UTC_OFFSET_FORMAT: do not rely on promotion of function calls (#704)

Author: RalfJung

time/src/serde/mod.rs

@@ -404,22 +404,27 @@ impl<'a> Deserialize<'a> for Time {
 // endregion Time
 
 // region: UtcOffset
+// FIXME: turn these constants into `const { ... }` blocks once we can depend on Rust 1.79.
+#[cfg(feature = "parsing")]
+const UTC_OFFSET_HOUR: modifier::OffsetHour = {
+    let mut m = modifier::OffsetHour::default();
+    m.sign_is_mandatory = true;
+    m
+};
+#[cfg(feature = "parsing")]
+const UTC_OFFSET_MINUTE: modifier::OffsetMinute = modifier::OffsetMinute::default();
+#[cfg(feature = "parsing")]
+const UTC_OFFSET_SECOND: modifier::OffsetSecond = modifier::OffsetSecond::default();
 /// The format used when serializing and deserializing a human-readable `UtcOffset`.
 #[cfg(feature = "parsing")]
 const UTC_OFFSET_FORMAT: &[BorrowedFormatItem<'_>] = &[
-    BorrowedFormatItem::Component(Component::OffsetHour({
-        let mut m = modifier::OffsetHour::default();
-        m.sign_is_mandatory = true;
-        m
-    })),
+    BorrowedFormatItem::Component(Component::OffsetHour(UTC_OFFSET_HOUR)),
     BorrowedFormatItem::Optional(&BorrowedFormatItem::Compound(&[
         BorrowedFormatItem::Literal(b":"),
-        BorrowedFormatItem::Component(Component::OffsetMinute(modifier::OffsetMinute::default())),
+        BorrowedFormatItem::Component(Component::OffsetMinute(UTC_OFFSET_MINUTE)),
         BorrowedFormatItem::Optional(&BorrowedFormatItem::Compound(&[
             BorrowedFormatItem::Literal(b":"),
-            BorrowedFormatItem::Component(Component::OffsetSecond(
-                modifier::OffsetSecond::default(),
-            )),
+            BorrowedFormatItem::Component(Component::OffsetSecond(UTC_OFFSET_SECOND)),
         ])),
     ])),
 ];