Source Replacement - The Cargo Book

## Source Replacement

This document is about replacing the crate index. You can read about overriding dependencies in the overriding dependencies section of this documentation.

Cargo supports the ability to replace one source with another to express strategies along the lines of mirrors or vendoring dependencies. Configuration is currently done through the `.cargo/config` configuration mechanism, like so:

    # The `source` table is where all keys related to source-replacement
    # are stored.
    [source]

    # Under the `source` table are a number of other tables whose keys are a
    # name for the relevant source. For example this section defines a new
    # source, called `my-awesome-source`, which comes from a directory
    # located at `vendor` relative to the directory containing this `.cargo/config`
    # file
    [source.my-awesome-source]
    directory = "vendor"

    # Git sources can optionally specify a branch/tag/rev as well
    git = "https://example.com/path/to/repo"
    # branch = "master"
    # tag = "v1.0.1"
    # rev = "313f44e8"

    # The crates.io default source for crates is available under the name
    # "crates-io", and here we use the `replace-with` key to indicate that it's
    # replaced with our source above.
    [source.crates-io]
    replace-with = "my-awesome-source"

With this configuration Cargo attempts to look up all crates in the directory "vendor" rather than querying the online registry at crates.io. Using source replacement Cargo can express:

- Vendoring - custom sources can be defined which represent crates on the local filesystem. These sources are subsets of the source that they're replacing and can be checked into projects if necessary.

- Mirroring - sources can be replaced with an equivalent version which acts as a cache for crates.io itself.

Cargo has a core assumption about source replacement that the source code is exactly the same from both sources. In our above example Cargo assumes that all of the crates coming from `my-awesome-source` are the exact same as the copies from `crates-io`. Note that this also means that `my-awesome-source` is not allowed to have crates which are not present in the `crates-io` source.

As a consequence, source replacement is not appropriate for situations such as patching a dependency or a private registry. Cargo supports patching dependencies through the usage of the `[replace]` key, and private registry support is planned for a future version of Cargo.

### Configuration

Configuration of replacement sources is done through `.cargo/config` and the full set of available keys are:

    # Each source has its own table where the key is the name of the source
    [source.the-source-name]

    # Indicate that `the-source-name` will be replaced with `another-source`,
    # defined elsewhere
    replace-with = "another-source"

    # Available kinds of sources that can be specified (described below)
    registry = "https://example.com/path/to/index"
    local-registry = "path/to/registry"
    directory = "path/to/vendor"

The `crates-io` represents the crates.io online registry (default source of crates) and can be replaced with:

    [source.crates-io]
    replace-with = 'another-source'

### Registry Sources

A "registry source" is one that is the same as crates.io itself. That is, it has an index served in a git repository which matches the format of the crates.io index. That repository then has configuration indicating where to download crates from.

Currently there is not an already-available project for setting up a mirror of crates.io. Stay tuned though!

### Local Registry Sources

A "local registry source" is intended to be a subset of another registry source, but available on the local filesystem (aka vendoring). Local registries are downloaded ahead of time, typically sync'd with a `Cargo.lock`, and are made up of a set of `*.crate` files and an index like the normal registry is.

The primary way to manage and create local registry sources is through the `cargo-local-registry` subcommand, available on crates.io and can be installed with `cargo install cargo-local-registry`.

Local registries are contained within one directory and contain a number of `*.crate` files downloaded from crates.io as well as an `index` directory with the same format as the crates.io-index project (populated with just entries for the crates that are present).

### Directory Sources

A "directory source" is similar to a local registry source where it contains a number of crates available on the local filesystem, suitable for vendoring dependencies. Also like local registries, directory sources can primarily be managed by an external subcommand, `cargo-vendor`, which can be installed with `cargo install cargo-vendor`.

Directory sources are distinct from local registries though in that they contain the unpacked version of `*.crate` files, making it more suitable in some situations to check everything into source control. A directory source is just a directory containing a number of other directories which contain the source code for crates (the unpacked version of `*.crate` files). Currently no restriction is placed on the name of each directory.

Each crate in a directory source also has an associated metadata file indicating the checksum of each file in the crate to protect against accidental modifications.