BTreeMap: document a curious assumption in test cases

library/alloc/src/collections/btree/map/tests.rs

@@ -1661,6 +1661,7 @@ create_append_test!(test_append_239, 239);
 create_append_test!(test_append_1700, 1700);
 
 fn rand_data(len: usize) -> Vec<(u32, u32)> {
+    assert!(len * 2 <= 70029); // from that point on numbers repeat
     let mut rng = DeterministicRng::new();
     Vec::from_iter((0..len).map(|_| (rng.next(), rng.next())))
 }

library/alloc/src/collections/btree/mod.rs

@@ -49,6 +49,7 @@ impl DeterministicRng {
         DeterministicRng { x: 0x193a6754, y: 0xa8a7d469, z: 0x97830e05, w: 0x113ba7bb }
     }
 
+    /// Guarantees that the first 70029 results are unique.
     fn next(&mut self) -> u32 {
         let x = self.x;
         let t = x ^ (x << 11);

library/alloc/src/collections/btree/set/tests.rs

@@ -680,6 +680,7 @@ fn test_first_last() {
 }
 
 fn rand_data(len: usize) -> Vec<u32> {
+    assert!(len <= 70029); // from that point on numbers repeat
     let mut rng = DeterministicRng::new();
     Vec::from_iter((0..len).map(|_| rng.next()))
 }