feat(tvix/eval): implement trivial attribute set literals

Implements attribute set literals without nesting. Technically this
already supports dynamic key fragments (evaluating to strings), though
the only way to create these (interpolation) is not yet implemented.

However, creating simple attribute sets like `{ }`, or `{ a = 15; }`
or `{ a = 10 * 2; }` works.

Recursive attribute sets are not yet implemented as we do not have any
kind of scope access yet anyways.

This is implemented using a new instruction that creates an attribute
set with a given number of elements by popping key/value pairs off the
stack.

Change-Id: I0f9aac7a131a112d3f66b131297686b38aaeddf2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6091
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/compiler.rs

@@ -4,9 +4,10 @@
 use crate::chunk::Chunk;
 use crate::errors::EvalResult;
 use crate::opcode::OpCode;
-use crate::value::Value;
+use crate::value::{NixString, Value};
+
 use rnix;
-use rnix::types::{TokenWrapper, TypedNode, Wrapper};
+use rnix::types::{EntryHolder, TokenWrapper, TypedNode, Wrapper};
 
 struct Compiler {
     chunk: Chunk,
@@ -46,6 +47,11 @@ impl Compiler {
                 self.compile_ident(node)
             }
 
+            rnix::SyntaxKind::NODE_ATTR_SET => {
+                let node = rnix::types::AttrSet::cast(node).unwrap();
+                self.compile_attr_set(node)
+            }
+
             kind => {
                 println!("visiting unsupported node: {:?}", kind);
                 Ok(())
@@ -125,6 +131,65 @@ impl Compiler {
 
         Ok(())
     }
+
+    // Compile attribute set literals into equivalent bytecode.
+    //
+    // This is complicated by a number of features specific to Nix
+    // attribute sets, most importantly:
+    //
+    // 1. Keys can be dynamically constructed through interpolation.
+    // 2. Keys can refer to nested attribute sets.
+    // 3. Attribute sets can (optionally) be recursive.
+    fn compile_attr_set(&mut self, node: rnix::types::AttrSet) -> EvalResult<()> {
+        let mut count = 0;
+
+        for kv in node.entries() {
+            count += 1;
+
+            // Because attribute set literals can contain nested keys,
+            // there is potentially more than one key fragment. If
+            // this is the case, a special operation to construct a
+            // runtime value representing the attribute path is
+            // emitted.
+            let mut key_count = 0;
+            for fragment in kv.key().unwrap().path() {
+                key_count += 1;
+
+                match fragment.kind() {
+                    rnix::SyntaxKind::NODE_IDENT => {
+                        let ident = rnix::types::Ident::cast(fragment).unwrap();
+
+                        // TODO(tazjin): intern!
+                        let idx = self
+                            .chunk
+                            .add_constant(Value::String(NixString(ident.as_str().to_string())));
+                        self.chunk.add_op(OpCode::OpConstant(idx));
+                    }
+
+                    // For all other expression types, we simply
+                    // compile them as normal. The operation should
+                    // result in a string value, which is checked at
+                    // runtime on construction.
+                    _ => self.compile(fragment)?,
+                }
+            }
+
+            // We're done with the key if there was only one fragment,
+            // otherwise we need to emit an instruction to construct
+            // the attribute path.
+            if key_count > 1 {
+                todo!("emit OpAttrPath(n) instruction")
+            }
+
+            // The value is just compiled as normal so that its
+            // resulting value is on the stack when the attribute set
+            // is constructed at runtime.
+            self.compile(kv.value().unwrap())?;
+        }
+
+        self.chunk.add_op(OpCode::OpAttrs(count));
+        Ok(())
+    }
 }
 
 pub fn compile(ast: rnix::AST) -> EvalResult<Chunk> {

tvix/eval/src/opcode.rs

@@ -29,4 +29,7 @@ pub enum OpCode {
 
     // Logical binary operators
     OpEqual,
+
+    // Attribute sets
+    OpAttrs(usize),
 }

tvix/eval/src/vm.rs

@@ -1,11 +1,13 @@
 //! This module implements the virtual (or abstract) machine that runs
 //! Tvix bytecode.
 
+use std::{collections::BTreeMap, rc::Rc};
+
 use crate::{
     chunk::Chunk,
     errors::{Error, EvalResult},
     opcode::OpCode,
-    value::Value,
+    value::{NixAttrs, NixString, Value},
 };
 
 pub struct VM {
@@ -114,6 +116,7 @@ impl VM {
                 OpCode::OpNull => self.push(Value::Null),
                 OpCode::OpTrue => self.push(Value::Bool(true)),
                 OpCode::OpFalse => self.push(Value::Bool(false)),
+                OpCode::OpAttrs(count) => self.run_attrset(count)?,
             }
 
             if self.ip == self.chunk.code.len() {
@@ -121,6 +124,20 @@ impl VM {
             }
         }
     }
+
+    fn run_attrset(&mut self, count: usize) -> EvalResult<()> {
+        let mut attrs: BTreeMap<NixString, Value> = BTreeMap::new();
+
+        for _ in 0..count {
+            let value = self.pop();
+            let key = self.pop().as_string()?; // TODO(tazjin): attrpath
+            attrs.insert(key, value);
+        }
+        // TODO(tazjin): extend_reserve(count) (rust#72631)
+
+        self.push(Value::Attrs(Rc::new(NixAttrs::Map(attrs))));
+        Ok(())
+    }
 }
 
 #[derive(Clone, Copy, Debug, PartialEq)]