Repro duration_since regression from issue 146228

Author: stepancheg

library/std/src/sys/pal/hermit/time.rs

@@ -26,15 +26,22 @@ impl Timespec {
     }
 
     fn sub_timespec(&self, other: &Timespec) -> Result<Duration, Duration> {
+        fn sub_ge_to_unsigned(a: i64, b: i64) -> u64 {
+            debug_assert!(a >= b);
+            a.wrapping_sub(b).cast_unsigned()
+        }
+
         if self >= other {
+            // Logic here is identical to Unix version of `Timestamp::sub_timespec`,
+            // check comments there why operations do not overflow.
             Ok(if self.t.tv_nsec >= other.t.tv_nsec {
                 Duration::new(
-                    (self.t.tv_sec - other.t.tv_sec) as u64,
+                    sub_ge_to_unsigned(self.t.tv_sec, other.t.tv_sec),
                     (self.t.tv_nsec - other.t.tv_nsec) as u32,
                 )
             } else {
                 Duration::new(
-                    (self.t.tv_sec - 1 - other.t.tv_sec) as u64,
+                    sub_ge_to_unsigned(self.t.tv_sec - 1, other.t.tv_sec),
                     (self.t.tv_nsec + NSEC_PER_SEC - other.t.tv_nsec) as u32,
                 )
             })

library/std/src/sys/pal/unix/time.rs

@@ -134,28 +134,25 @@ impl Timespec {
     }
 
     pub fn sub_timespec(&self, other: &Timespec) -> Result<Duration, Duration> {
+        // When a >= b, the difference fits in u64.
+        fn sub_ge_to_unsigned(a: i64, b: i64) -> u64 {
+            debug_assert!(a >= b);
+            a.wrapping_sub(b).cast_unsigned()
+        }
+
         if self >= other {
-            // NOTE(eddyb) two aspects of this `if`-`else` are required for LLVM
-            // to optimize it into a branchless form (see also #75545):
-            //
-            // 1. `self.tv_sec - other.tv_sec` shows up as a common expression
-            //    in both branches, i.e. the `else` must have its `- 1`
-            //    subtraction after the common one, not interleaved with it
-            //    (it used to be `self.tv_sec - 1 - other.tv_sec`)
-            //
-            // 2. the `Duration::new` call (or any other additional complexity)
-            //    is outside of the `if`-`else`, not duplicated in both branches
-            //
-            // Ideally this code could be rearranged such that it more
-            // directly expresses the lower-cost behavior we want from it.
             let (secs, nsec) = if self.tv_nsec.as_inner() >= other.tv_nsec.as_inner() {
                 (
-                    (self.tv_sec - other.tv_sec) as u64,
+                    sub_ge_to_unsigned(self.tv_sec, other.tv_sec),
                     self.tv_nsec.as_inner() - other.tv_nsec.as_inner(),
                 )
             } else {
+                // Following sequence of assertions explain why `self.tv_sec - 1` does not underflow.
+                debug_assert!(self.tv_nsec < other.tv_nsec);
+                debug_assert!(self.tv_sec > other.tv_sec);
+                debug_assert!(self.tv_sec > i64::MIN);
                 (
-                    (self.tv_sec - other.tv_sec - 1) as u64,
+                    sub_ge_to_unsigned(self.tv_sec - 1, other.tv_sec),
                     self.tv_nsec.as_inner() + (NSEC_PER_SEC as u32) - other.tv_nsec.as_inner(),
                 )
             };

library/std/tests/time.rs

@@ -227,3 +227,19 @@ fn big_math() {
     check(instant.checked_add(Duration::from_secs(100)), Instant::checked_sub);
     check(instant.checked_add(Duration::from_secs(i64::MAX as _)), Instant::checked_sub);
 }
+
+#[test]
+#[cfg(unix)]
+fn system_time_duration_since_max_range_on_unix() {
+    // Repro regression https://github.com/rust-lang/rust/issues/146228
+
+    // Min and max values of `SystemTime` on Unix.
+    let min = SystemTime::UNIX_EPOCH - (Duration::new(i64::MAX as u64 + 1, 0));
+    let max = SystemTime::UNIX_EPOCH + (Duration::new(i64::MAX as u64, 999_999_999));
+
+    let delta_a = max.duration_since(min).expect("duration_since overflow");
+    let delta_b = min.duration_since(max).expect_err("duration_since overflow").duration();
+
+    assert_eq!(Duration::MAX, delta_a);
+    assert_eq!(Duration::MAX, delta_b);
+}