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, 32 insertions, 0 deletions

diff --git a/tvix/Cargo.toml b/tvix/Cargo.toml
new file mode 100644
index 000000000000..4cc70f9040a6
--- /dev/null
+++ b/tvix/Cargo.toml
@@ -0,0 +1,32 @@
+# This Cargo file is a workspace configuration as per
+# https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html
+#
+# We add this file to get a coherent set of dependencies across Tvix
+# crates by sharing a Cargo.lock. This is necessary because of the
+# currently limited support for Rust builds in Nix.
+#
+# Note that this explicitly does *not* mean that //tvix should be
+# considered "one project": This is simply a workaround for a
+# technical limitation and it should be our aim to remove this
+# workspace file and make the subprojects independent.
+#
+# Note also that CI targets for actual projects should *not* be tied
+# to //tvix, but to its subprojects. A potential file at
+# //tvix/default.nix should likely *not* expose anything other than
+# extra steps or other auxiliary targets.
+
+[workspace]
+
+members = [
+  "eval",
+  "eval/builtin-macros",
+  "nix_cli",
+  "store",
+]
+
+# Add a profile to all targets that enables release optimisations, but
+# retains debug symbols. This is great for use with
+# benchmarking/profiling tools.
+[profile.release-with-debug]
+inherits = "release"
+debug = true