Add `StructType` for referencing defined structs in variable declarations

[omsherikar]

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Description

Enable variables to be declared with struct types by introducing a lightweight StructType reference that resolves to a previously defined struct.

This phase allows declaring struct variables, but does not introduce value semantics, copying, or passing structs by value.

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Background

• Phase 1 introduces named struct types in LLVM IR Add LLVM IR codegen for StructDefStmt (struct type definitions) #143
• Variables need a way to reference these types at the AST level
• Struct variables are pointer-based and are allocated on the stack
• Struct layout follows LLVM target data layout rules and is opaque at the language level

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Tasks

1. Add StructType

• Create StructType in src/irx/system.py
• Inherit from astx.DataType
• Store only:
  • struct_name: str

StructType acts purely as a symbolic reference.
Struct definitions are resolved during code generation.

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2. Update variable declaration codegen

Update visit(astx.VariableDeclaration) in llvmliteir.py:

• Detect when type_ is StructType
• Look up the corresponding LLVM struct type using struct_name
• Allocate the struct using ir_builder.alloca(llvm_struct_type)
• Register the variable as holding a pointer to the struct

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Error Handling (Required)

Raise a compile-time error if:

• The referenced struct name is not defined

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Example Usage

# Reference a previously defined struct
point_type = StructType(struct_name="Point")

# Declare a struct variable
point_var = astx.VariableDeclaration(name="p", type_=point_type)