feat(tvix/eval): implement builtins.currentSystem r/5190

This commit implements builtins.currentSystem, by capturing the
cargo environment variable `TARGET` and exposing it to rustc as
`TVIX_CURRENT_SYSTEM` so it can be inserted into the source code
using `env!()`.

The resulting value needs to be massaged a bit, since it is an "LLVM
triple".  The current code should work for all the platforms for
which cppnix works (thanks qyliss for generating the list!).  It
does *not* reject all of the triples that cppnix's configure.ac
rejects -- it is much more forgiving.  We can tighten this up in a
future commit.

Signed-off-by: Adam Joseph <adam@westernsemico.com>
Change-Id: I947f504b2af5a7fee8cf0cb301421d2fc9174ce1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6986
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

1 files changed, 12 insertions, 0 deletions

diff --git a/tvix/eval/src/tests/nix_tests/eval-okay-builtins.nix b/tvix/eval/src/tests/nix_tests/eval-okay-builtins.nix
new file mode 100644
index 000000000000..e9d65e88a817
--- /dev/null
+++ b/tvix/eval/src/tests/nix_tests/eval-okay-builtins.nix
@@ -0,0 +1,12 @@
+assert builtins ? currentSystem;
+assert !builtins ? __currentSystem;
+
+let {
+
+  x = if builtins ? dirOf then builtins.dirOf /foo/bar else "";
+
+  y = if builtins ? fnord then builtins.fnord "foo" else "";
+
+  body = x + y;
+  
+}