Implement a trait for Intl.RelativeTimeFormat

Fixes google#182

ecma402_traits/src/lib.rs

@@ -48,3 +48,9 @@ pub mod collator;
 ///
 /// The [datetimeformat] mod contains all the needed implementation bits for `Intl.DateTimeFormat`.
 pub mod datetimeformat;
+
+/// A Rust implementation of ECMA 402 RelativeTimeFormat API.
+///
+/// The [relativetimeformat] mod contains all the needed implementation bits for
+/// `Intl.RelativeTimeFormat`.
+pub mod relativetimeformat;

ecma402_traits/src/relativetimeformat.rs

@@ -0,0 +1,92 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::fmt;
+
+/// Contains the API configuration as prescribed by [ECMA 402][ecma].
+///
+///    [ecma]: https://www.ecma-international.org/publications/standards/Ecma-402.htm
+///
+/// The meaning of the options is the same as in the similarly named
+/// options in the JS version.
+///
+/// See [Options] for the contents of the options.  See the [RelativeTimeFormat::try_new]
+/// for the use of the options.
+pub mod options {
+    /// Whether to use numeric formatting in the display, like "1 day ago".
+    #[derive(Debug, Clone, PartialEq)]
+    pub enum Numeric {
+        /// Always use numeric formatting.
+        ALWAYS,
+        /// Allows not to use numeric formatting, and use "yesterday" instead of "1 day ago".
+        AUTO,
+    }
+
+    /// The length of the internationalized message.
+    #[derive(Debug, Clone, PartialEq)]
+    pub enum Style {
+        /// Default, e.g. "in 1 month"
+        LONG,
+        /// Short, e.g. "in 1 mo."
+        SHORT,
+        /// Short, e.g. "in 1 mo". The narrow style could be identical to [Style::SHORT] in some
+        /// locales.
+        NARROW,
+    }
+}
+
+/// The options set by the user at construction time.
+///
+/// Provides as a "bag of options" since we don't expect any
+/// implementations to be attached to this struct.
+pub struct Options {
+    pub numeric: options::Numeric,
+    pub style: options::Style,
+}
+
+impl Default for Options {
+    fn default() -> Self {
+        Options {
+            numeric: options::Numeric::AUTO,
+            style: options::Style::LONG,
+        }
+    }
+}
+
+pub trait RelativeTimeFormat {
+    /// The type of the error reported, if any.
+    type Error: std::error::Error;
+
+    /// Creates a new [RelativeTimeFormat].
+    ///
+    /// Creation may fail if the locale-specific data is not loaded, or if the supplied options are
+    /// inconsistent.
+    fn try_new<L>(l: L, opts: Options) -> Result<Self, Self::Error>
+    where
+        L: crate::Locale,
+        Self: Sized;
+
+    /// Formats `days` into the supplied writer.
+    ///
+    /// A positive value means days in the future.  A negative value means days in the past.
+    ///
+    /// The function implements [`Intl.RelativeDateFormat`][ref] from [ECMA 402][ecma].
+    ///
+    /// [ref]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/RelativeTimeFormat
+    /// [ecma]: https://www.ecma-international.org/publications/standards/Ecma-402.htm
+    ///
+    fn format<W>(&self, days: i32, writer: &mut W) -> fmt::Result
+    where
+        W: fmt::Write;
+}