seed_from_u64: use newpavlov's suggestion

Author: dhardy

rand_core/src/lib.rs

@@ -300,20 +300,30 @@ pub trait SeedableRng: Sized {
     /// considered a value-breaking change.
     fn seed_from_u64(mut state: u64) -> Self {
         // We use PCG32 to generate a u32 sequence, and copy to the seed
-        const MUL: u64 = 6364136223846793005;
-        const INC: u64 = 11634580027462260723;
+        fn pcg32(state: &mut u64) -> [u8; 4] {
+            const MUL: u64 = 6364136223846793005;
+            const INC: u64 = 11634580027462260723;
 
-        let mut seed = Self::Seed::default();
-        for chunk in seed.as_mut().chunks_mut(4) {
             // We advance the state first (to get away from the input value,
             // in case it has low Hamming Weight).
-            state = state.wrapping_mul(MUL).wrapping_add(INC);
+            *state = state.wrapping_mul(MUL).wrapping_add(INC);
+            let state = *state;
 
             // Use PCG output function with to_le to generate x:
             let xorshifted = (((state >> 18) ^ state) >> 27) as u32;
             let rot = (state >> 59) as u32;
             let x = xorshifted.rotate_right(rot);
-            chunk.copy_from_slice(&x.to_le_bytes()[..chunk.len()]);
+            x.to_le_bytes()
+        }
+
+        let mut seed = Self::Seed::default();
+        let mut iter = seed.as_mut().chunks_exact_mut(4);
+        for chunk in &mut iter {
+            chunk.copy_from_slice(&pcg32(&mut state));
+        }
+        let rem = iter.into_remainder();
+        if !rem.is_empty() {
+            rem.copy_from_slice(&pcg32(&mut state)[..rem.len()]);
         }
 
         Self::from_seed(seed)