# Rust MCP Debugger Test Report - January 14, 2025

## Executive Summary

Testing of the mcp-debugger with Rust language examples revealed a **critical bug** that prevents Rust debugging from functioning. The bug is a JavaScript context binding issue in the session manager that causes a runtime error when attempting to start any Rust debugging session.

## Test Environment

- **Date**: January 14, 2025, 4:37 PM EST
- **Platform**: Windows 11
- **Working Directory**: `C:\path\to\debug-mcp-server`
- **MCP Server**: mcp-debugger
- **Examples Tested**: 
  - `examples/rust/hello_world` - Simple Rust program with variables and functions
  - `examples/rust/async_example` - Async Rust with Tokio (not reached due to initial failure)

## Test Execution

### Test 1: Hello World Example

**Steps Performed:**
1. Created Rust debug session: `create_debug_session` with language="rust"
   - **Result**: ✅ SUCCESS - Session ID created: `a166456d-fc2e-46c3-8aea-39cc2f1304af`

2. Set breakpoint at line 12 in `hello_world/src/main.rs`
   - **Result**: ✅ SUCCESS - Breakpoint set (unverified)
   - Breakpoint ID: `b9ea28cb-6b88-49c9-9606-f79ad32a1428`

3. Started debugging with source file path
   - **Command**: `start_debugging` with scriptPath pointing to `main.rs`
   - **Result**: ❌ **FAILURE**
   - **Error**: `"Cannot read properties of undefined (reading 'lastToolchainValidation')"`

4. Retry with executable path  
   - **Command**: `start_debugging` with scriptPath pointing to compiled `.exe`
   - **Result**: ❌ **FAILURE**  
   - **Error**: Same error - `"Cannot read properties of undefined (reading 'lastToolchainValidation')"`

### Test 2: Async Example

Not attempted due to critical failure in Test 1.

## Root Cause Analysis

### The Bug

Located in `src/session/session-manager-operations.ts`, lines 115-122:

```typescript
const consumeLastToolchainValidation =
  (adapter as { consumeLastToolchainValidation?: () => unknown }).consumeLastToolchainValidation;
const toolchainValidation =
  typeof consumeLastToolchainValidation === 'function'
  ? (consumeLastToolchainValidation() as ToolchainValidationState)  // ❌ BUG HERE
  : undefined;
```

**Problem**: When the method `consumeLastToolchainValidation` is extracted from the adapter object and stored as a standalone function reference, it **loses its `this` binding**. When subsequently called as `consumeLastToolchainValidation()`, the `this` context inside the method is `undefined`, causing the error when trying to access `this.lastToolchainValidation`.

### Technical Details

From `packages/adapter-rust/src/rust-debug-adapter.ts`, line 108:

```typescript
public consumeLastToolchainValidation(): ToolchainValidationResult | undefined {
  const value = this.lastToolchainValidation;  // ❌ 'this' is undefined here
  this.lastToolchainValidation = undefined;
  return value;
}
```

When called without proper context, `this` is `undefined`, resulting in:
```
Cannot read properties of undefined (reading 'lastToolchainValidation')
```

### Why This Happens

This is a classic JavaScript/TypeScript issue with method references. When you do:
```typescript
const method = object.method;
method(); // 'this' inside method is undefined (in strict mode)
```

The proper way would be either:
```typescript
adapter.consumeLastToolchainValidation();  // Option 1: Call directly on object
```
or
```typescript
const bound = consumeLastToolchainValidation.bind(adapter);
bound();  // Option 2: Bind first, then call
```

### Impact

This bug affects **ALL Rust debugging attempts**. The error occurs during the `startProxyManager` phase, before:
- The debug adapter is launched
- Breakpoints are sent to the debugger
- Any actual debugging can begin

The bug is **100% reproducible** and makes Rust debugging completely non-functional.

## Additional Observations

### What Works
- ✅ Session creation (`create_debug_session`)
- ✅ Breakpoint queuing (`set_breakpoint`) 
- ✅ Rust adapter initialization
- ✅ CodeLLDB path resolution (would work if we got that far)

### What Fails
- ❌ **Any `start_debugging` call with Rust language**
- ❌ Cannot proceed to actual debugging operations

### Other Issues Not Tested
Due to the critical blocking bug, the following could not be evaluated:
- Toolchain compatibility checking (MSVC vs GNU)
- Actual breakpoint verification
- Variable inspection
- Stepping through code
- Stack traces
- Expression evaluation

## Comparison with Other Languages

Based on the codebase analysis, this bug appears to be **Rust-specific** because:

1. Only `RustDebugAdapter` has the `consumeLastToolchainValidation` method
2. The method exists to handle Rust's MSVC toolchain compatibility issues
3. Python and JavaScript adapters don't have this method, so they work fine
4. The session manager tries to call this method optionally (checking if it exists)

The optional calling pattern suggests this was added specifically for Rust, but the implementation has the context binding bug.

## Severity Assessment

**Severity**: 🔴 **CRITICAL**

- **Functional Impact**: Complete failure of Rust debugging functionality
- **Workaround Available**: ❌ No - bug must be fixed
- **Reproducibility**: 100% - fails every time
- **User Impact**: All users attempting Rust debugging

## Recommendations

### Immediate Fix Required

In `src/session/session-manager-operations.ts`, line 120-121, change:

```typescript
// BEFORE (broken):
const toolchainValidation =
  typeof consumeLastToolchainValidation === 'function'
  ? (consumeLastToolchainValidation() as ToolchainValidationState)
  : undefined;

// AFTER (fixed):
const toolchainValidation =
  typeof consumeLastToolchainValidation === 'function'
  ? (consumeLastToolchainValidation.call(adapter) as ToolchainValidationState)
  : undefined;
```

Or more elegantly:

```typescript
const toolchainValidation =
  typeof adapter.consumeLastToolchainValidation === 'function'
  ? (adapter.consumeLastToolchainValidation() as ToolchainValidationState)
  : undefined;
```

### Testing After Fix

Once fixed, comprehensive testing should include:
1. ✅ Basic hello_world example with breakpoints
2. ✅ Async example with Tokio
3. ✅ MSVC vs GNU toolchain detection
4. ✅ Variable inspection
5. ✅ Stepping operations
6. ✅ Expression evaluation
7. ✅ Stack traces

## Files Examined

1. `examples/rust/hello_world/src/main.rs` - Test subject
2. `examples/rust/async_example/src/main.rs` - Test subject (not reached)
3. `packages/adapter-rust/src/rust-debug-adapter.ts` - Bug location (method definition)
4. `src/session/session-manager-operations.ts` - Bug location (method invocation)

## Conclusion

The mcp-debugger has a **critical blocking bug** that prevents any Rust debugging from working. The bug is a straightforward JavaScript context binding issue that occurs when extracting a method reference without preserving its `this` context. 

**The bug must be fixed before Rust debugging can be tested or used.**

The fix is simple and well-understood, requiring only a small change to how the method is invoked. After fixing, full regression testing of Rust debugging features should be performed.

---

**Report Generated**: January 14, 2025, 4:40 PM EST  
**Tester**: Cline (AI Assistant)  
**Status**: Testing Blocked - Critical Bug Found
