feat(tvix/eval): Support builtins.length

Get the length of a list

Change-Id: I41d91e96d833269541a1b3c23b7cc879f96d1e5a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6407
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -45,6 +45,9 @@ fn pure_builtins() -> Vec<Builtin> {
             let a = args.pop().unwrap();
             Ok(arithmetic_op!(a, b, /))
         }),
+        Builtin::new("length", 1, |args| {
+            Ok(Value::Integer(args[0].as_list()?.len() as i64))
+        }),
         Builtin::new("isAttrs", 1, |args| {
             Ok(Value::Bool(matches!(args[0], Value::Attrs(_))))
         }),

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length.exp

@@ -0,0 +1 @@
+[ 0 1 3 ]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-length.nix

@@ -0,0 +1,5 @@
+[
+  (builtins.length [])
+  (builtins.length [ 1 ])
+  (builtins.length [ "one" "two" "three" ])
+]

tvix/eval/src/value/list.rs

@@ -28,6 +28,10 @@ impl NixList {
         lhs
     }
 
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
     pub fn construct(count: usize, stack_slice: Vec<Value>) -> Self {
         debug_assert!(
             count == stack_slice.len(),

tvix/eval/src/value/mod.rs

@@ -103,6 +103,17 @@ impl Value {
         }
     }
 
+    pub fn as_list(&self) -> EvalResult<&NixList> {
+        match self {
+            Value::List(xs) => Ok(xs),
+            other => Err(ErrorKind::TypeError {
+                expected: "list",
+                actual: other.type_of(),
+            }
+            .into()),
+        }
+    }
+
     pub fn to_string(self) -> EvalResult<NixString> {
         match self {
             Value::String(s) => Ok(s),