A crate was declared but cannot be found.

Erroneous code example:

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.