message: fix(compiler)  `std` and `core` are mutually exclusive features.
diff: )
  )]
  -#[cfg(all(not(feature = "std"), feature = "core"))]
  +#[cfg(all(feature = "std", feature = "core"))]
  +compile_error!(
  + "The `std` and `core` features are both enabled, which is an error. Please enable only once."
  +);
  +
  +#[cfg(all(not(feature = "std"), not(feature = "core")))]
  +compile_error!("Both the `std` and `core` features are disabled. Please enable one of them.");
  +
  +#[cfg(feature = "core")]
  extern crate alloc;
  mod lib {
  - #[cfg(all(not(feature = "std"), feature = "core"))]
  + #[cfg(feature = "core")]
  pub mod std {
  #[macro_use]
  pub use alloc::{boxed, string, vec};
  