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# Error code E0463

A crate was declared but cannot be found.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    extern crate foo; // error: can't find crate
    }

You need to link your code to the relevant crate in order to be able to use it (through Cargo or the `-L` option of rustc, for example).

## Common causes

- The crate is not present at all. If using Cargo, add it to `[dependencies]` in Cargo.toml.
- The crate is present, but under a different name. If using Cargo, look for `package =` under `[dependencies]` in Cargo.toml.

## Common causes for missing `std` or `core`

- You are cross-compiling for a target which doesn't have `std` prepackaged. Consider one of the following:
  - Adding a pre-compiled version of std with `rustup target add`
  - Building std from source with `cargo build -Z build-std`
  - Using `#![no_std]` at the crate root, so you won't need `std` in the first place.
- You are developing the compiler itself and haven't built libstd from source. You can usually build it with `x.py build library/std`. More information about x.py is available in the rustc-dev-guide.