fix: compilation failed in MSRV

rand_distr/tests/cdf.rs

@@ -426,19 +426,20 @@ fn hypergeometric() {
 
 #[test]
 fn poisson() {
-    fn cdf(k: i64, lambda: f64) -> f64 {
+    fn cdf(k: i64, lambda: f32) -> f64 {
         if k < 0 || lambda <= 0.0 {
             return 0.0;
         }
 
-        1.0 - lambda.inc_gamma(k as f64 + 1.0)
+        1.0 - lambda.inc_gamma(k as f32 + 1.0) as f64
     }
 
-    let parameters = [0.5, 1.0, 7.5, 32.0, 100.0];
+    let parameters = [0.5_f32, 1.0, 7.5, 32.0, 100.0, 1.844e19];
 
     for (seed, lambda) in parameters.into_iter().enumerate() {
         let dist = rand_distr::Poisson::new(lambda).unwrap();
-        test_discrete::<u64>(seed as u64, dist, |k| cdf(k, lambda));
+        // `f64` will unable to compile in MSRV 1.61.0
+        test_discrete(seed as u64, dist, |k| cdf(k, lambda));
     }
 }