feat(tvix/eval): add initial opcode representation

Change-Id: Ibc7685a6b0b92e08f0b6c82cf7d9b04fbb593a4e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6068
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/main.rs

@@ -6,6 +6,7 @@ use std::{
 
 mod errors;
 mod eval;
+mod opcode;
 mod value;
 
 fn main() {

tvix/eval/src/opcode.rs

@@ -0,0 +1,19 @@
+//! This module implements the instruction set running on the abstract
+//! machine implemented by tvix.
+
+#[derive(Clone, Copy, Debug)]
+pub struct ConstantIdx(pub usize);
+
+#[derive(Clone, Copy, Debug)]
+pub struct CodeIdx(pub usize);
+
+#[derive(Clone, Copy, Debug)]
+pub enum OpCode {
+    // Push a constant onto the stack.
+    OpConstant(ConstantIdx),
+
+    // Push a literal value.
+    OpNull,
+    OpTrue,
+    OpFalse,
+}