## Source Replacement

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 crate 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.