make sim::Compiler not print things to stdout unless you ask for it

crates/fayalite/src/sim.rs

@@ -1601,6 +1601,14 @@ impl MakeTraceDeclTarget {
     }
 }
 
+struct DebugOpaque<T>(T);
+
+impl<T> fmt::Debug for DebugOpaque<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str("<...>")
+    }
+}
+
 #[derive(Debug)]
 pub struct Compiler {
     insns: Insns<InsnsBuilding>,
@@ -1622,6 +1630,7 @@ pub struct Compiler {
     registers: Vec<Register>,
     traces: SimTraces<Vec<SimTrace<SimTraceKind, ()>>>,
     memories: Vec<Memory>,
+    dump_assignments_dot: Option<DebugOpaque<Box<dyn Fn(&dyn fmt::Debug)>>>,
 }
 
 impl Compiler {
@@ -1647,8 +1656,14 @@ impl Compiler {
             registers: Vec::new(),
             traces: SimTraces(Vec::new()),
             memories: Vec::new(),
+            dump_assignments_dot: None,
         }
     }
+    #[doc(hidden)]
+    /// This is explicitly unstable and may be changed/removed at any time
+    pub fn dump_assignments_dot(&mut self, callback: Box<dyn Fn(&dyn fmt::Debug)>) {
+        self.dump_assignments_dot = Some(DebugOpaque(callback));
+    }
     fn new_sim_trace(&mut self, kind: SimTraceKind) -> TraceScalarId {
         let id = TraceScalarId(self.traces.0.len());
         self.traces.0.push(SimTrace {
@@ -4650,12 +4665,11 @@ impl Compiler {
     fn process_assignments(&mut self) {
         self.assignments
             .finalize(self.insns.state_layout().ty.clone().into());
-        println!(
+        if let Some(DebugOpaque(dump_assignments_dot)) = &self.dump_assignments_dot {
-            "{:#?}",
+            let graph =
-            petgraph::dot::Dot::new(&petgraph::graph::DiGraph::<_, _, usize>::from_elements(
+                petgraph::graph::DiGraph::<_, _, usize>::from_elements(self.assignments.elements());
-                self.assignments.elements()
+            dump_assignments_dot(&petgraph::dot::Dot::new(&graph));
-            ))
+        }
-        );
         let assignments_order: Vec<_> = match petgraph::algo::toposort(&self.assignments, None) {
             Ok(nodes) => nodes
                 .into_iter()