Implement Error and Display for all errors

rand_distr/src/hypergeometric.rs

@@ -73,6 +73,10 @@ impl fmt::Display for Error {
     }
 }
 
+#[cfg(feature = "std")]
+#[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {}
+
 // evaluate fact(numerator.0)*fact(numerator.1) / fact(denominator.0)*fact(denominator.1)
 fn fraction_of_products_of_factorials(numerator: (u64, u64), denominator: (u64, u64)) -> f64 {
     let min_top = u64::min(numerator.0, numerator.1);

rand_distr/src/inverse_gaussian.rs

@@ -1,6 +1,7 @@
 use crate::{Distribution, Standard, StandardNormal};
 use num_traits::Float;
 use rand::Rng;
+use core::fmt;
 
 /// Error type returned from `InverseGaussian::new`
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -11,6 +12,19 @@ pub enum Error {
     ShapeNegativeOrNull,
 }
 
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            Error::MeanNegativeOrNull => "mean <= 0 or is NaN in inverse Gaussian distribution",
+            Error::ShapeNegativeOrNull => "shape <= 0 or is NaN in inverse Gaussian distribution",
+        })
+    }
+}
+
+#[cfg(feature = "std")]
+#[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {}
+
 /// The [inverse Gaussian distribution](https://en.wikipedia.org/wiki/Inverse_Gaussian_distribution)
 #[derive(Debug, Clone, Copy)]
 pub struct InverseGaussian<F>

rand_distr/src/normal_inverse_gaussian.rs

@@ -1,6 +1,7 @@
 use crate::{Distribution, InverseGaussian, Standard, StandardNormal};
 use num_traits::Float;
 use rand::Rng;
+use core::fmt;
 
 /// Error type returned from `NormalInverseGaussian::new`
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -11,6 +12,19 @@ pub enum Error {
     AbsoluteBetaNotLessThanAlpha,
 }
 
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            Error::AlphaNegativeOrNull => "alpha <= 0 or is NaN in normal inverse Gaussian distribution",
+            Error::AbsoluteBetaNotLessThanAlpha => "|beta| >= alpha or is NaN in normal inverse Gaussian distribution",
+        })
+    }
+}
+
+#[cfg(feature = "std")]
+#[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {}
+
 /// The [normal-inverse Gaussian distribution](https://en.wikipedia.org/wiki/Normal-inverse_Gaussian_distribution)
 #[derive(Debug, Clone, Copy)]
 pub struct NormalInverseGaussian<F>