feat(vm): add call stack depth guard to MirInterpreter

src/backend/mir_interpreter/exec.rs

@@ -13,6 +13,23 @@ impl MirInterpreter {
         func: &MirFunction,
         arg_vals: Option<&[VMValue]>,
     ) -> Result<VMValue, VMError> {
+        // Safety valve: cap nested exec_function_inner depth to avoid Rust stack overflow
+        // on accidental infinite recursion in MIR (e.g., self-recursive call chains).
+        const MAX_CALL_DEPTH: usize = 1024;
+        self.call_depth = self.call_depth.saturating_add(1);
+        if self.call_depth > MAX_CALL_DEPTH {
+            eprintln!(
+                "[vm-call-depth] exceeded {} in fn={} (depth={})",
+                MAX_CALL_DEPTH,
+                func.signature.name,
+                self.call_depth
+            );
+            self.call_depth = self.call_depth.saturating_sub(1);
+            return Err(VMError::InvalidInstruction(format!(
+                "vm call stack depth exceeded (max_depth={}, fn={})",
+                MAX_CALL_DEPTH, func.signature.name
+            )));
+        }
         // Phase 1: delegate cross-class reroute / narrow fallbacks to method_router
         if let Some(r) = super::method_router::pre_exec_reroute(self, func, arg_vals) { return r; }
         let saved_regs = mem::take(&mut self.regs);
@@ -96,6 +113,7 @@ impl MirInterpreter {
                 BlockOutcome::Return(result) => {
                     self.cur_fn = saved_fn;
                     self.regs = saved_regs;
+                    self.call_depth = self.call_depth.saturating_sub(1);
                     return Ok(result);
                 }
                 BlockOutcome::Next {

src/backend/mir_interpreter/mod.rs

@@ -41,6 +41,9 @@ pub struct MirInterpreter {
     pub(super) inst_count: u64,
     pub(super) branch_count: u64,
     pub(super) compare_count: u64,
+    /// Call stack depth (exec_function_inner nesting). Used as a safety valve
+    /// to prevent Rust stack overflow on accidental infinite recursion in MIR.
+    pub(super) call_depth: usize,
 }
 
 impl MirInterpreter {
@@ -58,6 +61,7 @@ impl MirInterpreter {
             inst_count: 0,
             branch_count: 0,
             compare_count: 0,
+            call_depth: 0,
         }
     }