Adding upstream version 124.0.1.upstream/124.0.1

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

7 files changed, 3316 insertions, 0 deletions

diff --git a/third_party/rust/ffi-support/src/error.rs b/third_party/rust/ffi-support/src/error.rs
new file mode 100644
index 0000000000..6bc2808b2c
--- /dev/null
+++ b/third_party/rust/ffi-support/src/error.rs
@@ -0,0 +1,365 @@
+/* Copyright 2018-2019 Mozilla Foundation
+ *
+ * Licensed under the Apache License (Version 2.0), or the MIT license,
+ * (the "Licenses") at your option. You may not use this file except in
+ * compliance with one of the Licenses. You may obtain copies of the
+ * Licenses at:
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *    http://opensource.org/licenses/MIT
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the Licenses is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the Licenses for the specific language governing permissions and
+ * limitations under the Licenses. */
+
+use crate::string::{destroy_c_string, rust_string_to_c};
+use std::os::raw::c_char;
+use std::{self, ptr};
+
+/// Represents an error that occured within rust, storing both an error code, and additional data
+/// that may be used by the caller.
+///
+/// Misuse of this type can cause numerous issues, so please read the entire documentation before
+/// usage.
+///
+/// ## Rationale
+///
+/// This library encourages a pattern of taking a `&mut ExternError` as the final parameter for
+/// functions exposed over the FFI. This is an "out parameter" which we use to write error/success
+/// information that occurred during the function's execution.
+///
+/// To be clear, this means instances of `ExternError` will be created on the other side of the FFI,
+/// and passed (by mutable reference) into Rust.
+///
+/// While this pattern is not particularly ergonomic in Rust (although hopefully this library
+/// helps!), it offers two main benefits over something more ergonomic (which might be `Result`
+/// shaped).
+///
+/// 1. It avoids defining a large number of `Result`-shaped types in the FFI consumer, as would
+///    be required with something like an `struct ExternResult<T> { ok: *mut T, err:... }`
+///
+/// 2. It offers additional type safety over `struct ExternResult { ok: *mut c_void, err:... }`,
+///    which helps avoid memory safety errors. It also can offer better performance for returning
+///    primitives and repr(C) structs (no boxing required).
+///
+/// It also is less tricky to use properly than giving consumers a `get_last_error()` function, or
+/// similar.
+///
+/// ## Caveats
+///
+/// Note that the order of the fields is `code` (an i32) then `message` (a `*mut c_char`), getting
+/// this wrong on the other side of the FFI will cause memory corruption and crashes.
+///
+/// The fields are public largely for documentation purposes, but you should use
+/// [`ExternError::new_error`] or [`ExternError::success`] to create these.
+///
+/// ## Layout/fields
+///
+/// This struct's field are not `pub` (mostly so that we can soundly implement `Send`, but also so
+/// that we can verify rust users are constructing them appropriately), the fields, their types, and
+/// their order are *very much* a part of the public API of this type. Consumers on the other side
+/// of the FFI will need to know its layout.
+///
+/// If this were a C struct, it would look like
+///
+/// ```c,no_run
+/// struct ExternError {
+///     int32_t code;
+///     char *message; // note: nullable
+/// };
+/// ```
+///
+/// In rust, there are two fields, in this order: `code: ErrorCode`, and `message: *mut c_char`.
+/// Note that ErrorCode is a `#[repr(transparent)]` wrapper around an `i32`, so the first property
+/// is equivalent to an `i32`.
+///
+/// #### The `code` field.
+///
+/// This is the error code, 0 represents success, all other values represent failure. If the `code`
+/// field is nonzero, there should always be a message, and if it's zero, the message will always be
+/// null.
+///
+/// #### The `message` field.
+///
+/// This is a null-terminated C string containing some amount of additional information about the
+/// error. If the `code` property is nonzero, there should always be an error message. Otherwise,
+/// this should will be null.
+///
+/// This string (when not null) is allocated on the rust heap (using this crate's
+/// [`rust_string_to_c`]), and must be freed on it as well. Critically, if there are multiple rust
+/// packages using being used in the same application, it *must be freed on the same heap that
+/// allocated it*, or you will corrupt both heaps.
+///
+/// Typically, this object is managed on the other side of the FFI (on the "FFI consumer"), which
+/// means you must expose a function to release the resources of `message` which can be done easily
+/// using the [`define_string_destructor!`] macro provided by this crate.
+///
+/// If, for some reason, you need to release the resources directly, you may call
+/// `ExternError::release()`. Note that you probably do not need to do this, and it's
+/// intentional that this is not called automatically by implementing `drop`.
+///
+/// ## Example
+///
+/// ```rust,no_run
+/// use ffi_support::{ExternError, ErrorCode};
+///
+/// #[derive(Debug)]
+/// pub enum MyError {
+///     IllegalFoo(String),
+///     InvalidBar(i64),
+///     // ...
+/// }
+///
+/// // Putting these in a module is obviously optional, but it allows documentation, and helps
+/// // avoid accidental reuse.
+/// pub mod error_codes {
+///     // note: -1 and 0 are reserved by ffi_support
+///     pub const ILLEGAL_FOO: i32 = 1;
+///     pub const INVALID_BAR: i32 = 2;
+///     // ...
+/// }
+///
+/// fn get_code(e: &MyError) -> ErrorCode {
+///     match e {
+///         MyError::IllegalFoo(_) => ErrorCode::new(error_codes::ILLEGAL_FOO),
+///         MyError::InvalidBar(_) => ErrorCode::new(error_codes::INVALID_BAR),
+///         // ...
+///     }
+/// }
+///
+/// impl From<MyError> for ExternError {
+///     fn from(e: MyError) -> ExternError {
+///         ExternError::new_error(get_code(&e), format!("{:?}", e))
+///     }
+/// }
+/// ```
+#[repr(C)]
+// Note: We're intentionally not implementing Clone -- it's too risky.
+#[derive(Debug, PartialEq)]
+pub struct ExternError {
+    // Don't reorder or add anything here!
+    code: ErrorCode,
+    message: *mut c_char,
+}
+
+impl std::panic::UnwindSafe for ExternError {}
+impl std::panic::RefUnwindSafe for ExternError {}
+
+/// This is sound so long as our fields are private.
+unsafe impl Send for ExternError {}
+
+impl ExternError {
+    /// Construct an ExternError representing failure from a code and a message.
+    #[inline]
+    pub fn new_error(code: ErrorCode, message: impl Into<String>) -> Self {
+        assert!(
+            !code.is_success(),
+            "Attempted to construct a success ExternError with a message"
+        );
+        Self {
+            code,
+            message: rust_string_to_c(message),
+        }
+    }
+
+    /// Returns a ExternError representing a success. Also returned by ExternError::default()
+    #[inline]
+    pub fn success() -> Self {
+        Self {
+            code: ErrorCode::SUCCESS,
+            message: ptr::null_mut(),
+        }
+    }
+
+    /// Helper for the case where we aren't exposing this back over the FFI and
+    /// we just want to warn if an error occurred and then release the allocated
+    /// memory.
+    ///
+    /// Typically, this is done if the error will still be detected and reported
+    /// by other channels.
+    ///
+    /// We assume we're not inside a catch_unwind, and so we wrap inside one
+    /// ourselves.
+    pub fn consume_and_log_if_error(self) {
+        if !self.code.is_success() {
+            // in practice this should never panic, but you never know...
+            crate::abort_on_panic::call_with_output(|| {
+                log::error!("Unhandled ExternError({:?}) {:?}", self.code, unsafe {
+                    crate::FfiStr::from_raw(self.message)
+                });
+                unsafe {
+                    self.manually_release();
+                }
+            })
+        }
+    }
+
+    /// Get the `code` property.
+    #[inline]
+    pub fn get_code(&self) -> ErrorCode {
+        self.code
+    }
+
+    /// Get the `message` property as a pointer to c_char.
+    #[inline]
+    pub fn get_raw_message(&self) -> *const c_char {
+        self.message as *const _
+    }
+
+    /// Get the `message` property as an [`FfiStr`][crate::FfiStr]
+    #[inline]
+    pub fn get_message(&self) -> crate::FfiStr<'_> {
+        // Safe because the lifetime is the same as our lifetime.
+        unsafe { crate::FfiStr::from_raw(self.get_raw_message()) }
+    }
+
+    /// Get the `message` property as a String, or None if this is not an error result.
+    ///
+    /// ## Safety
+    ///
+    /// You should only call this if you are certain that the other side of the FFI doesn't have a
+    /// reference to this result (more specifically, to the `message` property) anywhere!
+    #[inline]
+    pub unsafe fn get_and_consume_message(self) -> Option<String> {
+        if self.code.is_success() {
+            None
+        } else {
+            let res = self.get_message().into_string();
+            self.manually_release();
+            Some(res)
+        }
+    }
+
+    /// Manually release the memory behind this string. You probably don't want to call this.
+    ///
+    /// ## Safety
+    ///
+    /// You should only call this if you are certain that the other side of the FFI doesn't have a
+    /// reference to this result (more specifically, to the `message` property) anywhere!
+    pub unsafe fn manually_release(self) {
+        if !self.message.is_null() {
+            destroy_c_string(self.message)
+        }
+    }
+}
+
+impl Default for ExternError {
+    #[inline]
+    fn default() -> Self {
+        ExternError::success()
+    }
+}
+
+// This is the `Err` of std::thread::Result, which is what
+// `panic::catch_unwind` returns.
+impl From<Box<dyn std::any::Any + Send + 'static>> for ExternError {
+    fn from(e: Box<dyn std::any::Any + Send + 'static>) -> Self {
+        // The documentation suggests that it will *usually* be a str or String.
+        let message = if let Some(s) = e.downcast_ref::<&'static str>() {
+            (*s).to_string()
+        } else if let Some(s) = e.downcast_ref::<String>() {
+            s.clone()
+        } else {
+            "Unknown panic!".to_string()
+        };
+        log::error!("Caught a panic calling rust code: {:?}", message);
+        ExternError::new_error(ErrorCode::PANIC, message)
+    }
+}
+
+/// A wrapper around error codes, which is represented identically to an i32 on the other side of
+/// the FFI. Essentially exists to check that we don't accidentally reuse success/panic codes for
+/// other things.
+#[repr(transparent)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Default)]
+pub struct ErrorCode(i32);
+
+impl ErrorCode {
+    /// The ErrorCode used for success.
+    pub const SUCCESS: ErrorCode = ErrorCode(0);
+
+    /// The ErrorCode used for panics. It's unlikely you need to ever use this.
+    // TODO: Consider moving to the reserved region...
+    pub const PANIC: ErrorCode = ErrorCode(-1);
+
+    /// The ErrorCode used for handle map errors.
+    pub const INVALID_HANDLE: ErrorCode = ErrorCode(-1000);
+
+    /// Construct an error code from an integer code.
+    ///
+    /// ## Panics
+    ///
+    /// Panics if you call it with 0 (reserved for success, but you can use `ErrorCode::SUCCESS` if
+    /// that's what you want), or -1 (reserved for panics, but you can use `ErrorCode::PANIC` if
+    /// that's what you want).
+    pub fn new(code: i32) -> Self {
+        assert!(code > ErrorCode::INVALID_HANDLE.0 && code != ErrorCode::PANIC.0 && code != ErrorCode::SUCCESS.0,
+            "Error: The ErrorCodes `{success}`, `{panic}`, and all error codes less than or equal \
+            to `{reserved}` are reserved (got {code}). You may use the associated constants on this \
+            type (`ErrorCode::PANIC`, etc) if you'd like instances of those error codes.",
+            panic = ErrorCode::PANIC.0,
+            success = ErrorCode::SUCCESS.0,
+            reserved = ErrorCode::INVALID_HANDLE.0,
+            code = code,
+        );
+
+        ErrorCode(code)
+    }
+
+    /// Get the raw numeric value of this ErrorCode.
+    #[inline]
+    pub fn code(self) -> i32 {
+        self.0
+    }
+
+    /// Returns whether or not this is a success code.
+    #[inline]
+    pub fn is_success(self) -> bool {
+        self.code() == 0
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    #[should_panic]
+    fn test_code_new_reserved_success() {
+        ErrorCode::new(0);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_code_new_reserved_panic() {
+        ErrorCode::new(-1);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_code_new_reserved_handle_error() {
+        ErrorCode::new(-1000);
+    }
+    #[test]
+    #[should_panic]
+    fn test_code_new_reserved_unknown() {
+        // Everything below -1000 should be reserved.
+        ErrorCode::new(-1043);
+    }
+
+    #[test]
+    fn test_code_new_allowed() {
+        // Should not panic
+        ErrorCode::new(-2);
+    }
+
+    #[test]
+    fn test_code() {
+        assert!(!ErrorCode::PANIC.is_success());
+        assert!(!ErrorCode::INVALID_HANDLE.is_success());
+        assert!(ErrorCode::SUCCESS.is_success());
+        assert_eq!(ErrorCode::default(), ErrorCode::SUCCESS);
+    }
+}
diff --git a/third_party/rust/ffi-support/src/ffistr.rs b/third_party/rust/ffi-support/src/ffistr.rs
new file mode 100644
index 0000000000..fb60e1f72c
--- /dev/null
+++ b/third_party/rust/ffi-support/src/ffistr.rs
@@ -0,0 +1,252 @@
+/* Copyright 2018-2019 Mozilla Foundation
+ *
+ * Licensed under the Apache License (Version 2.0), or the MIT license,
+ * (the "Licenses") at your option. You may not use this file except in
+ * compliance with one of the Licenses. You may obtain copies of the
+ * Licenses at:
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *    http://opensource.org/licenses/MIT
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the Licenses is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the Licenses for the specific language governing permissions and
+ * limitations under the Licenses. */
+
+use std::ffi::CStr;
+use std::marker::PhantomData;
+use std::os::raw::c_char;
+
+/// `FfiStr<'a>` is a safe (`#[repr(transparent)]`) wrapper around a
+/// nul-terminated `*const c_char` (e.g. a C string). Conceptually, it is
+/// similar to [`std::ffi::CStr`], except that it may be used in the signatures
+/// of extern "C" functions.
+///
+/// Functions accepting strings should use this instead of accepting a C string
+/// directly. This allows us to write those functions using safe code without
+/// allowing safe Rust to cause memory unsafety.
+///
+/// A single function for constructing these from Rust ([`FfiStr::from_raw`])
+/// has been provided. Most of the time, this should not be necessary, and users
+/// should accept `FfiStr` in the parameter list directly.
+///
+/// ## Caveats
+///
+/// An effort has been made to make this struct hard to misuse, however it is
+/// still possible, if the `'static` lifetime is manually specified in the
+/// struct. E.g.
+///
+/// ```rust,no_run
+/// # use ffi_support::FfiStr;
+/// // NEVER DO THIS
+/// #[no_mangle]
+/// extern "C" fn never_do_this(s: FfiStr<'static>) {
+///     // save `s` somewhere, and access it after this
+///     // function returns.
+/// }
+/// ```
+///
+/// Instead, one of the following patterns should be used:
+///
+/// ```
+/// # use ffi_support::FfiStr;
+/// #[no_mangle]
+/// extern "C" fn valid_use_1(s: FfiStr<'_>) {
+///     // Use of `s` after this function returns is impossible
+/// }
+/// // Alternative:
+/// #[no_mangle]
+/// extern "C" fn valid_use_2(s: FfiStr) {
+///     // Use of `s` after this function returns is impossible
+/// }
+/// ```
+#[repr(transparent)]
+pub struct FfiStr<'a> {
+    cstr: *const c_char,
+    _boo: PhantomData<&'a ()>,
+}
+
+impl<'a> FfiStr<'a> {
+    /// Construct an `FfiStr` from a raw pointer.
+    ///
+    /// This should not be needed most of the time, and users should instead
+    /// accept `FfiStr` in function parameter lists.
+    ///
+    /// # Safety
+    ///
+    /// Dereferences a pointer and is thus unsafe.
+    #[inline]
+    pub unsafe fn from_raw(ptr: *const c_char) -> Self {
+        Self {
+            cstr: ptr,
+            _boo: PhantomData,
+        }
+    }
+
+    /// Construct a FfiStr from a `std::ffi::CStr`. This is provided for
+    /// completeness, as a safe method of producing an `FfiStr` in Rust.
+    #[inline]
+    pub fn from_cstr(cstr: &'a CStr) -> Self {
+        Self {
+            cstr: cstr.as_ptr(),
+            _boo: PhantomData,
+        }
+    }
+
+    /// Get an `&str` out of the `FfiStr`. This will panic in any case that
+    /// [`FfiStr::as_opt_str`] would return `None` (e.g. null pointer or invalid
+    /// UTF-8).
+    ///
+    /// If the string should be optional, you should use [`FfiStr::as_opt_str`]
+    /// instead. If an owned string is desired, use [`FfiStr::into_string`] or
+    /// [`FfiStr::into_opt_string`].
+    #[inline]
+    pub fn as_str(&self) -> &'a str {
+        self.as_opt_str()
+            .expect("Unexpected null string pointer passed to rust")
+    }
+
+    /// Get an `Option<&str>` out of the `FfiStr`. If this stores a null
+    /// pointer, then None will be returned. If a string containing invalid
+    /// UTF-8 was passed, then an error will be logged and `None` will be
+    /// returned.
+    ///
+    /// If the string is a required argument, use [`FfiStr::as_str`], or
+    /// [`FfiStr::into_string`] instead. If `Option<String>` is desired, use
+    /// [`FfiStr::into_opt_string`] (which will handle invalid UTF-8 by
+    /// replacing with the replacement character).
+    pub fn as_opt_str(&self) -> Option<&'a str> {
+        if self.cstr.is_null() {
+            return None;
+        }
+        unsafe {
+            match std::ffi::CStr::from_ptr(self.cstr).to_str() {
+                Ok(s) => Some(s),
+                Err(e) => {
+                    log::error!("Invalid UTF-8 was passed to rust! {:?}", e);
+                    None
+                }
+            }
+        }
+    }
+
+    /// Get an `Option<String>` out of the `FfiStr`. Returns `None` if this
+    /// `FfiStr` holds a null pointer. Note that unlike [`FfiStr::as_opt_str`],
+    /// invalid UTF-8 is replaced with the replacement character instead of
+    /// causing us to return None.
+    ///
+    /// If the string should be mandatory, you should use
+    /// [`FfiStr::into_string`] instead. If an owned string is not needed, you
+    /// may want to use [`FfiStr::as_str`] or [`FfiStr::as_opt_str`] instead,
+    /// (however, note the differences in how invalid UTF-8 is handled, should
+    /// this be relevant to your use).
+    pub fn into_opt_string(self) -> Option<String> {
+        if !self.cstr.is_null() {
+            unsafe { Some(CStr::from_ptr(self.cstr).to_string_lossy().to_string()) }
+        } else {
+            None
+        }
+    }
+
+    /// Get a `String` out of a `FfiStr`. This function is essential a
+    /// convenience wrapper for `ffi_str.into_opt_string().unwrap()`, with a
+    /// message that indicates that a null argument was passed to rust when it
+    /// should be mandatory. As with [`FfiStr::into_opt_string`], invalid UTF-8
+    /// is replaced with the replacement character if encountered.
+    ///
+    /// If the string should *not* be mandatory, you should use
+    /// [`FfiStr::into_opt_string`] instead. If an owned string is not needed,
+    /// you may want to use [`FfiStr::as_str`] or [`FfiStr::as_opt_str`]
+    /// instead, (however, note the differences in how invalid UTF-8 is handled,
+    /// should this be relevant to your use).
+    #[inline]
+    pub fn into_string(self) -> String {
+        self.into_opt_string()
+            .expect("Unexpected null string pointer passed to rust")
+    }
+}
+
+impl<'a> std::fmt::Debug for FfiStr<'a> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        if let Some(s) = self.as_opt_str() {
+            write!(f, "FfiStr({:?})", s)
+        } else {
+            write!(f, "FfiStr(null)")
+        }
+    }
+}
+
+// Conversions...
+
+impl<'a> From<FfiStr<'a>> for String {
+    #[inline]
+    fn from(f: FfiStr<'a>) -> Self {
+        f.into_string()
+    }
+}
+
+impl<'a> From<FfiStr<'a>> for Option<String> {
+    #[inline]
+    fn from(f: FfiStr<'a>) -> Self {
+        f.into_opt_string()
+    }
+}
+
+impl<'a> From<FfiStr<'a>> for Option<&'a str> {
+    #[inline]
+    fn from(f: FfiStr<'a>) -> Self {
+        f.as_opt_str()
+    }
+}
+
+impl<'a> From<FfiStr<'a>> for &'a str {
+    #[inline]
+    fn from(f: FfiStr<'a>) -> Self {
+        f.as_str()
+    }
+}
+
+// TODO: `AsRef<str>`?
+
+// Comparisons...
+
+// Compare FfiStr with eachother
+impl<'a> PartialEq for FfiStr<'a> {
+    #[inline]
+    fn eq(&self, other: &FfiStr<'a>) -> bool {
+        self.as_opt_str() == other.as_opt_str()
+    }
+}
+
+// Compare FfiStr with str
+impl<'a> PartialEq<str> for FfiStr<'a> {
+    #[inline]
+    fn eq(&self, other: &str) -> bool {
+        self.as_opt_str() == Some(other)
+    }
+}
+
+// Compare FfiStr with &str
+impl<'a, 'b> PartialEq<&'b str> for FfiStr<'a> {
+    #[inline]
+    fn eq(&self, other: &&'b str) -> bool {
+        self.as_opt_str() == Some(*other)
+    }
+}
+
+// rhs/lhs swap version of above
+impl<'a> PartialEq<FfiStr<'a>> for str {
+    #[inline]
+    fn eq(&self, other: &FfiStr<'a>) -> bool {
+        Some(self) == other.as_opt_str()
+    }
+}
+
+// rhs/lhs swap...
+impl<'a, 'b> PartialEq<FfiStr<'a>> for &'b str {
+    #[inline]
+    fn eq(&self, other: &FfiStr<'a>) -> bool {
+        Some(*self) == other.as_opt_str()
+    }
+}
diff --git a/third_party/rust/ffi-support/src/handle_map.rs b/third_party/rust/ffi-support/src/handle_map.rs
new file mode 100644
index 0000000000..b9b0df7c78
--- /dev/null
+++ b/third_party/rust/ffi-support/src/handle_map.rs
@@ -0,0 +1,1263 @@
+/* Copyright 2018-2019 Mozilla Foundation
+ *
+ * Licensed under the Apache License (Version 2.0), or the MIT license,
+ * (the "Licenses") at your option. You may not use this file except in
+ * compliance with one of the Licenses. You may obtain copies of the
+ * Licenses at:
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *    http://opensource.org/licenses/MIT
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the Licenses is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the Licenses for the specific language governing permissions and
+ * limitations under the Licenses. */
+
+//! This module provides a [`Handle`] type, which you can think of something
+//! like a dynamically checked, type erased reference/pointer type. Depending on
+//! the usage pattern a handle can behave as either a borrowed reference, or an
+//! owned pointer.
+//!
+//! They can be losslessly converted [to](Handle::into_u64) and
+//! [from](Handle::from_u64) a 64 bit integer, for ease of passing over the FFI
+//! (and they implement [`IntoFfi`] using these primitives for this purpose).
+//!
+//! The benefit is primarially that they can detect common misuse patterns that
+//! would otherwise be silent bugs, such as use-after-free, double-free, passing
+//! a wrongly-typed pointer to a function, etc.
+//!
+//! Handles are provided when inserting an item into either a [`HandleMap`] or a
+//! [`ConcurrentHandleMap`].
+//!
+//! # Comparison to types from other crates
+//!
+//! [`HandleMap`] is similar to types offered by other crates, such as
+//! `slotmap`, or `slab`. However, it has a number of key differences which make
+//! it better for our purposes as compared to the types in those crates:
+//!
+//! 1. Unlike `slab` (but like `slotmap`), we implement versioning, detecting
+//!    ABA problems, which allows us to detect use after free.
+//! 2. Unlike `slotmap`, we don't have the `T: Copy` restriction.
+//! 3. Unlike either, we can detect when you use a Key in a map that did not
+//!    allocate the key. This is true even when the map is from a `.so` file
+//!    compiled separately.
+//! 3. Our implementation of doesn't use any `unsafe` (at the time of this
+//!    writing).
+//!
+//! However, it comes with the following drawbacks:
+//!
+//! 1. `slotmap` holds its version information in a `u32`, and so it takes
+//!    2<sup>31</sup> colliding insertions and deletions before it could
+//!    potentially fail to detect an ABA issue, wheras we use a `u16`, and are
+//!    limited to 2<sup>15</sup>.
+//! 2. Similarly, we can only hold 2<sup>16</sup> items at once, unlike
+//!    `slotmap`'s 2<sup>32</sup>. (Considering these items are typically things
+//!    like database handles, this is probably plenty).
+//! 3. Our implementation is slower, and uses slightly more memory than
+//!    `slotmap` (which is in part due to the lack of `unsafe` mentioned above)
+//!
+//! The first two issues seem exceptionally unlikely, even for extremely
+//! long-lived `HandleMap`, and we're still memory safe even if they occur (we
+//! just might fail to notice a bug). The third issue also seems unimportant for
+//! our use case.
+
+use crate::error::{ErrorCode, ExternError};
+use crate::into_ffi::IntoFfi;
+use std::error::Error as StdError;
+use std::fmt;
+use std::ops;
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::{Mutex, RwLock};
+
+/// `HandleMap` is a collection type which can hold any type of value, and
+/// offers a stable handle which can be used to retrieve it on insertion. These
+/// handles offer methods for converting [to](Handle::into_u64) and
+/// [from](Handle::from_u64) 64 bit integers, meaning they're very easy to pass
+/// over the FFI (they also implement [`IntoFfi`] for the same purpose).
+///
+/// See the [module level docs](index.html) for more information.
+///
+/// Note: In FFI code, most usage of `HandleMap` will be done through the
+/// [`ConcurrentHandleMap`] type, which is a thin wrapper around a
+/// `RwLock<HandleMap<Mutex<T>>>`.
+#[derive(Debug, Clone)]
+pub struct HandleMap<T> {
+    // The value of `map_id` in each `Handle`.
+    id: u16,
+
+    // Index to the start of the free list. Always points to a free item --
+    // we never allow our free list to become empty.
+    first_free: u16,
+
+    // The number of entries with `data.is_some()`. This is never equal to
+    // `entries.len()`, we always grow before that point to ensure we always have
+    // a valid `first_free` index to add entries onto. This is our `len`.
+    num_entries: usize,
+
+    // The actual data. Note: entries.len() is our 'capacity'.
+    entries: Vec<Entry<T>>,
+}
+
+#[derive(Debug, Clone)]
+struct Entry<T> {
+    // initially 1, incremented on insertion and removal. Thus,
+    // if version is even, state should always be EntryState::Active.
+    version: u16,
+    state: EntryState<T>,
+}
+
+#[derive(Debug, Clone)]
+enum EntryState<T> {
+    // Not part of the free list
+    Active(T),
+    // The u16 is the next index in the free list.
+    InFreeList(u16),
+    // Part of the free list, but the sentinel.
+    EndOfFreeList,
+}
+
+impl<T> EntryState<T> {
+    #[cfg(any(debug_assertions, test))]
+    fn is_end_of_list(&self) -> bool {
+        match self {
+            EntryState::EndOfFreeList => true,
+            _ => false,
+        }
+    }
+
+    #[inline]
+    fn is_occupied(&self) -> bool {
+        self.get_item().is_some()
+    }
+
+    #[inline]
+    fn get_item(&self) -> Option<&T> {
+        match self {
+            EntryState::Active(v) => Some(v),
+            _ => None,
+        }
+    }
+
+    #[inline]
+    fn get_item_mut(&mut self) -> Option<&mut T> {
+        match self {
+            EntryState::Active(v) => Some(v),
+            _ => None,
+        }
+    }
+}
+
+// Small helper to check our casts.
+#[inline]
+fn to_u16(v: usize) -> u16 {
+    use std::u16::MAX as U16_MAX;
+    // Shouldn't ever happen.
+    assert!(v <= (U16_MAX as usize), "Bug: Doesn't fit in u16: {}", v);
+    v as u16
+}
+
+/// The maximum capacity of a [`HandleMap`]. Attempting to instantiate one with
+/// a larger capacity will cause a panic.
+///
+/// Note: This could go as high as `(1 << 16) - 2`, but doing is seems more
+/// error prone. For the sake of paranoia, we limit it to this size, which is
+/// already quite a bit larger than it seems like we're likely to ever need.
+pub const MAX_CAPACITY: usize = (1 << 15) - 1;
+
+// Never having to worry about capacity == 0 simplifies the code at the cost of
+// worse memory usage. It doesn't seem like there's any reason to make this
+// public.
+const MIN_CAPACITY: usize = 4;
+
+/// An error representing the ways a `Handle` may be invalid.
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord)]
+pub enum HandleError {
+    /// Identical to invalid handle, but has a slightly more helpful
+    /// message for the most common case 0.
+    NullHandle,
+
+    /// Returned from [`Handle::from_u64`] if [`Handle::is_valid`] fails.
+    InvalidHandle,
+
+    /// Returned from get/get_mut/delete if the handle is stale (this indicates
+    /// something equivalent to a use-after-free / double-free, etc).
+    StaleVersion,
+
+    /// Returned if the handle index references an index past the end of the
+    /// HandleMap.
+    IndexPastEnd,
+
+    /// The handle has a map_id for a different map than the one it was
+    /// attempted to be used with.
+    WrongMap,
+}
+
+impl StdError for HandleError {}
+
+impl fmt::Display for HandleError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        use HandleError::*;
+        match self {
+            NullHandle => {
+                f.write_str("Tried to use a null handle (this object has probably been closed)")
+            }
+            InvalidHandle => f.write_str("u64 could not encode a valid Handle"),
+            StaleVersion => f.write_str("Handle has stale version number"),
+            IndexPastEnd => f.write_str("Handle references a index past the end of this HandleMap"),
+            WrongMap => f.write_str("Handle is from a different map"),
+        }
+    }
+}
+
+impl From<HandleError> for ExternError {
+    fn from(e: HandleError) -> Self {
+        ExternError::new_error(ErrorCode::INVALID_HANDLE, e.to_string())
+    }
+}
+
+impl<T> HandleMap<T> {
+    /// Create a new `HandleMap` with the default capacity.
+    pub fn new() -> Self {
+        Self::new_with_capacity(MIN_CAPACITY)
+    }
+
+    /// Allocate a new `HandleMap`. Note that the actual capacity may be larger
+    /// than the requested value.
+    ///
+    /// Panics if `request` is greater than [`handle_map::MAX_CAPACITY`](MAX_CAPACITY)
+    pub fn new_with_capacity(request: usize) -> Self {
+        assert!(
+            request <= MAX_CAPACITY,
+            "HandleMap capacity is limited to {} (request was {})",
+            MAX_CAPACITY,
+            request
+        );
+
+        let capacity = request.max(MIN_CAPACITY);
+        let id = next_handle_map_id();
+        let mut entries = Vec::with_capacity(capacity);
+
+        // Initialize each entry with version 1, and as a member of the free list
+        for i in 0..(capacity - 1) {
+            entries.push(Entry {
+                version: 1,
+                state: EntryState::InFreeList(to_u16(i + 1)),
+            });
+        }
+
+        // And the final entry is at the end of the free list
+        // (but still has version 1).
+        entries.push(Entry {
+            version: 1,
+            state: EntryState::EndOfFreeList,
+        });
+        Self {
+            id,
+            first_free: 0,
+            num_entries: 0,
+            entries,
+        }
+    }
+
+    /// Get the number of entries in the `HandleMap`.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.num_entries
+    }
+
+    /// Returns true if the HandleMap is empty.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Returns the number of slots allocated in the handle map.
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        // It's not a bug that this isn't entries.capacity() -- We're returning
+        // how many slots exist, not something about the backing memory allocation
+        self.entries.len()
+    }
+
+    fn ensure_capacity(&mut self, cap_at_least: usize) {
+        assert_ne!(self.len(), self.capacity(), "Bug: should have grown by now");
+        assert!(cap_at_least <= MAX_CAPACITY, "HandleMap overfilled");
+        if self.capacity() > cap_at_least {
+            return;
+        }
+
+        let mut next_cap = self.capacity();
+        while next_cap <= cap_at_least {
+            next_cap *= 2;
+        }
+        next_cap = next_cap.min(MAX_CAPACITY);
+
+        let need_extra = next_cap.saturating_sub(self.entries.capacity());
+        self.entries.reserve(need_extra);
+
+        assert!(
+            !self.entries[self.first_free as usize].state.is_occupied(),
+            "Bug: HandleMap.first_free points at occupied index"
+        );
+
+        // Insert new entries at the front of our list.
+        while self.entries.len() < next_cap - 1 {
+            // This is a little wasteful but whatever. Add each new entry to the
+            // front of the free list one at a time.
+            self.entries.push(Entry {
+                version: 1,
+                state: EntryState::InFreeList(self.first_free),
+            });
+            self.first_free = to_u16(self.entries.len() - 1);
+        }
+
+        self.debug_check_valid();
+    }
+
+    #[inline]
+    fn debug_check_valid(&self) {
+        // Run the expensive validity check in tests and in debug builds.
+        #[cfg(any(debug_assertions, test))]
+        {
+            self.assert_valid();
+        }
+    }
+
+    #[cfg(any(debug_assertions, test))]
+    fn assert_valid(&self) {
+        assert_ne!(self.len(), self.capacity());
+        assert!(self.capacity() <= MAX_CAPACITY, "Entries too large");
+        // Validate that our free list is correct.
+
+        let number_of_ends = self
+            .entries
+            .iter()
+            .filter(|e| e.state.is_end_of_list())
+            .count();
+        assert_eq!(
+            number_of_ends, 1,
+            "More than one entry think's it's the end of the list, or no entries do"
+        );
+
+        // Check that the free list hits every unoccupied item.
+        // The tuple is: `(should_be_in_free_list, is_in_free_list)`.
+        let mut free_indices = vec![(false, false); self.capacity()];
+        for (i, e) in self.entries.iter().enumerate() {
+            if !e.state.is_occupied() {
+                free_indices[i].0 = true;
+            }
+        }
+
+        let mut next = self.first_free;
+        loop {
+            let ni = next as usize;
+
+            assert!(
+                ni <= free_indices.len(),
+                "Free list contains out of bounds index!"
+            );
+
+            assert!(
+                free_indices[ni].0,
+                "Free list has an index that shouldn't be free! {}",
+                ni
+            );
+
+            assert!(
+                !free_indices[ni].1,
+                "Free list hit an index ({}) more than once! Cycle detected!",
+                ni
+            );
+
+            free_indices[ni].1 = true;
+
+            match &self.entries[ni].state {
+                EntryState::InFreeList(next_index) => next = *next_index,
+                EntryState::EndOfFreeList => break,
+                // Hitting `Active` here is probably not possible because of the checks above, but who knows.
+                EntryState::Active(..) => unreachable!("Bug: Active item in free list at {}", next),
+            }
+        }
+        let mut occupied_count = 0;
+        for (i, &(should_be_free, is_free)) in free_indices.iter().enumerate() {
+            assert_eq!(
+                should_be_free, is_free,
+                "Free list missed item, or contains an item it shouldn't: {}",
+                i
+            );
+            if !should_be_free {
+                occupied_count += 1;
+            }
+        }
+        assert_eq!(
+            self.num_entries, occupied_count,
+            "num_entries doesn't reflect the actual number of entries"
+        );
+    }
+
+    /// Insert an item into the map, and return a handle to it.
+    pub fn insert(&mut self, v: T) -> Handle {
+        let need_cap = self.len() + 1;
+        self.ensure_capacity(need_cap);
+        let index = self.first_free;
+        let result = {
+            // Scoped mutable borrow of entry.
+            let entry = &mut self.entries[index as usize];
+            let new_first_free = match entry.state {
+                EntryState::InFreeList(i) => i,
+                _ => panic!("Bug: next_index pointed at non-free list entry (or end of list)"),
+            };
+            entry.version += 1;
+            if entry.version == 0 {
+                entry.version += 2;
+            }
+            entry.state = EntryState::Active(v);
+            self.first_free = new_first_free;
+            self.num_entries += 1;
+
+            Handle {
+                map_id: self.id,
+                version: entry.version,
+                index,
+            }
+        };
+        self.debug_check_valid();
+        result
+    }
+
+    // Helper to contain the handle validation boilerplate. Returns `h.index as usize`.
+    fn check_handle(&self, h: Handle) -> Result<usize, HandleError> {
+        if h.map_id != self.id {
+            log::info!(
+                "HandleMap access with handle having wrong map id: {:?} (our map id is {})",
+                h,
+                self.id
+            );
+            return Err(HandleError::WrongMap);
+        }
+        let index = h.index as usize;
+        if index >= self.entries.len() {
+            log::info!("HandleMap accessed with handle past end of map: {:?}", h);
+            return Err(HandleError::IndexPastEnd);
+        }
+        if self.entries[index].version != h.version {
+            log::info!(
+                "HandleMap accessed with handle with wrong version {:?} (entry version is {})",
+                h,
+                self.entries[index].version
+            );
+            return Err(HandleError::StaleVersion);
+        }
+        // At this point, we know the handle version matches the entry version,
+        // but if someone created a specially invalid handle, they could have
+        // its version match the version they expect an unoccupied index to
+        // have.
+        //
+        // We don't use any unsafe, so the worse thing that can happen here is
+        // that we get confused and panic, but still that's not great, so we
+        // check for this explicitly.
+        //
+        // Note that `active` versions are always even, as they start at 1, and
+        // are incremented on both insertion and deletion.
+        //
+        // Anyway, this is just for sanity checking, we already check this in
+        // practice when we convert `u64`s into `Handle`s, which is the only
+        // way we ever use these in the real world.
+        if (h.version % 2) != 0 {
+            log::info!(
+                "HandleMap given handle with matching but illegal version: {:?}",
+                h,
+            );
+            return Err(HandleError::StaleVersion);
+        }
+        Ok(index)
+    }
+
+    /// Delete an item from the HandleMap.
+    pub fn delete(&mut self, h: Handle) -> Result<(), HandleError> {
+        self.remove(h).map(drop)
+    }
+
+    /// Remove an item from the HandleMap, returning the old value.
+    pub fn remove(&mut self, h: Handle) -> Result<T, HandleError> {
+        let index = self.check_handle(h)?;
+        let prev = {
+            // Scoped mutable borrow of entry.
+            let entry = &mut self.entries[index];
+            entry.version += 1;
+            let index = h.index;
+            let last_state =
+                std::mem::replace(&mut entry.state, EntryState::InFreeList(self.first_free));
+            self.num_entries -= 1;
+            self.first_free = index;
+
+            if let EntryState::Active(value) = last_state {
+                value
+            } else {
+                // This indicates either a bug in HandleMap or memory
+                // corruption. Abandon all hope.
+                unreachable!(
+                    "Handle {:?} passed validation but references unoccupied entry",
+                    h
+                );
+            }
+        };
+        self.debug_check_valid();
+        Ok(prev)
+    }
+
+    /// Get a reference to the item referenced by the handle, or return a
+    /// [`HandleError`] describing the problem.
+    pub fn get(&self, h: Handle) -> Result<&T, HandleError> {
+        let idx = self.check_handle(h)?;
+        let entry = &self.entries[idx];
+        // This should be caught by check_handle above, but we avoid panicking
+        // because we'd rather not poison any locks we don't have to poison
+        let item = entry
+            .state
+            .get_item()
+            .ok_or_else(|| HandleError::InvalidHandle)?;
+        Ok(item)
+    }
+
+    /// Get a mut reference to the item referenced by the handle, or return a
+    /// [`HandleError`] describing the problem.
+    pub fn get_mut(&mut self, h: Handle) -> Result<&mut T, HandleError> {
+        let idx = self.check_handle(h)?;
+        let entry = &mut self.entries[idx];
+        // This should be caught by check_handle above, but we avoid panicking
+        // because we'd rather not poison any locks we don't have to poison
+        let item = entry
+            .state
+            .get_item_mut()
+            .ok_or_else(|| HandleError::InvalidHandle)?;
+        Ok(item)
+    }
+}
+
+impl<T> Default for HandleMap<T> {
+    #[inline]
+    fn default() -> Self {
+        HandleMap::new()
+    }
+}
+
+impl<T> ops::Index<Handle> for HandleMap<T> {
+    type Output = T;
+    #[inline]
+    fn index(&self, h: Handle) -> &T {
+        self.get(h)
+            .expect("Indexed into HandleMap with invalid handle!")
+    }
+}
+
+// We don't implement IndexMut intentionally (implementing ops::Index is
+// dubious enough)
+
+/// A Handle we allow to be returned over the FFI by implementing [`IntoFfi`].
+/// This type is intentionally not `#[repr(C)]`, and getting the data out of the
+/// FFI is done using `Handle::from_u64`, or it's implemetation of `From<u64>`.
+///
+/// It consists of, at a minimum:
+///
+/// - A "map id" (used to ensure you're using it with the correct map)
+/// - a "version" (incremented when the value in the index changes, used to
+///   detect multiple frees, use after free, and ABA and ABA)
+/// - and a field indicating which index it goes into.
+///
+/// In practice, it may also contain extra information to help detect other
+/// errors (currently it stores a "magic value" used to detect invalid
+/// [`Handle`]s).
+///
+/// These fields may change but the following guarantees are made about the
+/// internal representation:
+///
+/// - This will always be representable in 64 bits.
+/// - The bits, when interpreted as a signed 64 bit integer, will be positive
+///   (that is to say, it will *actually* be representable in 63 bits, since
+///   this makes the most significant bit unavailable for the purposes of
+///   encoding). This guarantee makes things slightly less dubious when passing
+///   things to Java, gives us some extra validation ability, etc.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Handle {
+    map_id: u16,
+    version: u16,
+    index: u16,
+}
+
+// We stuff this into the top 16 bits of the handle when u16 encoded to detect
+// various sorts of weirdness. It's the letters 'A' and 'S' as ASCII, but the
+// only important thing about it is that the most significant bit be unset.
+const HANDLE_MAGIC: u16 = 0x4153_u16;
+
+impl Handle {
+    /// Convert a `Handle` to a `u64`. You can also use `Into::into` directly.
+    /// Most uses of this will be automatic due to our [`IntoFfi`] implementation.
+    #[inline]
+    pub fn into_u64(self) -> u64 {
+        let map_id = u64::from(self.map_id);
+        let version = u64::from(self.version);
+        let index = u64::from(self.index);
+        // SOMEDAY: we could also use this as a sort of CRC if we were really paranoid.
+        // e.g. `magic = combine_to_u16(map_id, version, index)`.
+        let magic = u64::from(HANDLE_MAGIC);
+        (magic << 48) | (map_id << 32) | (index << 16) | version
+    }
+
+    /// Convert a `u64` to a `Handle`. Inverse of `into_u64`. We also implement
+    /// `From::from` (which will panic instead of returning Err).
+    ///
+    /// Returns [`HandleError::InvalidHandle`](HandleError) if the bits cannot
+    /// possibly represent a valid handle.
+    pub fn from_u64(v: u64) -> Result<Self, HandleError> {
+        if !Handle::is_valid(v) {
+            log::warn!("Illegal handle! {:x}", v);
+            if v == 0 {
+                Err(HandleError::NullHandle)
+            } else {
+                Err(HandleError::InvalidHandle)
+            }
+        } else {
+            let map_id = (v >> 32) as u16;
+            let index = (v >> 16) as u16;
+            let version = v as u16;
+            Ok(Self {
+                map_id,
+                version,
+                index,
+            })
+        }
+    }
+
+    /// Returns whether or not `v` makes a bit pattern that could represent an
+    /// encoded [`Handle`].
+    #[inline]
+    pub fn is_valid(v: u64) -> bool {
+        (v >> 48) == u64::from(HANDLE_MAGIC) &&
+        // The "bottom" field is the version. We increment it both when
+        // inserting and removing, and they're all initially 1. So, all valid
+        // handles that we returned should have an even version.
+        ((v & 1) == 0)
+    }
+}
+
+impl From<u64> for Handle {
+    fn from(u: u64) -> Self {
+        Handle::from_u64(u).expect("Illegal handle!")
+    }
+}
+
+impl From<Handle> for u64 {
+    #[inline]
+    fn from(h: Handle) -> u64 {
+        h.into_u64()
+    }
+}
+
+unsafe impl IntoFfi for Handle {
+    type Value = u64;
+    // Note: intentionally does not encode a valid handle for any map.
+    #[inline]
+    fn ffi_default() -> u64 {
+        0u64
+    }
+    #[inline]
+    fn into_ffi_value(self) -> u64 {
+        self.into_u64()
+    }
+}
+
+/// `ConcurrentHandleMap` is a relatively thin wrapper around
+/// `RwLock<HandleMap<Mutex<T>>>`. Due to the nested locking, it's not possible
+/// to implement the same API as [`HandleMap`], however it does implement an API
+/// that offers equivalent functionality, as well as several functions that
+/// greatly simplify FFI usage (see example below).
+///
+/// See the [module level documentation](index.html) for more info.
+///
+/// # Example
+///
+/// ```rust,no_run
+/// # #[macro_use] extern crate lazy_static;
+/// # extern crate ffi_support;
+/// # use ffi_support::*;
+/// # use std::sync::*;
+///
+/// // Somewhere...
+/// struct Thing { value: f64 }
+///
+/// lazy_static! {
+///     static ref ITEMS: ConcurrentHandleMap<Thing> = ConcurrentHandleMap::new();
+/// }
+///
+/// #[no_mangle]
+/// pub extern "C" fn mylib_new_thing(value: f64, err: &mut ExternError) -> u64 {
+///     // Most uses will be `ITEMS.insert_with_result`. Note that this already
+///     // calls `call_with_output` (or `call_with_result` if this were
+///     // `insert_with_result`) for you.
+///     ITEMS.insert_with_output(err, || Thing { value })
+/// }
+///
+/// #[no_mangle]
+/// pub extern "C" fn mylib_thing_value(h: u64, err: &mut ExternError) -> f64 {
+///     // Or `ITEMS.call_with_result` for the fallible functions.
+///     ITEMS.call_with_output(err, h, |thing| thing.value)
+/// }
+///
+/// #[no_mangle]
+/// pub extern "C" fn mylib_thing_set_value(h: u64, new_value: f64, err: &mut ExternError) {
+///     ITEMS.call_with_output_mut(err, h, |thing| {
+///         thing.value = new_value;
+///     })
+/// }
+///
+/// // Note: defines the following function:
+/// // pub extern "C" fn mylib_destroy_thing(h: u64, err: &mut ExternError)
+/// define_handle_map_deleter!(ITEMS, mylib_destroy_thing);
+/// ```
+pub struct ConcurrentHandleMap<T> {
+    /// The underlying map. Public so that more advanced use-cases
+    /// may use it as they please.
+    pub map: RwLock<HandleMap<Mutex<T>>>,
+}
+
+impl<T> ConcurrentHandleMap<T> {
+    /// Construct a new `ConcurrentHandleMap`.
+    pub fn new() -> Self {
+        Self {
+            map: RwLock::new(HandleMap::new()),
+        }
+    }
+
+    /// Get the number of entries in the `ConcurrentHandleMap`.
+    ///
+    /// This takes the map's `read` lock.
+    #[inline]
+    pub fn len(&self) -> usize {
+        let map = self.map.read().unwrap();
+        map.len()
+    }
+
+    /// Returns true if the `ConcurrentHandleMap` is empty.
+    ///
+    /// This takes the map's `read` lock.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Insert an item into the map, returning the newly allocated handle to the
+    /// item.
+    ///
+    /// # Locking
+    ///
+    /// Note that this requires taking the map's write lock, and so it will
+    /// block until all other threads have finished any read/write operations.
+    pub fn insert(&self, v: T) -> Handle {
+        // Fails if the lock is poisoned. Not clear what we should do here... We
+        // could always insert anyway (by matching on LockResult), but that
+        // seems... really quite dubious.
+        let mut map = self.map.write().unwrap();
+        map.insert(Mutex::new(v))
+    }
+
+    /// Remove an item from the map.
+    ///
+    /// # Locking
+    ///
+    /// Note that this requires taking the map's write lock, and so it will
+    /// block until all other threads have finished any read/write operations.
+    pub fn delete(&self, h: Handle) -> Result<(), HandleError> {
+        // We use `remove` and not delete (and use the inner block) to ensure
+        // that if `v`'s destructor panics, we aren't holding the write lock
+        // when it happens, so that the map itself doesn't get poisoned.
+        let v = {
+            let mut map = self.map.write().unwrap();
+            map.remove(h)
+        };
+        v.map(drop)
+    }
+
+    /// Convenient wrapper for `delete` which takes a `u64` that it will
+    /// convert to a handle.
+    ///
+    /// The main benefit (besides convenience) of this over the version
+    /// that takes a [`Handle`] is that it allows handling handle-related errors
+    /// in one place.
+    pub fn delete_u64(&self, h: u64) -> Result<(), HandleError> {
+        self.delete(Handle::from_u64(h)?)
+    }
+
+    /// Remove an item from the map, returning either the item,
+    /// or None if its guard mutex got poisoned at some point.
+    ///
+    /// # Locking
+    ///
+    /// Note that this requires taking the map's write lock, and so it will
+    /// block until all other threads have finished any read/write operations.
+    pub fn remove(&self, h: Handle) -> Result<Option<T>, HandleError> {
+        let mut map = self.map.write().unwrap();
+        let mutex = map.remove(h)?;
+        Ok(mutex.into_inner().ok())
+    }
+
+    /// Convenient wrapper for `remove` which takes a `u64` that it will
+    /// convert to a handle.
+    ///
+    /// The main benefit (besides convenience) of this over the version
+    /// that takes a [`Handle`] is that it allows handling handle-related errors
+    /// in one place.
+    pub fn remove_u64(&self, h: u64) -> Result<Option<T>, HandleError> {
+        self.remove(Handle::from_u64(h)?)
+    }
+
+    /// Call `callback` with a non-mutable reference to the item from the map,
+    /// after acquiring the necessary locks.
+    ///
+    /// # Locking
+    ///
+    /// Note that this requires taking both:
+    ///
+    /// - The map's read lock, and so it will block until all other threads have
+    ///   finished any write operations.
+    /// - The mutex on the slot the handle is mapped to.
+    ///
+    /// And so it will block if there are ongoing write operations, or if
+    /// another thread is reading from the same handle.
+    ///
+    /// # Panics
+    ///
+    /// This will panic if a previous `get()` or `get_mut()` call has panicked
+    /// inside it's callback. The solution to this
+    ///
+    /// (It may also panic if the handle map detects internal state corruption,
+    /// however this should not happen except for bugs in the handle map code).
+    pub fn get<F, E, R>(&self, h: Handle, callback: F) -> Result<R, E>
+    where
+        F: FnOnce(&T) -> Result<R, E>,
+        E: From<HandleError>,
+    {
+        self.get_mut(h, |v| callback(v))
+    }
+
+    /// Call `callback` with a mutable reference to the item from the map, after
+    /// acquiring the necessary locks.
+    ///
+    /// # Locking
+    ///
+    /// Note that this requires taking both:
+    ///
+    /// - The map's read lock, and so it will block until all other threads have
+    ///   finished any write operations.
+    /// - The mutex on the slot the handle is mapped to.
+    ///
+    /// And so it will block if there are ongoing write operations, or if
+    /// another thread is reading from the same handle.
+    ///
+    /// # Panics
+    ///
+    /// This will panic if a previous `get()` or `get_mut()` call has panicked
+    /// inside it's callback. The only solution to this is to remove and reinsert
+    /// said item.
+    ///
+    /// (It may also panic if the handle map detects internal state corruption,
+    /// however this should not happen except for bugs in the handle map code).
+    pub fn get_mut<F, E, R>(&self, h: Handle, callback: F) -> Result<R, E>
+    where
+        F: FnOnce(&mut T) -> Result<R, E>,
+        E: From<HandleError>,
+    {
+        // XXX figure out how to handle poison...
+        let map = self.map.read().unwrap();
+        let mtx = map.get(h)?;
+        let mut hm = mtx.lock().unwrap();
+        callback(&mut *hm)
+    }
+
+    /// Convenient wrapper for `get` which takes a `u64` that it will convert to
+    /// a handle.
+    ///
+    /// The other benefit (besides convenience) of this over the version
+    /// that takes a [`Handle`] is that it allows handling handle-related errors
+    /// in one place.
+    ///
+    /// # Locking
+    ///
+    /// Note that this requires taking both:
+    ///
+    /// - The map's read lock, and so it will block until all other threads have
+    ///   finished any write operations.
+    /// - The mutex on the slot the handle is mapped to.
+    ///
+    /// And so it will block if there are ongoing write operations, or if
+    /// another thread is reading from the same handle.
+    pub fn get_u64<F, E, R>(&self, u: u64, callback: F) -> Result<R, E>
+    where
+        F: FnOnce(&T) -> Result<R, E>,
+        E: From<HandleError>,
+    {
+        self.get(Handle::from_u64(u)?, callback)
+    }
+
+    /// Convenient wrapper for [`Self::get_mut`] which takes a `u64` that it will
+    /// convert to a handle.
+    ///
+    /// The main benefit (besides convenience) of this over the version
+    /// that takes a [`Handle`] is that it allows handling handle-related errors
+    /// in one place.
+    ///
+    /// # Locking
+    ///
+    /// Note that this requires taking both:
+    ///
+    /// - The map's read lock, and so it will block until all other threads have
+    ///   finished any write operations.
+    /// - The mutex on the slot the handle is mapped to.
+    ///
+    /// And so it will block if there are ongoing write operations, or if
+    /// another thread is reading from the same handle.
+    pub fn get_mut_u64<F, E, R>(&self, u: u64, callback: F) -> Result<R, E>
+    where
+        F: FnOnce(&mut T) -> Result<R, E>,
+        E: From<HandleError>,
+    {
+        self.get_mut(Handle::from_u64(u)?, callback)
+    }
+
+    /// Helper that performs both a
+    /// [`call_with_result`][crate::call_with_result] and
+    /// [`get`](ConcurrentHandleMap::get_mut).
+    pub fn call_with_result_mut<R, E, F>(
+        &self,
+        out_error: &mut ExternError,
+        h: u64,
+        callback: F,
+    ) -> R::Value
+    where
+        F: std::panic::UnwindSafe + FnOnce(&mut T) -> Result<R, E>,
+        ExternError: From<E>,
+        R: IntoFfi,
+    {
+        use crate::call_with_result;
+        call_with_result(out_error, || -> Result<_, ExternError> {
+            // We can't reuse get_mut here because it would require E:
+            // From<HandleError>, which is inconvenient...
+            let h = Handle::from_u64(h)?;
+            let map = self.map.read().unwrap();
+            let mtx = map.get(h)?;
+            let mut hm = mtx.lock().unwrap();
+            Ok(callback(&mut *hm)?)
+        })
+    }
+
+    /// Helper that performs both a
+    /// [`call_with_result`][crate::call_with_result] and
+    /// [`get`](ConcurrentHandleMap::get).
+    pub fn call_with_result<R, E, F>(
+        &self,
+        out_error: &mut ExternError,
+        h: u64,
+        callback: F,
+    ) -> R::Value
+    where
+        F: std::panic::UnwindSafe + FnOnce(&T) -> Result<R, E>,
+        ExternError: From<E>,
+        R: IntoFfi,
+    {
+        self.call_with_result_mut(out_error, h, |r| callback(r))
+    }
+
+    /// Helper that performs both a
+    /// [`call_with_output`][crate::call_with_output] and
+    /// [`get`](ConcurrentHandleMap::get).
+    pub fn call_with_output<R, F>(
+        &self,
+        out_error: &mut ExternError,
+        h: u64,
+        callback: F,
+    ) -> R::Value
+    where
+        F: std::panic::UnwindSafe + FnOnce(&T) -> R,
+        R: IntoFfi,
+    {
+        self.call_with_result(out_error, h, |r| -> Result<_, HandleError> {
+            Ok(callback(r))
+        })
+    }
+
+    /// Helper that performs both a
+    /// [`call_with_output`][crate::call_with_output] and
+    /// [`get_mut`](ConcurrentHandleMap::get).
+    pub fn call_with_output_mut<R, F>(
+        &self,
+        out_error: &mut ExternError,
+        h: u64,
+        callback: F,
+    ) -> R::Value
+    where
+        F: std::panic::UnwindSafe + FnOnce(&mut T) -> R,
+        R: IntoFfi,
+    {
+        self.call_with_result_mut(out_error, h, |r| -> Result<_, HandleError> {
+            Ok(callback(r))
+        })
+    }
+
+    /// Use `constructor` to create and insert a `T`, while inside a
+    /// [`call_with_result`][crate::call_with_result] call (to handle panics and
+    /// map errors onto an [`ExternError`][crate::ExternError]).
+    pub fn insert_with_result<E, F>(&self, out_error: &mut ExternError, constructor: F) -> u64
+    where
+        F: std::panic::UnwindSafe + FnOnce() -> Result<T, E>,
+        ExternError: From<E>,
+    {
+        use crate::call_with_result;
+        call_with_result(out_error, || -> Result<_, ExternError> {
+            // Note: it's important that we don't call the constructor while
+            // we're holding the write lock, because we don't want to poison
+            // the entire map if it panics!
+            let to_insert = constructor()?;
+            Ok(self.insert(to_insert))
+        })
+    }
+
+    /// Equivalent to
+    /// [`insert_with_result`](ConcurrentHandleMap::insert_with_result) for the
+    /// case where the constructor cannot produce an error.
+    ///
+    /// The name is somewhat dubious, since there's no `output`, but it's
+    /// intended to make it clear that it contains a
+    /// [`call_with_output`][crate::call_with_output] internally.
+    pub fn insert_with_output<F>(&self, out_error: &mut ExternError, constructor: F) -> u64
+    where
+        F: std::panic::UnwindSafe + FnOnce() -> T,
+    {
+        // The Err type isn't important here beyond being convertable to ExternError
+        self.insert_with_result(out_error, || -> Result<_, HandleError> {
+            Ok(constructor())
+        })
+    }
+}
+
+impl<T> Default for ConcurrentHandleMap<T> {
+    #[inline]
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+// Returns the next map_id.
+fn next_handle_map_id() -> u16 {
+    let id = HANDLE_MAP_ID_COUNTER
+        .fetch_add(1, Ordering::SeqCst)
+        .wrapping_add(1);
+    id as u16
+}
+
+// Note: These IDs are only used to detect using a key against the wrong HandleMap.
+// We ensure they're randomly initialized, to prevent using them across separately
+// compiled .so files.
+lazy_static::lazy_static! {
+    // This should be `AtomicU16`, but those aren't stablilized yet.
+    // Instead, we just cast to u16 on read.
+    static ref HANDLE_MAP_ID_COUNTER: AtomicUsize = {
+        // Abuse HashMap's RandomState to get a strong RNG without bringing in
+        // the `rand` crate (OTOH maybe we should just bring in the rand crate?)
+        use std::collections::hash_map::RandomState;
+        use std::hash::{BuildHasher, Hasher};
+        let init = RandomState::new().build_hasher().finish() as usize;
+        AtomicUsize::new(init)
+    };
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[derive(PartialEq, Debug)]
+    pub(super) struct Foobar(usize);
+
+    #[test]
+    fn test_invalid_handle() {
+        assert_eq!(Handle::from_u64(0), Err(HandleError::NullHandle));
+        // Valid except `version` is odd
+        assert_eq!(
+            Handle::from_u64((u64::from(HANDLE_MAGIC) << 48) | 0x1234_0012_0001),
+            Err(HandleError::InvalidHandle)
+        );
+
+        assert_eq!(
+            Handle::from_u64((u64::from(HANDLE_MAGIC) << 48) | 0x1234_0012_0002),
+            Ok(Handle {
+                version: 0x0002,
+                index: 0x0012,
+                map_id: 0x1234,
+            })
+        );
+    }
+
+    #[test]
+    fn test_correct_value_single() {
+        let mut map = HandleMap::new();
+        let handle = map.insert(Foobar(1234));
+        assert_eq!(map.get(handle).unwrap(), &Foobar(1234));
+        map.delete(handle).unwrap();
+        assert_eq!(map.get(handle), Err(HandleError::StaleVersion));
+    }
+
+    #[test]
+    fn test_correct_value_multiple() {
+        let mut map = HandleMap::new();
+        let handle1 = map.insert(Foobar(1234));
+        let handle2 = map.insert(Foobar(4321));
+        assert_eq!(map.get(handle1).unwrap(), &Foobar(1234));
+        assert_eq!(map.get(handle2).unwrap(), &Foobar(4321));
+        map.delete(handle1).unwrap();
+        assert_eq!(map.get(handle1), Err(HandleError::StaleVersion));
+        assert_eq!(map.get(handle2).unwrap(), &Foobar(4321));
+    }
+
+    #[test]
+    fn test_wrong_map() {
+        let mut map1 = HandleMap::new();
+        let mut map2 = HandleMap::new();
+
+        let handle1 = map1.insert(Foobar(1234));
+        let handle2 = map2.insert(Foobar(1234));
+
+        assert_eq!(map1.get(handle1).unwrap(), &Foobar(1234));
+        assert_eq!(map2.get(handle2).unwrap(), &Foobar(1234));
+
+        assert_eq!(map1.get(handle2), Err(HandleError::WrongMap));
+        assert_eq!(map2.get(handle1), Err(HandleError::WrongMap));
+    }
+
+    #[test]
+    fn test_bad_index() {
+        let map: HandleMap<Foobar> = HandleMap::new();
+        assert_eq!(
+            map.get(Handle {
+                map_id: map.id,
+                version: 2,
+                index: 100
+            }),
+            Err(HandleError::IndexPastEnd)
+        );
+    }
+
+    #[test]
+    fn test_resizing() {
+        let mut map = HandleMap::new();
+        let mut handles = vec![];
+        for i in 0..1000 {
+            handles.push(map.insert(Foobar(i)))
+        }
+        for (i, &h) in handles.iter().enumerate() {
+            assert_eq!(map.get(h).unwrap(), &Foobar(i));
+            assert_eq!(map.remove(h).unwrap(), Foobar(i));
+        }
+        let mut handles2 = vec![];
+        for i in 1000..2000 {
+            // Not really related to this test, but it's convenient to check this here.
+            let h = map.insert(Foobar(i));
+            let hu = h.into_u64();
+            assert_eq!(Handle::from_u64(hu).unwrap(), h);
+            handles2.push(hu);
+        }
+
+        for (i, (&h0, h1u)) in handles.iter().zip(handles2).enumerate() {
+            // It's still a stale version, even though the slot is occupied again.
+            assert_eq!(map.get(h0), Err(HandleError::StaleVersion));
+            let h1 = Handle::from_u64(h1u).unwrap();
+            assert_eq!(map.get(h1).unwrap(), &Foobar(i + 1000));
+        }
+    }
+
+    /// Tests that check our behavior when panicing.
+    ///
+    /// Naturally these require panic=unwind, which means we can't run them when
+    /// generating coverage (well, `-Zprofile`-based coverage can't -- although
+    /// ptrace-based coverage like tarpaulin can), and so we turn them off.
+    ///
+    /// (For clarity, `cfg(coverage)` is not a standard thing. We add it in
+    /// `automation/emit_coverage_info.sh`, and you can force it by adding
+    /// "--cfg coverage" to your RUSTFLAGS manually if you need to do so).
+    #[cfg(not(coverage))]
+    mod panic_tests {
+        use super::*;
+
+        struct PanicOnDrop(());
+        impl Drop for PanicOnDrop {
+            fn drop(&mut self) {
+                panic!("intentional panic (drop)");
+            }
+        }
+
+        #[test]
+        fn test_panicking_drop() {
+            let map = ConcurrentHandleMap::new();
+            let h = map.insert(PanicOnDrop(())).into_u64();
+            let mut e = ExternError::success();
+            crate::call_with_result(&mut e, || map.delete_u64(h));
+            assert_eq!(e.get_code(), crate::ErrorCode::PANIC);
+            let _ = unsafe { e.get_and_consume_message() };
+            assert!(!map.map.is_poisoned());
+            let inner = map.map.read().unwrap();
+            inner.assert_valid();
+            assert_eq!(inner.len(), 0);
+        }
+
+        #[test]
+        fn test_panicking_call_with() {
+            let map = ConcurrentHandleMap::new();
+            let h = map.insert(Foobar(0)).into_u64();
+            let mut e = ExternError::success();
+            map.call_with_output(&mut e, h, |_thing| {
+                panic!("intentional panic (call_with_output)");
+            });
+
+            assert_eq!(e.get_code(), crate::ErrorCode::PANIC);
+            let _ = unsafe { e.get_and_consume_message() };
+
+            {
+                assert!(!map.map.is_poisoned());
+                let inner = map.map.read().unwrap();
+                inner.assert_valid();
+                assert_eq!(inner.len(), 1);
+                let mut seen = false;
+                for e in &inner.entries {
+                    if let EntryState::Active(v) = &e.state {
+                        assert!(!seen);
+                        assert!(v.is_poisoned());
+                        seen = true;
+                    }
+                }
+            }
+            assert!(map.delete_u64(h).is_ok());
+            assert!(!map.map.is_poisoned());
+            let inner = map.map.read().unwrap();
+            inner.assert_valid();
+            assert_eq!(inner.len(), 0);
+        }
+
+        #[test]
+        fn test_panicking_insert_with() {
+            let map = ConcurrentHandleMap::new();
+            let mut e = ExternError::success();
+            let res = map.insert_with_output(&mut e, || {
+                panic!("intentional panic (insert_with_output)");
+            });
+
+            assert_eq!(e.get_code(), crate::ErrorCode::PANIC);
+            let _ = unsafe { e.get_and_consume_message() };
+
+            assert_eq!(res, 0);
+
+            assert!(!map.map.is_poisoned());
+            let inner = map.map.read().unwrap();
+            inner.assert_valid();
+            assert_eq!(inner.len(), 0);
+        }
+    }
+}
diff --git a/third_party/rust/ffi-support/src/into_ffi.rs b/third_party/rust/ffi-support/src/into_ffi.rs
new file mode 100644
index 0000000000..96c2037e71
--- /dev/null
+++ b/third_party/rust/ffi-support/src/into_ffi.rs
@@ -0,0 +1,287 @@
+/* Copyright 2018-2019 Mozilla Foundation
+ *
+ * Licensed under the Apache License (Version 2.0), or the MIT license,
+ * (the "Licenses") at your option. You may not use this file except in
+ * compliance with one of the Licenses. You may obtain copies of the
+ * Licenses at:
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *    http://opensource.org/licenses/MIT
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the Licenses is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the Licenses for the specific language governing permissions and
+ * limitations under the Licenses. */
+
+use crate::string::*;
+use std::os::raw::{c_char, c_void};
+use std::ptr;
+
+/// This trait is used to return types over the FFI. It essentially is a mapping between a type and
+/// version of that type we can pass back to C (`IntoFfi::Value`).
+///
+/// The main wrinkle is that we need to be able to pass a value back to C in both the success and
+/// error cases. In the error cases, we don't want there to need to be any cleanup for the foreign
+/// code to do, and we want the API to be relatively easy to use.
+///
+/// Additionally, the mapping is not consistent for different types. For some rust types, we want to
+/// convert them to JSON. For some, we want to return an opaque `*mut T` handle. For others,
+/// we'd like to return by value.
+///
+/// This trait supports those cases by adding some type-level indirection, and allowing both cases
+/// to be provided (both cases what is done in the error and success cases).
+///
+/// We implement this for the following types:
+///
+/// - `String`, by conversion to `*mut c_char`. Note that the caller (on the other side of the FFI)
+///   is expected to free this, so you will need to provide them with a destructor for strings,
+///   which can be done with the [`define_string_destructor!`] macro.
+///
+/// - `()`: as a no-op conversion -- this just allows us to expose functions without a return type
+///   over the FFI.
+///
+/// - `bool`: is implemented by conversion to `u8` (`0u8` is `false`, `1u8` is `true`, and
+///   `ffi_default()` is `false`). This is because it doesn't seem to be safe to pass over the FFI
+///   directly (or at least, doing so might hit a bug in JNA).
+///
+/// - All numeric primitives except  `isize`, `usize`, `char`, `i128`, and `u128` are implememented
+///   by passing directly through (and using `Default::default()` for `ffi_default()`).
+///     - `isize`, `usize` could be added, but they'd be quite easy to accidentally misuse, so we
+///       currently omit them.
+///     - `char` is less easy to misuse, but it's also less clear why you'd want to be doing this.
+///       If we did ever add this, we'd probably want to convert to a `u32` (similar to how we
+///       convert `bool` to `u8`) for better ABI stability.
+///     - `i128` and `u128` do not have a stable ABI, so they cannot be returned across the FFI.
+///
+/// - `Option<T>` where `T` is `IntoFfi`, by returning `IntoFfi::ffi_default()` for `None`.
+///
+/// None of these are directly helpful for user types though, so macros are provided for the
+/// following cases:
+///
+/// 1. For types which are passed around by an opaque pointer, the macro
+///    [`implement_into_ffi_by_pointer!`] is provided.
+///
+/// 2. For types which should be returned as a JSON string, the macro
+///    [`implement_into_ffi_by_json!`] is provided.
+///
+/// See the "Examples" section below for some other cases, such as returning by value.
+///
+/// ## Safety
+///
+/// This is an unsafe trait (implementing it requires `unsafe impl`). This is because we cannot
+/// guarantee that your type is safe to pass to C. The helpers we've provided as macros should be
+/// safe to use, and in the cases where a common pattern can't be done both safely and generically,
+/// we've opted not to provide a macro for it. That said, many of these cases are still safe if you
+/// meet some relatively basic requirements, see below for examples.
+///
+/// ## Examples
+///
+/// ### Returning types by value
+///
+/// If you want to return a type by value, we don't provide a macro for this, primarially because
+/// doing so cannot be statically guarantee that it is safe. However, it *is* safe for the cases
+/// where the type is either `#[repr(C)]` or `#[repr(transparent)]`. If this doesn't hold, you will
+/// want to use a different option!
+///
+/// Regardless, if this holds, it's fairly simple to implement, for example:
+///
+/// ```rust
+/// # use ffi_support::IntoFfi;
+/// #[derive(Default)]
+/// #[repr(C)]
+/// pub struct Point {
+///     pub x: i32,
+///     pub y: i32,
+/// }
+///
+/// unsafe impl IntoFfi for Point {
+///     type Value = Self;
+///     #[inline] fn ffi_default() -> Self { Default::default() }
+///     #[inline] fn into_ffi_value(self) -> Self { self }
+/// }
+/// ```
+///
+/// ### Conversion to another type (which is returned over the FFI)
+///
+/// In the FxA FFI, we used to have a `SyncKeys` type, which was converted to a different type before
+/// returning over the FFI. (The real FxA FFI is a little different, and more complex, but this is
+/// relatively close, and more widely recommendable than the one the FxA FFI uses):
+///
+/// This is fairly easy to do by performing the conversion inside `IntoFfi`.
+///
+/// ```rust
+/// # use ffi_support::{self, IntoFfi};
+/// # use std::{ptr, os::raw::c_char};
+/// pub struct SyncKeys(pub String, pub String);
+///
+/// #[repr(C)]
+/// pub struct SyncKeysC {
+///     pub sync_key: *mut c_char,
+///     pub xcs: *mut c_char,
+/// }
+///
+/// unsafe impl IntoFfi for SyncKeys {
+///     type Value = SyncKeysC;
+///     #[inline]
+///     fn ffi_default() -> SyncKeysC {
+///         SyncKeysC {
+///             sync_key: ptr::null_mut(),
+///             xcs: ptr::null_mut(),
+///         }
+///     }
+///
+///     #[inline]
+///     fn into_ffi_value(self) -> SyncKeysC {
+///         SyncKeysC {
+///             sync_key: ffi_support::rust_string_to_c(self.0),
+///             xcs:      ffi_support::rust_string_to_c(self.1),
+///         }
+///     }
+/// }
+///
+/// // Note: this type manages memory, so you still will want to expose a destructor for this,
+/// // and possibly implement Drop as well.
+/// ```
+pub unsafe trait IntoFfi: Sized {
+    /// This type must be:
+    ///
+    /// 1. Compatible with C, which is to say `#[repr(C)]`, a numeric primitive,
+    ///    another type that has guarantees made about it's layout, or a
+    ///    `#[repr(transparent)]` wrapper around one of those.
+    ///
+    ///    One could even use `&T`, so long as `T: Sized`, although it's
+    ///    extremely dubious to return a reference to borrowed memory over the
+    ///    FFI, since it's very difficult for the caller to know how long it
+    ///    remains valid.
+    ///
+    /// 2. Capable of storing an empty/ignorable/default value.
+    ///
+    /// 3. Capable of storing the actual value.
+    ///
+    /// Valid examples include:
+    ///
+    /// - Primitive numbers (other than i128/u128)
+    ///
+    /// - #[repr(C)] structs containing only things on this list.
+    ///
+    /// - `Option<Box<T>>`, but only if `T` is `Sized`. (Internally this is
+    ///   guaranteed to be represented equivalently to a pointer)
+    ///
+    /// - Raw pointers such as `*const T`, and `*mut T`, but again, only if `T`
+    ///   is `Sized` (`*const [T]`, `*mut dyn SomeTrait` etc are not valid).
+    ///
+    /// - Enums with a fixed `repr`, although it's a good idea avoid
+    ///   `#[repr(C)]` enums in favor of, say, `#[repr(i32)]` (for example, any
+    ///   fixed type there should be fine), as it's potentially error prone to
+    ///   access `#[repr(C)]` enums from Android over JNA (it's only safe if C's
+    ///   `sizeof(int) == 4`, which is very common, but not universally true).
+    ///
+    /// - `&T`/`&mut T` where `T: Sized` but only if you really know what you're
+    ///   doing, because this is probably a mistake.
+    ///
+    /// Invalid examples include things like `&str`, `&[T]`, `String`, `Vec<T>`,
+    /// `std::ffi::CString`, `&std::ffi::CStr`, etc.
+    type Value;
+
+    /// Return an 'empty' value. This is what's passed back to C in the case of an error,
+    /// so it doesn't actually need to be "empty", so much as "ignorable". Note that this
+    /// is also used when an empty `Option<T>` is returned.
+    fn ffi_default() -> Self::Value;
+
+    /// Convert ourselves into a value we can pass back to C with confidence.
+    fn into_ffi_value(self) -> Self::Value;
+}
+
+unsafe impl IntoFfi for String {
+    type Value = *mut c_char;
+
+    #[inline]
+    fn ffi_default() -> Self::Value {
+        ptr::null_mut()
+    }
+
+    #[inline]
+    fn into_ffi_value(self) -> Self::Value {
+        rust_string_to_c(self)
+    }
+}
+
+// Implement IntoFfi for Option<T> by falling back to ffi_default for None.
+unsafe impl<T: IntoFfi> IntoFfi for Option<T> {
+    type Value = <T as IntoFfi>::Value;
+
+    #[inline]
+    fn ffi_default() -> Self::Value {
+        T::ffi_default()
+    }
+
+    #[inline]
+    fn into_ffi_value(self) -> Self::Value {
+        if let Some(s) = self {
+            s.into_ffi_value()
+        } else {
+            T::ffi_default()
+        }
+    }
+}
+
+// We've had problems in the past returning booleans over the FFI (specifically to JNA), and so
+// we convert them to `u8`.
+unsafe impl IntoFfi for bool {
+    type Value = u8;
+    #[inline]
+    fn ffi_default() -> Self::Value {
+        0u8
+    }
+    #[inline]
+    fn into_ffi_value(self) -> Self::Value {
+        self as u8
+    }
+}
+
+unsafe impl IntoFfi for crate::ByteBuffer {
+    type Value = crate::ByteBuffer;
+    #[inline]
+    fn ffi_default() -> Self::Value {
+        crate::ByteBuffer::default()
+    }
+    #[inline]
+    fn into_ffi_value(self) -> Self::Value {
+        self
+    }
+}
+
+// just cuts down on boilerplate. Not public.
+macro_rules! impl_into_ffi_for_primitive {
+    ($($T:ty),+) => {$(
+        unsafe impl IntoFfi for $T {
+            type Value = Self;
+            #[inline] fn ffi_default() -> Self { Default::default() }
+            #[inline] fn into_ffi_value(self) -> Self { self }
+        }
+    )+}
+}
+
+// See IntoFfi docs for why this is not exhaustive
+impl_into_ffi_for_primitive![(), i8, u8, i16, u16, i32, u32, i64, u64, f32, f64];
+
+// just cuts down on boilerplate. Not public.
+macro_rules! impl_into_ffi_for_pointer {
+    ($($T:ty),+) => {$(
+        unsafe impl IntoFfi for $T {
+            type Value = Self;
+            #[inline] fn ffi_default() -> Self { ptr::null_mut() }
+            #[inline] fn into_ffi_value(self) -> Self { self }
+        }
+    )+}
+}
+
+impl_into_ffi_for_pointer![
+    *mut i8,
+    *const i8,
+    *mut u8,
+    *const u8,
+    *mut c_void,
+    *const c_void
+];
diff --git a/third_party/rust/ffi-support/src/lib.rs b/third_party/rust/ffi-support/src/lib.rs
new file mode 100644
index 0000000000..cf50f0cd0d
--- /dev/null
+++ b/third_party/rust/ffi-support/src/lib.rs
@@ -0,0 +1,625 @@
+/* Copyright 2018-2019 Mozilla Foundation
+ *
+ * Licensed under the Apache License (Version 2.0), or the MIT license,
+ * (the "Licenses") at your option. You may not use this file except in
+ * compliance with one of the Licenses. You may obtain copies of the
+ * Licenses at:
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *    http://opensource.org/licenses/MIT
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the Licenses is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the Licenses for the specific language governing permissions and
+ * limitations under the Licenses. */
+
+#![deny(missing_docs)]
+#![allow(unknown_lints)]
+#![warn(rust_2018_idioms)]
+
+//! # FFI Support
+//!
+//! This crate implements a support library to simplify implementing the patterns that the
+//! `mozilla/application-services` repository uses for it's "Rust Component" FFI libraries.
+//!
+//! It is *strongly encouraged* that anybody writing FFI code in this repository read this
+//! documentation before doing so, as it is a subtle, difficult, and error prone process.
+//!
+//! ## Terminology
+//!
+//! For each library, there are currently three parts we're concerned with. There's no clear correct
+//! name for these, so this documentation will attempt to use the following terminology:
+//!
+//! - **Rust Component**: A Rust crate which does not expose an FFI directly, but may be may be
+//!   wrapped by one that does. These have a `crate-type` in their Cargo.toml (see
+//!   https://doc.rust-lang.org/reference/linkage.html) of `lib`, and not `staticlib` or `cdylib`
+//!   (Note that `lib` is the default if `crate-type` is not specified). Examples include the
+//!   `fxa-client`, and `logins` crates.
+//!
+//! - **FFI Component**: A wrapper crate that takes a Rust component, and exposes an FFI from it.
+//!   These typically have `ffi` in the name, and have `crate-type = ["lib", "staticlib", "cdylib"]`
+//!   in their Cargo.toml. For example, the `fxa-client/ffi` and `logins/ffi` crates (note:
+//!   paths are subject to change). When built, these produce a native library that is consumed by
+//!   the "FFI Consumer".
+//!
+//! - **FFI Consumer**: This is a low level library, typically implemented in Kotlin (for Android)
+//!   or Swift (for iOS), that exposes a memory-safe wrapper around the memory-unsafe C API produced
+//!   by the FFI component. It's expected that the maintainers of the FFI Component and FFI Consumer
+//!   be the same (or at least, the author of the consumer should be completely comfortable with the
+//!   API exposed by, and code in the FFI component), since the code in these is extremely tightly
+//!   coupled, and very easy to get wrong.
+//!
+//! Note that while there are three parts, there may be more than three libraries relevant here, for
+//! example there may be more than one FFI consumer (one for Android, one for iOS).
+//!
+//! ## Usage
+//!
+//! This library will typically be used in both the Rust component, and the FFI component, however
+//! it frequently will be an optional dependency in the Rust component that's only available when a
+//! feature flag (which the FFI component will always require) is used.
+//!
+//! The reason it's required inside the Rust component (and not solely in the FFI component, which
+//! would be nice), is so that types provided by that crate may implement the traits provided by
+//! this crate (this is because Rust does not allow crate `C` to implement a trait defined in crate
+//! `A` for a type defined in crate `B`).
+//!
+//! In general, examples should be provided for the most important types and functions
+//! ([`call_with_result`], [`IntoFfi`],
+//! [`ExternError`], etc), but you should also look at the code of
+//! consumers of this library.
+//!
+//! ### Usage in the Rust Component
+//!
+//! Inside the Rust component, you will implement:
+//!
+//! 1. [`IntoFfi`] for all types defined in that crate that you want to return
+//!    over the FFI. For most common cases, the [`implement_into_ffi_by_json!`] and
+//!    [`implement_into_ffi_by_protobuf!`] macros will do the job here, however you
+//!    can see that trait's documentation for discussion and examples of
+//!    implementing it manually.
+//!
+//! 2. Conversion to [`ExternError`] for the error type(s) exposed by that
+//!    rust component, that is, `impl From<MyError> for ExternError`.
+//!
+//! ### Usage in the FFI Component
+//!
+//! Inside the FFI component, you will use this library in a few ways:
+//!
+//! 1. Destructors will be exposed for each types that had [`implement_into_ffi_by_pointer!`] called
+//!    on it (using [`define_box_destructor!`]), and a destructor for strings should be exposed as
+//!    well, using [`define_string_destructor`]
+//!
+//! 2. The body of every / nearly every FFI function will be wrapped in either a
+//!    [`call_with_result`] or [`call_with_output`].
+//!
+//!    This is required because if we `panic!` (e.g. from an `assert!`, `unwrap()`, `expect()`, from
+//!    indexing past the end of an array, etc) across the FFI boundary, the behavior is undefined
+//!    and in practice very weird things tend to happen (we aren't caught by the caller, since they
+//!    don't have the same exception behavior as us).
+//!
+//!    If you don't think your program (or possibly just certain calls) can handle panics, you may
+//!    also use the versions of these functions in the [`abort_on_panic`] module, which
+//!    do as their name suggest.
+//!
+//! Additionally, c strings that are passed in as arguments may be represented using [`FfiStr`],
+//! which contains several helpful inherent methods for extracting their data.
+//!
+
+use std::{panic, thread};
+
+mod error;
+mod ffistr;
+pub mod handle_map;
+mod into_ffi;
+#[macro_use]
+mod macros;
+mod string;
+
+pub use crate::error::*;
+pub use crate::ffistr::FfiStr;
+pub use crate::into_ffi::*;
+pub use crate::macros::*;
+pub use crate::string::*;
+
+// We export most of the types from this, but some constants
+// (MAX_CAPACITY) don't make sense at the top level.
+pub use crate::handle_map::{ConcurrentHandleMap, Handle, HandleError, HandleMap};
+
+/// Call a callback that returns a `Result<T, E>` while:
+///
+/// - Catching panics, and reporting them to C via [`ExternError`].
+/// - Converting `T` to a C-compatible type using [`IntoFfi`].
+/// - Converting `E` to a C-compatible error via `Into<ExternError>`.
+///
+/// This (or [`call_with_output`]) should be in the majority of the FFI functions, see the crate
+/// top-level docs for more info.
+///
+/// If your function doesn't produce an error, you may use [`call_with_output`] instead, which
+/// doesn't require you return a Result.
+///
+/// ## Example
+///
+/// A few points about the following example:
+///
+/// - We need to mark it as `#[no_mangle] pub extern "C"`.
+///
+/// - We prefix it with a unique name for the library (e.g. `mylib_`). Foreign functions are not
+///   namespaced, and symbol collisions can cause a large number of problems and subtle bugs,
+///   including memory safety issues in some cases.
+///
+/// ```rust,no_run
+/// # use ffi_support::{ExternError, ErrorCode, FfiStr};
+/// # use std::os::raw::c_char;
+///
+/// # #[derive(Debug)]
+/// # struct BadEmptyString;
+/// # impl From<BadEmptyString> for ExternError {
+/// #     fn from(e: BadEmptyString) -> Self {
+/// #         ExternError::new_error(ErrorCode::new(1), "Bad empty string")
+/// #     }
+/// # }
+///
+/// #[no_mangle]
+/// pub extern "C" fn mylib_print_string(
+///     // Strings come in as an `FfiStr`, which is a wrapper around a null terminated C string.
+///     thing_to_print: FfiStr<'_>,
+///     // Note that taking `&mut T` and `&T` is both allowed and encouraged, so long as `T: Sized`,
+///     // (e.g. it can't be a trait object, `&[T]`, a `&str`, etc). Also note that `Option<&T>` and
+///     // `Option<&mut T>` are also allowed, if you expect the caller to sometimes pass in null, but
+///     // that's the only case when it's currently to use `Option` in an argument list like this).
+///     error: &mut ExternError
+/// ) {
+///     // You should try to to do as little as possible outside the call_with_result,
+///     // to avoid a case where a panic occurs.
+///     ffi_support::call_with_result(error, || {
+///         let s = thing_to_print.as_str();
+///         if s.is_empty() {
+///             // This is a silly example!
+///             return Err(BadEmptyString);
+///         }
+///         println!("{}", s);
+///         Ok(())
+///     })
+/// }
+/// ```
+pub fn call_with_result<R, E, F>(out_error: &mut ExternError, callback: F) -> R::Value
+where
+    F: panic::UnwindSafe + FnOnce() -> Result<R, E>,
+    E: Into<ExternError>,
+    R: IntoFfi,
+{
+    call_with_result_impl(out_error, callback)
+}
+
+/// Call a callback that returns a `T` while:
+///
+/// - Catching panics, and reporting them to C via [`ExternError`]
+/// - Converting `T` to a C-compatible type using [`IntoFfi`]
+///
+/// Note that you still need to provide an [`ExternError`] to this function, to report panics.
+///
+/// See [`call_with_result`] if you'd like to return a `Result<T, E>` (Note: `E` must
+/// be convertible to [`ExternError`]).
+///
+/// This (or [`call_with_result`]) should be in the majority of the FFI functions, see
+/// the crate top-level docs for more info.
+pub fn call_with_output<R, F>(out_error: &mut ExternError, callback: F) -> R::Value
+where
+    F: panic::UnwindSafe + FnOnce() -> R,
+    R: IntoFfi,
+{
+    // We need something that's `Into<ExternError>`, even though we never return it, so just use
+    // `ExternError` itself.
+    call_with_result(out_error, || -> Result<_, ExternError> { Ok(callback()) })
+}
+
+fn call_with_result_impl<R, E, F>(out_error: &mut ExternError, callback: F) -> R::Value
+where
+    F: panic::UnwindSafe + FnOnce() -> Result<R, E>,
+    E: Into<ExternError>,
+    R: IntoFfi,
+{
+    *out_error = ExternError::success();
+    let res: thread::Result<(ExternError, R::Value)> = panic::catch_unwind(|| {
+        ensure_panic_hook_is_setup();
+        match callback() {
+            Ok(v) => (ExternError::default(), v.into_ffi_value()),
+            Err(e) => (e.into(), R::ffi_default()),
+        }
+    });
+    match res {
+        Ok((err, o)) => {
+            *out_error = err;
+            o
+        }
+        Err(e) => {
+            *out_error = e.into();
+            R::ffi_default()
+        }
+    }
+}
+
+/// This module exists just to expose a variant of [`call_with_result`] and [`call_with_output`]
+/// that aborts, instead of unwinding, on panic.
+pub mod abort_on_panic {
+    use super::*;
+
+    // Struct that exists to automatically process::abort if we don't call
+    // `std::mem::forget()` on it. This can have substantial performance
+    // benefits over calling `std::panic::catch_unwind` and aborting if a panic
+    // was caught, in addition to not requiring AssertUnwindSafe (for example).
+    struct AbortOnDrop;
+    impl Drop for AbortOnDrop {
+        fn drop(&mut self) {
+            std::process::abort();
+        }
+    }
+
+    /// A helper function useful for cases where you'd like to abort on panic,
+    /// but aren't in a position where you'd like to return an FFI-compatible
+    /// type.
+    #[inline]
+    pub fn with_abort_on_panic<R, F>(callback: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        let aborter = AbortOnDrop;
+        let res = callback();
+        std::mem::forget(aborter);
+        res
+    }
+
+    /// Same as the root `call_with_result`, but aborts on panic instead of unwinding. See the
+    /// `call_with_result` documentation for more.
+    pub fn call_with_result<R, E, F>(out_error: &mut ExternError, callback: F) -> R::Value
+    where
+        F: FnOnce() -> Result<R, E>,
+        E: Into<ExternError>,
+        R: IntoFfi,
+    {
+        with_abort_on_panic(|| match callback() {
+            Ok(v) => {
+                *out_error = ExternError::default();
+                v.into_ffi_value()
+            }
+            Err(e) => {
+                *out_error = e.into();
+                R::ffi_default()
+            }
+        })
+    }
+
+    /// Same as the root `call_with_output`, but aborts on panic instead of unwinding. As a result,
+    /// it doesn't require a [`ExternError`] out argument. See the `call_with_output` documentation
+    /// for more info.
+    pub fn call_with_output<R, F>(callback: F) -> R::Value
+    where
+        F: FnOnce() -> R,
+        R: IntoFfi,
+    {
+        with_abort_on_panic(callback).into_ffi_value()
+    }
+}
+
+/// Initialize our panic handling hook to optionally log panics
+#[cfg(feature = "log_panics")]
+pub fn ensure_panic_hook_is_setup() {
+    use std::sync::Once;
+    static INIT_BACKTRACES: Once = Once::new();
+    INIT_BACKTRACES.call_once(move || {
+        #[cfg(all(feature = "log_backtraces", not(target_os = "android")))]
+        {
+            std::env::set_var("RUST_BACKTRACE", "1");
+        }
+        // Turn on a panic hook which logs both backtraces and the panic
+        // "Location" (file/line). We do both in case we've been stripped,
+        // ).
+        std::panic::set_hook(Box::new(move |panic_info| {
+            let (file, line) = if let Some(loc) = panic_info.location() {
+                (loc.file(), loc.line())
+            } else {
+                // Apparently this won't happen but rust has reserved the
+                // ability to start returning None from location in some cases
+                // in the future.
+                ("<unknown>", 0)
+            };
+            log::error!("### Rust `panic!` hit at file '{}', line {}", file, line);
+            #[cfg(all(feature = "log_backtraces", not(target_os = "android")))]
+            {
+                log::error!("  Complete stack trace:\n{:?}", backtrace::Backtrace::new());
+            }
+        }));
+    });
+}
+
+/// Initialize our panic handling hook to optionally log panics
+#[cfg(not(feature = "log_panics"))]
+pub fn ensure_panic_hook_is_setup() {}
+
+/// ByteBuffer is a struct that represents an array of bytes to be sent over the FFI boundaries.
+/// There are several cases when you might want to use this, but the primary one for us
+/// is for returning protobuf-encoded data to Swift and Java. The type is currently rather
+/// limited (implementing almost no functionality), however in the future it may be
+/// more expanded.
+///
+/// ## Caveats
+///
+/// Note that the order of the fields is `len` (an i64) then `data` (a `*mut u8`), getting
+/// this wrong on the other side of the FFI will cause memory corruption and crashes.
+/// `i64` is used for the length instead of `u64` and `usize` because JNA has interop
+/// issues with both these types.
+///
+/// ### `Drop` is not implemented
+///
+/// ByteBuffer does not implement Drop. This is intentional. Memory passed into it will
+/// be leaked if it is not explicitly destroyed by calling [`ByteBuffer::destroy`], or
+/// [`ByteBuffer::destroy_into_vec`]. This is for two reasons:
+///
+/// 1. In the future, we may allow it to be used for data that is not managed by
+///    the Rust allocator\*, and `ByteBuffer` assuming it's okay to automatically
+///    deallocate this data with the Rust allocator.
+///
+/// 2. Automatically running destructors in unsafe code is a
+///    [frequent footgun](https://without.boats/blog/two-memory-bugs-from-ringbahn/)
+///    (among many similar issues across many crates).
+///
+/// Note that calling `destroy` manually is often not needed, as usually you should
+/// be passing these to the function defined by [`define_bytebuffer_destructor!`] from
+/// the other side of the FFI.
+///
+/// Because this type is essentially *only* useful in unsafe or FFI code (and because
+/// the most common usage pattern does not require manually managing the memory), it
+/// does not implement `Drop`.
+///
+/// \* Note: in the case of multiple Rust shared libraries loaded at the same time,
+/// there may be multiple instances of "the Rust allocator" (one per shared library),
+/// in which case we're referring to whichever instance is active for the code using
+/// the `ByteBuffer`. Note that this doesn't occur on all platforms or build
+/// configurations, but treating allocators in different shared libraries as fully
+/// independent is always safe.
+///
+/// ## Layout/fields
+///
+/// This struct's field are not `pub` (mostly so that we can soundly implement `Send`, but also so
+/// that we can verify rust users are constructing them appropriately), the fields, their types, and
+/// their order are *very much* a part of the public API of this type. Consumers on the other side
+/// of the FFI will need to know its layout.
+///
+/// If this were a C struct, it would look like
+///
+/// ```c,no_run
+/// struct ByteBuffer {
+///     // Note: This should never be negative, but values above
+///     // INT64_MAX / i64::MAX are not allowed.
+///     int64_t len;
+///     // Note: nullable!
+///     uint8_t *data;
+/// };
+/// ```
+///
+/// In rust, there are two fields, in this order: `len: i64`, and `data: *mut u8`.
+///
+/// For clarity, the fact that the data pointer is nullable means that `Option<ByteBuffer>` is not
+/// the same size as ByteBuffer, and additionally is not FFI-safe (the latter point is not
+/// currently guaranteed anyway as of the time of writing this comment).
+///
+/// ### Description of fields
+///
+/// `data` is a pointer to an array of `len` bytes. Note that data can be a null pointer and therefore
+/// should be checked.
+///
+/// The bytes array is allocated on the heap and must be freed on it as well. Critically, if there
+/// are multiple rust shared libraries using being used in the same application, it *must be freed
+/// on the same heap that allocated it*, or you will corrupt both heaps.
+///
+/// Typically, this object is managed on the other side of the FFI (on the "FFI consumer"), which
+/// means you must expose a function to release the resources of `data` which can be done easily
+/// using the [`define_bytebuffer_destructor!`] macro provided by this crate.
+#[repr(C)]
+pub struct ByteBuffer {
+    len: i64,
+    data: *mut u8,
+}
+
+impl From<Vec<u8>> for ByteBuffer {
+    #[inline]
+    fn from(bytes: Vec<u8>) -> Self {
+        Self::from_vec(bytes)
+    }
+}
+
+impl ByteBuffer {
+    /// Creates a `ByteBuffer` of the requested size, zero-filled.
+    ///
+    /// The contents of the vector will not be dropped. Instead, `destroy` must
+    /// be called later to reclaim this memory or it will be leaked.
+    ///
+    /// ## Caveats
+    ///
+    /// This will panic if the buffer length (`usize`) cannot fit into a `i64`.
+    #[inline]
+    pub fn new_with_size(size: usize) -> Self {
+        // Note: `Vec` requires this internally on 64 bit platforms (and has a
+        // stricter requirement on 32 bit ones), so this is just to be explicit.
+        assert!(size < i64::MAX as usize);
+        let mut buf = vec![];
+        buf.reserve_exact(size);
+        buf.resize(size, 0);
+        ByteBuffer::from_vec(buf)
+    }
+
+    /// Creates a `ByteBuffer` instance from a `Vec` instance.
+    ///
+    /// The contents of the vector will not be dropped. Instead, `destroy` must
+    /// be called later to reclaim this memory or it will be leaked.
+    ///
+    /// ## Caveats
+    ///
+    /// This will panic if the buffer length (`usize`) cannot fit into a `i64`.
+    #[inline]
+    pub fn from_vec(bytes: Vec<u8>) -> Self {
+        use std::convert::TryFrom;
+        let mut buf = bytes.into_boxed_slice();
+        let data = buf.as_mut_ptr();
+        let len = i64::try_from(buf.len()).expect("buffer length cannot fit into a i64.");
+        std::mem::forget(buf);
+        Self { data, len }
+    }
+
+    /// View the data inside this `ByteBuffer` as a `&[u8]`.
+    // TODO: Is it worth implementing `Deref`? Patches welcome if you need this.
+    #[inline]
+    pub fn as_slice(&self) -> &[u8] {
+        if self.data.is_null() {
+            &[]
+        } else {
+            unsafe { std::slice::from_raw_parts(self.data, self.len()) }
+        }
+    }
+
+    #[inline]
+    fn len(&self) -> usize {
+        use std::convert::TryInto;
+        self.len
+            .try_into()
+            .expect("ByteBuffer length negative or overflowed")
+    }
+
+    /// View the data inside this `ByteBuffer` as a `&mut [u8]`.
+    // TODO: Is it worth implementing `DerefMut`? Patches welcome if you need this.
+    #[inline]
+    pub fn as_mut_slice(&mut self) -> &mut [u8] {
+        if self.data.is_null() {
+            &mut []
+        } else {
+            unsafe { std::slice::from_raw_parts_mut(self.data, self.len()) }
+        }
+    }
+
+    /// Deprecated alias for [`ByteBuffer::destroy_into_vec`].
+    #[inline]
+    #[deprecated = "Name is confusing, please use `destroy_into_vec` instead"]
+    pub fn into_vec(self) -> Vec<u8> {
+        self.destroy_into_vec()
+    }
+
+    /// Convert this `ByteBuffer` into a Vec<u8>, taking ownership of the
+    /// underlying memory, which will be freed using the rust allocator once the
+    /// `Vec<u8>`'s lifetime is done.
+    ///
+    /// If this is undesirable, you can do `bb.as_slice().to_vec()` to get a
+    /// `Vec<u8>` containing a copy of this `ByteBuffer`'s underlying data.
+    ///
+    /// ## Caveats
+    ///
+    /// This is safe so long as the buffer is empty, or the data was allocated
+    /// by Rust code, e.g. this is a ByteBuffer created by
+    /// `ByteBuffer::from_vec` or `Default::default`.
+    ///
+    /// If the ByteBuffer were allocated by something other than the
+    /// current/local Rust `global_allocator`, then calling `destroy` is
+    /// fundamentally broken.
+    ///
+    /// For example, if it were allocated externally by some other language's
+    /// runtime, or if it were allocated by the global allocator of some other
+    /// Rust shared object in the same application, the behavior is undefined
+    /// (and likely to cause problems).
+    ///
+    /// Note that this currently can only happen if the `ByteBuffer` is passed
+    /// to you via an `extern "C"` function that you expose, as opposed to being
+    /// created locally.
+    #[inline]
+    pub fn destroy_into_vec(self) -> Vec<u8> {
+        if self.data.is_null() {
+            vec![]
+        } else {
+            let len = self.len();
+            // Safety: This is correct because we convert to a Box<[u8]> first,
+            // which is a design constraint of RawVec.
+            unsafe { Vec::from_raw_parts(self.data, len, len) }
+        }
+    }
+
+    /// Reclaim memory stored in this ByteBuffer.
+    ///
+    /// You typically should not call this manually, and instead expose a
+    /// function that does so via [`define_bytebuffer_destructor!`].
+    ///
+    /// ## Caveats
+    ///
+    /// This is safe so long as the buffer is empty, or the data was allocated
+    /// by Rust code, e.g. this is a ByteBuffer created by
+    /// `ByteBuffer::from_vec` or `Default::default`.
+    ///
+    /// If the ByteBuffer were allocated by something other than the
+    /// current/local Rust `global_allocator`, then calling `destroy` is
+    /// fundamentally broken.
+    ///
+    /// For example, if it were allocated externally by some other language's
+    /// runtime, or if it were allocated by the global allocator of some other
+    /// Rust shared object in the same application, the behavior is undefined
+    /// (and likely to cause problems).
+    ///
+    /// Note that this currently can only happen if the `ByteBuffer` is passed
+    /// to you via an `extern "C"` function that you expose, as opposed to being
+    /// created locally.
+    #[inline]
+    pub fn destroy(self) {
+        // Note: the drop is just for clarity, of course.
+        drop(self.destroy_into_vec())
+    }
+}
+
+impl Default for ByteBuffer {
+    #[inline]
+    fn default() -> Self {
+        Self {
+            len: 0 as i64,
+            data: std::ptr::null_mut(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    #[test]
+    fn test_bb_access() {
+        let mut bb = ByteBuffer::from(vec![1u8, 2, 3]);
+        assert_eq!(bb.as_slice(), &[1u8, 2, 3]);
+        assert_eq!(bb.as_mut_slice(), &mut [1u8, 2, 3]);
+        bb.as_mut_slice()[2] = 4;
+
+        // Use into_vec to cover both into_vec and destroy_into_vec.
+        #[allow(deprecated)]
+        {
+            assert_eq!(bb.into_vec(), &[1u8, 2, 4]);
+        }
+    }
+
+    #[test]
+    fn test_bb_empty() {
+        let mut bb = ByteBuffer::default();
+        assert_eq!(bb.as_slice(), &[]);
+        assert_eq!(bb.as_mut_slice(), &[]);
+        assert_eq!(bb.destroy_into_vec(), &[]);
+    }
+
+    #[test]
+    fn test_bb_new() {
+        let bb = ByteBuffer::new_with_size(5);
+        assert_eq!(bb.as_slice(), &[0u8, 0, 0, 0, 0]);
+        bb.destroy();
+
+        let bb = ByteBuffer::new_with_size(0);
+        assert_eq!(bb.as_slice(), &[]);
+        assert!(!bb.data.is_null());
+        bb.destroy();
+
+        let bb = ByteBuffer::from_vec(vec![]);
+        assert_eq!(bb.as_slice(), &[]);
+        assert!(!bb.data.is_null());
+        bb.destroy();
+    }
+}
diff --git a/third_party/rust/ffi-support/src/macros.rs b/third_party/rust/ffi-support/src/macros.rs
new file mode 100644
index 0000000000..1eeff5771e
--- /dev/null
+++ b/third_party/rust/ffi-support/src/macros.rs
@@ -0,0 +1,362 @@
+/* Copyright 2018-2019 Mozilla Foundation
+ *
+ * Licensed under the Apache License (Version 2.0), or the MIT license,
+ * (the "Licenses") at your option. You may not use this file except in
+ * compliance with one of the Licenses. You may obtain copies of the
+ * Licenses at:
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *    http://opensource.org/licenses/MIT
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the Licenses is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the Licenses for the specific language governing permissions and
+ * limitations under the Licenses. */
+
+/// Implements [`IntoFfi`][crate::IntoFfi] for the provided types (more than one
+/// may be passed in) by allocating `$T` on the heap as an opaque pointer.
+///
+/// This is typically going to be used from the "Rust component", and not the
+/// "FFI component" (see the top level crate documentation for more
+/// information), however you will still need to implement a destructor in the
+/// FFI component using [`define_box_destructor!`][crate::define_box_destructor].
+///
+/// In general, is only safe to do for `send` types (even this is dodgy, but
+/// it's often necessary to keep the locking on the other side of the FFI, so
+/// Sync is too harsh), so we enforce this in this macro. (You're still free to
+/// implement this manually, if this restriction is too harsh for your use case
+/// and you're certain you know what you're doing).
+#[macro_export]
+macro_rules! implement_into_ffi_by_pointer {
+    ($($T:ty),* $(,)*) => {$(
+        unsafe impl $crate::IntoFfi for $T where $T: Send {
+            type Value = *mut $T;
+
+            #[inline]
+            fn ffi_default() -> *mut $T {
+                std::ptr::null_mut()
+            }
+
+            #[inline]
+            fn into_ffi_value(self) -> *mut $T {
+                Box::into_raw(Box::new(self))
+            }
+        }
+    )*}
+}
+
+/// Implements [`IntoFfi`][crate::IntoFfi] for the provided types (more than one
+/// may be passed in) by converting to the type to a JSON string.
+///
+/// Additionally, most of the time we recomment using this crate's protobuf
+/// support, instead of JSON.
+///
+/// This is typically going to be used from the "Rust component", and not the
+/// "FFI component" (see the top level crate documentation for more
+/// information).
+///
+/// Note: Each type passed in must implement or derive `serde::Serialize`.
+///
+/// Note: for this to works, the crate it's called in must depend on `serde` and
+/// `serde_json`.
+///
+/// ## Panics
+///
+/// The [`IntoFfi`][crate::IntoFfi] implementation this macro generates may
+/// panic in the following cases:
+///
+/// - You've passed a type that contains a Map that has non-string keys (which
+///   can't be represented in JSON).
+///
+/// - You've passed a type which has a custom serializer, and the custom
+///   serializer failed.
+///
+/// These cases are both rare enough that this still seems fine for the majority
+/// of uses.
+#[macro_export]
+macro_rules! implement_into_ffi_by_json {
+    ($($T:ty),* $(,)*) => {$(
+        unsafe impl $crate::IntoFfi for $T where $T: serde::Serialize {
+            type Value = *mut std::os::raw::c_char;
+            #[inline]
+            fn ffi_default() -> *mut std::os::raw::c_char {
+                std::ptr::null_mut()
+            }
+            #[inline]
+            fn into_ffi_value(self) -> *mut std::os::raw::c_char {
+                // This panic is inside our catch_panic, so it should be fine.
+                // We've also documented the case where the IntoFfi impl that
+                // calls this panics, and it's rare enough that it shouldn't
+                // matter that if it happens we return an ExternError
+                // representing a panic instead of one of some other type
+                // (especially given that the application isn't likely to be
+                // able to meaningfully handle JSON serialization failure).
+                let as_string = serde_json::to_string(&self).unwrap();
+                $crate::rust_string_to_c(as_string)
+            }
+        }
+    )*}
+}
+
+/// Implements [`IntoFfi`][crate::IntoFfi] for the provided types (more than one
+/// may be passed in) implementing `prost::Message` (protobuf auto-generated
+/// type) by converting to the type to a [`ByteBuffer`][crate::ByteBuffer]. This
+/// [`ByteBuffer`][crate::ByteBuffer] should later be passed by value.
+///
+/// Note: for this to works, the crate it's called in must depend on `prost`.
+///
+/// Note: Each type passed in must implement or derive `prost::Message`.
+#[macro_export]
+macro_rules! implement_into_ffi_by_protobuf {
+    ($($FFIType:ty),* $(,)*) => {$(
+        unsafe impl $crate::IntoFfi for $FFIType where $FFIType: prost::Message {
+            type Value = $crate::ByteBuffer;
+            #[inline]
+            fn ffi_default() -> Self::Value {
+                Default::default()
+            }
+
+            #[inline]
+            fn into_ffi_value(self) -> Self::Value {
+                use prost::Message;
+                let mut bytes = Vec::with_capacity(self.encoded_len());
+                // Unwrap is safe, since we have reserved sufficient capacity in
+                // the vector.
+                self.encode(&mut bytes).unwrap();
+                bytes.into()
+            }
+        }
+    )*}
+}
+
+/// Implement [`InfoFfi`][crate::IntoFfi] for a type by converting through
+/// another type.
+///
+/// The argument `$MidTy` argument must implement `From<$SrcTy>` and
+/// [`InfoFfi`][crate::IntoFfi].
+///
+/// This is provided (even though it's trivial) because it is always safe (well,
+/// so long as `$MidTy`'s [`IntoFfi`][crate::IntoFfi] implementation is
+/// correct), but would otherwise require use of `unsafe` to implement.
+#[macro_export]
+macro_rules! implement_into_ffi_by_delegation {
+    ($SrcTy:ty, $MidTy:ty) => {
+        unsafe impl $crate::IntoFfi for $SrcTy
+        where
+            $MidTy: From<$SrcTy> + $crate::IntoFfi,
+        {
+            // The <$MidTy as SomeTrait>::method is required even when it would
+            // be ambiguous due to some obscure details of macro syntax.
+            type Value = <$MidTy as $crate::IntoFfi>::Value;
+
+            #[inline]
+            fn ffi_default() -> Self::Value {
+                <$MidTy as $crate::IntoFfi>::ffi_default()
+            }
+
+            #[inline]
+            fn into_ffi_value(self) -> Self::Value {
+                use $crate::IntoFfi;
+                <$MidTy as From<$SrcTy>>::from(self).into_ffi_value()
+            }
+        }
+    };
+}
+
+/// For a number of reasons (name collisions are a big one, but, it also wouldn't work on all
+/// platforms), we cannot export `extern "C"` functions from this library. However, it's pretty
+/// common to want to free strings allocated by rust, so many libraries will need this, so we
+/// provide it as a macro.
+///
+/// It simply expands to a `#[no_mangle] pub unsafe extern "C" fn` which wraps this crate's
+/// [`destroy_c_string`][crate::destroy_c_string] function.
+///
+/// ## Caveats
+///
+/// If you're using multiple separately compiled rust libraries in your application, it's critical
+/// that you are careful to only ever free strings allocated by a Rust library using the same rust
+/// library. Passing them to a different Rust library's string destructor will cause you to corrupt
+/// multiple heaps.
+///
+/// Additionally, be sure that all strings you pass to this were actually allocated by rust. It's a
+/// common issue for JNA code to transparently convert Pointers to things to Strings behind the
+/// scenes, which is quite risky here. (To avoid this in JNA, only use `String` for passing
+/// read-only strings into Rust, e.g. it's for passing `*const c_char`. All other uses should use
+/// `Pointer` and `getString()`).
+///
+/// Finally, to avoid name collisions, it is strongly recommended that you provide an name for this
+/// function unique to your library.
+///
+/// ## Example
+///
+/// ```rust
+/// # use ffi_support::define_string_destructor;
+/// define_string_destructor!(mylib_destroy_string);
+/// ```
+#[macro_export]
+macro_rules! define_string_destructor {
+    ($mylib_destroy_string:ident) => {
+        /// Public destructor for strings managed by the other side of the FFI.
+        ///
+        /// # Safety
+        ///
+        /// This will free the string pointer it gets passed in as an argument,
+        /// and thus can be wildly unsafe if misused.
+        ///
+        /// See the documentation of `ffi_support::destroy_c_string` and
+        /// `ffi_support::define_string_destructor!` for further info.
+        #[no_mangle]
+        pub unsafe extern "C" fn $mylib_destroy_string(s: *mut std::os::raw::c_char) {
+            // Note: This should never happen, but in the case of a bug aborting
+            // here is better than the badness that happens if we unwind across
+            // the FFI boundary.
+            $crate::abort_on_panic::with_abort_on_panic(|| {
+                if !s.is_null() {
+                    $crate::destroy_c_string(s)
+                }
+            });
+        }
+    };
+}
+
+/// Define a (public) destructor for a type that was allocated by
+/// `Box::into_raw(Box::new(value))` (e.g. a pointer which is probably opaque).
+///
+/// ## Caveats
+///
+/// When called over the FFI, this can go wrong in a ridiculous number of ways,
+/// and we can't really prevent any of them. But essentially, the caller (on the
+/// other side of the FFI) needs to be extremely careful to ensure that it stops
+/// using the pointer after it's freed.
+///
+/// Also, to avoid name collisions, it is strongly recommended that you provide
+/// an name for this function unique to your library. (This is true for all
+/// functions you expose).
+///
+/// However, when called from rust, this is safe, as it becomes a function that
+/// just drops a `Option<Box<T>>` with some panic handling.
+///
+/// ## Example
+///
+/// ```rust
+/// # use ffi_support::define_box_destructor;
+/// struct CoolType(Vec<i32>);
+///
+/// define_box_destructor!(CoolType, mylib_destroy_cooltype);
+/// ```
+#[macro_export]
+macro_rules! define_box_destructor {
+    ($T:ty, $destructor_name:ident) => {
+        /// # Safety
+        /// This is equivalent to calling Box::from_raw with panic handling, and
+        /// thus inherits [`Box::from_raw`]'s safety properties. That is to say,
+        /// this function is wildly unsafe.
+        #[no_mangle]
+        pub unsafe extern "C" fn $destructor_name(v: *mut $T) {
+            // We should consider passing an error parameter in here rather than
+            // aborting, but at the moment the only case where we do this
+            // (interrupt handles) should never panic in Drop, so it's probably
+            // fine.
+            $crate::abort_on_panic::with_abort_on_panic(|| {
+                if !v.is_null() {
+                    drop(Box::from_raw(v))
+                }
+            });
+        }
+    };
+}
+
+/// Define a (public) destructor for the [`ByteBuffer`][crate::ByteBuffer] type.
+///
+/// ## Caveats
+///
+/// If you're using multiple separately compiled rust libraries in your application, it's critical
+/// that you are careful to only ever free `ByteBuffer` instances allocated by a Rust library using
+/// the same rust library. Passing them to a different Rust library's string destructor will cause
+/// you to corrupt multiple heaps.
+/// One common ByteBuffer destructor is defined per Rust library.
+///
+/// Also, to avoid name collisions, it is strongly recommended that you provide an name for this
+/// function unique to your library. (This is true for all functions you expose).
+///
+/// ## Example
+///
+/// ```rust
+/// # use ffi_support::define_bytebuffer_destructor;
+/// define_bytebuffer_destructor!(mylib_destroy_bytebuffer);
+/// ```
+#[macro_export]
+macro_rules! define_bytebuffer_destructor {
+    ($destructor_name:ident) => {
+        #[no_mangle]
+        pub extern "C" fn $destructor_name(v: $crate::ByteBuffer) {
+            // Note: This should never happen, but in the case of a bug aborting
+            // here is better than the badness that happens if we unwind across
+            // the FFI boundary.
+            $crate::abort_on_panic::with_abort_on_panic(|| v.destroy())
+        }
+    };
+}
+
+/// Define a (public) destructor for a type that lives inside a lazy_static
+/// [`ConcurrentHandleMap`][crate::ConcurrentHandleMap].
+///
+/// Note that this is actually totally safe, unlike the other
+/// `define_blah_destructor` macros.
+///
+/// A critical difference, however, is that this dtor takes an `err` out
+/// parameter to indicate failure. This difference is why the name is different
+/// as well (deleter vs destructor).
+///
+/// ## Example
+///
+/// ```rust
+/// # use lazy_static::lazy_static;
+/// # use ffi_support::{ConcurrentHandleMap, define_handle_map_deleter};
+/// struct Thing(Vec<i32>);
+/// // Somewhere...
+/// lazy_static! {
+///     static ref THING_HANDLES: ConcurrentHandleMap<Thing> = ConcurrentHandleMap::new();
+/// }
+/// define_handle_map_deleter!(THING_HANDLES, mylib_destroy_thing);
+/// ```
+#[macro_export]
+macro_rules! define_handle_map_deleter {
+    ($HANDLE_MAP_NAME:ident, $destructor_name:ident) => {
+        #[no_mangle]
+        pub extern "C" fn $destructor_name(v: u64, err: &mut $crate::ExternError) {
+            $crate::call_with_result(err, || {
+                // Force type errors here.
+                let map: &$crate::ConcurrentHandleMap<_> = &*$HANDLE_MAP_NAME;
+                map.delete_u64(v)
+            })
+        }
+    };
+}
+
+/// Force a compile error if the condition is not met. Requires a unique name
+/// for the assertion for... reasons. This is included mainly because it's a
+/// common desire for FFI code, but not for other sorts of code.
+///
+/// # Examples
+///
+/// Failing example:
+///
+/// ```compile_fail
+/// ffi_support::static_assert!(THIS_SHOULD_FAIL, false);
+/// ```
+///
+/// Passing example:
+///
+/// ```
+/// ffi_support::static_assert!(THIS_SHOULD_PASS, true);
+/// ```
+#[macro_export]
+macro_rules! static_assert {
+    ($ASSERT_NAME:ident, $test:expr) => {
+        #[allow(dead_code, nonstandard_style)]
+        const $ASSERT_NAME: [u8; 0 - (!$test as bool as usize)] =
+            [0u8; 0 - (!$test as bool as usize)];
+    };
+}
diff --git a/third_party/rust/ffi-support/src/string.rs b/third_party/rust/ffi-support/src/string.rs
new file mode 100644
index 0000000000..b1311669f5
--- /dev/null
+++ b/third_party/rust/ffi-support/src/string.rs
@@ -0,0 +1,162 @@
+/* Copyright 2018-2019 Mozilla Foundation
+ *
+ * Licensed under the Apache License (Version 2.0), or the MIT license,
+ * (the "Licenses") at your option. You may not use this file except in
+ * compliance with one of the Licenses. You may obtain copies of the
+ * Licenses at:
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *    http://opensource.org/licenses/MIT
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the Licenses is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the Licenses for the specific language governing permissions and
+ * limitations under the Licenses. */
+
+use crate::FfiStr;
+use std::ffi::CString;
+use std::os::raw::c_char;
+use std::ptr;
+
+/// Convert a rust string into a NUL-terminated utf-8 string suitable for passing to C, or to things
+/// ABI-compatible with C.
+///
+/// Important: This string must eventually be freed. You may either do that using the
+/// [`destroy_c_string`] method (or, if you must, by dropping the underlying [`std::ffi::CString`]
+/// after recovering it via [`std::ffi::CString::from_raw`]).
+///
+/// It's common to want to allow the consumer (e.g. on the "C" side of the FFI) to be allowed to
+/// free this memory, and the macro [`define_string_destructor!`] may be used to do so.
+///
+/// ## Panics
+///
+/// This function may panic if the argument has an interior null byte. This is fairly rare, but
+/// is possible in theory.
+#[inline]
+pub fn rust_string_to_c(rust_string: impl Into<String>) -> *mut c_char {
+    CString::new(rust_string.into())
+        .expect("Error: Rust string contained an interior null byte.")
+        .into_raw()
+}
+
+/// Variant of [`rust_string_to_c`] which takes an Option, and returns null for None.
+#[inline]
+pub fn opt_rust_string_to_c(opt_rust_string: Option<impl Into<String>>) -> *mut c_char {
+    if let Some(s) = opt_rust_string {
+        rust_string_to_c(s)
+    } else {
+        ptr::null_mut()
+    }
+}
+
+/// Free the memory of a string created by [`rust_string_to_c`] on the rust heap. If `c_string` is
+/// null, this is a no-op.
+///
+/// See the [`define_string_destructor!`] macro which may be used for exposing this function over
+/// the FFI.
+///
+/// ## Safety
+///
+/// This is inherently unsafe, since we're deallocating memory. Be sure
+///
+/// - Nobody can use the memory after it's deallocated.
+/// - The memory was actually allocated on this heap (and it's not a string from the other side of
+///   the FFI which was allocated on e.g. the C heap).
+///     - If multiple separate rust libraries are in use (for example, as DLLs) in a single program,
+///       you must also make sure that the rust library that allocated the memory is also the one
+///       that frees it.
+///
+/// See documentation for [`define_string_destructor!`], which gives a more complete overview of the
+/// potential issues.
+#[inline]
+pub unsafe fn destroy_c_string(cstring: *mut c_char) {
+    // we're not guaranteed to be in a place where we can complain about this beyond logging,
+    // and there's an obvious way to handle it.
+    if !cstring.is_null() {
+        drop(CString::from_raw(cstring))
+    }
+}
+
+/// Convert a null-terminated C string to a rust `str`. This does not take ownership of the string,
+/// and you should be careful about the lifetime of the resulting string. Note that strings
+/// containing invalid UTF-8 are replaced with the empty string (for many cases, you will want to
+/// use [`rust_string_from_c`] instead, which will do a lossy conversion).
+///
+/// If you actually need an owned rust `String`, you're encouraged to use [`rust_string_from_c`],
+/// which, as mentioned, also behaves better in the face of invalid UTF-8.
+///
+/// ## Safety
+///
+/// This is unsafe because we read from a raw pointer, which may or may not be valid.
+///
+/// We also assume `c_string` is a null terminated string, and have no way of knowing if that's
+/// actually true. If it's not, we'll read arbitrary memory from the heap until we see a '\0', which
+/// can result in a enormous number of problems.
+///
+/// ## Panics
+///
+/// Panics if it's argument is null, see [`opt_rust_str_from_c`] for a variant that returns None in
+/// this case instead.
+///
+/// Note: This means it's forbidden to call this outside of a `call_with_result` (or something else
+/// that uses [`std::panic::catch_unwind`]), as it is UB to panic across the FFI boundary.
+#[inline]
+#[deprecated(since = "0.3.0", note = "Please use FfiStr::as_str instead")]
+pub unsafe fn rust_str_from_c<'a>(c_string: *const c_char) -> &'a str {
+    FfiStr::from_raw(c_string).as_str()
+}
+
+/// Same as `rust_string_from_c`, but returns None if `c_string` is null instead of asserting.
+///
+/// ## Safety
+///
+/// This is unsafe because we read from a raw pointer, which may or may not be valid.
+///
+/// We also assume `c_string` is a null terminated string, and have no way of knowing if that's
+/// actually true. If it's not, we'll read arbitrary memory from the heap until we see a '\0', which
+/// can result in a enormous number of problems.
+#[inline]
+#[deprecated(since = "0.3.0", note = "Please use FfiStr::as_opt_str instead")]
+pub unsafe fn opt_rust_str_from_c<'a>(c_string: *const c_char) -> Option<&'a str> {
+    FfiStr::from_raw(c_string).as_opt_str()
+}
+
+/// Convert a null-terminated C into an owned rust string, replacing invalid UTF-8 with the
+/// unicode replacement character.
+///
+/// ## Safety
+///
+/// This is unsafe because we dereference a raw pointer, which may or may not be valid.
+///
+/// We also assume `c_string` is a null terminated string, and have no way of knowing if that's
+/// actually true. If it's not, we'll read arbitrary memory from the heap until we see a '\0', which
+/// can result in a enormous number of problems.
+///
+/// ## Panics
+///
+/// Panics if it's argument is null. See also [`opt_rust_string_from_c`], which returns None
+/// instead.
+///
+/// Note: This means it's forbidden to call this outside of a `call_with_result` (or something else
+/// that uses `std::panic::catch_unwind`), as it is UB to panic across the FFI boundary.
+#[inline]
+#[deprecated(since = "0.3.0", note = "Please use FfiStr::into_string instead")]
+pub unsafe fn rust_string_from_c(c_string: *const c_char) -> String {
+    FfiStr::from_raw(c_string).into_string()
+}
+
+/// Same as `rust_string_from_c`, but returns None if `c_string` is null instead of asserting.
+///
+/// ## Safety
+///
+/// This is unsafe because we dereference a raw pointer, which may or may not be valid.
+///
+/// We also assume `c_string` is a null terminated string, and have no way of knowing if that's
+/// actually true. If it's not, we'll read arbitrary memory from the heap until we see a '\0', which
+/// can result in a enormous number of problems.
+#[inline]
+#[deprecated(since = "0.3.0", note = "Please use FfiStr::into_opt_string instead")]
+pub unsafe fn opt_rust_string_from_c(c_string: *const c_char) -> Option<String> {
+    FfiStr::from_raw(c_string).into_opt_string()
+}