feat: Improve parser error messages for better user experience

[jade-codes]

Problem

Currently, pest parsing errors are displayed directly to users, which can be cryptic and unhelpful. For example:

ParseError { variant: Expected(keyword), positives: [keyword], negatives: [] }

This tells the user almost nothing about what went wrong or how to fix it.

Current State

• ParseError struct in core/parse_result.rs captures pest errors
• error_codes.rs defines error code constants (P001-P007) but they're not used for parser errors
• The LSP displays parse_error.variant directly as the message (see syntax/sysml/parser.rs:63)

Proposed Solution

1. Error Code Integration

Map pest error patterns to our defined error codes:

• P001 (PARSER_SYNTAX_ERROR) - General syntax errors
• P002 (PARSER_UNEXPECTED_TOKEN) - When pest reports unexpected input
• P003 (PARSER_EXPECTED_TOKEN) - When pest reports expected tokens
• P006 (PARSER_UNTERMINATED) - Unterminated strings/comments

2. Human-Readable Error Messages

Create a PestErrorTranslator that:

• Analyzes pest's positives and negatives lists
• Maps grammar rule names to user-friendly terms (e.g., package_declaration → "package declaration")
• Provides context-aware suggestions based on error location

3. Example Transformations

Pest Error	Current Display	Improved Display
Expected([keyword])	"Expected(keyword)"	"P003: Expected a keyword (e.g., 'part', 'port', 'package')"
Expected([identifier])	"Expected(identifier)"	"P003: Expected an identifier"
Expected([";"])	"Expected(";")`	"P003: Missing semicolon at end of statement"
Unexpected(";")	"Unexpected(;)"	"P002: Unexpected ';' - check for missing expression"

4. Contextual Help

For common errors, provide hints:

• Missing semicolon → "Did you forget a ';' at the end?"
• Unmatched brace → "Unmatched '{' - check for missing '}'"
• Reserved word as identifier → "'part' is a reserved keyword and cannot be used as a name"

5. Implementation Plan

1. Phase 1: Create parser/error_translator.rs module

   • Rule name → friendly name mapping
   • Basic error message templates

2. Phase 2: Integrate with ParseError

   • Add code field to ParseError
   • Add hint field for suggestions
   • Update LSP diagnostics to use new format

3. Phase 3: Context-aware improvements

   • Analyze surrounding tokens for better suggestions
   • Add "did you mean?" for near-matches
   • Track common error patterns for tailored hints

Success Criteria

• All parser errors display with error codes (P001-P007)
• Error messages are in plain English, not grammar rule names
• Common errors have helpful hints
• LSP diagnostics show improved messages

Related

• Error codes defined in core/error_codes.rs
• Current parsing: syntax/sysml/parser.rs, syntax/kerml/parser.rs
• LSP diagnostics: syster-lsp/src/server/diagnostics.rs