Make nalgebra-sparse unit tests require io feature

nalgebra-sparse/tests/unit.rs

@@ -1,6 +1,13 @@
 //! Unit tests
-#[cfg(any(not(feature = "proptest-support"), not(feature = "compare")))]
-compile_error!("Tests must be run with features `proptest-support` and `compare`");
+#[cfg(not(all(
+feature = "proptest-support",
+feature = "compare",
+feature = "io",
+)))]
+compile_error!(
+    "Please enable the `proptest-support`, `compare` and `io` features in order to compile and run the tests.
+     Example: `cargo test -p nalgebra-sparse --features proptest-support,compare,io`"
+);
 
 mod unit_tests;
 

nalgebra-sparse/tests/unit_tests/mod.rs

@@ -3,7 +3,6 @@ mod convert_serial;
 mod coo;
 mod csc;
 mod csr;
-#[cfg(feature = "io")]
 mod matrix_market;
 mod ops;
 mod pattern;