From f78eb20ef1e9b92de430b78c5a828ff0f2694ce1 Mon Sep 17 00:00:00 2001
From: Paul Dicker <paul@pitdicker.nl>
Date: Mon, 2 Oct 2023 13:17:55 +0200
Subject: [PATCH] Make `NaiveTime::overflowing_sub_signed` const

---
 src/duration.rs       | 19 ++++++++++++++-----
 src/naive/time/mod.rs |  4 ++--
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/src/duration.rs b/src/duration.rs
index be3dc0ce..7c371b17 100644
--- a/src/duration.rs
+++ b/src/duration.rs
@@ -435,6 +435,15 @@ impl Duration {
         }
         Ok(StdDuration::new(self.secs as u64, self.nanos as u32))
     }
+
+    /// This duplicates `Neg::neg` because trait methods can't be const yet.
+    pub(crate) const fn neg(self) -> Duration {
+        let (secs_diff, nanos) = match self.nanos {
+            0 => (0, 0),
+            nanos => (1, NANOS_PER_SEC - nanos),
+        };
+        Duration { secs: -self.secs - secs_diff, nanos }
+    }
 }
 
 impl Neg for Duration {
@@ -442,11 +451,11 @@ impl Neg for Duration {
 
     #[inline]
     fn neg(self) -> Duration {
-        if self.nanos == 0 {
-            Duration { secs: -self.secs, nanos: 0 }
-        } else {
-            Duration { secs: -self.secs - 1, nanos: NANOS_PER_SEC - self.nanos }
-        }
+        let (secs_diff, nanos) = match self.nanos {
+            0 => (0, 0),
+            nanos => (1, NANOS_PER_SEC - nanos),
+        };
+        Duration { secs: -self.secs - secs_diff, nanos }
     }
 }
 
diff --git a/src/naive/time/mod.rs b/src/naive/time/mod.rs
index f6861c03..b47639ce 100644
--- a/src/naive/time/mod.rs
+++ b/src/naive/time/mod.rs
@@ -639,8 +639,8 @@ impl NaiveTime {
     /// ```
     #[inline]
     #[must_use]
-    pub fn overflowing_sub_signed(&self, rhs: OldDuration) -> (NaiveTime, i64) {
-        let (time, rhs) = self.overflowing_add_signed(-rhs);
+    pub const fn overflowing_sub_signed(&self, rhs: OldDuration) -> (NaiveTime, i64) {
+        let (time, rhs) = self.overflowing_add_signed(rhs.neg());
         (time, -rhs) // safe to negate, rhs is within +/- (2^63 / 1000)
     }