Add support to Cargo for alternative registries

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+- Feature Name: cargo_alternative_registries
+- Start Date: 2017-09-06
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+This RFC proposes the addition of the support for alternative crates.io servers to be used
+alongside the public crates.io server. This would allow users to publish crates to their own
+private instance of crates.io, while still able to use the public instance of crates.io.
+
+# Motivation
+[motivation]: #motivation
+
+Cargo currently has support for getting crates from a public server, which works well for open
+source projects using Rust, however is problematic for closed source code. A workaround for this is
+to use Git repositories to specify the packages, but that means that the helpful versioning and
+discoverability that Cargo and crates.io provides is lost. We would like to change this such that
+it is possible to have a local crates.io server which crates can be pushed to, while still making
+use of the public crates.io server.
+
+We would also like to support the use of crates.io mirrors. These differ from alternative
+registries in that a mirror completely replicates the functionality and content of crates.io. A
+mirror would be useful if we ever need a fallback for when crates.io goes down, or in areas of the
+world where crates.io is blocked.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+## Registry definition specification
+[registry-definition-specification]: #registry-definition-specification
+
+We need a way to define what registries are valid for Cargo to pull from and publish to. For this
+purpose, we propose that users would be able to define multiple registries in a [`.cargo/config`
+file](http://doc.crates.io/config.html). This allows the user to specify the locations of
+registries in one place, in a parent directory of all projects, rather than needing to configure
+the registry location within each project's `Cargo.toml`. Once a registry has been configured with
+a name, each `Cargo.toml` can use the registry name to refer to that registry.
+
+Another benefit of using `.cargo/config` is that these files are not typically checked in to the
+projects' source control. The registries might have credentials associated with them, which should
+not be checked in. Separating the URLs and the use of the URLs in this way encourages good security
+practices of not checking in credentials.
+
+In order to tell Cargo about a registry other than crates.io, you can specify and name it in a
+`.cargo/config` as follows:
+
+```toml
+[registry.$choose-a-name]
+index = "https://username:password@my-intranet:8080/index"
+```
+
+Instead of `$choose-a-name`, place the name you'd like to use to refer to this registry in your
+`Cargo.toml` files. The `index` key should contain the location of the registry index for this
+registry; the registry format is specified in the [Registry Index Format Specification
+section][registry-index-format-specification].
+
+### CI
+
+Because this system discourages checking in the registry configuration, the registry configuration
+won't be immediately available to continuous integration systems like TravisCI. However, Cargo
+currently supports configuring any key in `.cargo/config` using environment variables instead:
+
+> Cargo can also be configured through environment variables in addition to the TOML syntax above.
+> For each configuration key above of the form `foo.bar` the environment variable `CARGO_FOO_BAR`
+> can also be used to define the value. For example the build.jobs key can also be defined by
+> `CARGO_BUILD_JOBS`.
+
+To configure TravisCI to use an alternate registry named `my-registry` for example, you can use
+[Travis' encrypted environment variables feature](https://docs.travis-ci.com/user/environment-variables/#Defining-encrypted-variables-in-.travis.yml) to set:
+
+`CARGO_REGISTRY_MY_REGISTRY_INDEX=https://username:password@my-intranet:8080/index`
+
+## Using a dependency from another registry
+
+*Note: this syntax will initially be implemented as an [unstable cargo
+feature](https://github.com/rust-lang/cargo/pull/4433) available in nightly cargo only and
+stabilized as it becomes ready.*
+
+Once you've configured a registry (with a name, for example, `my-registry`) in `.cargo/config`, you
+can specify that a dependency comes from an alternate registry by using the `registry` key:
+
+```toml
+[dependencies]
+secret-crate = { version = "1.0", registry = "my-registry" }
+```
+
+## Publishing to another registry; preventing unwanted publishes
+
+In order to specify that a crate should only be published to a particular set of registries,
+specify in the `[package]` section the allowed registries using the `publish-registries` key and
+specifying the list of registries that are allowed with `cargo publish`.
+
+```
+publish-registries = ["my-registry"]
+```
+
+If you run `cargo publish` without specifying an `--index` argument pointing to an allowed
+registry, the command will fail. This prevents accidental publishes of private crates to crates.io,
+for example.
+
+## Running a minimal registry
+
+The most minimal form of a registry that Cargo can use will consist of:
+
+- A registry in the format specified in the [Registry index format specification
+  section][registry-index-format-specification], which contains a pointer to:
+- A location containing the `.crate` files for the crates in the registry.
+
+## Running a fully-featured registry
+
+This RFC does not attempt to standardize or specify any of crates.io's APIs, but it should be
+possible to take crates.io's codebase and run it along with a registry index in order to provide
+crates.io's functionality as an alternate registry.
+
+## Crates.io
+
+Because crates.io's purpose is to be a reliable host for open source crates, crates that have
+dependencies from registries other than crates.io will be rejected at publish time. Crates.io
+cannot make availability guarantees about alternate registries, so much like git dependencies
+today, publishing with dependencies from other registries won't be allowed.
+
+In crates.io's codebase, we will add a configuration option that specifies a list of approved
+alternate registry locations that dependencies may use. For private registries run using
+crates.io's code, this will likely include the private registry itself plus crates.io, so that
+private crates are allowed to depend on open source crates. Any crates with dependencies from
+registries not specified in this configuration option will be rejected at publish time.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+## Related issues
+
+In order to make working with multiple registries more convenient, we would also like to support:
+
+- [Being able to specify the API host rather than the index
+  location](https://github.com/rust-lang/cargo/issues/4208), so that, for example, you could
+  specify `https://crates.io` rather than `https://github.com/rust-lang/crates.io-index`. We do not
+  want to *require* specifying the API host, since some registries will choose not to have an API
+  host at all and only supply an index and a location for crate files. This would require the API
+  to have a way to tell Cargo where the associated registry index is located.
+- [Being able to save multiple tokens in
+  `.cargo/credentials`](https://github.com/rust-lang/cargo/issues/3365), one per registry, so that
+  people publishing to multiple registries don't need to log in over and over or specify tokens on
+  every publish.
+- Being able to specify `--registry registry-name` for all Cargo commands that currently take
+  `--index`
+- Being able to use a dependency under a different name. Alternate registries that are not mirrors
+  should be allowed to have crates with the same name as crates in any other registry, including
+  crates.io. In order to allow a crate to depend on both, say, the `http` crate from crates.io and
+  the `http` crate from a private registry, at least one will need to be renamed when listed as a
+  dependency in `Cargo.toml`. [RFC
+  2126](https://github.com/aturon/rfcs/blob/path-clarity/text/0000-path-clarity.md#basic-changes)
+  proposes this change as follows:
+
+  > Cargo will provide a new crate key for aliasing dependencies, so that e.g. users who want to
+  > use the `rand` crate but call it `random` instead can now write `random = { version = "0.3",
+  > crate = "rand" }`.
+
+## Registry index format specification
+[registry-index-format-specification]: #registry-index-format-specification
+
+Cargo needs to be able to get a registry index containing metadata for all crates and their
+dependencies available from an alternate registry in order to perform offline version resolution.
+The registry index for crates.io is available at
+[https://github.com/rust-lang/crates.io-index](https://github.com/rust-lang/crates.io-index), and
+this section aims to specify the format of this registry index so that other registries can provide
+their own registry index that Cargo will understand.
+
+This is version 1 of the registry index format specification. There may be other versions of the
+specification someday. Along with a new specification version will be a plan for supporting
+registries using the older specification and a migration plan for registries to upgrade the
+specification version their index is using.
+
+A valid registry index meets the following criteria:
+
+- The registry index is stored in a git repository so that Cargo can efficiently fetch incremental
+  updates to the index.
+- There will be a file at the top level named `config.json`. This file will be a valid JSON object
+  with the following keys:
+
+  ```json
+  {
+    "dl": "https://my-crates-server.com/api/v1/crates",
+    "api": "https://my-crates-server.com/",
+    "allowed-registries": ["https://crates.io", "https://my-other-crates-server.com"]
+  }
+  ```
+
+  The `dl` key is required specifies where Cargo can download the tarballs containing the source
+  files of the crates listed in the registry.
+
+  The `api` key is optional and specifies where Cargo can find the API server that provides the
+  same API functionality that crates.io does today, such as publishing and searching. Without the
+  `api` key, these features will not be available. This RFC is not attempting to standardize
+  crates.io's API in any way, although that could be a future enhancement.
+
+  The `allowed-registries` key is optional and specifies the other registries that crates in this
+  index are allowed to have dependencies on. The default will be nothing, which will mean only
+  crates that depend on other crates in the current registry are allowed. This is currently the
+  case for crates.io and will remain the case for crates.io going forward. Alternate registries
+  will probably want to add crates.io to this list.
+
+- There will be a number of directories in the git repository.
+  - `1/` - holds files for all crates whose names have one letter.
+  - `2/` - holds files for all crates whose names have two letters.
+  - `3/` - holds files for all crates whose names have three letters.
+  - `aa/aa/` etc - for all crates whose names have four or more letters, their
+    files will be in a directory named with the first and second letters of
+    their name, then in a subdirectory named with the third and fourth letters
+    of their name. For example, a file for a crate named `sample` would be
+    found in `sa/mp/`.
+
+- For each crate in the registry, there will be a file with the name of that crate in the directory
+  structure as specified above. The file will contain metadata about each version of the crate,
+  with one version per line. Each line will be valid JSON with, minimally, the keys as shown. More
+  keys may be added, but Cargo may ignore them. The contents of one line are pretty-printed here
+  for readability.
+
+  ```json
+  {
+      "name": "my_serde",
+      "vers": "1.0.11",
+      "deps": [
+          {
+              "name": "serde",
+              "req": "^1.0",
+              "registry": "https://crates.io",
+              "features": [],
+              "optional": true,
+              "default_features": true,
+              "target": null,
+              "kind": "normal"
+          }
+      ],
+      "cksum": "f7726f29ddf9731b17ff113c461e362c381d9d69433f79de4f3dd572488823e9",
+      "features": {
+          "default": [
+              "std"
+          ],
+          "derive": [
+              "serde_derive"
+          ],
+          "std": [
+
+          ],
+      },
+      "yanked": false
+  }
+  ```
+
+  The top-level keys for a crate are:
+
+    - `name`: the name of the crate
+    - `vers`: the version of the crate this row is describing
+    - `deps`: a list of all dependencies of this crate
+    - `cksum`: a checksum of this version's files
+    - `features`: a list of the features available from this crate
+    - `yanked`: whether or not this version has been yanked
+
+  Within the `deps` list, each dependency should be listed as an item in the `deps` array with the
+  following keys:
+
+    - `name`: the name of the dependency
+    - `req`: the semver version requirement string on this dependency
+    - `registry`: **New to this RFC: the registry from which this crate is available**
+    - `features`: a list of the features available from this crate
+    - `optional`: whether this dependency is optional or not
+    - `default_features`: whether the parent uses the default features of this dependency or not
+    - `target`: on which target this dependency is needed
+    - `kind`: can be `normal`, `build`, or `dev` to be a regular dependency, a build-time
+      dependency, or a development dependency
+
+If a dependency's registry is not specified, Cargo will assume the dependency can be located in the
+current registry. By specifying the registry of a dependency in the index, cargo will have the
+information it needs to fetch crate files from the registry indices involved without needing to
+involve an API server.
+
+## New command: `cargo generate-index-metadata`
+
+Currently, the knowledge of how to create a file in the registry index format is spread between
+Cargo and crates.io. This RFC proposes the addition of a Cargo command that would generate this
+file locally for the current crate so that it can be added to the git repository using a mechanism
+other than a server running crates.io's codebase.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Supporting alternative registries, and having multiple public registries, could fracture the
+ecosystem. However, we feel that supporting private registries, and the Rust adoption that could
+enable, outweighs the potential downsides of having multiple public registries.
+
+# Rationale and Alternatives
+[alternatives]: #alternatives
+
+A [previous RFC](https://github.com/rust-lang/rfcs/pull/2006) proposed having the registry
+information completely defined within `Cargo.toml` rather than using `.cargo/config`. This requires
+repeating the same information multiple times for multiple projects, and encourages checking in
+credentials that might be needed to access the registries. That RFC also didn't specify the format
+for the registry index, which needs to be shared among all registries.
+
+An alternative design could be to support specifying the registry URL in either `.cargo/config` or
+`Cargo.toml`. This has the downsides of creating more choices for the user and potentially
+encouraging poor practices such as checking credentials into a project's source control. The
+implementation of this feature would also be more complex. The upside would be supporting
+configuration in ways that would be more convenient in various situations.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+- Are the names of everything what we want?
+  - `cargo generate-index-metadata`?
+  - `registry = my-registry`?
+  - `publish-registries = []`?