Add a benchmark

See unicode-org/icu4x#4230 (review)

Results look like:
instanciate every time  time:   [9.5168 µs 9.5523 µs 9.5949 µs]
instanciate once        time:   [936.66 ns 942.79 ns 950.65 ns]
instanciate every time (typed)
                        time:   [9.1276 µs 9.1405 µs 9.1541 µs]
instanciate once (typed)
                        time:   [897.98 ns 916.17 ns 940.64 ns]

Cargo.toml

@@ -16,4 +16,9 @@ intl-memoizer = "0.5"
 unic-langid = "0.9"
 
 [dev-dependencies]
+criterion = "0.5"
 fluent = "0.16"
+
+[[bench]]
+name = "icu-formatter-instanciation"
+harness = false

benches/icu-formatter-instanciation.rs

@@ -0,0 +1,57 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use icu_calendar::{AnyCalendar, DateTime, Gregorian};
+use icu_datetime::options::length;
+use icu_datetime::{DateTimeFormatter, TypedDateTimeFormatter};
+use icu_provider::DataLocale;
+
+fn make_formatter(locale: &DataLocale) -> DateTimeFormatter {
+    // Using Time::Short otherwise try_new returns an Err(UnsupportedField(TimeZone(LowerZ)))
+    let length = length::Bag::from_date_time_style(length::Date::Full, length::Time::Short);
+    icu_datetime::DateTimeFormatter::try_new(locale, length.into()).unwrap()
+}
+
+fn format(formatter: &DateTimeFormatter, dt: &DateTime<AnyCalendar>) -> String {
+    formatter.format_to_string(dt).unwrap()
+}
+
+fn make_typed_formatter(locale: &DataLocale) -> TypedDateTimeFormatter<Gregorian> {
+    let length = length::Bag::from_date_time_style(length::Date::Full, length::Time::Short);
+    // Using Gregorian, not Iso, otherwise I get:
+    // the trait `CldrCalendar` is not implemented for `icu_calendar::Iso`
+    icu_datetime::TypedDateTimeFormatter::try_new(locale, length.into()).unwrap()
+}
+
+fn format_typed(formatter: &TypedDateTimeFormatter<Gregorian>, dt: &DateTime<Gregorian>) -> String {
+    formatter.format_to_string(dt)
+}
+
+pub fn criterion_benchmark(c: &mut Criterion) {
+    let locale = icu_locid::langid!("fr-FR");
+    let locale = &locale.into();
+    let datetime = DateTime::try_new_gregorian_datetime(1989, 11, 9, 23, 30, 0).unwrap();
+    let dt_any = datetime.clone().to_any();
+
+    c.bench_function("instanciate every time", |b| {
+        b.iter(|| format(&make_formatter(black_box(locale)), black_box(&dt_any)))
+    });
+    let formatter = make_formatter(locale);
+    c.bench_function("instanciate once", |b| {
+        b.iter(|| format(black_box(&formatter), black_box(&dt_any)))
+    });
+
+    c.bench_function("instanciate every time (typed)", |b| {
+        b.iter(|| {
+            format_typed(
+                &make_typed_formatter(black_box(locale)),
+                black_box(&datetime),
+            )
+        })
+    });
+    let formatter = make_typed_formatter(locale);
+    c.bench_function("instanciate once (typed)", |b| {
+        b.iter(|| format_typed(black_box(&formatter), black_box(&datetime)))
+    });
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);