feat(tvix/eval): construct internal attribute path representation r/4439

This is required for constructing nested attribute sets at runtime.

There'll be quite a lot of optimisation potential with this solution
eventually, if it should turn out to be a bottleneck.

This introduces a conceptual change, in that the `Value` enum is now
an enum representing "all runtime values" instead of "all Nix language
types". This makes sense in general, as this type will also contain
Chunk representations etc. which are not exposed to users.

Change-Id: Ic5f72b2a0965b146c6a451efad34c6a81ca1aad8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6103
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

4 files changed, 30 insertions, 1 deletions

diff --git a/tvix/eval/src/compiler.rs b/tvix/eval/src/compiler.rs
index 5da844448860..5b6f748dc72d 100644
--- a/tvix/eval/src/compiler.rs
+++ b/tvix/eval/src/compiler.rs
@@ -224,7 +224,7 @@ impl Compiler {
             // otherwise we need to emit an instruction to construct
             // the attribute path.
             if key_count > 1 {
-                todo!("emit OpAttrPath(n) instruction")
+                self.chunk.add_op(OpCode::OpAttrPath(2));
             }
 
             // The value is just compiled as normal so that its
diff --git a/tvix/eval/src/opcode.rs b/tvix/eval/src/opcode.rs
index 0af8f23fc79d..622a02ac85f8 100644
--- a/tvix/eval/src/opcode.rs
+++ b/tvix/eval/src/opcode.rs
@@ -32,6 +32,7 @@ pub enum OpCode {
 
     // Attribute sets
     OpAttrs(usize),
+    OpAttrPath(usize),
 
     // Lists
     OpList(usize),
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index d00fc56e2a73..8a95d00f416a 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -21,6 +21,10 @@ pub enum Value {
     String(NixString),
     Attrs(Rc<NixAttrs>),
     List(NixList),
+
+    // Internal values that, while they technically exist at runtime,
+    // are never returned to or created directly by users.
+    AttrPath(Vec<NixString>),
 }
 
 impl Value {
@@ -41,6 +45,9 @@ impl Value {
             Value::String(_) => "string",
             Value::Attrs(_) => "set",
             Value::List(_) => "list",
+
+            // Internal types
+            Value::AttrPath(_) => "internal",
         }
     }
 
@@ -76,6 +83,9 @@ impl Display for Value {
             Value::String(s) => s.fmt(f),
             Value::Attrs(attrs) => attrs.fmt(f),
             Value::List(list) => list.fmt(f),
+
+            // internal types
+            Value::AttrPath(_) => f.write_str("internal"),
         }
     }
 }
diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index d6d3de154c7f..58ef67cb5531 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -121,6 +121,7 @@ impl VM {
                 OpCode::OpTrue => self.push(Value::Bool(true)),
                 OpCode::OpFalse => self.push(Value::Bool(false)),
                 OpCode::OpAttrs(count) => self.run_attrset(count)?,
+                OpCode::OpAttrPath(count) => self.run_attr_path(count)?,
                 OpCode::OpList(count) => self.run_list(count)?,
                 OpCode::OpInterpolate(count) => self.run_interpolate(count)?,
             }
@@ -218,6 +219,23 @@ impl VM {
         Ok(())
     }
 
+    // Construct runtime representation of an attr path (essentially
+    // just a list of strings).
+    //
+    // The difference to the list construction operation is that this
+    // forces all elements into strings, as attribute set keys are
+    // required to be strict in Nix.
+    fn run_attr_path(&mut self, count: usize) -> EvalResult<()> {
+        let mut path = vec![NixString(String::new()); count];
+
+        for idx in 0..count {
+            path[count - idx - 1] = self.pop().as_string()?
+        }
+
+        self.push(Value::AttrPath(path));
+        Ok(())
+    }
+
     // Construct runtime representation of a list. Because the list
     // items are on the stack in reverse order, the vector is created
     // initialised and elements are directly assigned to their