Reorganize some code and add comments.

Author: sunfishcode

src/ffi/mod.rs

@@ -1,11 +1,16 @@
 //! Utilities related to FFI bindings.
 
-#[cfg(not(feature = "std"))]
-pub use alloc::ffi::{CString, NulError};
-#[cfg(not(feature = "std"))]
-pub use core::ffi::{c_char, CStr, FromBytesWithNulError};
-
+// If we have std, use it.
 #[cfg(feature = "std")]
-pub use std::ffi::{CStr, CString, FromBytesWithNulError, NulError};
-#[cfg(feature = "std")]
-pub use std::os::raw::c_char;
+pub use {
+    std::ffi::{CStr, CString, FromBytesWithNulError, NulError},
+    std::os::raw::c_char,
+};
+
+// If we don't have std, we can depend on core and alloc having these features
+// in new versions of Rust.
+#[cfg(not(feature = "std"))]
+pub use {
+    alloc::ffi::{CString, NulError},
+    core::ffi::{c_char, CStr, FromBytesWithNulError},
+};