emit compile error when none of std and hashbrown is enabled

Author: CrazyboyQCD

src/allocator/mod.rs

@@ -10,9 +10,7 @@ use crate::result::*;
 pub(crate) mod dedicated_block_allocator;
 pub(crate) use dedicated_block_allocator::DedicatedBlockAllocator;
 
-#[cfg(any(feature = "std", feature = "hashbrown"))]
 pub(crate) mod free_list_allocator;
-#[cfg(any(feature = "std", feature = "hashbrown"))]
 pub(crate) use free_list_allocator::FreeListAllocator;
 
 #[derive(PartialEq, Copy, Clone, Debug)]

src/lib.rs

@@ -225,6 +225,9 @@ extern crate alloc;
 #[cfg(all(not(feature = "std"), feature = "visualizer"))]
 compile_error!("Cannot enable \"visualizer\" feature in \"no_std\" environment.");
 
+#[cfg(not(any(feature = "std", feature = "hashbrown")))]
+compile_error!("Either `std` or `hashbrown` feature must be enabled");
+
 mod result;
 pub use result::*;