feat(tvix/eval): implement builtins.dirOf

This commit causes the test eval-okay-builtins.nix to pass.

It also adds tests/tvix_tests/eval-okay-dirof.nix which has better
coverage than the nix tests for this builtin.

Signed-off-by: Adam Joseph <adam@westernsemico.com>
Change-Id: I71d96b48680696fd6e4fea3a9861742b35cfaa66
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6987
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -194,6 +194,21 @@ fn pure_builtins() -> Vec<Builtin> {
             &[false, false],
             |args: Vec<Value>, vm: &mut VM| arithmetic_op!(&*args[0].force(vm)?, &*args[1].force(vm)?, /),
         ),
+        Builtin::new("dirOf", &[true], |args: Vec<Value>, vm: &mut VM| {
+            let s = args[0].coerce_to_string(CoercionKind::Weak, vm)?;
+            let result = s
+                .rsplit_once('/')
+                .map(|(x, _)| match x {
+                    "" => "/",
+                    _ => x,
+                })
+                .unwrap_or(".");
+            if args[0].is_path() {
+                Ok(Value::Path(result.into()))
+            } else {
+                Ok(result.into())
+            }
+        }),
         Builtin::new("elem", &[true, true], |args: Vec<Value>, vm: &mut VM| {
             for val in args[1].to_list()? {
                 if val.nix_eq(&args[0], vm)? {