Add From impls to convert between FormattingSpecs

src/format/formatting.rs

@@ -9,7 +9,7 @@ use core::borrow::Borrow;
 use core::fmt::{self, Display, Write};
 use core::marker::PhantomData;
 
-use crate::offset::{FixedOffset, Offset};
+use crate::offset::{FixedOffset, Offset, Utc};
 use crate::{DateTime, NaiveDate, NaiveDateTime, NaiveTime};
 use crate::{Datelike, ParseError, SecondsFormat, TimeZone, Timelike, Weekday};
 
@@ -344,6 +344,21 @@ formatting_spec_impls!(NaiveDateTime, true, true, false);
 formatting_spec_impls!(NaiveDate, true, false, false);
 formatting_spec_impls!(NaiveTime, false, true, false);
 
+macro_rules! formatting_spec_from_impls {
+    ($src:ty, $dst:ty) => {
+        impl<I> From<FormattingSpec<$src, I>> for FormattingSpec<$dst, I> {
+            fn from(value: FormattingSpec<$src, I>) -> Self {
+                Self { items: value.items, date_time_type: PhantomData, locale: value.locale }
+            }
+        }
+    };
+}
+formatting_spec_from_impls!(NaiveTime, NaiveDateTime);
+formatting_spec_from_impls!(NaiveDate, NaiveDateTime);
+formatting_spec_from_impls!(NaiveTime, DateTime<Utc>);
+formatting_spec_from_impls!(NaiveDate, DateTime<Utc>);
+formatting_spec_from_impls!(NaiveDateTime, DateTime<Utc>);
+
 /// A *temporary* object which can be used as an argument to [`format!`] or others.
 #[derive(Debug)]
 pub struct Formatter<I, Off> {