Add struct, enum, method, match, and array code generation

[benpayne]

Summary

This PR implements comprehensive code generation support for structs, enums, methods, pattern matching, and arrays in the LLVM backend. It extends the compiler to handle complex type definitions and their operations, moving beyond basic function and primitive type support.

Key Changes

• Struct Type System: Added getOrCreateStructType() to create LLVM struct types from AST definitions, with support for empty structs via dummy bytes. Struct types are registered during module generation and cached for reuse.

• Method Code Generation: Implemented method generation from impl blocks as mangled LLVM functions (e.g., StructName_methodName). Methods are generated with proper parameter handling including self parameter conversion to struct types, and automatic return value insertion.

• Struct Literals and Field Access: Added genStructLiteral() to allocate and initialize structs on the stack, and genFieldAccess() to read struct fields using GEP (GetElementPtr) instructions.

• Method Calls: Implemented genMethodCall() to resolve and invoke struct methods with proper argument passing, including the implicit self argument.

• Pattern Matching: Added genMatchExpression() with switch-based code generation for integer subjects, support for wildcard patterns, and variable binding in match arms.

• Error Handling: Implemented genTryExpression() for the ? operator (simplified pass-through for now, with comments for full Result/Option handling).

• Array Support: Added genArrayLiteral() to allocate and initialize arrays on the stack, and genIndexExpression() for array element access via GEP.

• Type Resolution: Enhanced getLLVMType() to resolve struct and enum types from definition maps, with fallback to i32 for unknown types.

• Type Casting: Added implicit type casting in variable declarations and return statements for integer, float, and struct types to handle type mismatches gracefully.

• Control Flow Safety: Added checks to prevent code generation after terminators (unreachable code detection) in genBlock() and genStatement().

• Build Configuration: Added BLANG_ENABLE_LLVM CMake option to allow building in parse-only mode without LLVM, with CI updated to test both configurations.

• Module Scope Tracking: Stored module scope in Module for type resolution during code generation.

Notable Implementation Details

• Struct fields are accessed via GEP with field indices resolved from struct definitions
• Methods use name mangling to avoid conflicts with top-level functions
• Match expressions create separate basic blocks for each arm with proper control flow merging
• Array literals are stack-allocated with element-wise initialization
• Generic functions and methods are skipped during code generation (templates, not concrete code)
• Empty structs receive a dummy i8 field to satisfy LLVM's struct layout requirements

https://claude.ai/code/session_01FRtRs941FT95yKVtd2oEgX