feat(tvix/eval): implement tvix's user-agent, err, nixVersion r/5038

nixpkgs has hardcoded references to Nix versions, we need to provide
it with something that looks like a Nix version while actually being a
Tvix version.

For now, we do this by stealing a trick out of the browser book and
constructing a version that looks like a Nix version to Nix, but like
a Tvix version to people who know what they are looking for.

Nevermind that we don't actually have any kind of versioning for
Tvix (yet?), other than depot revisions.

Change-Id: I7ce8079dd8164a2079891d38e707f09a45f0bbc1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6858
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

1 files changed, 6 insertions, 0 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 848ac9e25d..61ff60fa05 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -374,6 +374,12 @@ fn pure_builtins() -> Vec<Builtin> {
 fn builtins_set() -> NixAttrs {
     let mut map: BTreeMap<NixString, Value> = BTreeMap::new();
 
+    // Pure-value builtins
+    map.insert(
+        "nixVersion".into(),
+        Value::String("2.3-compat-tvix-0.1".into()),
+    );
+
     let mut add_builtins = |builtins: Vec<Builtin>| {
         for builtin in builtins {
             map.insert(builtin.name().into(), Value::Builtin(builtin));