refactor(tvix): share a Cargo.lock file between Rust projects r/5419

This relates to the (abandoned) cl/7256.

Introduces a Cargo workspace at //tvix that is primarily intended to
be used as a workaround for the annoying Nix+Rust tooling while having
a consistent set of dependencies.

This is driven in part by a desire to adopt crate2nix and get more
granular Nix builds for Tvix's Rust projects, and in part by a need to
split //tvix/eval into something providing the CLI (REPL etc.), and a
library providing eval, without significantly altering the structure
of build targets.

To accomplish this the workspace has been designed to allow projects
to remain independent build targets. I want to avoid lumping all the
projects together - something like //tvix/eval should always be
independent of other parts of tvix.

A helper function in //tvix/default.nix lets downstream naersk
projects construct a sparse root for the project which combines the
workspace's `Cargo.lock` with the project's own `Cargo.toml`.

Note that cargo commands in the workspace itself require the build
dependencies of _all_ projects to be present, which is currently a bit
annoying to accomplish.

This introduces some breakage:

1. It breaks usage of rust-analyser without being in a shell with the
   dependencies of *all* Tvix projects, as it is not capable of
   respecting only the subset of dependencies for a part of the
   workspace.

2. It is no longer possible to run tests using `cargo test`, as the
   test generation crate we use does not work with workspaces:

   https://github.com/frehberg/test-generator/issues/6

   This still works in the Nix build as we construct a Cargo project
   that looks like it's not in a workspace there. Until somebody fixes
   that crate / writes a new macro / does something else with the test
   suite, the way to run the tests is through the Nix build.

Long-term we'll probably want to get rid of cargo completely, it's
just a big wart and most tooling works without it if correctly
configured, but we don't have time for that now.

Change-Id: I846bff7a8429a25c077fd1e9ef4e3c34a299a4a1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7533
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 1 insertions, 1 deletions

diff --git a/tvix/eval/default.nix b/tvix/eval/default.nix
index 42722e5660..f7480e8268 100644
--- a/tvix/eval/default.nix
+++ b/tvix/eval/default.nix
@@ -3,7 +3,7 @@
 lib.fix (self: depot.third_party.naersk.buildPackage (lib.fix (naerskArgs: {
   src = depot.third_party.gitignoreSource ./.;
   # see https://github.com/nix-community/naersk/issues/169
-  root = depot.nix.sparseTree ./. [ ./Cargo.lock ./Cargo.toml ];
+  root = depot.tvix.naerskRootFor ./Cargo.toml;
 
   doCheck = true;