7 files changed, 964 insertions, 0 deletions
diff --git a/intl/l10n/rust/fluent-ffi/Cargo.toml b/intl/l10n/rust/fluent-ffi/Cargo.toml
new file mode 100644
index 0000000000..dad9498efd
--- /dev/null
+++ b/intl/l10n/rust/fluent-ffi/Cargo.toml
@@ -0,0 +1,16 @@
+[package]
+name = "fluent-ffi"
+version = "0.1.0"
+authors = ["Zibi Braniecki <zibi@braniecki.net>"]
+edition = "2018"
+license = "MPL-2.0"
+
+[dependencies]
+fluent = { version = "0.16.0", features = ["fluent-pseudo"] }
+fluent-pseudo = "0.3.1"
+intl-memoizer = "0.5.1"
+unic-langid = "0.9"
+nsstring = { path = "../../../../xpcom/rust/nsstring" }
+cstr = "0.2"
+xpcom = { path = "../../../../xpcom/rust/xpcom" }
+thin-vec = { version = "0.2.1", features = ["gecko-ffi"] }
diff --git a/intl/l10n/rust/fluent-ffi/cbindgen.toml b/intl/l10n/rust/fluent-ffi/cbindgen.toml
new file mode 100644
index 0000000000..5384a81b0a
--- /dev/null
+++ b/intl/l10n/rust/fluent-ffi/cbindgen.toml
@@ -0,0 +1,24 @@
+header = """/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */"""
+autogen_warning = """/* DO NOT MODIFY THIS MANUALLY! This file was generated using cbindgen. See RunCbindgen.py */
+#ifndef mozilla_intl_l10n_FluentBindings_h
+#error "Don't include this file directly, instead include FluentBindings.h"
+#endif
+"""
+include_version = true
+braces = "SameLine"
+line_length = 100
+tab_width = 2
+language = "C++"
+namespaces = ["mozilla", "intl", "ffi"]
+
+[parse]
+parse_deps = true
+include = ["fluent", "fluent-bundle", "intl-memoizer"]
+
+[enum]
+derive_helper_methods = true
+
+[export.rename]
+"ThinVec" = "nsTArray"
diff --git a/intl/l10n/rust/fluent-ffi/src/builtins.rs b/intl/l10n/rust/fluent-ffi/src/builtins.rs
new file mode 100644
index 0000000000..c7ffe8c3ee
--- /dev/null
+++ b/intl/l10n/rust/fluent-ffi/src/builtins.rs
@@ -0,0 +1,389 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use crate::ffi;
+use fluent::types::{FluentNumberOptions, FluentType, FluentValue};
+use fluent::FluentArgs;
+use intl_memoizer::IntlLangMemoizer;
+use intl_memoizer::Memoizable;
+use nsstring::nsCString;
+use std::borrow::Cow;
+use std::ptr::NonNull;
+use unic_langid::LanguageIdentifier;
+
+pub struct NumberFormat {
+    raw: Option<NonNull<ffi::RawNumberFormatter>>,
+}
+
+/**
+ * According to http://userguide.icu-project.org/design, as long as we constrain
+ * ourselves to const APIs ICU is const-correct.
+ */
+unsafe impl Send for NumberFormat {}
+unsafe impl Sync for NumberFormat {}
+
+impl NumberFormat {
+    pub fn new(locale: LanguageIdentifier, options: &FluentNumberOptions) -> Self {
+        let loc: String = locale.to_string();
+        Self {
+            raw: unsafe {
+                NonNull::new(ffi::FluentBuiltInNumberFormatterCreate(
+                    &loc.into(),
+                    &options.into(),
+                ))
+            },
+        }
+    }
+
+    pub fn format(&self, input: f64) -> String {
+        if let Some(raw) = self.raw {
+            unsafe {
+                let mut byte_count = 0;
+                let mut capacity = 0;
+                let buffer = ffi::FluentBuiltInNumberFormatterFormat(
+                    raw.as_ptr(),
+                    input,
+                    &mut byte_count,
+                    &mut capacity,
+                );
+                if buffer.is_null() {
+                    return String::new();
+                }
+                String::from_raw_parts(buffer, byte_count, capacity)
+            }
+        } else {
+            String::new()
+        }
+    }
+}
+
+impl Drop for NumberFormat {
+    fn drop(&mut self) {
+        if let Some(raw) = self.raw {
+            unsafe { ffi::FluentBuiltInNumberFormatterDestroy(raw.as_ptr()) };
+        }
+    }
+}
+
+impl Memoizable for NumberFormat {
+    type Args = (FluentNumberOptions,);
+    type Error = &'static str;
+    fn construct(lang: LanguageIdentifier, args: Self::Args) -> Result<Self, Self::Error> {
+        Ok(NumberFormat::new(lang, &args.0))
+    }
+}
+
+#[repr(C)]
+#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq)]
+pub enum FluentDateTimeStyle {
+    Full,
+    Long,
+    Medium,
+    Short,
+    None,
+}
+
+impl Default for FluentDateTimeStyle {
+    fn default() -> Self {
+        Self::None
+    }
+}
+
+impl From<&str> for FluentDateTimeStyle {
+    fn from(input: &str) -> Self {
+        match input {
+            "full" => Self::Full,
+            "long" => Self::Long,
+            "medium" => Self::Medium,
+            "short" => Self::Short,
+            _ => Self::None,
+        }
+    }
+}
+
+#[repr(C)]
+#[derive(Debug, Clone, Hash, PartialEq, Eq)]
+pub enum FluentDateTimeHourCycle {
+    H24,
+    H23,
+    H12,
+    H11,
+    None,
+}
+
+impl Default for FluentDateTimeHourCycle {
+    fn default() -> Self {
+        Self::None
+    }
+}
+
+impl From<&str> for FluentDateTimeHourCycle {
+    fn from(input: &str) -> Self {
+        match input {
+            "h24" => Self::H24,
+            "h23" => Self::H23,
+            "h12" => Self::H12,
+            "h11" => Self::H11,
+            _ => Self::None,
+        }
+    }
+}
+
+#[repr(C)]
+#[derive(Debug, Clone, Hash, PartialEq, Eq)]
+pub enum FluentDateTimeTextComponent {
+    Long,
+    Short,
+    Narrow,
+    None,
+}
+
+impl Default for FluentDateTimeTextComponent {
+    fn default() -> Self {
+        Self::None
+    }
+}
+
+impl From<&str> for FluentDateTimeTextComponent {
+    fn from(input: &str) -> Self {
+        match input {
+            "long" => Self::Long,
+            "short" => Self::Short,
+            "narrow" => Self::Narrow,
+            _ => Self::None,
+        }
+    }
+}
+
+#[repr(C)]
+#[derive(Debug, Clone, Hash, PartialEq, Eq)]
+pub enum FluentDateTimeNumericComponent {
+    Numeric,
+    TwoDigit,
+    None,
+}
+
+impl Default for FluentDateTimeNumericComponent {
+    fn default() -> Self {
+        Self::None
+    }
+}
+
+impl From<&str> for FluentDateTimeNumericComponent {
+    fn from(input: &str) -> Self {
+        match input {
+            "numeric" => Self::Numeric,
+            "2-digit" => Self::TwoDigit,
+            _ => Self::None,
+        }
+    }
+}
+
+#[repr(C)]
+#[derive(Debug, Clone, Hash, PartialEq, Eq)]
+pub enum FluentDateTimeMonthComponent {
+    Numeric,
+    TwoDigit,
+    Long,
+    Short,
+    Narrow,
+    None,
+}
+
+impl Default for FluentDateTimeMonthComponent {
+    fn default() -> Self {
+        Self::None
+    }
+}
+
+impl From<&str> for FluentDateTimeMonthComponent {
+    fn from(input: &str) -> Self {
+        match input {
+            "numeric" => Self::Numeric,
+            "2-digit" => Self::TwoDigit,
+            "long" => Self::Long,
+            "short" => Self::Short,
+            "narrow" => Self::Narrow,
+            _ => Self::None,
+        }
+    }
+}
+
+#[repr(C)]
+#[derive(Debug, Clone, Hash, PartialEq, Eq)]
+pub enum FluentDateTimeTimeZoneNameComponent {
+    Long,
+    Short,
+    None,
+}
+
+impl Default for FluentDateTimeTimeZoneNameComponent {
+    fn default() -> Self {
+        Self::None
+    }
+}
+
+impl From<&str> for FluentDateTimeTimeZoneNameComponent {
+    fn from(input: &str) -> Self {
+        match input {
+            "long" => Self::Long,
+            "short" => Self::Short,
+            _ => Self::None,
+        }
+    }
+}
+
+#[repr(C)]
+#[derive(Default, Debug, Clone, Hash, PartialEq, Eq)]
+pub struct FluentDateTimeOptions {
+    pub date_style: FluentDateTimeStyle,
+    pub time_style: FluentDateTimeStyle,
+    pub hour_cycle: FluentDateTimeHourCycle,
+    pub weekday: FluentDateTimeTextComponent,
+    pub era: FluentDateTimeTextComponent,
+    pub year: FluentDateTimeNumericComponent,
+    pub month: FluentDateTimeMonthComponent,
+    pub day: FluentDateTimeNumericComponent,
+    pub hour: FluentDateTimeNumericComponent,
+    pub minute: FluentDateTimeNumericComponent,
+    pub second: FluentDateTimeNumericComponent,
+    pub time_zone_name: FluentDateTimeTimeZoneNameComponent,
+}
+
+impl FluentDateTimeOptions {
+    pub fn merge(&mut self, opts: &FluentArgs) {
+        for (key, value) in opts.iter() {
+            match (key, value) {
+                ("dateStyle", FluentValue::String(n)) => {
+                    self.date_style = n.as_ref().into();
+                }
+                ("timeStyle", FluentValue::String(n)) => {
+                    self.time_style = n.as_ref().into();
+                }
+                ("hourCycle", FluentValue::String(n)) => {
+                    self.hour_cycle = n.as_ref().into();
+                }
+                ("weekday", FluentValue::String(n)) => {
+                    self.weekday = n.as_ref().into();
+                }
+                ("era", FluentValue::String(n)) => {
+                    self.era = n.as_ref().into();
+                }
+                ("year", FluentValue::String(n)) => {
+                    self.year = n.as_ref().into();
+                }
+                ("month", FluentValue::String(n)) => {
+                    self.month = n.as_ref().into();
+                }
+                ("day", FluentValue::String(n)) => {
+                    self.day = n.as_ref().into();
+                }
+                ("hour", FluentValue::String(n)) => {
+                    self.hour = n.as_ref().into();
+                }
+                ("minute", FluentValue::String(n)) => {
+                    self.minute = n.as_ref().into();
+                }
+                ("second", FluentValue::String(n)) => {
+                    self.second = n.as_ref().into();
+                }
+                ("timeZoneName", FluentValue::String(n)) => {
+                    self.time_zone_name = n.as_ref().into();
+                }
+                _ => {}
+            }
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Clone)]
+pub struct FluentDateTime {
+    epoch: f64,
+    options: FluentDateTimeOptions,
+}
+
+impl FluentType for FluentDateTime {
+    fn duplicate(&self) -> Box<dyn FluentType + Send> {
+        Box::new(self.clone())
+    }
+    fn as_string(&self, intls: &IntlLangMemoizer) -> Cow<'static, str> {
+        let result = intls
+            .with_try_get::<DateTimeFormat, _, _>((self.options.clone(),), |dtf| {
+                dtf.format(self.epoch)
+            })
+            .expect("Failed to retrieve a DateTimeFormat instance.");
+        result.into()
+    }
+    fn as_string_threadsafe(
+        &self,
+        _: &intl_memoizer::concurrent::IntlLangMemoizer,
+    ) -> Cow<'static, str> {
+        unimplemented!()
+    }
+}
+
+impl std::fmt::Display for FluentDateTime {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "DATETIME: {}", self.epoch)
+    }
+}
+
+impl FluentDateTime {
+    pub fn new(epoch: f64, options: FluentDateTimeOptions) -> Self {
+        Self { epoch, options }
+    }
+}
+
+pub struct DateTimeFormat {
+    raw: Option<NonNull<ffi::RawDateTimeFormatter>>,
+}
+
+/**
+ * According to http://userguide.icu-project.org/design, as long as we constrain
+ * ourselves to const APIs ICU is const-correct.
+ */
+unsafe impl Send for DateTimeFormat {}
+unsafe impl Sync for DateTimeFormat {}
+
+impl DateTimeFormat {
+    pub fn new(locale: LanguageIdentifier, options: FluentDateTimeOptions) -> Self {
+        // ICU needs null-termination here, otherwise we could use nsCStr.
+        let loc: nsCString = locale.to_string().into();
+        Self {
+            raw: unsafe { NonNull::new(ffi::FluentBuiltInDateTimeFormatterCreate(&loc, options)) },
+        }
+    }
+
+    pub fn format(&self, input: f64) -> String {
+        if let Some(raw) = self.raw {
+            unsafe {
+                let mut byte_count = 0;
+                let buffer =
+                    ffi::FluentBuiltInDateTimeFormatterFormat(raw.as_ptr(), input, &mut byte_count);
+                if buffer.is_null() {
+                    return String::new();
+                }
+                String::from_raw_parts(buffer, byte_count as usize, byte_count as usize)
+            }
+        } else {
+            String::new()
+        }
+    }
+}
+
+impl Drop for DateTimeFormat {
+    fn drop(&mut self) {
+        if let Some(raw) = self.raw {
+            unsafe { ffi::FluentBuiltInDateTimeFormatterDestroy(raw.as_ptr()) };
+        }
+    }
+}
+
+impl Memoizable for DateTimeFormat {
+    type Args = (FluentDateTimeOptions,);
+    type Error = &'static str;
+    fn construct(lang: LanguageIdentifier, args: Self::Args) -> Result<Self, Self::Error> {
+        Ok(DateTimeFormat::new(lang, args.0))
+    }
+}
diff --git a/intl/l10n/rust/fluent-ffi/src/bundle.rs b/intl/l10n/rust/fluent-ffi/src/bundle.rs
new file mode 100644
index 0000000000..21bf0d52e9
--- /dev/null
+++ b/intl/l10n/rust/fluent-ffi/src/bundle.rs
@@ -0,0 +1,331 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use crate::builtins::{FluentDateTime, FluentDateTimeOptions, NumberFormat};
+use cstr::cstr;
+pub use fluent::{FluentArgs, FluentBundle, FluentError, FluentResource, FluentValue};
+use fluent_pseudo::transform_dom;
+pub use intl_memoizer::IntlLangMemoizer;
+use nsstring::{nsACString, nsCString};
+use std::borrow::Cow;
+use std::ffi::CStr;
+use std::mem;
+use std::rc::Rc;
+use thin_vec::ThinVec;
+use unic_langid::LanguageIdentifier;
+use xpcom::interfaces::nsIPrefBranch;
+
+pub type FluentBundleRc = FluentBundle<Rc<FluentResource>>;
+
+#[derive(Debug)]
+#[repr(C, u8)]
+pub enum FluentArgument<'s> {
+    Double_(f64),
+    String(&'s nsACString),
+}
+
+#[derive(Debug)]
+#[repr(C)]
+pub struct L10nArg<'s> {
+    pub id: &'s nsACString,
+    pub value: FluentArgument<'s>,
+}
+
+fn transform_accented(s: &str) -> Cow<str> {
+    transform_dom(s, false, true, true)
+}
+
+fn transform_bidi(s: &str) -> Cow<str> {
+    transform_dom(s, false, false, false)
+}
+
+fn format_numbers(num: &FluentValue, intls: &IntlLangMemoizer) -> Option<String> {
+    match num {
+        FluentValue::Number(n) => {
+            let result = intls
+                .with_try_get::<NumberFormat, _, _>((n.options.clone(),), |nf| nf.format(n.value))
+                .expect("Failed to retrieve a NumberFormat instance.");
+            Some(result)
+        }
+        _ => None,
+    }
+}
+
+fn get_string_pref(name: &CStr) -> Option<nsCString> {
+    let mut value = nsCString::new();
+    let prefs_service =
+        xpcom::get_service::<nsIPrefBranch>(cstr!("@mozilla.org/preferences-service;1"))?;
+    unsafe {
+        prefs_service
+            .GetCharPref(name.as_ptr(), &mut *value)
+            .to_result()
+            .ok()?;
+    }
+    Some(value)
+}
+
+fn get_bool_pref(name: &CStr) -> Option<bool> {
+    let mut value = false;
+    let prefs_service =
+        xpcom::get_service::<nsIPrefBranch>(cstr!("@mozilla.org/preferences-service;1"))?;
+    unsafe {
+        prefs_service
+            .GetBoolPref(name.as_ptr(), &mut value)
+            .to_result()
+            .ok()?;
+    }
+    Some(value)
+}
+
+pub fn adapt_bundle_for_gecko(bundle: &mut FluentBundleRc, pseudo_strategy: Option<&nsACString>) {
+    bundle.set_formatter(Some(format_numbers));
+
+    bundle
+        .add_function("PLATFORM", |_args, _named_args| {
+            if cfg!(target_os = "linux") {
+                "linux".into()
+            } else if cfg!(target_os = "windows") {
+                "windows".into()
+            } else if cfg!(target_os = "macos") {
+                "macos".into()
+            } else if cfg!(target_os = "android") {
+                "android".into()
+            } else {
+                "other".into()
+            }
+        })
+        .expect("Failed to add a function to the bundle.");
+    bundle
+        .add_function("NUMBER", |args, named| {
+            if let Some(FluentValue::Number(n)) = args.get(0) {
+                let mut num = n.clone();
+                num.options.merge(named);
+                FluentValue::Number(num)
+            } else {
+                FluentValue::None
+            }
+        })
+        .expect("Failed to add a function to the bundle.");
+    bundle
+        .add_function("DATETIME", |args, named| {
+            if let Some(FluentValue::Number(n)) = args.get(0) {
+                let mut options = FluentDateTimeOptions::default();
+                options.merge(&named);
+                FluentValue::Custom(Box::new(FluentDateTime::new(n.value, options)))
+            } else {
+                FluentValue::None
+            }
+        })
+        .expect("Failed to add a function to the bundle.");
+
+    enum PseudoStrategy {
+        Accented,
+        Bidi,
+        None,
+    }
+    // This is quirky because we can't coerce Option<&nsACString> and Option<nsCString>
+    // into bytes easily without allocating.
+    let strategy_kind = match pseudo_strategy.map(|s| &s[..]) {
+        Some(b"accented") => PseudoStrategy::Accented,
+        Some(b"bidi") => PseudoStrategy::Bidi,
+        _ => {
+            if let Some(pseudo_strategy) = get_string_pref(cstr!("intl.l10n.pseudo")) {
+                match &pseudo_strategy[..] {
+                    b"accented" => PseudoStrategy::Accented,
+                    b"bidi" => PseudoStrategy::Bidi,
+                    _ => PseudoStrategy::None,
+                }
+            } else {
+                PseudoStrategy::None
+            }
+        }
+    };
+    match strategy_kind {
+        PseudoStrategy::Accented => bundle.set_transform(Some(transform_accented)),
+        PseudoStrategy::Bidi => bundle.set_transform(Some(transform_bidi)),
+        PseudoStrategy::None => bundle.set_transform(None),
+    }
+
+    // Temporarily disable bidi isolation due to Microsoft not supporting FSI/PDI.
+    // See bug 1439018 for details.
+    let default_use_isolating = false;
+    let use_isolating =
+        get_bool_pref(cstr!("intl.l10n.enable-bidi-marks")).unwrap_or(default_use_isolating);
+    bundle.set_use_isolating(use_isolating);
+}
+
+#[no_mangle]
+pub extern "C" fn fluent_bundle_new_single(
+    locale: &nsACString,
+    use_isolating: bool,
+    pseudo_strategy: &nsACString,
+) -> *mut FluentBundleRc {
+    let id = match locale.to_utf8().parse::<LanguageIdentifier>() {
+        Ok(id) => id,
+        Err(..) => return std::ptr::null_mut(),
+    };
+
+    Box::into_raw(fluent_bundle_new_internal(
+        &[id],
+        use_isolating,
+        pseudo_strategy,
+    ))
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn fluent_bundle_new(
+    locales: *const nsCString,
+    locale_count: usize,
+    use_isolating: bool,
+    pseudo_strategy: &nsACString,
+) -> *mut FluentBundleRc {
+    let mut langids = Vec::with_capacity(locale_count);
+    let locales = std::slice::from_raw_parts(locales, locale_count);
+    for locale in locales {
+        let id = match locale.to_utf8().parse::<LanguageIdentifier>() {
+            Ok(id) => id,
+            Err(..) => return std::ptr::null_mut(),
+        };
+        langids.push(id);
+    }
+
+    Box::into_raw(fluent_bundle_new_internal(
+        &langids,
+        use_isolating,
+        pseudo_strategy,
+    ))
+}
+
+fn fluent_bundle_new_internal(
+    langids: &[LanguageIdentifier],
+    use_isolating: bool,
+    pseudo_strategy: &nsACString,
+) -> Box<FluentBundleRc> {
+    let mut bundle = FluentBundle::new(langids.to_vec());
+    bundle.set_use_isolating(use_isolating);
+
+    bundle.set_formatter(Some(format_numbers));
+
+    adapt_bundle_for_gecko(&mut bundle, Some(pseudo_strategy));
+
+    Box::new(bundle)
+}
+
+#[no_mangle]
+pub extern "C" fn fluent_bundle_get_locales(
+    bundle: &FluentBundleRc,
+    result: &mut ThinVec<nsCString>,
+) {
+    for locale in &bundle.locales {
+        result.push(locale.to_string().as_str().into());
+    }
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn fluent_bundle_destroy(bundle: *mut FluentBundleRc) {
+    let _ = Box::from_raw(bundle);
+}
+
+#[no_mangle]
+pub extern "C" fn fluent_bundle_has_message(bundle: &FluentBundleRc, id: &nsACString) -> bool {
+    bundle.has_message(id.to_string().as_str())
+}
+
+#[no_mangle]
+pub extern "C" fn fluent_bundle_get_message(
+    bundle: &FluentBundleRc,
+    id: &nsACString,
+    has_value: &mut bool,
+    attrs: &mut ThinVec<nsCString>,
+) -> bool {
+    match bundle.get_message(&id.to_utf8()) {
+        Some(message) => {
+            attrs.reserve(message.attributes().count());
+            *has_value = message.value().is_some();
+            for attr in message.attributes() {
+                attrs.push(attr.id().into());
+            }
+            true
+        }
+        None => {
+            *has_value = false;
+            false
+        }
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn fluent_bundle_format_pattern(
+    bundle: &FluentBundleRc,
+    id: &nsACString,
+    attr: &nsACString,
+    args: &ThinVec<L10nArg>,
+    ret_val: &mut nsACString,
+    ret_errors: &mut ThinVec<nsCString>,
+) -> bool {
+    let args = convert_args(&args);
+
+    let message = match bundle.get_message(&id.to_utf8()) {
+        Some(message) => message,
+        None => return false,
+    };
+
+    let pattern = if !attr.is_empty() {
+        match message.get_attribute(&attr.to_utf8()) {
+            Some(attr) => attr.value(),
+            None => return false,
+        }
+    } else {
+        match message.value() {
+            Some(value) => value,
+            None => return false,
+        }
+    };
+
+    let mut errors = vec![];
+    bundle
+        .write_pattern(ret_val, pattern, args.as_ref(), &mut errors)
+        .expect("Failed to write to a nsCString.");
+    append_fluent_errors_to_ret_errors(ret_errors, &errors);
+    true
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn fluent_bundle_add_resource(
+    bundle: &mut FluentBundleRc,
+    r: *const FluentResource,
+    allow_overrides: bool,
+    ret_errors: &mut ThinVec<nsCString>,
+) {
+    // we don't own the resource
+    let r = mem::ManuallyDrop::new(Rc::from_raw(r));
+
+    if allow_overrides {
+        bundle.add_resource_overriding(Rc::clone(&r));
+    } else if let Err(errors) = bundle.add_resource(Rc::clone(&r)) {
+        append_fluent_errors_to_ret_errors(ret_errors, &errors);
+    }
+}
+
+pub fn convert_args<'s>(args: &[L10nArg<'s>]) -> Option<FluentArgs<'s>> {
+    if args.is_empty() {
+        return None;
+    }
+
+    let mut result = FluentArgs::with_capacity(args.len());
+    for arg in args {
+        let val = match arg.value {
+            FluentArgument::Double_(d) => FluentValue::from(d),
+            FluentArgument::String(s) => FluentValue::from(s.to_utf8()),
+        };
+        result.set(arg.id.to_string(), val);
+    }
+    Some(result)
+}
+
+fn append_fluent_errors_to_ret_errors(ret_errors: &mut ThinVec<nsCString>, errors: &[FluentError]) {
+    for error in errors {
+        ret_errors.push(error.to_string().into());
+    }
+}
diff --git a/intl/l10n/rust/fluent-ffi/src/ffi.rs b/intl/l10n/rust/fluent-ffi/src/ffi.rs
new file mode 100644
index 0000000000..a264ad11b7
--- /dev/null
+++ b/intl/l10n/rust/fluent-ffi/src/ffi.rs
@@ -0,0 +1,154 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use crate::builtins::FluentDateTimeOptions;
+use fluent::types::FluentNumberCurrencyDisplayStyle;
+use fluent::types::FluentNumberOptions;
+use fluent::types::FluentNumberStyle;
+use nsstring::nsCString;
+
+pub enum RawNumberFormatter {}
+
+#[repr(C)]
+pub enum FluentNumberStyleRaw {
+    Decimal,
+    Currency,
+    Percent,
+}
+
+impl From<FluentNumberStyle> for FluentNumberStyleRaw {
+    fn from(input: FluentNumberStyle) -> Self {
+        match input {
+            FluentNumberStyle::Decimal => Self::Decimal,
+            FluentNumberStyle::Currency => Self::Currency,
+            FluentNumberStyle::Percent => Self::Percent,
+        }
+    }
+}
+
+#[repr(C)]
+#[derive(Clone, Copy)]
+pub enum FluentNumberCurrencyDisplayStyleRaw {
+    Symbol,
+    Code,
+    Name,
+}
+
+impl From<FluentNumberCurrencyDisplayStyle> for FluentNumberCurrencyDisplayStyleRaw {
+    fn from(input: FluentNumberCurrencyDisplayStyle) -> Self {
+        match input {
+            FluentNumberCurrencyDisplayStyle::Symbol => Self::Symbol,
+            FluentNumberCurrencyDisplayStyle::Code => Self::Code,
+            FluentNumberCurrencyDisplayStyle::Name => Self::Name,
+        }
+    }
+}
+
+#[repr(C)]
+pub struct FluentNumberOptionsRaw {
+    pub style: FluentNumberStyleRaw,
+    pub currency: nsCString,
+    pub currency_display: FluentNumberCurrencyDisplayStyleRaw,
+    pub use_grouping: bool,
+    pub minimum_integer_digits: usize,
+    pub minimum_fraction_digits: usize,
+    pub maximum_fraction_digits: usize,
+    pub minimum_significant_digits: isize,
+    pub maximum_significant_digits: isize,
+}
+
+fn get_number_option(val: Option<usize>, min: usize, max: usize, default: usize) -> usize {
+    if let Some(val) = val {
+        if val >= min && val <= max {
+            val
+        } else {
+            default
+        }
+    } else {
+        default
+    }
+}
+
+impl From<&FluentNumberOptions> for FluentNumberOptionsRaw {
+    fn from(input: &FluentNumberOptions) -> Self {
+        let currency: nsCString = if let Some(ref currency) = input.currency {
+            currency.into()
+        } else {
+            nsCString::new()
+        };
+
+        //XXX: This should be fetched from currency table.
+        let currency_digits = 2;
+
+        // Keep it aligned with ECMA402 NumberFormat logic.
+        let minfd_default = if input.style == FluentNumberStyle::Currency {
+            currency_digits
+        } else {
+            0
+        };
+        let maxfd_default = match input.style {
+            FluentNumberStyle::Decimal => 3,
+            FluentNumberStyle::Currency => currency_digits,
+            FluentNumberStyle::Percent => 0,
+        };
+        let minid = get_number_option(input.minimum_integer_digits, 1, 21, 1);
+        let minfd = get_number_option(input.minimum_fraction_digits, 0, 20, minfd_default);
+        let maxfd_actual_default = std::cmp::max(minfd, maxfd_default);
+        let maxfd = get_number_option(
+            input.maximum_fraction_digits,
+            minfd,
+            20,
+            maxfd_actual_default,
+        );
+
+        let (minsd, maxsd) = if input.minimum_significant_digits.is_some()
+            || input.maximum_significant_digits.is_some()
+        {
+            let minsd = get_number_option(input.minimum_significant_digits, 1, 21, 1);
+            let maxsd = get_number_option(input.maximum_significant_digits, minsd, 21, 21);
+            (minsd as isize, maxsd as isize)
+        } else {
+            (-1, -1)
+        };
+
+        Self {
+            style: input.style.into(),
+            currency,
+            currency_display: input.currency_display.into(),
+            use_grouping: input.use_grouping,
+            minimum_integer_digits: minid,
+            minimum_fraction_digits: minfd,
+            maximum_fraction_digits: maxfd,
+            minimum_significant_digits: minsd,
+            maximum_significant_digits: maxsd,
+        }
+    }
+}
+
+pub enum RawDateTimeFormatter {}
+
+extern "C" {
+    pub fn FluentBuiltInNumberFormatterCreate(
+        locale: &nsCString,
+        options: &FluentNumberOptionsRaw,
+    ) -> *mut RawNumberFormatter;
+    pub fn FluentBuiltInNumberFormatterFormat(
+        formatter: *const RawNumberFormatter,
+        input: f64,
+        out_count: &mut usize,
+        out_capacity: &mut usize,
+    ) -> *mut u8;
+    pub fn FluentBuiltInNumberFormatterDestroy(formatter: *mut RawNumberFormatter);
+
+    pub fn FluentBuiltInDateTimeFormatterCreate(
+        locale: &nsCString,
+        options: FluentDateTimeOptions,
+    ) -> *mut RawDateTimeFormatter;
+    pub fn FluentBuiltInDateTimeFormatterFormat(
+        formatter: *const RawDateTimeFormatter,
+        input: f64,
+        out_count: &mut u32,
+    ) -> *mut u8;
+    pub fn FluentBuiltInDateTimeFormatterDestroy(formatter: *mut RawDateTimeFormatter);
+}
diff --git a/intl/l10n/rust/fluent-ffi/src/lib.rs b/intl/l10n/rust/fluent-ffi/src/lib.rs
new file mode 100644
index 0000000000..6cf56c2e4a
--- /dev/null
+++ b/intl/l10n/rust/fluent-ffi/src/lib.rs
@@ -0,0 +1,11 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+mod builtins;
+mod bundle;
+mod ffi;
+mod resource;
+
+pub use bundle::*;
+pub use resource::*;
diff --git a/intl/l10n/rust/fluent-ffi/src/resource.rs b/intl/l10n/rust/fluent-ffi/src/resource.rs
new file mode 100644
index 0000000000..dc011b9462
--- /dev/null
+++ b/intl/l10n/rust/fluent-ffi/src/resource.rs
@@ -0,0 +1,39 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+pub use fluent::FluentResource;
+use nsstring::nsACString;
+use std::{
+    mem::{self, ManuallyDrop},
+    rc::Rc,
+};
+
+#[no_mangle]
+pub extern "C" fn fluent_resource_new(
+    name: &nsACString,
+    has_errors: &mut bool,
+) -> *const FluentResource {
+    let res = match FluentResource::try_new(name.to_string()) {
+        Ok(res) => {
+            *has_errors = false;
+            res
+        }
+        Err((res, _)) => {
+            *has_errors = true;
+            res
+        }
+    };
+    Rc::into_raw(Rc::new(res))
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn fluent_resource_addref(res: *const FluentResource) {
+    let raw = ManuallyDrop::new(Rc::from_raw(res));
+    mem::forget(Rc::clone(&raw));
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn fluent_resource_release(res: *const FluentResource) {
+    let _ = Rc::from_raw(res);
+}