feat(tvix/eval): Implement builtins.listToAttrs

Implement the listToAttrs builtin, which constructs an attribute set
from a list of attribute sets with keys name and value.

This is tested using an adaptation of the nix `eval-ok-listtoattrs.nix`,
with the utilities from `lib.nix` inlined.

Change-Id: Ib5bf743466dda9722c2c1e00797df4b58448cf0f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6894
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/builtins/mod.rs

@@ -294,6 +294,23 @@ fn pure_builtins() -> Vec<Builtin> {
             let value = args[0].force(vm)?;
             Ok(Value::Bool(matches!(*value, Value::String(_))))
         }),
+        Builtin::new("listToAttrs", &[true], |args: Vec<Value>, vm: &mut VM| {
+            let list = args[0].to_list()?;
+            let mut map = BTreeMap::new();
+            for val in list {
+                let attrs = val.force(vm)?.to_attrs()?;
+                let get = |key| {
+                    attrs
+                        .select(key)
+                        .ok_or(ErrorKind::AttributeNotFound { name: key.into() })
+                };
+                let name = get("name")?.to_str()?;
+                let value = get("value")?.clone();
+                // Map entries earlier in the list take precedence over entries later in the list
+                map.entry(name).or_insert(value);
+            }
+            Ok(Value::attrs(NixAttrs::from_map(map)))
+        }),
         Builtin::new(
             "mul",
             &[false, false],

tvix/eval/src/tests/tvix_tests/eval-okay-listtoattrs.exp

@@ -0,0 +1 @@
+"AAbar"

tvix/eval/src/tests/tvix_tests/eval-okay-listtoattrs.nix

@@ -0,0 +1,15 @@
+with builtins;
+let
+  fold = op: nul: list:
+    if list == []
+    then nul
+    else op (head list) (fold op nul (tail list));
+  concat =
+    fold (x: y: x + y) "";
+  asi = name: value : { inherit name value; };
+  list = [ ( asi "a" "A" ) ( asi "b" "B" ) ];
+  a = builtins.listToAttrs list;
+  b = builtins.listToAttrs ( list ++ list );
+  r = builtins.listToAttrs [ (asi "result" [ a b ]) ( asi "throw" (throw "this should not be thrown")) ];
+  x = builtins.listToAttrs [ (asi "foo" "bar") (asi "foo" "bla") ];
+in concat (map (x: x.a) r.result) + x.foo