Rollup merge of rust-lang#84843 - wcampbell0x2a:use-else-if-let, r=dtolnay

use else if in std library

Decreases indentation and improves readability

Author: Dylan-DPC

library/core/src/time.rs

@@ -518,13 +518,11 @@ impl Duration {
         if let Some(mut secs) = self.secs.checked_sub(rhs.secs) {
             let nanos = if self.nanos >= rhs.nanos {
                 self.nanos - rhs.nanos
+            } else if let Some(sub_secs) = secs.checked_sub(1) {
+                secs = sub_secs;
+                self.nanos + NANOS_PER_SEC - rhs.nanos
             } else {
-                if let Some(sub_secs) = secs.checked_sub(1) {
-                    secs = sub_secs;
-                    self.nanos + NANOS_PER_SEC - rhs.nanos
-                } else {
-                    return None;
-                }
+                return None;
             };
             debug_assert!(nanos < NANOS_PER_SEC);
             Some(Duration { secs, nanos })