rust: time: Add Instant::from_ktime()

For implementing Rust bindings which can return a point in time.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Link: https://lore.kernel.org/r/20250821193259.964504-7-lyude@redhat.com
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Author: Lyude

rust/kernel/time.rs

@@ -200,6 +200,29 @@ impl<C: ClockSource> Instant<C> {
     pub(crate) fn as_nanos(&self) -> i64 {
         self.inner
     }
+
+    /// Create an [`Instant`] from a `ktime_t` without checking if it is non-negative.
+    ///
+    /// # Panics
+    ///
+    /// On debug builds, this function will panic if `ktime` is not in the range from 0 to
+    /// `KTIME_MAX`.
+    ///
+    /// # Safety
+    ///
+    /// The caller promises that `ktime` is in the range from 0 to `KTIME_MAX`.
+    #[expect(unused)]
+    #[inline]
+    pub(crate) unsafe fn from_ktime(ktime: bindings::ktime_t) -> Self {
+        debug_assert!(ktime >= 0);
+
+        // INVARIANT: Our safety contract ensures that `ktime` is in the range from 0 to
+        // `KTIME_MAX`.
+        Self {
+            inner: ktime,
+            _c: PhantomData,
+        }
+    }
 }
 
 impl<C: ClockSource> core::ops::Sub for Instant<C> {