time_util: add functions to parse format spec and apply it later

In templater, it's easier to handle invalid format string at parsing stage, so
I want to build formatting items upfront. Since the formatting items borrow
the input string by reference, we need to manually convert them to the owned
variants.

src/time_util.rs

@@ -3,6 +3,44 @@ use chrono::{DateTime, FixedOffset, LocalResult, TimeZone, Utc};
 use jujutsu_lib::backend::Timestamp;
 use once_cell::sync::Lazy;
 
+/// Parsed formatting items which should never contain an error.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct FormattingItems<'a> {
+    items: Vec<chrono::format::Item<'a>>,
+}
+
+impl<'a> FormattingItems<'a> {
+    /// Parses strftime-like format string.
+    pub fn parse(format: &'a str) -> Option<Self> {
+        // If the parsed format contained an error, format().to_string() would panic.
+        let items = StrftimeItems::new(format)
+            .map(|item| match item {
+                chrono::format::Item::Error => None,
+                _ => Some(item),
+            })
+            .collect::<Option<_>>()?;
+        Some(FormattingItems { items })
+    }
+
+    pub fn into_owned(self) -> FormattingItems<'static> {
+        use chrono::format::Item;
+        let items = self
+            .items
+            .into_iter()
+            .map(|item| match item {
+                Item::Literal(s) => Item::OwnedLiteral(s.into()),
+                Item::OwnedLiteral(s) => Item::OwnedLiteral(s),
+                Item::Space(s) => Item::OwnedSpace(s.into()),
+                Item::OwnedSpace(s) => Item::OwnedSpace(s),
+                Item::Numeric(spec, pad) => Item::Numeric(spec, pad),
+                Item::Fixed(spec) => Item::Fixed(spec),
+                Item::Error => Item::Error, // shouldn't exist, but just copy
+            })
+            .collect();
+        FormattingItems { items }
+    }
+}
+
 fn datetime_from_timestamp(context: &Timestamp) -> Option<DateTime<FixedOffset>> {
     let utc = match Utc.timestamp_opt(
         context.timestamp.0.div_euclid(1000),
@@ -24,10 +62,14 @@ fn datetime_from_timestamp(context: &Timestamp) -> Option<DateTime<FixedOffset>>
 }
 
 pub fn format_absolute_timestamp(timestamp: &Timestamp) -> String {
-    static FORMAT_ITEMS: Lazy<Vec<chrono::format::Item>> =
-        Lazy::new(|| StrftimeItems::new("%Y-%m-%d %H:%M:%S.%3f %:z").collect());
+    static DEFAULT_FORMAT: Lazy<FormattingItems> =
+        Lazy::new(|| FormattingItems::parse("%Y-%m-%d %H:%M:%S.%3f %:z").unwrap());
+    format_absolute_timestamp_with(timestamp, &DEFAULT_FORMAT)
+}
+
+pub fn format_absolute_timestamp_with(timestamp: &Timestamp, format: &FormattingItems) -> String {
     match datetime_from_timestamp(timestamp) {
-        Some(datetime) => datetime.format_with_items(FORMAT_ITEMS.iter()).to_string(),
+        Some(datetime) => datetime.format_with_items(format.items.iter()).to_string(),
         None => "<out-of-range date>".to_string(),
     }
 }