feat(tvix/eval): add helper for selecting required attributes r/5262

Change-Id: Idd4ae78ef55891d89b72b5c2f3afc8b697b4b26e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7189
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

1 files changed, 7 insertions, 0 deletions

diff --git a/tvix/eval/src/value/attrs.rs b/tvix/eval/src/value/attrs.rs
index 59e477db4a..d8e3001f4a 100644
--- a/tvix/eval/src/value/attrs.rs
+++ b/tvix/eval/src/value/attrs.rs
@@ -217,6 +217,13 @@ impl NixAttrs {
         self.0.select(key)
     }
 
+    /// Select a required value from an attribute set by key, return
+    /// an `AttributeNotFound` error if it is missing.
+    pub fn select_required(&self, key: &str) -> Result<&Value, ErrorKind> {
+        self.select(key)
+            .ok_or_else(|| ErrorKind::AttributeNotFound { name: key.into() })
+    }
+
     pub fn contains(&self, key: &str) -> bool {
         self.0.contains(key)
     }