Update src/lib.rs

Co-authored-by: Tobin C. Harding <[email protected]>

src/lib.rs

@@ -48,7 +48,7 @@ pub extern crate serde;
 use alloc::borrow::Cow;
 use core::{fmt, str};
 
-#[cfg(not(feature = "std"))]
+#[cfg(all(feature = "alloc", not(feature = "std")))]
 use alloc::{string::ToString, vec::Vec};
 
 /// We support a wide range of dependency versions for `rand` and `rand_core` and not