feat(tvix/vm): add opcodes for new Value::NotFound sentinel

This sentinel value is going to be used for certain nested accesses
into attribute sets.

There is a new instruction similar to `OpAttrsSelect` which leaves the
sentinel on the stack if a key is not found, instead of raising an
error.

Additionally, a new jump instruction makes its jump operation
conditional on finding such a sentinel value.

Change-Id: I2642f0a0bcc85bbe0ead68ea09a7dd794dbedeac
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6166
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/opcode.rs

@@ -41,12 +41,14 @@ pub enum OpCode {
     OpJump(usize),
     OpJumpIfTrue(usize),
     OpJumpIfFalse(usize),
+    OpJumpIfNotFound(usize),
 
     // Attribute sets
     OpAttrs(usize),
     OpAttrPath(usize),
     OpAttrsUpdate,
     OpAttrsSelect,
+    OpAttrOrNotFound,
     OpAttrsIsSet,
 
     // Lists

tvix/eval/src/value/mod.rs

@@ -26,6 +26,7 @@ pub enum Value {
     // are never returned to or created directly by users.
     AttrPath(Vec<NixString>),
     Blackhole,
+    NotFound,
 }
 
 impl Value {
@@ -48,7 +49,7 @@ impl Value {
             Value::List(_) => "list",
 
             // Internal types
-            Value::AttrPath(_) | Value::Blackhole => "internal",
+            Value::AttrPath(_) | Value::Blackhole | Value::NotFound => "internal",
         }
     }
 
@@ -116,8 +117,9 @@ impl Display for Value {
             )),
 
             // internal types
-            Value::AttrPath(_) => f.write_str("internal<attrpath>"),
+            Value::AttrPath(_) => f.write_str("internal[attrpath]"),
-            Value::Blackhole => f.write_str("internal<blackhole>"),
+            Value::Blackhole => f.write_str("internal[blackhole]"),
+            Value::NotFound => f.write_str("internal[not found]"),
         }
     }
 }

tvix/eval/src/vm.rs

@@ -173,6 +173,16 @@ impl VM {
                     }
                 }
 
+                OpCode::OpAttrOrNotFound => {
+                    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 => self.push(Value::NotFound),
+                    }
+                }
+
                 OpCode::OpAttrsIsSet => {
                     let key = self.pop().as_string()?;
                     let attrs = self.pop().as_attrs()?;
@@ -210,6 +220,13 @@ impl VM {
                     }
                 }
 
+                OpCode::OpJumpIfNotFound(offset) => {
+                    if matches!(self.peek(0), Value::NotFound) {
+                        self.pop();
+                        self.ip += offset;
+                    }
+                }
+
                 // These assertion operations error out if the stack
                 // top is not of the expected type. This is necessary
                 // to implement some specific behaviours of Nix