Add assert_one_feature! macro

Cargo.toml

@@ -24,5 +24,11 @@ members = ["proc"]
 nightly = []
 proc = ["proc_static_assertions"]
 
+# These are used for doctests in assert_one_feature
+feature-1 = []
+feature-2 = []
+
+default = ["feature-1", "feature-2"]
+
 [badges]
 maintenance = { status = "actively-maintained" }

src/assert_one_feature.rs

@@ -0,0 +1,39 @@
+/// Asserts that exactly one feature of a set is enabled.
+/// 
+/// ```compile_fail
+/// # #[macro_use] extern crate static_assertions; fn main() {}
+/// // Both feature-1 and feature-2 are set, so this fails
+/// 
+/// assert_one_feature!("feature-1","foo","bar","feature-2");
+/// ```
+/// 
+/// ```compile_fail
+/// # #[macro_use] extern crate static_assertions; fn main() {}
+/// // None of these features are set, so this fails
+/// 
+/// assert_one_feature!("foo", "bar", "baz");
+/// ```
+/// 
+/// ```
+/// # #[macro_use] extern crate static_assertions; fn main() {}
+/// // Exactly one of these features ("feature-1") is set, so this is fine
+/// 
+/// assert_one_feature!("foo", "bar", "baz", "qup", "feature-1");
+/// assert_one_feature!("foo", "bar", "baz", "qup", "feature-2");
+/// ```
+#[macro_export]
+macro_rules! assert_one_feature {
+    () => {};
+    ($($feature:literal),+) => {
+        #[cfg(not(any($(
+            feature = $feature,
+        )+)))]
+        compile_error!(concat!("Must have one of the following features: ", stringify!($($feature,)+)));
+        const _: () = {
+            $(
+                #[cfg(feature = $feature)]
+                const ASSERT_ONE_FEATURE:() = ();
+            )+
+        };
+    };
+}

src/lib.rs

@@ -116,6 +116,7 @@ mod assert_cfg;
 mod assert_fields;
 mod assert_impl;
 mod assert_obj_safe;
+mod assert_one_feature;
 mod assert_size;
 mod assert_trait;
 mod assert_type;