feat(tvix/eval): implement chunk disassembler output

This makes for a much nicer view of an execution if `--feature
disassembler` is set, for example:

  tvix-repl> let value = [ 1 2 { a = 1; } ]; in value ++ [ 1 ]
  === compiled bytecode (11 operations) ===
  02   OpConstant(1)
  02   OpConstant(2)
  02   OpConstant("a")
  02   OpConstant(1)
  02   OpAttrs(1)
  02   OpList(3)
  02   OpGetLocal(0)
  02   OpConstant(1)
  02   OpList(1)
  02   OpConcat
  02   OpCloseScope(1)
  === runtime trace ===
  0001 OpConstant(ConstantIdx(0))  [ 1 ]
  0002 OpConstant(ConstantIdx(1))  [ 1 2 ]
  0003 OpConstant(ConstantIdx(2))  [ 1 2 "a" ]
  0004 OpConstant(ConstantIdx(3))  [ 1 2 "a" 1 ]
  0005 OpAttrs(1)                  [ 1 2 { a = 1; } ]
  0006 OpList(3)                   [ [ 1 2 { a = 1; } ] ]
  0007 OpGetLocal(0)               [ [ 1 2 { a = 1; } ] [ 1 2 { a = 1; } ] ]
  0008 OpConstant(ConstantIdx(4))  [ [ 1 2 { a = 1; } ] [ 1 2 { a = 1; } ] 1 ]
  0009 OpList(1)                   [ [ 1 2 { a = 1; } ] [ 1 2 { a = 1; } ] [ 1 ] ]
  0010 OpConcat                    [ [ 1 2 { a = 1; } ] [ 1 2 { a = 1; } 1 ] ]
  0011 OpCloseScope(1)             [ [ 1 2 { a = 1; } 1 ] ]
  => [ 1 2 { a = 1; } 1 ] :: list

Change-Id: If79c7fd1f0f18255ddb3763c1ba585fda8041b1b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6195
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/chunk.rs

@@ -4,7 +4,7 @@ use crate::value::Value;
 #[derive(Debug, Default)]
 pub struct Chunk {
     pub code: Vec<OpCode>,
-    constants: Vec<Value>,
+    pub constants: Vec<Value>,
 }
 
 impl Chunk {

tvix/eval/src/disassembler.rs

@@ -4,6 +4,7 @@
 use std::io::{Stderr, Write};
 use tabwriter::TabWriter;
 
+use crate::chunk::Chunk;
 use crate::opcode::OpCode;
 use crate::value::Value;
 
@@ -35,3 +36,33 @@ impl Drop for Tracer {
         self.0.flush().ok();
     }
 }
+
+fn disassemble_op(tw: &mut TabWriter<Stderr>, chunk: &Chunk, offset: usize) {
+    let code_width = format!("{}", chunk.code.len()).len();
+    write!(tw, "{:0width$}\t ", width = code_width).ok();
+
+    match chunk.code[offset] {
+        OpCode::OpConstant(idx) => write!(tw, "OpConstant({})\n", chunk.constant(idx)).ok(),
+
+        op => write!(tw, "{:?}\n", op).ok(),
+    };
+}
+
+/// Disassemble a chunk of code, printing out the operations in a
+/// reasonable, human-readable format.
+pub fn disassemble_chunk(chunk: &Chunk) {
+    let mut tw = TabWriter::new(std::io::stderr());
+
+    write!(
+        &mut tw,
+        "=== compiled bytecode ({} operations) ===\n",
+        chunk.code.len()
+    )
+    .ok();
+
+    for (idx, _) in chunk.code.iter().enumerate() {
+        disassemble_op(&mut tw, chunk, idx);
+    }
+
+    tw.flush().ok();
+}

tvix/eval/src/eval.rs

@@ -17,7 +17,9 @@ pub fn interpret(code: &str, location: Option<PathBuf>) -> EvalResult<Value> {
     }
 
     let result = crate::compiler::compile(ast, location)?;
-    println!("code: {:?}", result.chunk);
+
+    #[cfg(feature = "disassembler")]
+    crate::disassembler::disassemble_chunk(&result.chunk);
 
     for warning in result.warnings {
         eprintln!(