scripts: add script to help distros use global Rust packages

Some distros prefer to avoid vendored crate sources, and instead use
local sources from e.g. ``/usr/share/cargo/registry``.  Add a
script, inspired by the Mesa spec file(*), that automatically
performs this task.  The script is meant to be invoked after unpacking
the QEMU tarball.

(*) This is the hack that Mesa uses:

    export MESON_PACKAGE_CACHE_DIR="%{cargo_registry}/"
    %define inst_crate_nameversion() %(basename %{cargo_registry}/%{1}-*)
    %define rewrite_wrap_file() sed -e "/source.*/d" -e "s/%{1}-.*/%{inst_crate_nameversion %{1}}/" -i subprojects/%{1}.wrap
    %rewrite_wrap_file proc-macro2
    ... more %rewrite_wrap_file invocations follow ...

Reviewed-by: Neal Gompa <ngompa@fedoraproject.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/docs/about/build-platforms.rst b/docs/about/build-platforms.rst
index 8ecbd6b..8671c3b 100644
--- a/docs/about/build-platforms.rst
+++ b/docs/about/build-platforms.rst
@@ -127,6 +127,14 @@ Rust build dependencies
   (or newer) package.  The path to ``rustc`` and ``rustdoc`` must be
   provided manually to the configure script.
 
+  Some distros prefer to avoid vendored crate sources, and instead use
+  local sources from e.g. ``/usr/share/cargo/registry``.  QEMU includes a
+  script, ``scripts/get-wraps-from-cargo-registry.py``, that automatically
+  performs this task.  The script is meant to be invoked after unpacking
+  the QEMU tarball.  QEMU also includes ``rust/Cargo.toml`` and
+  ``rust/Cargo.lock`` files that can be used to compute QEMU's build
+  dependencies, e.g. using ``cargo2rpm -p rust/Cargo.toml buildrequires``.
+
 Optional build dependencies
   Build components whose absence does not affect the ability to build QEMU
   may not be available in distros, or may be too old for our requirements.