docs/manual/adding-packages-cargo.adoc

// -*- mode:doc; -*-
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=== Infrastructure for Cargo-based packages

Cargo is the package manager for the Rust programming language. It allows the
user to build programs or libraries written in Rust, but it also downloads and
manages their dependencies, to ensure repeatable builds. Cargo packages are
called "crates".

[[cargo-package-tutorial]]

==== +cargo-package+ tutorial

The +Config.in+ file of Cargo-based package 'foo' should contain:

----
01: config BR2_PACKAGE_FOO
02: 	bool "foo"
03: 	depends on BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS
04: 	select BR2_PACKAGE_HOST_RUSTC
05: 	help
06: 	  This is a comment that explains what foo is.
07:
08: 	  http://foosoftware.org/foo/
----

And the +.mk+ file for this package should contain:

----
01: ################################################################################
02: #
03: # foo
04: #
05: ################################################################################
06:
07: FOO_VERSION = 1.0
08: FOO_SOURCE = foo-$(FOO_VERSION).tar.gz
09: FOO_SITE = http://www.foosoftware.org/download
10: FOO_LICENSE = GPL-3.0+
11: FOO_LICENSE_FILES = COPYING
12:
13: $(eval $(cargo-package))
----

The Makefile starts with the definition of the standard variables for
package declaration (lines 7 to 11).

As seen in line 13, it is based on the +cargo-package+
infrastructure. Cargo will be invoked automatically by this
infrastructure to build and install the package.

It is still possible to define custom build commands or install
commands (i.e.  with FOO_BUILD_CMDS and FOO_INSTALL_TARGET_CMDS).
Those will then replace the commands from the cargo infrastructure.

==== +cargo-package+ reference

The main macros for the Cargo package infrastructure are
+cargo-package+ for target packages and +host-cargo-package+ for host
packages.

Just like the generic infrastructure, the Cargo infrastructure works
by defining a number of variables before calling the +cargo-package+
or +host-cargo-package+ macros.

All the package metadata information variables that exist in the
xref:generic-package-reference[generic package infrastructure] also
exist in the Cargo infrastructure.

A few additional variables, specific to the Cargo infrastructure, can
also be defined. Many of them are only useful in very specific cases,
typical packages will therefore only use a few of them.

* +FOO_SUBDIR+ may contain the name of a subdirectory inside the package
  that contains the Cargo.toml file. This is useful, if for example, it
  is not at the root of the tree extracted by the tarball. If
  +HOST_FOO_SUBDIR+ is not specified, it defaults to +FOO_SUBDIR+.

* +FOO_CARGO_ENV+ can be used to pass additional variables in the
  environment of +cargo+ invocations. It used at both build and
  installation time

* +FOO_CARGO_BUILD_OPTS+ can be used to pass additional options to
  +cargo+ at build time.

* +FOO_CARGO_INSTALL_OPTS+ can be used to pass additional options to
  +cargo+ at install time.

A crate can depend on other libraries from crates.io or git
repositories, listed in its +Cargo.toml+ file. Buildroot automatically
takes care of downloading such dependencies as part of the download
step of packages that use the +cargo-package+ infrastructure. Such
dependencies are then kept together with the package source code in
the tarball cached in Buildroot's +DL_DIR+, and therefore the hash of
the package's tarball doesn't only cover the source of the package
itself, but also covers the sources of the dependencies. Thus, a change
injected into one of the dependencies will also be discovered by the
hash check. In addition,  this mechanism allows the build to be
performed completely offline since cargo will not do any downloads
during the build. This mechanism is called vendoring the dependencies.