feat: detect unused expression errors in statement context

[aofei]

• Add ElemFlags to track expression kinds (type cast, map index, comma-ok)
• Add isValidStmtExpr() to validate statements (only calls and receive ops)
• Add exprDescription() and helpers for Go-compatible error messages
• Report "xxx (type) is not used" errors in EndStmt()

[gemini-code-assist]

Summary of Changes

Hello @aofei, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the compiler's diagnostic capabilities by introducing robust detection and reporting of unused expression errors in statement contexts. It categorizes different types of expressions using new flags and implements validation logic to ensure that only appropriate expressions (like function calls or channel receives) can stand alone as statements, thereby improving code quality and adherence to Go language specifications.

Highlights

• Expression Kind Tracking: Introduced ElemFlags to categorize expressions, specifically marking type cast, map index, and comma-ok expressions for special handling.
• Statement Context Validation: Added isValidStmtExpr() to determine if an expression is valid when used as a standalone statement, adhering to Go's rules (only function calls and channel receive operations are permitted).
• Go-Compatible Error Messages: Implemented exprCode() and exprDescription() helper functions to generate error messages that closely match the Go compiler's format for unused expressions.
• Unused Expression Error Reporting: Modified EndStmt() to detect and report 'xxx (type) is not used' errors for expressions that are not valid as statements, improving compiler diagnostics.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.19%. Comparing base (03c926d) to head (fc79eee).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #552      +/-   ##
==========================================
+ Coverage   96.00%   96.19%   +0.18%     
==========================================
  Files          23       23              
  Lines        6482     6592     +110     
==========================================
+ Hits         6223     6341     +118     
+ Misses        193      183      -10     
- Partials       66       68       +2     

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[gemini-code-assist]

Code Review

This pull request introduces a valuable feature to detect and report unused expression values as errors, aligning the behavior more closely with the standard Go compiler. The implementation is thorough, including new flags to track expression kinds, helper functions to generate Go-compatible error messages, and comprehensive tests for both valid and invalid statement expressions. I've identified a few areas for improvement in the helper functions to enhance correctness and maintainability, particularly in isAddressable and exprCode.

[Copilot]

Pull request overview

This PR adds compile-time detection of unused expressions in statement context, matching Go compiler behavior. When an expression that produces a value is used as a standalone statement (e.g., a + 1 instead of _ = a + 1), the compiler now reports an error with descriptive messages like "a + 1 (value of type int) is not used".

Key changes:

• Added ElemFlags enum to track expression operand modes (type cast, map index, comma-ok)
• Implemented isValidStmtExpr() to validate that only function calls and receive operations can appear as statements
• Added exprDescription() and helper functions to generate Go-compatible error messages with proper type and value formatting

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/stack.go	Adds ElemFlags type and field to Elem struct to track expression kinds for error reporting
codebuild.go	Implements core validation logic including isValidStmtExpr(), exprDescription(), and integration into EndStmt()
ast.go	Sets ElemFlagTypeCast flag for type conversion expressions
builtin.go	Adds clarifying comment about channel receive operations being valid as statements
codebuild_test.go	Adds unit tests for helper functions like isValidStmtExpr(), exprCode(), and isAddressable()
error_msg_test.go	Adds comprehensive test cases covering various unused expression scenarios with expected error messages
package_test.go	Fixes existing test and adds valid statement expression tests for builtin functions

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[xgopilot]

Code Review Summary

Excellent implementation! This PR successfully adds unused expression error detection with Go-compatible error messages. The code demonstrates strong engineering practices with comprehensive test coverage and clean design.

Strengths:

• Outstanding design: ElemFlags provides a clean, performant way to track expression kinds
• Comprehensive testing: 30+ test cases with exact error message validation
• Go-compatible: Error messages accurately match Go compiler format
• Well-documented: Functions have clear explanations matching implementation

Minor suggestions (2 documentation improvements noted inline - both optional)

All review agents (code quality, performance, documentation, security) found the implementation to be sound with no blocking issues.