, instead of taking `*const T` by value, to use
+/// as a lifetime bound, such that the returned `&U` reference has a bounded
+/// lifetime when used to call the implementation method.
+#[inline]
+pub unsafe fn transmute_from_vtable_ptr<'a, T, U>(
+    this: &'a *const T,
+    vtable_index: usize,
+) -> &'a U {
+    &*((*this as *const *const ()).sub(vtable_index) as *const U)
+}
+
+/// On some ABIs, extra information is included before the vtable's function
+/// pointers which are used to implement RTTI. We build Gecko with RTTI
+/// disabled, however these fields may still be present to support
+/// `dynamic_cast<void*>` on our rust VTables in case they are accessed.
+///
+/// Itanium ABI Layout: https://refspecs.linuxbase.org/cxxabi-1.83.html#vtable
+#[repr(C)]
+pub struct VTableExtra<T> {
+    #[cfg(not(windows))]
+    pub offset: isize,
+    #[cfg(not(windows))]
+    pub typeinfo: *const libc::c_void,
+    pub vtable: T,
+}
diff --git a/xpcom/rust/xpcom/src/refptr.rs b/xpcom/rust/xpcom/src/refptr.rs
new file mode 100644
index 0000000000..8549b6d2f0
--- /dev/null
+++ b/xpcom/rust/xpcom/src/refptr.rs
@@ -0,0 +1,388 @@
+/* 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 crate::interfaces::nsISupports;
+use libc;
+use nserror::{nsresult, NS_OK};
+use std::cell::Cell;
+use std::convert::TryInto;
+use std::fmt;
+use std::marker::PhantomData;
+use std::mem;
+use std::ops::Deref;
+use std::ptr::{self, NonNull};
+use std::sync::atomic::{self, AtomicUsize, Ordering};
+use threadbound::ThreadBound;
+
+// This should match the definition in mfbt/RefCountType.h, modulo the delicate
+// effort at maintaining binary compatibility with Microsoft COM on Windows.
+pub type MozExternalRefCountType = u32;
+
+/// A trait representing a type which can be reference counted invasively.
+/// The object is responsible for freeing its backing memory when its
+/// reference count reaches 0.
+pub unsafe trait RefCounted {
+    /// Increment the reference count.
+    unsafe fn addref(&self);
+    /// Decrement the reference count, potentially freeing backing memory.
+    unsafe fn release(&self);
+}
+
+/// A smart pointer holding a RefCounted object. The object itself manages its
+/// own memory. RefPtr will invoke the addref and release methods at the
+/// appropriate times to facilitate the bookkeeping.
+#[repr(transparent)]
+pub struct RefPtr<T: RefCounted + 'static> {
+    _ptr: NonNull<T>,
+    // Tell dropck that we own an instance of T.
+    _marker: PhantomData<T>,
+}
+
+impl<T: RefCounted + 'static> RefPtr<T> {
+    /// Construct a new RefPtr from a reference to the refcounted object.
+    #[inline]
+    pub fn new(p: &T) -> RefPtr<T> {
+        unsafe {
+            p.addref();
+        }
+        RefPtr {
+            _ptr: p.into(),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Construct a RefPtr from a raw pointer, addrefing it.
+    #[inline]
+    pub unsafe fn from_raw(p: *const T) -> Option<RefPtr<T>> {
+        let ptr = NonNull::new(p as *mut T)?;
+        ptr.as_ref().addref();
+        Some(RefPtr {
+            _ptr: ptr,
+            _marker: PhantomData,
+        })
+    }
+
+    /// Construct a RefPtr from a raw pointer, without addrefing it.
+    #[inline]
+    pub unsafe fn from_raw_dont_addref(p: *const T) -> Option<RefPtr<T>> {
+        Some(RefPtr {
+            _ptr: NonNull::new(p as *mut T)?,
+            _marker: PhantomData,
+        })
+    }
+
+    /// Write this RefPtr's value into an outparameter.
+    #[inline]
+    pub fn forget(self, into: &mut *const T) {
+        *into = Self::forget_into_raw(self);
+    }
+
+    #[inline]
+    pub fn forget_into_raw(this: RefPtr<T>) -> *const T {
+        let into = &*this as *const T;
+        mem::forget(this);
+        into
+    }
+}
+
+impl<T: RefCounted + 'static> Deref for RefPtr<T> {
+    type Target = T;
+    #[inline]
+    fn deref(&self) -> &T {
+        unsafe { self._ptr.as_ref() }
+    }
+}
+
+impl<T: RefCounted + 'static> Drop for RefPtr<T> {
+    #[inline]
+    fn drop(&mut self) {
+        unsafe {
+            self._ptr.as_ref().release();
+        }
+    }
+}
+
+impl<T: RefCounted + 'static> Clone for RefPtr<T> {
+    #[inline]
+    fn clone(&self) -> RefPtr<T> {
+        RefPtr::new(self)
+    }
+}
+
+impl<T: RefCounted + 'static + fmt::Debug> fmt::Debug for RefPtr<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "RefPtr<{:?}>", self.deref())
+    }
+}
+
+// Both `Send` and `Sync` bounds are required for `RefPtr<T>` to implement
+// either, as sharing a `RefPtr<T>` also allows transferring ownership, and
+// vice-versa.
+unsafe impl<T: RefCounted + 'static + Send + Sync> Send for RefPtr<T> {}
+unsafe impl<T: RefCounted + 'static + Send + Sync> Sync for RefPtr<T> {}
+
+macro_rules! assert_layout_eq {
+    ($T:ty, $U:ty) => {
+        const _: [(); std::mem::size_of::<$T>()] = [(); std::mem::size_of::<$U>()];
+        const _: [(); std::mem::align_of::<$T>()] = [(); std::mem::align_of::<$U>()];
+    };
+}
+
+// Assert that `RefPtr<nsISupports>` has the correct memory layout.
+assert_layout_eq!(RefPtr<nsISupports>, *const nsISupports);
+// Assert that the null-pointer optimization applies to `RefPtr<nsISupports>`.
+assert_layout_eq!(RefPtr<nsISupports>, Option<RefPtr<nsISupports>>);
+
+/// A wrapper that binds a RefCounted value to its original thread,
+/// preventing retrieval from other threads and panicking if the value
+/// is dropped on a different thread.
+///
+/// These limitations enable values of this type to be Send + Sync, which is
+/// useful when creating a struct that holds a RefPtr<T> type while being
+/// Send + Sync.  Such a struct can hold a ThreadBoundRefPtr<T> type instead.
+pub struct ThreadBoundRefPtr<T: RefCounted + 'static>(ThreadBound<*const T>);
+
+impl<T: RefCounted + 'static> ThreadBoundRefPtr<T> {
+    pub fn new(ptr: RefPtr<T>) -> Self {
+        let raw: *const T = &*ptr;
+        mem::forget(ptr);
+        ThreadBoundRefPtr(ThreadBound::new(raw))
+    }
+
+    pub fn get_ref(&self) -> Option<&T> {
+        self.0.get_ref().map(|raw| unsafe { &**raw })
+    }
+}
+
+impl<T: RefCounted + 'static> Drop for ThreadBoundRefPtr<T> {
+    fn drop(&mut self) {
+        unsafe {
+            RefPtr::from_raw_dont_addref(self.get_ref().expect("drop() called on wrong thread!"));
+        }
+    }
+}
+
+/// A helper struct for constructing `RefPtr<T>` from raw pointer outparameters.
+/// Holds a `*const T` internally which will be released if non null when
+/// destructed, and can be easily transformed into an `Option<RefPtr<T>>`.
+///
+/// It many cases it may be easier to use the `getter_addrefs` method.
+pub struct GetterAddrefs<T: RefCounted + 'static> {
+    _ptr: *const T,
+    _marker: PhantomData<T>,
+}
+
+impl<T: RefCounted + 'static> GetterAddrefs<T> {
+    /// Create a `GetterAddrefs`, initializing it with the null pointer.
+    #[inline]
+    pub fn new() -> GetterAddrefs<T> {
+        GetterAddrefs {
+            _ptr: ptr::null(),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Get a reference to the internal `*const T`. This method is unsafe,
+    /// as the destructor of this class depends on the internal `*const T`
+    /// being either a valid reference to a value of type `T`, or null.
+    #[inline]
+    pub unsafe fn ptr(&mut self) -> &mut *const T {
+        &mut self._ptr
+    }
+
+    /// Get a reference to the internal `*const T` as a `*mut libc::c_void`.
+    /// This is useful to pass to functions like `GetInterface` which take a
+    /// void pointer outparameter.
+    #[inline]
+    pub unsafe fn void_ptr(&mut self) -> *mut *mut libc::c_void {
+        &mut self._ptr as *mut *const T as *mut *mut libc::c_void
+    }
+
+    /// Transform this `GetterAddrefs` into an `Option<RefPtr<T>>`, without
+    /// performing any addrefs or releases.
+    #[inline]
+    pub fn refptr(self) -> Option<RefPtr<T>> {
+        let p = self._ptr;
+        // Don't run the destructor because we don't want to release the stored
+        // pointer.
+        mem::forget(self);
+        unsafe { RefPtr::from_raw_dont_addref(p) }
+    }
+}
+
+impl<T: RefCounted + 'static> Drop for GetterAddrefs<T> {
+    #[inline]
+    fn drop(&mut self) {
+        if !self._ptr.is_null() {
+            unsafe {
+                (*self._ptr).release();
+            }
+        }
+    }
+}
+
+/// Helper method for calling XPCOM methods which return a reference counted
+/// value through an outparameter. Takes a lambda, which is called with a valid
+/// outparameter argument (`*mut *const T`), and returns a `nsresult`. Returns
+/// either a `RefPtr<T>` with the value returned from the outparameter, or a
+/// `nsresult`.
+///
+/// # NOTE:
+///
+/// Can return `Err(NS_OK)` if the call succeeded, but the outparameter was set
+/// to NULL.
+///
+/// # Usage
+///
+/// ```
+/// let x: Result<RefPtr<T>, nsresult> =
+///     getter_addrefs(|p| iosvc.NewURI(uri, ptr::null(), ptr::null(), p));
+/// ```
+#[inline]
+pub fn getter_addrefs<T: RefCounted, F>(f: F) -> Result<RefPtr<T>, nsresult>
+where
+    F: FnOnce(*mut *const T) -> nsresult,
+{
+    let mut ga = GetterAddrefs::<T>::new();
+    let rv = f(unsafe { ga.ptr() });
+    if rv.failed() {
+        return Err(rv);
+    }
+    ga.refptr().ok_or(NS_OK)
+}
+
+/// The type of the reference count type for xpcom structs.
+///
+/// `#[xpcom(nonatomic)]` will use this type for the `__refcnt` field.
+#[derive(Debug)]
+pub struct Refcnt(Cell<usize>);
+impl Refcnt {
+    /// Create a new reference count value. This is unsafe as manipulating
+    /// Refcnt values is an easy footgun.
+    pub unsafe fn new() -> Self {
+        Refcnt(Cell::new(0))
+    }
+
+    /// Increment the reference count. Returns the new reference count. This is
+    /// unsafe as modifying this value can cause a use-after-free.
+    pub unsafe fn inc(&self) -> MozExternalRefCountType {
+        // XXX: Checked add?
+        let new = self.0.get() + 1;
+        self.0.set(new);
+        new.try_into().unwrap()
+    }
+
+    /// Decrement the reference count. Returns the new reference count. This is
+    /// unsafe as modifying this value can cause a use-after-free.
+    pub unsafe fn dec(&self) -> MozExternalRefCountType {
+        // XXX: Checked sub?
+        let new = self.0.get() - 1;
+        self.0.set(new);
+        new.try_into().unwrap()
+    }
+
+    /// Get the current value of the reference count.
+    pub fn get(&self) -> usize {
+        self.0.get()
+    }
+}
+
+/// The type of the atomic reference count used for xpcom structs.
+///
+/// `#[xpcom(atomic)]` will use this type for the `__refcnt` field.
+///
+/// See `nsISupportsImpl.h`'s `ThreadSafeAutoRefCnt` class for reasoning behind
+/// memory ordering decisions.
+#[derive(Debug)]
+pub struct AtomicRefcnt(AtomicUsize);
+impl AtomicRefcnt {
+    /// Create a new reference count value. This is unsafe as manipulating
+    /// Refcnt values is an easy footgun.
+    pub unsafe fn new() -> Self {
+        AtomicRefcnt(AtomicUsize::new(0))
+    }
+
+    /// Increment the reference count. Returns the new reference count. This is
+    /// unsafe as modifying this value can cause a use-after-free.
+    pub unsafe fn inc(&self) -> MozExternalRefCountType {
+        let result = self.0.fetch_add(1, Ordering::Relaxed) + 1;
+        result.try_into().unwrap()
+    }
+
+    /// Decrement the reference count. Returns the new reference count. This is
+    /// unsafe as modifying this value can cause a use-after-free.
+    pub unsafe fn dec(&self) -> MozExternalRefCountType {
+        let result = self.0.fetch_sub(1, Ordering::Release) - 1;
+        if result == 0 {
+            // We're going to destroy the object on this thread, so we need
+            // acquire semantics to synchronize with the memory released by
+            // the last release on other threads, that is, to ensure that
+            // writes prior to that release are now visible on this thread.
+            if cfg!(feature = "thread_sanitizer") {
+                // TSan doesn't understand atomic::fence, so in order to avoid
+                // a false positive for every time a refcounted object is
+                // deleted, we replace the fence with an atomic operation.
+                self.0.load(Ordering::Acquire);
+            } else {
+                atomic::fence(Ordering::Acquire);
+            }
+        }
+        result.try_into().unwrap()
+    }
+
+    /// Get the current value of the reference count.
+    pub fn get(&self) -> usize {
+        self.0.load(Ordering::Acquire)
+    }
+}
+
+#[cfg(feature = "gecko_refcount_logging")]
+pub mod trace_refcnt {
+    extern "C" {
+        pub fn NS_LogCtor(aPtr: *mut libc::c_void, aTypeName: *const libc::c_char, aSize: u32);
+        pub fn NS_LogDtor(aPtr: *mut libc::c_void, aTypeName: *const libc::c_char, aSize: u32);
+        pub fn NS_LogAddRef(
+            aPtr: *mut libc::c_void,
+            aRefcnt: usize,
+            aClass: *const libc::c_char,
+            aClassSize: u32,
+        );
+        pub fn NS_LogRelease(
+            aPtr: *mut libc::c_void,
+            aRefcnt: usize,
+            aClass: *const libc::c_char,
+            aClassSize: u32,
+        );
+    }
+}
+
+// stub inline methods for the refcount logging functions for when the feature
+// is disabled.
+#[cfg(not(feature = "gecko_refcount_logging"))]
+pub mod trace_refcnt {
+    #[inline]
+    #[allow(non_snake_case)]
+    pub unsafe extern "C" fn NS_LogCtor(_: *mut libc::c_void, _: *const libc::c_char, _: u32) {}
+    #[inline]
+    #[allow(non_snake_case)]
+    pub unsafe extern "C" fn NS_LogDtor(_: *mut libc::c_void, _: *const libc::c_char, _: u32) {}
+    #[inline]
+    #[allow(non_snake_case)]
+    pub unsafe extern "C" fn NS_LogAddRef(
+        _: *mut libc::c_void,
+        _: usize,
+        _: *const libc::c_char,
+        _: u32,
+    ) {
+    }
+    #[inline]
+    #[allow(non_snake_case)]
+    pub unsafe extern "C" fn NS_LogRelease(
+        _: *mut libc::c_void,
+        _: usize,
+        _: *const libc::c_char,
+        _: u32,
+    ) {
+    }
+}
diff --git a/xpcom/rust/xpcom/src/statics.rs b/xpcom/rust/xpcom/src/statics.rs
new file mode 100644
index 0000000000..27f66fdb4b
--- /dev/null
+++ b/xpcom/rust/xpcom/src/statics.rs
@@ -0,0 +1,75 @@
+/* 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 crate::interfaces::{nsIComponentManager, nsIComponentRegistrar, nsIServiceManager};
+use crate::{GetterAddrefs, RefPtr, XpCom};
+use std::ffi::CStr;
+
+/// Get a reference to the global `nsIComponentManager`.
+///
+/// Can return `None` during shutdown.
+#[inline]
+pub fn component_manager() -> Option<RefPtr<nsIComponentManager>> {
+    unsafe { RefPtr::from_raw(Gecko_GetComponentManager()) }
+}
+
+/// Get a reference to the global `nsIServiceManager`.
+///
+/// Can return `None` during shutdown.
+#[inline]
+pub fn service_manager() -> Option<RefPtr<nsIServiceManager>> {
+    unsafe { RefPtr::from_raw(Gecko_GetServiceManager()) }
+}
+
+/// Get a reference to the global `nsIComponentRegistrar`
+///
+/// Can return `None` during shutdown.
+#[inline]
+pub fn component_registrar() -> Option<RefPtr<nsIComponentRegistrar>> {
+    unsafe { RefPtr::from_raw(Gecko_GetComponentRegistrar()) }
+}
+
+/// Helper for calling `nsIComponentManager::CreateInstanceByContractID` on the
+/// global `nsIComponentRegistrar`.
+///
+/// This method is similar to `do_CreateInstance` in C++.
+#[inline]
+pub fn create_instance<T: XpCom>(id: &CStr) -> Option<RefPtr<T>> {
+    unsafe {
+        let mut ga = GetterAddrefs::<T>::new();
+        if component_manager()?
+            .CreateInstanceByContractID(id.as_ptr(), &T::IID, ga.void_ptr())
+            .succeeded()
+        {
+            ga.refptr()
+        } else {
+            None
+        }
+    }
+}
+
+/// Helper for calling `nsIServiceManager::GetServiceByContractID` on the global
+/// `nsIServiceManager`.
+///
+/// This method is similar to `do_GetService` in C++.
+#[inline]
+pub fn get_service<T: XpCom>(id: &CStr) -> Option<RefPtr<T>> {
+    unsafe {
+        let mut ga = GetterAddrefs::<T>::new();
+        if service_manager()?
+            .GetServiceByContractID(id.as_ptr(), &T::IID, ga.void_ptr())
+            .succeeded()
+        {
+            ga.refptr()
+        } else {
+            None
+        }
+    }
+}
+
+extern "C" {
+    fn Gecko_GetComponentManager() -> *const nsIComponentManager;
+    fn Gecko_GetServiceManager() -> *const nsIServiceManager;
+    fn Gecko_GetComponentRegistrar() -> *const nsIComponentRegistrar;
+}
diff --git a/xpcom/rust/xpcom/xpcom_macros/Cargo.toml b/xpcom/rust/xpcom/xpcom_macros/Cargo.toml
new file mode 100644
index 0000000000..4b5bcdc315
--- /dev/null
+++ b/xpcom/rust/xpcom/xpcom_macros/Cargo.toml
@@ -0,0 +1,16 @@
+[package]
+name = "xpcom_macros"
+version = "0.1.0"
+authors = ["Nika Layzell <nika@thelayzells.com>"]
+edition = "2018"
+license = "MPL-2.0"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+syn = "1"
+quote = "1"
+proc-macro2 = "1"
+lazy_static = "1.0"
+mozbuild = "0.1"
diff --git a/xpcom/rust/xpcom/xpcom_macros/src/lib.rs b/xpcom/rust/xpcom/xpcom_macros/src/lib.rs
new file mode 100644
index 0000000000..2511ba9103
--- /dev/null
+++ b/xpcom/rust/xpcom/xpcom_macros/src/lib.rs
@@ -0,0 +1,818 @@
+/* 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/. */
+
+//! This crate provides the `#[xpcom]` custom attribute. This custom attribute
+//! is used in order to implement [`xpcom`] interfaces.
+//!
+//! # Usage
+//!
+//! The easiest way to explain this crate is probably with a usage example. I'll
+//! show you the example, and then we'll destructure it and walk through what
+//! each component is doing.
+//!
+//! ```ignore
+//! // Declaring an XPCOM Struct
+//! #[xpcom(implement(nsIRunnable), atomic)]
+//! struct ImplRunnable {
+//!     i: i32,
+//! }
+//!
+//! // Implementing methods on an XPCOM Struct
+//! impl ImplRunnable {
+//!     unsafe fn Run(&self) -> nsresult {
+//!         println!("{}", self.i);
+//!         NS_OK
+//!     }
+//! }
+//! ```
+//!
+//! ## Declaring an XPCOM Struct
+//!
+//! ```ignore
+//! // This derive should be placed on the initialization struct in order to
+//! // trigger the procedural macro.
+//! #[xpcom(
+//!     // The implement argument should be passed the names of the IDL
+//!     // interfaces which you want to implement. These can be separated by
+//!     // commas if you want to implement multiple interfaces.
+//!     //
+//!     // Some methods use types which we cannot bind to in rust. Interfaces
+//!     // like those cannot be implemented, and a compile-time error will occur
+//!     // if they are listed in this attribute.
+//!     implement(nsIRunnable),
+//!
+//!     // The refcount kind can be specified as one of the following values:
+//!     //  * `atomic` == atomic reference count
+//!     //    ~= NS_DECL_THREADSAFE_ISUPPORTS in C++
+//!     //  * `nonatomic` == non atomic reference count
+//!     //    ~= NS_DECL_ISUPPORTS in C++
+//!     atomic,
+//! )]
+//!
+//! // It is a compile time error to put the `#[xpcom]` attribute on
+//! // an enum, union, or tuple struct.
+//! //
+//! // The macro will generate both the named struct, as well as a version with
+//! // its name prefixed with `Init` which can be used to initialize the type.
+//! struct ImplRunnable {
+//!     i: i32,
+//! }
+//! ```
+//!
+//! The above example will generate `ImplRunnable` and `InitImplRunnable`
+//! structs. The `ImplRunnable` struct will implement the [`nsIRunnable`] XPCOM
+//! interface, and cannot be constructed directly.
+//!
+//! The following methods will be automatically implemented on it:
+//!
+//! ```ignore
+//! // Automatic nsISupports implementation
+//! unsafe fn AddRef(&self) -> MozExternalRefCountType;
+//! unsafe fn Release(&self) -> MozExternalRefCountType;
+//! unsafe fn QueryInterface(&self, uuid: &nsIID, result: *mut *mut libc::c_void) -> nsresult;
+//!
+//! // Allocates and initializes a new instance of this type. The values will
+//! // be moved from the `Init` struct which is passed in.
+//! fn allocate(init: InitImplRunnable) -> RefPtr<Self>;
+//!
+//! // Helper for performing the `query_interface` operation to case to a
+//! // specific interface.
+//! fn query_interface<T: XpCom>(&self) -> Option<RefPtr<T>>;
+//!
+//! // Coerce function for cheaply casting to our base interfaces.
+//! fn coerce<T: ImplRunnableCoerce>(&self) -> &T;
+//! ```
+//!
+//! The [`RefCounted`] interface will also be implemented, so that the type can
+//! be used within the [`RefPtr`] type.
+//!
+//! The `coerce` and `query_interface` methods are backed by the generated
+//! `*Coerce` trait. This trait is impl-ed for every interface implemented by
+//! the trait. For example:
+//!
+//! ```ignore
+//! pub trait ImplRunnableCoerce {
+//!     fn coerce_from(x: &ImplRunnable) -> &Self;
+//! }
+//! impl ImplRunnableCoerce for nsIRunnable { .. }
+//! impl ImplRunnableCoerce for nsISupports { .. }
+//! ```
+//!
+//! ## Implementing methods on an XPCOM Struct
+//!
+//! ```ignore
+//! // Methods should be implemented directly on the generated struct. All
+//! // methods other than `AddRef`, `Release`, and `QueryInterface` must be
+//! // implemented manually.
+//! impl ImplRunnable {
+//!     // The method should have the same name as the corresponding C++ method.
+//!     unsafe fn Run(&self) -> nsresult {
+//!         // Fields defined on the template struct will be directly on the
+//!         // generated struct.
+//!         println!("{}", self.i);
+//!         NS_OK
+//!     }
+//! }
+//! ```
+//!
+//! XPCOM methods implemented in Rust have signatures similar to methods
+//! implemented in C++.
+//!
+//! ```ignore
+//! // nsISupports foo(in long long bar, in AString baz);
+//! unsafe fn Foo(&self, bar: i64, baz: *const nsAString,
+//!               _retval: *mut *const nsISupports) -> nsresult;
+//!
+//! // AString qux(in nsISupports ham);
+//! unsafe fn Qux(&self, ham: *const nsISupports,
+//!               _retval: *mut nsAString) -> nsresult;
+//! ```
+//!
+//! This is a little tedious, so the `xpcom_method!` macro provides a convenient
+//! way to generate wrappers around more idiomatic Rust methods.
+//!
+//! [`xpcom`]: ../xpcom/index.html
+//! [`nsIRunnable`]: ../xpcom/struct.nsIRunnable.html
+//! [`RefCounted`]: ../xpcom/struct.RefCounted.html
+//! [`RefPtr`]: ../xpcom/struct.RefPtr.html
+
+use lazy_static::lazy_static;
+use proc_macro2::{Span, TokenStream};
+use quote::{format_ident, quote, ToTokens};
+use std::collections::{HashMap, HashSet};
+use syn::punctuated::Punctuated;
+use syn::{
+    parse_macro_input, parse_quote, AttributeArgs, Field, Fields, Ident, ItemStruct, Meta,
+    MetaList, NestedMeta, Path, Token, Type,
+};
+
+macro_rules! bail {
+    (@($t:expr), $s:expr) => {
+        return Err(syn::Error::new_spanned(&$t, &$s[..]))
+    };
+    (@($t:expr), $f:expr, $($e:expr),*) => {
+        return Err(syn::Error::new_spanned(&$t, &format!($f, $($e),*)[..]))
+    };
+    ($s:expr) => {
+        return Err(syn::Error::new(Span::call_site(), &$s[..]))
+    };
+    ($f:expr, $($e:expr),*) => {
+        return Err(syn::Error::new(Span::call_site(), &format!($f, $($e),*)[..]))
+    };
+}
+
+/* These are the structs generated by the rust_macros.py script */
+
+/// A single parameter to an XPCOM method.
+#[derive(Debug)]
+struct Param {
+    name: &'static str,
+    ty: &'static str,
+}
+
+/// A single method on an XPCOM interface.
+#[derive(Debug)]
+struct Method {
+    name: &'static str,
+    params: &'static [Param],
+    ret: &'static str,
+}
+
+/// An XPCOM interface. `methods` will be `Err("reason")` if the interface
+/// cannot be implemented in rust code.
+#[derive(Debug)]
+struct Interface {
+    name: &'static str,
+    base: Option<&'static str>,
+    methods: Result<&'static [Method], &'static str>,
+}
+
+impl Interface {
+    fn base(&self) -> Option<&'static Interface> {
+        Some(IFACES[self.base?])
+    }
+
+    fn methods(&self) -> Result<&'static [Method], syn::Error> {
+        match self.methods {
+            Ok(methods) => Ok(methods),
+            Err(reason) => Err(syn::Error::new(
+                Span::call_site(),
+                &format!(
+                    "Interface {} cannot be implemented in rust \
+                     because {} is not supported yet",
+                    self.name, reason
+                ),
+            )),
+        }
+    }
+}
+
+lazy_static! {
+    /// This item contains the information generated by the procedural macro in
+    /// the form of a `HashMap` from interface names to their descriptions.
+    static ref IFACES: HashMap<&'static str, &'static Interface> = {
+        let lists: &[&[Interface]] =
+            include!(mozbuild::objdir_path!("dist/xpcrs/bt/all.rs"));
+
+        let mut hm = HashMap::new();
+        for &list in lists {
+            for iface in list {
+                hm.insert(iface.name, iface);
+            }
+        }
+        hm
+    };
+}
+
+/// The type of the reference count to use for the struct.
+#[derive(Debug, Eq, PartialEq, Copy, Clone)]
+enum RefcntKind {
+    Atomic,
+    NonAtomic,
+}
+
+/// Produces the tokens for the type representation.
+impl ToTokens for RefcntKind {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        match *self {
+            RefcntKind::NonAtomic => quote!(xpcom::Refcnt).to_tokens(tokens),
+            RefcntKind::Atomic => quote!(xpcom::AtomicRefcnt).to_tokens(tokens),
+        }
+    }
+}
+
+/// Extract the fields list from the input struct.
+fn get_fields(si: &ItemStruct) -> Result<&Punctuated<Field, Token![,]>, syn::Error> {
+    match si.fields {
+        Fields::Named(ref named) => Ok(&named.named),
+        _ => bail!(@(si), "The initializer struct must be a standard named \
+                          value struct definition"),
+    }
+}
+
+/// Takes the template struct in, and generates `ItemStruct` for the "real" and
+/// "init" structs.
+fn gen_structs(
+    template: &ItemStruct,
+    bases: &[&Interface],
+    refcnt_ty: RefcntKind,
+) -> Result<(ItemStruct, ItemStruct), syn::Error> {
+    let real_ident = &template.ident;
+    let init_ident = format_ident!("Init{}", real_ident);
+    let vis = &template.vis;
+
+    let bases = bases.iter().map(|base| {
+        let ident = format_ident!("__base_{}", base.name);
+        let vtable = format_ident!("{}VTable", base.name);
+        quote!(#ident : *const xpcom::interfaces::#vtable)
+    });
+
+    let fields = get_fields(template)?;
+    let (impl_generics, _, where_clause) = template.generics.split_for_impl();
+    Ok((
+        parse_quote! {
+           #[repr(C)]
+           #vis struct #real_ident #impl_generics #where_clause {
+               #(#bases,)*
+               __refcnt: #refcnt_ty,
+               #fields
+           }
+        },
+        parse_quote! {
+           #vis struct #init_ident #impl_generics #where_clause {
+               #fields
+           }
+        },
+    ))
+}
+
+/// Generates the `extern "system"` methods which are actually included in the
+/// VTable for the given interface.
+///
+/// `idx` must be the offset in pointers of the pointer to this vtable in the
+/// struct `real`. This is soundness-critical, as it will be used to offset
+/// pointers received from xpcom back to the concrete implementation.
+fn gen_vtable_methods(
+    real: &ItemStruct,
+    iface: &Interface,
+    vtable_index: usize,
+) -> Result<TokenStream, syn::Error> {
+    let base_ty = format_ident!("{}", iface.name);
+
+    let base_methods = if let Some(base) = iface.base() {
+        gen_vtable_methods(real, base, vtable_index)?
+    } else {
+        quote! {}
+    };
+
+    let ty_name = &real.ident;
+    let (impl_generics, ty_generics, where_clause) = real.generics.split_for_impl();
+
+    let mut method_defs = Vec::new();
+    for method in iface.methods()? {
+        let ret = syn::parse_str::<Type>(method.ret)?;
+
+        let mut params = Vec::new();
+        let mut args = Vec::new();
+        for param in method.params {
+            let name = format_ident!("{}", param.name);
+            let ty = syn::parse_str::<Type>(param.ty)?;
+
+            params.push(quote! {#name : #ty,});
+            args.push(quote! {#name,});
+        }
+
+        let name = format_ident!("{}", method.name);
+        method_defs.push(quote! {
+            unsafe extern "system" fn #name #impl_generics (
+                this: *const #base_ty, #(#params)*
+            ) -> #ret #where_clause {
+                let this: &#ty_name #ty_generics =
+                    ::xpcom::reexports::transmute_from_vtable_ptr(&this, #vtable_index);
+                this.#name(#(#args)*)
+            }
+        });
+    }
+
+    Ok(quote! {
+        #base_methods
+        #(#method_defs)*
+    })
+}
+
+/// Generates the VTable for a given base interface. This assumes that the
+/// implementations of each of the `extern "system"` methods are in scope.
+fn gen_inner_vtable(real: &ItemStruct, iface: &Interface) -> Result<TokenStream, syn::Error> {
+    let vtable_ty = format_ident!("{}VTable", iface.name);
+
+    // Generate the vtable for the base interface.
+    let base_vtable = if let Some(base) = iface.base() {
+        let vt = gen_inner_vtable(real, base)?;
+        quote! {__base: #vt,}
+    } else {
+        quote! {}
+    };
+
+    // Include each of the method definitions for this interface.
+    let (_, ty_generics, _) = real.generics.split_for_impl();
+    let turbofish = ty_generics.as_turbofish();
+    let vtable_init = iface
+        .methods()?
+        .into_iter()
+        .map(|method| {
+            let name = format_ident!("{}", method.name);
+            quote! { #name : #name #turbofish, }
+        })
+        .collect::<Vec<_>>();
+
+    Ok(quote!(#vtable_ty {
+        #base_vtable
+        #(#vtable_init)*
+    }))
+}
+
+fn gen_root_vtable(
+    real: &ItemStruct,
+    base: &Interface,
+    idx: usize,
+) -> Result<TokenStream, syn::Error> {
+    let field = format_ident!("__base_{}", base.name);
+    let vtable_ty = format_ident!("{}VTable", base.name);
+
+    let (impl_generics, ty_generics, where_clause) = real.generics.split_for_impl();
+    let turbofish = ty_generics.as_turbofish();
+
+    let methods = gen_vtable_methods(real, base, idx)?;
+    let vtable = gen_inner_vtable(real, base)?;
+
+    // Define the `recover_self` method. This performs an offset calculation to
+    // recover a pointer to the original struct from a pointer to the given
+    // VTable field.
+    Ok(quote! {#field: {
+        // The method implementations which will be used to build the vtable.
+        #methods
+
+        // The actual VTable definition. This is in a separate method in order
+        // to allow it to be generic.
+        #[inline]
+        fn get_vtable #impl_generics () -> &'static ::xpcom::reexports::VTableExtra<#vtable_ty> #where_clause {
+            &::xpcom::reexports::VTableExtra {
+                #[cfg(not(windows))]
+                offset: {
+                    // NOTE: workaround required to avoid depending on the
+                    // unstable const expression feature `const {}`.
+                    const OFFSET: isize = -((::std::mem::size_of::<usize>() * #idx) as isize);
+                    OFFSET
+                },
+                #[cfg(not(windows))]
+                typeinfo: 0 as *const _,
+                vtable: #vtable,
+            }
+        }
+        &get_vtable #turbofish ().vtable
+    },})
+}
+
+/// Generate the cast implementations. This generates the implementation details
+/// for the `Coerce` trait, and the `QueryInterface` method. The first return
+/// value is the `QueryInterface` implementation, and the second is the `Coerce`
+/// implementation.
+fn gen_casts(
+    seen: &mut HashSet<&'static str>,
+    iface: &Interface,
+    real: &ItemStruct,
+    coerce_name: &Ident,
+    vtable_field: &Ident,
+) -> Result<(TokenStream, TokenStream), syn::Error> {
+    if !seen.insert(iface.name) {
+        return Ok((quote! {}, quote! {}));
+    }
+
+    // Generate the cast implementations for the base interfaces.
+    let (base_qi, base_coerce) = if let Some(base) = iface.base() {
+        gen_casts(seen, base, real, coerce_name, vtable_field)?
+    } else {
+        (quote! {}, quote! {})
+    };
+
+    // Add the if statment to QueryInterface for the base class.
+    let base_name = format_ident!("{}", iface.name);
+
+    let qi = quote! {
+        #base_qi
+        if *uuid == #base_name::IID {
+            // Implement QueryInterface in terms of coercions.
+            self.addref();
+            *result = self.coerce::<#base_name>()
+                as *const #base_name
+                as *const ::xpcom::reexports::libc::c_void
+                as *mut ::xpcom::reexports::libc::c_void;
+            return ::xpcom::reexports::NS_OK;
+        }
+    };
+
+    // Add an implementation of the `*Coerce` trait for the base interface.
+    let name = &real.ident;
+    let (impl_generics, ty_generics, where_clause) = real.generics.split_for_impl();
+    let coerce = quote! {
+        #base_coerce
+
+        impl #impl_generics #coerce_name #ty_generics for ::xpcom::interfaces::#base_name #where_clause {
+            fn coerce_from(v: &#name #ty_generics) -> &Self {
+                unsafe {
+                    // Get the address of the VTable field. This should be a
+                    // pointer to a pointer to a vtable, which we can then cast
+                    // into a pointer to our interface.
+                    &*(&(v.#vtable_field)
+                        as *const *const _
+                        as *const ::xpcom::interfaces::#base_name)
+                }
+            }
+        }
+    };
+
+    Ok((qi, coerce))
+}
+
+fn check_generics(generics: &syn::Generics) -> Result<(), syn::Error> {
+    for param in &generics.params {
+        let tp = match param {
+            syn::GenericParam::Type(tp) => tp,
+            syn::GenericParam::Lifetime(lp) => bail!(
+                @(lp),
+                "Cannot use #[xpcom] on types with lifetime parameters. \
+                Implementors of XPCOM interfaces must not contain non-'static \
+                lifetimes.",
+            ),
+            // XXX: Once const generics become stable, it may be as simple as
+            // removing this bail! to support them.
+            syn::GenericParam::Const(cp) => {
+                bail!(@(cp), "Cannot use #[xpcom] on types with const generics.")
+            }
+        };
+
+        let mut static_lt = false;
+        for bound in &tp.bounds {
+            match bound {
+                syn::TypeParamBound::Lifetime(lt) if lt.ident == "static" => {
+                    static_lt = true;
+                    break;
+                }
+                _ => {}
+            }
+        }
+
+        if !static_lt {
+            bail!(
+                @(param),
+                "Every generic parameter for xpcom implementation must have a \
+                'static lifetime bound declared in the generics. Implicit \
+                lifetime bounds or lifetime bounds in where clauses are not \
+                detected by the macro and will be ignored. \
+                Implementors of XPCOM interfaces must not contain non-'static \
+                lifetimes.",
+            );
+        }
+    }
+    Ok(())
+}
+
+#[derive(Default)]
+struct Options {
+    bases: Vec<&'static Interface>,
+    refcnt: Option<RefcntKind>,
+}
+
+impl Options {
+    fn parse_path_arg(&mut self, path: &Path) -> Result<(), syn::Error> {
+        if path.is_ident("atomic") || path.is_ident("nonatomic") {
+            if self.refcnt.is_some() {
+                bail!(@(path), "Duplicate refcnt atomicity specifier");
+            }
+            self.refcnt = Some(if path.is_ident("atomic") {
+                RefcntKind::Atomic
+            } else {
+                RefcntKind::NonAtomic
+            });
+            return Ok(());
+        }
+        bail!(@(path), "Unexpected path argument to #[xpcom]");
+    }
+
+    fn parse_list_arg(&mut self, list: &MetaList) -> Result<(), syn::Error> {
+        if list.path.is_ident("implement") {
+            for item in list.nested.iter() {
+                let iface = match *item {
+                    NestedMeta::Meta(syn::Meta::Path(ref iface)) => iface,
+                    _ => bail!(@(item), "Expected interface name to implement"),
+                };
+                let ident = match iface.get_ident() {
+                    Some(ref iface) => iface.to_string(),
+                    _ => bail!(@(iface), "Interface name must be unqualified"),
+                };
+                if let Some(&iface) = IFACES.get(ident.as_str()) {
+                    self.bases.push(iface);
+                } else {
+                    bail!(@(item), "Invalid base interface `{}`", ident);
+                }
+            }
+            return Ok(());
+        }
+        bail!(@(list), "Unexpected list argument to #[xpcom]");
+    }
+
+    fn parse(&mut self, args: &AttributeArgs) -> Result<(), syn::Error> {
+        for arg in args {
+            match arg {
+                NestedMeta::Meta(Meta::Path(path)) => self.parse_path_arg(path)?,
+                NestedMeta::Meta(Meta::List(list)) => self.parse_list_arg(list)?,
+                NestedMeta::Meta(Meta::NameValue(name_value)) => {
+                    bail!(@(name_value), "Unexpected name-value argument to #[xpcom]")
+                }
+                NestedMeta::Lit(lit) => bail!(@(lit), "Unexpected literal argument to #[xpcom]"),
+            }
+        }
+
+        if self.bases.is_empty() {
+            bail!(
+                "Types with #[xpcom(..)] must implement at least one \
+                interface. Interfaces can be implemented by adding an \
+                implements(nsIFoo, nsIBar) parameter to the #[xpcom] attribute"
+            );
+        }
+
+        if self.refcnt.is_none() {
+            bail!("Must specify refcnt kind in #[xpcom] attribute");
+        }
+
+        Ok(())
+    }
+}
+
+/// The root xpcom procedural macro definition.
+fn xpcom_impl(args: AttributeArgs, template: ItemStruct) -> Result<TokenStream, syn::Error> {
+    let mut options = Options::default();
+    options.parse(&args)?;
+
+    check_generics(&template.generics)?;
+
+    let bases = options.bases;
+
+    // Ensure that all our base interface methods have unique names.
+    let mut method_names = HashMap::new();
+    for base in &bases {
+        for method in base.methods()? {
+            if let Some(existing) = method_names.insert(method.name, base.name) {
+                bail!(
+                    "The method `{0}` is declared on both `{1}` and `{2}`,
+                     but a Rust type cannot implement two methods with the \
+                     same name. You can add the `[binaryname(Renamed{0})]` \
+                     XPIDL attribute to one of the declarations to rename it.",
+                    method.name,
+                    existing,
+                    base.name
+                );
+            }
+        }
+    }
+
+    // Determine what reference count type to use, and generate the real struct.
+    let refcnt_ty = options.refcnt.unwrap();
+    let (real, init) = gen_structs(&template, &bases, refcnt_ty)?;
+
+    let name_init = &init.ident;
+    let name = &real.ident;
+    let coerce_name = format_ident!("{}Coerce", name);
+
+    // Generate a VTable for each of the base interfaces.
+    let mut vtables = Vec::new();
+    for (idx, base) in bases.iter().enumerate() {
+        vtables.push(gen_root_vtable(&real, base, idx)?);
+    }
+
+    // Generate the field initializers for the final struct, moving each field
+    // out of the original __init struct.
+    let inits = get_fields(&init)?.iter().map(|field| {
+        let id = &field.ident;
+        quote! { #id : __init.#id, }
+    });
+
+    let vis = &real.vis;
+
+    // Generate the implementation for QueryInterface and Coerce.
+    let mut seen = HashSet::new();
+    let mut qi_impl = Vec::new();
+    let mut coerce_impl = Vec::new();
+    for base in &bases {
+        let (qi, coerce) = gen_casts(
+            &mut seen,
+            base,
+            &real,
+            &coerce_name,
+            &format_ident!("__base_{}", base.name),
+        )?;
+        qi_impl.push(qi);
+        coerce_impl.push(coerce);
+    }
+
+    let size_for_logs = if real.generics.params.is_empty() {
+        quote!(::std::mem::size_of::<Self>() as u32)
+    } else {
+        // Refcount logging requires all types with the same name to have the
+        // same size, and generics aren't taken into account when creating our
+        // name string, so we need to make sure that all possible instantiations
+        // report the same size. To do that, we fake a size based on the number
+        // of vtable pointers and the known refcount field.
+        let fake_size_npointers = bases.len() + 1;
+        quote!((::std::mem::size_of::<usize>() * #fake_size_npointers) as u32)
+    };
+
+    let (impl_generics, ty_generics, where_clause) = real.generics.split_for_impl();
+    let name_for_logs = quote!(
+        concat!(module_path!(), "::", stringify!(#name #ty_generics), "\0").as_ptr()
+            as *const ::xpcom::reexports::libc::c_char
+    );
+    Ok(quote! {
+        #init
+
+        #real
+
+        impl #impl_generics #name #ty_generics #where_clause {
+            /// This method is used for
+            fn allocate(__init: #name_init #ty_generics) -> ::xpcom::RefPtr<Self> {
+                #[allow(unused_imports)]
+                use ::xpcom::*;
+                #[allow(unused_imports)]
+                use ::xpcom::interfaces::*;
+                #[allow(unused_imports)]
+                use ::xpcom::reexports::{
+                    libc, nsACString, nsAString, nsCString, nsString, nsresult
+                };
+
+                // Helper for asserting that for all instantiations, this
+                // object has the 'static lifetime.
+                fn xpcom_types_must_be_static<T: 'static>(t: &T) {}
+
+                unsafe {
+                    // NOTE: This is split into multiple lines to make the
+                    // output more readable.
+                    let value = #name {
+                        #(#vtables)*
+                        __refcnt: #refcnt_ty::new(),
+                        #(#inits)*
+                    };
+                    let boxed = ::std::boxed::Box::new(value);
+                    xpcom_types_must_be_static(&*boxed);
+                    let raw = ::std::boxed::Box::into_raw(boxed);
+                    ::xpcom::RefPtr::from_raw(raw).unwrap()
+                }
+            }
+
+            /// Automatically generated implementation of AddRef for nsISupports.
+            #vis unsafe fn AddRef(&self) -> ::xpcom::MozExternalRefCountType {
+                let new = self.__refcnt.inc();
+                ::xpcom::trace_refcnt::NS_LogAddRef(
+                    self as *const _ as *mut ::xpcom::reexports::libc::c_void,
+                    new as usize,
+                    #name_for_logs,
+                    #size_for_logs,
+                );
+                new
+            }
+
+            /// Automatically generated implementation of Release for nsISupports.
+            #vis unsafe fn Release(&self) -> ::xpcom::MozExternalRefCountType {
+                let new = self.__refcnt.dec();
+                ::xpcom::trace_refcnt::NS_LogRelease(
+                    self as *const _ as *mut ::xpcom::reexports::libc::c_void,
+                    new as usize,
+                    #name_for_logs,
+                    #size_for_logs,
+                );
+                if new == 0 {
+                    // dealloc
+                    ::std::mem::drop(::std::boxed::Box::from_raw(self as *const Self as *mut Self));
+                }
+                new
+            }
+
+            /// Automatically generated implementation of QueryInterface for
+            /// nsISupports.
+            #vis unsafe fn QueryInterface(&self,
+                                          uuid: *const ::xpcom::nsIID,
+                                          result: *mut *mut ::xpcom::reexports::libc::c_void)
+                                          -> ::xpcom::reexports::nsresult {
+                #[allow(unused_imports)]
+                use ::xpcom::*;
+                #[allow(unused_imports)]
+                use ::xpcom::interfaces::*;
+
+                #(#qi_impl)*
+
+                ::xpcom::reexports::NS_ERROR_NO_INTERFACE
+            }
+
+            /// Perform a QueryInterface call on this object, attempting to
+            /// dynamically cast it to the requested interface type. Returns
+            /// Some(RefPtr<T>) if the cast succeeded, and None otherwise.
+            #vis fn query_interface<XPCOM_InterfaceType: ::xpcom::XpCom>(&self)
+                -> ::std::option::Option<::xpcom::RefPtr<XPCOM_InterfaceType>>
+            {
+                let mut ga = ::xpcom::GetterAddrefs::<XPCOM_InterfaceType>::new();
+                unsafe {
+                    if self.QueryInterface(&XPCOM_InterfaceType::IID, ga.void_ptr()).succeeded() {
+                        ga.refptr()
+                    } else {
+                        None
+                    }
+                }
+            }
+
+            /// Coerce this type safely to any of the interfaces which it
+            /// implements without `AddRef`ing it.
+            #vis fn coerce<XPCOM_InterfaceType: #coerce_name #ty_generics>(&self) -> &XPCOM_InterfaceType {
+                XPCOM_InterfaceType::coerce_from(self)
+            }
+        }
+
+        /// This trait is implemented on the interface types which this
+        /// `#[xpcom]` type can be safely ane cheaply coerced to using the
+        /// `coerce` method.
+        ///
+        /// The trait and its method should usually not be used directly, but
+        /// rather acts as a trait bound and implementation for the `coerce`
+        /// methods.
+        #[doc(hidden)]
+        #vis trait #coerce_name #impl_generics #where_clause {
+            /// Convert a value of the `#[xpcom]` type into the implementing
+            /// interface type.
+            fn coerce_from(v: &#name #ty_generics) -> &Self;
+        }
+
+        #(#coerce_impl)*
+
+        unsafe impl #impl_generics ::xpcom::RefCounted for #name #ty_generics #where_clause {
+            unsafe fn addref(&self) {
+                self.AddRef();
+            }
+
+            unsafe fn release(&self) {
+                self.Release();
+            }
+        }
+    })
+}
+
+#[proc_macro_attribute]
+pub fn xpcom(
+    args: proc_macro::TokenStream,
+    input: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+    let args = parse_macro_input!(args as AttributeArgs);
+    let input = parse_macro_input!(input as ItemStruct);
+    match xpcom_impl(args, input) {
+        Ok(ts) => ts.into(),
+        Err(err) => err.to_compile_error().into(),
+    }
+}