feat(tvix/eval): implement optional runtime tracing

This adds a `disassembler` feature to the crate configuration that
traces the operations executed and the state of the stack at runtime.

This can be enabled by compiling with `--feature disassembler`.

This will also gain a more sensible layout of code slices eventually.

Change-Id: I34c15e1cd346ecc4362b5afba6bf82dd49359d20
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6193
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/disassembler.rs

@@ -0,0 +1,37 @@
+//! Implements methods for disassembling and printing a representation
+//! of compiled code, as well as tracing the runtime stack during
+//! execution.
+use std::io::{Stderr, Write};
+use tabwriter::TabWriter;
+
+use crate::opcode::OpCode;
+use crate::value::Value;
+
+/// Helper struct to trace runtime values and automatically flush the
+/// output after the value is dropped (i.e. in both success and
+/// failure exits from the VM).
+pub struct Tracer(TabWriter<Stderr>);
+
+impl Tracer {
+    pub fn new() -> Self {
+        let mut tw = TabWriter::new(std::io::stderr());
+        write!(&mut tw, "=== runtime trace ===\n").ok();
+        Tracer(tw)
+    }
+
+    pub fn trace(&mut self, op: &OpCode, ip: usize, stack: &[Value]) {
+        write!(&mut self.0, "{:04} {:?}\t[ ", ip, op).ok();
+
+        for val in stack {
+            write!(&mut self.0, "{} ", val).ok();
+        }
+
+        write!(&mut self.0, "]\n").ok();
+    }
+}
+
+impl Drop for Tracer {
+    fn drop(&mut self) {
+        self.0.flush().ok();
+    }
+}