Maintenance: RegisterExpression - Add test coverage for readonly register errors

[claude]

Area Inspected

Randomly selected and inspected: vim-engine/src/main/kotlin/com/maddyhome/idea/vim/vimscript/model/expressions/RegisterExpression.kt

Issues Found

While reviewing RegisterExpression.kt and its test coverage, I discovered a significant gap in test coverage:

Missing Test Coverage:

• No tests for E354 error when attempting to assign to readonly registers
• The assign() method in RegisterExpression.kt correctly throws E354 for readonly registers (%, :, ., #, =), but this behavior was not tested
• Without these tests, regressions in this error handling could go undetected

Changes Made

Added comprehensive test coverage in LetCommandRegisterLValueTest.kt:

1. Tests for simple assignment to readonly registers:

   • @% (current filename register)
   • @: (last command register)
   • @. (last inserted text register)
   • @# (alternate buffer register)
   • @= (expression buffer register)

2. Tests for compound assignment to readonly registers:

   • @%.= (current filename register with string concatenation)
   • @:.= (last command register with string concatenation)

All tests verify that the appropriate E354 error is thrown with the correct error message.

Why This Improves the Code

1. Prevents Regressions: Ensures that readonly register protection cannot be accidentally broken in future refactorings
2. Documents Behavior: Tests serve as executable documentation of the expected error handling
3. Completeness: Brings test coverage in line with the implementation's error handling capabilities
4. Vim Compatibility: Ensures IdeaVim maintains compatibility with Vim's readonly register behavior

Code Review Notes

The implementation in RegisterExpression.kt is sound:

• Code style follows Kotlin conventions ✓
• Null safety is properly handled ✓
• Error handling is appropriate ✓
• The logic is clear and maintainable ✓

The only issue was the missing test coverage, which has now been addressed.

🤖 Generated with Claude Code