Potential integer overflow in Cursor::peek_n method

[coderabbitai]

Description

The peek_n method in src/parser/cursor.rs (lines 145-147) uses unchecked addition self.pos + n which could potentially overflow if n is extremely large. While .get() safely returns None for out-of-bounds indices, integer overflow itself can be undefined behavior in Rust (in release mode with overflow checks disabled).

References

• Pull Request: Merge pull request #195 from WebFirstLanguage/devin/1765135204-fix-clippy-warnings #196
• Comment: Merge pull request #195 from WebFirstLanguage/devin/1765135204-fix-clippy-warnings #196 (comment)
• Reported by: @logbie

Context

This issue was identified during code review of the parser refactor that introduces the new cursor-based token navigation system.

[logbie]

@claude

Investigate and fix this issue useing TDD.

[claude]

Claude finished @logbie's task —— View job • claude/issue-197-20251209-1025 • Create PR ➔

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Investigating and Fixing Integer Overflow in Cursor::peek_n ✅

Following TDD approach to fix potential integer overflow in src/parser/cursor.rs:146.

Analysis

Found the issue in peek_n method:

pub fn peek_n(&self, n: usize) -> Option<&'a TokenWithPosition> {
    self.tokens.get(self.pos + n)  // <- Unchecked addition can overflow
}

Test Results

✅ Tests confirmed the overflow issue:

• Test from position 0 with usize::MAX passed (addition wraps around)
• Test from position 3 with usize::MAX failed with overflow panic

Solution

Fixed using checked arithmetic:

pub fn peek_n(&self, n: usize) -> Option<&'a TokenWithPosition> {
    // Use checked addition to prevent integer overflow
    self.pos.checked_add(n)
        .and_then(|index| self.tokens.get(index))
}

Todo List:

• Read repository guidelines and current implementation
• Write failing tests that expose the overflow issue
• Implement safe fix using checked arithmetic
• Verify all tests pass (16/16 cursor tests passing)
• Run linting and type checking
• Commit and push changes

Summary

Successfully implemented TDD approach to fix the integer overflow vulnerability. The fix uses checked_add() to safely handle potentially large n values, returning None instead of panicking on overflow. All existing functionality is preserved and comprehensive tests ensure the fix works correctly.