Try to fix tests on stable Rust

This required introducing a new "nightly" feature.

.travis.yml

@@ -33,7 +33,12 @@ before_script:
 script:
   - cargo fmt --all -- --check
   - cargo build
-  - cargo test
+  - >
+    if [[ "$TRAVIS_RUST_VERSION" == "nightly" ]]; then
+      cargo test --features nightly
+    else
+      cargo test
+    fi
 
 after_success: >
   if [[ "$TRAVIS_OS_NAME" == "linux" ]] && [[ "$TRAVIS_RUST_VERSION" == "stable" ]]; then

Cargo.toml

@@ -13,10 +13,12 @@ repository = "https://github.com/boxtown/statrs"
 edition = "2018"
 
 [lib]
-
 name = "statrs"
 path = "src/lib.rs"
 
+[features]
+nightly = []
+
 [dependencies]
 rand = "0.8"
 nalgebra = { version = "0.26", features = ["rand"] }

src/distribution/beta.rs

@@ -369,7 +369,7 @@ impl Continuous<f64, f64> for Beta {
 }
 
 #[rustfmt::skip]
-#[cfg(test)]
+#[cfg(all(test, feature = "nightly"))]
 mod tests {
     use super::*;
     use crate::consts::ACC;

src/distribution/gamma.rs

@@ -322,7 +322,7 @@ pub fn sample_unchecked<R: Rng + ?Sized>(rng: &mut R, shape: f64, rate: f64) ->
     }
 }
 
-#[cfg(test)]
+#[cfg(all(test, feature = "nightly"))]
 mod tests {
     use super::*;
     use crate::consts::ACC;

src/distribution/internal.rs

@@ -18,6 +18,7 @@ pub mod test {
     use crate::consts::ACC;
     use crate::distribution::{Continuous, ContinuousCDF, Discrete, DiscreteCDF};
 
+    #[cfg(feature = "nightly")]
     #[macro_export]
     macro_rules! testing_boiler {
         ($arg:ty, $dist:ty) => {

src/lib.rs

@@ -29,8 +29,8 @@
 #![allow(clippy::many_single_char_names)]
 #![allow(unused_imports)]
 #![forbid(unsafe_code)]
-#![cfg_attr(test, feature(unboxed_closures))]
-#![cfg_attr(test, feature(fn_traits))]
+#![cfg_attr(all(test, feature = "nightly"), feature(unboxed_closures))]
+#![cfg_attr(all(test, feature = "nightly"), feature(fn_traits))]
 
 #[macro_use]
 extern crate approx;