Issue #1969 - Implement Intl.RelativeTimeFormat.

Based on Mozilla bugs 1270140, 1504656, 1483545, and 1504334.
Took note of Mozilla bug 1379222 which changed GetPrototypeFromCallableConstructor
 to GetPrototypeFromBuiltinConstructor.
There are many other changes I did myself since the initial implementation by
 Mozilla wouldn't work with this codebase.

13 files changed, 669 insertions, 20 deletions

diff --git a/config/check_spidermonkey_style.py b/config/check_spidermonkey_style.py
index 4667a1a2ff..eb272a81c6 100644
--- a/config/check_spidermonkey_style.py
+++ b/config/check_spidermonkey_style.py
@@ -86,12 +86,14 @@ included_inclnames_to_ignore = set([
     'unicode/ucol.h',           # ICU
     'unicode/udat.h',           # ICU
     'unicode/udatpg.h',         # ICU
+    'unicode/udisplaycontext.h',# ICU
     'unicode/uenum.h',          # ICU
     'unicode/uniset.h',         # ICU
     'unicode/unorm.h',          # ICU
     'unicode/unum.h',           # ICU
     'unicode/unumsys.h',        # ICU
     'unicode/upluralrules.h',   # ICU
+    'unicode/ureldatefmt.h',    # ICU
     'unicode/ustring.h',        # ICU
     'unicode/utypes.h',         # ICU
     'vtune/VTuneWrapper.h'      # VTune
diff --git a/config/external/icu/defs.mozbuild b/config/external/icu/defs.mozbuild
index a6249783f4..165564cf32 100644
--- a/config/external/icu/defs.mozbuild
+++ b/config/external/icu/defs.mozbuild
@@ -14,7 +14,6 @@ DEFINES.update(
     UCONFIG_NO_LEGACY_CONVERSION = True,
     UCONFIG_NO_TRANSLITERATION = True,
     UCONFIG_NO_REGULAR_EXPRESSIONS = True,
-    UCONFIG_NO_BREAK_ITERATION = True,
 
     # We don't need to pass data to and from legacy char* APIs.
     U_CHARSET_IS_UTF8 = True,
diff --git a/intl/icu-patches/bug-1504656-relativetimeformat-plural-other-fallback.diff b/intl/icu-patches/bug-1504656-relativetimeformat-plural-other-fallback.diff
new file mode 100644
index 0000000000..060ea5c0bf
--- /dev/null
+++ b/intl/icu-patches/bug-1504656-relativetimeformat-plural-other-fallback.diff
@@ -0,0 +1,45 @@
+Workaround for https://unicode-org.atlassian.net/browse/ICU-20253
+
+https://bugzilla.mozilla.org/show_bug.cgi?id=1504656
+
+diff --git a/intl/icu/source/i18n/reldatefmt.cpp b/intl/icu/source/i18n/reldatefmt.cpp
+--- a/intl/icu/source/i18n/reldatefmt.cpp
++++ b/intl/icu/source/i18n/reldatefmt.cpp
+@@ -157,24 +157,30 @@ const UnicodeString& RelativeDateTimeCac
+  }
+ 
+  // Use fallback cache for SimpleFormatter relativeUnits.
+  const SimpleFormatter* RelativeDateTimeCacheData::getRelativeDateTimeUnitFormatter(
+         int32_t fStyle,
+         URelativeDateTimeUnit unit,
+         int32_t pastFutureIndex,
+         int32_t pluralUnit) const {
+-    int32_t style = fStyle;
+-    do {
+-        if (relativeUnitsFormatters[style][unit][pastFutureIndex][pluralUnit] != nullptr) {
+-            return relativeUnitsFormatters[style][unit][pastFutureIndex][pluralUnit];
++    while (true) {
++        int32_t style = fStyle;
++        do {
++            if (relativeUnitsFormatters[style][unit][pastFutureIndex][pluralUnit] != nullptr) {
++                return relativeUnitsFormatters[style][unit][pastFutureIndex][pluralUnit];
++            }
++            style = fallBackCache[style];
++        } while (style != -1);
++
++        if (pluralUnit == StandardPlural::OTHER) {
++            return nullptr;  // No formatter found.
+         }
+-        style = fallBackCache[style];
+-    } while (style != -1);
+-    return nullptr;  // No formatter found.
++        pluralUnit = StandardPlural::OTHER;
++    }
+  }
+ 
+ static UBool getStringWithFallback(
+         const UResourceBundle *resource,
+         const char *key,
+         UnicodeString &result,
+         UErrorCode &status) {
+     int32_t len = 0;
diff --git a/intl/icu/source/i18n/reldatefmt.cpp b/intl/icu/source/i18n/reldatefmt.cpp
index 1a4da0f0b9..85637cb456 100644
--- a/intl/icu/source/i18n/reldatefmt.cpp
+++ b/intl/icu/source/i18n/reldatefmt.cpp
@@ -162,14 +162,20 @@ const UnicodeString& RelativeDateTimeCacheData::getAbsoluteUnitString(
         URelativeDateTimeUnit unit,
         int32_t pastFutureIndex,
         int32_t pluralUnit) const {
-    int32_t style = fStyle;
-    do {
-        if (relativeUnitsFormatters[style][unit][pastFutureIndex][pluralUnit] != nullptr) {
-            return relativeUnitsFormatters[style][unit][pastFutureIndex][pluralUnit];
+    while (true) {
+        int32_t style = fStyle;
+        do {
+            if (relativeUnitsFormatters[style][unit][pastFutureIndex][pluralUnit] != nullptr) {
+                return relativeUnitsFormatters[style][unit][pastFutureIndex][pluralUnit];
+            }
+            style = fallBackCache[style];
+        } while (style != -1);
+
+        if (pluralUnit == StandardPlural::OTHER) {
+            return nullptr;  // No formatter found.
         }
-        style = fallBackCache[style];
-    } while (style != -1);
-    return nullptr;  // No formatter found.
+        pluralUnit = StandardPlural::OTHER;
+    }
  }
 
 static UBool getStringWithFallback(
diff --git a/intl/patch-icu.sh b/intl/patch-icu.sh
index 4c8cd89cba..a8e9977ea5 100644..100755
--- a/intl/patch-icu.sh
+++ b/intl/patch-icu.sh
@@ -21,7 +21,9 @@ for patch in \
  suppress-warnings.diff \
  bug-1172609-timezone-recreateDefault.diff \
  bug-1198952-workaround-make-3.82-bug.diff \
+ bug-1504656-relativetimeformat-plural-other-fallback.diff \
 ; do
   echo "Applying local patch $patch"
   patch -d ${icu_dir}/../../ -p1 --no-backup-if-mismatch < ${icu_dir}/../icu-patches/$patch
 done
+
diff --git a/intl/update-icu.sh b/intl/update-icu.sh
index 7e4c833c7b..ae22076ab3 100755
--- a/intl/update-icu.sh
+++ b/intl/update-icu.sh
@@ -7,11 +7,11 @@ set -e
 
 # Update to an ICU release:
 #   Usage: update-icu.sh <URL of ICU GIT> <release tag name>
-#   E.g., for ICU 63.2: update-icu.sh https://github.com/unicode-org/icu.git release-63-2
+#   E.g., for ICU 62.2: update-icu.sh https://github.com/unicode-org/icu.git release-62-2
 #
 # Update to an ICU maintenance branch:
 #   Usage: update-icu.sh <URL of ICU GIT> <maintenance name>
-#   E.g., for ICU 63.2: update-icu.sh https://github.com/unicode-org/icu.git maint/maint-63
+#   E.g., for ICU 62.2: update-icu.sh https://github.com/unicode-org/icu.git maint/maint-62
 
 if [ $# -lt 2 ]; then
   echo "Usage: update-icu.sh <URL of ICU GIT> <release tag name>"
@@ -86,6 +86,7 @@ for patch in \
  suppress-warnings.diff \
  bug-1172609-timezone-recreateDefault.diff \
  bug-1198952-workaround-make-3.82-bug.diff \
+ bug-1504656-relativetimeformat-plural-other-fallback.diff \
 ; do
   echo "Applying local patch $patch"
   patch -d ${icu_dir}/../../ -p1 --no-backup-if-mismatch < ${icu_dir}/../icu-patches/$patch
diff --git a/js/public/Class.h b/js/public/Class.h
index 634e5a281d..d7e2ab40db 100644
--- a/js/public/Class.h
+++ b/js/public/Class.h
@@ -913,7 +913,7 @@ struct JSClass {
 // application.
 #define JSCLASS_GLOBAL_APPLICATION_SLOTS 5
 #define JSCLASS_GLOBAL_SLOT_COUNT                                             \
-    (JSCLASS_GLOBAL_APPLICATION_SLOTS + JSProto_LIMIT * 2 + 46)
+    (JSCLASS_GLOBAL_APPLICATION_SLOTS + JSProto_LIMIT * 2 + 47)
 #define JSCLASS_GLOBAL_FLAGS_WITH_SLOTS(n)                                    \
     (JSCLASS_IS_GLOBAL | JSCLASS_HAS_RESERVED_SLOTS(JSCLASS_GLOBAL_SLOT_COUNT + (n)))
 #define JSCLASS_GLOBAL_FLAGS                                                  \
diff --git a/js/src/builtin/Intl.cpp b/js/src/builtin/Intl.cpp
index 5455b3a854..494a0c0473 100644
--- a/js/src/builtin/Intl.cpp
+++ b/js/src/builtin/Intl.cpp
@@ -11,6 +11,7 @@
 #include "builtin/Intl.h"
 
 #include "mozilla/Casting.h"
+#include "mozilla/FloatingPoint.h"
 #include "mozilla/PodOperations.h"
 #include "mozilla/Range.h"
 
@@ -28,10 +29,12 @@
 #include "unicode/ucol.h"
 #include "unicode/udat.h"
 #include "unicode/udatpg.h"
+#include "unicode/udisplaycontext.h"
 #include "unicode/uenum.h"
 #include "unicode/unum.h"
 #include "unicode/unumsys.h"
 #include "unicode/upluralrules.h"
+#include "unicode/ureldatefmt.h"
 #include "unicode/ustring.h"
 #include "vm/DateTime.h"
 #include "vm/GlobalObject.h"
@@ -3205,6 +3208,303 @@ js::intl_GetPluralCategories(JSContext* cx, unsigned argc, Value* vp)
     return true;
 }
 
+/**************** RelativeTimeFormat *****************/
+
+static void relativeTimeFormat_finalize(FreeOp* fop, JSObject* obj);
+
+static const uint32_t URELATIVE_TIME_FORMAT_SLOT = 0;
+static const uint32_t RELATIVE_TIME_FORMAT_SLOTS_COUNT = 1;
+
+static const ClassOps RelativeTimeFormatClassOps = {
+    nullptr, /* addProperty */
+    nullptr, /* delProperty */
+    nullptr, /* getProperty */
+    nullptr, /* enumerate */
+    nullptr, /* newEnumerate */
+    nullptr, /* resolve */
+    nullptr, /* mayResolve */
+    relativeTimeFormat_finalize
+};
+
+static const Class RelativeTimeFormatClass = {
+    js_Object_str,
+    JSCLASS_HAS_RESERVED_SLOTS(RELATIVE_TIME_FORMAT_SLOTS_COUNT) |
+    JSCLASS_FOREGROUND_FINALIZE,
+    &RelativeTimeFormatClassOps
+};
+
+#if JS_HAS_TOSOURCE
+static bool
+relativeTimeFormat_toSource(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setString(cx->names().RelativeTimeFormat);
+    return true;
+}
+#endif
+
+static const JSFunctionSpec relativeTimeFormat_static_methods[] = {
+    JS_SELF_HOSTED_FN("supportedLocalesOf", "Intl_RelativeTimeFormat_supportedLocalesOf", 1, 0),
+    JS_FS_END
+};
+
+static const JSFunctionSpec relativeTimeFormat_methods[] = {
+    JS_SELF_HOSTED_FN("resolvedOptions", "Intl_RelativeTimeFormat_resolvedOptions", 0, 0),
+    JS_SELF_HOSTED_FN("format", "Intl_RelativeTimeFormat_format", 2, 0),
+#if JS_HAS_TOSOURCE
+    JS_FN(js_toSource_str, relativeTimeFormat_toSource, 0, 0),
+#endif
+    JS_FS_END
+};
+
+static const JSPropertySpec relativeTimeFormat_properties[] = {
+    JS_STRING_SYM_PS(toStringTag, "Intl.RelativeTimeFormat", JSPROP_READONLY),
+    JS_PS_END};
+
+/**
+ * RelativeTimeFormat constructor.
+ * Spec: ECMAScript 402 API, RelativeTimeFormat, 1.1
+ */
+static bool
+RelativeTimeFormat(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Step 1.
+    if (!ThrowIfNotConstructing(cx, args, "Intl.RelativeTimeFormat"))
+        return false;
+
+    // Step 2 (Inlined 9.1.14, OrdinaryCreateFromConstructor).
+    RootedObject proto(cx);
+    if (args.isConstructing() && !GetPrototypeFromCallableConstructor(cx, args, &proto))
+        return false;
+
+    if (!proto) {
+        proto = GlobalObject::getOrCreateRelativeTimeFormatPrototype(cx, cx->global());
+        if (!proto)
+            return false;
+    }
+
+    RootedObject relativeTimeFormat(cx);
+    relativeTimeFormat = NewObjectWithGivenProto(cx, &RelativeTimeFormatClass, proto);
+    if (!relativeTimeFormat)
+        return false;
+
+    relativeTimeFormat->as<NativeObject>().setReservedSlot(URELATIVE_TIME_FORMAT_SLOT, PrivateValue(nullptr));
+
+    RootedValue locales(cx, args.get(0));
+    RootedValue options(cx, args.get(1));
+
+    // Step 3.
+    if (!IntlInitialize(cx, relativeTimeFormat, cx->names().InitializeRelativeTimeFormat, locales, options))
+        return false;
+
+    args.rval().setObject(*relativeTimeFormat);
+    return true;
+}
+
+static void
+relativeTimeFormat_finalize(FreeOp* fop, JSObject* obj)
+{
+    MOZ_ASSERT(fop->onMainThread());
+
+    // This is-undefined check shouldn't be necessary, but for internal
+    // brokenness in object allocation code.  For the moment, hack around it by
+    // explicitly guarding against the possibility of the reserved slot not
+    // containing a private.  See bug 949220.
+    const Value& slot = obj->as<NativeObject>().getReservedSlot(URELATIVE_TIME_FORMAT_SLOT);
+    if (!slot.isUndefined()) {
+        if (URelativeDateTimeFormatter* rtf = static_cast<URelativeDateTimeFormatter*>(slot.toPrivate()))
+            ureldatefmt_close(rtf);
+    }
+}
+
+static JSObject*
+CreateRelativeTimeFormatPrototype(JSContext* cx, HandleObject Intl, Handle<GlobalObject*> global)
+{
+    RootedFunction ctor(cx);
+    ctor = global->createConstructor(cx, &RelativeTimeFormat, cx->names().RelativeTimeFormat, 0);
+    if (!ctor)
+        return nullptr;
+
+    RootedNativeObject proto(cx, GlobalObject::createBlankPrototype(cx, global, &RelativeTimeFormatClass));
+    if (!proto)
+        return nullptr;
+    proto->setReservedSlot(URELATIVE_TIME_FORMAT_SLOT, PrivateValue(nullptr));
+
+    if (!LinkConstructorAndPrototype(cx, ctor, proto))
+        return nullptr;
+
+    if (!JS_DefineFunctions(cx, ctor, relativeTimeFormat_static_methods))
+        return nullptr;
+
+    if (!JS_DefineFunctions(cx, proto, relativeTimeFormat_methods))
+        return nullptr;
+
+    if (!JS_DefineProperties(cx, proto, relativeTimeFormat_properties))
+        return nullptr;
+
+    RootedValue options(cx);
+    if (!CreateDefaultOptions(cx, &options))
+        return nullptr;
+
+    if (!IntlInitialize(cx, proto, cx->names().InitializeRelativeTimeFormat, UndefinedHandleValue,
+                        options))
+    {
+        return nullptr;
+    }
+
+    RootedValue ctorValue(cx, ObjectValue(*ctor));
+    if (!DefineProperty(cx, Intl, cx->names().RelativeTimeFormat, ctorValue, nullptr, nullptr, 0)) {
+        return nullptr;
+    }
+
+    return proto;
+}
+
+bool
+js::intl_RelativeTimeFormat_availableLocales(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    MOZ_ASSERT(args.length() == 0);
+
+    RootedValue result(cx);
+    // We're going to use ULocale availableLocales as per ICU recommendation:
+    // https://ssl.icu-project.org/trac/ticket/12756
+    if (!intl_availableLocales(cx, uloc_countAvailable, uloc_getAvailable, &result))
+        return false;
+    args.rval().set(result);
+    return true;
+}
+
+
+enum class RelativeTimeNumeric
+{
+    /**
+     * Only strings with numeric components like `1 day ago`.
+     */
+    Always,
+    /**
+     * Natural-language strings like `yesterday` when possible,
+     * otherwise strings with numeric components as in `7 months ago`.
+     */
+    Auto,
+};
+
+bool
+js::intl_FormatRelativeTime(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    MOZ_ASSERT(args.length() == 3);
+
+    RootedObject relativeTimeFormat(cx, &args[0].toObject());
+
+    RootedObject internals(cx, GetInternals(cx, relativeTimeFormat));
+    if (!internals)
+        return false;
+
+    RootedValue value(cx);
+
+    if (!GetProperty(cx, internals, internals, cx->names().locale, &value))
+        return false;
+    JSAutoByteString locale(cx, value.toString());
+    if (!locale)
+        return false;
+
+    if (!GetProperty(cx, internals, internals, cx->names().style, &value))
+        return false;
+    RootedLinearString style(cx, value.toString()->ensureLinear(cx));
+    if (!style)
+        return false;
+
+    double t = args[1].toNumber();
+
+    UDateRelativeDateTimeFormatterStyle relDateTimeStyle;
+
+    if (StringEqualsAscii(style, "short")) {
+        relDateTimeStyle = UDAT_STYLE_SHORT;
+    } else if (StringEqualsAscii(style, "narrow")) {
+        relDateTimeStyle = UDAT_STYLE_NARROW;
+    } else {
+        MOZ_ASSERT(StringEqualsAscii(style, "long"));
+        relDateTimeStyle = UDAT_STYLE_LONG;
+    }
+
+    URelativeDateTimeUnit relDateTimeUnit;
+    {
+        JSLinearString* unit = args[2].toString()->ensureLinear(cx);
+        if (!unit) {
+            return false;
+        }
+
+        if (StringEqualsAscii(unit, "second") || StringEqualsAscii(unit, "seconds")) {
+            relDateTimeUnit = UDAT_REL_UNIT_SECOND;
+        } else if (StringEqualsAscii(unit, "minute") || StringEqualsAscii(unit, "minutes")) {
+            relDateTimeUnit = UDAT_REL_UNIT_MINUTE;
+        } else if (StringEqualsAscii(unit, "hour") || StringEqualsAscii(unit, "hours")) {
+            relDateTimeUnit = UDAT_REL_UNIT_HOUR;
+        } else if (StringEqualsAscii(unit, "day") || StringEqualsAscii(unit, "days")) {
+            relDateTimeUnit = UDAT_REL_UNIT_DAY;
+        } else if (StringEqualsAscii(unit, "week") || StringEqualsAscii(unit, "weeks")) {
+            relDateTimeUnit = UDAT_REL_UNIT_WEEK;
+        } else if (StringEqualsAscii(unit, "month") || StringEqualsAscii(unit, "months")) {
+            relDateTimeUnit = UDAT_REL_UNIT_MONTH;
+        } else if (StringEqualsAscii(unit, "quarter") || StringEqualsAscii(unit, "quarters")) {
+            relDateTimeUnit = UDAT_REL_UNIT_QUARTER;
+        } else {
+            MOZ_ASSERT(StringEqualsAscii(unit, "year") || StringEqualsAscii(unit, "years"));
+            relDateTimeUnit = UDAT_REL_UNIT_YEAR;
+        }
+    }
+
+    if (!GetProperty(cx, internals, internals, cx->names().numeric, &value))
+        return false;
+    RootedLinearString numeric(cx, value.toString()->ensureLinear(cx));
+    if (!numeric)
+        return false;
+
+    RelativeTimeNumeric relDateTimeNumeric;
+
+    if (StringEqualsAscii(numeric, "auto")) {
+        relDateTimeNumeric = RelativeTimeNumeric::Auto;
+    } else {
+        MOZ_ASSERT(StringEqualsAscii(numeric, "always"));
+        relDateTimeNumeric = RelativeTimeNumeric::Always;
+    }
+
+    Vector<char16_t, INITIAL_CHAR_BUFFER_SIZE> chars(cx);
+    if (!chars.resize(INITIAL_CHAR_BUFFER_SIZE))
+        return false;
+    UErrorCode status = U_ZERO_ERROR;
+    URelativeDateTimeFormatter* rtf =
+        ureldatefmt_open(icuLocale(locale.ptr()), nullptr, relDateTimeStyle,
+                         UDISPCTX_CAPITALIZATION_FOR_STANDALONE, &status);
+    if (U_FAILURE(status)) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_INTERNAL_INTL_ERROR);
+        return false;
+    }
+
+    int32_t size;
+
+    if (relDateTimeNumeric == RelativeTimeNumeric::Auto) {
+        size = ureldatefmt_format(rtf, t, relDateTimeUnit, Char16ToUChar(chars.begin()),
+                                  INITIAL_CHAR_BUFFER_SIZE, &status);
+    } else {
+        MOZ_ASSERT(relDateTimeNumeric == RelativeTimeNumeric::Always);
+        size = ureldatefmt_formatNumeric(rtf, t, relDateTimeUnit, Char16ToUChar(chars.begin()),
+                                         INITIAL_CHAR_BUFFER_SIZE, &status);
+    }
+
+    ScopedICUObject<URelativeDateTimeFormatter, ureldatefmt_close> closeRelativeTimeFormat(rtf);
+
+    JSString* str = NewStringCopyN<CanGC>(cx, chars.begin(), size);
+    if (!str)
+        return false;
+
+    args.rval().setString(str);
+    return true;
+}
+
 bool
 js::intl_GetCalendarInfo(JSContext* cx, unsigned argc, Value* vp)
 {
@@ -3647,6 +3947,11 @@ GlobalObject::initIntlObject(JSContext* cx, Handle<GlobalObject*> global)
     if (!pluralRulesProto)
         return false;
 
+    RootedObject relativeTimeFmtProto(cx, CreateRelativeTimeFormatPrototype(cx, intl, global));
+    if (!relativeTimeFmtProto) {
+        return false;
+    }
+
     // The |Intl| object is fully set up now, so define the global property.
     RootedValue intlValue(cx, ObjectValue(*intl));
     if (!DefineProperty(cx, global, cx->names().Intl, intlValue, nullptr, nullptr,
@@ -3668,6 +3973,7 @@ GlobalObject::initIntlObject(JSContext* cx, Handle<GlobalObject*> global)
     global->setReservedSlot(DATE_TIME_FORMAT_PROTO, ObjectValue(*dateTimeFormatProto));
     global->setReservedSlot(NUMBER_FORMAT_PROTO, ObjectValue(*numberFormatProto));
     global->setReservedSlot(PLURAL_RULES_PROTO, ObjectValue(*pluralRulesProto));
+    global->setReservedSlot(RELATIVE_TIME_FORMAT_PROTO, ObjectValue(*relativeTimeFmtProto));
 
     // Also cache |Intl| to implement spec language that conditions behavior
     // based on values being equal to "the standard built-in |Intl| object".
diff --git a/js/src/builtin/Intl.h b/js/src/builtin/Intl.h
index fd1fc5da6a..330b8a4963 100644
--- a/js/src/builtin/Intl.h
+++ b/js/src/builtin/Intl.h
@@ -406,6 +406,31 @@ intl_SelectPluralRule(JSContext* cx, unsigned argc, Value* vp);
 extern MOZ_MUST_USE bool
 intl_GetPluralCategories(JSContext* cx, unsigned argc, Value* vp);
 
+/******************** RelativeTimeFormat ********************/
+
+/**
+ * Returns an object indicating the supported locales for relative time format
+ * by having a true-valued property for each such locale with the
+ * canonicalized language tag as the property name. The object has no
+ * prototype.
+ *
+ * Usage: availableLocales = intl_RelativeTimeFormat_availableLocales()
+ */
+extern MOZ_MUST_USE bool
+intl_RelativeTimeFormat_availableLocales(JSContext* cx, unsigned argc, Value* vp);
+
+/**
+ * Returns a relative time as a string formatted according to the effective
+ * locale and the formatting options of the given RelativeTimeFormat.
+ *
+ * t should be a number representing a number to be formatted.
+ * unit should be "second", "minute", "hour", "day", "week", "month", "quarter", or "year".
+ *
+ * Usage: formatted = intl_FormatRelativeTime(relativeTimeFormat, t, unit)
+ */
+extern MOZ_MUST_USE bool
+intl_FormatRelativeTime(JSContext* cx, unsigned argc, Value* vp);
+
 /**
  * Returns a plain object with calendar information for a single valid locale
  * (callers must perform this validation).  The object will have these
diff --git a/js/src/builtin/Intl.js b/js/src/builtin/Intl.js
index ef0aa986a6..e7d1b58ed6 100644
--- a/js/src/builtin/Intl.js
+++ b/js/src/builtin/Intl.js
@@ -23,6 +23,7 @@
          intl_FormatDateTime: false,
          intl_SelectPluralRule: false,
          intl_GetPluralCategories: false,
+         intl_FormatRelativeTime: false,
          intl_GetCalendarInfo: false,
 */
 
@@ -1275,7 +1276,8 @@ function setLazyData(internals, type, lazyData)
 {
     assert(internals.type === "partial", "can't set lazy data for anything but a newborn");
     assert(type === "Collator" || type === "DateTimeFormat" ||
-           type == "NumberFormat" || type === "PluralRules",
+           type === "NumberFormat" || type === "PluralRules" ||
+           type === "RelativeTimeFormat",
            "bad type");
     assert(IsObject(lazyData), "non-object lazy data");
 
@@ -1327,7 +1329,8 @@ function isInitializedIntlObject(obj) {
         assert(callFunction(std_Object_hasOwnProperty, internals, "type"), "missing type");
         var type = internals.type;
         assert(type === "partial" || type === "Collator" ||
-               type === "DateTimeFormat" || type === "NumberFormat" || type === "PluralRules",
+               type === "DateTimeFormat" || type === "NumberFormat" ||
+               type === "PluralRules" || type === "RelativeTimeFormat",
                "unexpected type");
         assert(callFunction(std_Object_hasOwnProperty, internals, "lazyData"), "missing lazyData");
         assert(callFunction(std_Object_hasOwnProperty, internals, "internalProps"), "missing internalProps");
@@ -1381,14 +1384,24 @@ function getInternals(obj)
 
     var internalProps;
     var type = internals.type;
-    if (type === "Collator")
-        internalProps = resolveCollatorInternals(lazyData)
-    else if (type === "DateTimeFormat")
-        internalProps = resolveDateTimeFormatInternals(lazyData)
-    else if (type === "PluralRules")
-        internalProps = resolvePluralRulesInternals(lazyData)
-    else
+    
+    switch (type) {
+      case "Collator":
+        internalProps = resolveCollatorInternals(lazyData);
+        break;
+      case "DateTimeFormat":
+        internalProps = resolveDateTimeFormatInternals(lazyData);
+        break;
+      case "PluralRules":
+        internalProps = resolvePluralRulesInternals(lazyData);
+        break;
+      case "RelativeTimeFormat":
+        internalProps = resolveRelativeTimeFormatInternals(lazyData);
+        break;
+      default: // type === "NumberFormat"
         internalProps = resolveNumberFormatInternals(lazyData);
+        break;
+    }
     setInternalProperties(internals, internalProps);
     return internalProps;
 }
@@ -3252,6 +3265,244 @@ function Intl_PluralRules_resolvedOptions() {
 }
 
 
+/********** Intl.RelativeTimeFormat **********/
+
+/**
+ * RelativeTimeFormat internal properties.
+ *
+ * Spec: ECMAScript 402 API, RelativeTimeFormat, 1.3.3.
+ */
+var relativeTimeFormatInternalProperties = {
+    localeData: relativeTimeFormatLocaleData,
+    _availableLocales: null,
+    availableLocales: function() // eslint-disable-line object-shorthand
+    {
+        var locales = this._availableLocales;
+        if (locales)
+            return locales;
+
+        locales = intl_RelativeTimeFormat_availableLocales();
+        addSpecialMissingLanguageTags(locales);
+        return (this._availableLocales = locales);
+    },
+    relevantExtensionKeys: [],
+};
+
+function relativeTimeFormatLocaleData() {
+    // RelativeTimeFormat doesn't support any extension keys.
+    return {};
+}
+
+/**
+ * Compute an internal properties object from |lazyRelativeTimeFormatData|.
+ */
+function resolveRelativeTimeFormatInternals(lazyRelativeTimeFormatData) {
+    assert(IsObject(lazyRelativeTimeFormatData), "lazy data not an object?");
+
+    var internalProps = std_Object_create(null);
+
+    var RelativeTimeFormat = relativeTimeFormatInternalProperties;
+
+    // Steps 7-8.
+    const r = ResolveLocale(callFunction(RelativeTimeFormat.availableLocales, RelativeTimeFormat),
+                            lazyRelativeTimeFormatData.requestedLocales,
+                            lazyRelativeTimeFormatData.opt,
+                            RelativeTimeFormat.relevantExtensionKeys,
+                            RelativeTimeFormat.localeData);
+
+    // Step 9-10.
+    internalProps.locale = r.locale;
+
+    // Step 11.
+    assert(r.locale === r.dataLocale,
+           "resolved locale matches the resolved data-locale when no extension-keys are present");
+
+    // Step 13.
+    internalProps.style = lazyRelativeTimeFormatData.style;
+
+    // Step 15.
+    internalProps.numeric = lazyRelativeTimeFormatData.numeric;
+
+    // Steps 16-20 (Not relevant in our implementation).
+
+    return internalProps;
+}
+
+/**
+ * Returns an object containing the RelativeTimeFormat internal properties of |obj|,
+ * or throws a TypeError if |obj| isn't RelativeTimeFormat-initialized.
+ */
+function getRelativeTimeFormatInternals(obj, methodName) {
+    var internals = getIntlObjectInternals(obj, "RelativeTimeFormat", methodName);
+    assert(internals.type === "RelativeTimeFormat", "bad type escaped getIntlObjectInternals");
+
+    var internalProps = maybeInternalProperties(internals);
+    if (internalProps)
+        return internalProps;
+
+    internalProps = resolveRelativeTimeFormatInternals(internals.lazyData);
+    setInternalProperties(internals, internalProps);
+    return internalProps;
+}
+
+/**
+ * Initializes an object as a RelativeTimeFormat.
+ *
+ * This method is complicated a moderate bit by its implementing initialization
+ * as a *lazy* concept.  Everything that must happen now, does -- but we defer
+ * all the work we can until the object is actually used as a RelativeTimeFormat.
+ * This later work occurs in |resolveRelativeTimeFormatInternals|; steps not noted
+ * here occur there.
+ *
+ * Spec: ECMAScript 402 API, RelativeTimeFormat, 1.1.1.
+ */
+function InitializeRelativeTimeFormat(relativeTimeFormat, locales, options) {
+    assert(IsObject(relativeTimeFormat), "InitializeRelativeTimeFormat");
+
+    if (isInitializedIntlObject(relativeTimeFormat))
+        ThrowTypeError(JSMSG_INTL_OBJECT_REINITED);
+
+    let internals = initializeIntlObject(relativeTimeFormat);
+
+    // Lazy RelativeTimeFormat data has the following structure:
+    //
+    //   {
+    //     requestedLocales: List of locales,
+    //     style: "long" / "short" / "narrow",
+    //     numeric: "always" / "auto",
+    //
+    //     opt: // opt object computed in InitializeRelativeTimeFormat
+    //       {
+    //         localeMatcher: "lookup" / "best fit",
+    //       }
+    //   }
+    //
+    // Note that lazy data is only installed as a final step of initialization,
+    // so every RelativeTimeFormat lazy data object has *all* these properties, never a
+    // subset of them.
+    const lazyRelativeTimeFormatData = std_Object_create(null);
+
+    // Step 1.
+    let requestedLocales = CanonicalizeLocaleList(locales);
+    lazyRelativeTimeFormatData.requestedLocales = requestedLocales;
+
+    // Steps 2-3.
+    if (options === undefined)
+        options = std_Object_create(null);
+    else
+        options = ToObject(options);
+
+    // Step 4.
+    let opt = new Record();
+
+    // Steps 5-6.
+    let matcher = GetOption(options, "localeMatcher", "string", ["lookup", "best fit"], "best fit");
+    opt.localeMatcher = matcher;
+
+    lazyRelativeTimeFormatData.opt = opt;
+
+    // Steps 12-13.
+    const style = GetOption(options, "style", "string", ["long", "short", "narrow"], "long");
+    lazyRelativeTimeFormatData.style = style;
+
+    // Steps 14-15.
+    const numeric = GetOption(options, "numeric", "string", ["always", "auto"], "always");
+    lazyRelativeTimeFormatData.numeric = numeric;
+
+    setLazyData(internals, "RelativeTimeFormat", lazyRelativeTimeFormatData)
+}
+
+/**
+ * Returns the subset of the given locale list for which this locale list has a
+ * matching (possibly fallback) locale. Locales appear in the same order in the
+ * returned list as in the input list.
+ *
+ * Spec: ECMAScript 402 API, RelativeTimeFormat, 1.3.2.
+ */
+function Intl_RelativeTimeFormat_supportedLocalesOf(locales /*, options*/) {
+    var options = arguments.length > 1 ? arguments[1] : undefined;
+
+    // Step 1.
+    var availableLocales = callFunction(relativeTimeFormatInternalProperties.availableLocales,
+                                        relativeTimeFormatInternalProperties);
+    // Step 2.
+    let requestedLocales = CanonicalizeLocaleList(locales);
+
+    // Step 3.
+    return SupportedLocales(availableLocales, requestedLocales, options);
+}
+
+/**
+ * Returns a String value representing the written form of a relative date
+ * formatted according to the effective locale and the formatting options
+ * of this RelativeTimeFormat object.
+ *
+ * Spec: ECMAScript 402 API, RelativeTImeFormat, 1.4.3.
+ */
+function Intl_RelativeTimeFormat_format(value, unit) {
+    // Step 1.
+    let relativeTimeFormat = this;
+
+    // Step 2.
+    let internals = getRelativeTimeFormatInternals(relativeTimeFormat, "format");
+
+    // Step 3.
+    let t = ToNumber(value);
+
+    // Step 4.
+    let u = ToString(unit);
+
+    // PartitionRelativeTimePattern, step 4.
+    if (!Number_isFinite(t)) {
+        ThrowRangeError(JSMSG_DATE_NOT_FINITE, "RelativeTimeFormat");
+    }
+
+    // PartitionRelativeTimePattern, step 5.
+    switch (u) {
+      case "second":
+      case "seconds":
+      case "minute":
+      case "minutes":
+      case "hour":
+      case "hours":
+      case "day":
+      case "days":
+      case "week":
+      case "weeks":
+      case "month":
+      case "months":
+      case "quarter":
+      case "quarters":
+      case "year":
+      case "years":
+        break;
+      default:
+        ThrowRangeError(JSMSG_INVALID_OPTION_VALUE, "unit", u);
+    }
+
+    // Step 5.
+    return intl_FormatRelativeTime(relativeTimeFormat, t, u, internals.numeric);
+}
+
+/**
+ * Returns the resolved options for a PluralRules object.
+ *
+ * Spec: ECMAScript 402 API, RelativeTimeFormat, 1.4.4.
+ */
+function Intl_RelativeTimeFormat_resolvedOptions() {
+    var internals = getRelativeTimeFormatInternals(this, "resolvedOptions");
+
+    // Steps 4-5.
+    var result = {
+        locale: internals.locale,
+        style: internals.style,
+        numeric: internals.numeric,
+    };
+
+    // Step 6.
+    return result;
+}
+
 function Intl_getCanonicalLocales(locales) {
   let codes = CanonicalizeLocaleList(locales);
   let result = [];
diff --git a/js/src/vm/CommonPropertyNames.h b/js/src/vm/CommonPropertyNames.h
index 445b8a6bb3..8ecd3a2083 100644
--- a/js/src/vm/CommonPropertyNames.h
+++ b/js/src/vm/CommonPropertyNames.h
@@ -182,6 +182,7 @@
     macro(InitializeDateTimeFormat, InitializeDateTimeFormat, "InitializeDateTimeFormat") \
     macro(InitializeNumberFormat, InitializeNumberFormat, "InitializeNumberFormat") \
     macro(InitializePluralRules, InitializePluralRules, "InitializePluralRules") \
+    macro(InitializeRelativeTimeFormat, InitializeRelativeTimeFormat, "InitializeRelativeTimeFormat") \
     macro(innermost, innermost, "innermost") \
     macro(inNursery, inNursery, "inNursery") \
     macro(input, input, "input") \
@@ -297,6 +298,8 @@
     macro(Reify, Reify, "Reify") \
     macro(reject, reject, "reject") \
     macro(rejected, rejected, "rejected") \
+    macro(RelativeTimeFormat, RelativeTimeFormat, "RelativeTimeFormat") \
+    macro(RelativeTimeFormatFormat, RelativeTimeFormatFormat, "Intl_RelativeTimeFormat_Format") \
     macro(RequireObjectCoercible, RequireObjectCoercible, "RequireObjectCoercible") \
     macro(resolve, resolve, "resolve") \
     macro(resumeGenerator, resumeGenerator, "resumeGenerator") \
@@ -364,6 +367,7 @@
     macro(unescape, unescape, "unescape") \
     macro(uneval, uneval, "uneval") \
     macro(unicode, unicode, "unicode") \
+    macro(unit, unit, "unit") \
     macro(uninitialized, uninitialized, "uninitialized") \
     macro(unsized, unsized, "unsized") \
     macro(unwatch, unwatch, "unwatch") \
diff --git a/js/src/vm/GlobalObject.h b/js/src/vm/GlobalObject.h
index f9c0149f12..2e1b6ce87b 100644
--- a/js/src/vm/GlobalObject.h
+++ b/js/src/vm/GlobalObject.h
@@ -110,6 +110,7 @@ class GlobalObject : public NativeObject
         NUMBER_FORMAT_PROTO,
         DATE_TIME_FORMAT_PROTO,
         PLURAL_RULES_PROTO,
+        RELATIVE_TIME_FORMAT_PROTO,
         MODULE_PROTO,
         IMPORT_ENTRY_PROTO,
         EXPORT_ENTRY_PROTO,
@@ -513,6 +514,11 @@ class GlobalObject : public NativeObject
         return getOrCreateObject(cx, global, PLURAL_RULES_PROTO, initIntlObject);
     }
 
+    static JSObject*
+    getOrCreateRelativeTimeFormatPrototype(JSContext* cx, Handle<GlobalObject*> global) {
+        return getOrCreateObject(cx, global, RELATIVE_TIME_FORMAT_PROTO, initIntlObject);
+    }
+
     static bool ensureModulePrototypesCreated(JSContext *cx, Handle<GlobalObject*> global);
 
     JSObject* maybeGetModulePrototype() {
diff --git a/js/src/vm/SelfHosting.cpp b/js/src/vm/SelfHosting.cpp
index 29c75c2064..058cc6b6d0 100644
--- a/js/src/vm/SelfHosting.cpp
+++ b/js/src/vm/SelfHosting.cpp
@@ -2520,6 +2520,8 @@ static const JSFunctionSpec intrinsic_functions[] = {
     JS_FN("intl_PluralRules_availableLocales", intl_PluralRules_availableLocales, 0,0),
     JS_FN("intl_GetPluralCategories", intl_GetPluralCategories, 2, 0),
     JS_FN("intl_SelectPluralRule", intl_SelectPluralRule, 2,0),
+    JS_FN("intl_RelativeTimeFormat_availableLocales", intl_RelativeTimeFormat_availableLocales, 0,0),
+    JS_FN("intl_FormatRelativeTime", intl_FormatRelativeTime, 3,0),
 
     JS_INLINABLE_FN("IsRegExpObject",
                     intrinsic_IsInstanceOfBuiltin<RegExpObject>, 1,0,