feat(tvix/eval): builtins.storeDir

Returns the store directory through EvalIO::store_dir.

Note that this is _optional_ in Tvix, as an evaluation can occur in a
context where there simply is no store directory. In those contexts,
`builtins.storeDir` returns `null` in Tvix.

This would only happen in contexts like Tvixbolt (or completely
unrelated use-cases) in practice.

Co-Authored-By: Vincent Ambo <tazjin@tvl.su>
Change-Id: I5a752c7e89b2f75bd7efb082dbfa5b25e3b1ff3b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7452
Autosubmit: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/builtins/impure.rs

@@ -4,7 +4,7 @@ use smol_str::SmolStr;
 use std::{
     collections::BTreeMap,
     env,
-    rc::Weak,
+    rc::{Rc, Weak},
     time::{SystemTime, UNIX_EPOCH},
 };
 
@@ -74,6 +74,16 @@ pub(super) fn builtins() -> BTreeMap<&'static str, Value> {
         .map(|b| (b.name(), Value::Builtin(b)))
         .collect();
 
+    map.insert(
+        "storeDir",
+        Value::Thunk(Thunk::new_suspended_native(Rc::new(Box::new(
+            |vm: &mut VM| match vm.io().store_dir() {
+                None => Ok(Value::Null),
+                Some(dir) => Ok(Value::String(dir.into())),
+            },
+        )))),
+    );
+
     // currentTime pins the time at which evaluation was started
     {
         let seconds = match SystemTime::now().duration_since(UNIX_EPOCH) {