fix miri bootstrap build

Author: RalfJung

src/clock.rs

@@ -46,7 +46,21 @@ impl Instant {
                 InstantKind::Virtual { nanoseconds: earlier },
             ) => {
                 let duration = nanoseconds.saturating_sub(earlier);
-                Duration::from_nanos_u128(duration)
+                cfg_select! {
+                    bootstrap => {
+                         // `Duration` does not provide a nice constructor from a `u128` of nanoseconds,
+                        // so we have to implement this ourselves.
+                        // It is possible for second to overflow because u64::MAX < (u128::MAX / 1e9).
+                        // It will be saturated to u64::MAX seconds if the value after division exceeds u64::MAX.
+                        let seconds = u64::try_from(duration / 1_000_000_000).unwrap_or(u64::MAX);
+                        // It is impossible for nanosecond to overflow because u32::MAX > 1e9.
+                        let nanosecond = u32::try_from(duration.wrapping_rem(1_000_000_000)).unwrap();
+                        Duration::new(seconds, nanosecond)
+                    }
+                    _ => {
+                        Duration::from_nanos_u128(duration)
+                    }
+                }
             }
             _ => panic!("all `Instant` must be of the same kind"),
         }

src/lib.rs

@@ -18,7 +18,7 @@
 #![feature(derive_coerce_pointee)]
 #![feature(arbitrary_self_types)]
 #![feature(iter_advance_by)]
-#![feature(duration_from_nanos_u128)]
+#![cfg_attr(not(bootstrap), feature(duration_from_nanos_u128))]
 // Configure clippy and other lints
 #![allow(
     clippy::collapsible_else_if,