feat(tvix/eval): Support builtins.getAttr r/4928

Support looking-up values from attrsets by their keys.

Change-Id: Ib37a472a511dab145f99ebc849879b3494e8e89f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6554
Reviewed-by: wpcarro <wpcarro@gmail.com>
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

3 files changed, 18 insertions, 0 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index f6af16f96f9d..9714c96d05d8 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -131,6 +131,17 @@ fn pure_builtins() -> Vec<Builtin> {
                 }
             }
         }),
+        Builtin::new("getAttr", &[true, true], |args, _| {
+            let k = args[0].to_str()?;
+            let xs = args[1].to_attrs()?;
+
+            match xs.select(k.as_str()) {
+                Some(x) => Ok(x.clone()),
+                None => Err(ErrorKind::AttributeNotFound {
+                    name: k.to_string(),
+                }),
+            }
+        }),
         Builtin::new("length", &[true], |args, _| {
             Ok(Value::Integer(args[0].to_list()?.len() as i64))
         }),
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.exp
new file mode 100644
index 000000000000..89fa6c681064
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.exp
@@ -0,0 +1 @@
+[ 1 2 3 { bar = { baz = 3; }; } ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.nix
new file mode 100644
index 000000000000..87a2adbcd3dd
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-getattr.nix
@@ -0,0 +1,6 @@
+[
+  (builtins.getAttr "foo" { foo = 1; bar = 2; baz = 3; })
+  (builtins.getAttr "bar" { foo = 1; bar = 2; baz = 3; })
+  (builtins.getAttr "baz" { foo = 1; bar = 2; baz = 3; })
+  (builtins.getAttr "foo" { foo = { bar = { baz = 3; }; }; })
+]