README: document cfg_attr for cases (#236)

* README: document cfg_attr for cases

Fixes #234.

* rstest_macros: document cfg_attr for cases

README.md

@@ -117,6 +117,25 @@ fn it_works(#[case] a: u32, #[case] b: u32) {
 
 See [`rstest_reuse`][reuse-crate-link] for more details.
 
+#### Feature flagged cases
+
+In case you want certain test cases to only be present if a certain feature is
+enabled, use `#[cfg_attr(feature = …, case(…))]`:
+
+```rust
+use rstest::rstest;
+
+#[rstest]
+#[case(2, 2)]
+#[cfg_attr(feature = "frac", case(4/2, 2))]
+#[case(4/2, 2)]
+fn it_works(#[case] a: u32, #[case] b: u32) {
+    assert!(a == b);
+}
+```
+
+This also works with [`rstest_reuse`][reuse-crate-link].
+
 ### Magic Conversion
 
 If you need a value where its type implement `FromStr()` trait you can use a literal

rstest_macros/src/lib.rs

@@ -483,6 +483,24 @@ pub fn fixture(
 ///     assert_eq!(s.as_ref().len(), len);
 /// }
 /// ```
+/// ### Feature flagged cases
+///
+/// In case you want certain test cases to only be present if a certain feature is
+/// enabled, use `#[cfg_attr(feature = …, case(…))]`:
+///
+/// ```
+/// use rstest::rstest;
+///
+/// #[rstest]
+/// #[case(2, 2)]
+/// #[cfg_attr(feature = "frac", case(4/2, 2))]
+/// #[case(4/2, 2)]
+/// fn it_works(#[case] a: u32, #[case] b: u32) {
+///     assert!(a == b);
+/// }
+/// ```
+///
+/// This also works with [`rstest_reuse`](https://crates.io/crates/rstest_reuse).
 ///
 /// ### Magic Conversion
 ///