feat(tvix/eval): implement attribute set access operator

Fairly straightforward, handling the optimised representations
manually and otherwise delegating to BTreeMap.

Note that parsing of raw identifiers is not yet implemented.

Encountering an identifier node usually means that there is locals
access going on, so we need a special case for compiling a node in
such a way that an identifier's literal value ends up on the stack.

Change-Id: I13fbab7ac657b17ef3f4c5859fe737c321890c8a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6158
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/compiler.rs

@@ -75,6 +75,11 @@ impl Compiler {
                 self.compile_attr_set(node)
             }
 
+            rnix::SyntaxKind::NODE_SELECT => {
+                let node = rnix::types::Select::cast(node).unwrap();
+                self.compile_select(node)
+            }
+
             rnix::SyntaxKind::NODE_LIST => {
                 let node = rnix::types::List::cast(node).unwrap();
                 self.compile_list(node)
@@ -85,10 +90,7 @@ impl Compiler {
                 self.compile_if_else(node)
             }
 
-            kind => {
-                println!("visiting unsupported node: {:?}", kind);
-                Ok(())
-            }
+            kind => panic!("visiting unsupported node: {:?}", kind),
         }
     }
 
@@ -282,6 +284,20 @@ impl Compiler {
         Ok(())
     }
 
+    fn compile_select(&mut self, node: rnix::types::Select) -> EvalResult<()> {
+        // Push the set onto the stack
+        self.compile(node.set().unwrap())?;
+
+        // Push the key and emit the access instruction.
+        //
+        // This order matters because the key needs to be evaluated
+        // first to fail in the correct order on type errors.
+        self.compile(node.index().unwrap())?;
+        self.chunk.add_op(OpCode::OpAttrsSelect);
+
+        Ok(())
+    }
+
     // Compile list literals into equivalent bytecode. List
     // construction is fairly simple, composing of pushing code for
     // each literal element and an instruction with the element count.

tvix/eval/src/errors.rs

@@ -10,6 +10,10 @@ pub enum Error {
         given: &'static str,
     },
 
+    AttributeNotFound {
+        name: String,
+    },
+
     TypeError {
         expected: &'static str,
         actual: &'static str,

tvix/eval/src/opcode.rs

@@ -46,6 +46,7 @@ pub enum OpCode {
     OpAttrs(usize),
     OpAttrPath(usize),
     OpAttrsUpdate,
+    OpAttrsSelect,
 
     // Lists
     OpList(usize),

tvix/eval/src/value/attrs.rs

@@ -45,6 +45,26 @@ impl AttrsRep {
             }
         }
     }
+
+    fn select(&self, key: &str) -> Option<&Value> {
+        match self {
+            AttrsRep::Empty => None,
+
+            AttrsRep::KV { name, value } => {
+                if key == "name" {
+                    return Some(&name);
+                }
+
+                if key == "value" {
+                    return Some(&value);
+                }
+
+                None
+            }
+
+            AttrsRep::Map(map) => map.get(&key.to_string().into()),
+        }
+    }
 }
 
 #[derive(Clone, Debug)]
@@ -133,6 +153,11 @@ impl NixAttrs {
         }
     }
 
+    // Select a value from an attribute set by key.
+    pub fn select(&self, key: &str) -> Option<&Value> {
+        self.0.select(key)
+    }
+
     /// Implement construction logic of an attribute set, to encapsulate
     /// logic about attribute set optimisations inside of this module.
     pub fn construct(count: usize, mut stack_slice: Vec<Value>) -> EvalResult<Self> {

tvix/eval/src/vm.rs

@@ -158,6 +158,21 @@ impl VM {
                     self.push(Value::Attrs(Rc::new(lhs.update(&rhs))))
                 }
 
+                OpCode::OpAttrsSelect => {
+                    let key = self.pop().as_string()?;
+                    let attrs = self.pop().as_attrs()?;
+
+                    match attrs.select(key.as_str()) {
+                        Some(value) => self.push(value.clone()),
+
+                        None => {
+                            return Err(Error::AttributeNotFound {
+                                name: key.as_str().to_string(),
+                            })
+                        }
+                    }
+                }
+
                 OpCode::OpList(count) => {
                     let list =
                         NixList::construct(count, self.stack.split_off(self.stack.len() - count));