atom: Display timestamps ourselves

chrono hides iso8601 output behind the alloc feature (or fragile Debug
output) so we annoyingly have to implement it ourselves

src/atom.rs

@@ -1,14 +1,18 @@
 use crate::git::ModifyMap;
 use chrono::{DateTime, NaiveDateTime};
 use html_escaper::Escape;
-use std::{collections::BTreeMap, path::PathBuf};
+use std::{
+    collections::BTreeMap,
+    fmt::{self, Write},
+    path::PathBuf,
+};
 
 #[derive(boilerplate::Boilerplate)]
 pub struct FeedXml<'a> {
     pub title: &'a str,
     pub id: &'a str,
     pub url: &'a str,
-    pub updated: &'a NaiveDateTime,
+    pub updated: &'a AtomDateTime,
     pub entries: &'a [AtomEntry<'a>],
 }
 
@@ -17,7 +21,29 @@ pub struct AtomEntry<'a> {
     pub title: &'a str,
     pub path: &'a str,
     pub author: &'a str,
-    pub updated: NaiveDateTime,
+    pub updated: AtomDateTime,
+}
+
+/// NaiveDateTime that `Display`s to an atom feed compatible date (iso8601/rfc3339 subset)
+/// without unnecessary allocation, as chrono gates iso8601 output behind the `alloc` feature
+#[derive(Debug)]
+pub struct AtomDateTime(pub NaiveDateTime);
+
+impl AtomDateTime {
+    /// create a new AtomDateTime from a unix timestamp
+    pub fn new(unix: i64) -> Option<Self> {
+        let ts = DateTime::from_timestamp(unix, 0)?;
+        Some(Self(ts.naive_utc()))
+    }
+}
+
+impl fmt::Display for AtomDateTime {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.0.date().fmt(f)?;
+        f.write_char('T')?;
+        self.0.time().fmt(f)?;
+        f.write_char('Z')
+    }
 }
 
 pub fn entries<'a>(
@@ -32,9 +58,7 @@ pub fn entries<'a>(
             None => continue,
         };
         let (updated, author) = mtime.get(old).ok_or("missing modification info")?;
-        let updated = DateTime::from_timestamp(updated.seconds(), 0)
-            .ok_or("broken modification date")?
-            .naive_utc();
+        let updated = AtomDateTime::new(updated.seconds()).ok_or("broken modification date")?;
 
         entries.push(AtomEntry {
             title,
@@ -44,6 +68,6 @@ pub fn entries<'a>(
         });
     }
 
-    entries.sort_by(|x, y| y.updated.cmp(&x.updated));
+    entries.sort_by(|x, y| y.updated.0.cmp(&x.updated.0));
     Ok(entries)
 }

templates/feed.xml

@@ -3,14 +3,14 @@
 <title>{{ self.title }}</title>
 <id>{{ self.id }}/feed.xml</id>
 <link rel="self" href="{{ self.url }}/feed.xml"/>
-<updated>{{ format!("{:?}Z", self.updated) }}</updated>
+<updated>{{ self.updated }}</updated>
 %% for entry in self.entries {
 <entry>
 <title>{{ entry.title }}</title>
 <id>{{ self.id }}/{{ entry.path }}</id>
 <link rel="alternate" href="{{ self.url }}/{{ entry.path }}"/>
 <author><name>{{ entry.author }}</name></author>
-<updated>{{ format!("{:?}Z", entry.updated) }}</updated>
+<updated>{{ entry.updated }}</updated>
 </entry>
 %% }
 </feed>