can't find crate for `core`

As an AI language model, I don't have access to your specific development environment or project. However, if you're having trouble finding a crate for `core`, it's possible that you need to add it to your `Cargo.toml` file as a dependency.

To add the `core` crate as a dependency, you can add the following line to your `Cargo.toml` file:

[dependencies]
core = "1.0.0"

After adding this line, you can run `cargo build` to download and build the `core` crate and any other dependencies you have specified in your `Cargo.toml` file.

If you continue to have issues, it's possible that there is a problem with your Rust installation or with your project structure. You may want to check the Rust documentation or forums for additional support.