[PATCH v6 6/7] rust: time: Add Instant::from_nanos()

[Lyude Paul]

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

Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>

---
V4:
* Turn from_nanos() into an unsafe function in order to ensure that we
  uphold the invariants of Instant
V5:
* Add debug_assert!() to from_nanos

Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
---
 rust/kernel/time.rs | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 64c8dcf548d63..75088d080b834 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -200,6 +200,29 @@ pub fn elapsed(&self) -> Delta {
     pub(crate) fn as_nanos(&self) -> i64 {
         self.inner
     }
+
+    /// Create an [`Instant`] from a time duration specified in nanoseconds without checking if it
+    /// is positive.
+    ///
+    /// # Panics
+    ///
+    /// On debug builds, this function will panic if `nanos` violates our safety contract.
+    ///
+    /// # Safety
+    ///
+    /// The caller promises that `nanos` is in the range from 0 to `KTIME_MAX`.
+    #[expect(unused)]
+    #[inline]
+    pub(crate) unsafe fn from_nanos(nanos: i64) -> Self {
+        debug_assert!(nanos >= 0);
+
+        // INVARIANT: Our safety contract ensures that `nanos` is in the range from 0 to
+        // `KTIME_MAX`.
+        Self {
+            inner: nanos as bindings::ktime_t,
+            _c: PhantomData,
+        }
+    }
 }
 
 impl<C: ClockSource> core::ops::Sub for Instant<C> {
-- 
2.50.0