Re-export cfg_if macro from libstd and test

Author: gnzlbg

src/libstd/lib.rs

@@ -327,6 +327,10 @@ pub use core::{assert_eq, assert_ne, debug_assert, debug_assert_eq, debug_assert
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::{unreachable, unimplemented, write, writeln, r#try, todo};
 
+#[unstable(feature = "cfg_if", issue = "59442")]
+pub use core::cfg_if;
+
+
 #[allow(unused_imports)] // macros from `alloc` are not used on all platforms
 #[macro_use]
 extern crate alloc as alloc_crate;

src/libstd/tests/cfg_if.rs

@@ -0,0 +1,14 @@
+pub use std::cfg_if;
+
+cfg_if! {
+    if #[cfg(test)] {
+        fn foo() -> bool { true }
+    } else {
+        fn foo() -> bool { false }
+    }
+}
+
+#[test]
+fn cfg_if_test {
+    assert!(foo());
+}